What causes porposing? Motor too deep?

[ssv1761982]

A friend just bought a 2000 Four Winns 18' fish & ski with a 150 Yamaha. At 30 mph you can't trim it any to get the bow up or it starts bouncing down the lake. The motor is on the transom and it has a 4 blade 18" Turbo prop. Hole shot is good for skiing but it only tops out at 5200rpm and 43mph on the GPS.

Thanks for any advise,
Dave Smith

[pyro]

Sounds about right. Porpoising happens in two ways, the slow porpoise and the fast porpoise:

At slower speeds, the rear of the hull may "dig in" and push a little wall of water out in front of itself, then it climbs up this wave and the bow drops down. The stern digs in again, and starts the cycle all over again. Hop, hop, hop. Positive trim makes it worse. Large passenger loads also make it worse.

High-speed porpoise happens with tunnel hulls between 50 and 85 mph. The tunnels are trying to pack air, but the stern is not un-wetted enough, so pressure builds in the tunnels, then gets released at the stern, then builds pressure again. The driver has to actually trim UP a little bit to drive through this porpoising stage until the boat has reached a speed where it consistently pack air in the tunnels from bow to stern, for a uniform lifting effect.


Why the heck are you giving a Four Winns positive trim at 30 mph?

[ssv1761982]

It just feels bow heavy. Seems like it should carry a little higher. We were waterskiing last night before the big storms came through NE Ohio.

[baja200merk]

can setback help with the slow porposing? my brothers 12ft minihawk wit 25hp merc porposes continuously... motors on the transom was thinking about seting it back and jackin it up what do you think?

[pyro]

It's a Four Winns, of course it's bow heavy. That's why it cuts through the chop so nice. It's not made to fly, it's made to float. Trim it in to keep the bow DOWN when you're skiing.

as for the little boat with the 25, not sure about that one. For lower motor settings and cruising speeds, a planing fin will help the porpoising. Otherwise, a small, light boat with a lot of weight in the rear will tend to porpoise a lot for the reasons I mentioned above, especially with passenger load.

-Chad

[ssv1761982]

Thanks, this is the 1st Four Winns I have had any experience in. I thought maybe raising the motor a hole or 2 may help but it sounds like it isn't worth it.

Dave

[150aintenuff]

it is worth trying... as it doesnt cost anything to rais the motor if you do it yourself.. and I have seen where 1 or 2 holes will help the prop bite better as it places it in the water colum at the designed height it wants to work in... BTW it sounds deep to me and I have a heavier hull with same HP on it and went up 2 holes on mine and it helped so its is worth a try.

[ssv1761982]

Thanks, I will talk him into jacking it up. I have an engine hoist so we should be able to hold it up.

Dave