401 Hull Speed

Hull Speed

There are two types of boats, planing vessels and displacement vessels.

A planing vessel has a hull that basically consists of an equilateral triangle and relatively flat bottom.  The propulsion system pushes the vessel forward and eventually with enough speed the boat will skim or plane across the water.  The only limits to speed are the power of the propulsion system, the drag between the hull and the water, and the insanity level of the pilot.

Displacement vessels, of which your average sailboat is a member, have to push their way through the water.  Pushing generates waves and thus wave making resistance.  At low speed the bow wave dissipates before a buildup occurs.  As the boat increases speed the amplitude of the wave increases because the boat is generating a bow wave faster than the wave can dissipate.  At higher and higher speeds the boat must cut through an increasingly bigger bow wave which in turn creates greater and greater drag on the hull of the boat.

 

 

It used to be believed that Hull Speed was dependant only on the length of the vessel and the rule of thumb formula was:

Where v = velocity in knots and LWL = Length at WaterLine in feet.

For your Catalina 22 the Hull Speed computes as follows:

V = 1.34(22)^ ½

V = 6.29 knots

Some folks use 1.51 as the constant which would yield a slightly higher hull speed.

V = 1.51(22)^ ½

V = 7.08 knots

Current nautical science has shown that there are various factors that can be modified to increase hull speeds on non-planing vessels.  These designs include hulls with very fine ends, very long hulls with narrow beams, and wave piercing designs.  (There is even a bulbus nose design that you can see on big vessels that cut the waves more efficiently.  I need to do more research but I believe that the bulb works sort of like a snow plow on the front of a train.  It throws the wave away from the bow thus reducing the drag of a large wave traveling down the length of the hull.  That's my current theory.  I'll update if I discover more info.)

The top speed that I have reached in the Christopher-Jin is 6.5 knots at a near broach and once we hit 7.1 knots planing down some rather big waves (4ft. plus).  (Yes, it was unbelievably rough sailing and yes, I did drop my cookies but when situations get dangerious and if you get sick, get it over with and keep the sailboat under control.  I was at the helm and I wanted to be able go sailing another day.)
 
 
 
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