70hp Enivrude VRO, prop ventilating!

[maddognt]

I have a 4.8 polycraft cubby cab, with a 1990 (old) 70hp Evinrude VRO. Polycraft are very good boats, stable and ride very good. BUT!!! The prop ventilates! Even on small turns. The dealer where I bought the boat said too put the motor 3 holes down from the top. Ventilates out of the hole. So I put the motor in the top hole (down all the way) was better no ventilating at of the hole, but on a small turn ventilates!!!, then I installed a Permatrim made it a lot better but still not right. Now I’m thinking a bigger anti-ventilating plate like a Boatright or Nedski. Maybe a 4 bladed prop, I have a 13 3/8 x 17 x 3 blades. So has anyone else had trouble with ventilation on polycraft boats? Thanks

[Pirate Pete]

Most owners find their boats drive like they are on rails & only have any hint of ventilation if they dont trim down in tight turns.

Do you have any pic of the hight of the motor compared to the bottom of the hull.

If you take some pics sit a straight edge along the side of the keel & bring it out past you motor. The cav plate should be about 25mm or so above the top of that straight edge

[maddognt]

Most owners find their boats drive like they are on rails & only have any hint of ventilation if they dont trim down in tight turns.

Do you have any pic of the hight of the motor compared to the bottom of the hull.

If you take some pics sit a straight edge along the side of the keel & bring it out past you motor. The cav plate should be about 25mm or so above the top of that straight edge

The motor is about 25mm blow the hull, any higher it ventilates! too much, even 25mm below it ventilates at about 1/3 steering lock, trimmed all the way down, If I’d known the was going to be this bad I would of bought a quintrex. It is next door to useless the way it is at the moment.

[chin]

My 4.55 was ventilating, dropped it 1 hole which made it worse, put it up 1 hole from where it started and it's almost perfect. It still ventilates a bit in tight turns, but I can live with it.
A good S/S prop from Solas would probably cure it.

[maddognt]

And another thing, the two middle upside down "U" things in the hull could be making turbulence making it ventilate. They are 15" apart, my prop is a 13 3/8” so a sight turn the prop is in line with them. So has anyone thought this, or filled them up with poly and tried it.

[maddognt]

And another thing, the two middle upside down "U" things in the hull could be making turbulence making it ventilate. They are 15" apart, my prop is a 13 3/8” so a sight turn the prop is in line with them. So has anyone thought this, or filled them up with poly and tried it.

I’m going to fill them with silicon, and go take it of a try

[bushbeachboy]

That's an interesting thought maddog. Those sections must be in there for a reason. Maybe someone could advise? Perhaps directional stability? Polycraft wouldn't have gone to the expense of having the mould made that way for nothing, so be a bit careful.

Something else you might check is the alignment of the prop blades. Remove the prop and place it face down on a flat surface, then measure the gap between the blade tips and the flat surface.

I mention this because a mate has a Quintrex with a 60 Evinrude, about a 95 model. It would cavitate for no reason also. When we removed the prop and put it face down, you could see without measurement the difference in the blade alignment. It measured to about 10mm. He had the prop fixed and problem solved. All this was done on the advice from a bloke in the fishing club, and it worked a treat.

[DvrDve]

1 st make sure the the prop is balanced and true
spinning on a shaft in a lathe is preferred the blades might be as lil as .4mm out of true -hard to measure
i had strees with 3 props from Johnson -straight out the factory the rubber bush inserts were out of tru -only by nothing but enough on my big boat to have stuffed it 27ft banana style boat
eventually got my money back-on mates boats- different designs it made no difference
so i ddint want to believe it was the prop at 1st

i went to a 4 blader solas
lost about 5-8% top end -but out of hole /following seas etc it is now an absolute pleasure
i went ali as S/STEEL is heavier-more effort on motor etc gotta go down a further at least 1/4 inch pitch normally
also it is easier to carry an ali spare prop and break a blade than it is to replace a gearbox/skeg and use the same S/S prop again mho

[Andy]

Ventialtion means one thing, you are getting air onto your prop. Get yourself onto flat water, boat on the plane and take a good look at where the cav plate is. The cav plate should be on the surface of the water when planing, and it should be parallel to the surface. When turning hard, your boat leans over, and so the prop starts pulling air. Trim down. There is no way to avoid this on a planing boat unless you have an inboard and the prop is always in the water.

Your motor may well be the correct height, but it has to be at the correct angle at the correct height.