crispyspa

I've searched and searched but I can't find a 3 blade propeller that would be good for a 92cc engine.
The engine is an old Homelite chainsaw, 5+ HP, I suppose the RPM is right around 6000. I build large scale RC airboats that usually have 36-40 cc engines on them, but this time I want to go REALLY big. The problem is finding the proper propeller to fit this behemoth engine. I want a 3 blade to lower the cg of the boat, and a variable pitch (if available) would be great.

thanks in advance.

krayzc-RCU

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that is going to be one bad boat

Cryhavoc

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I'd love to see some pics of those airboats!

av8tor1977

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There's a company in Canada I believe, that sells variable pitch props (ground adjustable) in two and three blade versions. I've seen them used on belt reduction drive engines in model airplanes. I believe Dick Petit of R/C Report magazine used one on a Zenoah G-62. You might also check with Mick Reeves in Europe, as it was on his reduction drive.

Post pics!

AV8TOR

arobatx

There are various manufacturers that make props large enough for your engine, no problem there....even in three blade. The trick is the variable pitch. Here is a place that specializes in some high end variable pitch setups...but it might scare you away

http://www.soloprops.com/

av8tor1977

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BINGO! That's the company I was thinking of... I've got it in my favorites now. I've been dreaming about building a belt reduced twin cylinder setup for my (not yet built) 1/4 scale Sopwith Pup, and when I do I'll need an adjustable pitch prop to get it all dialed in. One of these days...

AV8TOR

crispyspa

I re-read my original post and I neglected to mention that the prop cannot be wood. I was looking at the NRG site and...well...maybe I don't NEED variable pitch.LOL 186 bux for a 3 blade hub....uhhhhhh maybe not. I did not see composite props either.

any other links to props?

here are some pics.

av8tor1977

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Yeah, wood props and water spray don't get along well.... Nice boats. Do they turn ok with just the air rudders? I had a small one years ago and it didn't turn very well. (Air rudder only.)

AV8TOR

crispyspa

Thanks. They turn rather well actually.
I have videos....If you're interested.

av8tor1977

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Thanks, but I've got a video card problem, or something. Videos just won't run right. New computer time....

AV8TOR

arobatx

How about Bolly, Air Models, or Mejzlik? They are available on many sites, try www.desertaircraft.com for example.

crispyspa

Thank you...just what I was looking for.......uhhhhhh...Any idea on what size I should get? Remember, because it will be on a boat, the propeller has no chance to unload. I was thinking of maybe an 18 x 10 3-blade, but you guys would probably know better than I.

av8tor1977

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It seems to me I once read that to use a three blade prop in place of a two blade, you decrease the diameter by one inch, if that helps any. Also, an 18 x 10 would be too light of a prop load in general for that size engine. The 100cc range of engines use a 24 to 26" diameter 3 bladed prop or a 26 to 28" two blade prop.

A 10 inch pitch seems high for an airboat. You can either mathematically figure pitch speed, or you can use a free download program called Thrust HP. It's static thrust numbers are notoriously inaccurate, but the pitch speed it provides should be close. Estimate your boat max speed, then use the pitch that will provide that speed at the engine's max rpms. You then choose a prop diameter that will allow the engine to turn it's max rpms and not overload nor underload the engine.

For example, Thrust HP shows that a 10" pitch prop at 7000 rpms would theoretically be just over 66 mph. In real life, the speed would likely be lower because of water drag.

If you over pitch your prop, you are giving away acceleration while attempting to arrive at a speed that may not be realistic. However, design restraints as to prop length may prevent you from using the ideal prop in your installation.

AV8TOR

shaunmichaeljohnson

hi chrispy!
i thought about doing this also with a 125cc chainsaw but the prop was WAY too $$$
if i remember corectly it was a website closely related to the "DA" gas engines.
that company sold props for the DA engines that were made of holow carbon fiber.

here http://www.desertaircraft.com/page.php?Page=Bolly

try that, it is for a carbon fiber props way up in the 24x12" #3 blade props[X(]

shaun

arobatx

I would think 24x12 or 26x10 in three blade would be about right for your engine. Did you notice that they also offer 4 blade? This would let you take your CG down even lower with a smaller diameter prop.

rangerfredbob

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what about a toothed belt driving the prop and putting the engine at the bottom of the boat? toothed sprockets shouldn't be too hard to find in a junk yard as a timing belt, I'm sure several cam pulleys are aluminum and light.