Naval Architecture Question??????????

[Greg Maier]

According to the Guide to High Performance Powerboating, Porpoising is defined as
The rhythmic rise and fall that a boat may exhibit when the center of gravity is forward of the center of buoyancy. The solution has generally been to move the principal weight, the engines, as far to the stern as possible, moving the boat's center of gravity aft and over the planing surface or over the center of hydrodynamic lift Through experience, this does not seem true to me. I think my Blackhawk porpoises more when I have passengers in the back seat. The reason that I am asking is that I am remounting my batteries, and would like to mount them in an appropriate place that provides for minimal porpoising but also will not have an effect on top end. Or, is the weight of the batteries so minimal that relocation won't have an effect. Currently, the batteries are mounted right next to the transom on both sides of the engine. Also, I am getting ready to put an aftermarket exhaust system on (which will reduce weight). Let me know what everybody's thoughts are.

[blackhawk]

Greg, unfortunately the rocker bottom of the blackhawk makes it porpoise at cruise speeds. I always drag a little tab when I am running at cruise speeds and it works fine. At WFO I bring the tabs up as high as possible for conditions. If you start moving weight forward I think you will lose top speed. One time my brother and I were on flat water and he would get up and move to the back seat and I would pick up .4 - .6 mph. We did it 5 times and it was faster everytime with him in the back!

[GEOO]

I agree; the batteries were put back there to help move the CG back, moving the batteries foward would reduce porpoising and slow down top speed.
You could just run a little tab, or instal a water balast tank in the bow!!

[Greg Maier]

Thanks for the replys. I have no problem keeping the porpoise under control with the 280 K-planes, and from from 65 to 80 mph, I use absolutely no tabs. I was just wondering if the batteries were put that far back for a purpose or just convenience and if they could be moved to a better spot. In reference to the qoute from the book, it seems to me like moving weight forward would reduce purposing, but the auther says that moving weight backwards is what needs to be done.