Hi Geoff ,

I’ll work it out for you . Send me all the details in pm .

MikeW

Geoff – got pm ok .
Richard – yes .

Expect answer in a day or two .

MikeW

Horsepower by a compound steam engine, whether it has two cylinders or several cylinders can be calculated in the same manner as that of a steam engine with a cylinder, in that it is estimated that the vapor total expansion takes place in the last cylinder and thence calculate horse power by the effective vapor pressure in this cylinder, its piston area and piston speed. Which will easily be appreciated that the only gain, which is obtained by applying several cylinders, the steam that is saved as one gets smaller cylinder condensation, while that one gets more crankpins on a shaft gets more steady operation of the steam engine.

So………………………………..no "plain English" calculations then ?

Richard.

Like I said…………………no "plain English" calculations.

Richard.

Just thought..can you not just work from volume of steam consumed?

"Everything should be made as simple as possible, but not simpler"

We can make simple calculations for this sort of thing, but the more we simplify, the less accurate they will be. You may recall that early calculations of steam plant horsepower were often made on the basis of heating surface. (or was it grate area?) By the time these were applied to traction engines, the art had been improved to the extent that a 10 nhp traction engine could probably produce at least forty brake horsepower.

However, for our purposes, a simple calculation that gives a rough idea of the power to be produced is probably close enough…unless you actually plan to put the engine on a brake later and actually measure the power?

So…people are worrying about indicator diagrams. Let's simplify things by assuming that we admit steam for the whole stroke. After all if we are trying to get maximum power we will go for a late cutoff. Let's also assume some pressures…we need to know the admission pressure, the intermediate pressure (or pressures for a triple), and the exhaust pressure..all should be absolute. Ok, so simple subtraction gives us the pressure difference across each cylinder. Don't bank on getting a vaccum better than 20" (About 5psi absolute) on a small plant

So apply the formula from Roy's earlier post for each cylinder, add the resulting powers together and you have a number. (This is also what Jason is suggesting.) This will be on the high side, since we did not allow for pressure drops across the valves or between cylinders. We also probably won't run at 100% cutoff! Note that things like cylinder condensation will not reduce the power when the cutoff is late, they just mean that more steam is used.

But bear in mind that you probably don't know how much power you will actually need for a given hull anyway. Any figures provided for driving the hull with a petrol engine are pretty irrelevant when comparing to a steam plant. This is because the steam plant will operate at much lower rpm, with a much larger and much more efficient propellor. So for instance the Leak compound, described in ME in 1983 and still available as casting, is recommended for boats over twentyfive feet long. My own back of an envelope calculations suggest that at the sort of pressure and rpm likely to be used this engine will probably produce about ten brake horsepower at the shaft. That does not sound like much, but if you check out the Steam Boat Associations register, you will see that plenty of people have successfully used this engine in boats that size and bigger. If you were going to substitute a petrol engine or small diesel you would probably need more than twice the shaft power to get the same performance, since the smaller high speed propellor would be much less efficient.

All this is by way of suggesting that comparing the proposed plant to others that are already working is probably a better way of deciding what hull size the engine can push than calculating the horsepower.

John

To make short and clear answer: Horsepower is determined by the size of the boiler and not vice versa. Compound steam engine does not get more horsepower than a single cylinder steam engine. Difference between a single steam engine and compound steam engine: Increased steam economy and a smoother running steam engine in a compound steam engine. The simple method to measure horsepower to example by Prony's brake dynamometer or similar instrument to measure effect to example use difference propeller and a tachometer + a fixed size of the boiler to learn out how effective is steam engine.

The boiler is the source of the power; the engine is only a medium for converting the latent power in the steam into useful work.

It is better to have high pressure in boiler than the steam engine is designed to run at work pressure due there is loss of steam pressure from boiler to steam engine by condensation, resistant in steam pipe etc, etc..

… and the answer was?