Newboy Posted March 31, 2005 Report Share Posted March 31, 2005 Crouch's Formula V = C/((DISP/HP)**.5) Where V = boat speed in knots (1 knot=1.15 mph) C = Constant (depends on boat type) DISP = Displacement (pounds) Note that boat manufacturers usually give innacurate numbers for displacement, typically on the low side HP = Horsepower available at the propeller You can use this to work out (approx) max speed of your boat. Quote Link to comment Share on other sites More sharing options...
Adam F Posted March 31, 2005 Report Share Posted March 31, 2005 ....So what is the 'constant' how do you find that out? I have the HP, and the lb's displacement.....?? Quote Link to comment Share on other sites More sharing options...
Barry Posted March 31, 2005 Report Share Posted March 31, 2005 The consant is the waterline length. This formula only works for full displacement boats. Planning type hulls are a different kettle of fish altogether. Even then its not totally accurate as you rally need to take into account the thrust from different propellers. There is another formula that will give the theoretical top speed of a full displacent hull using the waterline length. A little prog on the Steel Developments site is a bit more applicable to planning hulls as it will give prop sizes for given hull lengths/motor hp outputs. But then prop sizes is a whole subject by itself. Quote Link to comment Share on other sites More sharing options...
Adam F Posted March 31, 2005 Report Share Posted March 31, 2005 Ill forget it then! Thanks Barry. Anyway - I know what the top speed of my boat is....its when the throttle is hard down! and the tide is behind me! Quote Link to comment Share on other sites More sharing options...
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