|
I just looked up Bowditch because I was surprised at the generality you
implied. Bowditch is clearly talking about hydraulic current which _adds_ to the tidal current. The overall effect is as I described in my other response. Cheers Jeff Morris wrote: Then you're disagreeing with Bowditch: "A slight departure from the sine form is exhibited by the reversing current in a strait, such as East River, New York, that connects two tidal basins. The tides at the two ends of a strait are seldom in phase or equal in range, and the current, called hydraulic current, is generated largely by the continuously changing difference in height of water at the two ends. The speed of a hydraulic current varies nearly as the square root of the difference in height. The speed reaches a maximum more quickly and remains at strength for a longer period than shown in Figure 914b, and the period of weak current near the time of slack is considerably shortened." "Nav" wrote in message ... That sounds like an estimate based on an energy argument. However, I don't think that case could be considered to be a closed system so such energy balance need not apply. Cheers Jeff Morris wrote: "Thom Stewart" wrote in message ... Scott, You've seem to have forgotten "The Gulf Stream Master" Jax. I'm sure he can help you ride the eddies:^) Maybe even find you a stray current heading south (g) Jax also insisted that the time of high water and slack current must be the same, to be otherwise would violate the laws of physics (on his planet). Now, for what's it worth. The Tide Flow is a sine wave. Us common folk understand that slack water is at the top and bottom of the wave. So slack is, roughly about a hour and it is roughly in the upper 30% counting the end of the rising tide and the beginning of the Ebb. 15% to the HW and 15% roughly to the Max Flow. That means about two hours to max flow, 8 hours of max flow and then another couple of hour of diminishing flow to LW. While the current flow in "simple harbors" may follow a sine curve, in cuts between two bodies of water the flow is different. The curve is more "squarish," that is, the flow ramps up quicker and stays high longer. The duration of slack is reduced. The best example is Hell Gate in New York. IIRC, the flow rate is proportional to the square root of the different of height of the two bodies. |
"Nav" wrote in message ... Bowditch? How about Google? Did you find anything? This is basic seamanship on tides. The most likely time of max stream is when tide height changes most rapidly. Well, duh! but when is the fastest heighth change? Scooter Beers |
Scott Vernon wrote: "Nav" wrote in message ... Bowditch? How about Google? Did you find anything? This is basic seamanship on tides. The most likely time of max stream is when tide height changes most rapidly. Well, duh! but when is the fastest heighth change? It depends on the type of tide. Many tides do not have simple sine wave forms. Dougs 7/12ths was his typical BS. But one can make the generality that in the open sea and at the end of estuaries it is most likely to be at half tide -OK? Cheers |
Nav wrote:
Tide tables are hardly rocket science! Well actually they are. Mankind does not yet posses the full knowledge to produce exact tide predictions. Have a look at: Tidal Science - 1996, proceedings of a symposium held in London, 21-22 October 1996. Progress in Oceanography, Elsevier Press. 'Special issue', vol. 40, Issue 1-4, 1998 This should give an overview of the outlook for tidal science just before the start of the new millennium. Peter S/Y Anicula "Nav" skrev i en meddelelse ... You should complain to the hydrographic offic. Tide tables are hardly rocket science! Cheers DSK wrote: One of the most frustrating things around here is that the tide tables are usually in error by at least half an hour. The tidal current in Snow's Cut (for example) is very strong, and so far I have yet to see it run on schedule. It's annoying after planning a departure at an inconvenient time so as to arrive at slack, only to find the current running like a champ with no sign of slacking. Fresh Breezes- Doug King |
Nav wrote:
Tide tables are hardly rocket science! Well actually they are. Mankind does not yet posses the full knowledge to produce exact tide predictions. Have a look at: Tidal Science - 1996, proceedings of a symposium held in London, 21-22 October 1996. Progress in Oceanography, Elsevier Press. 'Special issue', vol. 40, Issue 1-4, 1998 This should give an overview of the outlook for tidal science just before the start of the new millennium. Peter S/Y Anicula "Nav" skrev i en meddelelse ... You should complain to the hydrographic offic. Tide tables are hardly rocket science! Cheers DSK wrote: One of the most frustrating things around here is that the tide tables are usually in error by at least half an hour. The tidal current in Snow's Cut (for example) is very strong, and so far I have yet to see it run on schedule. It's annoying after planning a departure at an inconvenient time so as to arrive at slack, only to find the current running like a champ with no sign of slacking. Fresh Breezes- Doug King |
Nav wrote:
What a sad little man you are. ??? So bound up in jealousy. ??? What am I supposed to be jealous of? Your superior knowledge about the USS Constitution? DSK |
Thom Stewart wrote:
Doug, Pardon me if I sound argumentative. I'm not trying to be. What are you doing, *not* arguing? Wanna get kicked out of the club?? 7/12 isn't the way I figure the Tides. I use the RMS and the 12 hour cycle. I round the 707 to 70%. This leaves 30% for the curve at the top and bottom and since we are only thinking of a 1/2 sine wave 15% on the top and 15% on the bottom. So, .15x12=1.8 hours from the middle of Slack Tide, 8.4 hours max flow, 1.8 to Slack again (1.8+8.4+1.8=12 hour Tide Run) That's a bit more complicated but still simple enough to do in your head. A good method. That leaves only (g) the problem of Mid Slack.This I think is where we start to lay the blame on the Tide Tables. There can be HW by sight for at least an hour. I really don't know how to find this mid point and an hour difference at a inlet can be the difference of passage or no. Up there, heck yeah. I was very intimidated by tides, currents, and rocks, the times we chartered in the PacNW. Fortunately it's a lot easier around here. You might find the southeast coast boring. Allow time and be early. I let my boat "Lady Bug" until the right time. I like GF's explanation, but I still don't know what that means. Fresh Breezes- Doug King |
Holy Backpedal, Nav!
Of course the total current is the sum of a variety of effects. However, these are certain places where the hydraulic current dominates the flow. Snow's Cut, I suspect, is one. There are others, and they tend to be very significant for sailors. Most simple tidal currents around here max at under 2 knots, while the hydraulics are frequently over 4, sometimes 6 or more. For simple currents, the "50-90" rule, which approximates a sine curve, can be used. This says that the current reaches half strength in the first third between slack and max, and 90% in the second third. However, when a hydraulic current dominates, the current changes very quickly, often with virtually no slack. Cape Cod Canal this morning went from a .9 knot ebb to a 1 knot flood in 10 minutes. It went from 50% ebb to 50% flood strength in less than an hour, or twice as fast as a sine wave would predict. Hell Gate changes even faster than that. This is a very significant effect for the sailor, who often wishes to transit such areas at slack. Perhaps they are not common where you are, Nav, but I'm rather surprised you heard of it. jeff "Nav" wrote in message ... I just looked up Bowditch because I was surprised at the generality you implied. Bowditch is clearly talking about hydraulic current which _adds_ to the tidal current. The overall effect is as I described in my other response. Cheers Jeff Morris wrote: Then you're disagreeing with Bowditch: "A slight departure from the sine form is exhibited by the reversing current in a strait, such as East River, New York, that connects two tidal basins. The tides at the two ends of a strait are seldom in phase or equal in range, and the current, called hydraulic current, is generated largely by the continuously changing difference in height of water at the two ends. The speed of a hydraulic current varies nearly as the square root of the difference in height. The speed reaches a maximum more quickly and remains at strength for a longer period than shown in Figure 914b, and the period of weak current near the time of slack is considerably shortened." "Nav" wrote in message ... That sounds like an estimate based on an energy argument. However, I don't think that case could be considered to be a closed system so such energy balance need not apply. Cheers Jeff Morris wrote: "Thom Stewart" wrote in message ... Scott, You've seem to have forgotten "The Gulf Stream Master" Jax. I'm sure he can help you ride the eddies:^) Maybe even find you a stray current heading south (g) Jax also insisted that the time of high water and slack current must be the same, to be otherwise would violate the laws of physics (on his planet). Now, for what's it worth. The Tide Flow is a sine wave. Us common folk understand that slack water is at the top and bottom of the wave. So slack is, roughly about a hour and it is roughly in the upper 30% counting the end of the rising tide and the beginning of the Ebb. 15% to the HW and 15% roughly to the Max Flow. That means about two hours to max flow, 8 hours of max flow and then another couple of hour of diminishing flow to LW. While the current flow in "simple harbors" may follow a sine curve, in cuts between two bodies of water the flow is different. The curve is more "squarish," that is, the flow ramps up quicker and stays high longer. The duration of slack is reduced. The best example is Hell Gate in New York. IIRC, the flow rate is proportional to the square root of the different of height of the two bodies. |
Nav?
If the max flow is when the height distance is the greatest, wouldn't the tide change wave be like a pyramid instead of a sine wave? Guys, I think more thought needs to be given to what cause Tides (Moon) and rotation of the earth. It isn't just a simple hydraulic solution. Now a question; 1 point (to be awarded by Donal): If the Earth makes one rotation in 24 hours and the Moon rotates around the Earth making one revolution, why does high tide time vary daily? Ole Thom |
Thom Stewart wrote:
Nav? If the max flow is when the height distance is the greatest, wouldn't the tide change wave be like a pyramid instead of a sine wave? Guys, I think more thought needs to be given to what cause Tides (Moon) and rotation of the earth. It isn't just a simple hydraulic solution. Now a question; 1 point (to be awarded by Donal): If the Earth makes one rotation in 24 hours and the Moon rotates around the Earth making one revolution, why does high tide time vary daily? Ole Thom Well Thom, the moon makes one revolution around the earth in 28 days, always keeping the same side facing the earth, thus the time of the overhead position of the moon moves by about about 1/2 hour per day. Cheers Marty |
Marty,
Right you are! Tell Donal to mark you up for One Point. The Tides use the Moon Calender:^) That is also the reason that Tide rides are possible. Ole Thom |
Thom Stewart wrote: Nav? If the max flow is when the height distance is the greatest, wouldn't the tide change wave be like a pyramid instead of a sine wave? Thom Max flow is most likely when the _rate of change_ of height is fastest. This is most likely at half tide in the open sea, but there is no firm rule for this hence my *most likely*. The current due to tides is also summed with other currents for example. Cheers |
DSK wrote: Nav wrote: What a sad little man you are. ??? So bound up in jealousy. ??? What am I supposed to be jealous of? Your superior knowledge about the USS Constitution? Constellation actually, little man. Cheers |
That's not a backpedal. Tidal current is discrete from hydraulic current
and the two sum as I said. Cheers Jeff Morris wrote: Holy Backpedal, Nav! Of course the total current is the sum of a variety of effects. However, these are certain places where the hydraulic current dominates the flow. Snow's Cut, I suspect, is one. There are others, and they tend to be very significant for sailors. Most simple tidal currents around here max at under 2 knots, while the hydraulics are frequently over 4, sometimes 6 or more. For simple currents, the "50-90" rule, which approximates a sine curve, can be used. This says that the current reaches half strength in the first third between slack and max, and 90% in the second third. However, when a hydraulic current dominates, the current changes very quickly, often with virtually no slack. Cape Cod Canal this morning went from a .9 knot ebb to a 1 knot flood in 10 minutes. It went from 50% ebb to 50% flood strength in less than an hour, or twice as fast as a sine wave would predict. Hell Gate changes even faster than that. This is a very significant effect for the sailor, who often wishes to transit such areas at slack. Perhaps they are not common where you are, Nav, but I'm rather surprised you heard of it. jeff "Nav" wrote in message ... I just looked up Bowditch because I was surprised at the generality you implied. Bowditch is clearly talking about hydraulic current which _adds_ to the tidal current. The overall effect is as I described in my other response. Cheers Jeff Morris wrote: Then you're disagreeing with Bowditch: "A slight departure from the sine form is exhibited by the reversing current in a strait, such as East River, New York, that connects two tidal basins. The tides at the two ends of a strait are seldom in phase or equal in range, and the current, called hydraulic current, is generated largely by the continuously changing difference in height of water at the two ends. The speed of a hydraulic current varies nearly as the square root of the difference in height. The speed reaches a maximum more quickly and remains at strength for a longer period than shown in Figure 914b, and the period of weak current near the time of slack is considerably shortened." "Nav" wrote in message ... That sounds like an estimate based on an energy argument. However, I don't think that case could be considered to be a closed system so such energy balance need not apply. Cheers Jeff Morris wrote: "Thom Stewart" wrote in message ... Scott, You've seem to have forgotten "The Gulf Stream Master" Jax. I'm sure he can help you ride the eddies:^) Maybe even find you a stray current heading south (g) Jax also insisted that the time of high water and slack current must be the same, to be otherwise would violate the laws of physics (on his planet). Now, for what's it worth. The Tide Flow is a sine wave. Us common folk understand that slack water is at the top and bottom of the wave. So slack is, roughly about a hour and it is roughly in the upper 30% counting the end of the rising tide and the beginning of the Ebb. 15% to the HW and 15% roughly to the Max Flow. That means about two hours to max flow, 8 hours of max flow and then another couple of hour of diminishing flow to LW. While the current flow in "simple harbors" may follow a sine curve, in cuts between two bodies of water the flow is different. The curve is more "squarish," that is, the flow ramps up quicker and stays high longer. The duration of slack is reduced. The best example is Hell Gate in New York. IIRC, the flow rate is proportional to the square root of the different of height of the two bodies. |
"Nav" wrote in message ... The reason I ask is that wind and air pressure affect the tide height and I would expect that that effect could alter time of peak tide and stream. The question, would such an effect also alter time of slack water differently from time of high water? I suspect not much. I tend to agree with you. However, I cannot think of any other explanation. Any ideas? Regards Donal -- |
You mean a line parted and it wasn't even chafing on the boat or dock??? Good
Grief!!! This is the most pathetic confession since jaxie admitted he didn't know how to use a compass! "Nav" wrote in message ... That's not a backpedal. Tidal current is discrete from hydraulic current and the two sum as I said. Cheers Jeff Morris wrote: Holy Backpedal, Nav! Of course the total current is the sum of a variety of effects. However, these are certain places where the hydraulic current dominates the flow. Snow's Cut, I suspect, is one. There are others, and they tend to be very significant for sailors. Most simple tidal currents around here max at under 2 knots, while the hydraulics are frequently over 4, sometimes 6 or more. For simple currents, the "50-90" rule, which approximates a sine curve, can be used. This says that the current reaches half strength in the first third between slack and max, and 90% in the second third. However, when a hydraulic current dominates, the current changes very quickly, often with virtually no slack. Cape Cod Canal this morning went from a .9 knot ebb to a 1 knot flood in 10 minutes. It went from 50% ebb to 50% flood strength in less than an hour, or twice as fast as a sine wave would predict. Hell Gate changes even faster than that. This is a very significant effect for the sailor, who often wishes to transit such areas at slack. Perhaps they are not common where you are, Nav, but I'm rather surprised you heard of it. jeff "Nav" wrote in message ... I just looked up Bowditch because I was surprised at the generality you implied. Bowditch is clearly talking about hydraulic current which _adds_ to the tidal current. The overall effect is as I described in my other response. Cheers Jeff Morris wrote: Then you're disagreeing with Bowditch: "A slight departure from the sine form is exhibited by the reversing current in a strait, such as East River, New York, that connects two tidal basins. The tides at the two ends of a strait are seldom in phase or equal in range, and the current, called hydraulic current, is generated largely by the continuously changing difference in height of water at the two ends. The speed of a hydraulic current varies nearly as the square root of the difference in height. The speed reaches a maximum more quickly and remains at strength for a longer period than shown in Figure 914b, and the period of weak current near the time of slack is considerably shortened." "Nav" wrote in message ... That sounds like an estimate based on an energy argument. However, I don't think that case could be considered to be a closed system so such energy balance need not apply. Cheers Jeff Morris wrote: "Thom Stewart" wrote in message ... Scott, You've seem to have forgotten "The Gulf Stream Master" Jax. I'm sure he can help you ride the eddies:^) Maybe even find you a stray current heading south (g) Jax also insisted that the time of high water and slack current must be the same, to be otherwise would violate the laws of physics (on his planet). Now, for what's it worth. The Tide Flow is a sine wave. Us common folk understand that slack water is at the top and bottom of the wave. So slack is, roughly about a hour and it is roughly in the upper 30% counting the end of the rising tide and the beginning of the Ebb. 15% to the HW and 15% roughly to the Max Flow. That means about two hours to max flow, 8 hours of max flow and then another couple of hour of diminishing flow to LW. While the current flow in "simple harbors" may follow a sine curve, in cuts between two bodies of water the flow is different. The curve is more "squarish," that is, the flow ramps up quicker and stays high longer. The duration of slack is reduced. The best example is Hell Gate in New York. IIRC, the flow rate is proportional to the square root of the different of height of the two bodies. |
Sorry Nav, that was obviously meant for Booby.
Your backpedal was pretty pathetic, also. You most pathetic since claiming to be a charter member of the "Save the Constitution" committee! "Jeff Morris" wrote in message ... You mean a line parted and it wasn't even chafing on the boat or dock??? Good Grief!!! This is the most pathetic confession since jaxie admitted he didn't know how to use a compass! "Nav" wrote in message ... That's not a backpedal. Tidal current is discrete from hydraulic current and the two sum as I said. Cheers Jeff Morris wrote: Holy Backpedal, Nav! Of course the total current is the sum of a variety of effects. However, these are certain places where the hydraulic current dominates the flow. Snow's Cut, I suspect, is one. There are others, and they tend to be very significant for sailors. Most simple tidal currents around here max at under 2 knots, while the hydraulics are frequently over 4, sometimes 6 or more. For simple currents, the "50-90" rule, which approximates a sine curve, can be used. This says that the current reaches half strength in the first third between slack and max, and 90% in the second third. However, when a hydraulic current dominates, the current changes very quickly, often with virtually no slack. Cape Cod Canal this morning went from a .9 knot ebb to a 1 knot flood in 10 minutes. It went from 50% ebb to 50% flood strength in less than an hour, or twice as fast as a sine wave would predict. Hell Gate changes even faster than that. This is a very significant effect for the sailor, who often wishes to transit such areas at slack. Perhaps they are not common where you are, Nav, but I'm rather surprised you heard of it. jeff "Nav" wrote in message ... I just looked up Bowditch because I was surprised at the generality you implied. Bowditch is clearly talking about hydraulic current which _adds_ to the tidal current. The overall effect is as I described in my other response. Cheers Jeff Morris wrote: Then you're disagreeing with Bowditch: "A slight departure from the sine form is exhibited by the reversing current in a strait, such as East River, New York, that connects two tidal basins. The tides at the two ends of a strait are seldom in phase or equal in range, and the current, called hydraulic current, is generated largely by the continuously changing difference in height of water at the two ends. The speed of a hydraulic current varies nearly as the square root of the difference in height. The speed reaches a maximum more quickly and remains at strength for a longer period than shown in Figure 914b, and the period of weak current near the time of slack is considerably shortened." "Nav" wrote in message ... That sounds like an estimate based on an energy argument. However, I don't think that case could be considered to be a closed system so such energy balance need not apply. Cheers Jeff Morris wrote: "Thom Stewart" wrote in message ... Scott, You've seem to have forgotten "The Gulf Stream Master" Jax. I'm sure he can help you ride the eddies:^) Maybe even find you a stray current heading south (g) Jax also insisted that the time of high water and slack current must be the same, to be otherwise would violate the laws of physics (on his planet). Now, for what's it worth. The Tide Flow is a sine wave. Us common folk understand that slack water is at the top and bottom of the wave. So slack is, roughly about a hour and it is roughly in the upper 30% counting the end of the rising tide and the beginning of the Ebb. 15% to the HW and 15% roughly to the Max Flow. That means about two hours to max flow, 8 hours of max flow and then another couple of hour of diminishing flow to LW. While the current flow in "simple harbors" may follow a sine curve, in cuts between two bodies of water the flow is different. The curve is more "squarish," that is, the flow ramps up quicker and stays high longer. The duration of slack is reduced. The best example is Hell Gate in New York. IIRC, the flow rate is proportional to the square root of the different of height of the two bodies. |
Peter S/Y Anicula wrote: Nav wrote: Tide tables are hardly rocket science! Well actually they are. Mankind does not yet posses the full knowledge to produce exact tide predictions. There's a world of difference between "exact" and coatal tide tables accurate to 15 minutes. Most tables are still produced by measurement of local harmonic constants not by modelling. The local constants if correct will produce sufficient accuaracy for navigation purposes. That is my poinmt, not that research into hydrographics is not sophisticated. Cheers |
"Thom Stewart" wrote in message ... Marty, Right you are! Tell Donal to mark you up for One Point. The Tides use the Moon Calender:^) That is also the reason that Tide rides are possible. I'm afraid that I can only award Marty 0.75 of a point. I'm about to dissappear for a couple of days, so I'll explain when I get back. [hint] The sun has a much greater gravitational effect on the Earth than the moon. So why does the moon seem to have a greater impact on the tides? Regards Donal -- |
Nav,
Height is a measurement of distance from a reference point. It isn't a rate. It is a static measurement. It just doesn't compute. I don't follow you terminology? Ole Thom |
Donal,
Any ideas How about the Increase of Tidal Flow and height during Hurricanes? Ole Thom |
"Donal" wrote in message ... "Thom Stewart" wrote in message ... Marty, Right you are! Tell Donal to mark you up for One Point. The Tides use the Moon Calender:^) That is also the reason that Tide rides are possible. I'm afraid that I can only award Marty 0.75 of a point. I'm about to dissappear for a couple of days, so I'll explain when I get back. [hint] The sun has a much greater gravitational effect on the Earth than the moon. So why does the moon seem to have a greater impact on the tides? Regards Donal -- |
"Donal" wrote in message
... "Thom Stewart" wrote in message ... Marty, Right you are! Tell Donal to mark you up for One Point. The Tides use the Moon Calender:^) That is also the reason that Tide rides are possible. I'm afraid that I can only award Marty 0.75 of a point. I'm about to dissappear for a couple of days, so I'll explain when I get back. [hint] The sun has a much greater gravitational effect on the Earth than the moon. So why does the moon seem to have a greater impact on the tides? The tides are not created by what is normally called the "Gravitational Force." (Of course, the tides are caused by gravitational forces, but they are not calculated the same way as the "inverse square" rule that is used for calculating orbits.) I leave the rest as an exercise for the reader. |
Sure. There was I thinking you would discuss a scientific subject
intelligently. Silly me. Cheers Jeff Morris wrote: Sorry Nav, that was obviously meant for Booby. Your backpedal was pretty pathetic, also. You most pathetic since claiming to be a charter member of the "Save the Constitution" committee! "Jeff Morris" wrote in message ... You mean a line parted and it wasn't even chafing on the boat or dock??? Good Grief!!! This is the most pathetic confession since jaxie admitted he didn't know how to use a compass! "Nav" wrote in message ... That's not a backpedal. Tidal current is discrete from hydraulic current and the two sum as I said. Cheers Jeff Morris wrote: Holy Backpedal, Nav! Of course the total current is the sum of a variety of effects. However, these are certain places where the hydraulic current dominates the flow. Snow's Cut, I suspect, is one. There are others, and they tend to be very significant for sailors. Most simple tidal currents around here max at under 2 knots, while the hydraulics are frequently over 4, sometimes 6 or more. For simple currents, the "50-90" rule, which approximates a sine curve, can be used. This says that the current reaches half strength in the first third between slack and max, and 90% in the second third. However, when a hydraulic current dominates, the current changes very quickly, often with virtually no slack. Cape Cod Canal this morning went from a .9 knot ebb to a 1 knot flood in 10 minutes. It went from 50% ebb to 50% flood strength in less than an hour, or twice as fast as a sine wave would predict. Hell Gate changes even faster than that. This is a very significant effect for the sailor, who often wishes to transit such areas at slack. Perhaps they are not common where you are, Nav, but I'm rather surprised you heard of it. jeff "Nav" wrote in message ... I just looked up Bowditch because I was surprised at the generality you implied. Bowditch is clearly talking about hydraulic current which _adds_ to the tidal current. The overall effect is as I described in my other response. Cheers Jeff Morris wrote: Then you're disagreeing with Bowditch: "A slight departure from the sine form is exhibited by the reversing current in a strait, such as East River, New York, that connects two tidal basins. The tides at the two ends of a strait are seldom in phase or equal in range, and the current, called hydraulic current, is generated largely by the continuously changing difference in height of water at the two ends. The speed of a hydraulic current varies nearly as the square root of the difference in height. The speed reaches a maximum more quickly and remains at strength for a longer period than shown in Figure 914b, and the period of weak current near the time of slack is considerably shortened." "Nav" wrote in message ... That sounds like an estimate based on an energy argument. However, I don't think that case could be considered to be a closed system so such energy balance need not apply. Cheers Jeff Morris wrote: "Thom Stewart" wrote in message ... Scott, You've seem to have forgotten "The Gulf Stream Master" Jax. I'm sure he can help you ride the eddies:^) Maybe even find you a stray current heading south (g) Jax also insisted that the time of high water and slack current must be the same, to be otherwise would violate the laws of physics (on his planet). Now, for what's it worth. The Tide Flow is a sine wave. Us common folk understand that slack water is at the top and bottom of the wave. So slack is, roughly about a hour and it is roughly in the upper 30% counting the end of the rising tide and the beginning of the Ebb. 15% to the HW and 15% roughly to the Max Flow. That means about two hours to max flow, 8 hours of max flow and then another couple of hour of diminishing flow to LW. While the current flow in "simple harbors" may follow a sine curve, in cuts between two bodies of water the flow is different. The curve is more "squarish," that is, the flow ramps up quicker and stays high longer. The duration of slack is reduced. The best example is Hell Gate in New York. IIRC, the flow rate is proportional to the square root of the different of height of the two bodies. |
Nav wrote:
Constellation actually, little man. Are you sure? For how many posts & how many days did you rant and rave about the Consitution, not noticing (or just as likely, not knowing) the difference, little boy? DSK |
Sorry Nav, I missed that. Perhaps you should have a special annotation for when
you're trying to be intelligent. "Nav" wrote in message ... Sure. There was I thinking you would discuss a scientific subject intelligently. Silly me. Cheers Jeff Morris wrote: Sorry Nav, that was obviously meant for Booby. Your backpedal was pretty pathetic, also. You most pathetic since claiming to be a charter member of the "Save the Constitution" committee! "Jeff Morris" wrote in message ... You mean a line parted and it wasn't even chafing on the boat or dock??? Good Grief!!! This is the most pathetic confession since jaxie admitted he didn't know how to use a compass! "Nav" wrote in message ... That's not a backpedal. Tidal current is discrete from hydraulic current and the two sum as I said. Cheers Jeff Morris wrote: Holy Backpedal, Nav! Of course the total current is the sum of a variety of effects. However, these are certain places where the hydraulic current dominates the flow. Snow's Cut, I suspect, is one. There are others, and they tend to be very significant for sailors. Most simple tidal currents around here max at under 2 knots, while the hydraulics are frequently over 4, sometimes 6 or more. For simple currents, the "50-90" rule, which approximates a sine curve, can be used. This says that the current reaches half strength in the first third between slack and max, and 90% in the second third. However, when a hydraulic current dominates, the current changes very quickly, often with virtually no slack. Cape Cod Canal this morning went from a .9 knot ebb to a 1 knot flood in 10 minutes. It went from 50% ebb to 50% flood strength in less than an hour, or twice as fast as a sine wave would predict. Hell Gate changes even faster than that. This is a very significant effect for the sailor, who often wishes to transit such areas at slack. Perhaps they are not common where you are, Nav, but I'm rather surprised you heard of it. jeff "Nav" wrote in message ... I just looked up Bowditch because I was surprised at the generality you implied. Bowditch is clearly talking about hydraulic current which _adds_ to the tidal current. The overall effect is as I described in my other response. Cheers Jeff Morris wrote: Then you're disagreeing with Bowditch: "A slight departure from the sine form is exhibited by the reversing current in a strait, such as East River, New York, that connects two tidal basins. The tides at the two ends of a strait are seldom in phase or equal in range, and the current, called hydraulic current, is generated largely by the continuously changing difference in height of water at the two ends. The speed of a hydraulic current varies nearly as the square root of the difference in height. The speed reaches a maximum more quickly and remains at strength for a longer period than shown in Figure 914b, and the period of weak current near the time of slack is considerably shortened." "Nav" wrote in message ... That sounds like an estimate based on an energy argument. However, I don't think that case could be considered to be a closed system so such energy balance need not apply. Cheers Jeff Morris wrote: "Thom Stewart" wrote in message ... Scott, You've seem to have forgotten "The Gulf Stream Master" Jax. I'm sure he can help you ride the eddies:^) Maybe even find you a stray current heading south (g) Jax also insisted that the time of high water and slack current must be the same, to be otherwise would violate the laws of physics (on his planet). Now, for what's it worth. The Tide Flow is a sine wave. Us common folk understand that slack water is at the top and bottom of the wave. So slack is, roughly about a hour and it is roughly in the upper 30% counting the end of the rising tide and the beginning of the Ebb. 15% to the HW and 15% roughly to the Max Flow. That means about two hours to max flow, 8 hours of max flow and then another couple of hour of diminishing flow to LW. While the current flow in "simple harbors" may follow a sine curve, in cuts between two bodies of water the flow is different. The curve is more "squarish," that is, the flow ramps up quicker and stays high longer. The duration of slack is reduced. The best example is Hell Gate in New York. IIRC, the flow rate is proportional to the square root of the different of height of the two bodies. |
DSK wrote: Nav wrote: Constellation actually, little man. Are you sure? Yes, and with each snipe you get smaller. Cheers |
Good idea -then you can kill file it. I have yet to have a good
scientific discussion about nautical matters with anyone here (except Gilligan). Cheers Jeff Morris wrote: Sorry Nav, I missed that. Perhaps you should have a special annotation for when you're trying to be intelligent. "Nav" wrote in message ... Sure. There was I thinking you would discuss a scientific subject intelligently. Silly me. Cheers Jeff Morris wrote: Sorry Nav, that was obviously meant for Booby. Your backpedal was pretty pathetic, also. You most pathetic since claiming to be a charter member of the "Save the Constitution" committee! "Jeff Morris" wrote in message ... You mean a line parted and it wasn't even chafing on the boat or dock??? Good Grief!!! This is the most pathetic confession since jaxie admitted he didn't know how to use a compass! "Nav" wrote in message ... That's not a backpedal. Tidal current is discrete from hydraulic current and the two sum as I said. Cheers Jeff Morris wrote: Holy Backpedal, Nav! Of course the total current is the sum of a variety of effects. However, these are certain places where the hydraulic current dominates the flow. Snow's Cut, I suspect, is one. There are others, and they tend to be very significant for sailors. Most simple tidal currents around here max at under 2 knots, while the hydraulics are frequently over 4, sometimes 6 or more. For simple currents, the "50-90" rule, which approximates a sine curve, can be used. This says that the current reaches half strength in the first third between slack and max, and 90% in the second third. However, when a hydraulic current dominates, the current changes very quickly, often with virtually no slack. Cape Cod Canal this morning went from a .9 knot ebb to a 1 knot flood in 10 minutes. It went from 50% ebb to 50% flood strength in less than an hour, or twice as fast as a sine wave would predict. Hell Gate changes even faster than that. This is a very significant effect for the sailor, who often wishes to transit such areas at slack. Perhaps they are not common where you are, Nav, but I'm rather surprised you heard of it. jeff "Nav" wrote in message ... I just looked up Bowditch because I was surprised at the generality you implied. Bowditch is clearly talking about hydraulic current which _adds_ to the tidal current. The overall effect is as I described in my other response. Cheers Jeff Morris wrote: Then you're disagreeing with Bowditch: "A slight departure from the sine form is exhibited by the reversing current in a strait, such as East River, New York, that connects two tidal basins. The tides at the two ends of a strait are seldom in phase or equal in range, and the current, called hydraulic current, is generated largely by the continuously changing difference in height of water at the two ends. The speed of a hydraulic current varies nearly as the square root of the difference in height. The speed reaches a maximum more quickly and remains at strength for a longer period than shown in Figure 914b, and the period of weak current near the time of slack is considerably shortened." "Nav" wrote in message ... That sounds like an estimate based on an energy argument. However, I don't think that case could be considered to be a closed system so such energy balance need not apply. Cheers Jeff Morris wrote: "Thom Stewart" wrote in message ... Scott, You've seem to have forgotten "The Gulf Stream Master" Jax. I'm sure he can help you ride the eddies:^) Maybe even find you a stray current heading south (g) Jax also insisted that the time of high water and slack current must be the same, to be otherwise would violate the laws of physics (on his planet). Now, for what's it worth. The Tide Flow is a sine wave. Us common folk understand that slack water is at the top and bottom of the wave. So slack is, roughly about a hour and it is roughly in the upper 30% counting the end of the rising tide and the beginning of the Ebb. 15% to the HW and 15% roughly to the Max Flow. That means about two hours to max flow, 8 hours of max flow and then another couple of hour of diminishing flow to LW. While the current flow in "simple harbors" may follow a sine curve, in cuts between two bodies of water the flow is different. The curve is more "squarish," that is, the flow ramps up quicker and stays high longer. The duration of slack is reduced. The best example is Hell Gate in New York. IIRC, the flow rate is proportional to the square root of the different of height of the two bodies. |
The moon are a lot closer than the sun. Therefore the gravitational
force of the moon varies more over the earth's surface. It is the variation in the gravitational force and not the force in itself that creates the tides. The moons pull on a water-molecule directly under the moon is larger than on a molecule on the far side of the earth, actually it is larger than "the average pull on the whole earth", and here the moon pulls away from the earth. On the far side of the earth (seen from the moon) the gravitation from the moon is less than average and at this point the moon pulls toward the earth. In the middle (when the moon is in the horizon) the moons pulls with the same as on the earth as a whole, and there is no vertical component, so here the water-molecule is "unaffected" by the moon. On both sides the change in gravitational pull from the moon reduces or counteracts the gravitational force of the earth on the water-molecule(making it lighter, so to speak). This should explain why there is to tides a day, one when the moon is culminating and one when it is on the other side. Peter S/Y Anicula "Donal" skrev i en meddelelse ... "Thom Stewart" wrote in message ... Marty, Right you are! Tell Donal to mark you up for One Point. The Tides use the Moon Calender:^) That is also the reason that Tide rides are possible. I'm afraid that I can only award Marty 0.75 of a point. I'm about to dissappear for a couple of days, so I'll explain when I get back. [hint] The sun has a much greater gravitational effect on the Earth than the moon. So why does the moon seem to have a greater impact on the tides? Regards Donal -- |
If you look at a curve of the "tidal streams" of a given location, or
at the ATSA, (Admirallity Tidal Stream Atlas), it incorporates the effect of landmasses and sea-bottom topography on the water-flow. Doesn't it ? Otherwise it wouldn't be very useful ! Peter S/Y Anicula "Nav" skrev i en meddelelse ... That's not a backpedal. Tidal current is discrete from hydraulic current and the two sum as I said. Cheers Jeff Morris wrote: Holy Backpedal, Nav! Of course the total current is the sum of a variety of effects. However, these are certain places where the hydraulic current dominates the flow. Snow's Cut, I suspect, is one. There are others, and they tend to be very significant for sailors. Most simple tidal currents around here max at under 2 knots, while the hydraulics are frequently over 4, sometimes 6 or more. For simple currents, the "50-90" rule, which approximates a sine curve, can be used. This says that the current reaches half strength in the first third between slack and max, and 90% in the second third. However, when a hydraulic current dominates, the current changes very quickly, often with virtually no slack. Cape Cod Canal this morning went from a .9 knot ebb to a 1 knot flood in 10 minutes. It went from 50% ebb to 50% flood strength in less than an hour, or twice as fast as a sine wave would predict. Hell Gate changes even faster than that. This is a very significant effect for the sailor, who often wishes to transit such areas at slack. Perhaps they are not common where you are, Nav, but I'm rather surprised you heard of it. jeff "Nav" wrote in message ... I just looked up Bowditch because I was surprised at the generality you implied. Bowditch is clearly talking about hydraulic current which _adds_ to the tidal current. The overall effect is as I described in my other response. Cheers Jeff Morris wrote: Then you're disagreeing with Bowditch: "A slight departure from the sine form is exhibited by the reversing current in a strait, such as East River, New York, that connects two tidal basins. The tides at the two ends of a strait are seldom in phase or equal in range, and the current, called hydraulic current, is generated largely by the continuously changing difference in height of water at the two ends. The speed of a hydraulic current varies nearly as the square root of the difference in height. The speed reaches a maximum more quickly and remains at strength for a longer period than shown in Figure 914b, and the period of weak current near the time of slack is considerably shortened." "Nav" wrote in message ... That sounds like an estimate based on an energy argument. However, I don't think that case could be considered to be a closed system so such energy balance need not apply. Cheers Jeff Morris wrote: "Thom Stewart" wrote in message ... Scott, You've seem to have forgotten "The Gulf Stream Master" Jax. I'm sure he can help you ride the eddies:^) Maybe even find you a stray current heading south (g) Jax also insisted that the time of high water and slack current must be the same, to be otherwise would violate the laws of physics (on his planet). Now, for what's it worth. The Tide Flow is a sine wave. Us common folk understand that slack water is at the top and bottom of the wave. So slack is, roughly about a hour and it is roughly in the upper 30% counting the end of the rising tide and the beginning of the Ebb. 15% to the HW and 15% roughly to the Max Flow. That means about two hours to max flow, 8 hours of max flow and then another couple of hour of diminishing flow to LW. While the current flow in "simple harbors" may follow a sine curve, in cuts between two bodies of water the flow is different. The curve is more "squarish," that is, the flow ramps up quicker and stays high longer. The duration of slack is reduced. The best example is Hell Gate in New York. IIRC, the flow rate is proportional to the square root of the different of height of the two bodies. |
http://groups.google.com/groups?hl=e...com%26rnum%3D2
Or since long URLs can be troublesome http://tinyurl.com/3uc5a The discussion was originally about "Old Ironsides" and went on for a dozen or more posts about the "USS Constitution" ver specifically. Amazing what just a few seconds in the archives can reveal. Nav wrote: Constellation actually, little man. Are you sure? Nav wrote: Yes, and with each snipe you get smaller. And with each denial and/or backpedal, you get.... DSK |
Peter, thanks for your educational posts.
Peter S/Y Anicula wrote: On both sides the change in gravitational pull from the moon reduces or counteracts the gravitational force of the earth on the water-molecule(making it lighter, so to speak). This should explain why there is to tides a day, one when the moon is culminating and one when it is on the other side. I would think that when the moon is on the opposite side, it's gravitation effect would be cumulative, acting to depress the water level. But it would be far less than when it's overhead, and the water has been put in motion. My (relatively vague) understanding of the science behind tides is that it's partly gravity and partly harmonics. Fresh Breezes- Doug King |
Donal wrote:
"Thom Stewart" wrote in message ... Marty, Right you are! Tell Donal to mark you up for One Point. The Tides use the Moon Calender:^) That is also the reason that Tide rides are possible. I'm afraid that I can only award Marty 0.75 of a point. I'm about to dissappear for a couple of days, so I'll explain when I get back. [hint] The sun has a much greater gravitational effect on the Earth than the moon. So why does the moon seem to have a greater impact on the tides? Well duh! Remember F=G*(m'*m")/(d^2), d is very much smaller in the earth moon case than the sun earth case. IOW, it's because the moon is closer. Give my point to Joe, he needs all the help he can get. ;-o Cheers Marty ------------ And now a word from our sponsor ------------------ Want to have instant messaging, and chat rooms, and discussion groups for your local users or business, you need dbabble! -- See http://netwinsite.com/sponsor/sponsor_dbabble.htm ---- |
"DSK" wrote in message ... Peter, thanks for your educational posts. Peter S/Y Anicula wrote: On both sides the change in gravitational pull from the moon reduces or counteracts the gravitational force of the earth on the water-molecule(making it lighter, so to speak). This should explain why there is to tides a day, one when the moon is culminating and one when it is on the other side. I would think that when the moon is on the opposite side, it's gravitation effect would be cumulative, acting to depress the water level. But it would be far less than when it's overhead, and the water has been put in motion. My (relatively vague) understanding of the science behind tides is that it's partly gravity and partly harmonics. Gravity is the force that drives it, harmonics determines the timing. Here's a site that describes the Differential Gravity in a fairly simple way: http://burro.astr.cwru.edu/Academics...ity/tides.html |
DSK wrote: Peter, thanks for your educational posts. Peter S/Y Anicula wrote: On both sides the change in gravitational pull from the moon reduces or counteracts the gravitational force of the earth on the water-molecule(making it lighter, so to speak). This should explain why there is to tides a day, one when the moon is culminating and one when it is on the other side. I would think that when the moon is on the opposite side, it's gravitation effect would be cumulative, acting to depress the water level. But it would be far less than when it's overhead, and the water has been put in motion. My (relatively vague) understanding of the science behind tides is that it's partly gravity and partly harmonics. The key to understanding resides in where the center of mass of the earth-moon system resides. Cheers |
Jeff Morris wrote: Here's a site that describes the Differential Gravity in a fairly simple way: http://burro.astr.cwru.edu/Academics...ity/tides.html That page shows a (trivial) spatial differentiation of a gravity field. Not a good explanation IMHO -the terms used are not described at all... What is the "center of mass force" and why is that different from gravity? The force vector diagram makes no sense at all. How to you add the vectors: - -- --- to the center of mass force -- X -- to get -- X -- (note the reversal in direction at the left!) Must be some new maths! (If it were so why don't people fly of into to space at the equator?). Even worse, the site then goes on to use the _differential_ expression to calculate the ratio of forces between the moon and sun! Cheers |
OK let me put it another way, when the tide height is changing fastest
that is when the maximum flow is more likely to occur. The change in height is due to a change in volume of water (at a given point). So, if the volume is chaning faster it must be due to some volume of water moving away faster? Does that help? Cheers Thom Stewart wrote: Nav, Height is a measurement of distance from a reference point. It isn't a rate. It is a static measurement. It just doesn't compute. I don't follow you terminology? Ole Thom |
Jeff, you really cannot explain two tides a day unless you also include
the centripetal forces of the earth moon pair -this is the key that is seems repeatedly lost. Cheers Jeff Morris wrote: "DSK" wrote in message ... Peter, thanks for your educational posts. Peter S/Y Anicula wrote: On both sides the change in gravitational pull from the moon reduces or counteracts the gravitational force of the earth on the water-molecule(making it lighter, so to speak). This should explain why there is to tides a day, one when the moon is culminating and one when it is on the other side. I would think that when the moon is on the opposite side, it's gravitation effect would be cumulative, acting to depress the water level. But it would be far less than when it's overhead, and the water has been put in motion. My (relatively vague) understanding of the science behind tides is that it's partly gravity and partly harmonics. Gravity is the force that drives it, harmonics determines the timing. Here's a site that describes the Differential Gravity in a fairly simple way: http://burro.astr.cwru.edu/Academics...ity/tides.html |
And your point is what, that I was describing a different ship with a
very similar name? Big deal. It's hilarious that you still don't see the sarcasm in my refence to her "motor". But if it makes you feel good I'll say it again, I _was_ talking about the Constellation in _BALTIMORE_ and not the Constitution -although why you seemed to continually miss the fact that I was talking about a ship in Baltimore is beyond me. I guess you only see what you want. At least I can acknowlege when I'm wrong. Now why don't you tell us again about how great an engineer you are? Cheers DSK the wiper wrote: http://groups.google.com/groups?hl=e...com%26rnum%3D2 Or since long URLs can be troublesome http://tinyurl.com/3uc5a The discussion was originally about "Old Ironsides" and went on for a dozen or more posts about the "USS Constitution" ver specifically. Amazing what just a few seconds in the archives can reveal. Nav wrote: Constellation actually, little man. Are you sure? Nav wrote: Yes, and with each snipe you get smaller. And with each denial and/or backpedal, you get.... DSK |
Of course. While topography can have big effects in estuaries (except at
their end and mouth) the sea bottom has almost no effect on the time of peak tide current along the coast. That is determined by the addition of tide and ocean currents. Did you wonder why charts show tide and ocean currents with separate symbols and often only a flood arrow? Could these pieces of information be used by a navigator to estimate current at other times than high tide? Cheers Peter S/Y Anicula wrote: If you look at a curve of the "tidal streams" of a given location, or at the ATSA, (Admirallity Tidal Stream Atlas), it incorporates the effect of landmasses and sea-bottom topography on the water-flow. Doesn't it ? Otherwise it wouldn't be very useful ! Peter S/Y Anicula "Nav" skrev i en meddelelse ... That's not a backpedal. Tidal current is discrete from hydraulic current and the two sum as I said. Cheers Jeff Morris wrote: Holy Backpedal, Nav! Of course the total current is the sum of a variety of effects. However, these are certain places where the hydraulic current dominates the flow. Snow's Cut, I suspect, is one. There are others, and they tend to be very significant for sailors. Most simple tidal currents around here max at under 2 knots, while the hydraulics are frequently over 4, sometimes 6 or more. For simple currents, the "50-90" rule, which approximates a sine curve, can be used. This says that the current reaches half strength in the first third between slack and max, and 90% in the second third. However, when a hydraulic current dominates, the current changes very quickly, often with virtually no slack. Cape Cod Canal this morning went from a .9 knot ebb to a 1 knot flood in 10 minutes. It went from 50% ebb to 50% flood strength in less than an hour, or twice as fast as a sine wave would predict. Hell Gate changes even faster than that. This is a very significant effect for the sailor, who often wishes to transit such areas at slack. Perhaps they are not common where you are, Nav, but I'm rather surprised you heard of it. jeff "Nav" wrote in message ... I just looked up Bowditch because I was surprised at the generality you implied. Bowditch is clearly talking about hydraulic current which _adds_ to the tidal current. The overall effect is as I described in my other response. Cheers Jeff Morris wrote: Then you're disagreeing with Bowditch: "A slight departure from the sine form is exhibited by the reversing current in a strait, such as East River, New York, that connects two tidal basins. The tides at the two ends of a strait are seldom in phase or equal in range, and the current, called hydraulic current, is generated largely by the continuously changing difference in height of water at the two ends. The speed of a hydraulic current varies nearly as the square root of the difference in height. The speed reaches a maximum more quickly and remains at strength for a longer period than shown in Figure 914b, and the period of weak current near the time of slack is considerably shortened." "Nav" wrote in message ... That sounds like an estimate based on an energy argument. However, I don't think that case could be considered to be a closed system so such energy balance need not apply. Cheers Jeff Morris wrote: "Thom Stewart" wrote in message ... Scott, You've seem to have forgotten "The Gulf Stream Master" Jax. I'm sure he can help you ride the eddies:^) Maybe even find you a stray current heading south (g) Jax also insisted that the time of high water and slack current must be the same, to be otherwise would violate the laws of physics (on his planet). Now, for what's it worth. The Tide Flow is a sine wave. Us common folk understand that slack water is at the top and bottom of the wave. So slack is, roughly about a hour and it is roughly in the upper 30% counting the end of the rising tide and the beginning of the Ebb. 15% to the HW and 15% roughly to the Max Flow. That means about two hours to max flow, 8 hours of max flow and then another couple of hour of diminishing flow to LW. While the current flow in "simple harbors" may follow a sine curve, in cuts between two bodies of water the flow is different. The curve is more "squarish," that is, the flow ramps up quicker and stays high longer. The duration of slack is reduced. The best example is Hell Gate in New York. IIRC, the flow rate is proportional to the square root of the different of height of the two bodies. |
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