Theory Question - AV Plate

[Palssonater]

I believe this is the best forum for this question as it has to do with the lower end of the motor.

First off, I have a 1984 Evinrude 35HP 20" shaft motor. I am running a 10X15 prop on it right now and am convinced that is too much pitch. I will be trying out a 10x13 this weekend and see how the boat likes it.

I have a 14' Edson fishing boat. Its a shallow transom @ 15". Here lies my question. Am I damaging anything by running a 20" shaft on a 15" transom. I am trimmed to the second lowest hole and have noticed the boat usually runs about 20mph. I believe this to be too slow, but am wondering if the fact that AV plate is below the transom/keel if I am suffering top end because of that. Or if I am doing irreversible damage to the motor by doing this. The motor does run cool and does not seem to labor besides not being able to push us faster through the water.

I looked through the FAQ section and could not find theory pertaining to the AV plate, etc.

Could somone please enlighten me on some theory and my issue? This is the way I bought the entire unit, so it wasn't my decision to match this motor to this boat.

 

[WillyBWright]

Re: Theory Question - AV Plate

Dragging that extra 5" through the water is definately slowing you down and loading the powerhead more than it would otherwise. It is also having a more difficult time breathing with extra effort needed to push the exhaust out. Is it harming the motor? Not a ton, but yes. It's also making it that much more likely to hit bottom. You might consider getting it up out of the hole with one of these. It's about the same price as a prop and it might make the prop you have right for the job.

 

[steelespike]

Re: Theory Question - AV Plate

Even with the right setup an inch increase in height could add about 3 mph.
Imagine what 5 inches might do.

 

[BF]

Re: Theory Question - AV Plate

For comparision, I have a 16' edson aluminum fishing boat (16" transom, tiller steer, no floor) and a 1970 25 hp evinrude short shaft. It spins a 9x9 prop and I do just under 25 mph with 2 people, 10 gallons of gas, and not too much gear. Adding more weight/people in the boat does slow me down a bit, but I think your rig should be outperforming mine by far... so I think you're losing a lot of performance by running the wrong length motor.

 

[Esox]

Re: Theory Question - AV Plate

You should check what your WOT rpm's are with a tach. If you aren't in the rpm range of the motor - you could damage the powerhead, especially if there is more of a load than normal. If your rpm's are too low I would suggest taking Willy's advice and look into one of those brackets to get the motor up higher. Then look into a new prop if needed.​

 

[Palssonater]

Re: Theory Question - AV Plate

Switched to a 10x13 prop and it made a huge difference. I am up to 26mph and very quick to get on step.

Without ordering a Tiny Tach can't really check the engine RPM's, but by ear I did gain a bit. I imagine it is running cleaner now. Plugs are still perfect color.

I think for the age of the motor and boat combo, I'll run it for now. I may buy a used 25horse 4stroke next year.