Unusual Rear Thruster Problem

[DBBRanger]

My rear thruster gave out one time I was leaving the dock in the middle of using it. I feared the worst: a broken shear pin. So I did the dreaded motor removal using many tips received in this forum. Once I knew how to do it, it wasn't tooooo bad! However, I got it apart and found the shear pin was not broken. While the motor was off, I got in the water and turned the prop while my wife held the shaft on the inside of the boat. She couldn't hold it with her hands, so she used a wrench to hold it and discovered that I could turn the prop without having the shaft inside turn. I thought the worst and decided that it was the gear leg that went bad.....$480. I tried to get the prop off while in the water. I could get the nut off but couldn't bang the back of the prop adequately to remove the prop so I left it for when the boat was back on its trailer at home. I noted that the thruster would move some water but not nearly enough to effect movement of the boat. At home I removed the prop and found that it was the spline style and that the splines were ripped off allowing the prop to spin on the shaft with a bit of friction. I checked the gear leg and found that it indeed did transmit force through it. So my problem would be solved by a new prop.......... Well, not quite. It turned out the new prop (a 5 bladed one instead of the old 4 bladed style I had) would not fit in the tunnel because the shaft was not centered in the tunnel. The new prop was about .080" larger in diameter in addition to the offset. With the 4 blade in place there was a good 1/4" gap at the bottom of the tunnel and almost touching at the top. I found that with the motor in place, I could lift it enough to make a difference in the gap on the outside. I don't know why the motor moves, but the bolts holding on the gear leg are tight nor can I move the gear leg from the outside by hand. Clearly the cantilevered motor seems to move more than it should. I think maybe all the traveling on bumpy road with the trailer might have done it. The manual that came with the thruster stresses that there has to be a support for the motor for proper installation. Of course there is none installed but that requirement is for thrusters that are not ignition protected and therefore do not have a black plastic case covering the motor. Imtra does not recommend a support for ignition protected (and therefore waterproof) thrusters since the case may break. Seems like a "can't fix it so ignore it" solution. In any case, I put a support under the motor in order to lift it enough so that the prop fits.

All is well now although I have only tried it on land - not in the water. We'll see if there is more to the story when I launch it!

 

[doke01]

Yeah, I had that problem with the splines once. I was not close to home or anywhere I could get a replacement so I drilled a small hole through the side if the prop and inserted a thin screw. I screwed it in just far enough to lock on the shaft splines. It actually worked so well I kept using it for a year. Now it's my spare.

 

[NorthernFocus]

"Interesting" design that the splines fail before the pin shears. Thanks for the heads up. One more little tidbit to file away.