boiler wood consumption

[Peter H]

My boat is 16 ft and weighs about 900LBs loaded. I am using 4 1/2 lbs of dry maple per mile. My boiler is run with an electric blower in the firebox. Is that what you would expect for wood consumption?

[TahoeSteam]

Hi Peter, I think we need more information;

-boiler type?
-engine type/dimensions/rpm?
-Condensing y/n?
-feed heater or economizer?
-speed?

[Peter H]

Fire tube boiler, all copper, stainless superheater in the firebox, about 9 sq ft heating surface, condensing , engine is 2 1/4 bore by 3 inch stroke . Running about 300 -400 rpm and 3-4 mph

[Peter H]

Also has a feed water heater

[fredrosse]

Running some numbers from the FAQ section of this website:

Boat, 15 feet waterline length gives a hull speed = 1.34 x SQRT (LWL) = 5.2 Knots = 6 Miles per Hour

Hull speed requires about 1.5 horsepower per ton (2240 Pounds) of displacement, and taking your displacement at 1/2 ton ( 1120 pounds, 508 kg) , = 0.75 horsepower at hull speed (with proper propeller and reasonable hull, (See FAQ section discussions)

Traveling at speed less than hull speed is a cubic function, so 4 MPH only requires about 0.25 horsepower (190 Watts) shaft horsepower to the propeller.

The engine you have, at 700 power strokes per minute, is capable of about 1 horsepower with a brake mean effective pressure of 50 psid (generally corresponding to about 100 psig main steam pressure with proper valve setting) . for 1 BHP conditions you would need about 62 PPH (pounds per hour) main steam flow, but at 1/4 horsepower only about 37 PPH is required.

The feedwater heater reduces fuel consumption, and to make 1/4 horsepower would require about 32,000 BTU/hr (13 kW) boiler output, or about 45,000 BTU / hr fuel heat input. With dry hardwood about 7 PPH fuel flow would be required here. That level of heat consumption is very high for a 9 square ft firetube boiler, and more appropriate for a watertube boiler.

In summary, your stated wood fuel consumption, which is in the right vicinity for your 9 square foot boiler, would produce only about half the required steam flow calculated above. Perhaps you have a very economical steam engine, short cutoff, good insulation throughout, good superheat, etc.

[Peter H]

Thank you for taking the time to answer my question. So, if I understand you, my wood consumption is not excessive. One thing about my boat is that I have never obtained 6mph. I believe my prop is not ideal (12x15) so I am thinking of making a prop this winter, maybe 12x20 with narrow blades. I believe i could fabricate in steel. I have machine shop equipment and a forge.