Technically a plain bush should be better as you have better support and the surface of the sintered bronze bush should absorb some lubrication and keep the bearing lubricated.

Unfortunately however far too many of the manufactured plain bush tubes are not machined to close enough tolerances and too frequently they are not even made of sintered bronze. In fact some are just plain brass. You can get away with this on slow turning shafts but performance ones such as you are talking about require the best possible alignement and closest tolerances.

Never having had any experience with performance models I am not in the best position to offer advice however I would say that ball bearings are probably the best chance of getting the closest clearances. They should also be double shielded although there is also a school of thought that the inside face should not be shielded to allow lubrication from the tube to access the balls.

My apologies, I've probably only confused things even more!

Edited By Richard Simpson on 11/06/2021 13:16:58

Ray, I'm not so sure it is the size that is the biggest concern but more the speed at which the shaft is rotating. In a performance model with a surface piercing prop where the shaft will be rotating very fast I would not use the same shaft arrangement that I consider suitable for a large diameter brass prop with a big pitch such as you might fit to a tug.

The higher the rotational speed the more critical the balance and alignment of all the propulsion system becomes, hence the quality of the shafts, bearings, couplings and motor etc. are all more of a consideration.

As an example the bog standard splined universal coupling is great for transmitting larger loads to a big prop but they are notoriously poorly balanced and would not be suitable for a high performance model. I have been through three on a particular Club 500 in an attempt to cut down vibration.

Here we are Ray!

obviously a ball raced shaft would be best, but not necessarily warranted.

A decent shaft as per J.perkins will/should be ok. How long is the shaft? Personally, anything over about 9 inches or so and I use a 5mm diameter one to prevent shaft-whip, especially at high revs, like 2000Kv and 3s.

As Richard suggests, the standard red plastic u/j are not up to very high revs, and a safer option is to get solid metal u/j types or a specialist type. Or even use two u/j.

My current build is a near 20,000 rpm job and, unable to get a die straight shaft I have used a rubber coupling one end and it runs as smooth as anyfink.

Ashley

Trevor, with all due respect, Mobile Marine deal with tugs, they are not the best to consult when it comes to revolutions over 5-6k , brushless are not within their remit.

For high reving brushless motors, the better firm to talk to would be Prestwich model boats.

Hi All,

If nothing else we may have scared Wayne off asking a question

Regards Ray

Hi Ashley,

Yes my serious IC boat has a flexible shaft and a 26cc Zenoah engine , I havn't run it yet it still scare's me just looking at it sitting on the stand

Regards Ray