Is my motor too high?

[jdeloney]

1984 Sunfire V-15FS, 15.25 feet long and weighs 725 (pounds dry). 1984 Merc inline 6 with a hydrofoil. Just bought the boat and it wants to porpoise at lower speeds. That's probably why it has the foil. I have to give it negative tilt at low speeds (5 to 10) to help it run level. Above 15 or so it planes normally.
I don't care about top speed at all (it goes about 45 now.) It looks like the motor was originally 2 bolts lower. I have included photos with the boat and motor fairly level. I did run it w/o the foil and that was way worse. I have no easy way to lower the motor unless there are tricks that I don't know about. Let me know what to try first.

 

[Art Bernard]

Re: Is my motor too high?

Looks fine to me, differant hulls handle differently so your going to have to get used to this one's handling. You might try redistributing the weight when you load her up, put more weight up front and play with the til/trim. Boats are like aircraft in that weight distribution really has an effect on them. If all your weight is middle of boat to stern it can cause the bow to want to rise.

Art

 

[BaileysBoat]

Re: Is my motor too high?

As long as trimming in controls porpoising you are fine. If this happens off plane it's not an issue. Most outboards are rigged with the engine too low, and the best height has to be determined by water testing. Engines mounted too high cause problems blow out/cavitation/ventilation at higher speed. You are lucky having the jackplate to experiment with.

 

[Expidia]

Re: Is my motor too high?

I'd start by getting that large cavitation plate (the big one above the prop) to within 1 inch above or below the bottom line of the boat (you can just hold a straight edge under the boat.

Like others have said it's tough to know what effect it's going to have becasue of various hull shapes.

A general rule of thumb can sometimes be no more than an inch above. Yours looks pretty high to me (but I'm no hull expert). Nice to have a jack plate to experiment with. I used a small hydraulic jack to move mine around with a 2 x 4 on top of it under the skeg to move it up and down myself. A scissors jack or bottle jack works well two. Just make sure you tie the head snugly to the rig. I used the two loops on top of the power head and some chain with those adjustable links that hold the rode chain to my anchors.

You will know when it's too high. My first try I went up two bolt holes and I was hearing the prop catching air on sharp turns, so I dropped it a hole and found my rigs sweet spot.

Mine was mounted 2 inches too low from the dealer and I was getting a lot of back spray hitting the transom off the lower unit. One would think a dealer would test run a rig after they mount these expensive outboards. Mine didn't.

A little tweaking up and down and you can easily find the best trade off for handling and speed. It just looks from my "untrained" eye that the jack plate was installed to get better top end speed.

I did the same for my Daughter's rig last year. Went to a metal shop and for $20 they made me up a custom aluminum block 12 inches long and 1 1/2 inches higher than how the factory mounted outboard sat. Amazing how much better it handles now with just the stock prop. I wish I had your jack plate though it would have made the job real simple.

There are many well seasoned members on this iboats forum that can give you much better insight on your hull design as to motor height. I'm just passing along my own personal experience and how I tweaked mine for better handling.