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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Old 01-24-2013
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Lengthening the prop shaft

I have a somewhat unusual question. I had a new shaft installed several years ago on my Marshall 22 catboat. Power is a Yanmar 3GMD, with a 1" shaft running in a full skeg such that there is only an inch or so of shaft exposed before the propellor. The propellor nut has only another 1 1/2 inch or so of clearance before the rudder. My problem is that there is not quite enough shaft exposed to fit a zinc doughnut unless I hacksaw it down to about 3/4" width. Is there a simple way to add a small spacer between the shaft coupler and transmission flange? It seems to me that it wold be an easy solution that would allow me to just bolt in a new zinc, but don't know if there would be problems, or if there is anything pre-made in the market to do it with. Any thoughts? by the way, the original shaft did nave just enough clearance for a zinc.
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Old 01-24-2013
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Re: Lengthening the prop shaft

I have exactly the same problem! My solution is to make a steel disk (with holes for the coupling bolts) that will sit between the gearbox flange and the shaft flange. It is easy enough to make with a piece of scrap 1/2 inch or so steel plate. However, it will require longer bolts when installed.
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Old 01-24-2013
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Re: Lengthening the prop shaft

Mike,
I had the exact same problem on my Tartan 27' - no clearance for an off the shelf zinc between the prop and deadwood.

One option is to buy a Drive Saver (see: http://www.globecomposite.com/pages/products_drivesaver ) which should add about 3/4" to the length of your shaft. The Drive Saver mounts between the engine output flange and the drive coupling.
I found a used one but never got around to installing it and trying it out and subsequently sold it. It would have given me the room I needed to mount an off the shelf collar zinc though.
We ended up needing to replace the bronze shaft due to 43 years of wear at the cutless so I had the prop shop add 3/4" to the overall length and now I am as happy as can be when replacing the collar zinc. No customization needed.
I found the price of new Drive Savers to be fairly high (around $200 or so) and our new SS shaft was only $225. For this reason I'd suggest checking your prop for wear at the stuffing box and cutless. If it is in good shape then I'd consider getting the right drive saver for your set up otherwise just replacing the shaft (cutless, shaft log etc) does not cost that much for materials - it is just a bit of work to do.
Another item to consider is if your motor mounts have loosened. It is possible that your engine has slid forward a small amount (from the prop pushing when in forward). If that is the case then pushing the engine back with a crow bar and re-tightening the engine bolts might do the trick. The danger with moving the engine is that you might mess up the engine-shaft alignment.
Good luck.
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Re: Lengthening the prop shaft

Thanks, guys. Glad to know that this is not completely off the wall. Caleb, good points about mounts, new shaft, etc., but I had everything replaced and the real trigger for this space problem was that I had the engine shifted towards the rear by about 2 inches for better access to front of motor for service (belts, pump, etc). That meant a new length for the shaft, not just measuring the old. I suppose I could complain to the mechanic, but too late now and he did a great job otherwise (and still does). All of the mounts, shaft, etc. were original and 25 years old, so I don't want to rush to replace again, and its a bit of bother to get a shaft in and out without dropping the rudder. Shifting the motor was atually an idea from Marshall Marine, so it is not messing up the boat at all. It's just a little hard for them to squeeze a 3 cylinder diesel into a 22' boat

Does the install of a drive saver require any other things like new motor ground plate? I see in their info that the DS completely blocks electrical flow, but I thought that there was a positive benefit from having the motor/electric system grounded right to the shaft. Thanks again, Mike
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Re: Lengthening the prop shaft

Mike,

You are correct and observant. The Drive Saver does isolate the shaft from the motor electrically speaking. I believe that the Drive Saver website mentions this as if it were a benefit and feature of their gizmo. I'm not sure that I buy that either.
I don' have a Masters in EE, nor am I a naval architect but I also like the idea that the engine is connected to the water via the shaft and prop in the event of a lightning strike. Then again, with lightning carrying around 10 Zillion volts it can surely jump, or arc through an inch or so of air on it's way to an eventual ground.
They claim that the DS prevents electrolysis by separating the engine from the shaft but it is usually the prop (bronze) that seems to bear the worst of it rather than the engine block. I think that the shaft zinc is more to protect my prop than my engine and replacing a zinc (< $20) is a lot cheaper than replacing a prop.

My plan (never implemented) was to run some wire(s) from the bolt heads on the shaft coupling side of the DS to the bolt heads on the output flange, thus connecting both sides electrically. I'd ask your friends at Marshall Marine what they thought about this issue.

Copacabana's idea about having a metal spacer (say 1/2" - 3/4") made of metal would leave everything electrically connected. I'd speak to a prop shop about this as the fabrication process would have to be pretty rigorous. As you know, you need a very close tolerance (+/- .001" or something) for the mating surface between the shaft coupling and the output flange. I'm not sure what a prop shop would charge for making you a custom metal spacer. Would it be more than an "out of the box" Drive Saver? I just don't know.
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