1.34 * sqrt (20) = 5.99
1.34 * sqrt (22) = 6.28
1.34 * sqrt (21.75) = 6.25
You have to understand what is being decried by the hull speed
formula, it is a inflection point on a power curve, power needed to
achieve a give speed at a given LWL, the physical phenomena is a hull
as it is passing thought water and is "displacing" water out of it
way, and in doing so forming a wave pattern. If you watch you boat in
the water, each speed has it's own pattern, first you might see 8
waves, then 4 waves, then 2 waves and then one that keeps lengthening
unlit it reaches the transom. This is said to be hull speed because
the boat will start to go up wave (boat is going uphill) being formed
and the power needed is going up dramatically. The question is where
is the "reasonable cutoff" or the inflection point.
Now, to go faster.
As an aside, an aircraft carrier has a rated HULL speed of 35+- knots,
in fact that is the number the DOD publishes; but the fact is, they
go faster by using twin (or 3) 80,000 hp reactors, this is the brute
force way (they are also using their displacement, length and hull
games but that's aside)
The second way is by having the boat ride up the wave, pushing less
water out of the way, going faster, riding up more of the wave,
pushing less water out of the way, faster, etc.... this is a planing
boat (i.e. speedboat), that is also why the "speedboat races" have
the great pictures of the back of the boat coming out from under them,
pin wheeling.
Sail some sailboats can plain also, but instead of using the motor to
get the force there are using their sails general with wind aft of
beam, if the naval architect designed the boat with this in mind, it
can be fun, (but they are general lighter boats to climb the wave)
there is one difference, when sail are overpower there forces are up
front and they pitch pole (the bow digs in and the stern comes up) or
broach (bow digs in and the stern is brought around by a following
wave, turns broadside and bad things happen).
The third trick to use is that sailboats heel, increasing the LWL,
this was a favorite trick for racing boats and the rules they used to
build them (out of fashion, rules changed), the Rhodes 22 LWL does
increase with heeling, but not much.
LWL describes most of the phenomena. That is why it is used in the
formula, but in truth it is the complete hull shape and balance (the
hull reacting with the wave pattern). The naval architect can shape
the hull to push the inflection point a little forward. Philip
Rhodes did some of this, but each hull is a set of compromises (sea
keeping, waves, load, speed, trailing, dryness, comfort etc...).
I hope this answers some of the questions. At this point I am about
at my limit of knowledge.
MJM
Michael, 5.99 would be the maximum hull speed for a boat with a 20-
foot water line. If the Rhodes is heeling it could have as much as
22 feet in the water. The hull speed would then be 6.25. Are you
saying it is possible to get a Rhodes to 6.25 upright?
Bill Effros
