Airpacker

Its all a result of physics. Plain and simple. As boat speed increases, air is compressed in the tunnel from front to rear. As more air is compressed, it attempts to lift the hull out of the water. If, force of compression matches or exceeds weight, the boat flies. If its a number less than boat weight, the boat rides on the hull. If its a number somewhere between then the hull starts to lift, lowers the compressive force number and starts to settle back down. As it does this, the compressive force rises again due to the tunnel getting closer to the water surface thusly lifting the hull again and lowering the compressive force.

Stay at a speed where this happens and it will porpoise indefinately. Slow down and you loose the lift and ride on the sponsons. Speed up and the compressive force ( lift ) becomes greater than the boat weight and you ride on the colum of compressed air.

CcanDo

1. Rule of thumb....Props turning out are more stable than turning in....Tripod effect. Also, when props turn out, toe should be out.
2. Rule of thumb....Prop blade tip should be 1/4" inside of tunnel, regulated by prop diameter. Toe in/toe out with also dictate that dimension.

Porpoise and bucking are sometimes confused. Bucking is a condition that lifts the transom out of the water...not good. Porpoise is the transition between hydrodynamics and aerodynamics....Safer, subject to degree.

Prop rake will mask most problems....More rake will create more bow lift, pull the transom down and should be more stable. However, subject to CG location, planing may require ballast tanks/nose weight.

CcanDo

Have you changed your set up yet ?

Naughty Kitty

Nicely written AP...one thing I learned as a marinal HP cat owner is that you can change the dynamics of the hull with the use of tunnel tabs that act as a governor to the release of the air pack out the back. I get a little hop in calm water around 80, but add a little tab to choke off the air pack vent out the back and it smooths out.

TuckerDog2

AirPacker's explanations is as good as it gets. I have a 36ft Spectre with twin 500efi's. It hops pretty good at 70-80mph. I can drive thru it without much problem, but many times I want to "cruise" at that speed, and you just can't unless the water is really choppy. The smoother it is, the more "hop" you get as your speed approachs that range. I was told a large tunnel tab will minimize the "hop", but to be careful as you get past 80mph, as the tunnel tab and a "errant" wave could send the nose into the water. I have seen alot of discussion in these forums as to the dangers of tunnel tabs, so I have pretty much resigned myself to cruising at 70mph or less, or 80 MPH or more. I have played with my set-up, props in and out, toe in, etc. It seems the biggest factors with my boat come from wind direction, fuel/passenger weight, and water conditions/choppy water.

kneeslider74

I changed the toe to even, checked the LW pu's as well as ran a laser across the bottom of the hull. Hull checked out nice no hook, I have yyet to get it to the lake. I am guessing it may be something to do w/ the toe. The drives were way toed out when I got it and it seemed to start bucking at about 80. I truned them in a bit and bucking started at a lower speed so I figure ill try in between. As far as things go I need more pilot time NO Doubt! I wont be able to get it on the water till the 18th now. I will keep this link going as I find out the details. I may just need to borrow some balls and do just what you said. Trim in till 75 or so and then bring the props out of the water and WOT.
Thanks to all