Lost Thrust! Lost Prop?

[Bash]

If there's a lesson here for all of us, it's not to put a new prop on an old shaft-saver. That new prop was probably delivering twice the torque that the shaft-saver had been subjected to recently.

a better lesson might be not to rely on shaft-savers to eliminate vibration. Get the engine mounts adjusted right, add the right prop, and suddenly you don't need rubber in your drive train.

[Evan]

A rubber torsionally flexible coupling is very common on flexibly mounted engines, though the one pictured is very long.

Bash is not correct. If you have soft engine mounts and a rigid coupling, you have eliminated the benefit of the soft mounts and all the engine vibrations get into the hull structure. You must align the engine with shaft properly with any sort of coupling, flexible or not.

For those that doubt the efficiency of flexible couplings, we designed two sistership tugboats, around 3500 HP each. One had rigidly mounted engines, and one had soft mounts and a flexible coupling. The noise levels between the two boats were about 8 - 10 dBA. (3 dBA increase is a doubling of sound levels). Guess which was louder!

I doubt you can successfully glue back the rubber to the steel. Bonding rubber to steel is pretty tricky. You really need a new flexible coupling - though you could use a shorter one and a small "spool piece" (a make up piece to take up the extra length) - but it might be cheaper to buy a longer prop shaft than fabricate the custom spool piece.

R&D designs makes them for boats, as does Globe. Google "Drivesaver".

If you are lucky you might find an off the shelf industrial unit that bolts up with minimal machining required.

I design shaft lines for all sorts of ships and boats for a living by the way

[Cheechako]

I doubt if the epoxy solution would work long term. You probably want to be done with this and fix it right, rather than have it break loose while you are going through a pass with 4 knots of current up there in BC! There are possible fixes I suppose but need some research. Looks like a good spot to put in an automotive style U joint, but finding the right setup might be tough... If you try to repair it I would have the metal plates sandblasted and would probably use 1 part polyurethane moisture cure glue. Obviously the problem with this joint .... even from the factory.... is getting glue to bond metal to rubber...

[Hydra]

Bash,
Shaft misalignment isn't the only cause for vibrations. In one of my posts, I tried to explain that the main source is the irregular inflow of water through the propeller. The usual solution on ships (generally at the design stage) is to increase the number of blades, exactly as 4-cylinder engines vibrate less than 2-cylinder ones. To avoid detection by submarines, many warships have 5-bladed propellers. Another solution is to have skewed blades, bent like a saber.

In the present case, both solutions would not be practical because they would increase drag when sailing, without even considering the cost.

Alain

[sww914]

Lord Fusor makes adhesives for that sort of thing, they developed a 2 part adhesive to cold vulcanize automotive motor mounts together. They also make adhesive to glue a bedside on to a pickup or a quarter panel on to a car without welding. Very fast, money making process. It may be possible to repair that coupler but 2 things are important to know. No glue could ever hold anything that dirty together. You would still need to remove the coupler for the repair to have adequate access to the pieces that need cleaning. The right adhesive may cost more than the part. The Fusor panel bonding adhesive that I bought recently was $40.00, the mixing tips are about $1.50 each and the gun to dispense it costs $100.00. The gun wouldn't be absolutely necessary in your case but I just like hearing myself type.

[OffSeason]

Thanks for all the input guys! I am going to call and see if I can get the part from the prop company in Victoria.

I am also considering the merits of going engineless. More space, less time in port monkeying around with the iron lung, thrown props, dirty bilge, sudden breakdowns. I know this is probably a frowned upon practise by many, but just the thought of it has got me thinking about how I would do things differently, and safely under wind and sail power alone.



All the best,

Mike