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Old 12-04-2009, 18:07   #1
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Shaft Length Between Cutlass and Prop?

I added a flexible coupling which has a 1.25" thickness, and now I'm thinking I may have too much exposed shaft aft of the cutlass.

The shaft is 30mm diameter and it's swinging a 19x16" 3-blade folding prop. I now have about 1.5" of exposed shaft, in addition to a line cutter that's approximately 1/2" thick and shaft zinc. The zinc takes about 1.25", so the total shaft between cutlass and prop hub is approximately 3".

If I remove the flexible coupling I may only have about 1/4" gap to the zinc, so that might be too short especially if that space is reduced under full thrust load underway. So without the flexible coupling I may need to do without either the zinc or line cutter. Both are important, as based on my experience here in New England it is difficult to avoid all the traps.

Please look at the image -- educated opinions would e appreciated. Boat is still a few weeks from launch date, so it's easy to make changes now.

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Old 12-04-2009, 18:34   #2
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The first thing that comes to mind, would be to find out what your blade tip clearance is now. If you decided to remove the flex coupling, you may be too close to the hull with the prop tips. Other than that, I would not be concerned about the exposed shaft that is shown in the photo. I have about 24 inches of shaft exposed on my shaft.
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Old 12-04-2009, 18:47   #3
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I have sailed from LIS to ME and only snagged a trap line 2x in 23 years when I was on autopilot and not looking out (My bad).

I there are other ways to block trap lines. You can connect a small wire from the keel to the skeg (if you have one) to deflect any line away from the prop.
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Old 12-04-2009, 19:05   #4
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The first thing that comes to mind, would be to find out what your blade tip clearance is now. If you decided to remove the flex coupling, you may be too close to the hull with the prop tips. Other than that, I would not be concerned about the exposed shaft that is shown in the photo. I have about 24 inches of shaft exposed on my shaft.
Tip clearance will not be an issue. Also, just to clarify, the bronze piece to the right in my photo is the prop hub. There is no shaft support aft of the zinc, and I don't think one can have 24" of unsupported shaft without having a severe vibration issue.

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Old 12-04-2009, 19:15   #5
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I have a full keel and have had two prop foulings from trap lines in the Chesapeake. Both were free floating line or something because there was no float or thunk noise.

Anyway, If you have room for the cutter I'd keep it on.

Shaft length is only a problem if there's a clearance issue for the blades. Check full range of motion. The flexible coupling sounds like it did a good thing by giving you clearance for the hardware and enough room for water to get into the cutlass.

If there are clearance issues you can always lose the coupling and go with a separate zinc on the prop or totally separate one with a brush on the shaft for electrical continuity.

BTW, do you have any idea how many off color jokes I didn't make about your post?

My personal favorite was the flexible coupling / Kama Sutra train of thought. :-)
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Old 12-04-2009, 19:52   #6
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Why did you add the flexible coupling?? It shouldn't be added in lieu of aligning the shaft and engine. Usually they are added if the engine can't be aligned.
Recommended distance for a shaft to extend past the cutless bearing is one shaft diameter. If it is more than that, any slight imbalance in the prop will cause the shaft to whip back and forth and possibly cause vibration and accelerate cutless bearing wear.
Best to try to solve the problem rather than mask it with the flexible coupling.
Brian
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Old 12-04-2009, 21:10   #7
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Why did you add the flexible coupling?? It shouldn't be added in lieu of aligning the shaft and engine. Usually they are added if the engine can't be aligned.
Recommended distance for a shaft to extend past the cutless bearing is one shaft diameter. If it is more than that, any slight imbalance in the prop will cause the shaft to whip back and forth and possibly cause vibration and accelerate cutless bearing wear.
Best to try to solve the problem rather than mask it with the flexible coupling.
Brian
I added the coupling just so I could have the zinc and cutter, and leave enough clearance for the water to enter the cutlass bearing. There was no vibration problem.
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Old 16-04-2009, 09:50   #8
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The rule of thumb is 1.5 X shaft diameter of exposed shaft between the cutless and the prop hub. On a 30mm shaft this would be 45mm.
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Old 24-04-2009, 16:32   #9
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An emergency repair forced my boat's shaft out 4" from the cutlass bearing

My wife and I are presently cruising in Mexico. As we were pulling up to the fuel dock at San Jose del Cabo, the slip coupling on our 25-year old Volvo failed and the prop shaft tried very hard to leave the boat entirely. The remnants of the old shaft coupling kept it from doing so.

Since the engine has long since been discontinued, no new replacement parts were available. Used parts were questionable and were likely going to require machine work on the shaft and there are no haul-out facilities there. Ultimately, I found an R&D Flex Coupling and shaft coupling from PYI, Inc. in Lynnwood, Washington. They had replacement, bolt-on parts on the shelf and they overnighted them to us the same day (getting caught in customs in Guadalajara but that's a different story). After fitting all the parts together, it left our shaft about 4" longer than it had been. Since then, we have logged over 150 hours of motoring time (way too much but, again, that's another story) and have had no problems and it seems smoother than it was before.

By the way, our Dutch-built Contest 48 came from the factory with very little clearance between the prop and the cutlass bearing - just enough for an L/C zinc anode. The cutlass bearing has no other source for cooling other than the water that makes it's way past the zinc and we've never had a problem.

Fair winds and calm seas.
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Old 26-04-2009, 02:57   #10
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The rule of thumb is 1.5 X shaft diameter of exposed shaft between the cutless and the prop hub. On a 30mm shaft this would be 45mm.
This was the advice I received from the slipstream people after I was concerned that too much shaft was exposed.
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Old 26-09-2009, 19:46   #11
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I know what the book will say but??

Read this and was thinking .... (not saying good thoughts).
If one added a flexible coupling and provided there was no prop clearance issues, could you use a longer cutlass bearing and have it extend out past the the aft end of the propeller shaft strut support? Maybe it is not possible to get longer ones but ...
This may not be optimum but it may provide some stabilization and I'm not talking a 4 inch cantilever of anything like that.

Comments / Opinions / negatives?

Extemp.
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Old 05-12-2009, 14:17   #12
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Half shaft to shaft x 1.5 is the accepted range of unsupported shaft as others have said. Any longer and with torque and thrust it is very likely you will have vibration and accelerated bearing wear.

The Bearing only needs 3.00mm gap to get water out or 1/8th"

Bearings are designed to have support so extending them doesn't add that much support and risks damaging the bearing. The weight and torque of a prop produces large forces trying to bend the shaft. The only solution is a shaft that is well supported in close toleranced bearings operating as close as possible in line with where the thrust is taken. Stainless steel is a very bendy material.

Alignment is still important with flexible couplings.

Shaver cutters need less space than scissor cutters.
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Old 05-12-2009, 15:36   #13
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Half shaft to shaft x 1.5 is the accepted range of unsupported shaft as others have said. Any longer and with torque and thrust it is very likely you will have vibration and accelerated bearing wear.

The Bearing only needs 3.00mm gap to get water out or 1/8th" ...
INDEED!


See also ➥ Study Hall:
and Richh (#4) ➥ cutlass bearing
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Old 16-12-2009, 11:49   #14
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I agree that flexible shaft couplers are frequently improperly applied to mask drivtrain issues, but they can reduce the amount of engine noise transmitted to the hull and provide some some protection for the transmission.

The 1.5x diameter thing is ok with three or 4 bladed props. I would shoot for half that ammount with a 2 bladed prop or ungeared folder to prevent whipping.
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Old 16-12-2009, 12:53   #15
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...The 1.5x diameter thing is ok with three or 4 bladed props. I would shoot for half that ammount with a 2 bladed prop or ungeared folder to prevent whipping.
The diagram indicates 1.0x to 1.5x shaft diameter.

Would 1x dia. be close enough, or would you still recommend no more than 0.75 x dia. (1/2 of 1.5x) for 2-blades etc?

My own 2-Blade was between 1.33 to 2.33 x shaft dia (as I recall) from the strut, and never "seemed" to have a problem with whipping or shimmy, even with a 10 - 12 year old cutlass*, and no flexible coupler.

*Even at 12 years, I couldn’t manually deflect my shaft in the cutlass bearing. Since rapid bearing wear signifies a problem in the drive train (& I appeared to have none), I assumed my installation was ok.
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