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#1




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 
#2




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 
#3




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 
#4





#6




On 12 Apr 2005 17:45:57 0700, wrote:
One of these days I'd like to look into the derivation of hull speed. ===================================== I'm guessing a bit but I'm pretty sure that you and James Clerk Maxwell could figure it out with the aid of some wave equations and the density of water. 
#7





#8




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 nonrigid 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. 
#9




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 
#10




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? 
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