R 22

Rhodes 22


Hull Speed

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.


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

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