Flexible Shaft Coupling

[rleslie]

Thanks for the advise. I hadn't thought about cutting/filing down the lenght of the allen wrench. I have an extremely tight fit, only about 1/4" to work with.

There are options other than pulling the boat.... have a diver pull the shaft back, remove the flexible coupling, repair it and reinstall. This is the route I'm considering.

The purpose of my post was to warn those considering a flex. coupling of potential issues in the future. I do appreciate the suggestions!!!

Thanks
Roger

rleslie, it sounds like a modification to your design is definitely in order. Good luck on that. Appreciate the info as well.

[Evan]

If your stuffing box is of the rigid type, it's foolish to have a flexibly mounted engine and a rigid coupling. If you have a hose type stuffing box, then some of the shaft vibrations are absorbed there and don't make it into the boat structure.

I routinely engineer 5000 HP tugboats with flexible mounts. We would never think of rigidly connecting the shaft to the engine - the shaft would fail in short order.

The difference between a hard mounted engine and a soft mounted one is about 10 dB - and 10 dB is a LOT louder.

[SkiprJohn]

Aloha Evan et al,

I have a rigid stuffing box and the engine has flexible mounts.

In another thread I mentioned cutting down the prop shaft to have the proper distance from prop to aft end of the cutlass bearing and at this point I'm pretty much committed to install a flexible coupling afterall. I need the space between trans coupling and shaft coupling to be filled a bit with something. A flexible coupling is the proper thickness.

Thanks to all for your input. Sometimes each question brings out personal preferences and its good to understand the reasoning behind them. It makes decisions like mine a bit easier.

Kind Regards,

JohnL

[dkall]

If memory serves me correct, ABYC said to NOT use a flexible coupling w/ flexible mounts. I personally witnessed one boat that did and when the harmonic vibration took hold I thought the engine would bounce off the mounts.

Just an FYI.

[knottybuoyz]

Originally Posted by dkall

If memory serves me correct, ABYC said to NOT use a flexible coupling w/ flexible mounts. I personally witnessed one boat that did and when the harmonic vibration took hold I thought the engine would bounce off the mounts.

Just an FYI.

ABYC Standard P-4 Marine Inboard Engines and Transmissions and P-6 Propeller Shafting Systems make no mention of flexible motor mounts or flexible shaft couplings except P-6 .5.5.1 which refers to alighment clearances and calls on SAE Standard J756, Marine Propeller Shaft Couplings, which I'm sorry I don't have a copy of but I'll try to locate them in our library.

I would think that 1 or the other would be sufficient.

[SkiprJohn]

Aloha dkall,

I think that there must have been something else wrong with the drivetrain you described. I've been doing a lot of research on this trying to understand all the nuances. I have to consider Evan's statement:

"If your stuffing box is of the rigid type, it's foolish to have a flexibly mounted engine and a rigid coupling. If you have a hose type stuffing box, then some of the shaft vibrations are absorbed there and don't make it into the boat structure."

This really made sense to me. There is a caution from Never Monday about using manufacturer's tolerance recommendation. All the books I've read indicate .001 times the diameter of the flanges. That, in my case is .005". The tolerances by R&D who makes the flexible coupling I intend to install (when West Marine sends me the correct one - second try) is .010". I'll use the tighter tolerance.

I think I have to agree with what has been stated that if you have flexible feet on your engine mount then you need to have something flexible between your engine and the rigid stuffing tube otherwise that slight wiggle will transfer all the way along your drivetrain to the propeller. Without the flexible coupling the strain then comes on your rigid stuffing box and your cutless bearing.

dkall, How is your Westsail set up? Westsail 42 is a great boat by the way and far and away heavier built than my Cascade 42. My engine is a Mercedes 1800cc (same size as a Perkins 4-107 or 4-108), trans is Velvet Drive 2.57:1, 1 1/4" shaft, rigid stuffing box, exposed shaft to cutless bearing and an 18x13 two blade Gori folding prop.

Kind Regards,

JohnL

[Alan Wheeler]

OK from an engineering standpoint, a flexy coupling should have no bearing on the fact that an engine is hard mounted or mounted with proper engine mounts. If an engine vibrates at "some harmonic freq" then there is an issue with either engine or it's mounts. The engine mounts MUST be speced for the engine weight and in larger engines, it's hp also comes into play. The only major difference with a marine installation is that the mounts should be of a "captive" design. This ensures should the boat ever roll over that the engine can not rip the mount apart and drop through the bott....err top of the boat.
Properly spec'd mounts should maintain good vibration dampening, but should also be such that the engine does not dramaticly move around. If it "bounces" around, then the mounts are either damaged, too light or wrong type for the job.
IMO, having a solid fixed shaft to engine whether soft mounted or not, is asking for trouble. A coupling is easy to do and it removes so many issues and can make an enormouse difference in vibration transmission control.

[dkall]

JohnL,
My engine is a Perkins 4-236 with a 1 1/4" shaft and a 21" Autoprop. The Autoprop is giving me some serious vibration. I've contacted Autoprop and am waiting yet for a resolution. They recommended the 21" prop and a 20" prop was what was on there. My tranny is 1.91:1. Engine mounts are flexible, fixed coupling and a bronze stuffing shaft log with a stuffing box on a hose.

Always thought the Cascade was a pretty boat. I've not seen one in person but the pics show a great boat w/ classic lines.

Fair Winds

[SkiprJohn]

Thanks Dave. Will look forward to posts on how your are coming along on your refit if you are not done already. Sounds as if you'll have plenty of power once you get the prop vibration sorted out.
Kind Regards,
JohnL

[Boracay]

To dkall,
If the autoprop is secondhand it may have worn unevenly.
You may be getting more thrust with the Autoprop and this would exacerbate existing problems.
Have you considered that the problem may not be the prop.
The causes that come to mind are bent shaft, worn bearings and too big a distance between bearings not to mention a misaligned shaft coupling.
The Autoprop is heavier than the fixed prop and this may exacerbate existing problems.
If it were me I would first see if there were any speed power combination where the problem did not occur (possibly low power/low speed) and try running the boat with that combination for a while. Then I would check the boat at a range of speeds and powers and possibly different radius turns, reverse etc.

[dkall]

Considering that none of these issues were an issue w/ the fixed prop. Same bearing, same shaft, same engine same shaft log. New prop. I have considered that it might be something else but I know of another WS that has an Autoprop same exact setup as mine and his is a 20" model. Don't know if I reported that memory of ABYC recommendations have a prop dia 16 to 1 and the prop on my boat that Autoprop recommended was a 21" model. I was concerened when I ordered it but accented to their knowledge. The vibrations mentioned above on a flexible mount and flexible coupling were on a different boat an not mine.

On mine now at 1400 rpm I get more than a vibration, more than a hum the shaft begins to bow and I feel the shutter thru the entire boat. At 1400 I'm moving at 7.5 knots or there abouts as reported on a GPS traveling two ways. I don't have any vibrations below 1400. At the dock the vibration occurs approx at about 1600 in forward and 1800 in reverse. All other speeds are ok up to those rpms.

I filled out Autoprops forms for the engineers and while they said "Maybe the prop is too heavy", I still know one WS that has the 20" Autoprop and he reports no problems.

Appreciate your input and will keep this board posted on Autoprops response.

[SkiprJohn]

Hi Dave,
Is the Autoprop one of those that you can adjust the pitch yourself?
1400 engine rpm is kind of slow to be at 90 percent hull speed so I'm thinking the pitch might be somewhat too high. If you are getting severe vibration then the prop is out of balance or cutlass bearing is gone.
Just some thoughts.
Have you checked out our thread Proper Prop Size?
Hope you get a good response from Autoprop.
Kind Regards,
JohnL

[Alan Wheeler]

Hi Dave, OK mate, we need some more info here.
I need to know what the boat is, planning or what.
The WL Length of the hull.
Max RPM of engine and does she achieve that or within 200RPM of max?
Once I get that info, I may be able to help further.

One other point is, are you absolutely sure all the blades are set to the same identical pitch? If one is out by a fraction, the prop is going to be screwing itself out of balance. With a diameter of 21" on only a 1 1/4" shaft, there is no room for error with pitch differences between blades. The faster the engine revs, the more the force on one side of the shaft and the likelyhood of you bowing the shaft. This will be putting severe strain on shaft, bearing and gearbox and flexi shaft coupling if you have one.

[dkall]

I need to know what the boat is, planning or what.
The boat is a Westsail 42 30k lbs full keel cutaway forefoot and full rudder. Definitely not planing.

The WL Length of the hull.33.4'

Max RPM of engine and does she achieve that or within 200RPM of max?
Perkins 4-236 Tack Reads 2600 with no load. Can't get anywhere near that with the prop spinning. The reduction gear is 1:91 to 1. If you're interested I can send you a video I took of the shaft and what happens at the critical juncture. I made a QT movie of it and can email to you. Don't remember the size but it's short.

Once I get that info, I may be able to help further.

One other point is, are you absolutely sure all the blades are set to the same identical pitch?
All blades are self pitching for the water state and boat speed. See Autoprops site. Each blade swings freely around a 360 degree axis.

If one is out by a fraction, the prop is going to be screwing itself out of balance. With a diameter of 21" on only a 1 1/4" shaft, there is no room for error with pitch differences between blades.
Prop is brand new from the factory

The faster the engine revs, the more the force on one side of the shaft and the likelyhood of you bowing the shaft. This will be putting severe strain on shaft, bearing and gearbox and flexi shaft coupling if you have one.

Additionally. The shaft log had no play in it when I launched. The coupling is fixed. I don't really have room to increase the dia of the shaft. The length of the shaft from coupling to the stern bearing is about 42'.