Need fir a Key Way

[Goofeyfoot]

I am having a flexible coupling installed on a 33 foot sailboat with a 25 hp Betamarine diesel engine. It is a direct shaft drive.

When the yard removed the existing coupling, they observed that there is no keyway cut into the shaft. The coupling, as designed clearly contemplates a key. But there hasn’t been a key in there for over 20 years.

The yard wants to pull the prop, pull the shaft out, and cut a keyway. My layman‘s logic is that if it hasn’t been a problem for 20 years it shouldn’t be a problem now.

What do you think I should do? By the way, i changed from 20 hp to 25 hp which is the only significant change in the stock install.

Thanks.

Michael

[n33077]

If it is a Vetus coupler, the instructions say no key. They use pins

[Goofeyfoot]

Well I guess I should have said “the flange as designed…” contemplates a keyway. So do I need to have a keyway cut to accommodate the flange?

Sorry for confusion.

[capt jgw]

If you've increased the horsepower, maybe. The key makes sure the shaft turns with the coupler, but apparently the set screws have been doing the job till now. That is not a very secure connection and eventually would shear the ends off the screws. They're only supposed to keep the shaft from backing out, not take all the rotational force.

I once had to shorten a 1" shaft and then have a new key way cut in it. That only cost about $50 as I recall, but I did the removal and reinstall myself.

[Wotname]

Is it a taper or parallel fit.

If taper and the fit is good i.e. more than 90% contact between the two surfaces, then the key doesn't take any torque anyway.

If parallel, I would be far more cautious and fit a key.

[HeywoodJ]

Might be far cheaper to purchase and fit a split coupling that is designed from the start for use without a keyway. Lots of options in that department, R&D link below, but Vetus, Volvo and lots of other places sell them. We’ve had them on our 40HP engines (30mm shaft) going on 8 years with no issues (he says, knocking on wood). R&D seems to add a key by default above about 1”, 1” and below are default without key (although you can get a key if you like).

https://www.generalpropeller.com/inb...plings/202-153

R and D MARINE : Flexible Shaft Couplings

[Goofeyfoot]

Is a split coupling the one that comes apart into halves and clamps together? Because that’s what this one is.

[Wotname]

Originally Posted by Goofeyfoot

Is a split coupling the one that comes apart into halves and clamps together? Because that’s what this one is.

Yes.

Me - I would use some bearing blue to ensure you are getting a good surface to surface fit and if so (say 70+%), ignore the key for this sized shaft / hp.

[CAR54]

Two years ago I installed a split coupling on my J28 from General Propeller on a one inch propeller shaft with a 2GM20F 18 hp. Engine. It was manufactured with two slots cut into it 180 degrees apart, that by design requires a steel wedge inserted into each slot and then tap the wedges in with a hammer that opens the coupling allowing it to slide on the propeller shaft to position it properly and requires no key way. Then torque the four bolts to specifications as per instructions. After an hour of operation I re torqued the bolts and picked up 3/4 of a turn on each of them. Customer support at General Propeller was excellent, they were concerned, but also said they had never had a customer re torque the bolts. I told them after a few more hours I would check the torque the bolts again and informed them that the bolts remained torqued to spec and they seemed relieved and thanked me for keeping them posted.

I would highly recommend using their coupling, it saved me from paying the yard to pick the boat, drop the rudder, take the propeller shaft out to a machine shop to have a standard coupling machined to fit the prop shaft and then re install.

[jhulmer]

Yes it needs the keyway. If the designer of the coupling didn't think it need a key he wouldn't have put a keyway in the coupling. I've never seen one without a key.

[Cadence]

Add a keyway. JMHO