Anti Cavitation plates.... do they work?

[akwalker]

I picked up a 78 Bonito 16' bowrider with a '60 Johnson 40. The engine runs great, but cavitates like all get out. Do those bolt on (Dolphin, etc) anti-cavitation plates work? Its like anything above 1/2 throttle spins the prop,,,,,, any ideas/suggestions? Thanks in advance.

 

[AlanR]

Re: Anti Cavitation plates.... do they work?

Sounds like your motor is to high out of the water... Is it a longshaft boat with a short shaft motor because that will cause it to cavitate. Also the dolphins are more for making the boat plane faster. They work great for making the boat plane faster and REDUCING cavitation

 

[akwalker]

Re: Anti Cavitation plates.... do they work?

Sounds like your motor is to high out of the water... Is it a longshaft boat with a short shaft motor because that will cause it to cavitate. Also the dolphins are more for making the boat plane faster. They work great for making the boat plane faster and REDUCING cavitation

Thats what Im afraid of. The existing cavitation plate on the engine is about 3 inches higher that the lowest point of the keel, if that helps. I'm afraid that if I add a longshaft kit, the cav plate will be too far below the keel. Should the cav plate be even with the keel?

 

[AlanR]

Re: Anti Cavitation plates.... do they work?

To get the best preformance out of the motor / boat combination the STOCK cavitation plate should be level with the lowest point of your boat. (unless its a sail boat Lol)

 

[AlanR]

Re: Anti Cavitation plates.... do they work?

Well what i would do is Get the long shaft conversion (as long is its not electric shift) and then see how much i have to come up so the cavitation plate is level with the bottom of the boat and then cut a peice of wood to go the full length of the top of the transom so the wood would bring the motor up (x) amount of inches then i would just drill some holes and bolt the motor back down. Ive done this a couple of times when i had a short shaft boat and a long shaft motor

 

[AlanR]

Re: Anti Cavitation plates.... do they work?

O also is your motor an electric shift because thats a real mess to change to a long shaft,

 

[akwalker]

Re: Anti Cavitation plates.... do they work?

To get the best preformance out of the motor / boat combination the STOCK cavitation plate should be level with the lowest point of your boat. (unless its a sail boat Lol)

Any idea how to achieve that, if I've got a "tweener"? 15" is too short, and 20" is too long

 

[akwalker]

Re: Anti Cavitation plates.... do they work?

No, its manual shift. I just measured from the cav plate on the motor to the lowest point on the keel, and the motor is only 2" higher than the bottom of the boat. This seems negligable to me, but I could be wrong.

 

[AlanR]

Re: Anti Cavitation plates.... do they work?

Before going to the extent of my idea id purcash the $40 Dolphins and try it.

 

[akwalker]

Re: Anti Cavitation plates.... do they work?

Okay, Alan, I think I'll give the bolt on anti-cav plate a try first. Even if I have to go long shaft, it'll still be benificial to have the ant-cav plate, I guess,,,,,,, thanks for the advice.

 

[nevd]

Re: Anti Cavitation plates.... do they work?

I picked up a 78 Bonito 16' bowrider with a '60 Johnson 40. The engine runs great, but cavitates like all get out. Do those bolt on (Dolphin, etc) anti-cavitation plates work? Its like anything above 1/2 throttle spins the prop,,,,,, any ideas/suggestions? Thanks in advance.

This old motor may also have a damaged prop which would have no cupping. AV plate 2" up was very high for the technology used in production props in those days. By todays standards 2" is not very high and with a good cupped prop (stainless if you can get it), this should be workable without the cost of shaft extensions or the problems foils can create.

That motor was also prone to spinning the rubber hub in the prop, so please check it by marking bush and prop and checking the marks are still in line following a run.

 

[akwalker]

Re: Anti Cavitation plates.... do they work?

Thanks, Nevd, I'll check the hub. I didn't think a pin drive prop had a rubber hub. I'm thinking of maybe cutting a 2" notch in the transom and lowering the motor that way.

 

[tschamp20]

Re: Anti Cavitation plates.... do they work?

have you tried lowering the motor by changing the position of the metal rod that the motor locks onto when lowered for use.there should be a couple of adjustment points.good luck

 

[akwalker]

Re: Anti Cavitation plates.... do they work?

have you tried lowering the motor by changing the position of the metal rod that the motor locks onto when lowered for use.there should be a couple of adjustment points.good luck

Yes sir, I have,,,,,, at the lowest trim setting it's still 2 inches above the keel. I think I'm going to notch the transom as the longshaft conversion is almost 400.00

 

[Paul Moir]

Re: Anti Cavitation plates.... do they work?

Your 40hp measures 18" from transom clamps to anti-ventilation plate for a good reason. The water pump must be submerged to prime - so it's designed to fit a 15" transom. This puts the ventilation plate 3" below the hull unlike more modern outboards where it can be even.

You can experiment with higher settings but make sure it reliably primes when started, keeps water flow in full speed reverse and full throttle forward. Reverse water is sucked through the plate on the side of the lower unit just above the AV plate so that must be submerged during reverse.

Dolphin type plates are worse than useless on these outboards since the A/V plate must be below the centre of the hull. You want those fins to exit the water completely once the boat is on plane or they'll drag severly and cause weird handling problems.

Back in the day, racers looking for performance would normally completely abandon this lower unit for a custom one.


Contact TCoutboards.com or other online junk yards for conversion parts. They are very common. You need a driveshaft, shift rod, 5" water pump extension, 5" spacer section and the bolts that hold it on.

 

[akwalker]

Re: Anti Cavitation plates.... do they work?

Paul,
Thanks for the concise response, as always. The engine pumps water quite well in all conditions, it just cavitates or ventilates to the extreme. Do you think I'm going to gain anything by setting the vent plate level with the keel, or am I going to have to lower it 3 inches BELOW the keel as your post suggests? What do you think?

 

[akwalker]

Re: Anti Cavitation plates.... do they work?

Also, Paul, I tried tcoutboards, and can't find the site. Do you have a link? Thanks

 

[Paul Moir]

Re: Anti Cavitation plates.... do they work?

As long as you can get the pump to prime and operate reliably, only lower the engine as much as you need to to get it to stop ventilating. The higher up, the better the efficiency since there's less drag. And that big gearcase presents a lot of drag.

Sorry about that, it's tcoutboard.com I cannot provide a direct link as that is against the Iboats message forums rules about links to commercial sites.

 

[akwalker]

Re: Anti Cavitation plates.... do they work?

Thank you, Paul. I am going to lower the engine the 2 inches. If that doesn't work, then I guess I'll raise it back up ans convert it to a longshaft.

 

[akwalker]

Re: Anti Cavitation plates.... do they work?

Here's the latest. Thanks again to everyone who responded. I wound up cutting a 2 1/2"deep notch out of the transom and lowered the motor by taht much. This put the AV plate about1/2 inch below the keel. WOW, what a differance!! Boat planes off immediatly, and will do about 32 MPH wide open with just me on board. Not too bad for a 46 year old engine on a 1000 lb boat. Thanks again to all for the help.

Andy