The Steamboating Forum

Good point.
I'm quite happy about the initial choice of propellor as identical boats have it, but the 0.75 " diameter shaft is my major concern as 7/8 and larger seem to be what others are referring to.
Your thoughts ......
Ian

Lopez Mike wrote: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.

Find a good prop shop. Any competent machinist with a large enough lathe can bore your prop out. They will need your shaft to match the taper. Unless they do lots of props and leave their taper attachment set up, they will need to cut and try. Cutting the new keyway is a more specialized operation but that's why you should go find a prop shop as they will have done this before or have found a shop that can.

Myself, I'm not that obsessed with keyways anyway. If the tapers are a good match, and you can make sure of this yourself with some lapping compound, it will never move anyway. There no keyways in a lathe tailstock and I've put some big drill bits through some ridiculous shock loads without them coming loose. No nut to keep them socked down either. Just a good whack with a hunk of lead or my big old lead hammer.

True enough, but on the other hand if it's 7/8 or larger that I should be looking at, maybe it's better to wait until the right diameter prop comes along.
I haven't started building the boat yet, but I do like to have as many of the components sorted before I start.
TTFN
Ian

Lopez Mike wrote:Find a good prop shop. Any competent machinist with a large enough lathe can bore your prop out. They will need your shaft to match the taper. Unless they do lots of props and leave their taper attachment set up, they will need to cut and try. Cutting the new keyway is a more specialized operation but that's why you should go find a prop shop as they will have done this before or have found a shop that can.

Myself, I'm not that obsessed with keyways anyway. If the tapers are a good match, and you can make sure of this yourself with some lapping compound, it will never move anyway. There no keyways in a lathe tailstock and I've put some big drill bits through some ridiculous shock loads without them coming loose. No nut to keep them socked down either. Just a good whack with a hunk of lead or my big old lead hammer.

Without looking over the prop I don't know how much metal is in the hub for potential machining but, again, I'd find a good prop shop.

Totally agree about the good tapers need no keys. Most Jacobs drill chucks simply go on the #33 Jacobs taper with no other means of securing. I have seen them come off, but it was an unusual event that caused it - bit hung in the work etc. And that is the reason an endmill should never be used in a drill chuck, the helical flutes are pulling down on it. If it comes off spinning it can cause injury.

The morse tapers in the tailstocks on most lathes don't have keyways, but do have a recess for a flat tang in the bottom that is for locking to the tools. If the tapers are in good shape it really isn't needed and should not be relied upon to hold the bit shank of whatever it is in there. The tang also serves as a way to get the tool out. When the tailstock fully retracts, there is a pin that pushes against that tang and ejects the tool.

For tapers in a prop, I do them in a CNC mill. Just create a helical toolpath with proper starting and ending diameter and correct height and let it go, no trial and error, comes out with perfect angle within a few thou ID accuracy.

For our very low horsepower engines, it would probably be fine to omit the taper altogether. Select the shaft size in stainless, run a drill and same size ream through the prop on center. Mill a short slot either side of the hole on the front of the prop (away from the propnut). Then drill and run a pin through the shaft that fits in the slot on the prop. Put a castle nut on it with cotter pin - no taper needed. Use a big outboard shear pin, may save the prop from getting damaged as well.

-Ron

I never thought about using a shear pin. It works on about a zillion outboards of all sizes. Too bad the splines for an outboard prop are such a mess to machine. I've seen some outboard props that were fairly long pitch.

ianrichards wrote:True enough, but on the other hand if it's 7/8 or larger that I should be looking at, maybe it's better to wait until the right diameter prop comes along.
I haven't started building the boat yet, but I do like to have as many of the components sorted before I start.

That's how I did it - I got opinions on the propeller size/pitch that would be appropriate for hull+engine and then bought a prop that was close enough to that. With my prop I had a shaft size and taper

-CB.

We should remember that most of the propellers for I/C engine boats are OK for a few hundred horsepower, and the shaft size is OK for that service too. A 1/2 inch (12mm) shaft size would be fine for a 5A Stuart, and the only reason to go bigger is because most available propellers have a far larger bore, or to be less likely to bend the shaft when grounding.

From Ron, "….. that is the reason an endmill should never be used in a drill chuck…"

Thanks for that bit of knowledge. I have used a drill chick for holding an end mill, and although it was a Morse taper with draw bolt, I experienced a ruined part as the mill dug too deep into the work.

From Ron, "For our very low horsepower engines, it would probably be fine to omit the taper altogether….."

For my next steamboat project I am planning on 15 horsepower, far more than 95% of the existing steam launches in this world. In order to test things, I installed a 14 horsepower IC engine, geared 3:1 to the prop, a 14 x 17 aluminum prop from an I/O drive which had a splined shaft.

I bored out the splines, and installed a one inch (25.4Mmm) straight steel shaft, fastened only with a typical roll pin and loctite. Bearing centers are at 83 inches. It is plenty strong enough for that service, so would be good for just about any steam launch.

This setup is on video on "Boat design.net" website, thread is "So much for the "Nay Sayers" (Powered O'Day Javelin 14.9 mph)" http://www.boatdesign.net/forums/

fredrosse wrote:"So much for the "Nay Sayers" (Powered O'Day Javelin 14.9 mph)" http://www.boatdesign.net/forums/

I had a nice chuckle at this thread because I usually only see you posting in the other position.
-CB

I assume you are referring to my asking a question rather than answering them? I am a decent engineer, but no Naval Architect, so I sometimes go on that forum hopefully for some knowledge. The boat design forum is often frustrating because so many people like to chime into the thread, without even answering the stated question. I notice that after all those people posted, I still have no answer to my question. The steamboating forum is far more civilized.