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Old 08-10-2013
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Coupler Sandwich?

Replacing a MD7A with MD7B that has common gear box setup. I need to replace the shaft as well. Since engine size and gear box are same I was thinking a same size shaft is needed. Here is what the coupler on the engine side should look like


The MD7B has that same end but has another intermediate coupler (?) tacked on. Here it is



What is this? What is it for? Do I keep it? If I keep it, I need the replacement shaft to be cut about 5/8" shorter to allow for this extra piece. If I remove it, then the new shaft will be identical to the old.
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Old 08-10-2013
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Re: Coupler Sandwich?

That, my friend, is a genuine drive saver coupling insert. http://www.globecomposite.com/pages/...cts_Drivesaver
The web page extols the virtues of this product.

I'm not sure I buy all their claims though. I don't buy that electrically isolating the shaft and prop is a good thing. If you are running a diesel I doubt that this doo-hickey would do anything noticeable to lessen vibrations or noise. I also question their claims that the drive saver will break before your engine pulls up a mount when a line wraps around your prop.

In short, I think that this is just another "marine" doo-dad that we could probably all live happily without. A new one costs almost as much as a new shaft too ($200+).
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Old 08-10-2013
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Re: Coupler Sandwich?

Thanks! At least I know what it is and that it's optional. I'll read up on it at the link. My first impression was that it would make alignment more difficult since it is an inch bigger in diameter than the shaft side coupler, unless I order the new shaft with a bigger coupler to match.
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Old 08-11-2013
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Re: Coupler Sandwich?

Quote:
Originally Posted by CalebD View Post
That, my friend, is a genuine drive saver coupling insert. http://www.globecomposite.com/pages/...cts_Drivesaver
The web page extols the virtues of this product.

I'm not sure I buy all their claims though. I don't buy that electrically isolating the shaft and prop is a good thing. If you are running a diesel I doubt that this doo-hickey would do anything noticeable to lessen vibrations or noise. I also question their claims that the drive saver will break before your engine pulls up a mount when a line wraps around your prop.

In short, I think that this is just another "marine" doo-dad that we could probably all live happily without. A new one costs almost as much as a new shaft too ($200+).
The first time you get a cable wrapped in your prop and the Drive Saver pops rather than yanking the engine off its mounts and/or the shaft out of the coupler/packing gland, you'll appreciate that little "doo-dad".
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Re: Coupler Sandwich?

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Originally Posted by cas206 View Post
make alignment more difficult since it is an inch bigger in diameter
Turns out not to be true. I read the online instructions and you apparently align the engine without the saver, then you put the saver in the middle. So I would still be working with a 4" coupler to 4" coupler during alignment. It also looks like I have plenty of space between prop and rudder blade to accommodate an extra 5/8 inch length. So I think I will order same size shaft, align as required without saver. Because I already have the saver, put it back in the middle and see how it goes.
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