Porpoising probems on a 34 Eliminator Eagle
 

I have a 01 Eliminator 34 Eagle that I am having some porpoising problems with. I can tab down in order to get it to level out, but as soon I give it any trim at all it starts right back up again. I am running 31 pitch props. The boat rides bow high almost constantly. It will do it at just about any speed, trim, or tab level. But the most violent is when you try to run north of 50 and pull some of the tab out of it and trim it up.

Have you tried props with less bow / more stern lift? Brett at Bblades will probably be able to fix that issue for you.

Are your props turning in or out? If in, try out.

it sounds like the boat has some hook in the bottom.

Like Mild Thunder said, try turning your props out first. I've heard that the 33 cats had a porpoising problem and the bottoms needed to rework on those.Contact Bob Leach at Eliminator, he would be your best bet. Hook would force the bow down at higher speeds loosing speed caused by more vetted surface.

I have been fighting this with my Eagle for about 2 years since I've owned it. BBlades did some custom prop work for me (Added bow lift) and solved the issue. Then I went with a -2 shorty and the porpoise came back. I have another prop on the way to BBblades to add more bow lift to cure it again. I too have the tabs and know exactly what your going through. From what I have been reading alot of eagles have this problem and added bow lift usually cures it. Good luck

Thanks for the input guys, but I can't see me needing anymore bow lift. It seems to have way too much as it is currently. And my props are turning outward already as well.

My research over the years on this subject has shown that porpoising is a result of the CG being too far aft for the current condition, as defined by the parameters of loading (lift coefficent), speed, and deadrise angle. If you are porpoising in the 50-60 mph range, that's fairly normal for a boat that is set up to run 75+mph. You must either drag the tabs at these midrange speeds, or move weight forward. Moving the weight forward will improve the situation at the midrange speed, but will end up costing you speed on the top end if you can't develop suffiicient bow lift to get to the optimal trim angle to minimize wetted surface area. Stepped bottoms are able to skirt these rules somewhat, and playing with the prop rake/drive depth parameters can change the effect for the better.

In my research, I've run both full-scale and tow-tank model tests. The model testing allows you to remove the variables associated with the drives, and concentrate on the hull dynamics. When you do that, you realize that a particular hull is going to porpoise at a certain threshold, and the other things we do (i.e. the props, tabs, etc) are merely trying to make the best of it by giving bow or stern lift to delay it's onset, and narrow the speed window at which it will happen. Some setups obviously have been able to all but eliminate it.

If anyone is interested in further reading, you can find my papers in the 1998 SNAME Transactions (Society of Naval Architects and Marine Engineers), or the full report is on file at the U.S. Naval Academy Library, Trident Scholar report #254.

Nice post, I will look up your papers when I get a chance........ thanks