Outboard Position

[Miroash]

I have a 17 foot center console with a 90hp Johnson outboard motor. I noticed that the Cavitation plate is about 1/4" higher than the bottom of the transom, will this make a difference in any way if it were even with the bottom of the transom? I was going to lower it but then I realized that it is such a small distance that I would onlt be able to make it further above the bottom of the transom or about a 1/2" below it. The only other option I can think of, is to install a jack plate which is about $200. Is it really worth that musc to move the motor 1/4"? I was considering adding a Dole-Fin just to ensure the motor would not cavitate.

 

[miker-gr]

Re: Outboard Position

the cavitation plate supposed to be on the same plane with the bottom of your transom! The only case that cavitation plate can be higher than the bottom is if you have an extension on your transom that moves your engine further backward! You can also check the installation manual of your engine!
Hope i help you

 

[tashasdaddy]

Re: Outboard Position

i disagree with miker-gr, somewhat. it is recommended that the AV Plate be on the Parrallel with the bottom of the hull. and any where between inline with the bottom to as much as 2 inches above the bottom. it is an effiecentcy experiment, as to position. but be sure you are still picking up cooling water.

 

[bustermin]

Re: Outboard Position

I have basically the same set-up, and it works fine for me.

 

[wilde1j]

Re: Outboard Position

You don't set motor height based on measurements, it's done based on performance. You keep moving the motor up until the motor starts breaking free (cavitates) in hard turns, then back down one hole. If the cavitation plate is covered with water at high power, it's too deep and you can clearly move up more. The benefits of getting it right are better top end speed (lower drag), better fuel consumption and easier steering. It's entirely an empirical process, and measurements can only provide a reasonable start point.

The motors on my boat are mounted on a bracket, and at the top holes. If I could come up more, I would, although the cav plate is uncovered at WOT, which is a good thing. Fuel use is down 20 to 25% now, compared to the prior setup (motors were at the deepest setting).

 

[emdsapmgr]

Re: Outboard Position

Tas and Wilde has some good comments. It's a little misleading to compare the antiventilation plate with the keel when the engine is tucked in and sitting on the trailer. Usually when underway and the engine is trimmed out, the flow of water under that plate changes. Next time you are out, check the water flow when on plane and trimmed out. The water should just flow under that plate with an aluminum prop. If you run a high tech stainless, you can run the engine slightly higher on the transom.

 

[kenmyfam]

Re: Outboard Position

I'm with TD on the set up. Then it's trial and error from there.

 

[Miroash]

Re: Outboard Position

i disagree with miker-gr, somewhat. it is recommended that the AV Plate be on the Parrallel with the bottom of the hull. and any where between inline with the bottom to as much as 2 inches above the bottom. it is an effiecentcy experiment, as to position. but be sure you are still picking up cooling water.

Thanks guys, on my boat right in the center there is a little spot that is lower than the rest of the transom, it is a rib like protrusion that starts at the bow and extends to the transom (Keel?). at the transom it protrudes maybe 1/2" below the rest of the transom, I am measuring to this "Rib" (please forgive my lack of marine vocabulary) if I measure to just the bottom of the transom it is about an 1/8" below the bottom of the transom itself. I dont believe it will have a negative effect as far as cooling, but just to be safe I added a "Doel-Fin" to help. Do you think the fin will help ensure it has plenty of water, or make things worse?

 

[junior1113]

Re: Outboard Position

fins shorten transom life

 

[emdsapmgr]

Re: Outboard Position

Make sure that absolutely nothing on the fin is dragging in the water when the boat is on plane. If you have excess spray from it, it's too low and will affect your speed and fuel economy.

 

[countvlad]

Re: Outboard Position

fins shorten transom life

just removed mine few days ago and gained about 300 rpms WOT trimmed up...