UP IN SMOKE

[farmerden]

hey Mike -good point.Boiler pressure 140 then I throttle down so the HP pressure would not be 140.Never thought of that before but when the engine is throttled up she runs good til the pressure drops .That's why I'm still looking for that magical 4 blade prop to slow the engine down and keep the pressure up! Thinking about that pressure thing as I write wouldn't the pressure still be 140 ? I've only adjusted the volume of steam not the pressure. Den

[Lopez Mike]

Den,
Not to ask you to add any more stuff to your system but a gauge at the delivery point to the engine is informative. You will find that it pretty much follows the throttle setting. It's amazing how little pressure it takes to have the engine turn over dead slow at the dock!

Ron,
As it happens, I just got off of the phone with Keith. Our weekly exercise in aggravating our respective spouses by tying up the phone for an hour at a time!

His take is that the reason locomotives have such a wide range of cut off available is that they are often in the situation of needing to start a heavy train at essentially zero rpm. Hence they can be set up at almost full stroke (80+% cutoff!). Efficiency be damned. As the speed builds, then the cut off is adjusted to have the power (torque/drawbar pull) match the needs of the situation. And also, as I said, trains have hills to deal with both going up and going down. Not a major problem for our boats.

So our consensus, after an hour of chewing on this, is that between the fact that our loads pretty much follow our engine rpm's and that the valve events on Stevenson valve gear get screwy at short cutoffs, we don't have a great need for big variations in cut off. Staying in the range of 60% works.

That said, we are going out this Spring with a vise grips to hold the reversing lever and do some crude testing of what happens when we hook this engine up shorter and shorter.

And yes, Den, a good match between the prop and the rest of the power plant is something to be sought after. A throttle is an inherently inefficient was to control things. A well balanced system will run smoothly at full throttle. Reading some other threads on here about monotube boilers and sophisticated control systems reminds me that there are many designs where the engine speed is controlled by the firing valve.

[wsmcycle]

These are all very thoughtful and thought provoking questions and statements. Let me correct something. My boat burns coal. I heat it up with a propane burner before i add coal. I put a pot over the flue. I leave the ash door wide open (in fact, I remove the door on my Semple boiler). This allows me to heat-up on the way to the river without having any sparks or smoke that might worry drivers.

If I reduce the throttle opening,(which is positioned at the engine entrance) I am not changing the entrance Pressure "directly", I am reducing the flow rate.

I never ran the stuart 6 at 140 lbs. I might could get there with my Semple VFT boiler but I could never maintain it. I guess, as Ron points out, I never kicked in the LP cylinder. That is probably the reason The Stuart6 did not have sufficient power to move the boat at max speed
.
I installed some helix baffles in the vft's one time (removed them). The problem with these type restrictors is that you cannot simply remove them to get the fire rolling. A Flue damper on the other hand could be regulated at any time.

What do y'all mean by the term "cut-off"? Valve throw reduction? That would change the "cut-on" also correct?

[Lopez Mike]

I still think that when you restrict the flow, it unavoidably drops the pressure when the pressure is measured after the throttle valve. It doesn't matter where the valve is. The pressure in the valve chest will drop when you restrict the flow. I don't see a way around that.

I like the idea of building a fire on the way to the ramp. I'm not sure leaving the door open would work for me. I fire with wood. But I think that having a second fire door with a propane weed burner integral to it would work or me. Just a low fire to get things up to where the pressure is just starting to show on the gauge. Then swap doors and build my wood fire in a hot boiler. I would need a really big steam gauge so that i could watch it in my rear view mirror!

I cannot imagine the response of a regular highway policeman to this though. Fortunately I live in a rural area on an island and the sheriff's deputies are friends and would mostly want to come and go for a ride!

If you look into the timing events of most valve gear designs, they allow adjusting the point where steam stops entering the cylinder without as big an effect on the entrance event. A large help with this is that the piston is nearly stationary at the point when the valve is opening and is moving at significant speeds then the valve is closing. Really sophisticated valve systems like the Corliss allow almost independent event settings. A few locomotives tried poppet valves driven with cam shafts like an internal combustion engine but the poor Irishman trying keep them running in the field was baffled and the experiment never went anywhere.

[wsmcycle]

I do not build a fire on the way to the ramp. I light a propane burner placed in the ash pit. I leave the ashpit door off with no wood or coal in the fire box. And I put a bucket over the top of the flue. I have yet to arrive at the river with any appreciable pressure. However, the boiler is on the verge of steaming.

If i slightly open the throttle, and the engine turns very slowly. The valve box pressure will be very near the same as the boiler side of the throttle. This is because there is sufficient back pressure in the cylinder to restrict the flow. If I open it wider, the engine takes off and there is much less back pressure. Now, the flow rate is much higher and the the pressure falls off because the boiler and piping can not supply enough flow to maintain the pressure.
The point is that the throttle did not directly cause the pressure drop. The inability to supply flow is what causes the pressure drop. As I am steaming, I often change the throttle position in order reduce the volume requirement. I primarily use my steam injector to refill the boiler so before I open the injector, I slow the engine consumption. I also make throttle adjustments when I feel that I am overrunning the prop'a ability to screw the boat forward without undue slippage. This reduces the consumption of water in the boiler and thus requires a little less boiler attention. I can move to the stern and enjoy gliding along.

[artemis]

VFT boiler: 10 sq.ft. heating surface per engine HP;
WT boiler: 5 sq.ft. heating surface per engine HP.
A Semple 40VFT boiler (40 sq.ft. heating surface) will power a 4 - 5 HP engine at 150psig. Do not use a higher pressure or overfire. Period.

Get a bigger boiler.

[Lopez Mike]

So why not run a higher pressure of the boiler is designed, built and tested for it? And what constitutes over firing?

I get enough output from my boiler with natural draft and hunks of wood but I wouldn't worry about turning on a stack blower if I needed more steam for any number of reasons. As long as the water level is right and I'm not getting carry through of liquid water to the engine, I don't know what the safe and practical limits are. I have a VFT with a water leg and I doubt that I would be likely to overheat the segments of the flues above the water level. Might though. But then all economizers run dry and they seem to handle it.

Do you think that running say 200 psi would be a problem? I'm pretty sure it would still be saturated steam. Not likely to hurt the insides of my engine.

[artemis]

So why not run a higher pressure of the boiler is designed, built and tested for it? And what constitutes over firing?

I get enough output from my boiler with natural draft and hunks of wood but I wouldn't worry about turning on a stack blower if I needed more steam for any number of reasons. As long as the water level is right and I'm not getting carry through of liquid water to the engine, I don't know what the safe and practical limits are. I have a VFT with a water leg and I doubt that I would be likely to overheat the segments of the flues above the water level. Might though. But then all economizers run dry and they seem to handle it.

Do you think that running say 200 psi would be a problem? I'm pretty sure it would still be saturated steam. Not likely to hurt the insides of my engine.

As I recall, Semple boilers were rated for 150psi. Will check some of their ads in old issues of Steamboats and Modern Steam Launches.

[Lopez Mike]

Misunderstanding. I didn't mean that I intended to run higher pressure with this boiler. I took your advice as saying that pressures of over 150 psi in general were contraindicated.

And looking at the design and construction of this boiler (Beckman VFT), I don't see any reason I couldn't use a fair degree of forced draft without overheating things. The areas with fire on one side and water on the other all seem to have decent circulation. No narrow gaps. And the parts of the flues above the water level get only convective heating. I doubt that there would be much going on up there anyway.

It does occur to me that at some level of flow one might get into liquid entrainment to the throttle and the engine.

As far as I know, Beckman's h.p. ratings are based on natural draft. I'll give this some thought and talk to a couple of gray beards in my area (Frank Orr and Keith Sternberg).

Not that I'm planning on trying to get 10 steam h.p. out of this little pot any more than I would run over 150 psi. Just am curious as to what the limits are.

[wsmcycle]

Fredrosse

”I am able to cover the flue because the propane burn rate is not dependent upon increased, turbulent air flow. “

Question>>>Trying to understand your statement, how does the air get into the propane flame, and where do your exhaust gasses get out when you have shutoff the stack?

Answer>>>(the propane setup is only for startup) The ash door is open and the propane flame is fed combustion air through it. There is no place for the exhaust to go except that the system leaks a little and there is very complete combustion of the propane. So "practically", there is no exhaust. just like a gas stove in you garage without a vent. Eventually it may get smelly but remember, I have the ash door off so there is probably a two way communication of gas and air through that opening.

Back to the question. the boiler makes steam much better with the flue closed than open and I reason that heat is forced into the boiler water instead of out the flue. IF turbulent combustion air were not required to burn/stoke the fuel in the firebox on our steam boats, we would get more complete heat exchange with the flue closed. Therefore, the turbulent combustion air is a two edged sword. It stokes the fuel but it carries much heat up and out the flue. Could there be a system devised to cycle the combustion? on off on off pulsing/stoking, pulsing/heat exchanging.
If the boiler were designed for "complete heat exchange", there would be no turbulent air flow and the boiler would fail because it would not stoke the fire.
For a given piston size combined with a sufficient boiler size, the HP output of the engine is directly proportional to the HP output of the boiler.
I have a 5 hp Semple boiler and a 5 hp Semple d34 engine. What i find is that I often operate at the edge of the boiler's HP capacity. At 2/3 steam throttle position, the boiler pressure will climb (meaning the HP capability of the boiler is greater than the current throttled output power of the engine. Open it up to 3/4 and the boiler pressure starts to drop thus the Boiler HP is insufficient for the engine requirement. I beleive i can feel some acceleration when the throttle is opened from 2/3 to 3/4 and I think that means i have not achieved "hull speed". All of this together tells me my boiler is not large enough to power my boat at hull speed.