The Steamboating Forum

What would be the most suitable prop shaft diameter for a Stuart 5a engine?
Are other details required to determine this ?
Thanks
Ian

The torque requirement isn't high, and 7/8" would be enough using most normal prop shaft materials. However, prop shafts vary a lot in length, and more importantly in unsupported length, and you may need to go up to 1" or even 1.25" to avoid whipping. A centre bearing would sort that with a thin shaft, but it's a horrible place to seat and maintain a bearing, and most of us with small boats try to avoid such things. I think the SBA Steamboating Handbook has a paragraph or two on this.

I would have thought 7/8 would be easily enough. I have an inch shaft swinging a 23 inch prop, so 7/8 would be quite sufficent. The standard rule for bearing spacings is: Bearings spaced no closer than 20 times the shaft diameter, and no further apart than 40 times the shaft diameter.
Daniel

Food for thought
Many thanks
Ian

Is that a prohibition of having the bearings closer than 20 diameters or that it's not needed.

I'm trying to figure out why you couldn't have one bearing after another side by side if you were nuts enough to do it.

Mike, if Daniel will forgive me, I think the 20 to 40 times diameter rule is straight from the Steamboating Handbook, and it isn't worded very well there either. I think it just means that 20x or less diameter is just getting a bit silly if you aren't doing it for other reasons.

I suspect the 40x upper limit is a (very reasonable) limit set by observed problems and a safety margin. It's not directly to do with Euler buckling from end loading according to my calibrated back of envelope, whereas it clearly will be affected by off centre loading from both the engine and prop, and those things will vary wildly.

Perhaps we also ought to postulate a maximum rigid length, with central bearings, because our small boats, particularly wooden and fibreglass ones, do distort quite a lot during use and over time.

On the heavy tug, I have a 1" stainless shaft swinging a 21" 4-blade, never had any issues with it. The cutless bearing is right at the prop mounted in the strut. One main consideration for shaft diameter is running the prop aground and bending the shaft, it should be able to withstand this to a reasonable extent. My Chris Craft has a 215 hp engine and uses a 1-1/8" shaft, and how I arrived at 1" being ample for the tug at roughly 8hp, I accidentally ran over a submerged rock with it and it wadded the prop up, but the shaft was unharmed. If the prop is considerable distance from the strut or other supporting bearing, then up-sizing the shaft should be considered. On what is needed for the simple transmission of power, smaller launches with shorter propshafts could probably get by with 5/8" diameter, but it would be damaged easily.

-Ron

Yeah, the 40x number makes sense from the point of shaft whip and such.

My launch has an 18 x 24 prop and a 1.25" shaft which is way over kill. I think the guy that collected the bits to build it got his prop first and it was bored for 1.25" and had not a clue that you can sleeve a prop for a smaller shaft.

If I ever burn this hull I will more than likely go down to 1".

I've come across a 3 blades 14" diameter, 20" pitch, which another builder has used on an identical boat to the one I'll be building.
It is .75" diameter.
Would this be ok for my. 15ft 5a powered launch ?
Thanks
Ian

There's all sorts of graphs and formulas that attempt to help with this stuff but I suspect that they can only get you in the ball park. It's not so cut and dried.

Until you get the beast steaming you won't know about the difficult to predict things like whether the boat has a sweet spot at a certain r.p.m. It's not an electric motor.

I'm only on my second prop.