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Old 07-08-2022, 08:30   #1
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Sheared Prop Key? Or Transmission Issue?

So. Launched the boat about a month ago, took her to a temporary marina to paint the decks before heading to the permanent marina that has more stringent rules. While at the temporary marina, also got an oil and transmission fluid change, and paid someone to do it. Last week, we went to go pull out of the temporary marina to head over to the new marina, and reversed entirely out of the slip before realizing that I didn't have any forward on the engine, and then managed to muscle back into a neighboring slip without engine.

First, we suspected the transmission fluid. It was reading about double the amount that should have been in there so the contractor must have overfilled it, and we thought the transmission was probably not gripping with the extra fluid so we siphoned out half of it to get to the right level. Didn't fix it.

Second, we dove this morning to remove a somewhat risky prop shaft zinc I had installed before launch (there's very little clearance between the prop and the cutlass bearing). It didn't have any issues on the 3 hour run from the yard to the temporary marina, but perhaps it had shifted and was now causing a problem. Sounds better (less noise and vibration, especially on deck), but still didn't fix it.

Currently, the engine and transmission sound and function normally in neutral and reverse. In forward idle, it's also fine. Transmission fluid is now at the right level, and the risky zinc has been removed. But in forward, I'm getting plenty of prop wash, but not any substantial amount of forward thrust; not enough to put any strain on the docklines or even enough to counteract the current even at 1500-2000 rpms, and there is a lot of noise and vibration. It's hard to tell for sure if the noise and vibration is coming from the transmission itself or the transom area as a whole because the steel hull echoes. We have three theories:

1) There are already a decent amount of barnacles on the prop (it's a very high-growth, nutrient-polluted river), which could be affecting thrust. But, it doesn't make sense to me that reverse would be normal and forward would be so affected. I feel like it would have to be both.

2) The propeller key sheared due to the forces exerted on the risky zinc before it was removed, potentially in a way that causes reverse to seem normal but forward not to.

3) There is an issue internal to the transmission, potentially caused by the initial overfilling of the transmission fluid.

Any thoughts? I've never seen an issue where I'm obviously getting plenty of prop wash in forward, but not enough thrust to move the boat even an inch. Also, the engine, transmission, prop, shaft—the whole drivetrain—is new as of 2020, with under 500 hours, and has had all the right service at all the right times.
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Old 07-08-2022, 08:42   #2
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Re: Sheared Prop Key? Or Transmission Issue?

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Originally Posted by KelseyB View Post
There are already a decent amount of barnacles on the prop (it's a very high-growth, nutrient-polluted river), which could be affecting thrust. But, it doesn't make sense to me that reverse would be normal and forward would be so affected. I feel like it would have to be both.
This is your most likely cause. Not sure why you wouldn't have cleaned the prop while you were down there.

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The propeller key sheared due to the forces exerted on the risky zinc before it was removed, potentially in a way that causes reverse to seem normal but forward not to.
Possibly but not due to your anode situation. Is the prop badly corroded?
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Old 07-08-2022, 08:44   #3
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Re: Sheared Prop Key? Or Transmission Issue?

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Originally Posted by fstbttms View Post
This is your most likely cause. Not sure why you wouldn't have cleaned the prop while you were down there.



Possibly but not due to your anode situation. Is the prop badly corroded?
We wanted to clean it, just didn't have a good tool for it on hand at the moment.

And the prop doesn't have any corrosion on it, last I checked about a month ago.
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Old 07-08-2022, 08:47   #4
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Re: Sheared Prop Key? Or Transmission Issue?

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And the prop doesn't have any corrosion on it, last I checked about a month ago.
I have never seen an overridden key (which is rare in any case) that wasn't very badly corroded, along with the prop.
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Old 07-08-2022, 11:58   #5
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Re: Sheared Prop Key? Or Transmission Issue?

Had the same symptoms with a Borg Warner transmission which we had rebuilt and still no joy. Even had a diver watch to confirm the prop was turning in forward and reverse.
Finally hauled the boat to see if the Max prop was ok and it was. However the key had sheared and when replaced problem solved.
$5 solution but it cost a lot more.
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Old 07-08-2022, 12:04   #6
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Re: Sheared Prop Key? Or Transmission Issue?

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I have never seen an overridden key (which is rare in any case) that wasn't very badly corroded, along with the prop.
Or from a really loose prop nut...
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Old 07-08-2022, 12:05   #7
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Re: Sheared Prop Key? Or Transmission Issue?

Just to avoid making an assumption... this is a fixed prop, right? Not a feathering or folding one???
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Old 07-08-2022, 12:30   #8
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Re: Sheared Prop Key? Or Transmission Issue?

which gear box reference ?
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Old 07-08-2022, 12:35   #9
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Re: Sheared Prop Key? Or Transmission Issue?

Thrust in reverse but not in forward could be the forward clutch in the transmission.
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Old 07-08-2022, 13:25   #10
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Re: Sheared Prop Key? Or Transmission Issue?

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Originally Posted by ItDepends View Post
Just to avoid making an assumption... this is a fixed prop, right? Not a feathering or folding one???
Yup. Fixed 3-blade. Should've clarified!
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Old 07-08-2022, 13:27   #11
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Re: Sheared Prop Key? Or Transmission Issue?

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which gear box reference ?
It's a Volvo MS-15L. We also suspected that the diaphragm could've blown from the excess fluid, but then dismissed it because if that wee the case we wouldn't have forward or reverse.
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Old 07-08-2022, 13:28   #12
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Re: Sheared Prop Key? Or Transmission Issue?

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Originally Posted by JimsCAL View Post
Thrust in reverse but not in forward could be the forward clutch in the transmission.
We did check all the cables, both shifter and throttle, and they both appear to be working normally.
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Old 07-08-2022, 13:42   #13
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Re: Sheared Prop Key? Or Transmission Issue?

Get yourself a photo or laser tachometer ($20 on Amazon) and compare the engine RPM-to-shaft RPM ratio at idle and high-ish RPM in both forward and reverse. This'll tell you for sure whether the transmission is slipping.

If the engine to shaft ratio is constant, either you've got a sheared (or missing) prop key, plastic bag or other flotsam wrapped on your prop or it's REALLY fouled.

There's also likely a key at shaft coupling. Make sure you're not spinning your coupling on your shaft (although that would have almost surely backed the shaft out of the coupling when you were in reverse.)
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Old 07-08-2022, 13:46   #14
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Re: Sheared Prop Key? Or Transmission Issue?

so, have you disconnected the cables and operated the control lever by hand?

and what color the oil?
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Old 07-08-2022, 13:53   #15
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Re: Sheared Prop Key? Or Transmission Issue?

Take your strobe tachometer and put a piece of reflective tape on the crankshaft pulley, and another piece of tape on the prop shaft or prop shaft coupling.

Start and rev the engine in forward gear to about 2000 or 2500 rpm and use your strobe tach to read the rpm of the crank pulley, and then also read the rpm of the shaft.

If the transmission is OK, the shaft rpm will be the engine RPM divided by the forward gear reduction ratio.

Opps.....same idea as bellinghamster, although he assumes the ratios are the same in forward and reverse, which they are not for this transmission.
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