Flexible shaft coupling

[Nasher]

Hello all.

The little restoration project I'm working on at the moment involves a small inboard engine and shaft drive prop.

I've never had the engine fitted to the boat so have no original fittings.

I've sourced some aluminium to machine up a flexible coupling, and will be aligning the engine output shaft and propshaft really well.

However today when looking at it I had a thought.

The propshaft and engine output shaft are only 5/8in in diameter.
The engine itself is very low power and torque, a 4.5HP 2-stroke, and it will drive a small 8x7 prop.

So the thought occurred, that considering the low torque/HP, and the good alignment I'll achieve, why don't I just join the two shafts with a length of thick wall rubber hose pushed over and clamped to each shaft with a small gap between the shafts inside the hose.

There is no fwd/rev gearbox or even clutch in the craft, it wasn't designed with one in mind, the engine is permanently connected to the prop.
I do want a flexible coupling, mainly as the craft is of aluminium riveted construction and I'll need to isolate the engine from the bearers on some sort of rubber mount to keep the vibration and noise down. To my mind that means I have to have a flexible coupling to the shaft as well.

What do you think?
Would it last very long?
Am I being daft?

I am also still looking for a shaft seal/stuffing box, but it's so small I think I'm going to have to machine that from scratch as well.

Nasher.

[sean-nós]

Hi and welcome, as the engine will be on rubber mounts could you run the prop shaft straight out to the P-bracket without having a join or it coming into contact with the hull. I don't have a photo of mine with the stuffing box on but it is connected to the end of the fiberglass tube sticking up by a rubber pipe and no vibration is felt.

[Nasher]

Thanks for that, it's worth a thought.

I think I need to take some images so you guys can see what I'm up against, but I won't be home during daylight until Saturday.

Nasher.

[solitaire]

Bear in mind that any conventional inboard engine installations uses the propeller thrust transmitted via the shaft, to the engine bearers to move the vessel - the thrust needs to be absorbed into the bearers before it gets to the end of the engine crankshaft, often on the shaft, or gearbox with a thrust bearing.
Even the stuat turner 11/2hp and 4hp engines have thrust bearings in their gearboxes.
Engines are not normally designed to take thrust on the end of their crankshafts, and the internal crank thrust bearings are only designed to take small amounts of end-float in the crank
Flexible shaft couplings that have the prop thrusting through them usually have a means of keeping the parts of the coupling in line, - a spigot!, and universal couplings (u/j's) can only be used where there is no thrust.
The normal acceptable shaft to engine coupling misalignment should be no more than 0.002" (old money) on the coupling face and side to side - - remember to machine a register on the coupling faces to retain alignment.
Hope this helps..........

[solitaire]

The smallest shaft I have seen is 3/4" - used on the Stuart Turner 11/2hp and 4hp engines - if you take a look at the pictures attached to item no 181636889323 currently on ebay you will see some installation ideas - I have a s/h 3/4" shaft log like the one in the pictures, if your interested - pm me

[pspeed]

Hi,

Even a 4.5 hp will develop a fair amount of torque, I reckon it would just tear the tube apart in no time, that's even if the clamps could hold on.

I've never seen it done that way and have worked in the marine industry in some way for years, and as someone else has said, you would have to have some means of taking the thrust other than directly on the end of the crank.
Personally I would use one of the more recognised methods.

Regds,

Phil.

[Nasher]

All

I've finally managed to get some images of what I'm trying to do.

These are what the engine goes into.
A Mk9 Military canoe from the late 40s which is a development of the Cockleshell heroes Canoes.

This is a historic image of what it looks like fully kitted out.

Another historic image of the engine in the boat

My engine/Gearbox combination without exhaust or fuel tank fitted.

Fits in here

More images in next post.

[Nasher]

The engine goes in like so

And this is where I need to connect it to the prop shaft.
Luckily all the angles etc are correct, I just need to adjust the height of the unit with shims.
Note the integral water pump on the gearbox.

I've discovered Range Rovers have a Rubber doughnut type coupling in their steering column, so will get hold of one to see how big it is.

I'm also going to see what other small rubber Doughnut type couplings I can find.
I have a Hillman Imp one in the garage but it's too big.

Nasher

[sean-nós]

Very interesting little project, I love it My shaft is fitted direct to the engine with the coupling and any vibration is absorbed it the rubber in the cutlass bearing and engine mounts, if you can get the engine lined up right with the shaft it should be no problem. My stuffing box is filled with grease and packing you might be able to double up on the packing to get a good seal around your shaft.

[Nasher]

Sean-nós.

Hi and thanks for your reply.

We really couldn’t be at more opposite ends of the spectrum with regard to power, performance, etc, but it’s really interesting to see your set-up. You’re youtube vids are great by the way.

Because the shaft is so short, and my space to work so tight, I can’t see how I can get the shafts to line up closely enough to be able to do away with a flexible coupling, so want to persist with fitting one.
Plus being an Aluminium tube I need to do all I can to isolate the engine vibration from it.

Nasher