Wm Watt wrote:

I don't know the actual limits to Froude's formula or if
there is an adjusment factor incorporating light displacement and
extreme length-to-beam ratio.

The mistake people make is to assume that Froude's formula for hull speed
actually represents a meaningful number for analysis or design. It is simply an
observation that there is a speed-length ratio where the bow wavelength is the
same as the waterline length. It is only useful in comparing two nominally
identical hulls of different length. It is of no real value otherwise. Marine
architects and engineers do not use hull speed for design.

In real vessels, if you tow them and measure the bow wavelength and then
determine the speed at which it equals the waterline length, you will find that
is is not likely to be precisely 1.34. It may be more or less, depending on the
shape of the hull.

If you look at a graph of speed versus resistance measured from a towing tank
test, you cannot find a point on the graph that represents "hull speed". The
curve is smooth and shows no change in magnitude or slope that would show where
hull speed occurs. There is no manifestation that would suggest a rapid
increase in resistance. There is no indication that the vessels is "climbing
its bow wave".

Vessels do not climb their bow wave - you cannot climb a wave that you create.
That would be like holding a rope up with your left hand and claiming you can
climb it with your right. You cannot push through the bow wave for the same
reason. What happens is that the vessels changes apparent trim angle to match
the wave and you continue pushing the water out of the way. This starts with
_any_ motion of the vessel - it does not start at hull speed. The faster you
go, the more energy it takes.

It's too bad that the term used is "hull speed". It does not represent the
speed of the hull. I wish the term would go away as it has generated far more
bull**** than meaningful discussion on boat performance.

Mike