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post #14 of Old 10-17-2011
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The quotes from Gary Mull pretty much sum up why "hull speed" shouldn't be called "maximum hull speed". Let me just add a bit to why hull speed works as a concept. As a displacement hull moves through the water it creates a wave, which boaters/sailors usually call the "bow wave" or "bow wake". This bow wave initially has a celerity equal to the velocity of the vessel. The celerity of a wave actually slowly increases with time, but we only need to worry about what happens when the wave is first created. The wavelength of a surface wave is proportional the celerity of the wave; the faster the wave, the longer the wavelength. There is actually a fairly complicated formula for this relationship, involving the celerity, the force of gravity, and the density, viscosity, and surface tension of the water. But, since everything is pretty much constant except celerity and wavelength, the whole thing boils down to C = 1.34*sqrt(wavelength).

Now, what exactly does that mean? As the boat moves through the water the wavelength of the bow wave gets longer as the boat moves faster. The first crest is always at the bow, with the next crest being one wavelength aft of the bow. When the speed of the boat (and hence the celerity of the bow wave) increases to the point that the next crest is near the stern of the boat this crest is often called the "quarter wave", but it is really just the second crest of the bow wave; the stern quarter isn't causing the wave, it's being cause by the bow. The velocity at which this crest is right at the stern is called the "hull speed" or HS, and is described simply by substituting HS for C and length at the waterline (LWL) for wavelength in the above equation, to get, HS = 1.34*sqrt(LWL). The important thing here is what happens when the boat is going fast enough (and hence the bow wave has a high enough celerity) that the wave has a wavelength longer than the dynamic waterline of the boat? Now the stern of the boat will no longer be riding on the second crest of the bow wave (or on the quarter wave, if you prefer), and the boat will have to pitch up slightly. This takes a power input to maintain speed and so the power/speed ratio of the boat increases due to this added factor. The power/speed ratio has been increasing with speed all along due to other factors, but now this increased pitch of the hull and the forces generated by the bow having to push "through" more of the initial crest of the bow wave causes a sharp increase in the relationship.

So, it always takes more power (energy/time) to make a boat go faster, but above hull speed the relationship between power and boat speed gets steeper and steeper. This is essentially because above hull speed the boat is forced to climb is own wake. Next time you're in a planing boat you can actually demonstrated this. Give the boat a bit of gas and initially the bow will pitch up, the stern will squat down, and it will feel like the boat is trying to climb out of a hole. Give it enough gas and the boat will "jump" onto a plane and the rules will suddenly change; once the hull is actually planing you can ease the throttle back a bit (lower the power) and stay on the plane fairly easily. But, kill the engine (or go to idle) and the boat will plane for a bit then suddenly slow as it can't maintain a plane anymore and has to crawl up its bow wake, until it gets below hull speed again and more easily glides along as a "true" displacement hull again.

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Last edited by SlowButSteady; 10-17-2011 at 01:57 AM.
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