> Hi John, The problem is that these "power required" calculators don't actually work for many of our boats. We had a long discussion last year about this, I even discussed the problem with Dave Gerr himself. Dave admitted that his formulas won't work at the lower speeds and powers that we typically operate at. Here's my proof. The caculator that you suggested says that my 30' ketch with a 24' LWL and 10,200 lb displacement would need more than 6.81kW or 9.13hp. But I know that the boat only draws 2500W from the batteries to motor at 5kts, less than 37% of the predicted value. >
One should use eg. Michlet for better hull drag estimation. Michlet was actually made for narrow hulls estimation, but it works pretty well for wider hulls too.
http://boats.yyrtti.com/performance/hulldrag/hulldrag.html
Give length, beam, weight, max speed, and you get predicted drag curve (not wetted trasom stern or pointer stern hull). You get 170lb at 5kn for your example ketch.
Additionally you must calculate required power from speed and drag figures, and you must know your prop efficiency:
Power = Drag * Speed / prop efficiency
Prop efficiency may vary 30% to 60% depending …
Drag to Power equation (european units way):
2 * 0.001 * 0.5148 * 0.4536 * 9.81 * 170 lb * 5 kn = 3.89 kW
clarifications:
2 is: 50% prop efficiency assumed
0.001 is: Watts to Kilowatts
0.5148 is: Knots to m/s
0.4536 is: lb to kg
8.81 is: kg to Newton
Simplified version in case of 50% efficiency and lb and knots:
0.0046 * 170 lb * 5 kn = 3.89 kW
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