Shaft couplings for repower

[Mancave2011]

I am repowering my Columbia 8.3 with a Yanmar 2GM20 that uses a Kanzaki KM2P transmission. I removed the old Atomic 4 so my boat has an existing 3/4 shaft with a three bolt coupling but the KM2P has a four bolt flange. What is the best plan? Change the shaft coupling or adapt the transmission flange?

[Jim Cate]

It is likely that it would be less expensive to change the coupling than to have a shop make up the adapter. If you can fabricate it yourself, that's different!

BTW, have you checked that the 3/4 inch shaft is adequate for the new engine/reduction gear? Seems intuitively a bit skinny, but I don't have the references handy.

Cheers,

Jim

[Mancave2011]

Thanks for the quick reply, Jim. I questioned the shaft diameter myself but the Columbia 8.3 offered a Volvo Penta option using the same shaft as the stock Atomic 4. I have been considering replacing the shaft with something thicker.

[weephee]

I am in the process of doing the same thing with my 8.3 only I am re-powering with a 2qm15. My engine came with a shaft flange but was for a 1 inch shaft. I took it to a machine shop and had an insert made thereby reducing it to 3/4 inch. If you don't have one, it might be easier to look on ebay or elsewhere and buy the proper flange and adapt it. My biggest problem has been getting the engine compartment reduced in width and getting the new engine brackets positioned properly for correct engine 15 degree alignment. Don't know where you are in the process but I do have some pictures and experience now in doing this if you need any help.

[Mancave2011]

How is your repower progressing? So far most of my efforts have gone well however pulling the old Atomic 4 out stretched the limits of my abilities. I had planned to look for inserts to reduce the 1 inch transmission flange but the real problem for me is the bolt pattern. Going from a 4 bolt to a 3 bolt flange doesn't seem likely. I have an aluminum cradle that looks like it will accept the motor mounts. This past week I have been going over the motor replacing filters, impellers, gaskets and any other thing that needs regular maintenance before I drop her in. If its not too much of a problem I would love to see photos of your project.

Thanks
JC

[Boracay]

You don't say if there is a flexible coupler. If not then I'd consider using one.

If there is a flexible coupler that adapts from four to three to four bolts then that could be used.

If it's possible to redrill the transmission or shaft flange then that's going to be the cheapest way.

Otherwise a new shaft flange properly sized and drilled to fit may be the best way.

[Terra Nova]

Replace the shaft coupling. Yanmar has the correct mating 1/2. Don't bush an oversize coupling--get the correct size or bore an un-bored one.

[weephee]

I will send you some pictures later today. BTW have you considered that you will need a new prop. The A4 prop is RH 12 x 6 and the yanmar prop is RH 15 x 9 for the Columbia 8.3. As per the previous comment on getting the correct flange from Yanmar, I was unable to find the 4 hole flange for a 3/4 shaft. If someone knows the part number that would be great.

[OceanSeaSpray]

Originally Posted by Boracay

You don't say if there is a flexible coupler. If not then I'd consider using one.

I have thrown several shaft coupling out on boats with great benefits... they are not the "universal, no-brainer solution one should always adopt" as often touted.

If your shaft is supported by the rear cutless bearing and the gearbox only (floating shaft seal), you should not use a flexible coupling, but you may get away with one with short shafts. If the shaft is also long, it can cause it to wobble out of control. Many floating shaft installations run much smoother without a flexible coupling.

If your shaft is also supported at the inboard end of the tube by a bearing or the shaft seal arrangement, then you must use a flexible coupling, because this is not a floating shaft any more and it can't follow the movements of the engine.

I initially found this the hard way and... it happens to be fully consistent with engine installation guidelines from Volvo.

[weephee]

Oceanseaspray, just for clarification. I have a cutlass bearing on the outboard end and a stuffing box about a foot out from the flange on the inboard end. The stuffing box is attached to a rubber hose which is attached to the rigid stern tube. I guess you could say the stuffing box is semi floating. In this case would you recommend a flex coupling.

[hpeer]

Split shaft coupling

-- PYI Inc. Max-Prop PSS Shaft Seal Seaview Radar Mounts R&D --

[weephee]

This is the flex coupler I have which came with the 2qm15 yanmar engine I am installing. The shaft length in my case is 46 inches long. Not sure if I should use it or go with the straight flange to flange rigid coupling.

[OceanSeaSpray]

Originally Posted by weephee

Oceanseaspray, just for clarification. I have a cutlass bearing on the outboard end and a stuffing box about a foot out from the flange on the inboard end. The stuffing box is attached to a rubber hose which is attached to the rigid stern tube. I guess you could say the stuffing box is semi floating. In this case would you recommend a flex coupling.

It depends on the amount of "give" you have with the stuffing box, but in general this would be termed a floating shaft. Making the rubber hose just a little longer can help relieving any loads on the stuffing box.

With a 46' shaft, you are probably still just in the zone where it would work either way. Disconnect the shaft and give it a pull sideways to see how much you can move it without using a lot of force. It just needs to be able to follow the engine when everything is coupled.

Regards,

Eric

[Mancave2011]

Well, things are moving along nicely and the Yanmar is nearly ready for installation. Thanks to all for the help. Now I need to consider the propeller. Clearly the Atomic 4 prop will no longer do but I would like to test the system using it before finding a new one. The original prop has a keyway with a nylon sleeve that goes about 3/4 of the way into the prop shaft opening. The original key seems to be fitted for that sleeve. Is this a normal setup or is this somehow rigged to make it work?

[weephee]

If I understand you correctly, it sounds like the prop is/was oversized for the 3/4 inch diameter shaft so someone used a nylon insert. You can also get them in bronze which is better IMHO. If I am assuming correctly, and if I were doing it, I would either remove the nylon insert or reducer, determine its size ie. outside dia to inside diameter ratio and order a bronze one for approx $50 or alternatively find the right size prop with the proper match to your existing 3/4 inch drive shaft.
http://www.tacomapropeller.com/parts...anchor-bushing