The Steamboating Forum

I'm building a 62-inch steam powered model of the snagboat W.T. Preston. I need some advise on building the steam engine for this model. I'd like to build it with a 5/8" bore and 2.25" stroke and sliding valve setup for the two cylinders. Can an engine be built with such a long stroke and small bore that will run smoothly at this size? I need it to start and run without hesitating with a max operating speed of around 110 rpm. The paddlwheel is 6.5 inches in diameter and 7.5 inches wide. I'm building the paddlewheel out of brass, steel and wood and will include scale nuts and bolts (356, 00-90 brass hex nuts alone) to hold the 48 paddlewheel blades in place. The hull is completed and most of the paddlewheel parts cut. See photos of the actual W.T. Preston at: http://museum.cityofanacortes.org/preston.htm

Build photos will be placed on my website at: web.me.com/hookpilot. Click on "Hobbies"

If anyone has a set of drawings for steam engines with a bore/stroke ratio of around 1:3.75 that I could scale to my size, I'd appreciate it.

Thanks

Mike in Seattle, WA USA

Hello Hookpilot,

My guess is that your engine won't run smoothly at such a low RPM because ot the small scale, because of the high stroke to bore ratio and because of the high steam condensation rate - that phenomenon occurs especially in small models. When steam condensates at a high rate, its available working power plummets.

I think you can partly avoid this by superheating steam and by turning your engine into a uniflow one: its better thermal efficiency should reduce the condensation, resulting into a higher working power available, hence a smoother run. The counterpart will be a seriously shortened stroke...

I'm wondering if 110 RPM is sustainably achievable with such a little engine.

Nevertheless, it's a very interesting project! Keep us posted!

Akitene

Akitene wrote:Hello Hookpilot,

My guess is that your engine won't run smoothly at such a low RPM because ot the small scale, because of the high stroke to bore ratio and because of the high steam condensation rate - that phenomenon occurs especially in small models. When steam condensates at a high rate, its available working power plummets.

I think you can partly avoid this by superheating steam and by turning your engine into a uniflow one: its better thermal efficiency should reduce the condensation, resulting into a higher working power available, hence a smoother run. The counterpart will be a seriously shortened stroke...

I'm wondering if 110 RPM is sustainably achievable with such a little engine.

Nevertheless, it's a very interesting project! Keep us posted!

Akitene

Thanks for the information. The boiler I'm using has a superheater, a unit built in England. See http://www.maccsteam.com. I can't shorten the stroke and still keep the paddlewheel anywhere near scale. I have seen a similar cut-off non condensing steam engine turning the same size paddlewheel. It was operating on compressed air though. See it at: www.youtube.com/watch?v=p0OmPBQMUXU

I could increase the bore without changing the stroke and that might make it work smoothly.

Mike

I have to disagree here, a two cylinder double acting steam engine will run very well with saturated steam at moderate pressure. The trick here is to have nearly zero angle of advance, so that steam is admitted almost throughout the power stroke, and exhaust is open almost throughout the return stroke. This arrangement will provide steady motion to the paddlewheel. This setup is not economical of steam, but will work well, and the small toy steam engine boilers or slightly larger will do the trick.

A uniflow engine would, by its very nature, have considerable compression, and hence tend to be erratic in its rotational speed, unless running at higher RPM with a decent flywheel.

Fred:
I hope I've got your name correct... I like your analysis. Especially if I can keep the 5/8" bore. It sounds like accurate valving will be the key to a smooth running low rpm engine. If I understand you correctly, the steam chest valving has to admit steam throughout the entire power stroke while at the same time allowing exhaust steam to exit the cylinder for all the entire exhaust stroke. The boiler I'm looking at is able to produce more than enough steam to run the engine. I will also have sensors to keep track of water level and steam demand and be able to adjust the flame for both safety and increased run time. I'll also have a water injector to fill the boiler under steam.

Do you know of plans I can look over showing the valving I need to incorporate?

Thanks,

Mike in Edmonds WA

You understand correctly. I have a Jensen model 55 engine (2 cylinder, double acting, 1/2 inch bore x 5/8 inch stroke, with Stephenson reverse valve gear) which can be throttled down to under 100 RPM without any noticable oscillations in speed. It is self starting in forward or reverse. You can look on Youtube to see this engine running, although the views I found were with the engine running fast.

I would think you could copy this engine, and its valve events (steam and exhaust lap, eccentrics, advance angles, etc.), with a longer stroke, and have a very satisfactory paddlewheel driver.

fredrosse wrote:You understand correctly. I have a Jensen model 55 engine (2 cylinder, double acting, 1/2 inch bore x 5/8 inch stroke, with Stephenson reverse valve gear) which can be throttled down to under 100 RPM without any noticable oscillations in speed. It is self starting in forward or reverse. You can look on Youtube to see this engine running, although the views I found were with the engine running fast.

I would think you could copy this engine, and its valve events (steam and exhaust lap, eccentrics, advance angles, etc.), with a longer stroke, and have a very satisfactory paddlewheel driver.

Thanks for the information. It looks like a straight forward modification of the Jensen 55. One last question: Do I set the paddlewheel shaft cranks exactly 90 degrees apart? Someone told me to set them a few degrees off the 90 degree mark. That doesn't make sense.

Mike

I would set the cranks at 90 degrees, there would give the most uniform turning torque to the machine. Being off by a few degrees will not matter much, but closer to 90 is better.

A reasonable amount of flywheel is one thing that will contribute to steady running at low speed...you probably can't include a flywheel as such, but that brass paddlewheel should provide a substantial amount of flywheel effect. It seems to me that you should be OK, if you follow the ideas above, eg dry steam (But not too hot) and admission for most of the stroke.

regards
John