Twin Compound

[SteamGuy]

I looked at the Stuart 6A, and some of Ranier's engines over the weekend, and got the twin compound bug.

I roughed out a quick assembly just to see if I could come up with something that looked good enough to continue working on. Not an accurate assembly (yet), but good enough to get a look and feel.

Here is what a came up with, which is a morph of many different steam launche engines, old and new that I have seen over the years and recently.

Here are the proposed specs:

Pat J's Compound Twin
(Approximate Dimensions/Values)
Height: 24"
Width: 16"
Length: 22"
Bore: 3" and 5" (Changed to 3" and 6")
Stroke: 3.75" (Changed to 5")
6 bhp @150 psi and 750 rpm
No condenser
Balanced D-valves for hp and lp cylinders

Does this layout pass the "Steamboat" test?

[S. Weaver]

Pat:

My two cents:
- Slow the thing down. Steam has a torque curve that IC can only envy;
- Widen out the LP to 5.5 or six inches in order to harness the expansion (or shrink the HP);
- Stroke it to at least four inches, maybe five;
- After you do all that, you may have to widen out the front columns for strength and stability.

Isn't computer modeling fun? Good work - keep dreaming!

[SteamGuy]

Steve-

I am a big fan of slow speed steam engines, and really just pulled the 750 rpm out of the air (maybe a Stuart rating that I copied). I pay more attention to maximum rpm for sizing ports than for really operating the engine.

Now that I think about it, probably 150 rpm would be more like the maximum, with a cruise at 60 rpm.

As far as rpm, it seems like the YouTube group almost without fail run their Stuarts at what seems like 10,000 rpm. The videos generally show an engine that is more akin to a high speed sewing machine than a steam engine, and it makes my hair stand on end to see a nice steam engine reved up like that. The general public seems to be completely clueless about steam engine rpm.
My favorite engine is a large Corliss running about 50 rpm.

The public domain book by Porter talks about the first high speed steam engine that he designed that ran at the unheard of speed of 150 rpm in the mid 1800's, and it was widely anticipated that Porter's engine would fly apart, but Porter got it right and the modern steam engine was born.

A 6" lp piston is going to push my equipment, especially the cylinder head, but I can probably do that.

A longer stroke will be more difficult to change, since the 3" cylinder is complete already with passages, but I will look at going with a 5" stroke. I generally begin all parts of any give part at the midpoint of planes, and so can generally just lengthen out from the midpoint without too much trouble.

A 5" stroke seems normal for the old engines (lots of old advertisements for 3x5 engines), but probably is a little long for a marine engine. I like it though and will go with that.

I like 3D modeling a lot. Although it was rough to learn, it is ultimately much easier to use as far as designing an engine than a 2D CAD program, and changing any part of the model automatically propogates to every drawing.

[SteamGuy]

Steve-

Thanks for the feedback.

I am a big fan of slow speed steam engines, and really just pulled the 750 rpm out of the air (maybe a Stuart rating that I copied). I pay more attention to maximum rpm for sizing ports than for really operating the engine.

Now that I think about it, probably 150 rpm would be more like the maximum, with a cruise at 60 rpm.

As far as rpm, it seems like the YouTube group almost without fail run their Stuarts at what seems like 10,000 rpm. The videos generally show an engine that is more akin to a high speed sewing machine than a steam engine, and it makes my hair stand on end to see a nice steam engine reved up like that. The general public seems to be completely clueless about steam engine rpm.
My favorite engine is a large Corliss running about 50 rpm.

The public domain book by Porter talks about the first high speed steam engine that he designed that ran at the unheard of speed of 150 rpm in the mid 1800's, and it was widely anticipated that Porter's engine would fly apart, but Porter got it right and the modern steam engine was born.

A 6" lp piston is going to push my equipment, especially the cylinder head, but I can probably do that.

A longer stroke will be more difficult to change, since the 3" cylinder model is complete already with passages. I generally begin all parts of any give part at the midpoint of planes, and so I can lengthen out from the midpoint without too much trouble. A 5" stroke seems normal for the old engines (lots of old advertisements for 3x5 engines), but probably is a little long for a marine engine. I like it though and will go with that. I am a big fan of more torque and less rpm. I will have to keep the frame very rigid with this much bore and stroke or it will be flexing all over the place.

I like 3D modeling a lot. Although it was rough to learn, it is ultimately much easier to use as far as designing an engine than a 2D CAD program, and changing any part of the model automatically propogates to every drawing.

[SteamGuy]

Here is the partially revised hp cylinder.
Luckily, I started this cylinder from the midplane, and then drew the flanges from the outside in, so when I change the bore from 3.75" to 5", everything goes with the new cylinder length automatically except the bosses for the condensate drains (easy to fix) and the passage length (I think also easy to change probably).

I am assuming that the steam chest will remain the same size.
The ports can be enlarged slightly if necessary, but I may leave those as-is, since I typically generously size those anyway.

As far as 3D modeling techniques, beginning a model from midplane has saved my a lot of trouble as far as making changes later.

I tried the "top-down" approach, but the associations that get made can make changes cascade in ways that you don't necessarily want, so I use the "bottom-up" approach and create assemblies from collections of parts. "Bottom-up" is a safer approach, and gives isolation between parts (a fire wall of sorts).

I have not been able to figure out tables yet other than in theory.
Does anyone use those?

[SL Ethel]

While I agree that you'll be happier running an engine slower, the folks with higher RPM engines generally know what they're doing. Remember that piston speed is probably the most important aspect of engine speed - so that 50 rpm Corliss you like with (I'm guessing) a 24" or longer stroke is doing 200 feet/minute. With a 4" stroke, that would equate to 300 rpm. Under load in original spec, that Corliss might have been running twice as fast, so you might double the above numbers.

A properly designed, well balanced small compound can certainly run durably and reliably at 750 rpm if you want it to. And especially at our small sizes with so much surface area for heat loss, the high energy sewing machine engines will be more thermally efficient than a larger, slower turning engine producing the same power.

That said, I run my 3x5 simple around 275 rpm because that's where it looks pretty to me. For many of us (highly trained engineers like Mr. Maltby respectfully excepted), the object is much more aesthetics than efficiency or even best engineering practice. In other words, if it looks good to you, and your boat moves along safely, you are doing it right!

Cheers,
Scott

[S. Weaver]

Scott, well said - your last paragraph captures what our boats and engineering represent to us.

[SteamGuy]

Scott-

I agree. People ask me why I bother to try and make an engine that is like the old engines, when a new-style engine would work just fine.

Esthetics mean a great deal to me, don't ask me why.
I like a steam engine that is in every respect exactly like it was built in 1890 (some of the later steam engine designs are too modern for me, especially the closed crankcase designs, I like the old look, but not too old).
The sight and sound of a slow running steam engine under load is also highly appealing to me, something about that exhaust sound.

I do like a very clean looking engine though, sort of a minimalistic approach.
I have seen too many nice looking engines that get completely covered in piping, tubing, pumps, valves, levers, you name it, to the point where you might as well have a gas engine in there, since nobody will ever see it anyway.

[DetroiTug]

I do like a very clean looking engine though, sort of a minimalistic approach.
I have seen too many nice looking engines that get completely covered in piping, tubing, pumps, valves, levers, you name it, to the point where you might as well have a gas engine in there, since nobody will ever see it anyway.

I think everyone starts out wanting to keep it simple. Believe me, if I could get by with a boiler, engine and a hank of copper in between, that is how I'd have it. Unfortunately, for the system to operate correctly and efficiently, there needs to be peripheral equipment. And the better we need it to run and the more ease in operation desired, the more stuff there is to add.

-Ron

[SteamGuy]

Ron-

I have seen people run boats with just an engine and a boiler (African Queen style), but no doubt the extra equipment would keep you from having to babysit the engine and boiler so much.

For a small boat, I am not sure I would add a condenser just for efficiency (if you can afford a steam launch, then you probably are not that worried about fuel cost), but if it is a matter of running out of water for boiler feed, then a condenser woud make sense from having a closed system, and allowing an all day steam without having to re-water.

I guess running a boiler all day long would add up efficiency-wise, and you can only carry so much fuel and water.

I like propane, although it is not "the old way", it is so very convenient to control, start and stop. I like the smell of burning coal though, it has a certain air of athenticity (maybe authentic air polution).