New to steam. Putting engine and boiler in Navy whaleboat.

[Whistledoc]

Have 26 foot ex US Navy whaleboat weight approx 6000 pounds. Currently has running Westerbeke 4.108 diesel , 1" shaft and high speed prop. Hull speed about 6.5 to 7 knots. Using antiquated steam engine by Willard. Bore is 5" and stroke is 6". Boiler is Semple 40 sq. ft boiler. Estimated rpm is 350 and boiler pressure estimated at 80 psi. The gentleman that I bought the steam plant from says that it will push the boat to hull speed.

In my reading I found information that for a wide beam boat with a hull length to beam ratio of
25/8 = 3+ that I needed about 1 bhp per 1000# weight or roughly 6 hp.

Doing the calculations to figure horsepower My calculations suggest this is too small for a boat like this. Can someone review my math.

Bore = 5". Piston area is pi x 2.5 x 2.5 or 19.6 sq. in.

Force equals pressure x area 19.6 sq in x 80 psi = 1570 #

Work equals force x stroke x rpm 1570# x 0.5 feet x 350 rpm = 274,750 ft #

274,750/ 33000 =8.32 bhp.

With these numbers the plant should be adequate. But in my readings I have come across a monkey wrench called MEP which stands for mean effective pressure. One source suggests that the effective operating pressure inside the cylinder is is about 50% of the boiler psig. Now if I throw that factor in it cuts my Effective bhp in half to about 4 bhp. Bottom line can some shed some light on this issue.

[fredrosse]

Look in the FAQ section of this forum for details on hull power requirements, engine and boiler output, and engine steam consumption. You are basically correct in your calculations, for a single acting single cylinder engine. Many (probably most) steam engines are double acting, and the PLAN formula uses N = number of power strokes per minute, which is double the RPM if you have a double acting single cylinder engine.

Any pictures?

[artemis]

My previous boat was a 26' Navy Motor Whaleboat. I had the original 3" + 6" * 4.5" engine (built in 1965) rebuilt (1996). I installed a new watertube boiler in 1997 (ASME code) of about 56 sq.ft. heating surface. Used an inboard keel condenser and pulled a vacuum of about 20"-HG. Swung a 23" * 32" wheel and made about 6 knots at 320 rpm (engine was happiest at 290rpm). Best example in print is Capt McCready's Little Effie, described in the Jan/Feb 1961 issue of Steamboats and Modern Steam Launches. This is the "bible" of modern steamboating and is available from Elliott Bay Launch http://www.steamlaunch.com/ - you need this book if you're going to build a steamboat. Capt. McCready used a triple expansion engine, which was really overkill in the sense that a compound would have done as well and been a lot easier to maintain and given as good economy.

Picture of Artemis below was taken in the summer of 1998 on the Columbia River near Portland, OR.

[87gn@tahoe]

Welcome to the forum

[Whistledoc]

Some pic of whaleboat and engine

[Whistledoc]

Pics of engine and boiler.

[artemis]

My suggestions would be:
1. Find a good propeller shop nearby and determine the largest diamweter of propeller you can swing without altering the boat.
2. The best crusing speed for that hull (based on two existing examples I know) is about 5.5 knots. Allowing for a heavy, displacement hull assume a propeller efficiency of about 75% and a overall weight of about 6,000 lbs, determine the best pitch and rpms for the propeller (try not to exceed a diameter-to-pitch ratio of 1 : 1.5).
3. From the diameter, pitch, and rpm you can determine the HP required (probably less that 7HP).

If you live in Kiwi land, I can put you in touch with several persons there that have/are into hobby steamboating. There are quite a few steamboaters in NZ and they have several get togethers. You can p.m. me from this website (or through the website below) and I'll provide you with more info.

[87gn@tahoe]

That's a nice looking hull. Are you going to put a cabin on it or leave it open?

Here's a nice prop calculator I found:

http://vicprop.com/displacement_size.php

With a 5" bore and a 6" stroke supplied by a Semple VFT40 you are probably going to want to utilize a feedwater heater and an economizer

[fredrosse]

"Doing the calculations to figure horsepower My calculations suggest this is too small for a boat like this. Can someone review my math.

Bore = 5". Piston area is pi x 2.5 x 2.5 or 19.6 sq. in.

Force equals pressure x area 19.6 sq in x 80 psi = 1570 #

Work equals force x stroke x rpm 1570# x 0.5 feet x 350 rpm = 274,750 ft #

274,750/ 33000 =8.32 bhp.

With these numbers the plant should be adequate. But in my readings I have come across a monkey wrench called MEP which stands for mean effective pressure. One source suggests that the effective operating pressure inside the cylinder is is about 50% of the boiler psig. Now if I throw that factor in it cuts my Effective bhp in half to about 4 bhp. Bottom line can some shed some light on this issue?"

Running some crude numbers for your boat, at 350 RPM and 5.5 knots, you would like to have a 3 blade 24 inch dia x 26 inch pitch prop. If you only have room for a smaller prop, then use a four blade prop with a larger blade area. Propeller slip would probably increase, so use 28 to 30 inch pitch.

Power required is only about 4 Horsepower with an efficient prop, somewhat more with a smaller prop. See attached spreadsheet pic.

Your 5 x 6 engine is indeed double acting, so the number of power strokes per minute is 700, not 350. This compensates for using a MEP that is half of your steam pressure. Another attached spreadsheet shows what would be expected of this engine with 350 RPM, 70 PSIG admission, non-condensing, at about 6.5 horsepower, a comfortable match for the 4 horsepower requirement, with excess power for wind and waves.

Looks like a good (and seaworthy) project. Good luck.

[artemis]

My only quarrel whith Fred's calculations is simply that 30% cuttoff for a single cylinder marine engine is quite short, 60% to 65% is closer to a desireable figure and also guarantees easier starting from "stop". The Semple boiler was very conservatively rated at 125 psig - psi gauge - (or 140 psia - psi absolute, at perfect vacuum, what the steam tables are measured in) and it's not out of line to run the boiler at the usual recommended 10% below max for 127.5 psia; allowing for moderately insulated, straight, close pipes then probably 120 psia is being delivered to the throttle. Using an MEP (Mean Effective Power) factor chart a cutoff of 60% (or 1.667 expansions) yields an MEP factor of 0.9065. This multiplied by by the valve chest pressure (in psi absolute) or 120 * 0.9065 = 108.78 minus the back pressure (here's where condensing shows up) and let's just make it atmospheric or 15 psi for 93.78 psi MEP. This takes care of all the 50% to 75% "fudge factors" or "Finagle's Constant" and such other guesswork, giving a more reliable figure and permitting a smaller engine to be used