Michael Daly wrote:
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
Mike,

I totally agree, and I stand chastised and corrected for using the term
"hullspunik" or whatever. Since it is by your definition impossible to
climb over the wave, what is happening when the point at which the hull
separates from the water moves from up on the knuckle of the bow to a
point several inches (or more) behind the knuckle? BTW, at this point
my energy output to sustain this position has decreased below max, and
the speed of the boat exceeds the speed at which this max energy output
occurs.