Motor too low

[Barracuda Bill]

What happens if the anti-ventilation plate is too low? I have a 1979 Spirit 20 hp on a 1982 Lund 16' aluminum fishing boat, the AV plate is 1.5" below the bottom of the boat.

 

[Mac525]

Re: Motor too low

The preassure pushing up on the plate can cause the boat to porpise, poor top end, high fuel burn, hard to trim, and very twitchy steering. Oh and we had one boat that the 50hp was 1 1/4 below when I bought it and it would lay over hard to one side or the other at speed, scared the heck out of the guy I bought it from. He sold it to me cheap so i bought it fixed it and sold it back to him a week later for what I paided for it he couldnt believe it was the same boat.

 

[Barracuda Bill]

Re: Motor too low

The service manual for my motor says to "Locate the cavitation plate 1.2 to 2.0 in. below the bottom of the boat."
Has there been a change of thought over the years as to the positioning?

 

[Mac525]

Re: Motor too low

The flater bottom of your tin boat will be more forgiving but I would start at 1/4 inch above and see how you like it when adjusting above flush check for the tattle tail to make wure its pumping water at speed

 

[steelespike]

Re: Motor too low

I usually suggest raising the motor to a point just before venting becomes a problem.
You can just lay a stick on top of the transom to hold up the motor while you tighten the clamps.
Sounds like you could go the max just being sure the clamps have a good grip.
Motors with through hub exhaust can be raised higher than the old style non through hub.
Just be sure it pumps water and has a good grip.The less motor in the water the better the performance.Better hole shot,easier handling,less likely to porpois,and higher top speed.
These improvements will vary from obvious to very slight.
Generally the antivent plate(just above the prop)should be visable on plane.
Usually if you experience no venting(cavitation) at all your motor is too low.

 

[Barracuda Bill]

Re: Motor too low

The most I could raise the motor is 1/2 inch, since that is the distance until the top of the clamp feet are at the top of the transom. The water inlet for this motor is above the antivent plate, so I can't raise the motor too much anyway.

 

[steelespike]

Re: Motor too low

The Spirit is by Suzuki and I think the 20 is the old style leg non through hub exhaust.
I think there is a water intake on the front of the exhaust snout as well as above the plate.
Above the plate is to assist flow in reverse. Could be wrong but probably could block the upper intake. But the old style does limit raising the motor so probably not worth the effort.
The non through hub Evinrude/Johnson motors are very similar.
My 5hp Gale (an E/J relative) measures a full 17" to the antivent plate.
Your not likely to have any height problems so just leave well enough alone.