So the home-made mud motor I picked up doesn't have a cav plate. Yesterday was my first time testing it out, and for it to not cavitate I had to BURY that thing in the water. The square frame also seemed like it might be helping the prop cavitate by giving a funnel for air to get down there. At any sort of respectable speed (for a 9hp long tail on a 14ft semi-V hull) it was a chore to keep it deep enough to keep speed. I was going almost as fast at half throttle because the prop wasn't pushing as much air.

Would adding a cavitation plate make a world of difference, or does this sound like design flaw so I should look into a different frame or a whole new motor?



Side note, how easy is it to shear the shear pin? Because it broke on me last night trying to go through a little section of cat tails... as a storm was rolling in... and I didn't have a hammer and punch to get the old pin out... and the 9.9 'rude that I brought along just in case wouldn't start either. I got wet, but at least the lightning stayed in the clouds

sheer pin, shear pin, whatever it is called. Never said I could spell. The pin that actually spins the prop, so if you hit a log, rock, etc the pin breaks off so you don't F up everything.


If you were meaning where when you said were


Anyone have anything on the cavitation stuff?

my motor dont have a sheer pin, its got a key on the keyway in the shaft and crank but it wont sheer.

Are you talking about a flywheel key?

No, it's a home made mud motor, and has an actual sheer pin at the prop, just like a trolling motor has. I thought it was funny that it would break that easy and seeing what others throw theirs through that theirs didn't break.

I don't know about running one without a cav plate but I would thank that it would be a bitch to drive if your running wide open bc thats what pretty well keeps your prop in the water. Without it and when and if your boat plains out you going to half to fight the motor to keep that prop in the that water and from sucking air.

Also with the sheer pin I don't know how well thats going to work with a LT. I know it want work with me. I had a sheer bolt on a 6.5hp on the U joint and motor and you could count on that thing breaking everytime you hit the slightest little thing. Even in the thick veg I would sheer em.

ny chance we could see a pic of this shear pin? idk why he would build it like that. most props are made to be stron enough to survive a pretty good hit and bounce off. and if you runnin at a good speed the skeg shoud hit most things before your prop.

Girls, reach down and gently unwad panties.

Naw, I wasn't tryin to be a dick. I actually didn't know that most... er.. real ones, didn't have the shear(sheer? how the F are you supposed to spell that?) pin so that was why I was saying that.

I put a new pin in and put the prop back on once I got home so I'll try and pull it off to take a picture quick. Are most props threaded on then? And if that's the case, are they reverse threaded or are the blades backwards to most outboards?

I dunno, it's a B&S 9hp motor on it so maybe I'll just look into the longtail kits online and put together one that had some actual R&D into it...

It's normal threads, these motors spin opposite of outboard powerheads.

The industry standard is a 3/4" shaft 10 tpi with several nuts washers and a locknut holding it on.