Help with Cavitation

[Jim Hawkins]

I had problems with my boat that was classic "motor too low" There was spray coming up and over the transom. Measuring I found my ventilation plate was about 2 1/4 inches below the keel. So, I redid the transom and raised the motor. The ventilation plate is still sometimes about 1/4" below the keel (depending on the motor tilt) but the boat ran wonderful on flat water, left a wake that was, I thought perfect and added 3 MPH to my speed and no splashing over the transom.

However,

Giving it too much throttle too quickly now the motor revs due to cavitation. I know many of you will say just add a hydrofoil and I may but that's not the only problem. The big problem is weeds keep wrapping around the bottom end in front of the prop (Not In the Prop) and then the motor revs and cavitates due to the disturbed water. Again, the weeds are not in the prop but wrapped around the front of the lower unit.. Before I lowered the motor this would also happen but it did not cause the motor to rev it only made for more splashing over the transom and slower speed.

I'm tired of having to stop and tilt the motor up to free it from weed every mile or two. What can a boater do?

The boat is a 16' trihull and the motor a 30 HP Johnson.

Thanks

 

[ondarvr]

Cavitation is something different, you're experiencing ventilation.

Can you lower the motor one hole?

Different props can help, but they may not make any for that motor.

Don't motor through weeds. Weeds can plug the water inlet and destroy the motor.

A foil will help with ventilation, but may collect more weeds.

 

[Jim Hawkins]

Oh my dog, hate to sound stupid by I always thought Cavitation and Ventilation were about the same thing. I just educated myself by reading an article that explains both so I'm up to speed now.

I'm not motoring through beds of weeds. Really it only needs to collect about a half dozen single blades of grass about 10 to 12 inches long to make trouble. Here's a link to the article I just read, it gave me some things to check.

http://www.louisianasportsman.com/details.php?id=1795

 

[steelespike]

I have an old Johnson TD owners manual from the 1940s where they actually refer to ventilation as cavitation.

 

[Sea Rider]

Best transom/OB height for any OB is when water flow at speed passes right under small upper water deflector plate, forget all about what's indicated on manuals. It's a matter of dialiing proper lower leg to water flow at speed once boat rises from water out of the hole. Post a pic of lower leg (anticav plate area) shot sideways to hava e look.

You have confirmed that when sitting OB too low will experience over transom water splashes and when doing so too high will experience aereation issues, worst at tight close turns at speed or under choppy water cond.

Happy Boating

 

[Jim Hawkins]

Hello Sea Rider, you've helped me before and you've helped me again.

Yesterday, while out and about and while running on plane I leaned back over the transom to see what I could see and saw the water passing from under the transom in a very smooth sheet but about 1 to 1 1/2 inches below the ventilation plate. Now I know what some of you are thinking but I checked and double checked with a straight edge while the boat was on the trailer and the Ventilation plate was only 1/4 inch above the straight line coming from the keel. What I believe now is measuring only gets you close and you may only find the true water flow by testing. In my case I seem to have a slight hook at the rear of the hull, possibly from the boat on the trailer without the transom supported or possibly built in but either way I have to deal with it. I guess it works similar to trim tabs. by the way, here's the boat. Continued below picture.

So, with sawzall in hand, I will cut back through all my nice new fiberglass/gelcoat work and lower the transom an inch. That will put it halfway between where it was and where I raised it to. This time, I will not glass it back in until I can try it out to see if I hit the sweet spot.

Also, I looked at the prop and it was not what you would call nice. Nothing major but a small chip out here, a small sharp burr there and lots of pitting from cavitation. So with file in hand I got down in the canal and smoothed the rough spots. I didn't try to sharpen it but just tried to give it a nice even curve disturbing it as little as possible, more rounded on the leading edge, sharper on the trailing edge. Then I wiped it down with acetone and gave it a few coats of paint. I'm sure the paint won't last long on the blade it was more to fill the small pits. I'll give it another shot. Thanks

 

[jimmbo]

I have an old Johnson TD owners manual from the 1940s where they actually refer to ventilation as cavitation.

Either a Typo, or the writer just didn't know back then

 

[Jim Hawkins]

What a difference an inch makes.

Today I took friends out to Looe Key for snorkeling and had a chance to test drive the boat with the motor lowered again. As I said in my earlier post I had moved the motor up 2" and it was too high so I cut the transom down and lowered it 1" Well that worked swell. No more splashing over the back of the transom AND no more ventilating AND as an added bonus it did not collect as many weeds and the one time it did, it did not make the motor ventilate.

While I was in the water with snorkel gear on I had a fish eye view of the ventilation plate and the keel ( with the tilt at midway). The plate is a full 1" maybe 1 1/2 inch below the keel but hey, it works and as you know 1" higher or lower did not so I found the sweet spot. Thanks everyone for the help. Tomorrow I can drill the holes to bolt the motor on. I hate running with only those screw things, what do you call them?

 

[Sea Rider]

Hello JH,

Nice to read that what posted worked for you. If wanting to be spot on the sweet transom/engine height post a pic of motor shot sideways showing both upper and lower plates to have a look. Really liked your disposal to look what's going on out transom, that's the way to go.

Don't finish glassing the cut transom till some final tests are done, for that will need a pic.

Happy Boating

 

[Jim Hawkins]

Hey Sea Rider, didn't see that last post. The boat is still in the water and will be for awhile so no pictures but I don't think I need any more adjustment. I've been using the boat with no problems so I'm happy as is. Thanks.