Torque versus speed for steam engine

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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Torque versus speed for steam engine

Post by wsmcycle » Tue Apr 11, 2017 5:45 pm

Since the pressure of the steam is 100% at zero rpm, how does the Hp unfold as the rpm increases? I have a Navy K engine that "seems" to run too fast (rpm). I was told by the builder that the 28 inch pitch of the 26 inch prop was probably to low. I want the engine to run more slowly and I have found a prop on ebay that is 26X38. I am not looking to race anyone out of the hole so i think the 38 pitch is greater, but not too great at cruising speed. What do you think?
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Re: Torque versus speed for steam engine

Post by TahoeSteam » Tue Apr 11, 2017 6:38 pm

Generally the rule of thumb for steam propellers is around 1.2-1.5 diameter to pitch ratio, I's say that other prop would be about perfect.
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Re: Torque versus speed for steam engine

Post by wsmcycle » Tue Apr 11, 2017 7:57 pm

Thanks for looking Wesley. You may remember that my boat has the Navy K engine for which you so kindly supplied me prints. It runs too fast and I am going to try this 26X38 prop. Not too hard to try!
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Re: Torque versus speed for steam engine

Post by TahoeSteam » Wed Apr 12, 2017 7:31 am

I do remember! I'm glad you're getting her out and running her! I think it is also a rule of thumb that 1" in pitch is equivalent to about 2" in diameter...

If you could fit a larger diameter prop it would be better probably... There is a gentleman out on the west coast here that has a N.O.S. "K" in a 26' surf boat replica. I'm not exactly sure the diameter of his prop (24" maybe?), but he has it run through a transmission of sorts to speed up the prop about 2:1 I believe. That boat sure scoots, and the engine runs a a pleasurable RPM.
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Re: Torque versus speed for steam engine

Post by wsmcycle » Wed Apr 12, 2017 12:49 pm

My 26" prop clears about 1/2". So bigger diameter is not possible. But going from 28 pitch to 38 pitch is a 35% increase. I have purchased the Prop off of EBAY and I will have to make a sleeve for the bore. My shaft is 1.5 and the big end of the bore is 2.25. It does have a three bolt pattern on the inner face of the new prop so that i can "bolt-on" a sleeve. Then bore my shaft taper into the sleeve. in place.
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Re: Torque versus speed for steam engine

Post by DetroiTug » Wed Apr 12, 2017 6:22 pm

Quote: "shaft is 1.5 and the big end of the bore is 2.25"

The marine places sell plastic tapered ID/OD sleeves to adapt larger props. They work on boats with hundreds of horsepower and thousands of shaft RPM.

Quote: "how does the Hp unfold as the rpm increases?"

According to the PLAN formula, horsepower increases with RPM.

P= Pressure at the steamchest.
X
L= Length of stroke in decimal equivalent
X
A = Area of piston in square inches πXRXR = area of circle
X
N = Number of strokes per minute X 2 if double acting
/33,000 = the theoretic horsepower.

A taller pitch prop is harder on the engine wear surfaces.

-Ron
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Re: Torque versus speed for steam engine

Post by wsmcycle » Fri Apr 14, 2017 3:07 pm

You have said >>> "A taller pitch prop is harder on the engine wear surfaces."

I AGREE More torque & slower RPM equals same "HP" as higher RPM and lower torque. I want slower engine speed so it's a trade-off. I think the K engine has enough torque to satisfy the Prop BUT, if it doesn't, I wont burn any bridges.
Thanks for the input of a great bunch of enthusiasts!
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Re: Torque versus speed for steam engine

Post by Lopez Mike » Fri Apr 14, 2017 9:19 pm

In an I.C. engine the forces go up with R.P.M. as they run in a regime where the the inertial forces rapidly exceed the cylinder pressure loads.

Steam engines very seldom reach such speeds so letting them spin a bit can be easier on them. Most boats i've been on run into boat vibration issues though. It's all a compromise.

I agree with Ron about using a sleeve rather than depending on bolting anything to the prop hub. I've not used the plastic sleeves and am a little wary of them as my experience with plastics has been that they are slippery. I'd stick with bronze.

Your question about r.p.m. and h.p.: Our engines run at speeds where the port flow starts to be a serious limitation if you try to spin them up fast. All real high power to weight reciprocating engines of any sort run at high speed. The formula is inescapable.

My boat runs at a relatively moderate r.p.m. Unless, of course, there is another steam boat within sight! Barbara has noted this strange effect.

One of the funniest things in steaming is watching another skipper smiling at me with one side of his face while the other side is peering at his engine hoping it won't actually disassemble it'self.
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Re: Torque versus speed for steam engine

Post by fredrosse » Sun Apr 16, 2017 12:36 pm

"According to the PLAN formula, horsepower increases with RPM.

P= Pressure at the steamchest.
X
L= Length of stroke in decimal equivalent
X
A = Area of piston in square inches πXRXR = area of circle
X
N = Number of strokes per minute X 2 if double acting
/33,000 = the theoretic horsepower. "

Actually, in the PLAN/33000 formula, the following definitions apply:

P = Mean Effective Pressure (MEP) in pounds per square inch. This number could be as high as steam chest pressure, but that would result in a very very low efficiency engine. With good design and steam cutoff before full stroke the "P" value should be about 50 – 60% of the steam chest pressure.

L = Length of stroke, in Feet

A = Piston Area, in square inches

N = Number of power strokes per minute, for however many working cylinders are in action.

33000 = A constant to convert the above mentioned units into horsepower according to James Watt definition from the 18th Century.

For a compound engine, each piston will have different values for MEP and cylinder bore, so the total power of the compound engine is calculated as two different outputs and added together.
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Re: Torque versus speed for steam engine

Post by fredrosse » Sun Apr 16, 2017 12:52 pm

"In an I.C. engine the forces go up with R.P.M. as they run in a regime where the the inertial forces rapidly exceed the cylinder pressure loads."

Actually that is an "old wives tale", or should I say: an "Old hot rodder's tale". The big IC engines typically have more than 90% of their loads on the machine as static loads considering the basic force of gas pressure acting on the piston area. The only places where the inertia loads get big are in very high performance racing engines.

When a "hot rodder" soups up an engine and pushes it far beyond the design RPM, then indeed the inertial forces can get big enough to help in blowing up the engine, when acting in conjunction with the cylinder pressures, but even then the cylinder pressure forces typically are much larger than the inertial forces.

The inertia loads do contribute to the total loads, and for some reason the hot-rod community assumed that these loads are higher than the pressure loads, however that is almost always the wrong assumption.

There is a full discussion of the actual numbers on another forum, I will forward if you like.
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