new boat launch-propeller question

[Peter H1956]

Launched my boat yesterday. It is assassin by Wes Farmer with a Hasbrouck #1 engine. My prop is a 12 x 12. 300rpm gives me 3 mph at 60-100 psi. 400 rpm gives me 4mph at 60-100 psi. 500rpm gives me no increase in speed. At 400 rpm I have to keep turning my steam nozzle (1/16 dia) off and on to keep the psi from going over 100psi. Most steamboats would have more pitch than I have. Am I being held back by my prop? My boiler is on the small size (9sq ft heating surface) , all copper, but seems adequate if I feed it good wood. If I need a prop can anyone recommend a source?

[RNoe]

Nice work on your Assassin! Congrats on getting it onto the water.

My instant thought is that your prop has insufficient pitch.
It is difficult to "over prop" a steam engine, and I think a higher pitch will get you improved performance.
Sources? Not sure for the smaller props. Making your own prop a is possibility and there are people on this discussion board that could assist you.
(Just not me...}

And assuming your hull is fully a displacement hull at 16 feet long, top water speed (in knots) will be approximately 1.34 times the square root of the water line length.
That calculates to about 5.3 knots max.
Go for a higher pitch prop, and please let us know the results.
RussN

[Kelly Anderson]

A rule of thumb for steam launch propellers is to make the pitch 1.5 times the diameter, or 18" in your case. That may not be exactly right for your boat, but it is almost certain to be satisfactory.

Do you have a pressure gauge on your engine's steam chest? That can be very helpful in determining how much the propeller is loading your engine. My first propeller on my first boat was a 16" x 16", and I was shooting for a top engine speed of 500 rpm. On the first trail, the engine was nearing 500 rpm with a steam chest pressure of roughly 40 psi ( the boiler pressure was 150 psi). By the time I got the engine loaded down to 500 rpm while working at near maximum boiler pressure, the propeller had grown to 17" x 27", 1.59 to 1 pitch to diameter ratio.

If you are obsessed and good at math, buy a copy of Propeller Handbook by Dave Gerr. It is very readable and the formulas are understandable to the common man. Note that finding the best propeller can be a matter of trial and error for even the largest ships. IIRC, even the Queen Mary needed a couple of propeller swaps until they found the optimum combination.

[barts]

"Propeller Handbook" is indeed well worth examining; the math is pretty empirical. In general, for displacement boats you want to run the largest practical propeller diameter. One thing to note; the most efficient props in abstract are typically near 1:1 pitch/diameter ratio, but most small steamboats cannot swing a large enough propeller so the designers choose the diameter that fits and has adequate tip clearance and increase the pitch instead to load the engine down to a practical and comfortable rpm.

A larger diameter propeller will provide more bollard pull (stalled thrust) and will be affected less by increased resistance due to wave or wind actions.

- Bart

[Peter H1956]

Is there any problem with using a left hand prop? They seem to be more available on E-bay

[Kelly Anderson]

That depends on the engine. Many designs have a definite forward direction built in, and that should be respected. For example, some crossheads are designed with only half the bearing area when turning counterclockwise (when viewed from astern) as they do when turning clockwise.

20230618_141720.jpg (104.02 KiB) Viewed 11558 times

Likewise, valve gear can have a definite forward direction built in. For example on Stephenson valve gear, where the the reversing links connect to one end of the link, then the valve gear should be set up with that end of the link as forward gear. The reason for that is that the link block slip is much less on that end compared to the other end, meaning that the valve events will be more accurate. Since the engine will spend 99% of its life going forward, it pays off to pay attention to that detail.

20230618_141800.jpg (148.56 KiB) Viewed 11556 times

If your engine has equal crosshead bearing area in either direction, and valve gear that works equally well in either direction, then you are free to run rhe engine in either direction that is convenient.

[Kelly Anderson]

In an edit to my post above, after rereading your first post saying that you have a Hasbrook #1 engine, and finding this video of it in operation, I'll modify my statement to say that the tunnel crosshead will allow the engine to run equally well in either direction. If it was mine, I would reset the eccentrics to have the forward motion end of the link timed to drive the engine in the opposite direction.

[Peter H1956]

There is a 14 week wait to have a new prop made so I will search e-bay for one. I the meantime I will explore the possibility of making one . In the 1961Nov-Dec issue of Steamboats and Modern Steam launches there is an article where he mentions about the possibility of making one out of fiberglass. In the 1962 September-October issue there is an article by Wes Farmer on how to layout a prop. Might be a fun project.

[RNoe]

Here is a YouTube video of Steam Launch "Reciproca."

https://www.youtube.com/watch?v=BDv_J1RXTB4

Note the home made 2-blade prop and how it propels that 20 ft. long(?) launch at 6+ knots with just 2 horse power.
The prop is easily visible at the beginning of this video.

I have not found a description of how it was laid out and fabricated, and would like to learn that.
But it illustrates what is possible.
RussN

[Peter H1956]

A two blade prop would be alot easier to make than a 3 blade but I do not know if that would work for my boat. I would think I would need a larger diameter than 12".