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Prop Cavitating 1990 30hp

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SomeRandomDude

New member
I have a Johnson 1990 30 HP motor on a peterbourough jetstar boat. I will describe the problem best i can and i do have pictures of when it does or does not cavitate.

Normal rooster tail
DSCF0462.JPG
When the rooster tail appears like this, kinda flat, I can go any speed without cavitating.

Weird spray coming from motor
DSCF0463.JPG
If I hit a wave or corner tightly, the motor begins to have a weird spray coming up and out from it as you can kind of see from this picture. If I go full speed while this is happening, the motor will cavitate and shoot up to 5000 RPM. The only way to stop the spray or the cavition is to slow right down and out of plane and speed up again.

Metal plate
DSCF0461.JPG
My motor is mounted on a metal plate to help the transom. However, this plate needed to be mounted about 1 or so inches higher than the transom so it would not cover the hole that allows water to flow out of the well.

Putting the motor down one notch down does keep this from happening but the angle is puts the motor at is not good. There is much drag from the motor and it works too hard for the speed it goes. It goes 5 mph faster with this level which by the way is the middle hole, 3 from the top and bottom of the 5 trim holes. Lowering the plate and motor can be done but it would be hard because the plate and motor are bolted right through the transom. It would also be hard to modify the plate to allow water out of the well.

So my question is, is there anything I can do to stop this from happening other than finding a way to lower the plate? Would lowering it even make a difference?

Any suggestion would be greatly appreciated.
 
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Re: Prop Cavitating

Lot's of things can cause that problem. The most typical problem is that the motor is mounted too high for the boat.
Since your motor is mounted directly on the transom, the anti-cavitation plate should be exactly even with the bottom-most point on your hull at the transom. When you are running at WOT, the anti-cavitation plate should be riding just at, or slightly under the surface of the water coming off the hull.

You may be able to improve your turning performance with a hydrofoil, which you can bolt on directly to the anti-cavitation plate of your engine. They typically cost between $50.00 and $90.00 depending on size and type. Many of the new ones don't even require that you drill holes in the cavitation plate. But first, make sure your motor is set up correctly on your boat.
 
Re: Prop Cavitating

The plate raised the engine and violated the rule described well by CHAWK. The best fix would be to correctly relocate the engine on the transom.

don't really understand why the plate needed to be elevated...I'd lower it and just cut it to clear the drain. Even if it is stainless, it cuts easily if you go slow.

Tilting it may help but isn't the preferred fix, for the various reasons you have found.
 
Re: Prop Cavitating

Do I just fiberglass the holes of the bolts? Do you suggest for the bolts of the plate i just make new holes in the plate and use the existing holes through the transom? Does this inch or less of elevation really make a difference?
 
Re: Prop Cavitating

I fully understand your reluctance to put more holes in your transom. I'm of the same school - the fewer holes in the transom, the longer it will last.

How far is your anti-cavitation plate above the lowest point on your hull at the transom? Park the boat on level ground. Trim the engine to vertical. Use a level from the lowest point on your hull to lower unit. Then measure the vertical distance from the level to your anti-cavitation plate. That will tell you how much you need to lower your engine.

If your anti-cavitation plate is one inch or less above the bottom point of your hull, then the hydrofoil MAY help you a lot (no guarantees.) It it is significantly more than an inch, then you need to lower your engine. There are several ways to accomplish that. You can change your current mounting plate. If you don't mind spending the bucks, you could install a hydraulic jack plate. That will probably solve all your issues because it allows you to vary the height of your motor based on how you are operating while you are underway. You can also install a manual jackplate, which allows you to change engine height manually, before you launch, or while adrift.

If you need to lower your engine, then read up on jack plates, how they operate, and how they can improve performance.
 
Re: Prop Cavitating

I only really need to put 2 new fairly small holes that bolt on the engine. For the plate bolt holes i can just put new holes in the plate and use the current transom ones. Would these new holes weaken the transom significantly? Do I even need these bolts for the engine or can i just use the clamps? I don't know why the bolts are there, they've been like that forever, i didn't put them on.
 
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Re: Prop Cavitating

I'd bet you have another set of holes in the transom, covered by the plate, assuming the same engine was installed before the plates were added. a couple more holes won't make any significant difference in the transom's strength.

Whatever you do, make sure any unused holes are properly filled and sealed. Same for sealing the holes with the thru bolts. Failure to seal them properly causes a lot of transom failures down the road.

The thru bolts are a 'safety' feature in case the clamps loosen up. Personally, with that size engine, I'd put them back in.
 
Re: Prop Cavitating

Agree with Makomark. Did you ever measure the distance from the anti-cavitation plate to the bottom of the hull?
 
Re: Prop Cavitating

That would be more difficult, but it can be done if you can put the boat in water where you can stand up and run a straight edge off the bottom of the hull to the anti-cavitation plate.
 
Hello SomeRandomDude,

here is a inexpensive alliterative Set Back Plate this plate will allow you to move you motor back using existing holes and by proxy of moving it back it will slightly lower the motor due to the transom angle and allow te motor to run slightly higher due to the greater distance from the transom. In affect you are adding a jack plate.
 
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