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Go Back   SailNet Community > General Interest > General Discussion (sailing related)
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  #11  
Old 08-02-2002
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Hull Speed

This is a good discussion but a couple points here.

While some boats with long over hangs increase in speed as the boat heels not all do. In the current thinking, the boats that do increase in speed, do not increase in speed because the waterline is getting longer as previously thought, but because the counter is submerged increasing the buoyance aft and helping prevent the stern from squatting.

Moving crew forward as a boat comes off the top of a wave can help a boat surf sooner. (Similar to walking toward the front of a surfboard.)

I think that it is not just a matter of lower displacements per se but the degree of fineness of the bow and stern. There are boats that are considered semi-planing or semi-displacement hulls. These boats easily achieve speeds well over the theortical 1.34 times the square root of the waterline length. The issue that creates hull speed is about the energy required to push a boat against the resistance of its own combined bow and stern wave. When you talk about semi-displacement hulls great effort is made to reduce the height of the bow and stern wave at speed so that while the boat may not actually climb up on its own combined bow and stern wave (i.e. plane) it takes less energy to overcome the drag of its own combined bow and stern wave. Typically these are boats with comparatively fine bows and waterline beams, comparatively shallow canoe bodies and fairly powerful stern sections. If you look at most IMS typeform boats or the Volvo Ocean race boats that is where they get their speeds well in excess of the theoretical 1.34 number. Almost all catamarrans do not plane but infact get their speed though small wave making (i.e. semi-planning).

Jeff
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Old 08-03-2002
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Hull Speed

Jeff,

The authority that Duane cited said "it all depends on displacement." If we assume that is true, just for the sake of argument, then the one quality that distinguishes a heavy displacement boat from a light displacement boat is that the heavy boat moves more water out of its way than the latter. If the hull has to move more water out of its way, more energy is used up to drive the hull through the water. Perhaps a light displacement boat is capable of pushing its own, smaller, bow wave through the water at a faster speed than a heavy displacement boat can push its bigger bow wave through the water, using the same amount of energy. I don''t know whether this reasoning is correct, but it seems to be consistent with what you and Duane''s authority are both saying.

Also, Duane''s authority says, "Everyone is familiar with Anthony Deane''s original formula for heavy displacement hulls, and people are slow to catch that non-planing boats go faster than Deane''s formula predicts, despite our observations that boats sometimes do go faster than they''re supposed to." You said, "Waterline''s affect on hull speed is theoretical and not absolute." Is this an indication that, as boat design becomes more sophisticated, the authorities are coming to the conclusion that Deane''s formula should only be taken as an approximation, and that, with future advances in boat design, the upper limits of speed can be raised, and that perhaps Deane''s formula itself should be modified to reflect current technology and thinking? This would certainly explain the huge amount of anecdotal evidence that boats do in fact exceed the speeds predicted by Deane''s formula, without planing.
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