Boiler for a Stuart 5A

[Lopez Mike]

I have a friend who is collecting the bits for a small launch. He has a Stuart 5A and a 14 foot hull intended for electrical power with a 12" x 15" three blade prop.

He is considering an Ofeldt boiler but is not sure how much heating area he will need for this boat and engine.

There is one local boat here in the N.W. U.S., 'Charlie', with a 5A and about 18 sq. ft. of heating area and the operator says the boiler is working hard to keep up with the engine. He suspects bad rings or a worn D valve.

Any thoughts from the group about heating area?

[barts]

I have a friend who is collecting the bits for a small launch. He has a Stuart 5A and a 14 foot hull intended for electrical power with a 12" x 15" three blade prop.

He is considering an Ofeldt boiler but is not sure how much heating area he will need for this boat and engine.

There is one local boat here in the N.W. U.S., 'Charlie', with a 5A and about 18 sq. ft. of heating area and the operator says the boiler is working hard to keep up with the engine. He suspects bad rings or a worn D valve.

Any thoughts from the group about heating area?

My boiler (17 sq feet) keeps up w/ my 2.5" x 2.5" engine, but I have to force it pretty hard if I want to run wide open throttle.

That 12 x 15 will spin pretty freely w/ that 5A..

It really depends on how fast he wants to go. Small boats often get driven hard trying to keep up...

- Bart

[Lopez Mike]

The hull is a 14 x 5 with fairly high freeboard intended for electric drive. I suspect it will hit the wall pretty hard above hull speed. No racer.

As of last night when I talked with him on the phone he was wavering between an Ofeldt and a wet leg VFT. I've never bent sch. 40 tubing in such a tight radius before as would be needed for an Ofeldt. Would he need to heat it or could he just wind it around a suitable hunk of pipe? Or should it be thinner stuff?

[Weber]

="Lopez Mike"]I have a friend who is collecting the bits for a small launch. He has a Stuart 5A and a 14 foot hull intended for electrical power with a 12" x 15" three blade prop... Any thoughts from the group about heating area?

Interestingly, I'm also working on a launch adapted from a 13'-6" x 5' "International Flying Junior" hull and had considered using a Stuart 5A.

I decided to forgo the co$t of the $tuart ca$ting set and design/build my own engine of a two-cylinder, double-acting, 2" bore x 1.75" stroke piston valve arrangement. And, just last week, I tested my vertical, fire-tube, coal-fired boiler and engine together for the first time. Aside from a few minor plumbing leaks, all worked quite well. There's a video posted on this website of it running.

My heated surface area is only 6.5 sq. ft. But at 100 psi and an evap. rate approaching 40 cubic inches per minute, the engine ticks along nicely.

[Lopez Mike]

Thanks for the info. It will be interesting to see how well that small of a boiler keeps up with the engine under load. I've told him he needs about 20 sq. ft.

[Weber]

Thanks for the info. It will be interesting to see how well that small of a boiler keeps up with the engine under load. I've told him he needs about 20 sq. ft.

Agreed. 20 sq ft seems enough for his 14-footer, but that's just my gut feel.

We know that his 14' l.w.l. hull speed is about 5 knots (5.7 mph). Since the 5A can work nicely at 800 rpm and assuming a prop slip of 25%, his 15"-pitch prop could theoretically propel the boat up to 8.5 mph.

To feed this 8 cu. in. double-acting engine at 800 rpm (with no early cutoff) at 100 psi, the boiler must convert about 64 cu. in. (about 2 pints) of water per minute into 12,700 cu. in. of steam per minute. (That is, if my calculations are right.)

So, for me, the question is: just how "productive" (in converting water into steam) are the boilers being considered for his 14-ft boat? Harris says 1 cu. in. water per minute per 100 sq. in. heated surface area is good for model boilers. My 13'-6" launch's coal-fired vertical fire tube boiler (bigger than a model boiler but relatively small) converts about 4 cu. in/min/100sq.in. surf area. With my slower revving engine, that should get me hull speed (5.6 mph) with a 15" pitch prop.

Where can I find numbers on how much steam can be produced from a specified volume of water per heated surface area per unit of time in various boilers--water tube and fire tube-- commonly used in 14 to 30 foot launches?

[fredrosse]

"My heated surface area is only 6.5 sq. ft. But at 100 psi and an evap. rate approaching 40 cubic inches per minute, the engine ticks along nicely."

That evaporation rate (87 pounds per hour) from a 6.5 square foot boiler seems way too high.

My 20 square foot boiler (vertical fire tube) generates about 60 PPH of steam, producing about 1 horsepower at the engine shaft. Water tube boilers can run at about twice that steaming rate, but still far below the steam generation rate quoted above.

That power level (one shaft horsepower) is about what would be wanted for this 14 ft boat with a propeller drive. The FAQ section on this forum has details on engine sizing, engine steam consumption, and boiler sizing information that is fairly accurate.

[Weber]

"My heated surface area is only 6.5 sq. ft. But at 100 psi and an evap. rate approaching 40 cubic inches per minute, the engine ticks along nicely."

That evaporation rate (87 pounds per hour) from a 6.5 square foot boiler seems way too high.

My 20 square foot boiler (vertical fire tube) generates about 60 PPH of steam, producing about 1 horsepower at the engine shaft. Water tube boilers can run at about twice that steaming rate, but still far below the steam generation rate quoted above.

That power level (one shaft horsepower) is about what would be wanted for this 14 ft boat with a propeller drive. The FAQ section on this forum has details on engine sizing, engine steam consumption, and boiler sizing information that is fairly accurate.

Thank you for the info. Obviously, there is a big error in my calcs or observations. When testing my boiler, I watched the water glass level over a period of time and judged it to drop about 30 to 40 cu. in. per minute with the engine running and pressure holding fairly steady, but this must not have been the case. I'll check out the FAQ first and then watch more closely in future tests of the power plant. I suspect my engine was running much slower than 800 rpm and the 40 cu in per minute estimate is way high, as you suggest.

[Lopez Mike]

But your engine didn't have a load on it, did it?

[Weber]

But your engine didn't have a load on it, did it?

Nope... and I wasn't running any feed water in when watching the level in the glass go down. I read the FAQ on testing boiler output here and will do more testing. Thanks for the input.