WOT Motor cavitation

[bigpoppakdog]

115 hp evinrude on 16 ft larson mirado. At WOT I notice my boat will increase in rpm's while not increasing in speed. Gives a feeling like motor comes out of water. After a few minutes overheat alarm will start kicking in. When I throttle down just a hair, feels like motor catches the water and rpms drop and overheat alarm will stop shortly after. I can't see what the prop dimensions are. Does this scenario indicate that my prop is too small? It's an aluminum prop.

 

[steelespike]

Re: WOT Motor cavitation

Cavitation is a different animal most props experience cavitation to some extent.That is ventilation air disrupting the prop and aparently even fouling the water intake. Is this a new developement?Is the prop in good shape?Have you tried trimming in just a little before it developes or not trimming out so far?Anything added to the transom or bottom that might disrupt the flow past the motor?Is the bottom clean?Is there a hydrofoil on the motor?

 

[bigpoppakdog]

Re: WOT Motor cavitation

prop is very good, nothing added to motor or boat as far as hydrofoils. It seems as though I have the motor low to begin with. When I am in neutral the motor is way down in the water. I have one of those new tilt plates added so the motor is farther away from the transom. when I lower the motor more i don't get that higher rpm feeling. It just seems the motor is so low in the water. I thought the recommendation is to have that cavitation plate parallel with the water. I've just taken this boat out so I can't comment on if this is a new development. The thing that I notice is when I trim the motor lower I get rooster tail water almost shooting straight up under the motor and in between the trim plate and transom. Boat seems to run better but that rooster tail water shooting up like that bothers me. Could I add hydrofoils to help this situation?

 

[Randybeall]

Re: WOT Motor cavitation

The over heat is your biggest clue. Drop the motor down an inch and see if the overheat goes away. Most likely your boat rises up a lot when under way and you are getting the water intake out of the water. Get someone else that you trust to drive the boat and observe the lower unit while crusing, the wate intake must be below the surface, if not you will get to overheat and cavitation. The position of the engine on a boat standing still out of the water can occasionaly decieve us as to the dynamic position.

 

[steelespike]

Re: WOT Motor cavitation

Motor height varies.Antivent plate (not called cavitation plate)even with the bottom of the boat is considered a starting point usually you can go up from there sometimes you must go down.The actual angle of the plate may vary as well depending on the setup.Ideally parallel with the keel as a starting point but the angle depends on the type of boat weight distribution etc.Usually you trim out to the fastest speed without ventillation.That rooster tail splashing you speak of is an indication something is wrong.Perhaps you have a longshaft motor on a short shaft transom.20" motor 15" in transom
Perhaps your setback has put the motor at a critical point of disturbance.