Another cavitation question

Some of you might have seen the pics of my Grumman SportBoat. Its esentially a fat canoe with a transom.

Mfgr. rating for the motor is 7 HP. My current motor is a 5 HP 2-stroke Merc.

Due to the constant change in the way the boat sits in the water, I can't really nail down an optimal angle to set the motor at. Not a big issue as that comes with having a canoe with an outboard. My problem is, I can't get the full potential out of the 5HP Merc. no matter what angle I set the motor at or where I place the passenger, I get some cavitation (especially when turning) before I hit top end. I still got some throttle left and I start getting some cavitation.

I'm not looking to get up on plane. Its a canoe. I just don't want to screw up the Merc, and I don't like having to feather the throttle when I get into some chop or wake of another boat.

My questions are:

1, With weight in the front and the boat sitting rather level, should I set the angle more verticle (lower adjustment - Pic 1) or a set it at a little more angle (higher setting - Pic 2)?

2, Does the cavitation plate look like its in the correct location? Should I raise or lower the Merc? (currently mounted as low as possible. Lowering it more will require a long shaft or modification of the transom).

3, Should I shut up, accept the current performance, and be happy that I'm on the water and not fishing frome shore? :mrgreen:





For the record, with me at the rear and one adult up front, fishing gear, and maybe 4 gallons of gas it tops out at 11 MPH on the GPS with no wind and calm water.

Thanks for lookin'

Chris

The cavitation plate needs to be 1 to 1.5 inches lower than the bottom of the transom.In the pics it appears that the motor is to high and needs to be lowered a little.Those Mercs have a reputation of being a little shorter than most others.There was an episode of Ship Shape TV that covered it.

I agree with Ben.
Looks like you can cut the mounting board on the boat even with the top of the transom, that should fix your problem.

Thanks for the help guys. As it stands, I'm stuck with the current location. The screws are already at the bottom of the mounting board and resting on the transom.

Chris

Can you not place the screws on the transom or are you afraid that they will mar the surface?

Quackrstackr said:

Can you not place the screws on the transom or are you afraid that they will mar the surface?

Click to expand...

To be honest, I'm not sure about mounting it to the transom. I've never really looked at it.

There is a lip/rail (not sure what its called) across the top of the transom. If the mount fits over it I could. My concern is not with marring the surface. Its more with the strength of the transom without the wood mounting board in place.

The transom is a single wall with a triangular brace at the bottom center and the rail at the top. My biggest concern would be the amount of torque applied to the two screws on the single wall aluminum. With the wood mount, the weight, torque and any stress is applied over a wider surface area. I know a 5 HP isn't a torque monster but I am dealing with a canoe with a transom so I want to be careful.

I might be able to add some wood to the inside of the transom to help stiffen everything up.

I can't get pics 'til this weekend. I'll get some so I can better explain how the boat is set up.

Thanks again for your time.

Chris

I would leave the backing board and just trim it down to the top of the transom but maybe you can't do that. I will wait to see the pics.