What is causing cavitation?

jjt8784

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I'm running a 15 foot tull hull flat bottom with a 25hp evinrude.

I bought the evinrude for the boat a couple of months ago and had the transform cut a couple of inches because the prop wasn't in the water enough. With the motor lowered I picked up a lot more speed and runs much better. Tops out at 32 unloaded on the best conditions. I'm on the water by myself most of the time and a couple of things I've noticed.
Unless I keep my battery in the front of the boat it will porpoise bad. Battery in front with me and the gas tank in the back it'll run fine. While I can operate with the existing cavitation I don't think it's operating at its fullest performance. At high speeds slight to moderate left turns will make it cavitate. At hard right turns it will cavitate (assuming the difference is due to weight distribution on me sitting on the right on the motor). If I'm following somebody or vis verse they can make that turn a lot faster and I never had the same problem with my old motor running wide open around the same turns. When I'm making a left turn and it starts to cavitate, there is water coming over the right top of antiventilation plate. Not sure about left side bc I can't see it from sitting. The few times I have someone plus ice chest and gear with me there is no cavitation. Running about 25 when weighted down.

Unless I am mistaken I believe the antiventilation plate should be even with the top of the tunnel hull. Right now it sits about 1.5 inches from the top of the tunnel. Just to see what would happen I added a .5 inch piece of wood under the motor and tested it. Running wide open and straight it would start to cavitate. Should the antiventilation be positioned at the top of the hull because moving it closer seemed to make it worse?

Someone has suggested getting the prop cupped and after reading through other forums I've thought about adding a hydrofin. Any suggestions?
 

jbcurt00

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Do you mean Ventilates in the conditions you mentioned? The prop blows out of the water and risks over-reving because the prop is out of the water?

Cavitation is something else, but is often used to describe what you did, but thats done incorrectly.
 

Sea Rider

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Welcome to Iboats,

That's a new 2 or 4 strokes OB ? Can you post a side pic of lower leg showing both vertical plates. OB's of different brands have not same lower leg lengths, vary from manufacturer to manufacturer. Prop aeration is generally speaking due to improper transom/OB height. Anti Ventilation plate must run parallel to water level when combo is on plane, for that OB needs to be well trimmed to achieve that position.

Happy Boating
 
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ondarvr

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You mentioned cutting down the transom and a tunnel hull, so I assume you went from a 20" transom to a 15" to better fit the length of the new short shaft motor.

​Tunnel hull boats frequently require more fine tuning to get them set up correctly, and from what you're saying it sounds like the motor is still too high, so the transom would be need to be cut down more.
 

Watermann

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You're using the term cavitation but not describing what the problem your boat is having other than porpoising and that weight distribution helps with that.

Cavitation is normally seen as a burning on the front side of the prop or impeller as a void or bubble is formed in the water and rapidly collapses. As seen below it causes damage. Are you having cavitation burn problems? We could use some pics of what your working on.

cavitacao-helice.jpg
 

jjt8784

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Feb 20, 2017
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Thanks for the replies. Attached are a few pics. If I need different angles let me know. The .5in board is still between the motor and transform but hopefully you can still get an idea. As for the cavitation vs ventilation I guess it would be ventilation. There is no physical damage to the prop aside from wear.
 

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Watermann

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The thrust bar is set in the 3rd hole up, try putting it down in the next lower hole and if it helps some go to the lowest hole. Do you have power trim on the motor?

fetch
 

jjt8784

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No power trim. Before resetting the thrust bar, would it be better to leave the half inch block in place or remove it?
 

ondarvr

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You have a bunch of stuff going on there with that tunnel, you need to lower the motor more.
 

Watermann

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No power trim. Before resetting the thrust bar, would it be better to leave the half inch block in place or remove it?

Yeah I would loose the wood block, raising a motor closer to the surface is a cause for the prop to ventilate.

Personally speaking I don't like that tunnel separator plate that sticks out, I can see that causing problems with disrupted water flow to the prop and can't see it's purpose..
 

jjt8784

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It was there when I bought the boat. The previous owner said it use to be longer but has been worn with age and use. The purpose is for the metal to take a hit to a log before the motor does. At it's length now I'm not sure how much it helps. Where I run there is a lot of logs and it has taken some solid hits.
 

Watermann

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Oh I see a welded in river runner that could save your bacon I guess.
 

jjt8784

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To lower the motor more I'd have to have the transform cut again. Also, where the top of the transform is now, there is a brace attached to the transform. Forgot to get a picture of it this morning. So that too would have to be cut and redone.

I'll try the resetting the thrust bar before anything to see if that helps. But seeing how waters comes over the ventilation plate, would a hydrofin help since it is widening the ventilation plate?
 

ondarvr

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If you went from a long shaft to a short motor it will be 5" shorter, did you cut 5" off the transom so the new motor (prop) sits in a about the same place?

​Your best bet would have been to get a long shaft motor.

The tunnel will create turbulents and air pockets as you turn, so will that added fin in front of the motor, the faster you go the more of an issue both will become.

​Was the old motor lower in HP and slower, plus lower in the water (long shaft)? This would account for it working better before and having problems now.

​Adding a foil might help slightly, but the air is most likely coming from the fin in front of the motor and the tunnel, so more from the side than the top where the foil would be mounted.
 

jjt8784

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I wish I would've wrote most of this down when first figuring it out....

The old motor was a tohatsu 25hp and also topped out at 32. When measuring the length starting from where it attaches to the transform to the bottom the new motor was I think 2.5 inches shorter give or take. Definitely not 5 inches. Brought it to the welder and he suggested not cutting the transform as much as I had measured because of how it would affect with the tunnel. He was very reputable and works on almost entirely on boats so I had no objections.

I really can't remember how much he cut it. After looking through old pictures of the boat with original motor I noticed that it did have about a one inch wood block between the motor attachment and transform. When I put the new motor on, the ventilation plate was even with the top of the hull. Neglected to pay attention how the old motor was set on the ventilation plate and bottom of the boat.
 

Sea Rider

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Some issues :

If you can see the anti ventilation plate exposed (X) while combo rides, definitely OB sits too high, produces too much prop aeration/ventilation Ideal transom/OB height is when water flow at speed skims right under front border (*) and water deflector plate (green line).

Perform a wot test on calm, flat no wind water cond with deck weight evenly distributed and OB trimmed at a hole for combo and anti ventilation plate to ride parallel to water level once on plane, pull head out transom and check at which lower leg height is water flow passing by. Will know how much will needed to chop transom down.

Tunnel.JPG

Was the fin located on middle tunnel some sort of failed experiment ) Anyway, water flow at speed should pass flat undisturbed right through middle tunnel so lower leg receives it properly flat under upper plate to be cut clean through the sharp edge. If wooden edge proofs useless, remove it, sand well the whole rear tunnel for water to pass smooth through.

Installing a doel fin is useless, is money straight into the blue..

Happy Boating
 
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Motor looks too low for a tunnel setup. Any tunnel I've had the motor had to sit higher because the water tends to be higher due to the angle of the motor. I'm not familiar with the extra fin setup since I'm a saltwater guy but it basically acting as a rudder I would think and that would make the boat want to go straight. Back to the motor height think about a bracket setup. The farther back the motor it is the higher the motor sits. There is a formula for it maybe someone will chime in with it. The tunnel is effect is forcing water up as it exits the stern. They're tricky and even 1/8" can make a difference.
 

jjt8784

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Thanks again for the replies! Last night I adjusted the thrust bar from the 3rd to 2nd hole. Had to cut the tip of the fin so the motor wouldn't hit it but that was no problem. Put it in the water and could make all the zig zags I wanted without the prop ever slipping and catching air. On calm flat water with no wind or current it topped out at 27. Normally it would be 28-29. I can live with the little bit of speed loss since before I had to constantly slow down and straighten out when making hard turns which there is plenty of on the river and in the marsh.

Now that, that part is fixed I think I will try raising it again. As boatdrinks and said I know purpose of a tunnel hull is so that the motor can be higher which helps when running shallow water, logs, ect. I'll see what happens but either way I'm satisfied with where it is at now
.
 

Watermann

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Glad to hear it, thanks for coming back to update us and your thread.
 

Silvertip

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It sounds like you had the setup fairly close before you adjusted the thrust bar. Many boats set up "on the edge" need to slow a bit in hard turns and it is quite simple to explain. As you turn the boat banks in the direction of the turn. That effectively "raises" the prop height slightly which can be enough to cause it to ventilate. A cupped prop if you don't have one, may reduce or eliminate the ventilation problem. Another way to look at this to hold a two foot long piece of string with a weight on the end and then swing the weight right or left. You can see the weight rise because of the arc. Wider boats are more prone to this than narrow ones but either can be affected.
 
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