17' 4" Tri hull Evinrude 115 HP O/B porpusing

[jm.jones]

I just pick this old MFG Gypsy 17' 4" Tri hull and it has a great running Evinrude 115 HP O/B. We took it out and it ran great, BUT it seems that at WOT or close to it (around 35mph or so), it starts porpusing. This is an older motor without and tilt and trims on it, just the old school cross bar tab on the bottom of it to adjust how the angle, we have it the second hole from the back of the boat. How do I stop this thing from jumping in the water??

Thanks everybody for your help, John

 

[kenmyfam]

Re: 17' 4" Tri hull Evinrude 115 HP O/B porpusing

Could be a motor height issue and a trim issue. Other causes could be a hooked hull.
Welcome to iboats.

 

[jm.jones]

Re: 17' 4" Tri hull Evinrude 115 HP O/B porpusing

The motor is the one that came on it from the deal back in 1976, 25" shaft. How can I check for a hooked hull??? and why does that just sound bad.... Should I put a whale tail on it and would that help?

 

[kenmyfam]

Re: 17' 4" Tri hull Evinrude 115 HP O/B porpusing

Where is the cav plate in relation to the bottom of the hull ??? above, even or below and if not even how much above or below. Use a straight edge to determine.

 

[jm.jones]

Re: 17' 4" Tri hull Evinrude 115 HP O/B porpusing

the top plate of the foot, (lower unit) sits just under 2" higher then the bottom of the boat, it is in the middle of the drain plug, not sure of how to add my own pics on here, but I did take a few of the lower unit and where it sits on the boat

 

[NYBo]

Re: 17' 4" Tri hull Evinrude 115 HP O/B porpusing

Welcome to iboats!

It sounds like your motor is a little high on the transom. Not sure if that extra inch or so will make a big difference, but it might be worht a shot to lower it one mounting hole if you can.

 

[jm.jones]

Re: 17' 4" Tri hull Evinrude 115 HP O/B porpusing

I can go to the top hole, after that what else can I do, do they sell trim tabs of whale tails for this thing, or should I go with a four blade prop, or a higher pitch?