cavitation

[blcknspit1]

I have a 13' Dargel skooter with a 35 hp johnson, the boat is awsome if I can only get it to move on the water. My problem is when I get out of the no wake zone and give it some power the rpm's rise but the boat does not move. When sitting in the water the prop is about 2 1/2 to 3 inches under the water. i was thinking maybe I need to lower the engine I dont no please if anybody can help it will be appreciated.

 

[QC]

Re: cavitation

Well what you are calling cavitation is probably ventilation, your prop is catching more air than water . . . Bummer. There is a big horizontal plate above your propeller, interestingly enough called an anti-ventilation plate. Where is that in relationship to the bottom? That is the reference point you should be discussing. Also, what about the trim? Does this motor have hydraulic trim? Or manual with a pin and some holes? You may have the height right and the trim too high. Could cause the same thing . . .

 

[blcknspit1]

Re: cavitation

The boat has a full tunnel, do I have to look at the relationship between the anti-ventilation plate and the top of the tunnel or the bottom of the boat? Also I it is a manual trim and one more hole to go on it. I will try adjusting the trim to the last hole and see what happens thanks.

 

[Texasmark]

Re: cavitation

Stand at the front (bow) of the boat and look under the hull at the skeg/antivent plate/propeller. You should be able to see the skeg and most if not all of the propeller. Depending on the year of the 35 hp Johnson, you may also need to see the antivent plate and about a half inch above it....if it is of the old '50's style where the water pickup was in front of the exhaust snauze. You will need to have the engine lower in the water for the water pump to work properly.

The propeller has to have a clear stream of water into it at WOT. If not you will have a problem.

Other thing is you may have a slipping prop, like a sheared pin holding the prop to the propshaft, or a slipping rubber hub imbedded in the inside of the prop.

Easy to check. Get on the road side (port) of the engine with the shifter in Forward gear and stand on the propeller (engine ignition off). You should get significant resistance to your weight and hear the engine turn over as the blade of the prop heads for the ground. If not, disassemble the prop and look at what I said.

Mark

 

[steelespike]

Re: cavitation

I think it needs to be lower.Has it ever worked.What kind of prop are you running?Is it in good shape?A jack plate would get it back farther in better water and would allow more vertical height adjustment.
Are you sure it isn't a spun hub?

 

[ddaigle]

Re: cavitation

Those of you not familiar with a Dargel scooter need to know that it is a super shallow running tunnel boat. Since these boats are used in super shallow water the props take a lot of abuse. Usually a hole shot involves contact with the bottom. This will spin a hub and wear down a prop in a short time. If the hub is not spun then your prop probably has no cup left. A tunnel boat needs a heavy cupped prop preferably stainless due to the high elevation of the motor or the prop will just ventilate and spin. I have ruined aluminum props on the flats in a matter of hours. Down here in the Laguna madre we have to have props rebuilt regularly.