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Chesapeake Sailor
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579 Posts
1.34 x square root of the LWL. In this case 6.14 knots. (There is more than a little quibbling about the coefficient, but the standard deviation of the quibbling results in negligible differences anyway.)

wlw
 

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Chesapeake Sailor
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579 Posts
The calculation yields an approximation, but it should not be that far off. The more interesting of the two measurements you supply is the motoring in calm water example. That one precludes surfing or lengthening the effective LWL through angle of heel and thereby increasing theoretical speed. The first place to look is at the GPS measurement. Was there a favorable current running (tidal or river)? Remember GPS is measuring Speed Over Ground (SOG) not through the water which the calculation approximates. Also is the GPS measurement recent, i.e since GPS selective availability has been turned off (May 2000 I think)? Is your GPS a differential model using beacon or WAAS? The tighter geolocational accuracy of differential also yields more accurate velocity calculations.

wlw
 

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Chesapeake Sailor
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579 Posts
hervel,

I do not know a simple formula for a cat. As you know the catamaran is not restrained by displacing water - which the simplistic formula is based on. The ability to plane in a catamaran frees you of all that stuff. I suppose a much more complex formula based on drags (hydro and aero) and power is possible but it would be less general in nature i.e. requiring very specific measurements for your boat.

wlw
 
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