Motor cavitation. Prop or Boat?????????

Tom H.

Tom H.

Member
OK, here is the problem. I built this boat this spring.


I bought a new 15hp Suzuki short shaft for it. I have been having a problem getting it to hook up. The prop cavitates like crazy. I have tried 3 different props for it. The factory aluminum, 4 blade alum. Solas, and now a SS 3blade Solas. The factory aluminum has been the best at getting it to hook up. What did help a lot was I put on one of the dolphin tails. Well today I was wiping the boat and motor down and noticed this on the side of the lower unit.






There are two spots on the left side of the lower unit. Could the cavitation be causing this? I know it erodes the edges of the prop. I don't know where this is coming from. Nothing rubs up against it. I don't use one of the transom savers. I think it is something with the bottom of the boat. When I built it. When I put on the keelsons, I stopped them about a foot in fron of the transom. Could this be causing all off my problems?

Here is a pic of the bottom from the rear.


I am thinking of putting a jack plate on to help get in cleaner water. Thinking setting it back to let the water calm down a little?????????
 
Shaft is not bent. 3 different props. Same thing. The 4 blade was the worst. Would not even get on plane. The boat is front end heavy. It barely even rises when taking off.
 
Have u done a tank test on it while sitting on the trailer?

If it still cav's then there's an internal problem with the lower unit.
 
Tank test??

I think air is getting trapped under the boat. Maybe I'm wrong. I don't know.

When the motor finally hooks up and bites, you can feel it tug the handle and the rpms go down. And that is not until it is up to full speed and the boat is fully on plane. Thats why I think air is getting sucked under and is forming a pocket at the motor. How does that sound???
 
Take a 1/2 of a (55) fifty five gallon drum of a garbage can, fill it with water & run the motor.

If no cav, then it's a hull problem.

If cav, then a motor problem.

It's all about elimination.
 
It must be the boat then. I use the motor to power it onto the trailer and it does not do it.

So does anyone think that extending the keelsons to the edge of the transom will work? Or should I try the jack plate first?
 
Where is the motor's cavitation plate in relation to the bottom of the boat? Above, below, even? If it is above that's your culprit.
 
You may want to hold off on the jack plate and just shim the motor up on the transom. In the past I have set the outboard on a 1 x 2 or similar to get the cavitation plate flush with or slightly above the hull bottom. The bottom of the boat in the photo looks good but the cavitation issue suggests your are not getting undistrubed water to the prop. You may want to move your keelsons farther towards the side and possibly extend them back to insure undisturbed water flow to the prop. Good luck
 
IF it is a short shaft motor, your transom height shouldn't be taller than 16"...15" is the norm. 15 hp isn't a lot of motor...especially if it is a 4 stroke. If you have a bow heavy boat, it could very well be creating lots of turbulence under the hull and causing the motor to cavitate. Those 2 marks look like basic rub marks...nothing to worry about I expect. What is the weight of the hull and is it a 4 stroke? With all the boxes in it, it looks like it could be a HP issue to me.
 
The anti cavitation plate is 1.5" below the bottom. I have tried to play with the motor by lifting it up and with different trim angles. Yes it is a short shaft, but the lower is 17.5" Most motors are not what they are supposed to be. And yes it is a 4-stroke. I am going to build my jack plate next week hopefully. That will be the easiest and cheapest way first. The transom is made to be 16"

The marks are not rub marks. Thats what I cant understand. There is nothing back there that could rub on it. I don't think it is a HP issue. I am getting 22 mph out of it right now. It is not lugging the motor. It hits the rev limiter every once in a while so it is at the top of the rpm range.

If the jack plate doesn't fix it, I will then flip it over and work on extending the keelsons.
 
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The anti cavitation plate is 1.5" below the bottom. I have tried to play with the motor by lifting it up and with different trim angles. Yes it is a short shaft, but the lower is 17.5" Most motors are not what they are supposed to be. And yes it is a 4-stroke. I am going to build my jack plate next week hopefully. That will be the easiest and cheapest way first. The transom is made to be 16"

The marks are not rub marks. Thats what I cant understand. There is nothing back there that could rub on it. I don't think it is a HP issue. I am getting 22 mph out of it right now. It is not lugging the motor. It hits the rev limiter every once in a while so it is at the top of the rpm range.

If the jack plate doesn't fix it, I will then flip it over and work on extending the keelsons.
Click to expand...


IMO, a jack plate is not going to do a thing. I'd get a tach so you know what is happening with your RPMs.

Are the strakes on the bottom parallel, I'd assume so? Is the bottom of the hull flat from the stern forward, no rocker, no hook? A tiny bit of rocker or hook can do a lot.
 
The bottom is completely flat. There is no hook or dip. The keelson are straight. There is 8 feet of running surface before it bends up and tapers in.
 
Here are some more pics.
This is how it is mounted to the transom. The transom is 2 pieces of 3/4" then another piece of 3/4" to protect the hull from the motor digging in.



Here is a pic that shows how long the transom is. The tape is just hanging from the top of the transom.



Here is a pic that shows how far under the plate is.



I have tried raising the motor from 1/4" up to 1 1/4"

3/4" helps a little. Until you put water in the livewell. Then it barley planes.

I don't have a tac. But I do have a GPS. Usually at 16-17mph the prop finally bites and the rpms go down and you can feel the prop bite in the handle.

I am also getting a lot of water coming up the transom and being deflected by the next higher plate.
 
Tom, I think your motor is too deep in the water. Try taking off that wing thing and shim up your motor until the cavitation plate is just even with , or a little bit above the bottom of your boat. You are dragging too much through the water. Rich
 
Tried that today, made it worse. There is definitely air getting trapped.

Hopefully Monday I will be able to get the jack plate built and installed on the boat.
 
A very interesting puzzle, keep us all informed on your progress. I am sure glad to see you took that stupid fin off. Sometimes they will help get a fat a__ boat up out of the water with too small a motor BUT they really change the steering (IMHO). Making a boat work right sure can drive a guy to drink! It seems like you are getting some thoughtful advice, I don't have anything to add right now. Good Luck.
 
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