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Go Back   SailNet Community > On Board > Gear & Maintenance
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  #1  
Old 06-06-2008
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Shaft extension

Hello folks. I am in the midst of doing a repower. There is a possibility that the existing shaft is too short to reach the PTO on the new engine's gear box. I've been told by the mechanic doing the install that, if that is the case, I would need a whole new shaft that was the proper length. However, it seems to me that an extension piece with a flange on either end, could be used to bridge the gap between the existing shaft and the engine. It would only have to extend it a few inches. The mechanic couldn't really give me a good explanation as to why I wouldn't want to "bridge the gap", versus completely replacing a perfectly good shaft, hauling the boat or hiring divers, etc, etc.

Can anyone chime in on this?

Last edited by jr438234606; 06-06-2008 at 01:16 AM.
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Old 06-06-2008
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Old 06-06-2008
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I once read about a "shaft extender" which used a pneumatic pump to create a vacuum and extend the shaft. I can't remember which magazine I saw the add in but it was in the back.

I think it was made in Sweden.

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Old 06-06-2008
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Would this work??

Bullflex Flexible Couplings



Mfr. Vetus

$162.50 - $2,318.50
Smooths out torsional vibrations
Pretensioned rubber element ensures low noise and vibration-free transmission without backlash
Secured against shearing off, both axially and radially
Non-tapered clamping hub ensures easy installation and dismantling of the shaft assembly
Models 1, 2 and 4 have a 4" connection
Models 8 and 16 have both a 4" and 5" connection
Model 32 is provided with 6 threaded M16 holes on a pitch circle with diameter of 4-3/4"
Don't forget to buy Uniset 4/5 Adapter Bolts
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Old 06-06-2008
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Rockter will become famous soon enough
Jr...

The amount of machining and the accuracy needed to make that work would be well beyond the cost of a new shaft. The slightest error and the flanges will wobble like a jelly and at 15 times a second.

The flanges will have to be perpendicular to the shafts, and both flanges planar to one another and very closely so.

How much easier it would be to turn a new shaft.

I would not try it.
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Old 06-06-2008
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I met the same problem with a new engine. For various reasons, mostly to do with having the new engine use less space, I went for a Python drive to couple the engine to the shaft. This then meant that the shaft needed to be shortened; an easy job on a lathe. On pulling the shaft, it was found to have suffered wear and erosion just where the new stuffing box was supposed to go. (What 30 years' use can do). So I ended with a new shaft as well.

Why the new short shaft cost as much as the Python drive is one of those market driven rip-off issues.
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Old 06-06-2008
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A Drivesaver type of coupling may get you the distance you need, and offer it's own advantages as well.

But Seagull's point about shaft wear hidden within the stuffing box is a good one.
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Old 06-07-2008
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C'mon jr, you're replacing the power plant. Go for a real refit match your entire shaft ... like Rockter says. It's not worth a job !
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