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Riding the Tide
I found some current charts for some areas, they give slack, max ebb,
and max flow. this I understand. For other areas I find only the H & L tide chart. I want to make max use of the tide (ebb). Say H is at 0800 and L is at 1400. is it simply a matter of shoving off at 0800 and 'ride the out going tide till 1400? TIA Scotty |
Bobsprit wrote:
I want to make max use of the tide (ebb). Say H is at 0800 and L is at 1400. is it simply a matter of shoving off at 0800 and 'ride the out going tide till 1400? GADZOOKS!!!! Bwahahahahaha! RB Good answer! Cheeers Marty |
It depends of the area and of the duration of the trip.
There is 3 sources of tidal current: Tidal current tables (ATT), Tidal current charts and tidal currents shown in the sea charts. If you only know the time of HW and LW, you can assume that the curve of the tidal current will look a bit like a sinus curve. Generally there is a period of relatively weak currents around HW and LW and the current will run faster halfway between HW and LW. If the trip is expected to take 9 hours it would best to leave around 6.30 so you can sail in two periods of slack (weak currents) and one period of favourable current. If the trip is expected to take 3 hours, if you leave around 9.30 you should be able to sail the whole trip in favourable current. I'm sure Donald or some of the others who sail in areas of strong tides could elaborate further, but they probably think that you need the mental exercise of figuring it out yourself. Peter S/Y Anicula "Scott Vernon" skrev i en meddelelse ... I found some current charts for some areas, they give slack, max ebb, and max flow. this I understand. For other areas I find only the H & L tide chart. I want to make max use of the tide (ebb). Say H is at 0800 and L is at 1400. is it simply a matter of shoving off at 0800 and 'ride the out going tide till 1400? TIA Scotty |
"Peter S/Y Anicula" wrote in message ... It depends of the area and of the duration of the trip. There is 3 sources of tidal current: Tidal current tables (ATT), Tidal current charts and tidal currents shown in the sea charts. If you only know the time of HW and LW, you can assume that the curve of the tidal current will look a bit like a sinus curve. Generally there is a period of relatively weak currents around HW and LW and the current will run faster halfway between HW and LW. That's what I thought, but I want to be sure. If the trip is expected to take 9 hours it would best to leave around 6.30 so you can sail in two periods of slack (weak currents) and one period of favourable current. If the trip is expected to take 3 hours, if you leave around 9.30 you should be able to sail the whole trip in favourable current. Should take about 8 hrs., but you know sailing, could be longer. At a different location there is a narrow channel , with strong currents, to go through. this should be done at slack tide. I'm sure Donald or some of the others who sail in areas of strong tides could elaborate further, but they probably think that you need the mental exercise of figuring it out yourself. Nah, they know me better than that. Thanks, Peter. Scotty |
"Peter S/Y Anicula" wrote...
If you only know the time of HW and LW, you can assume that the curve of the tidal current will look a bit like a sinus curve. Yes, that's true. The current will also follow the 'rule of 12s' somewhat, lagging because of the momentum of the water in motion. In the absence of current tables, assume the max current is at 7/12s the interval between high & low water. DSK |
DSK wrote: "Peter S/Y Anicula" wrote... If you only know the time of HW and LW, you can assume that the curve of the tidal current will look a bit like a sinus curve. Yes, that's true. The current will also follow the 'rule of 12s' somewhat, lagging because of the momentum of the water in motion. In the absence of current tables, assume the max current is at 7/12s the interval between high & low water. Got that wrong Doug. Tidal stream is generally strongest at the time of the fastest rate of change of tide height. That is most often about halfway between high and low water. Look it up. Cheers |
"Scott Vernon" wrote in message ... I found some current charts for some areas, they give slack, max ebb, and max flow. this I understand. For other areas I find only the H & L tide chart. I want to make max use of the tide (ebb). Say H is at 0800 and L is at 1400. is it simply a matter of shoving off at 0800 and 'ride the out going tide till 1400? TIA Peter has given an excellent answer!(even if he got my name wrong). I would only add one thing. You need to use "slack water" as your reference, instead of HW, or LW. Geographical features can have a major impact on the tides. For example, at Portsmouth the current changes direction 2 hours before HW (or LW) because the proximity of the Isle of Wight. The Island produces a "circular" effect which completely changes the normal flow. It is also worth knowing that the current will be much smaller in shallow water. If the current is with you, then you should get into the deep water. If it is against you, then you should get into shallow water. Furthermore, you may be able to benefit from back eddies. A good tidal stream atlas for your local area should show where the back eddies are. Regards Donal -- |
"Donal" wrote ... "Scott Vernon" wrote in message ... I found some current charts for some areas, they give slack, max ebb, and max flow. this I understand. For other areas I find only the H & L tide chart. I want to make max use of the tide (ebb). Say H is at 0800 and L is at 1400. is it simply a matter of shoving off at 0800 and 'ride the out going tide till 1400? TIA Peter has given an excellent answer!(even if he got my name wrong). I would only add one thing. You need to use "slack water" as your reference, instead of HW, or LW. That's what I was asking, as I only have a HT- LT schedule for the one area. Geographical features can have a major impact on the tides. For example, at Portsmouth the current changes direction 2 hours before HW (or LW) because the proximity of the Isle of Wight. The Island produces a "circular" effect which completely changes the normal flow. It is also worth knowing that the current will be much smaller in shallow water. If the current is with you, then you should get into the deep water. If it is against you, then you should get into shallow water. Good point! Furthermore, you may be able to benefit from back eddies. A good tidal stream atlas for your local area should show where the back eddies are. I may not be going anyway , if these damn hurricanes don't stop. Scotty |
Nav wrote: DSK wrote: "Peter S/Y Anicula" wrote... If you only know the time of HW and LW, you can assume that the curve of the tidal current will look a bit like a sinus curve. Yes, that's true. The current will also follow the 'rule of 12s' somewhat, lagging because of the momentum of the water in motion. In the absence of current tables, assume the max current is at 7/12s the interval between high & low water. Got that wrong Doug. Tidal stream is generally strongest at the time of the fastest rate of change of tide height. That is most often about halfway between high and low water. Look it up. Cheers Personally, I disagree. You need to know where you are (narrow opening/wide opening) and various given conditions before you start to apply any particular rule. Compare your tables (tide/current) for the particular locale, then see if a particular rule applies at all times or only sometimes. otn |
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) 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. That doesn't make allowance for land masses, rivers, depth, etc. Also the time differences of where the HW takes place. So if you are sailing a Riding Tide you can increase your Max Flow by watching your location and tide tables times. From your fat, old sailor, Thom |
"Thom Stewart" wrote ...
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) Not forgotten, simply ignored. 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. That doesn't make allowance for land masses, rivers, depth, etc. Also the time differences of where the HW takes place. So if you are sailing a Riding Tide you can increase your Max Flow by watching your location and tide tables times. Thanks, Thom. Scotty |
"Nav" wrote in message ... DSK wrote: "Peter S/Y Anicula" wrote... If you only know the time of HW and LW, you can assume that the curve of the tidal current will look a bit like a sinus curve. Yes, that's true. The current will also follow the 'rule of 12s' somewhat, lagging because of the momentum of the water in motion. In the absence of current tables, assume the max current is at 7/12s the interval between high & low water. Got that wrong Doug. Tidal stream is generally strongest at the time of the fastest rate of change of tide height. That is most often about halfway between high and low water. Look it up. Isn't 7/12s 'about halfway'? look it up. Scotty |
Scott Vernon wrote:
Isn't 7/12s 'about halfway'? look it up. Not when it's Nav-math. And otnmbrd's post is really to the point... general rules are nice, but every location is a special case and will trump most of the general rules. 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 |
"DSK" wrote in message
.. . Scott Vernon wrote: Isn't 7/12s 'about halfway'? look it up. Not when it's Nav-math. And otnmbrd's post is really to the point... general rules are nice, but every location is a special case and will trump most of the general rules. 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. The current in cuts between two tidal domains is very difficult to predict. BTW, I was flying up from FL last week and had a great view of Snow's Cut from 7 miles up. In fact I had great views of the coast both down and back. And the Song Airlines off-season rate was less than what I paid for a 20 minute bi-plane ride on the Vineyard this summer. |
"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. |
"Scott Vernon" wrote in message ...
I found some current charts for some areas, they give slack, max ebb, and max flow. this I understand. For other areas I find only the H & L tide chart. I want to make max use of the tide (ebb). Say H is at 0800 and L is at 1400. is it simply a matter of shoving off at 0800 and 'ride the out going tide till 1400? TIA Scotty Perhaps if you modified your vessel and made it float like this one: http://pao.cnmoc.navy.mil/pao/n_onli...hotos/flip.jpg You would get a great current ride. This Navy ship has been in 80 foot seas! Joe |
Jeff,
You're not telling the full truth with "Hell's Gate" are you? You are leaving out the River flow. That is a constant and mentioned as a variation to Tidal flow. There are many variations on Tidal flows, including the Moon, which causes abnormal high and low tides. When coupled to large bodies of water constricted thru small inlets the sine wave will be knocked for a loop. Much like a clogged drain on a sink. LIS has its share of them. The Narrows, Plum Gut, Fisher Island to name a few. Every place in the World has its' variations, the higher the Latitudes, the greater the number. Fundy, Solvent, Gibraltar, Georgia Straits, Cape Horn, etc. The list is endless but local knowledge has learned to handle it. That's why we sail. It is still more of an Art than a Science! Ole Thom |
That's one heck of a big submarine!!!
-- "j" ganz @@ www.sailnow.com "Joe" wrote in message om... "Scott Vernon" wrote in message ... I found some current charts for some areas, they give slack, max ebb, and max flow. this I understand. For other areas I find only the H & L tide chart. I want to make max use of the tide (ebb). Say H is at 0800 and L is at 1400. is it simply a matter of shoving off at 0800 and 'ride the out going tide till 1400? TIA Scotty Perhaps if you modified your vessel and made it float like this one: http://pao.cnmoc.navy.mil/pao/n_onli...hotos/flip.jpg You would get a great current ride. This Navy ship has been in 80 foot seas! Joe |
Jeff Morris wrote:
The current in cuts between two tidal domains is very difficult to predict. I bet it is. Lots and lots of variables... BTW, I was flying up from FL last week and had a great view of Snow's Cut from 7 miles up. In fact I had great views of the coast both down and back. I've had the chance to do this too... a while back, though. The Outer Banks and the Eastern Shore are fascinating to fly over. Snow's Cut isn't really one of the highlights, visually speaking. ;) Fresh Breezes- Doug King |
"Joe" wrote
Scotty Perhaps if you modified your vessel and made it float like this one: http://pao.cnmoc.navy.mil/pao/n_onli...photos/flip.jp g You would get a great current ride. This Navy ship has been in 80 foot seas! What the heck is that? and where is it? SV |
"DSK" wrote in message .. . 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. A couple of years ago we encountered a tide that seemed to be an hour early. Five boats set off from St Peter port for the trip to Cherbourg. The trip took us through the Alderney Race, where the stream can reach more than 10 kts! We had planned to catch the last of the tide .... in fact we caught the first of the next tide! Afterwards, we all checked our figures, and we concluded that the tide had definitetly turned earlier than forecast. I suspect that weather systems can have an effect. In future, I will always leave half an hour earlier!!! Regards Donal -- |
"Scott Vernon" wrote in message ...
"Joe" wrote Scotty Perhaps if you modified your vessel and made it float like this one: http://pao.cnmoc.navy.mil/pao/n_onli...photos/flip.jp g You would get a great current ride. This Navy ship has been in 80 foot seas! What the heck is that? and where is it? SV It was buildt to research ocean currents. Here is the story that went with the photo: AG3 `flips' for research vessel duty with Scripps Institute by AG3 Michael DeMauro It's not too often that one encounters a vessel with bulkheads and decks designed for use in both the vertical and horizontal position…including heads and showers! Late on the afternoon of Jan. 15, I arrived at the Scripps Nimitz Marine Facility in San Diego. Having never been to sea, I had no idea what lay ahead as I made my way down the pier looking for the R/P FLIP with TAD orders in hand. In the distance appeared the most bizarre vessel I had ever seen. "This has got to be it," I thought. FLIP, or Floating Instrument Platform, is a 355-foot long, spoon-shaped buoy. Built in 1962, FLIP is owned by the U.S. Navy but operated by the Scripps Institute of Oceanography, University of California San Diego. The FLIP has a series of ballast tanks along its submarine-like hull, which can be strategically flooded with seawater. As the tanks fill, the "top" end of the ship slowly lifts out of the water (i.e. Flips) until the entire vessel becomes vertical, with 300 feet below the waterline. The remaining 55 feet become a stable platform, ideal for performing a variety of marine research experiments—so stable, in fact, that although the vessel was designed to operate in 30-foot seas, it has weathered 80 footers on more than one occasion. The Office of Naval Research hoped for 20-foot seas for some of the data collection periods during the planned underway. The bulk of the research during this underway period focused on Air-Sea Interface experiments, but in the past, FLIP was used in a variety of experiments, ranging from communications to ambient noise to the development of vertical arrays for acoustic sensors. …and another step toward a mutually beneficial relationship between the research and METOC communities was solidified. On reporting aboard for the January trip, I became a bit disoriented, to say the least. It's not too often that one encounters a vessel with bulkheads and decks designed for use in both the vertical and horizontal position…including heads and showers! Simply finding one of the three berthing spaces entangled in the forward superstructure proved difficult. Once settled in, however, I got straight to work establishing communications with Naval Pacific Meteorology and Oceanography Detachment Pt. Mugu, my home duty station. The small crew and full agenda of the research platform didn't allow me a free ride, and I was put to work straight away with various tasks, including responsibility for taking salinity soundings on a nightly watch from 2000 to 0300. During one three-day period I was even pressed into duty as the ship's cook, with no apparent casualties. The Scripps research vessel began working with Pt. Mugu in September 1999 when AGAN James Ross embarked on a similar trip off the Southern California coast. Tom Golfinos, FLIP OIC, has enthusiastically folded Pt. Mugu sailors into the crew, making the trips interesting and educational for the Sailors as well as the remainder of the crew. Simultaneously, civilian and military forecasters at NPMOD Pt. Mugu earned the trust of the crew by providing the vessel with tailored and precise forecasts for the environmentally sensitive operations. During the January underway period, a risky personnel and supply transfer was planned for the middle of the trip, which involved lowering a scientist down a rope ladder and dropping him into a rubber dinghy over open ocean, a dangerous operation even under ideal conditions. A go/no-go decision on executing the operation was based entirely on the Pt. Mugu Forecast. CAPT (Ret) Bill Gaines, Assistant Director Marine Physical Laboratory, Scripps Institute of Oceanography, relied on the accurate sea height, wind, and visibility forecasts provided by the detachment to eventually proceed with this critical operation. Although the weather didn't do its part by providing the FLIP with prolonged periods of high winds and seas during the 17-day drift, a good portion of the required data was collected, the personnel transfer went without incident, and another step toward a mutually beneficial relationship between the research and METOC communities was solidified. For a first-time underway period, my experience on the FLIP proved to be a valuable learning experience and a rare opportunity to ride one of the truly remarkable research platforms in existence. Joe |
Thanks for the link and the story. That is an unusual and interesting
vessel. DSK Joe wrote: http://pao.cnmoc.navy.mil/pao/n_onli...hotos/flip.jpg What the heck is that? and where is it? It was buildt to research ocean currents. Here is the story that went with the photo: AG3 `flips' for research vessel duty with Scripps Institute by AG3 Michael DeMauro It's not too often that one encounters a vessel with bulkheads and decks designed for use in both the vertical and horizontal position…including heads and showers! |
No, not in the context of Dougs ideas about max stream lagging behind
height changes. Max stream is most often at the maximaum rate of change of height. Look it up. Cheers Scott Vernon wrote: Got that wrong Doug. Tidal stream is generally strongest at the time of the fastest rate of change of tide height. That is most often about halfway between high and low water. Look it up. Isn't 7/12s 'about halfway'? look it up. Scotty |
How do you think it related to the change in tide height that day?
Cheers Donal wrote: "DSK" wrote in message .. . 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. A couple of years ago we encountered a tide that seemed to be an hour early. Five boats set off from St Peter port for the trip to Cherbourg. The trip took us through the Alderney Race, where the stream can reach more than 10 kts! We had planned to catch the last of the tide .... in fact we caught the first of the next tide! Afterwards, we all checked our figures, and we concluded that the tide had definitetly turned earlier than forecast. I suspect that weather systems can have an effect. In future, I will always leave half an hour earlier!!! Regards Donal -- |
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. |
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 |
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. |
Jeff Morris wrote:
Then you're disagreeing with Bowditch Hmm... Navvie, Bowditch... Bowditch or Navvie... A tough call but I'd have to say Bowditch has the slightly greater weight of authority. I'd agree that Bowditch is nowhere near as pompous nor as self-seeking. DSK |
Doug,
Pardon me if I sound argumentative. I'm not trying to be. 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 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. Allow time and be early. I let my boat "Lady Bug" until the right time. Ole Thom |
"Nav" wrote in message ... How do you think it related to the change in tide height that day? I've no idea. Our destination, Cherebourg, is a deep water port with 24 hour access to the marina. We had all done the trip many times, so we weren't watching the charts too closely. With hindsight, it would have been interesting to check the depth. Regards Donal -- |
"Nav" wrote in message ... No, not in the context of Dougs ideas about max stream lagging behind height changes. Max stream is most often at the maximaum rate of change of height. Look it up. Where? |
lady bug?
"Thom Stewart" wrote in message ... Doug, Pardon me if I sound argumentative. I'm not trying to be. 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 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. Allow time and be early. I let my boat "Lady Bug" until the right time. Ole Thom |
yep the lady bug
you know: while waiting for the tide to turn, the rum gets broken out from stowage, the crew gets naked and a regular party ensues. once the tide changes, everythings back to normal. hell you gotta do something while bouncing around out there. we usually leave 2-3 hours early ;) gf. "Scott Vernon" wrote in message ... lady bug? |
I think you are confusing a single example with a generality. While that
may be the case in Hell's gate it would be unlikely to apply to all bodies of water -for the energetic reason I explained. I suggest that a form that depends on the square root of the difference of a pair of curves is hardly a "small departure" from a sine. Consider this: A pair of tidal basins connected by a straight where the basin tides are out of phase and a current exists that flows through the straight. The flow through the straight would not be predictable without knowlege of the magnitude of the current -and it would not be proportional to the square root of the difference. OK? 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: Hmm... Navvie, Bowditch... Bowditch or Navvie... A tough call but I'd have to say Bowditch has the slightly greater weight of authority. I'd agree that Bowditch is nowhere near as pompous nor as self-seeking. What a sad little man you are. So bound up in jealousy. Cheers |
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. Cheers Donal wrote: "Nav" wrote in message ... How do you think it related to the change in tide height that day? I've no idea. Our destination, Cherebourg, is a deep water port with 24 hour access to the marina. We had all done the trip many times, so we weren't watching the charts too closely. With hindsight, it would have been interesting to check the depth. Regards Donal -- |
Don't worry, Doug was just BSing. Being a rather poor engineer he
assumed it would be compliant system and would lag. Cheers Thom Stewart wrote: Doug, Pardon me if I sound argumentative. I'm not trying to be. 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 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. Allow time and be early. I let my boat "Lady Bug" until the right time. Ole Thom |
Bowditch? How about Google?
This is basic seamanship on tides. The most likely time of max stream is when tide height changes most rapidly. Cheers Scott Vernon wrote: "Nav" wrote in message ... No, not in the context of Dougs ideas about max stream lagging behind height changes. Max stream is most often at the maximaum rate of change of height. Look it up. Where? |
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