Should cavitation plate be lower for 1967 Johnson 80hp? (pin-drive prop)

[BonairII]

I've been reading a bit about proper motor height and I think my '67 Johnson is setup way too low. Currently, the cavitation plate is 2.25" BELOW the hull.
I contacted the PO to ask him if there was a reason the motor was set so low. He told me that didn't set it to that height, but had asked a marine mechanic about it a few years ago and was told to "leave it alone"....which sounds to me like the mechanic wasn't interested in dealing with changing the height.

Sorry for the crappy pic, but it'll give you an idea of what my setup looks like

I'm guessing that the current setup is probably too low and creating quite a bit of drag.

I did read a thread here....where someone stated that "pin-drive" prop motors should be setup with the cav plate below the hull. The reasoning had something to do with the fact that these motors don't have thru-prop exhaust.

Anyone have any insight on the proper height for this old motor?

 

[Home Cookin']

Re: Should cavitation plate be lower for 1967 Johnson 80hp? (pin-drive prop)

I'd say that even if it is a little too low it's not enough to justify the cost and effort to raise/replace, including the risk to an old transom, unless you are racing. Someone presumably with knowledge already considered it and decided it was a good enough set-up. Spend the money on gas.

Can't speak to the shear-pin aspect; lots of old motor talent here, though.

 

[Chris1956]

Re: Should cavitation plate be lower for 1967 Johnson 80hp? (pin-drive prop)

I think that reasoning is crap. The exhaust from your motor is discharged under the antiventilation plate. The A-V plate should run on the surface of the water. I recommend a water test. Run near full throttle and look at the A-V plate. If it is underwater, then the motor is too low. Whether you gain performance by raising it is another question.

Also, the water pickup is right in front of the exhaust discharge. Make sure the water pickup is under water, so no raising the A-V plate out of the water, right?

 

[BonairII]

Re: Should cavitation plate be lower for 1967 Johnson 80hp? (pin-drive prop)

I'd say that even if it is a little too low it's not enough to justify the cost and effort to raise/replace, including the risk to an old transom, unless you are racing. Someone presumably with knowledge already considered it and decided it was a good enough set-up. Spend the money on gas.

Can't speak to the shear-pin aspect; lots of old motor talent here, though.

Other than the cost of some 3M 4200(to reseal the mounting bolts)...I'm not sure what other cost you may be referring to. From what I can see, I'm only going to have to remove the top 2 bolts and loosen the bottom 2 to raise the motor.
The reasoning for messing with the motor height is twofold. First was to increase performance, 2nd was to inspect the bolt holes to make sure they are still sealed properly.

The transom is rock rock solid from what I can see/tell.

I'll give it a water test before I change the height, though. If it doesn't need to be moved up, then I'll just seal around the edges of the transom bracket to make doubly sure that their is no chance of water getting into the transom bolt holes.

I'll wait to hear from a few others on the subject also.

 

[Home Cookin']

Re: Should cavitation plate be lower for 1967 Johnson 80hp? (pin-drive prop)

If there's already room on the transom, fine; give it a shot. I assumed there was more to it. My point was, think about the advice you will get to buy a shaft shortening kit, a jack plate, a transom rebuild, or other efforts and expenses over an inch.

 

[BonairII]

Re: Should cavitation plate be lower for 1967 Johnson 80hp? (pin-drive prop)

If there's already room on the transom, fine; give it a shot. I assumed there was more to it. My point was, think about the advice you will get to buy a shaft shortening kit, a jack plate, a transom rebuild, or other efforts and expenses over an inch.

This is what my transom bracket looks like.

If I end up having to raise the motor....can I just put a piece of wood on top of the transom to fill in the gap?

 

[Chris1956]

Re: Should cavitation plate be lower for 1967 Johnson 80hp? (pin-drive prop)

Sure, you can put a 1X2 or something under the motor to raise it up a bit. Don't raise it above where the clamps or bolts can be safely used to secure the motor.

 

[Chas0218]

Re: Should cavitation plate be lower for 1967 Johnson 80hp? (pin-drive prop)

I wouldn't raise it unless you don't have to drill new holes. The least amount of holes in a transom the better.

 

[BonairII]

Re: Should cavitation plate be lower for 1967 Johnson 80hp? (pin-drive prop)

I def wouldn't have to drill new holes. The bracket has 4 more bolt holes for the top bolt....and has a long channel for the bottom bolt.

 

[Home Cookin']

Re: Should cavitation plate be lower for 1967 Johnson 80hp? (pin-drive prop)

you don't have to fill the gap. Lots of motors, if bolted on, sit off the transom top, despite the shape of the "hook over" part. If it makes you feel better, cut a piece of lumber and put it in there, and replace it if it ages. I wouldn't do anything permanent.

 

[F_R]

Re: Should cavitation plate be lower for 1967 Johnson 80hp? (pin-drive prop)

Old OMC motors were designed to run the plate below the surface. That is a left-over from the wood-boat days engineering. However, 2 1/2" probably is a tad lower than necessary.

Now, c'mon guys, that motor has adjustable stern brackets. All you have to do is remove the upper ones and loosen the lower ones, raise it up to the next set of holes and re-bolt it. The idea is to raise it as much as possible without getting prop ventilation or cooling water problems. If you do experience those, drop it back down a set of holes. You DO NOT have to shim the top. It may look funky, but that's ok. Those hooks on the brackets are for temporary hanging the motor on a display stand or storage rack.

Once you have test run it and are satisfied, remove and seal the bolts, one at a time.

 

[Texasmark]

Re: Should cavitation plate be lower for 1967 Johnson 80hp? (pin-drive prop)

Old OMC motors were designed to run the plate below the surface. That is a left-over from the wood-boat days engineering. However, 2 1/2" probably is a tad lower than necessary.

Now, c'mon guys, that motor has adjustable stern brackets. All you have to do is remove the upper ones and loosen the lower ones, raise it up to the next set of holes and re-bolt it. The idea is to raise it as much as possible without getting prop ventilation or cooling water problems. If you do experience those, drop it back down a set of holes. You DO NOT have to shim the top. It may look funky, but that's ok. Those hooks on the brackets are for temporary hanging the motor on a display stand or storage rack.

Once you have test run it and are satisfied, remove and seal the bolts, one at a time.

Ditto. Just go to the "Old OMC" site maintained by a guy in Germany, and look at those sales brochures that he posts. Look at what F_R said about where the plate is. Heard several reasons. The best one was here today, in answer to your thread and that was to not only keep the exhaust under water to keep it quiet, but also to ensure you have water available at your inlet.

I too feel that 2 1/2 " below the transom is too low.

On where your engine sits on the transom, I have a 300+ lb outboard that doesn't sit on the transom at all. It is held in place with the 4 bolts. Boat was built in '01 and I have had it since Dec, '04, and I play hard with it. No problems with engine mounting.

HTH,

Mark

 

[BonairII]

Re: Should cavitation plate be lower for 1967 Johnson 80hp? (pin-drive prop)

Update:

Splashed the boat today and took her for a short cruise.

The cav plate is definitely too low. I took a look at the cav plate while we were WOT and it's def plowing water. It's not causing a huge amount of spray or anything, but def plowing thru the water.

Gonna raise the motor 2 holes and see what happens. I'll keep an eye on the tell-tale during WOT to make sure the motor's getting plenty of water.