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Engine revs but boat won't go.

markbabb

markbabb

Contributing Member
I'm trying to help a friend. He recently bought a 90 something Searay with 350 Mercruise I/O. Previous owner had same problem, my friend was told boat was repowered not very long ago before sale. The trouble is this, a mechanic replaced coupler feeling secure that was the problem. Problem remains the same. Prop shop says prop tests good.With the out drive off alignment tests good with tool insertion. I'm told the shaft on outdrive is the correct one and measurement shows it going far enough into coupler and splines on shaft are good and coupler is new.The gears in outdrive are visually good and no metal appears in lube oil. Outdrive seems to shift into gear correctly. Turning prop feels like it is engaged and meets the resistance expected. No noise is heard in outdrive. What else would cause this? Lower gears in outdrive? I'm not familiar with exactly weather the problem could exist down there or not. I used up all my guesses and am trying you guys for some help.
 
If you are not smelling burning rubber, I doubt that the coupler is gone.
The simplest and easiest thing to do, would be to try another propeller, and make sure that the diameter and pitch is correct.

The A drive is a dog clutch drive. These are either fully engaged, or not engaged.

If this boat has the B drive, these are cone clutch engagement. While the sliding sleeve and cups rarely fail this way, it could be a sliding sleeve or gear cup issue.
I'm fairly certain that 1988 or 1989 was the first year for the B drive.


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Ayuh,.... If this is an Alpha drive, tell yer Bud to drop the lower unit,...

I'm thinkin' the vertical shaft snapped off at the o-ring groove, near the top...

Sometimes if it snaps at an angle, what you explain can happen...
Any ground metal will be lost in the exhaust, 'n not seen in the oil...
 
bondo said:
Ayuh,.... If this is an Alpha drive, tell yer Bud to drop the lower unit,...

I'm thinkin' the vertical shaft snapped off at the o-ring groove, near the top...

Sometimes if it snaps at an angle, what you explain can happen...
Any ground metal will be lost in the exhaust, 'n not seen in the oil...
Click to expand...
Engine would over heat in a few minutes if the shaft was snapped.
 
Outdrive seems to shift into gear correctly. Turning prop feels like it is engaged and meets the resistance expected. No noise is heard in outdrive. What else would cause this? Lower gears in outdrive? I'm not familiar with exactly weather the problem could exist down there or not. I used up all my guesses and am trying you guys for some help. Im guessing the vert shaft right above the pinion gear ,with the engine off, shift into fwd gear and try to turn the prop counterclockwise with your foot. If it moves the shaft is broken.
 
Chris is correct.
If this is an A drive, and if the lower vertical shaft broke, there would be no sea water pumping action, nor would there be prop lock-up, as Mark suggests there is while in gear.

If B drive, the sliding sleeve/cup may hold during low torque, but may let loose during higher torque.

We need to know which drive this is.


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Icon is in this post but not my previous post my guess its broken just above the pinion gear. with the engine off, shift into fwd gear and try to move the prop counterclockwise.
prop moves the shaft is busted.
 
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My friends new mechanic figured out the problem...your going to like this...someone actually cut the end of the lower unit housing just before the prop. I'm guessing to allow for some kind of prop that should have never been used. Exhaust was now leaking or pouring out ahead of the prop causing complete cavitation as the rpms went up. He has now ordered a new lower unit to fix the problem. When the previous owner had the replacement outdrive installed he bought a new prop and stuck it on, had this problem, then sold the boat. My buddy now has a new coupler and lower unit and a big hole in his pocket but he did get the boat cheap. Who 'd have thunk it.
 
The previous owner probably forgot the large thrust washer behind the prop and the prop itself wore the housing away.........

Make sure the correct thrust washer is used.
 
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