I am trying to assist a boat friend of mine, and he is proposing to build a model of the steam tug Alva B.

He posted a few drawings of it, and I told him I could help him with the engine, however, the engine appears to have dual rocking valves, and a rather complex valve gear, with at least three eccentrics.

I assumed rocking valves were relatively simple affairs, and I understand the Corliss valve concept well.

When I look online for non-Corliss rocking valve information, I am coming up empty. The rocking valves used outside of Corliss engines seem to be far and few between.

I originally assumed that one rocker on the Alva B engine was steam, and the other exhaust, but it would seem that both valves much be both steam and exhaust, with the lower valve serving the lower part of the cylinder, and the upper valve serving the upper part of the cylinder.

Any ideas on how these valves function?

Edited By PatJ on 26/12/2021 17:09:19

These are the only photos of non-Corliss rocking valve engines that I have ever found, and I am not sure who owns these.

The function of these engines is not entirely clear, but certainly more understandable than the Alva B engine valves.

Jason-

Thanks much for the info.

I totally forgot about your rocking valve engine.

You builds just keep getting better.

I suspected the rocking valve may be a wrapped d-valve and port face, since I could not work out any other arrangement that would give the correct timing.

For the Alva B, there are clearly two rocking valves visible, and so I am guessing that the lower valve is connected to the passage on the lower end of the cylinder, and the upper valve is connected to the passage on the upper end of the cylinder.

There would be a common exhaust between the two valves, and you can see a band around the cylinder, which contains the exhaust passage, with the exhaust pipe coming off the side of the cylinder opposite of the valves.

Why two rocking valves, and three eccentrics?

I think using two rocking valves allows an early cutoff, without affecting the exhaust on the opposite side, which needs to close later than an early cutoff.

I think there is a reversing link in there too.

Thanks again; this helps a lot.

You across the pond guys put us to shame, but what can I say?

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