Small Steam Boat

[BlueBell]

You may want to try three blades before machining that hub.

If you make the transom joint articulating you can also make for steering.
But it gets complicated fast...

 

[jehardiman]

Hah, I was literally working on another set of blades today to upgrade to a four blade prop. I was considering trying surface piercing out first. If it doesn't work, I can keep the hole and make it a stern drive type thing. Someone else suggested making the transom connection a rotatable joint, so you can rotate the assembly up out of the water for launching and weed clearing.

No, because you have the torque in a steam engine, you want the largest, slowest turning prop you can fit...the Wageningen B-2.35 was not a random selection, but an optimal one. It would be better to fit a weed cage than a less effective prop.

 

[Rumars]

Because steam engines are a perfect match for large diameter very high efficiency (i.e. >85%) props.

It is as you say, I just don't see the benefits for this particular boat. It is much simpler to speed up the prop via a chain or belt drive and make use of the wide availability of cheap plastic trolling motor props. This also solves some installation problems like excessive shaft angle.

An option I didn't mention is to make a steam outboard. A short steam hose section will solve the issue of having the boiler in the boat and the engine on the transom.

 

[jehardiman]

While the prop shown is obviously not the correct one...(lots to comment about here)...this is much closer to the correct one than other models. While I don't support his options, I do support his selections.

 

[Rumars]

The "ideal" prop represents a problem. To achieve 5mph with 1/2hp at 300rpm a ~900lbs boat (he wants to take the family) needs a 20-24" x 24" prop. If shaft angle is to stay under 15° the engine and boiler end up somewhere in the first third of the boat. Weight distribution aside this invites ergonomy problems.
Everything can be solved of course, but is it worth the complications?

 

[rolphill]

The "ideal" prop represents a problem. To achieve 5mph with 1/2hp at 300rpm a ~400lbs boat (he wants to take the family) needs a 20-24" x 24" prop. If shaft angle is to stay under 15° the engine and boiler end up somewhere in the first third of the boat. Weight distribution aside this invites ergonomy problems.
Everything can be solved of course, but is it worth the complications?

Where did you get these numbers? I haven't really found any info on how to calculate this kind of thing, so I was just looking at other steam boats for inspiration.

 

[Rumars]

You should buy Gerr's Propeller Handbook but online calculators should give you similar results.

Displacement (weight) is an important factor, I made a mistake writing my previous post (corrected, was thinking in kg's). This kind of boat is usually rated around 900lbs, wich is realistic considering the hull weight, power plant, essential gear and 3-4 people.
The hull form with its immersed transom is what it is, can't change that.

 

[rolphill]

I managed to get a copy of the handbook, and I'm reading through it. The displacement speed formula gives me an estimated speed of 3 knots, which is about what my ideal speed goal was.

In the meantime, I got new wood for the gunnels, and for whatever reason it's much more flexible. I've got one on already. I also made 2 more blades, and started working on the stern tube. I printed a 4 blade hub, and am working on printing a shroud for it. The stern tube will use a standard mechanical shaft seal, and a PLA shaft bearing which doubles as the thrust bearing. The shaft will be 12mm stainless steel, with a keyway. PLA is hard and slippery, so it makes good bearings as long as it doesn't heat up.

 

[gonzo]

PLA is hard and slippery, so it makes good bearings as long as it doesn't heat up.

What is the compression strenght? Also, the bearing will get hot unless there is some kind of cooling, which would be rather complicated.

 

[Rumars]

That bushing will not act as a thrust bearing. If you don't want to use commercial roller bearings rated for axial loads, machine a traditional thrust block and insure lubrication.

 

[gonzo]

Thrust bearings are not expensive: McMaster-Carr https://www.mcmaster.com/products/bearing-races/for-load-direction~thrust/needle-roller-thrust-bearings/?s=thrust+bearing+races