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Old 03-06-2014, 13:11   #1
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Change of shaft from metric to SAE?

Refitting the entire shaft system (metric) on my vessel, from split coupler to the propeller (even removed the stern tube). This had to be done because of wear and tear from the previous neglectful owner. What a mess. I have 2 questions for anyone willing to help.

First, I’ve been told on numerous occasions that if I’m replacing everything that I should switch out to SAE and do away with metric solely for ease of finding parts. The closest my 30mm shaft comes to a standard measurement is almost 1-3/16”. Though I am finding that there are much more parts available (especially with shaft valves) if I go with either 1-1/8” or 1-1/4”. Looking to upgrade vs. downgrade, I am considering going from 30mm up to 1-1/4” on my 34 hp. Yanmar 3JH2E. Good? Bad? Suggestions?

Secondly, my original split coupler had a retaining hex bolt that was screwed on the inside of the coupler (very end of shaft) that would stop the shaft from disengaging with the coupling . I was told I could stick with this design or just use a stainless, keyed split coupler (without the retaining bolt). Pros or cons with using these threaded retaining bolts in conjunction with the split coupler?

Thanks in advance!

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Old 03-06-2014, 15:38   #2

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Re: Change of shaft from metric to SAE?

Apeman, I would guess the only issue is whether you can get a cutless bearing and shaft log that will fit into your boat, or how much work you'd need to open up the holes in the shaft support and hull. (Whatever your situation may be.)

And of course a new prop, unless you can also get the prop re-tapped for the larger SAE shaft.

I would think a single bolt would tend to unbalance the shaft some small amount, but an extra zinc perhaps? Or a stainless ring that could be bolted over the shaft perhaps? Or, having a machine shop tap 3 holes, and inserting three small bolts directly into it. (Which would prevent the loss, but I think a ring or zinc would do less damage to the shaft log when or if it spun back and hit it.)

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Old 03-06-2014, 16:10   #3
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Re: Change of shaft from metric to SAE?

I have a 3JH 39 hp also and the 1-1/8" x 60" long shaft, which is doing fine. Assuming your in America I'd go with SAE.
If you shaft is longer I would go with the 1-1/4" shaft.
And I have a Pythondrive so I did put in a center bolt just in case, but with a standard coupler all you need is a square head set screw in the side of the coupler that screws into a dimple in the shaft with a safety wire keeping the sets screw from backing out more then a half turn.
But here are the items you will need to change over;

SAE coupler of your choice (split couplers are no easier to get off once they rust up) keep well painted.

Stern tube that will have at least 1/4" clearance on each side so water can get in and keep the shaft seal cool.

An SAE packing gland, or a PSS if you put in a self centering shaft drive. E.G. Pythondrive or ?

An SAE strut the will take whatever bearing size you choose for the shaft. The bigger the better. You can up grade shaft size if you choose to.

The prop can be re machined if you go to 1-1/4".
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Old 03-06-2014, 18:36   #4
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Re: Change of shaft from metric to SAE?

All good info, though when referring to a center bolt holding the split coupler in place I literally mean in the center of the back of the shaft (not the side of the shaft like a set screw would be). The bolt is 'inside' the back of the coupler, and cannot be viewed unless taken off the transmission and stuck a mirror in there to look. If I do go with a split coupler again, I'm thinking of having it powder coated this time to help protect against rust (even if it was stainless). Perhaps this may help it's removal in the future.

Should shafts have keyways that are opposite each other? For instance, the keyway holding the propeller in place would be on the opposite side of where the keyway is for the split coupler for a better spinning balance?

Yes, I'm Florida, so SAE looks like the route to go. I'm having a damn hard time finding any fiberglass stern tubes (only G10, which have very limited sizes). I did find a fiberglass stern tube dealer in NZ (Chatfield marine), though just wish I could find something a bit closer. And as for a stern tube bearings it's a toss between Thordon or Vesconite...still figuring that out.

I did decide upon the Lasdrop Gen 2 for a shaft seal.

When you say the stern tube must have at least 1/4" clearance on each side...are you referring to the inside of the tube (space not occupied by stern tube bearings)?

Looked at Pythondrives. Their site seemed very vague, but the product looks well made. Do they also require a structural wall built like aqua drives? That wall was what nixed aqua drive systems for me unfortunately, engine compartments are too crammed as it is.

My shaft is only 45", though thinking of going with 1-1/4" diameter or else I gotta start looking for a new prop as well. At least I can have the current 30mm prop machined wider, but not smaller for a 1-1/8" diameter. Think this size shaft is getting too hefty for that 3JH2E? Or doesn't matter?

So many questions.....
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