Originally Posted by Stumble
The hull speed
formula that everyone throws out is a gross simplification that assumes that all boats are roughly the same shape. It used to work because all vessels were roughly the same shape and thus everything worked out fine. But as hull
shapes changed the simplification becomes less and less true, and eventually breaks down completely (cats, tri's, fast planing hulls).
The simplification is that the 1.34 used is not a constant, and really only applies to vessels with a maximum S/L ratio of 1.34. So instead of 1.34 what you should use is
max speed Length ratio=8.26 / (D/L)^.311
Where D/L = DLT ÷ (0.01 x LWL)³.
- DLT - Displacement
in Long Tons (2240lbs)
This is the formula derived by David Gerr and it does work for catamarans up to a Length/Beam of around 15:1. Past this and wave making resistance seems to fall off a cliff and a whole new range of even more complicated formulas takes over. But there are a vanishingly small number of boats that get into this range. Even fast cruising cats don't meet this normally.
You are correct of course, but in the case of the OP's boat
, a 1970's design era Cal
2-27, the simplified proforma formula works quite well enough within a few 10ths of a knot
That said, I think that if he goes into his "settings" on his GPS
, he'll find that his speed setting is KPH, rather than Knots. If one divides his reported speeds by 1.852 one gets reasonable numbers for Knots.
As for the OP "calibrating" his GPS
, I suspect he is merely resetting his trip/speed log to zero at the commencement of an outing.
The foregoing not withstanding, if he's "happy" with thinking he's zooming along I'm not sure what harm it does to anything save his credibility. One of my wife's friends has a bathroom scale that's so goofed up it under reports weight by upwards of 10%. (On her scale I weigh only 135#). Yet she beamingly proclaims that she only weighs XXX# and it must be so as that's what her scale reports. Where's the benefit in bursting her, or in this case, the OP's bubble?