Stuart 5A boiler size

[Lopez Mike]

I have a friend who is planning a small launch with a Stuart Turner 5A that he has had for some time.

The question is, how many square feet of heating surface should he design for in his boiler?

He is planning on a double pass Scotch boiler with propane firing. The one he has sketched out now has a little over 10 square feet counting the 6" combustion tube and all of the flues. Not included in the area total are the flue sheets.

The design he has on paper is 20" long with a 6" combustion chamber, an array of 1" flues on the first return pass and 5/8" flues on the next pass. No working pressure chosen as yet. Probably 150 psi max. Maybe more like 100.

For any of you not familiar with the 5A it is a single cylinder 2.25" bore x 2" stroke.

Any thoughts or design suggestions?

[barts]

Figure out BMEP.... @ 100 psi input, 50% cut-off I'd estimate at 80 psi if condensing... this yields about 1.25 hp at 400 rpm. For a Scotch boiler, I'd assume 10 sq. ft. per engine HP minimum.

This will work, but it won't be fast.... should work ok though in a small, easily driven boat. I'd probably consider smaller copper flues and more of them as a quick boost in effective heating surface. Stanley steamers used approx. 1/2" flues - if you're not firing with solid fuel, this will work well and pack in a lot more heating surface. .049" wall 1/2" steel hydraulic tubing would work here too.

The other thing that needs thought is supplying the 3700 btu/20 lb tank needed to vaporize propane. A water jacket is a reasonable approach although for sustained use water circulation may be needed; if the user understands propane he can also use a forklift tank that supplies liquid and arrange to vaporize the liquid outside rather than inside the tank.

- Bart

[barts]

Mulling over my previous answer, the 20" length on this is probably a little much for 1/2" OD tubes... but I'd skip the 1" tubes - you don't need 'em to carry soot - might as well just use all 5/8" tubes - you'll get more heating surface in, and with a Scotch boiler, more is definitely better.

- Bart