Seeking advice on boiler size for new steel tug build

[eugenewright]

Hello recently finished building glen-L fred murphy 30' steel hull tug, for my 8th boat I am planning on going back to steam, building reliable steams (salty) in steel. steam powered. Might stretch it out to 22 feet with the 7' 4" beam. My question is if I build a boiler vft 32 inches high , 20" in diameter with 22 fire tubes 1 1/2" will the boiler be big enough? I had this size in my 21' fantail cedar strip and it was great, but with the added weight of the steel boat, will it do the job? thanks!

[barts]

There are several rules of thumb that help size things.

Start with displacement; what is the anticipated displacement of the boat with the normal passenger count aboard in tons? For a adequately powered vessel, 2 hp per ton of displacement is pretty much standard; with a full load, approaching 1 hp/ton is manageable but slow.

For a firetube boiler, the suggested size is 10 square feet of heating surface per horsepower; for water tube boiler with their higher steaming rates, 5 square feet is usually adequate. In other case, more doesn't hurt except in terms of weight.

In your case, the the heating surface of your proposed boiler is 23 square feet, or 28 if you have a 12" tall wet leg firebox. This would provide steam for a two to three hp engine, or sufficient for a maximum displacement of about 3000 lbs or so. This may be a bit low for your small steel vessel as they tend to be heavier than boats built of wood.

- Bart

[fredrosse]

Do some simple math as outlined in the FAQ section of the forum, this can give you a reasonable perspective about your conditions and performance potential of your project. Boilers, hulls, engines, and propeller basics are covered there.

[Lopez Mike]

I have a 30 sq. ft. VFT driving a 3 x 4 single rated at 5 hp in a 24 ft hull weighing in the vicinity of a ton. Haven't weighed the boat yet and am hoping it will be less. The power to weight ratio does not seem out of balance.

Mike