Prop shaft size

[marinesteam]

Hello all,

Need a bit of guidance on selecting a prop shaft diameter. I currently have the prop flange chucked up and bored to 1-1/4" and would like to have a few questions answered before I remove it from the chuck.

So here is my dilemma. I have a lovely vintage 18x24 steam wheel http://www.thesteamboatingforum.net/for ... ?f=7&t=690. It appears to be bored for a 1-1/4" shaft (1.22"dia - 1.05" dia tapered bore w/ 1/4 keyway).

The plans for the York engine recommend a 1-1/2" dia prop shaft.

The hull that I have on the drawing board is a 27' fast gentleman's launch. Seems that 24" pitch may not be enough for this hull, I'm thinking 26"-28" would be more appropriate. Don't know if I can get much more diameter under her than 18" however (maybe 20"?). This may not be the hull I end up building but am currently heavily leaning that direction.

Being unfamiliar with what is possible regarding prop modifications would it be realistic to bore the prop for a 1-1/2 shaft and what is the maximum pitch I can expect to get if re-pitched? Is there any danger having this done on a vintage bronze prop? Should I leave this one alone and search for another with the right pitch and bore (looking like it may be hard to find)?

The best plan might be to leave the flange as it is currently with the 1-1/4" bore and bore to 1-1/2" later if needed. It would be easier, and would be more true to bore to 1-1/2" now while it's in the chuck, if boring to 1-1/2" is the more appropriate size for the engine.

What should I be looking at as the main driving factors in determining the prop shaft diameter?

Thanks

Ken

[gondolier88]

Ken,

At 10hp an 1 1/2" shaft is about right- you may be able to get that down to 1 1/4" if it was a fairly short run (unlikely in a 27ft'er!)

The main factors- first; torsion imparted through the reaction to propeller forces when making quick starts or fast manouvering in an emergency for example. Second- torsion and shock loading imparted by hitting a rock or other submersed object.

You can limp home with a bent propeller. You can't go anywhere with a badly bent prop shaft.

It is a fairly simple matter to re-bore the prop', it has been done many times- just be careful that it leaves enough land around the new bore- remembering the keyway will be that much nearer the OD of the hub.

If it were me I would be looking for about 30" pitch for the size of engine you have- interestingly, looking at the AG Mumford catalogue from 1900 which I have next to me:

Compound Surface-Condensing
Launch Machinery

3" + 6" x 5" Compound Engine, rated at 8hp

Required a prop' of 22" dia. x 30" pitch.

Length of stern tube- 2' 9"

Engine room space LOA- 7' 9" (using a large dia. single flue/cross tube vertical boiler of 120psi.)

An interesting comparison to your proposal I think.

Greg

[marinesteam]

Thanks Greg,

That's pretty much what I expected. I'll plan in using a 1-1/2" prop shaft and machine the flange to accommodate. I'll take care of the prop at a later time.

Ken