Eliminating High Speed Porpoising

[Jango]

For you Naval Architechs:

My 20 ft custom Inboard is starting to Porpoise at 82 mph, 12x15 prop turning 6200 rpm. I have plenty of throttle left for additional speed if it were not for the potentially dangerous bounce. Boat with driver and fuel weighs 2200 lbs with CG 87in from Transom.

My question: Would a Small diving plane, perhaps 6-10 in wide x 4 - 6 in. long welded to the bottom of my rudder Help this situation? I would think it would tend to pull the transom down overcoming the effects of the 16 deg. shaft angle? Has this been tried or will it just add drag. If this has a chance of working, How much down angle?
Adding weight (people) behind the Engine does tend to eliminate the Porpoise at the expense of speed

Thanks, Jango

[Rick Willoughby]

This site can help you play with the hull parameters:
http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html
This only considers the water forces. In your case the windage will also be a factor. Place the curser over the green "figure" to get the detail on input parameters.

There is a thread that deals with dynamic lcg that might provide more insight:
http://boatdesign.net/forums/showthread.php?t=22114&page=2&highlight=dynamic

Rick W.

[Jango]

Thanks Rick for your reply.

I probably should not have referred to my situation as Porpoissing. I am Not expierencing BOW LIFT, but rather TRANSOM LIFT caused by the 16 deg.(relative to the planing surface)Thrust angle of the Drive Shaft and Prop.