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Skipaway 07-09-2008 02:21 AM

Prop/Prop-Shaft Key Size?
The tow segment through the no-wind marina entrance was embarrassing (thanks so much to the runabout that almost passed me!), but once we got to the closest dock, I was able to pick up an active fender (also known as crew); we had a nice downwind leg in 3k wind and then turn into my slip. Thankfully, the appreciative audience missed the first part (most of those transients MOTORED :mad: down from Portland, so they should applaud).

But here’s the deal: Engine fine, prop shaft turns in forward & reverse (observed at the stuffing box), but no propulsion. The propeller is present, but does not spin.

This is an Islander 28 with a 3/4” prop shaft and 3 blade fixed prop. Location is lower Columbia River (30 miles to nearest haul-out facilities)

I suspect the prop retaining knots came loose (cotter pin gone?) and/or the prop/prop-shaft key has corroded or fallen out. (I am hoping it is not terminal galvanic corrosion.)

A sailing friend with scuba gear (and wetsuit) will dive to check it out. I’d like to be prepared ahead of time if possible: meaning have a prop/prop shaft key & nut cotter pin available; zinc's too. What else?

BTW, I did do a search before posting. Closest I’ve come is the thread on “Changing Prop Underwater” (March 2007) and I’ve printed the blow-by-blow how-to (thanks, Fstbttms). Also noted is Sue & Larry’s “pulling propellers” (March 2002).

So, FINALLY THE QUESTION: Can I determine ahead of time what size key is likely to be needed; and/or what range of sizes should I get? If not, what do I do next?


Rockter 07-09-2008 04:50 AM

The prop will almost certainly be on a taper, and will have a Woodruffe (spelling?) key between taper and prop. A Woodruffe is like a wee half-moon key the curved side of which will be embedded into the shaft, and the flat size keyed to the prop. I can't see a way of knowing what size the key is beforehand.

Do you have a tidal range you can use to your advantage? It would be easier that way.

Faster 07-09-2008 10:46 AM

Hopefully that's all that's gone wrong.. but it's pretty rare for that to happen, in fact usually the lock nut is cotter pinned in place precisely so what you suspect cannot occur.

I would think that the prop would occasionally "catch" and try to turn a bit even if it had slid down the taper enough to lose the keypiece. (but you'd have no drive - just some noise.

Hopefully the shaft is still in one piece!

Fstbttms 07-10-2008 02:25 AM

Shearing the key is not a common problem, but it does happen. Unfortunately, there is no way to tell the size and length of the key without pulling the prop. BTW, the keystock will be square, not curved as mentioned previously. You can buy a range of keystock pieces to have on hand (not expensive) and have a hacksaw and file close by to make any modifications necessary.

Skipaway 07-11-2008 12:21 AM

Thanks for the replies. Here’s an update: I drove to the big city today and went to a referred engine place; they sent me to a prop place; they said nobody uses 3/4” props anymore, but gave me some SS cotter pins, told me that I needed 3/16” key stock in SS or bronze and sent me to an industrial supply place; finally $2.19 for 1 foot of 3/16” SS key stock. 160 miles RT @ 30mpg x $4.99999 diesel = $26.67 for a $2 part.

Fortunatley I amortized the fuel cost: to complete my trip I stopped by WM for a new fuel filter (I’ll tag this info on to the WM thread, but the $35 R12T at NAPA cost $23 at WM, except WM didn’t have it in stock), and ended up buying a wet suit; at this point I’ll save money on fuel by not shoppping further and might be able to do more than sit on the dock watching a diver. Hey, my wife bitches about how much I spend so, when no one is interrupting, I love to ramble.

Another thought, after reading Rockter’s’s post, is beaching the boat on a high tide an alternative to under water repair? - I’ll post more questions on that in another thread (“Using the tides for bottom work”), however I would need very favorable winds to get out of the marina, and back if repair doesn’t work.

Rockter 07-11-2008 04:51 AM


What kind of key stock is it? Woodruffe keys are not square section. Perhaps I have got it wrong but on a taper it is normal to use a Woodruff key. A Woodruff is like a wee section of the moon, and it can rock a wee bit to follow the taper as you put the prop back on. This one isn't tapered, but it gives an idea....

Woodruff Metric key and Keyway Dimensions

This one is definitely for a tapered crankshaft....

[A]- - Vespa - Crankshaft - Woodruff Key - Flywheel Side - Vespa

In the future get a smaller car. 30 mpg is terrible. It should be twice that. End of sermon.

Fstbttms 07-11-2008 10:34 AM


Originally Posted by Rockter (Post 340088)
Perhaps I have got it wrong but on a taper it is normal to use a Woodruff key.

Not on any prop shaft I have ever seen.

Skipaway 07-29-2008 09:04 PM

Here’s an update: I kinda dove on the boat yesterday. The primary lesson is that 40 year old rubber does not hold up (my SCUBA cert & equipment dates to 1968). but with shock cord holding the mask on and a snorkel I discovered the prop is fine (zincs too). Bewildered, I started the engine and leaned that observing the transmission/prop flange rotating is not the same as the prop-shaft rotating. Duh!

The prop side of the flange has 2 screw holes, one of which has a loose set screw (no wire tie on it) the other is vacant. Tomorrow I’ll uncouple the flange and inspect for a missing key - if there was one, I would expect to hear it rattling in there.

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