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Effective speed much less than theoretical hull speed.
This is the situation: My 40' LWL boat (15 ton displacement) has a 150
PS engine. From the formula for speed I calculated a hull speed of sqrt(40)*1.34 = 8.47 knots *but*: when i did trials last week (absolutely calm water, almost no wind) those are the results: 800 rpm 5 knots no noticeable waves generated 1100 rpm 5.5 knots small waves 1800 rpm 6.5 knots (flat out) - huge waves generated, stern deep in the water, boat "running uphill". 1100 rpm is around 50/60 PS (from the engine rpm/PS table). Question: what could be the cause of the "slowness" of the boat ? I do not pretend to reach 8.4 knots cruising but at least 7 knots should be in. I'm thinging of dirty hull (green slime), incorrect weight distribution (bow tends to "point" upwards even when crossing small waves). Any experience ? Thanks Matteo |
Question: what could be the cause of the "slowness" of the boat ?
Expectations. Hull speed is the theoretical limit for any practical amount of power unless the hull shape is such that planing lift can start to reduce the displacement. You have far from that amount of power. The 1.34 figure would also be for a fairly slender (in flow terms, not necessarily length to beam) hull. The number goes down as the hull gets fatter. 1.25 is a more typical number for vessels as heavy as cruising sailboats and heavy power vessels but they will take a lot of power to get up to it. If the hull is making the waves and trimming as you describe, cleaning the bottom and fiddling with the prop probably won't make much of a difference. A lot has to do with the flow angles in the run. The bow makes rebound up and push the stern ahead, recovering some of the energy expended in making them and helping keep the stern up. Once the angle between the run and the direction of motion exceeds 12 - 15 degrees, the flow separates and the space between the smooth flow and the hull fills with lazy eddies of water that largely move along with the hull. Energy from the wave train can not be returned to the hull through the zone. The effective waterline of your hull is actually from the bow to the point where the flow lines (generally along the diagonals) exceeds this critical angle. -- Roger Long |
Oops. That's supposed to say, "The WAVES THE bow makes..." as
corrected below. -- Roger Long "Roger Long" wrote in message ... Question: what could be the cause of the "slowness" of the boat ? Expectations. Hull speed is the theoretical limit for any practical amount of power unless the hull shape is such that planing lift can start to reduce the displacement. You have far from that amount of power. The 1.34 figure would also be for a fairly slender (in flow terms, not necessarily length to beam) hull. The number goes down as the hull gets fatter. 1.25 is a more typical number for vessels as heavy as cruising sailboats and heavy power vessels but they will take a lot of power to get up to it. If the hull is making the waves and trimming as you describe, cleaning the bottom and fiddling with the prop probably won't make much of a difference. A lot has to do with the flow angles in the run. rebound up and push the stern ahead, recovering some of the energy expended in making them and helping keep the stern up. Once the angle between the run and the direction of motion exceeds 12 - 15 degrees, the flow separates and the space between the smooth flow and the hull fills with lazy eddies of water that largely move along with the hull. Energy from the wave train can not be returned to the hull through the zone. The effective waterline of your hull is actually from the bow to the point where the flow lines (generally along the diagonals) exceeds this critical angle. -- Roger Long |
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Roger Long:
One of these days I'd like to look into the derivation of hull speed. Can you suggest a very basic explanation (a source thereof). I'm just curious as to when it applies. Like: Does it apply to non-rigid hulls (hulls that might flex in the middle) Does it apply to totally submerged objects? Does it apply to towed objects, like dinghies? What happens when an object exceeds hull speed? Is there any way to "fool the water" into acting as if the boat is longer than it is? Thanks David OHara Wayne. B wrote: On 12 Apr 2005 11:46:08 -0700, (Matteo) wrote: I'm thinging of dirty hull (green slime), incorrect weight distribution (bow tends to "point" upwards even when crossing small waves). ================================== A dirty bottom and/or dirty props will definitely slow you down. It's also save to say that the effective waterline length of a 40 ft boat is actually less than 40 ft. Another factor is something called prismatic coefficient which if a fancy way of describing how sleek your hull form is. Obviously it's going to take more power to drive a 40 ft square box through the water than a 40 ft sailboat. The equation of 1.34 SQRT LWL is realy nothing more than an approximation and is not written in concrete. |
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The essential fact to understand about hull speed is that there is an
exact relationship between the length of a wave and how fast it moves through the water. If you time the crests as they go by a fixed point like a buoy, you can calculate the exact distance between the crests. Longer waves move faster. The hull makes wave as it disturbs the water. At low speeds, there is room for several crests and troughs along the hull. You can see by the large wave system even a small pebble sets up that it doesn't take a lot of energy to create a wave train. Hull resistance at low speeds is primarily skin friction. As speed increases, the waves the boat makes must become longer in order to maintain the speed / length relationship. Eventually there is room for just one wave at the bow and one quarter wave at the stern. When the speed length ratio is 1.0, there will be a crest at the bow and another at the stern. The boat will be sitting fairly symmetrically without trimming down by the stern and the wave rebounding up under the stern will actually be pushing the vessel ahead recovering some, but far from all, of the energy required to produce the wave train. Vessels can thus get up to this speed with fairly modest power. To go faster however, the crest of the wave at the stern has to start moving behind the boat. Two things happen. First, wave behind the hull can not return energy to it. This pushes power requirements up. Second, the hull now starts to squat by the stern which is moving into the trough. The bow wave always remains about in the same place so the boat has to start climbing up a hill that it is also making. The graph of power required starts to go straight up as the stern wave moves aft of the transom. The basic relationship is that it takes four times as much power to go twice as fast. If you graph this out, you'll see that hull speed is not a precise point but is a fairly narrow band. You quickly reach a point where doubling the size of the engine only gains you a quarter knot. If the boat is shaped so that water flow over the bottom creates dynamic lift instead of suction, the hull will start to lift up. With sufficient power, the vessel can be pushed up the hill of the bow wave on to the top where it can again ride level. It will still be producing a wave train but all the crests will be well behind it. A deep hull like a sailboat or a tug boat won't do this. The suction of steep flow lines in the stern will pull the stern down. Some hulls will actually pull themselves below the surface if enough power is applied. The waves created by hull will keep the water off the deck but, if something suddenly stops the hull, it can be swamped by its own wake. -- Roger Long Does it apply to non-rigid hulls (hulls that might flex in the middle) Very complex question. Can't be answered in general. Does it apply to totally submerged objects? No. Does it apply to towed objects, like dinghies? Yes. What happens when an object exceeds hull speed? See above. Is there any way to "fool the water" into acting as if the boat is longer than it is? If anybody has figured out how to fool the universe yet, I'd like to hear about it. |
Roger:
Thank you for a very lucid explanation. From this, is it correct to think that "hull Speed" is not some sort of value at which mathematics goes crazy and produces singularities but simply represents a speed range in which necesary power to produce a speed increase seriously increases? Is Hull Speed defined in some way relating to the slope of the power vs speed curve? Now, for the bizarre theory question. Consider a small boat that has a very long rigid extension on its stern that does not touch the water except far from the boat where it has a rigid float. Would this have a higher hull speed than the small boat alone? Could you arrange for this float at the end to gain back energy from the trough behind it? Could you arrange floats on this rigid extension at certain places to extract energy from the shorter period waves the boat produces? David |
Can we alter the properties of the water surface to change hull speeed.
What I have in mind is like spreading oil on water where oil is spread from the bow. I assume that what this does is to decrease the amplitude of the shorter period waves. Even if it didnt increaqse hull speed, would it reduce the energy going into the shorter period waves? |
The float would have a hull speed limitation based on it's length. If
it was shorter than the main hull, it would be a big drag. -- Roger Long wrote in message oups.com... Roger: Thank you for a very lucid explanation. From this, is it correct to think that "hull Speed" is not some sort of value at which mathematics goes crazy and produces singularities but simply represents a speed range in which necesary power to produce a speed increase seriously increases? Is Hull Speed defined in some way relating to the slope of the power vs speed curve? Now, for the bizarre theory question. Consider a small boat that has a very long rigid extension on its stern that does not touch the water except far from the boat where it has a rigid float. Would this have a higher hull speed than the small boat alone? Could you arrange for this float at the end to gain back energy from the trough behind it? Could you arrange floats on this rigid extension at certain places to extract energy from the shorter period waves the boat produces? David |
No.
-- Roger Long wrote in message oups.com... Can we alter the properties of the water surface to change hull speeed. What I have in mind is like spreading oil on water where oil is spread from the bow. I assume that what this does is to decrease the amplitude of the shorter period waves. Even if it didnt increaqse hull speed, would it reduce the energy going into the shorter period waves? |
Roger, you sure know how to kill my fun, but thanks
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However, there are games played with multihulls so that the waves from
one hull cancel the wave from the other. For one thing, this must be considered to understand how the chop will slap on the underside. However, advanced work has been done on more complex configurations of three or four hulls with an eye towards high speed and efficiency. I don't think this has led to any recreational sailboat designs. Roger Long wrote: The float would have a hull speed limitation based on it's length. If it was shorter than the main hull, it would be a big drag. |
I own a Newport 33 which has a waterline length of 27 ft. According to
the formula, the theoretical speed for the boat is 6.96 knots. I have a 16 HP diesel with a 2 bladed impeller, and a maximum engine RPM of 3300 RPM. Running the engine at 2700 RPM I can readily reach 6.5 knots. In a good wind I can go to 7 knots. The maximum speed I have ever done was 11 knots on the GPS surfing down a wave with full sails up on a very broad reach in about 30 knot wind. Many other boats of the same design ( relatively light displacemnt, fin keel and spade rudder) report he same thing. Racing boats in the around-the world alone race routinely exceeded hull speed for long periods surfing down waves. The hull speed for a 60 ft boat is 10.4 knots andthey were achieving more than 20 knots I seem to remember. So that is the way to go faster than hull speed, find a wave and then surf down. Catamarans also go faster than hull speed all the time. So if you put enough power into the boat in relation to the displacement and wetted surface, you can exceed the Hull speed. I think that traditional full keel boat with a high displacement would have a lot of trouble getting close to Hull speed. Rolf Jeff wrote: However, there are games played with multihulls so that the waves from one hull cancel the wave from the other. For one thing, this must be considered to understand how the chop will slap on the underside. However, advanced work has been done on more complex configurations of three or four hulls with an eye towards high speed and efficiency. I don't think this has led to any recreational sailboat designs. Roger Long wrote: The float would have a hull speed limitation based on it's length. If it was shorter than the main hull, it would be a big drag. |
Rolf wrote:
I own a Newport 33 which has a waterline length of 27 ft. According to the formula, the theoretical speed for the boat is 6.96 knots. I have a 16 HP diesel with a 2 bladed impeller, and a maximum engine RPM of 3300 RPM. Running the engine at 2700 RPM I can readily reach 6.5 knots. In a good wind I can go to 7 knots. The maximum speed I have ever done was 11 knots on the GPS surfing down a wave with full sails up on a very broad reach in about 30 knot wind. Many other boats of the same design ( relatively light displacemnt, fin keel and spade rudder) report he same thing. Racing boats in the around-the world alone race routinely exceeded hull speed for long periods surfing down waves. The hull speed for a 60 ft boat is 10.4 knots andthey were achieving more than 20 knots I seem to remember. So that is the way to go faster than hull speed, find a wave and then surf down. Catamarans also go faster than hull speed all the time. So if you put enough power into the boat in relation to the displacement and wetted surface, you can exceed the Hull speed. I think that traditional full keel boat with a high displacement would have a lot of trouble getting close to Hull speed. Rolf Hull speed is the absolute maximum that boat can travel through water. All your examples have the water moving forward also so the boat is not exceeding hull speed through the water. Stephen |
Actually, the water does not move forward in a wave but you are right
that the surfing examples are irrelevant examples since the waves are pushing the boat forward in other ways. The speed length ratio of the Newport 33 at 7 knots would be 1.35, just a hair above the generally accepted displacement hull maximum of 1.33. If the hull has an easy run, the counter becomes part of the waterline length as the stern waves rise up under it. Adding a foot brings the ratio down to 1.32, exactly what you would expect for an easy hull like that one. -- Roger Long "Stephen Trapani" wrote in message ... Rolf wrote: I own a Newport 33 which has a waterline length of 27 ft. According to the formula, the theoretical speed for the boat is 6.96 knots. I have a 16 HP diesel with a 2 bladed impeller, and a maximum engine RPM of 3300 RPM. Running the engine at 2700 RPM I can readily reach 6.5 knots. In a good wind I can go to 7 knots. The maximum speed I have ever done was 11 knots on the GPS surfing down a wave with full sails up on a very broad reach in about 30 knot wind. Many other boats of the same design ( relatively light displacemnt, fin keel and spade rudder) report he same thing. Racing boats in the around-the world alone race routinely exceeded hull speed for long periods surfing down waves. The hull speed for a 60 ft boat is 10.4 knots andthey were achieving more than 20 knots I seem to remember. So that is the way to go faster than hull speed, find a wave and then surf down. Catamarans also go faster than hull speed all the time. So if you put enough power into the boat in relation to the displacement and wetted surface, you can exceed the Hull speed. I think that traditional full keel boat with a high displacement would have a lot of trouble getting close to Hull speed. Rolf Hull speed is the absolute maximum that boat can travel through water. All your examples have the water moving forward also so the boat is not exceeding hull speed through the water. Stephen |
On 13 Apr 2005 20:55:30 -0700, "Rolf" wrote:
I own a Newport 33 which has a waterline length of 27 ft. According to the formula, the theoretical speed for the boat is 6.96 knots. I have a 16 HP diesel with a 2 bladed impeller, and a maximum engine RPM of 3300 RPM. Running the engine at 2700 RPM I can readily reach 6.5 knots. I have a Viking 33 with same waterline. My direct-drive Atomic 4 with a two-blade can drive the boat at 5.8 knots in flat water at half-throttle, but it's too damn noisy to get it to 6.4...that final half-knot is simply not worth the gas or the noise, as the A4 is quieter than a diesel at anything but full out. In a good wind I can go to 7 knots. The maximum speed I have ever done was 11 knots on the GPS surfing down a wave with full sails up on a very broad reach in about 30 knot wind. Many other boats of the same design ( relatively light displacemnt, fin keel and spade rudder) report he same thing. Your results match mine. I can hit 7.1 or 7.2 knots SOG sustained in 25 knots on the right point of sail, but she'll "surf" to 10+ briefly on a run. snip I think that traditional full keel boat with a high displacement would have a lot of trouble getting close to Hull speed. Not necessarily, but generally, that's correct. Full keelers can surf on a run as well, but they frequently can't helm quickly enough to maintain the right angle. On the other hand, in a three-day blow, I'd much prefer to heave to in a full keeler. Personal preference, location and experience play a huge role in getting the most out of your boat. In a full keeler, you may never go as fast as theory, but you may sail longer because the motion is less whippy and exhausting. Personally, I like cutaway forefoot, skeg hung, semi-full keelers. Best of all worlds if designed right. I even like the still rare idea of canted fixed dual bilge keels with extendable centerboards, but it's not common (yet). R. |
Personally, I like cutaway forefoot, skeg hung, semi-full keelers.
Best of all worlds if designed right. This boat designer agrees with you which is exactly why we bought the Endeavour 32. -- Roger Long |
On Thu, 14 Apr 2005 16:39:41 GMT, "Roger Long"
wrote: Personally, I like cutaway forefoot, skeg hung, semi-full keelers. Best of all worlds if designed right. This boat designer agrees with you which is exactly why we bought the Endeavour 32. OK, Mr. Designer...I am glad I am on the right track...I seem to be a lone voice in the wilderness advocating a number of older Ted Brewer/Bob Wallstrom/Robert Perry designs G. On the used boat market, what models would you recommend "like this" but in the 38-45 foot range? I also favour steel if well constructed and coated originally, which is admittedly a big "if". Your opinion would be most appreciated. R. |
I haven't followed yachts closely enough in the past couple of decades
to comment on individual designs. However, I can't remember ever seeing anything by Bob Perry that I didn't like. Brewer/Wallstrom have designed some nice boats but I know I had the "Why did they do that?" reaction much more often back when I looked at every yacht design I came across. Most of my career has been spent on metal vessels. If I were going around the world, I'd want to go in a steel or aluminum boat. I'd favor aluminum because of a more reliable compass and because you can patch it with a hand drill and sheet metal screws. Aluminum tends to bend flat and intact where steel fractures even though it is stronger in the stiffness sense. I once saw an aluminum yacht that went ashore on Nomans Land Island. The keel was torn off and one side was pounded in about five feet for three quarters of the length of the vessel. There were only about three six inch cracks that would have let water in. If she had been worth saving, she could have been made watertight and floated off with a roll of duct tape. A steel boat would have been in pieces all over the beach. The key thing I would look for is a full length skeg along the leading edge of the rudder all the way to the bottom. The directional stability comes from that fixed foil. Turning the rudder makes it a lifting surface in the direction you want to move the stern. A lot of the turning force then is created by something fixed to the hull instead of on a hinge where you have to resist it with your hands. The typical semi skeg with a bit of balance forward (as on the Endeavors) is a silly arrangement usually. There isn't enough balance on 90% of the rudders you see to effect the helm forces, the directional stability is reduced, and a line catcher created. The only rational for this kind of rudder is to look techie like an airplane. Our boat had glass added to the forward part of the rudder to increase the balance to an amount that will actually do something. 15 to 20 percent should be ahead of the hinge line. Some winter, before that trip around Newfoundland and up to Labrador, I'd like to cut it back and extend the skeg all the way down though. It's an easy conversion on most boats. For directional stability, you want lots of leading edge back there. I think my beef with a lot of Brewer/Wallstrom boats was that the cutout ahead of the rudder is often kind of a token so that there is very little leading edge. -- Roger Long "rhys" wrote in message ... On Thu, 14 Apr 2005 16:39:41 GMT, "Roger Long" wrote: Personally, I like cutaway forefoot, skeg hung, semi-full keelers. Best of all worlds if designed right. This boat designer agrees with you which is exactly why we bought the Endeavour 32. OK, Mr. Designer...I am glad I am on the right track...I seem to be a lone voice in the wilderness advocating a number of older Ted Brewer/Bob Wallstrom/Robert Perry designs G. On the used boat market, what models would you recommend "like this" but in the 38-45 foot range? I also favour steel if well constructed and coated originally, which is admittedly a big "if". Your opinion would be most appreciated. R. |
Here is a picture of how the rudder was enlarged on our boat with the
original line shown: http://home.maine.rr.com/rlma/Rudder.jpg The balance is still pretty minimal but you can see that there was effectively none before. The way I would modify it is shown in red. -- Roger Long |
In article ,
Stephen Trapani wrote: Hull speed is the absolute maximum that boat can travel through water. All your examples have the water moving forward also so the boat is not exceeding hull speed through the water. I thought I mentioned this before. Hope I'm not repeating myself. Hull speed is a suggestion for our boat, not the law. Though our theoretical hull speed is 6.65 knots, we regularly exceed that with aplomb, close hauled, close reach, broad reach, whatever point of sail. Spent a wonderful afternoon with 6 other sailors last season. As long as I was on the tiller, pushing her to where she likes to be, we were well above the theoretical hull speed. As we pinched to get back into the harbor, she insisted on doing over 7 knots directly into the wind (okay, about 15 degrees off). That last was our lovely lady showing off, of course, as what we did was clearly impossible. 1.34 was derived from observing boats about a century ago. Depending on the hull, that constant can be quite a bit different. As I recall, some multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp transom keeps her driving towards a 1.7 or so constant. -- Jere Lull Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD) Xan's Pages: http://members.dca.net/jerelull/X-Main.html Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/ |
On Sat, 16 Apr 2005 09:32:25 GMT, "Roger Long"
wrote: Comments below. Most of my career has been spent on metal vessels. If I were going around the world, I'd want to go in a steel or aluminum boat. I'd favor aluminum because of a more reliable compass and because you can patch it with a hand drill and sheet metal screws. Aluminum tends to bend flat and intact where steel fractures even though it is stronger in the stiffness sense. Interesting. I work with aluminum on the mast and I've fabbed up 1/4 in. backing plates for most of the deck gear, so I know simple hand tools will suffice, but usually the knock AGAINST aluminum is that it requires special welding gear and skills. I didn't think of it in terms of making a through bolted patch and running a bead of sealant around...but why not as a "get you home" metallic fothering? I once saw an aluminum yacht that went ashore on Nomans Land Island. The keel was torn off and one side was pounded in about five feet for three quarters of the length of the vessel. There were only about three six inch cracks that would have let water in. If she had been worth saving, she could have been made watertight and floated off with a roll of duct tape. A steel boat would have been in pieces all over the beach. I would think it would be worth saving for the aluminum alone...isn't "marine" aluminum a fairly expensive alloy? The key thing I would look for is a full length skeg along the leading edge of the rudder all the way to the bottom. The directional stability comes from that fixed foil. Turning the rudder makes it a lifting surface in the direction you want to move the stern. A lot of the turning force then is created by something fixed to the hull instead of on a hinge where you have to resist it with your hands. I'm a big fan of skegs for safety and directional reasons. If you ground by the stern with a spade rudder, usually it's game over. A skeg can help...maybe...to save it. The typical semi skeg with a bit of balance forward (as on the Endeavors) is a silly arrangement usually. There isn't enough balance on 90% of the rudders you see to effect the helm forces, the directional stability is reduced, and a line catcher created. The only rational for this kind of rudder is to look techie like an airplane. So you're no fan of the "Brewer Bite"? snip For directional stability, you want lots of leading edge back there. I think my beef with a lot of Brewer/Wallstrom boats was that the cutout ahead of the rudder is often kind of a token so that there is very little leading edge. I am not sure of the logic either, except that it makes otherwise traditional boats more "modern" looking on the undersides. R. |
On Sat, 16 Apr 2005 09:56:17 GMT, "Roger Long"
wrote: Here is a picture of how the rudder was enlarged on our boat with the original line shown: http://home.maine.rr.com/rlma/Rudder.jpg The balance is still pretty minimal but you can see that there was effectively none before. The way I would modify it is shown in red. Good picture! Thanks for your thoughts. R. |
I'm a big fan of skegs for safety and directional reasons. If you
ground by the stern with a spade rudder, usually it's game over. A skeg can help...maybe...to save it. Just to clarify: That is a fixed skeg shown on my proposed modification. On spade rudders: On power boats, I favor spade rudders. If the rudder has good clearance from the hull at the top, it will often remain functional after a grounding. The shaft may bend and the boat steer funny but it will still be steerable. With a bottom bearing, a little bit of bending will usually bind the whole thing up so if is useless. In a glass boat, it will be hard to make the skeg stiff enough to support the rudder. The whole thing can flex enough that the shaft will bend and the skeg will then bind the rudder. Even in metal, the sailboat type skeg will be hard to make sufficiently stiff. It doesn't take a lot of extra metal to make a rudder stock strong to be self supporting. If I were designing a boat that was not a weight critical racer, I would make the stock large enough to be a spade rudder. The skeg would then be structurally separate with just a line guard at the bottom. Grounding damage, which usually will bend the stock aft, would then leave the boat steerable in most cases. -- Roger Long |
Stephen Trapani wrote:
Hull speed is the absolute maximum that boat can travel through water. Not really. "Hull Speed" is sort of a convenient shorthand for indicating where the graph of a vessel's speed vs power begins to get inconveniently steep. All your examples have the water moving forward also so the boat is not exceeding hull speed through the water. Even catamarans? How about planing types? DSK |
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In article ,
DSK wrote: Usually making hulls non-rigid makes them slower for given power. One notable exception: a PortaBote, but that's not practical for most hulls. The hull deforms unbelievably, which is disconcerting. But, in general, it's true. -- Jere Lull Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD) Xan's Pages: http://members.dca.net/jerelull/X-Main.html Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/ |
On Wed, 20 Apr 2005 05:25:11 GMT, Jere Lull wrote:
In article , DSK wrote: Usually making hulls non-rigid makes them slower for given power. One notable exception: a PortaBote, but that's not practical for most hulls. The hull deforms unbelievably, which is disconcerting. It is hard for me to believe that is an exceoption. It would be notable if so. Indeed, it would be a miracle. Rodney Myrvaagnes NYC J36 Gjo/a "Religious wisdom is to wisdom as military music is to music." |
In article ,
Rodney Myrvaagnes wrote: On Wed, 20 Apr 2005 05:25:11 GMT, Jere Lull wrote: In article , DSK wrote: Usually making hulls non-rigid makes them slower for given power. One notable exception: a PortaBote, but that's not practical for most hulls. The hull deforms unbelievably, which is disconcerting. It is hard for me to believe that is an exceoption. It would be notable if so. Indeed, it would be a miracle. Obviously, you haven't run around on a PortaBote. 4 HP gives a solid 10+ knots speed with lots of load. When the boat goes on plane, the "floor" under the helmsman's feet drops a bunch of inches. Unlike a solid hull, the PortaBote expands into areas of low pressure to limit the drag (suction). Rodney Myrvaagnes NYC J36 Gjo/a "Religious wisdom is to wisdom as military music is to music." I *LIKE* military music, aka Marches. -- Jere Lull Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD) Xan's Pages: http://members.dca.net/jerelull/X-Main.html Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/ |
On Thu, 21 Apr 2005 05:40:38 GMT, Jere Lull wrote:
In article , Rodney Myrvaagnes wrote: On Wed, 20 Apr 2005 05:25:11 GMT, Jere Lull wrote: In article , DSK wrote: Usually making hulls non-rigid makes them slower for given power. One notable exception: a PortaBote, but that's not practical for most hulls. The hull deforms unbelievably, which is disconcerting. It is hard for me to believe that is an exceoption. It would be notable if so. Indeed, it would be a miracle. Obviously, you haven't run around on a PortaBote. 4 HP gives a solid 10+ knots speed with lots of load. When the boat goes on plane, the "floor" under the helmsman's feet drops a bunch of inches. Unlike a solid hull, the PortaBote expands into areas of low pressure to limit the drag (suction). If you say so. The age of miracles is not past. Rodney Myrvaagnes NYC J36 Gjo/a "Curse thee, thou quadrant. No longer will I guide my earthly way by thee." Capt. Ahab |
On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote:
In article , Stephen Trapani wrote: Hull speed is the absolute maximum that boat can travel through water. All your examples have the water moving forward also so the boat is not exceeding hull speed through the water. I thought I mentioned this before. Hope I'm not repeating myself. Hull speed is a suggestion for our boat, not the law. Though our theoretical hull speed is 6.65 knots, we regularly exceed that with aplomb, close hauled, close reach, broad reach, whatever point of sail. Spent a wonderful afternoon with 6 other sailors last season. As long as I was on the tiller, pushing her to where she likes to be, we were well above the theoretical hull speed. As we pinched to get back into the harbor, she insisted on doing over 7 knots directly into the wind (okay, about 15 degrees off). That last was our lovely lady showing off, of course, as what we did was clearly impossible. 1.34 was derived from observing boats about a century ago. Depending on the hull, that constant can be quite a bit different. As I recall, some multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp transom keeps her driving towards a 1.7 or so constant. Jere, It sounds like your speed-length parameter is higher than 1.34 - a testimonial to your hull designer. The 1.34 comes from the fact that speed-squared of a wave = g/2*pi times wavelength. If your hull's stern really places the stern wave a distance back from the bow wave equal to your design waterline length, then 1.34 is pretty accurate as the point where the curve of additional HP to yield additional speed for a displacement hull becomes almost vertical. However, with sweet butock lines, stern reflexes and other sophistications of hull design, the stern wave can actually be moved a bit aft of your transom. The wavelength thus becomes greater than your DWL. Since speed-squared is proportional to wavelength, and since your boat speed and the wave speed must match, you get a speed-length parameter that's higher than 1.34 as the effective multiplier times the square root of your DWL (since DWL is now less than the wavelength). At least that's my simplified understanding of a very complex subject. Al s/v Persephone Newburyport, MA |
In article ,
Albert P. Belle Isle wrote: On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote: In article , Stephen Trapani wrote: Hull speed is the absolute maximum that boat can travel through water. All your examples have the water moving forward also so the boat is not exceeding hull speed through the water. Hull speed is a suggestion for our boat, not the law. Though our theoretical hull speed is 6.65 knots, we regularly exceed that with aplomb, close hauled, close reach, broad reach, whatever point of sail. Spent a wonderful afternoon with 6 other sailors last season. As long as I was on the tiller, pushing her to where she likes to be, we were well above the theoretical hull speed. As we pinched to get back into the harbor, she insisted on doing over 7 knots directly into the wind (okay, about 15 degrees off). That last was our lovely lady showing off, of course, as what we did was clearly impossible. 1.34 was derived from observing boats about a century ago. Depending on the hull, that constant can be quite a bit different. As I recall, some multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp transom keeps her driving towards a 1.7 or so constant. Jere, It sounds like your speed-length parameter is higher than 1.34 - a testimonial to your hull designer. Full agreement. The 1.34 comes from the fact that speed-squared of a wave = g/2*pi times wavelength. Yes, I agree with the derivation of the formula -- as long as we include that wavelengths can differ. Swells have wavelengths 100s of feet and periods many seconds from crest to crest, while wind-driven waves have quite a bit shorter wavelengths and periods. And wind-driven waves have different periods and wavelengths. If your hull's stern really places the stern wave a distance back from the bow wave equal to your design waterline length, then 1.34 is pretty accurate as the point where the curve of additional HP to yield additional speed for a displacement hull becomes almost vertical. However, with sweet butock lines, stern reflexes and other sophistications of hull design, the stern wave can actually be moved a bit aft of your transom. The wavelength thus becomes greater than your DWL. Our resting WL is 24'. To maintain a 1.34 constant and get the speeds we've verified while definitely not surfing, our effective waterline would have to be greater than 35'. That's a LONG way behind our transom! Since speed-squared is proportional to wavelength, and since your boat speed and the wave speed must match, you get a speed-length parameter that's higher than 1.34 as the effective multiplier times the square root of your DWL (since DWL is now less than the wavelength). It feels like you left a bit out and mixed a couple of things here. Again, I agree that it probably has something to do with the wave speed, which has a certain value when the constant is 1.34. Change the wave's speed and you change the constant, and wave length. At least that's my simplified understanding of a very complex subject. Al s/v Persephone After having chased several NA's explanations for a few years, I finally gave up trying to explain and simply accept that it's more complex than 1.34, since other hull shapes have much higher observed constants. -- Jere Lull Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD) Xan's Pages: http://members.dca.net/jerelull/X-Main.html Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/ |
On Mon, 25 Apr 2005 04:06:45 GMT, Jere Lull wrote:
In article , Albert P. Belle Isle wrote: On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote: In article , Stephen Trapani wrote: Hull speed is the absolute maximum that boat can travel through water. All your examples have the water moving forward also so the boat is not exceeding hull speed through the water. Hull speed is a suggestion for our boat, not the law. Though our theoretical hull speed is 6.65 knots, we regularly exceed that with aplomb, close hauled, close reach, broad reach, whatever point of sail. Spent a wonderful afternoon with 6 other sailors last season. As long as I was on the tiller, pushing her to where she likes to be, we were well above the theoretical hull speed. As we pinched to get back into the harbor, she insisted on doing over 7 knots directly into the wind (okay, about 15 degrees off). That last was our lovely lady showing off, of course, as what we did was clearly impossible. 1.34 was derived from observing boats about a century ago. Depending on the hull, that constant can be quite a bit different. As I recall, some multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp transom keeps her driving towards a 1.7 or so constant. Jere, It sounds like your speed-length parameter is higher than 1.34 - a testimonial to your hull designer. Full agreement. The 1.34 comes from the fact that speed-squared of a wave = g/2*pi times wavelength. Yes, I agree with the derivation of the formula -- as long as we include that wavelengths can differ. Swells have wavelengths 100s of feet and periods many seconds from crest to crest, while wind-driven waves have quite a bit shorter wavelengths and periods. And wind-driven waves have different periods and wavelengths. If your hull's stern really places the stern wave a distance back from the bow wave equal to your design waterline length, then 1.34 is pretty accurate as the point where the curve of additional HP to yield additional speed for a displacement hull becomes almost vertical. However, with sweet butock lines, stern reflexes and other sophistications of hull design, the stern wave can actually be moved a bit aft of your transom. The wavelength thus becomes greater than your DWL. Our resting WL is 24'. To maintain a 1.34 constant and get the speeds we've verified while definitely not surfing, our effective waterline would have to be greater than 35'. That's a LONG way behind our transom! Since speed-squared is proportional to wavelength, and since your boat speed and the wave speed must match, you get a speed-length parameter that's higher than 1.34 as the effective multiplier times the square root of your DWL (since DWL is now less than the wavelength). It feels like you left a bit out and mixed a couple of things here. Again, I agree that it probably has something to do with the wave speed, which has a certain value when the constant is 1.34. Change the wave's speed and you change the constant, and wave length. At least that's my simplified understanding of a very complex subject. Al s/v Persephone After having chased several NA's explanations for a few years, I finally gave up trying to explain and simply accept that it's more complex than 1.34, since other hull shapes have much higher observed constants. The physics says that extra-long-wavelength swells just propagate more slowly. The c-squared = lambda*g/2pi is pretty fundamental for surface waves (as opposed to deep pressure waves, like tsunamis). There's a passage in John Craven's "The Silent War" where he gleefully chants the formula from the sail of a nuclear submarine as he watches her bow and stern waves demonstrate the validity of something he had drilled into his head in grad school. Maybe you have to be a geek to appreciate it g. However, the oversimplified nature of my "push-the-stern-wave-aft" explanation is, of course, quite true. IANA. If you haven't already read it, Jere, van Dorn's "Oceanography and Seamanship" has a pretty good discussion of the speed-power curves for planing, semiplaning and displacement hulls - as well as, among other things, a nifty nomograph for predicting sea-state from duration or fetch of sustained winds. Good sailing. Al s/v Persephone |
On Mon, 25 Apr 2005 20:16:09 GMT, Albert P. Belle Isle
wrote: Our resting WL is 24'. To maintain a 1.34 constant and get the speeds we've verified while definitely not surfing, our effective waterline would have to be greater than 35'. That's a LONG way behind our transom! Jere - As a quick calculation, 24ft DWL would yield a hull speed of about 6.6kt with a speed-length coefficient of 1.34. To get to 7kt, the effective DWL at 1.34 would be a little over 27ft - not 35ft. My previous boat had a DWL of 28ft, for which a speed-length coefficient of 1.34 would predict 7.1kt. I easily got 7.4kt on beam reaches, which would say my real coefficient was almost 1.4 (or that my effective DWL at 1.34 was a little over 30ft - a two foot "push-back" of the sten wave, which was roughly how the peak of the stern wave looked from my cockpit. Regards, Al |
On Mon, 25 Apr 2005 04:06:45 GMT, Jere Lull wrote:
In article , Albert P. Belle Isle wrote: On Sat, 16 Apr 2005 22:22:00 GMT, Jere Lull wrote: In article , Stephen Trapani wrote: Hull speed is the absolute maximum that boat can travel through water. All your examples have the water moving forward also so the boat is not exceeding hull speed through the water. Hull speed is a suggestion for our boat, not the law. Though our theoretical hull speed is 6.65 knots, we regularly exceed that with aplomb, close hauled, close reach, broad reach, whatever point of sail. Spent a wonderful afternoon with 6 other sailors last season. As long as I was on the tiller, pushing her to where she likes to be, we were well above the theoretical hull speed. As we pinched to get back into the harbor, she insisted on doing over 7 knots directly into the wind (okay, about 15 degrees off). That last was our lovely lady showing off, of course, as what we did was clearly impossible. 1.34 was derived from observing boats about a century ago. Depending on the hull, that constant can be quite a bit different. As I recall, some multi-hull boats' K is in the 2 or 3 range. Xan's fat ass and sharp transom keeps her driving towards a 1.7 or so constant. Jere, It sounds like your speed-length parameter is higher than 1.34 - a testimonial to your hull designer. Full agreement. The 1.34 comes from the fact that speed-squared of a wave = g/2*pi times wavelength. Yes, I agree with the derivation of the formula -- as long as we include that wavelengths can differ. Swells have wavelengths 100s of feet and periods many seconds from crest to crest, while wind-driven waves have quite a bit shorter wavelengths and periods. And wind-driven waves have different periods and wavelengths. If your hull's stern really places the stern wave a distance back from the bow wave equal to your design waterline length, then 1.34 is pretty accurate as the point where the curve of additional HP to yield additional speed for a displacement hull becomes almost vertical. However, with sweet butock lines, stern reflexes and other sophistications of hull design, the stern wave can actually be moved a bit aft of your transom. The wavelength thus becomes greater than your DWL. Our resting WL is 24'. To maintain a 1.34 constant and get the speeds we've verified while definitely not surfing, our effective waterline would have to be greater than 35'. That's a LONG way behind our transom! Since speed-squared is proportional to wavelength, and since your boat speed and the wave speed must match, you get a speed-length parameter that's higher than 1.34 as the effective multiplier times the square root of your DWL (since DWL is now less than the wavelength). It feels like you left a bit out and mixed a couple of things here. Again, I agree that it probably has something to do with the wave speed, which has a certain value when the constant is 1.34. Change the wave's speed and you change the constant, and wave length. At least that's my simplified understanding of a very complex subject. Al s/v Persephone After having chased several NA's explanations for a few years, I finally gave up trying to explain and simply accept that it's more complex than 1.34, since other hull shapes have much higher observed constants. It isn't a different constant. It is just that many boats nowadays are light enough so they aren't limited to the length of the wave they make. If you observe a tug traveling without a tow, you will see very easily what wave we are talking about. Rodney Myrvaagnes NYC J36 Gjo/a "Curse thee, thou quadrant. No longer will I guide my earthly way by thee." Capt. Ahab |
Hi All,
i post this update after changes to the trimming of the boat. I said the boat is poorly trimmed and seem to "sit" too much with the stern in the water. So i put around 3/4 of tons of water (750 liters) in the anchor room (the small cabin in the bow which contains the chain - is quite large and sealed). This had the effect to sink the bow around 8 inches and lift the stern around 3 inches. The boat is still NOT level but at least is getting closer. Now the effect on travelling speed: I could reach 5.5 knots at 900 rpm instead of 1100 rpm ! And my fuel consumption went down 40% at once (for the same speed). The benefits would be less and less trying to drive the boat faster and faster - basically the speed improvement at full throttle would be insignificant. Also waves were greatly reduced at 5.5 knots (almost reduced ripples). My theory: with the bow "lifted" too much the physical boat lenght starts quite some distance from the actual entry point in the water. Adding weight at the bow would push it down and effectively lenghten the WL. Now i have a long-range tank in the bow (1.5 ton) which is empty now (i normally use other two tanks, in the middle of the boat, each 1/2 ton). Next week i will bunker diesel and fill ALL the 3 tanks to the top - this should push down the boat another bit and eventually set it level - then make new trials. I also looked for green slime and the like on the hull - not much there. I will post the results in another 2 weeks or so. Matteo (Matteo) wrote in message om... This is the situation: My 40' LWL boat (15 ton displacement) has a 150 PS engine. From the formula for speed I calculated a hull speed of sqrt(40)*1.34 = 8.47 knots *but*: when i did trials last week (absolutely calm water, almost no wind) those are the results: 800 rpm 5 knots no noticeable waves generated 1100 rpm 5.5 knots small waves 1800 rpm 6.5 knots (flat out) - huge waves generated, stern deep in the water, boat "running uphill". 1100 rpm is around 50/60 PS (from the engine rpm/PS table). Question: what could be the cause of the "slowness" of the boat ? I do not pretend to reach 8.4 knots cruising but at least 7 knots should be in. I'm thinging of dirty hull (green slime), incorrect weight distribution (bow tends to "point" upwards even when crossing small waves). Any experience ? Thanks Matteo |
In article ,
Albert P. Belle Isle wrote: As a quick calculation, 24ft DWL would yield a hull speed of about 6.6kt with a speed-length coefficient of 1.34. To get to 7kt, the effective DWL at 1.34 would be a little over 27ft - not 35ft. We've sustained much higher speeds, but don't feel like opening myself up for someone saying that it's impossible, that I'm surfing, my knotmeter's off, there's a current, or some such. I would agree except that I eliminated all those things. We really have done "impossible" things. Would love to know how, but gave up and simply enjoy. -- Jere Lull Xan-a-Deux ('73 Tanzer 28 #4 out of Tolchester, MD) Xan's Pages: http://members.dca.net/jerelull/X-Main.html Our BVI FAQs (290+ pics) http://homepage.mac.com/jerelull/BVI/ |
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