How does one figure the conversion of effective thrust from a 2-bladed prop to a 3-bladed prop?

In other words, if I've got a 13x6 2-bladed prop, what diameter & pitch would I need on a 3-bladed prop to get the same effective thrust?

thanks!

You should be able to get that information on the prop list in the manual.

I had a few OS engines that gave 2 and 3 blade prob specs. for the corresponding engine.

The number of blades does not change the effective thrust by any noticeable amount, provided the diameter and RPM stay the same. A smaller diameter will result in less thrust, which may be necessary when using more blades, to keep the engine near peak torque RPM. If you can find a propeller with blades that are 2/3 the width of the two blade prop, you can use the same diameter and may even see a small increase in thrust, maybe only 1 or 2%. Here's a couple of reports on the subject

http://naca.larc.nasa.gov/reports/1938/naca-report-640/
http://naca.larc.nasa.gov/reports/1939/naca-tn-698/

There has been a rule of thumb that's been around forever.

Drop the diameter by an inch when going from a 2-blade to a 3-blade.

Make sure you retune the needle valve and test fly the sucker.

Those NACA reports are pretty neat. Thanks guys!

The rule is: Reduce diameter by 1 and incrcrease the pitch by 1

On the topis, I am building a scale avro lancaster. To be scale the props would have to be 3 blade and 8 inches in diameter. It is going to start off as a static model then when I can afford it will become a fully flying aircraft. What pitch would be most scale like?
The lancs scale is 1/10. Big sucker. It was a lot smaller in my mind when i designed it. It has a 3.10 meter wingspan. The reason i cant afford to get it flying yet is because I am an unemployed 17 year old. lol. any ideas for the design of the wings or would be fantastic. other plans would also be good.

also any ideas on where i can get 7 inch diamter wheels, or even make them. also trying to make a brake system for wheels not working to well. once i have completed the plans I am willing to distribute them for a trade of something, dont understand how to sell them. would like a pro builder to give it a go to see if my plans work for them aswell.

The Lancaster was like most bombers, it had variable pitch props. A static model would have the props pitched wherever the pilot left the setting when he parked. Or some might be different if the groundcrew had been working on them. If the model depicted a flying Lanc, then the pitch could be whatever was needed at that time in the flight. So...... any pitch would be correct.

ok then thanks will be going on holidays with family in a few days will try to convince my mother that some propellers would be a good investment.. ill see how it goes..