Not sure if any of you know this, but I am taking the Yacht Design course at
Westlawn. It certainly doesn't make me an authority, but the discussion is
timely. I'm currently working on the 5th module which is titled
'Resistance'. The value of 'about' 1.34 (not the 1.37 I mis-stated earlier)
is the number we're instructed to use.

There are many other factors that effect 'actual' hull speeds. These include
laminar/turbulent flow, relative wave forms (related to the
Displacement/Length ratio) which is also related to the prismatic
coefficient. The prismatic coefficient represents the distribution of the
underbody volume. It is derived by taking the maximum underbody sectional
area and multiplying it by the LWL. It is a factor used (in conjunction with
other coefficients) to give the designer a sense of the design's
performance. From the text: "A low prismatic coefficient indicates that a
hull has fine ends and a large midbody. A high value indicates fuller ends
and a more slender midbody." To picture this, if you had a prismatic
coefficient of 1.00, you'd have a floating brick representing the dimensions
LWL x Waterline Beam x Fairbody Draft.

It does not seem to provide any quantitative value in terms of hull speed.

These other factors, however, can be overcome with the application of enough
power. Regardless of these factors, the maximum (theoretical) displacement
hull speed is ONLY related to the bow/stern wave coincidence. I would assume
that the discrepancy in the multiplier probably has something to do with the
amplitude or some other physical characteristic of this wave coincidence.
Or, perhaps on certain vessels the waterline increases (or decreases?) at
displacement speeds. That seems counterintuitive, but could explain the
difference in the multiplier.

All of this is so much armchair engineering however. John McGrady
characterized it best. Nothing beats real-world, first-hand experience. I'm
just amazed that 7 tons of fiberglass can move along at all under wind power
alone.

Cheers,

Rick Beddoe
s/v Soņadora, 1978 Baba 30
http://sail2live.com





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Received on Sun Feb 12 19:13:45 2006