The Steamboating Forum

Good morning gentlemen.
Another question to chew over.
Cutlass Bearing V Bronze Outer Bearing ?
Which way to go ?
What are the advantages of each type?
Thanks as always.
Ian

The only bearing I've ever seen used in a strut is a cutlass bearing. Running a close fitting bronze bushing seems as though it would lock up on the slightest bit of sand etc. This is the kingdom of inland power boating here in Michigan surrounded by the Great lakes and 10,000 inland lakes, I've never seen anything other than a cutlass bearing used on an inboard propshaft.

-Ron

I’ve been advised both ways, but the local consensus is towards a cutlass bearing and I respect their advice ... and yours to.
So a cutless bearing it’ll be.
Something like this.
Thanks

The only thing I've ever had to do to a cutlass bearing is give it a splash of water if you are running the engine with the boat out of water. It will squeak and carry on to tell you it needs some moisture.

The prop shaft on my electric boat, one inch diameter, is supported by an external strut, and it uses an UHMW food grade bearing (Ultra High Molecular Weight). Water lubrication, in service since 2004 with no problems.

I never thought of UHMW in this service. I use that stuff all the time in all sorts of similar applications. Also, if the shaft is free of roughness, it can run happily with no lubrication.

The trick will be to come up with the right sized hole in the boat so that the UHMW doesn't shrink down too tight on the shaft. Or make it a slide fit and secure it in some other way like a set screw.

I have an external strut on the boat, fairly far from the hull proper. A metal strut, 2 inch x 1/4 inch flat bar in a "V" shape holds the shaft bearing about 10 inches away from the hull. Turning a 14 x 17 three blade propeller. The bearing has a self aligning feature, and uses no collars or set screws. The spherical housing is stainless steel.

This setup has worked well since 2004, with a 3/4 horsepower electric motor at 400 RPM prop speed. A couple of years ago I put in a 15 horsepower gasoline engine, turning the same shaft/prop arrangement at 1200 RPM, and everything seems to be holding up well.
Now if I can only find the time to put together the 15 horsepower steam plant.

I am at the point of putting in the stern tube in my new hull. I was going to order a nonmetallic cutlass bearing but I have plenty of UHMW about the shop. I wonder what the traditional clearance is between a cutlass bearing and a shaft? .001" per inch? The used one I have in hand seems to be almost zero clearance.

My stern packing needs a very small bit of water when under way to lubricate the flax. The usual rubber cutlass bearing has longitudinal grooves which allow free flow of water past the bearing. Perhaps I should provide some such flow opportunity.

With the relatively low power of our steam launches, I think the need for water circulation to cool the bearing is not necessary. Water lubrication yes, but that will happen as long as at least one end of the bearing is submerged.

Bearing clearance of 0.001 inch per inch of shaft diameter is the standard for typical journals with metal sleeve bearings and oil lubrication, like the crankshaft mains, connecting rod big end, etc. For the flexible plastic/rubber bearings on a marine prop shaft a a few hundred RPM, I would not be afraid with a couple of thousandths extra clearance, assuring water intrusion for lubrication, and allowing for any swelling of the bearing material.

I second or third the UHMW recommendation if used properly. I've worked with it a lot over the years in automotive tooling and its resiliency to wear is outstanding. As Mike points out, it may be a bit difficult to keep it secure in a sleeve, while it's wear characteristics are favorable, it's ability to hold shape in close fitting components is not. The way I've done it in the past is machine it about .005 oversize diameter and press it in the sleeve and then bore it to size. Standard Reaming or drilling is always going to result in an undersized hole through the UHMW, it takes very sharp tooling to achieve dimensional accuracy. Delrin acetal on the other hand has excellent machinability and better shape retention, but does not have the same wear ability as UHMW. Considering the service requirement, I think the Delrin would be a better option here running on a polished prop shaft. Both are easily damaged by abrasive surfaces, sharp steel edges etc. When securing either with threaded fasteners, the standard method is holes with helicoils, which works very well.

-Ron