SAE J755 1 1/4 Propellor Taper <-> 1" Shaft options

For technical tips, questions etc. on all subjects except Engines and Boilers.
Steamboat Mike
Steam on Deck
Steam on Deck
Posts: 54
Joined: Wed Apr 10, 2013 7:27 pm
Boat Name: Catawissa
Location: Sweetwater, New Jersey

Re: SAE J755 1 1/4 Propellor Taper <-> 1" Shaft options

Post by Steamboat Mike » Mon Sep 26, 2016 12:15 pm

Cyber Badger, a few thoughts on props and shafts......

Left hand threads on shafts intended for right hand propellers is recommended by prop manufacturers. I first saw it in a Columbia Propeller catalogue. While it may seem like belt and suspenders, the theory behind it is that a left hand thread has a tendency to tighten with right hand rotation. I have never had a vehicle with knock off wheels but is is my understanding that the knock off nut on the port side of the vehicle is left handed. Also Chrysler used left hand threads on the same side from the 1930's to 1971 for essentially the same reason. I used left hand threads on Catawissa because I have a fairwater nut (essentially a large, deep streamlined acorn nut) for the prop and it is not possible to use a cotter key since the end of the nut covers the shaft. I applied Loctite PST thread sealer to the threads, it is primarily a sealer rather than a locker, stays slightly gummy and makes removal easier, while (so far, for 21 years) has prevented undesired backing off of the un-cottered nut. I machined a shallow vee groove on the hex, like you find on fuel gas nuts, to remind me or others that it is left handed. Left hand threads may be not be strictly necessary, but I like the unusual solution to a problem.

As to the taper on the shaft and machining of same: the taper for all props in our range of sizes is the same, 3/4" per foot. If you have a piece of prop shaft with an existing taper of known accuracy it is easy to set the taper attatchment or compound rest very accurately and save the time and metal of trial and error. Set the known taper shaft in the chuck, adjust it for concentricity to your satisfaction with a dial test indicator. Next, bring the test indicator to contact the shaft by moving the carriage toward the headstock until it has a reading of ten or so thousandths. The exact number doesn't matter. Next use you carriage stop or improvise one with a machinist's clamp. 3/4" per foot is 1/16" per inch. Use a 1" gauge block or other other known inch such as selective pairing of two ground tool bits checked with a micrometer. The idea is to move the carriage exactly one repeatable inch and adjust the taper attachment or compound as needed. A tiny plastic mallet is very helpful for fine adjusting. Repeat until the indicator shows no change from your initial reading. It takes longer to write the procedure than it does to actually do it. On my lathe I just leave the attachment at 3/4" per foot, it is very useful for cutting N.P.T. pipe threads, as well as prop shafts, and stays untouched for long periods. I cut a long taper on a piece of 1 1/2" cold rolled steel as a test bar and have found it most useful as a lap for propeller bores. Spread a small amount of 220 grit lapping compound with your finger on the shaft placed upright in the bench vise. Gently place the prop on the taper and use a series of back and forth movements to lap it in. Check frequently, you don't want to enlarge the bore too much, just remove the ridges left by the reamer. It is quite surprising how rough the taper is in commercial props.

The other item of interest is taper shaft reducing bushing. I had a boat in the shop some time ago that had a 1 1/4" shaft and an unusual size bore on the prop that was to be fitted. It might have been 1 3/8" or something else odd, I was unable to find a bushing from the usual sources at a price that was acceptable to me or the boat owner. I was ready to machine one but hated to take a piece of solid stock and put most of it in the chip pan to make what is essentially a piece of sheet metal....... So through the magic of simple trigonometry I calculated the thickness of the space that would be filled by the bushing. I then found a piece of copper that was just the right thickness and made a paper pattern of the flat shape needed to roll a proper size cone. After annealing, the copper was formed around the 1 1/4" shaft by hand and tapped down with a leather mallet. The key way was filed out to clear the key and a compromise key made that fit the shaft and prop allowing for the bushing thickness. The description of the procedure is longer than the actual doing. The bushing has been in service for many years. Just another way to skin the proverbial cat.

Happy steamboating. Best regards, Steamboat Mike.
Post Reply