Porpoising 20 ft. trophy???

[Fasttoys]

Hi guys,
A friend of mine has a 2004 model 2002 20 ft. Trophy fishing rig with a 150 Optimax. He is complaining that it porpoises from 20 to 35 mph and it only has a top speed of 41. It also doesn't take rough water as well as it should.
I'm told the motor is mounted on the transom and the height is set in the center hole.(He thinks the entire prop is under the bottom of the vee so maybe propshaft is 6-8 inches under. That sounds like too much to me.) He also said the prop is the stock aluminum set up that came with the boat.

1)What can he do to stop the propoising?

2) What can he do to get a little more speed to get better gas mileage and a little more top end without sacrificing holeshot and ride quality?

Thanks in advance guys,
Steve

Also, he says he's at 5,800 rpm and trimmed until the prop starts to lose bite in order to get top speed of 41 and the porpoising to stop.

[Jimboat]

This is a common trait of performance hulls - tunnel boats, modified vee hulls and vee-pad hulls in particular. Each boat has it's own unique speed at which is can experience the transition through phases of planing performance. This transition speed is called the "hump zone" – and based on the description of your particular boat setup it is occurring at the velocity range between 20mph and 30mph.

All such hulls are planing on running surfaces and get their performance from a combination of hydrodynamic (water) and aerodynamic (air) Lift. In addition, the transition of the Vee-Pad hull to "running on the pad" can also cause dynamic instability. Here, the hump zone represents the speed at which the amount of Lift from the highly efficient "pad" section of the hull (aftward, center located, flat planing surface) becomes significant compared to the Lift generated by the veed (full length, higher deadrise) portion of the hull.

Propeller selection can often change the dynamic balance of the hull/setup. For example, a change to a prop that provides more aft-Lift can alter the dynamic balance of the hull, and similarly change the speed and range of the "hump zone" – often eliminating porpoising. Weight distribution changes can also have a positive effect on "where" the "hump zone" will occur, as can change in power application. Changing "trim" angle while driving through the "hump zone" even if less efficient, will also provide a better experience, and when well controlled, can "close up" the range of "hump zone" substantially – eliminating porpoising.

Check out the article on "Hump Zone/Why does my Boat Porpoise?". It may be of some help to understand the problem.
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[hsbob]

put some hydraulis trim tabs on. will tame the popoising down and help in rough water at wot.

the doel fins will also reduce the porpoising but may slow down the wot by 1-2 mph.

[Fasttoys]

Thanks for the porpoising article. It's very informative. Let's back up a little bit though.

What should the propshaft height be, and what's a good proven prop for this style boat with a 150 opti for starters? I want to get him some baseline that has been proven on his rig or something similar. Seven inches below seems like a lot to me but maybe on these types of rigs it's common.

Anybody???

Thanks in advance,
Steve

[blkmtrfan]

I had a 20ft trophy, but it had a 4.3l EFI with an alpha I/O drive, however I did try quite a few props on it and the one I liked the best was a merc Laser II

[WATERWINGS]

Don't we all hate the bounce.