Hi Sjoormen,
Gondolier, I tried to learn something new about steam plants, How do you know which one is best for ones purpose there are so many, so many types-which is most reliable . How do you calculate needed power? Just for fun:
http://www.aero-naut.net/index.php?id=634&L=1
Here it states that is for boats 100-200 cm long but 0,038 HP -that is not very much compared with electric motors (unless these boats are very narrow) or I am mistaken?
Your first lesson in steam has been learned- they don't produce much power- well not horsepower anyway- I'll give you an example in SY Gondola, the boat I work on- she has twin cylinder 'V' engine- cylinders are 9" X 8"- supplied with steam @ 40psi (2.8bar) on the valvechests (the chambers on each cylinder that control the admission and exhaust steam) giving a cruising RPM of approx 140-150 RPM and a speed of 6/7 Knots, at this speed the engine produces aprox just 30HP! And that’s a 45ton boat!!!
However if you were to compare the torque to an electric motor- it has the same line of acceleration- ie. no matter what power it's producing, it produces max torque (ie. from 0 RPM to max. RPM) yet a steam engine will produce MORE of it.
This means you can stick a slow revving but big diameter and good pitch propeller on the blunt end- which is after all what the prototype had!
Also- a few people have used those Saito engines in model tugs to good effect- you just have to be careful because although great value and beautifully built- they are on the fragile side of build quality and can't be fed more than 30psi steam pressure- this means it would use more steam than one that could work at steam at say 100psi.
You get what you pay for!
As regards power requirements- there are few engines which come to mind that have power specified by the manufacturer- other than a couple of the Stuart Turner ones- you could always get in touch with the manufacturer- i'm sure they would be more than happy to help.
There is a way to calculate the Horsepower made by a steam engine- this is called the PLAN method;
P- Mean effective pressure working on the pistons. (psi)
L- length of piston travel (Feet) (ie a piston travelling @ 3inch on it's downstroke and 3inch on it's upstroke, completing one full stroke of .5 Feet for every rev of the crankshaft)
A- Area of piston (Sq. Inch)
N- RPM
Then divide by 33,000
PLAN
33,000
Then further divide by around 15 at a generous estimate- this will give you the power minus the inefficiencies of the plant- frictional losses and such, in a full size plant this number would be far less- 6 or 7, but in a model plant frictional losses are of far greater detriment.
So a working example of a twin cylinder model steam engine- it has two cylinders- each are double acting. Each cylinder is .5 inch wide and a working stroke of .5 inch. The piston, then, has an area of Pi x r so 3.141 x .25 inch gives an area of .7853 inches.
In a model engine working on 100psi steam from the boiler it will typically have a mean effective pressure working on the piston of 20-35psi on its entire stroke, lets for arguments sake call it 30psi. (2 Bar) In a full size plant this figure is normally 50% the value of boiler pressure, so would normally be 50PSI- however scale pipe sizes and tight corners mean that in a model engine losses are much greater.
As we have a stroke of .5 inches, and it is a double acting cylinder- a piston will move 1 inch on one full rev of the shaft while doing work- and at a typical 400 RPM it will travel 33.3 Feet (1 / 12 x 400)
We have to double this figure at the end however as we are talking about a twin cylinder engine.
So our PLAN is thus;
30 (psi) x 33.3 (ft) x .7853 (sq. inch) x 400 (rpm)
33,000
/15
X 2
Equals a power of- 1.26Hp theoretically- however, as i've mentioned before the power doesn't really matter, it's the usable torque of the steam engine that is it's real advantage.
So now you know how to calculate your own power, which engine is the most reliable?
Well, you must ALWAYS have a twin cylinder (or more) SIMPLE engine (meaning that as it uses the steam it exhausts away from the engine) this is the complete opposite of COMPOUND (engines which use the steam twice), and TRIPLE EXPANSION engines (which use it three times), in full size even QUADRUPLE EXPANSION (steam expanded four times) engines were made, however it would be difficult to get to work in model sizes.
This is because only one cylinder will be supplied with steam at first and if it is at rest on it’s top or bottom dead centre (TDC or BDC) it WILL NOT start! Not what you want when your prised tug is being blown by the breeze to the other side of the boating pond!
Hope this is of some help. Any more questions and I’ll happily help- I’m quite enjoying teaching a newbie the intricacies of steam. ;)
Greg