Tuesday, July 8, 2014

A Steam Propeller

Some time ago, I picked up this Michigan 22 x 27 right-hand propeller for Iona.  The cost was cheaper than the scrap value and I had done enough reading on props to know that the prop had an rpm-limiting disk area, among other issues ...

Iona didn't mind, I wasn't afraid of modifications and there were enough other things to do so the prop collected dust.

Fast-forward to now - the propeller shaft is in - so it was time to re-visit the propeller.  Back to the bookmarks in the propeller tomes.  I decided on a surface area, plotted where I would modify (increase) the pitch and made a template.

Out came the center punch; a cut-off wheel was mounted on the small grinder - It was "bad haircut time."

Here's the prop with a "bad haircut."  I know about these things.  Summer of '70, the neighbor boys and I purchased black powder from the feed store.  You could do that kind of thing in those days.  We perfected our copper tubing skills by manufacturing bombs of various caliber and force.  Turns out we were challenged by that age-old problem of proper fusing.  (If we had only had the Internet!)  One of our devices experienced a premature - to us - detonation.  Half my summer-blond hair was frizzed.  Not to mention the powder grains embedded in temples, cheeks and shins.  [We are far afield from steam propellers ...]  It was the kind of thing the parents would certainly notice.  What to do?  Haircuts!  Of course, no one would notice!  So we similarly afflicted youth set about shearing each other - what could go wrong?  Followed by thorough scrubbing of the powder burns; there, we looked like we were ready for church ...

The photo doesn't do it quite justice: the chord is as thick as my little finger.  The next step was to grind a rolling foil from root to tip in a way that created lift or thrust on the back of the blade.  By dressing the foil in the way I imagined, it would increase the pitch slightly.  Oh, and I chopped the tips to fit the aperture.  Here's the first blade, with foil dressed.  (The tips will wait for balancing.)

The prop is sitting front side, up.  The major metal removal occurred on the back side in order to preserve the chord profile.

This photo uses different lighting and displays more of the overall profile.  I might keep the squarish tips, after a fashion.  They might add to "the look" whatever my profiling skills lack.  By the way, tools used were 4.5" grinder with razor cut-off wheel; 4.5" grinder with standard 0.250" wheel for roughing in; 7" slow-speed grinder with foam pad and 80 grit stick-on pads for fairing.

Balancing  and final tip dress will take place on a motorcycle wheel balancing stand.


  1. Good evening Steven, can you post some more pictures of the re-shaping of your prop? I've just found the propeller I was looking for so many years. My brand new prop has the right pitch for a steamer (27") but it's still to big to suit my launch as is, hence the need to cut it down to the right dimensions.

    1. Christophe, unfortunately, I did not take more pictures than this. The main resource that guided me was David Gerr's "Propeller Handbook" - http://www.amazon.com/Propeller-Handbook-Installing-Understanding-Propellers/dp/0071573232/ref=sr_1_2?s=books&ie=UTF8&qid=1455539940&sr=1-2&keywords=David+Gerr Gerr's book has a number of calculations that while directed primarily to internal combustion, nonetheless permit the use of torque and horsepower numbers from one's steam plant. I used the calculations to determine the surface area, ideal RPM's, etc. Then I made a blade template reflecting my goal. The template was transferred to each blade using center punch marks. After the basic cuts were made, it was a matter of sculpting each blade to maximize pitch, flow and balance between the blades. Those who have more experience than I say that propellers are an art as much as a science ... I trust this helps.