Propeller Math Reality Check

[Lopez Mike]

I would say that your example is to one end of the spectrum of I.C. engines. A very low rpm engine with heavy parts. If normal automotive engines were combustion force dominated, the upper rod bearing insert would wear disproportionately more. I have not seen that.

I just ran did some back of the envelope numbers on a 4" stroke at 4000 rpm with 2.5 lbs of reciprocating weight and got 3750 lbs. of inertial loads. A 4" bore and 100 BMEP gives 1256 pounds compression load. Of course, as you note, this is average gas loads and I note that I am assuming sinusoidal motion which is not accurate.

From my experience, the only automotive engines that have serious gas pressure loads compared to the inertia loads are fuel dragsters who are getting many thousands of h.p. from 500 cu. in. Their lower main bearings are good for only one four second run!

I will admit that when I worry about engine loads, I am coming from the point of view of a builder of rather extreme engines. I have seldom seen a racing engine connecting rod fail in compression. Almost always in bending combined with tension.

Ford did some deliberate destruction tests of V8 engines back when they were first contesting Indy in Lotus cars. Their findings were that the failures were disproportionately as the piston was slowing down at the top of the exhaust stroke. The extra compression loads on the power stroke were balancing the tensile loads enough to save the rod from failure. On the exhaust stroke, bingo.

So it's not ten times as much inertial as gas loads but the inertial loads are startlingly high.

It's sooo nice to be fooling with engines now that are so lightly loaded. My shop compressor is probably under more stress than my launch engine!

[fredrosse]

Mike, what inertial load does your formula give for the Yanmar engine of 75mm x 75mm with the 0.79 pound piston at 1800 RPM?

What is the formula you are using?

The general formula I am using is; Centrifugal Force = Mass * Radius * angular velocity^2, commonly; F = M x R x Omega Squared.
The maximum inertial load on the piston wrist pin occurs at TDC and BDC. A sinousoidal approximation is probably OK, a long rod would approach sinousoidal motion.

Where the force F is in pounds
Mass is slugs, or = PoundsMass / 32.2
R the radius of the crank, = half the stroke, units of feet
Omega = angular velocity, in radians per second. RPM x (2Pi radians / Revolution) x (1 Minute / 60 seconds ) = radians per second

To do the whole analysis and one should add the extra load due to the mass of the conn rod. That involves instant centers, and the integrated moment of inertia for the rod, this starts to get complicated. The instant center of the conn rod at TDC or BDC would be the rod bearing centers length, and the angular velocity of the rod is considerably less than for the crank at that point.

[cyberbadger]

Hmm,

Maybe I should start my own thread because I don't want to hijack this thread.

Very technical answers.

I guess I should present some of my assumptions of my question...

I am going to build probably a non historical launch.
I'm getting the steam side covered. I have two candidate engines that I have run at home on steam. I will be running non condensing.
The hull, well I don't have one yet and I'm not going to pay for a new really nice one. I'm waffling a bit on the style. I'm thinking maybe a sailboat hull that has a broken mast or something along that order. Between 20'-30' feet.

But as far as the engine RPM and the hullspeed, propellor etc and the ratio to get it all working fair I was thinking about "sea trials" on a small calm freshwater lake.

So when I said it might be easier to switch gears with a transmission, or with as I've also thought of, have a bunch of different sized pulleys/sprockets/chains/belts would be easier in the middle of a calm lake then changing the propeller.

I'd like to get in the ballpark for a "sea trial" and then be able to try it on the water and change it dynamically on the water and see how things worked.

-CB

[fredrosse]

"Maybe I should start my own thread because I don't want to hijack this thread."

It is Mike and I who have gotten off the propeller sizing issue and hijacked this thread, please excuse.

This is the proper place to discuss your propeller issues.

[DetroiTug]

it might be easier to switch gears with a transmission

Most transmissions ratios are so far apart, it would not be useful for testing where small increments are needed. Unless one used a Comet snowmobile type clutch (Constant velocity).

Our small steamers are not that difficult to size a prop for, we can go up and down many inches with satisfactory although less than optimum results. In the case of an I/C inboard or outboard due to the high RPM's, changing one inch up or down on diameter or pitch can produce an unsatisfactory result.

Use a calculation, ask a fellow steamer with a similar set up to arrive at a starting point for the correct prop and then go from there. A few inches on pitch will change things quite a bit, but it will still be quite usable even if the desired result is not achieved.

-Ron

[S. Weaver]

With railway engines, it was necessary to use heavy grease for the connecting rods. Towards the last, modern roller bearings began to take over but then the rising cost of fuel and exorbitant maintenance needs put the last nails in the coffins.

Mike, we're in the multi-year process of converting all of our rod bearings over to oil as grease and its quality become more "unobtainium." In our research, we found that historically, grease had labor, delivery and cost advantages, but no mechanical advantages. Simply, the bearing had to heat to X degrees in order to release the lubricating oil from the grease. By bypassing this "heating phase" we are able to keep lower average temperatures on main and con rod bearings with quality oil. BTW, this is also in the context of approximating historic loadings with the use of the locomotives in freight service during the summer.

[fredrosse]

Mr Weaver, what kind of oil do you use? Any additives?

[cyberbadger]

Thanks for the replies,

it might be easier to switch gears with a transmission

Most transmissions ratios are so far apart, it would not be useful for testing where small increments are needed.

What about a quick change gearbox from a lathe? Sometimes they have a lot of combinations.

-CB

[barts]

It really is not that critical or difficult to figure out unless your engine is far too small for the boat.

Do you have from 1 to 3 hp per ton of displacement? A traditional steam engine that revs to
something between 300 and 500 rpm? A reasonably easily driven hull (not a barge)?

Use the equation I posted... or if that is too difficult:

How long is the part of your boat that is in the water? How much does it weigh with everyone aboard?
How big is your steam engine (bore & stroke)? At what pressure is the safety set?

Post that and I can suggest a pitch that is likely to work well.
Remember - if you get it within 15% or so, it's more than close enough.

- Bart

[Lopez Mike]

In addition to what Bart is saying, consider that all of these possible gear box solutions will likely cost more than an extra prop, make noise and require some sort of lubrication and/or adjustment. I would keep it simple. Believe me, you will have more than enough to fool with!

Mike