cg on racing deep vee

[John Finch]

I race and design model deep vee boats that work by remote control. A 31 percent cg has worked extremely well for racing in the 40 to 50 mph range. The problem now is that speeds have increased to the 60 to 70 mph range where the boats blow off the water at that cg location. Moving the cg forward to 36 percent (distance measured from the transom forward as a percentage of full length) the boats stay on the water but tend to find holes in the water and submerge bow first at full speed without warning.
Dipping the bow to hold the boat to the water using aerodynamics works to keep the boat on the water, but the dip also causes the boat to submerge at times during full speed operation. The fix right now is to add a spoiler to the transom to keep the transom glued to the water. Any ideas? Anyone know what the big boats use for a cg or areodynamics?

John Finch

[Tom Lathrop]

Assuming that there is some linearity between large boats and models, the optimum trim angle of the aft hull bottom is usually given as 4 to 5 degrees positive to horizontal. This is given as the optimum balance between wavemaking drag and surface friction drag. At high speeds, your trim angle is probably generating too much lift to allow the hull to stay on the water, so reducing the trim angle by moving the CG forward keeps the hull on the water but gives the diving problem you see.

I'm disregarding aerodynamic effects since you can control them in other ways.

Perhaps the solution is to increase the bottom loading by decreasing the area of the aft planing surface. This should reduce the hydrodynaic lift without changing the hull trim angle. This can probably best be done by reducing the beam of the planing surface.

I'm just making educated guesses here and maybe someone else can come up with a better or more accurate analysis.

[John Finch]

Tom,
I have seen exactly what you said about the forward cg changing the angle of attack of the hull. The lesser angle of attack gives me about 15 mph over a standard cg boat. That is why I want to make this thing work! The narrower beam sounds like a good fix for straightaway running, but it creates problems in the turns. The narrow beam lets the boat sometimes roll over to the right in a right hand turn. These boats bank hard and run on the chine line for 80 percent of the length of the boat. The width is needed to keep the boat from rolling to the inside. We race clockwise with a 75 foot radious turn.
I have been experimenting with aerodynamic solution of a spoiler at the rear of the boat. My theory.......If the transom stays on the water, the boat can't stuff. Today I raced In Norfolk, Va. and found a spoiler that had just the right amount of angle and size to do the trick. I won every race: no problem. Still, I search for the ability to run the less than normal angle of attack without the use of a spoiler. Which leads to my next question....
Are the full size boats designed so the deck dips at the bow to keep the boat aerodynamically balanced at optimum speed?

Thanks,
John