The Steamboating Forum

I found a large twin cylinder compressor pump that I believe would be great for steam conversion. I have a few questions about this conversion process for the experts that are on these forums, if you guys wouldn't mind helping me.

I've never done a steam conversion before, the most I've ever done was converting a small single cylinder compressor pump to be a compressed air engine and assembling a single cylinder model steam engine from PM research. I am pretty mechanically inclined and I have tools, I don't have a lathe but I have a drill press and a welder and basically every hand tool that one will ever need for mechanical work.

The specific pump I am looking at is a rolair k-35. It is 65 cubic inches, is rated for 800 rpm max and 140 psi max. The bore is 3.54 inches and the stroke is 3.31 inches. It has an aluminum crankcase, aluminum cylinder head, cast iron cylinders, and ball main bearings. The flywheel is also nice and big, 15.16 inches in diameter.

Here's the pump if you want to look at more details:
https://www.ebay.com/itm/291454727005

So now that I gave all the specs for this monster, I have a few questions.

My first question, what kind of horsepower would this put out if I was running it on 120 psi of steam? I was estimating 8-10 horsepower at 600 rpm. I've only ever calculated steam engine horsepower for double acting steam engines, and I'm not great at math. I wouldn't want to crank it up any higher than 600 rpm most of the time because I want it to last a long time.

My second question, what would be the easiest way for me to convert it to a reliable steam engine? Is there a way I can convert it without having a vertical mill or other expensive equipment?

I was thinking of using a 4 way 2 position pneumatic spool valve somehow operated by a camshaft as the valve to control the inlet and exhaust of steam to each cylinder. Theoretically this would work, since they are designed to supply air to a double acting pneumatic cylinder, allowing flow into one side while routing the other side to the exhaust outlet. If it were attached to a single acting twin cylinder steam engine, it would basically work in the same manner, supplying steam to one cylinder while exhausting the other cylinder.

The only problem I foresee with this idea is the flow rate, the valve might be too restrictive to allow enough steam to enter the engine. I believe the openings on the valve are all 3/8 npt.

Would that be a problem or would that allow enough steam to go where it needs to in order to run this engine and produce good amounts of power?

Here's the valve I was thinking of using for this:
https://www.grainger.com/product/PARKER ... alve-3FEJ1

The valve is rated for 200 degrees Fahrenheit and 225 psi, so the temperature rating might be problematic.


I'm not trying to reinvent the wheel here, I want something that is simple to put together and works. The simpler it is the better in my opinion. This will not be going on a showboat, in fact it will be going on a large pontoon boat with a sternwheel. This will be mainly used on a large river where the sternwheel has a huge advantage over a large propeller.


If you're wondering what kind of boiler I'd be using, I'd use a wood fired water tube boiler. I haven't designed the boiler in great detail yet, but it will use 3/4 inch copper refrigeration tubing and a steam safety valve set at 200 psi. The shell will be made of steel. Like the engine I want it to be simple and reliable. I will hydrostatically test it to 400 or 500 psi before I ever fire it up.

Thanks for your help, I will really appreciate any expert advice from people who have experience with these conversions or any steam engine experience at all.

Bart?? Where are you? Help this person.

Hi Mike -

I've been clearing land for the new shop... chainsaw & tractor work. I've cut and stacked 4' rounds from a bunch of trees; I've got steamboat fuel for years - several cords worth. Excavator should show tomorrow.

I converted a twin cylinder refrigeration compressor 2 x 2.5" and ran it in my steam launch Otter for many years. It produced something like 2 hp at 170 psi; the bearings eventually failed. I'd guess you'll probably get 8 hp out of such a unit if you push it like I did that little compressor, but it'll use a fair amount of steam and won't last as long.

The flow rate on that valve isn't going to be what you want. Figure you want average flow velocities not to exceed 6000 fpm; you want your ports & passages to be at least a 1/2 square inch for this - 2" x 1/4" for a square port at 600 rpm.

I'd make a D-valve; they're not wonderful from a efficiency standpoint, but they're simple, self-adjusting and easy to understand. You do need a 'pocket', which would be easiest produced on a mill drill, but you can also use silver solder to assemble a bronze d-valve from pieces and lap it flat. Silver solder is the steamboater's friend; yes, it's not cheap but a little goes a long way and you can avoid using large hunks of expensive materials and producing a lot of expensive chips.

Here's some pictures of Otter's old engine: http://www.smaalders.net/bart/engine.html

Best valve materials are cast iron on cast iron, but bronze on cast iron works well, too.
Steam chest can be aluminum (6061 or other corrosion resistant alloy) .

You're going to need a LOT of steam to spin that motor quickly.

For a simple boiler, weld one up from steel pipe, esp. if you're using solid fuel. I've built a steel boiler with copper coiled tubes silver soldered in, and it works, but it's really easy to blow a tube if things go wrong.
Otter's boiler: http://www.smaalders.net/bart/boiler.html.

All steel watertube boilers are far more forgiving of momentary lapses of attention or feed pump issues or ...

I've got to race off for a picnic - happy to answer more questions later.

- Bart