Use 1.4 rather than 1.34 and you'll get closer to your real hull speed. as for climbing the wave....physics won't allow a displacement vessel to leave the wave trough. The reason motorboats can is because they ride on top of the water more or less. Unless u r a centerboard boat u will not be riding on the water. (Most centerboards at least)
The 1.34 "factor" is actually derived from a combination of a number of factors, including the density, viscosity, and surface tension of the water (which all vary with temperature and salinity), wavelength (lambda
), and gravity (g). As it turns out, if we set everything but lambda
to reasonable constants (and discount surface tension, which is too small a factor to worry about at the relevant scale), the whole thing boils down to the common 1.34 times SQRTlambda
. However, it will vary a bit with temperature and salinity.
Some folks, apparently you included, often add a bit to that 1.34 factor. This isn't terribly unreasonable, as the LWL is measured with the boat in a static condition and the dynamic waterline will increase a bit as the boat speeds up. However, there is a dramatic increase in the power required to propel a boat as it gets into the 1.34 to 1.5 times SQRTlambda
range. There is no actual inflection point in the curve, so we can debate as to exactly which factor is most appropriate. However, a displacement boat (any displacement boat) can "overcome" its hull speed with a large enough engine. This isn't to say that a Catalina 27 with an A4 running at full throttle will plane, but one look at the bow wave will demonstrate that the NEXT crest of that wave will be beyond the stern of the boat (i.e., the wave generated has a longer lambda
than the LWL of the boat). In this condition, the hull will be pitched slightly up, since the stern will be sitting in the trough of the bow wave and the bow will be perched (or "climbing up") that wave.
Note how the boat in the following pic is traveling a bit above hull speed (the stern is forward of the following crest), AND the hull appears to be pitched up slightly:
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Last edited by SlowButSteady; 04-27-2011 at 10:42 PM.