Re-pitching small diameter propellers

[gondolier88]

Hi,

As the title of the subject suggests, I have been given a couple of things to sort out for a new SBA member who's fitting out a GRP ex lifeboat hull with a VFT boiler and Cyril Taylor compound. On the list is a beautiful little brass prop' that was coupled to a Lister marine engine- it is the right diameter for the boat and is a good quality all cast 3 blader.

At a guess I would say 10" diameter x 9" pitch- of course the compound could do with a bit more water to push than that so I was wondering if anyone here had any experience of re-pitching prop's, and if so what should I be looking out for as to maximum pitch I can add, whether I should anneal first and what is the most accurate 'backyard' procedure I can use to get it done right?

Greg

[artemis]

I suggest that you read "Building Your Own Propeller" by Weston Farmer, Steamboats and Modern Steam Launches, September-October 1962, pps 18-20. Then sell the propeller you have for scrap metal price. The propeller, like the wheels on an automobile, needs to be carefully mated to the power train. What you're trying to do with a 10" x 9" prop and a Taylor compound is like trying to use a 5.30/4.50 R12 tire and rim with a Ford V-8, 351 engine. This propeller at 900 rpm might require 1.5 HP. Don't waste your time!

[fredrosse]

Re-pitching is an established technology, with formers available to commercial businesses, but re-pitching without the formers by an amature will be very crude at best.

As indicated by Ron, that prop is just not appropriate for a steamer. Steamboats need big and slow turning propellers for efficient propulsion.

Making a prop would be a good project for someone with lots of time and mechanical skill, but for a few dollars you can get a first rate manufactured propeller. I have bought many propellers on e-bay, at a small fraction of wholsale prices. Spend a little time on e-bay and what is needed will show up. I usually spend $20 to $50 for virtually new 12 - 18 inch three blade bronze props. (the new 34 x 36 prop I bought for $250 was probably the best bargain, but now I have to start a really big steamboat project)

What are the details of your application, boat size and displacement? What maximum prop size can be accomodated on the hull? We can figure out what might be a reasonable compromise in prop size.

[gondolier88]

I have no idea of displacement, havn't even seen a photo yet, it's a 23' GRP lifeboat hull that can accomodate upto an 11" dia. prop'.

I'm very aware of what he needs, however as we have a propellor to start with, if all it's worth is scrap or someone else's bargain on eBay then I see no reason why attempting something that was fairly commonplace in boatyards before hyper-revving stainless powerboat prop's etc. made the tiniest amount of pitch adjustment the difference between 120+mph and 130+mph and .0001" accurate equipment for carrying it out, however all I'd like to achieve is a couple more inches (who wouldn't...) per revolution off what is a decent little prop', if it doesn't work then we'll have to do what has been advised above.

Greg