The Poppet Valve Steam Engine Quest

I tend to favour the single-acting steam engine design because of its reduced part count and reduced friction compared to the double-acting variety. So far I have found nothing on the market, but I have found out about a twin-cylinder single-acting engine designed by Mr. Anton Bohaboy back in the twenties. Mr. Bohaboy's engines were mentioned a few times in the pages of Steamboats and Modern Steam Launches, but no surviving examples seem to be around.

-Photo #1 was copied from a vintage drawing of the Bohaboy model engine and shows the relationship of the major components. Although mostly enclosed, the exposed lifters and springs will provide the aesthetic necessary for a rabid steam enthusiast like myself. Its exhaust beat should be a pleasure to the ears as well!

To my delight I have made contact with Mr. Robert E. Pelfrey, through the Model Engineer's mailing list. Mr. Pelfrey knew Mr. Bohaboy and supplied castings for one of his model steam engines years ago. He is currently rennovating the patterns for this little engine and he has very kindly sent me drawings of this &quottypical" Bohaboy-designed model so I might have something to study. With luck, castings for this model engine will become available again, by year's end.

Although there are no drawings or castings available for a full-sized engine, Mr. Bohaboy described his ideas for one to Mr. Pelfrey. The larger engine is said to be easier to fabricate than the model depicted in the drawings he sent me. Six months ago this larger engine existed only as a series of notes, but we have been exchanging email for some time and now patterns are being made!.

In order to have a similar &quotfootprint" and the same displacement as the double-acting Stuart Swan engine, a single-acting poppet-valve design would need a larger bore and one more cylinder. Mr. Pelfrey is designing an engine that accomplishes this, without requiring much additional space in a vessel. By enclosing the crankcase and by positioning valves beside the cylinders, rather than before and behind like those on the Swan, the new engine has the same width, and length is only increased by an inch. Below are two of Mr. Pelfrey's renderings of this idea.

-Photo #2 illustrates the crankshaft
and general cylinder arrangement.
-Photo #3 details the overhead valve and steam passages to the top of each cylinder.

I took Intro. to Autocad this summer and I toyed with the idea of doing something that I have only recently learned is possible. That is, an Autocad file can be emailed to one of the various Stereolithography shops and the drawings can be &quotrendered" in plastic. The trick is to re-scale the drawings to compensate for shrinkage, then to do the 3-D rendering with the addition of &quotCore-prints", in this way generating instant foundry patterns! Well, the class was a bear and we only got as far as Chapter 4 in 20 hours of instruction, so my skills have a way to go yet! The other fly in the ointment is the cost: stereolithography is slow, relatively speaking and the time needed to make a full set of steam engine casting patterns would be several days, when they charge around $100 per hour!

I have no doubt that Mr. Pelfrey is more adept at patternmaking than I and certainly faster than I might crank them out via the computer, so I eagerly await further news!

Progress Report:
Date: Wed, 15 Oct 1997 20:24:33 -0400
From: Robert Pelfrey

Hi Ed: I thought I would give you a progress report - The Bohaboy patterns (the small ones) are almost completed and should be ready to pour soon.

I was able to hit a good lick on the Sea Sprite last week and whipped a couple knotty problems that have been bothering me. I like this design more and more and hope that it will be acceptable to you. I also hope you like to drill and thread holes as there are plenty of them! I am down to working on the cam and crank gears now. I may have the cutters to make them if they work out like I planned, i.e. diameter - DP - pitch- # of teeth. BTW, I am going to put our logo on the gear cover too so that when viewed from the front, it will be very visible.

I have licked the knotty problem of inlet manifold rather neatly and have the exhaust manifold yet to go, but it is not so complicated. I think that we may be able to use commercially available valves and guides as well as piston rings. You will need all the relief you can get!

Checked out your web site, very nice, "all the news that fits" I like that!

Cheers, Bob

Bobco Engineering and Machine

A day with nothing created is a day LOST.

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