cavitation plate

[seahorse197220]

First, is the plate above the prop called a cavitation plate or a ventilation plate? Second, how far below the boat should it sit? Thank you.

 

[JB]

Re: cavitation plate

The technically correct name is "anti ventilation plate". It prevents prop ventilation when the outboard is correctly rigged. It has nothing to do with cavitation.

The exact sweet spot in vertical adjustment changes from boat to boat and even with changes in rigging. I have never seen one with the AV plate below the bottom or keel. Most books and gurus say to start with it level with the keel, then ease it up until it starts to ventilate the prop and then back it down a bit. I have always used that procedure. I usually end with it about 1" above the bottom.

 

[seahorse197220]

Re: cavitation plate



This is what I am working with. It is a fiberglass boat. It is a 1972 20 hp Johnson short shaft. The problem I am having is, am I going to have to cut the boat in order to get the plate low enough?

 

[UncleWillie]

Re: cavitation plate

The Mercury Installation instructions recommend mounting the motor so the Anti-Ventilation plate is Zero to one inch (0.0" to 1.0") below the hull.
And, 1 to 4 inches below the hull at speeds from 50 to 80 mph.



If your water intake is on the botton of the Anti-ventilation Plate, having it above the keel could overheat the engine while on plane.

Before you start Cutting it up consider a Jack Plate.

 

[BonairII]

Re: cavitation plate

View attachment 134524

This is what I am working with. It is a fiberglass boat. It is a 1972 20 hp Johnson short shaft. The problem I am having is, am I going to have to cut the boat in order to get the plate low enough?

Can you take a pic of the entire motor/transom?

You could always convert the motor into a long shaft....or sell it and buy a long shaft motor.

 

[Philster]

Re: cavitation plate

The Mercury Installation instructions recommend mounting the motor so the Anti-Ventilation plate is Zero to one inch (0.0" to 1.0") below the hull.
And, 1 to 4 inches below the hull at speeds from 50 to 80 mph.



If your water intake is on the botton of the Anti-ventilation Plate, having it above the keel could overheat the engine while on plane.

Before you start Cutting it up consider a Jack Plate.

Would a jack plate lower it? The setting he is at is the lowest setting for a jackplate... no? He needs a different length shaft... well, the engine does anyway.

Lowering the engine stops proper tilt/trim. Jackplates enable raising the engine from its original spot....and lowering back to its lowest spot (which is where it is at now).

Anyone?

 

[seahorse197220]

Re: cavitation plate

Thanks for the advice. I am working with a 1967 fiberglass jon boat. It looks like the easiest and cheapest thing for me to do is cut it down about 4 inches. Hope it works.

 

[cyclops2]

Re: cavitation plate

Assuming you have a NORMAL lower unit housing.

The boat company was a bunch of jerks to build and sell that transom height. Bunch of real D I Y.

You would need a John ? Erude lower unit that comes with several spacers for goofy transoms like that. My old 9.9 hp had different lengths available. NOT a cheap job to change lengths. Borrow a long shaftmotor see if it fits better with no butchery or weakining the transom. You may have to do a lot of rot repair after you cut it open.

 

[UncleWillie]

Re: cavitation plate

Would a jack plate lower it? The setting he is at is the lowest setting for a jackplate... no? He needs a different length shaft... well, the engine does anyway.

Lowering the engine stops proper tilt/trim. Jackplates enable raising the engine from its original spot....and lowering back to its lowest spot (which is where it is at now).

Ooops!, You're Correct!:redface:
I guess what I really meant to suggest was a SetBack Plate.

What is really needed is a long shaft motor.
Cutting the transom increases the chances of swamping the boat.
A Jon boat doesn't have much freeboard to start with!

 

[kfa4303]

Re: cavitation plate

HI seahorse. I['m with JB on this. I have a '66 20 hp which is virtually identical to your motor and I also run it with the anti-vent plate level, or slightly higher than the keel of the boat for a variety of reasons. For starters, you want as little drag as possible to achieve the highest and most efficient speeds which means you want as little of the leg in the water as possible. Also, while it is true that there is a water inlet just above the plate, it it not self priming and is somewhat irrelevant when at speed and on plane. It is used to help cycle water through the motor at idle, low speed in reverse. Otherwise, water is under pressure forced up towards the waterpump by the propeller forcing water through the screen found on the lower unit (see pic). As a result, you don't want/need the intake found above the plate to be submerged at top speed. You should just barely be able to see the anti-vent plate break the surface of the water at top speed. Also, because most boats are stern heavy, when at rest the lower end will generally be well submerged allowing the inlet above the anti-vent plate to function. Here's a handy schematic that can explain it as well. Base on your pic, you actually look good to go. Your plate is about 1" higher than the bottom of the boat which is just about right. You certainly wouldn't want, or need to make it any lower than level with the keel.

 

[seahorse197220]

Re: cavitation plate

Thanks for the help. Without y'all I feel I could have messed up my boat. Thanks

 

[Outsider]

Re: cavitation plate

kfa4303 is on the money. Any reference to the keel should be considered a starting point! Run the boat at speed and look at the plate, optimum should be just slightly above the water (but getting splashed). If you're there, happy boating. If you're not, then figure out what to do about. Manufacturers don't usually produce designs that require modifications right out of the mold in order to be used with standard outboards, even back then ...

 

[kfa4303]

Re: cavitation plate

While the motor height can certainly make a difference, I think I only picked up a few mph on the top end by raising the motor, but it also seemed to help gas mileage, got me on plane a bit more quickly and seemed to allow the various tilt positions to have more of an effect. I think my anti-vent plate is just about level with the bottom of the boat and I have it tilted one position out from the transom and I get a little over 20 mph on a 14', aluminum v-hull boat. I could probably squeeze a few more out if I really wanted to fiddle with it and get a jack plate, etc..., but I figure anything over 20 mph is pretty good and plenty fast for me. The old racers would use 1/4" shims to raise/lower the motor to pick up microseconds of time, but that's a bit much for my needs. There really isn't any magic formula for getting the best performance. You just have to spend a day on the water trying all the various height, tilt, trim and weight combos until you find one that works best for your needs. It can be a little tedious, but you 'll learn tons about your boat and set it up exactly hos you like it.

 

[bob johnson]

Re: cavitation plate

is the motor cavitating now? the set up you have now is close...its only about 2" above the keel!!! not 5"!!! which is was a different shaft length would give you..

it seems like the boat might run good as is....

as for a jack plate ...most have 2 sets of holes on the transom side of the jack and 2 sets of holes on the motor side. that combination alone would "fix" any issue you have right now... you could mount the motor higher or lower with a manual jack plate..which you could buy for less than $200.

sooo is you boat having problems now like its set up???????????????????????


bob