Propeller Math Reality Check

[barts]

One of the nice things about steam engines is that they develop maximum torque at stall... so fitting an under pitched prop. to allow for headwinds, etc. is not needed. If your progress is slowed by either wind or seas, the boat (and engine) will slow somewhat... but since the torque curve is rising all the way down to stall, you will always make forward progress (within reason, of course). This is not the case with a car or truck pulling a hill in a gear that is too tall; once the engine slows past peak torque, downshifting is required.

- Bart

[SailorHarry]

I started out my steam experiences fifty years ago with any number of wonderful rules and ideas most of which have fallen by the wayside. The real world keeps sticking its nose in.

Wise words.

[guyvapeur]

I am observing this blog with some interest. I have recently put a 26 foot by 8.5 foot boat in the water and I am enjoying every minute of it.
Some of the boat data I can provide is that the engine is a 5x4 Troy Engberg with a 22 inch flywheel. (I could not locate an original and opted to install what was available.)
The boiler is a vertical fire tube with 40 sq. ft. of heating surface with about 2 sq. ft. of grate.
Now, I have installed a 20x18 3 blade prop. with good success.
I have tried and retried to install a 20x27 4 bladed prop. and always run into hammering in the drive train.
I have re-installed a 20x17 5 bladed prop. onto the boat and it works wonders.
I have not spent one minute trying to figure out what I should be using as, I have to use what is available to me.
Because the engine has such a big bore, I opted to have a 42 tooth gear on the engine and a 22 toothed gear on the propeller shaft which are chain connected.
I have obtained 7.5 mph with the 20x17 but one must factor in the current. Upstream I can get 4 mph. ( I have not even converted my GPS to give me knots per hour.)
I will be able to put the boat to the real test as I hope to be in Moultonborough. New Hampshire next month at which time I may be able to supply an update.
Best of luck in your endeavours in finding the right propeller.

[fredrosse]

19 x 22 is my vote

[cyberbadger]

I haven't even gotten a hull or prop yet, but I have been thinking that I may want to include a gearbox transmission from a lawn mower, or maybe different sized pulley's/sprockets so I can play around, change with on the water.

Changing a propeller on the water would be a lot harder.

Does this seem sensible?

-CB

[Lopez Mike]

I don't have any problem changing my prop as Folly spends much of its time on a trailer. I know, no help at all.

But it does make me wonder how many of our steamers are trailered on a regular basis. Or at least spend the off season on one. I have a couple of friends who rent a slip for the Summer.

Maybe it would be easier for those of you without trailers to chime in. I hadn't given it much thought until just now. Just assumed everyone trailered their boat.

Mike

[barts]

Picking a prop. isn't difficult, and with the very flat steam engine torque curve isn't particularly critical.

David Gerr's Propeller Handbook (less than $20 from Amazon) provides several methods of picking the perfect prop for a variety of different types of water craft.

My ad-hoc rule of thumb for a steamer is this:

Figure out what pitch you need, given hull speed in knots = 1.3 x square root of waterline length and assuming 30% slip and desired engine rpm....

For example, on our proposed Sea Lion, WL length is 30 feet, desired engine rpm is 400.

1.3 * sqrt(30) * 1.15 miles/knot * 5280 feet/mile * 1 hour/60 minutes * 1.3 slip * 1 minute/400 revs * 12 inches/foot = 28 inch pitch.

I like square wheels and we have the room, so we looked for a 28" x 28" prop on Ebay and got one delivered for about $100 .

If you don't have room for a square wheel, fit the biggest you can and kick up the pitch a little perhaps. The only reason to fit a gearbox is if you cannot fit a prop that has the right pitch or the pitch/diameter ratio gets over about 1.5.

For Otter, the same calculations suggest a 20.5" inch wheel - we have a 16 x 20, which works fine. We started with the prop from the old engine 13 x 17, which caused the engine to over-rev.

David Gerr's methods indicate we'll be up around 72% propeller efficiency on Sea Lion, which is much better than most installations (closer to 60%), but that's what happens w/ a lightly loaded square wheel.

- Bart

[DetroiTug]

One thing to keep in mind on steep pitched props, the calculations working from steam generated and desired rpm, speed etc. suggest a particular size and pitch; but what about the engine lower end? The optimum sized prop may be like driving a car in high gear all the time. The cylinders/pistons don't care, but the crank is going to suffer with very high loading on the bearing surfaces. It's true that steamboats and ships typically ran at very low RPM, but they had crank and rod bearings to handle the loading, most smaller steam engines do not in my opinion. There is a story in MS&SL IIRC about running a working tug at higher than normal pressure, even then the crank/rod bearings were ruined. I experimented with different sized props until I found one that the engine and I both like. Being a machinist I've listened to cutters long enough to know when they are running at adequate speeds, I did the same with the engine. My philosophy is let it spin, don't overload it, fuel is cheap and engines are expensive.

-Ron

[Lopez Mike]

It's a complicated and interesting problem. On relatively high speed reciprocating engines like Internal combustion ones, the inertial loads far exceed the loads from the cylinder pressure. Typically by perhaps ten to one. At low speeds the opposite is true.

That's one of the arguments for roller or ball bearings in steam engines. That they don't depend on a layer of oil to work. A plain bearing is at its worst at low speeds. No way for a 'hydrodynamic wedge' to form without quite large areas.

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

[fredrosse]

"On relatively high speed reciprocating engines like Internal combustion ones, the inertial loads far exceed the loads from the cylinder pressure. Typically by perhaps ten to one. At low speeds the opposite is true."

Just a few days ago I went thru this issue for an 1800 RPM marine Diessel Engine, 75mm bore and stroke. For the engine I evaluated, the peak inertial loads on the wrist pin, connecting rod big end bearings, and main crank bearings were only about 15% to 20% of the piston pressure loads. And the piston pressure loads were based on BMEP (Brake Mean Effective Pressure), while actual peak pressure loads are several times higher than BMEP in a Diessel (or spark ignition) engine.

So for that engine at least, the inertial loads are only a small fraction of the piston pressure loads. These loads vary as the RPM squared, so if the engine ran at 3600 RPM, the inertial loads would then be 4x higher, still far less than the BMEP piston pressure loads. At 7200 RPM, the loads are getting close to one another (inertial loads about equal to BMEP Pressure loads), and something like 11,000 RPM would be required for the peak inertial load to be equal to the peak piston pressure loads. The engine I evaluated was redlined at 2400 RPM, and many newer Diessel engines are running higher RPM, but they probably have lower piston and rod mass too.

I have herd this statement several times previously (that inertial loads are the big item, 10x the pressure loads), but as far as I can see with real calculations, this may be an old wives tale, or only applicable to screamers like chain saw engines and race engines? I would like to see any data on other real engines if possible.

Would anybody like to check my calculations?