Boiler for a Stuart 5A

[DetroiTug]

A steam engine spinning freely consumes very little energy to do so. Like an electric motor plated for i.e.1 hp and 10 amps, it only draws 10 amps when it is producing 1 horsepower - under load. Spinning freely at plated RPM results in a draw of less than ~1 amp.

I run a 3 + 3 X 4 twin on Iron Chief and a 42 sq ft VFT boiler and its the bare minimum on boiler output. I've made every practical enhancement except superheating. The wood has to be good quality for a hot fire.

Another comparison, the Locomobile steam car has a 2-1/2 + 2-1/2 X 3-5/8" twin. The boiler is a WT 24.9 sq ft heating surface. It runs real well, but it runs on a very high output kerosene burner. And a steamcar differs as the steam demand is intermittent unlike a boat which requires continual demand.

Your boiler can work with the engine, but its going to need every enhancement like feedwater heating, oil firing, superheating, very good insulation, steep pitch prop to keep the RPM low etc. It may be slow, but it will work. It comes down to whether or not the operator will be satisfied with the performance. I just wouldn't be expecting too much out of it. And who knows, you may be surprised. As my old steam buddy says: "Try it"..

-Ron

[VANYA]

To run a Stuart 5A with a smaller boiler, for a smaller steamboat, can the bore be reduced down to 2" or a little less.

Is there any problem to do this to match the engine with the boiler output to drive a smaller mass boat?

I ask this in relation to the Bill Durham Donkey Boiler which would maybe go with the Stuart 5A.

[fredrosse]

Actually you can get MORE horsepower from the 5A by keeping the larger bore, assuming the same boiler output limit. An engine that runs with short cutoff (length of stroke with full steam admission is a shorter part of the stroke) will use steam more efficiently than a smaller displacement engine with long cutoff.

Make the engine according to its design dimensions, and set the valve for shorter cutoff, get the same power output with less steam, or more power with the equal amount of steam, comparing identical engines, one with larger displacement will be more efficient iff cutoff is set correctly.

Note that the word "IFF" is not in error, it is a standard term in mathematics (at least it was when they taught it to me in the early 1960s.

[VANYA]

Thanks Fred.

It is similar to "notching up" on locomotives I guess.

I expect the 5a if more than enough power for what I want but they are the castings I have to hand and so if I beefed up the bearing surfaces it should provide an ideal power plant.

Thanks.

[DetroiTug]

Quote "To run a Stuart 5A with a smaller boiler, for a smaller steamboat, can the bore be reduced down to 2" or a little less."

I would try it first before altering the design, can always sleeve it later. Steam power application is full of options. Nothing is really known until it is tried. You're already helping the plant by selecting a smaller boat.

Steam engines are far different than their I/C counterparts. A small I/C engine is what you see is what you get. We can tweak steam plants with adjusting cutoff, economizers, superheaters, methods of feedwater admission and firing, change the prop pitch etc.

I would try it first, it may work satisfactorily with some tweaking.

-Ron