Prop blowing out

[Prentissboater]

96 model 25hp merc, short shaft on a short transom boat. My question is what is making the prop blow out at higher rpms? The prop isnt slipping but thats what it seems like. Verified this with a mark on the prop shaft and another on the prop. Anti cav playe is broken in half, the complete left side is missing. Previous owner also put prop on without the spacer and the front of the prop got into the lower unit, though it didnt mess up the blades on the prop. Any ideas? Thanks

 

[Scott Danforth]

96 model 25hp merc, short shaft on a short transom boat. My question is what is making the prop blow out at higher rpms? The prop isnt slipping but thats what it seems like. Verified this with a mark on the prop shaft and another on the prop. Anti cav playe is broken in half, the complete left side is missing. Previous owner also put prop on without the spacer and the front of the prop got into the lower unit, though it didnt mess up the blades on the prop. Any ideas? Thanks

your post says it all

the anti-cav plate is broken in half

the prop and lower became one and now you have an exhaust leak between the prop and the gear case

those two items alone will make your prop blow out

fix the anti-cav plate by taking it to a welder

if needed, fix the gear case

if the prop if borked, get a new prop

 

[Prentissboater]

Thank you

 

[Prentissboater]

Update, ordered a new prop, matched up to the gear case alot tighter. Not very much gap at all now. Forgot 2 mention before that it has a two piece whale tail on it. I suspect it was put on there due to the broken anti cav plate. Anyways, installed new prop and took it to the river, still having bad prop slip (prop blowing out) although not as bad as before. Any suggestions?

 

[Prentissboater]

I guess i should also note that this motor is on a 1852 jon boat.

 

[Prentissboater]

Anybody?

 

[Prentissboater]

Fixed anti cav plate, new prop, same troubles. Any suggestions?

 

[ondarvr]

Transom height measurements and outboard shaft measurements don't always pencil out to 15", so short shaft doesn't always mean the same thing. Also, some flat bottom boats (jon boats) require the motor to sit slightly lower or they'll blow out just like you described.

 

[Canufixit2]

I'll add a couple comments.

First - (if you have not already) find a installation/user manual for your engine - they usually have a section on mounting height of the Motor WRT the bottom of the transom and anti cav plate.

Second - I seem to remember an old installation manual that specifically mentioned to check the bottom of the boat for a bow (non flat boat bottom). It was said to screw with the prop bite. Can you quickly swap this to another boat to eliminate the jon boat hull ?/?

 

[TNRoostertail]

If your prop is blowing out at higher rpms and higher speed it means the prop is grabbing more than it can push or there is cavitation. Like others have said, try getting it a little lower. And do your props have cav holes in the front for hole shot? Usually smaller props do not but somtimes they do. It comes down to the prop is trying to grab water but the boat becomes to heavy or isnt going fast enough and the force need to push the boat exceeds what the prop can efficiently push.

 

[ondarvr]

It's not cavitating and they aren't cavitation holes, it's ventilation. They are very different things but the terms are frequently confused.

 

[TNRoostertail]

It's not cavitating and they aren't cavitation holes, it's ventilation. They are very different things but the terms are frequently confused.

You are absolutly right, I confuse them all the time lol. Thx

 

[TNRoostertail]

Also does your boat ever plane out?

 

[ondarvr]

You are absolutly right, I confuse them all the time lol. Thx

​The only reason I ever bring it up is because when describing problems and solutions the terms used need to be used in the best way possible so the answers make sense to the many people that will read these threads in the future.

 

[TNRoostertail]

I agree completely and throw the cavitate word around far to loosely. Usually cavitation is a problem while vent holes ventilate to let the prop turn faster with less resistance and that's usually a good thing.