I'll be all don moving in another couple of weeks, then I can get back to my conversion.

I have prices the props at about $10 each. That means that I can't afford to get more than 2 probably. SSSOOOOO...... I gotta have a pretty good idea of what size prop I'll need.

It's a STHIL 25.4 CC gonna be mounted on a air-boat or swamp buggy whatever you want to call it. It has a stock walbro carb, for now, but a free flow muffler. rtr wieght target is about 12 lbs. (considered light for most airboats this size)

I know that the pitch has to be lower than airplanes, because the boat never really overcomes more than 1/2 of the total drag. I know that with the .46 engines many use a 11x4 or 11x5.

CAn this engine swing a 16 inch prop fine? Should I go with the 15inch?

Hi,
If you want lower pitch you should go with an 18x6. A 16x8 is the most common size for the 25cc engine. A fifteen inch is to small for a 25cc unless it is a real dog.

Dave

Hi,

That engine should swing a 16" prop with no problem. Thrust increases greatly with diameter though, and in your case you could trade diameter for pitch. What is your expected speed of the boat? You can pitch for the expected speed at max rpms, and adjust diameter accordingly. That engine could probably swing an 18" x 4" prop at around 8500, which would provide a theoretical pitch speed of around 32 mph, and a good deal of static thrust.

Good luck and post pics if you can,
AV8TOR

I have converted and flown that engine on two planes and I like the 18x6 Master Airscrew Classic and the 18x6W APC on it. It will swing a 18x8 Zinger Pro but it loads it down a bit. If this is a swamp buggy go with the 18x6 and you will be very happy. You want to move a lot of air and the 18" prop will do just that. A 16" prop is too small for that engine IMO.

If I were using an 18" prop, what size cone do I need so I can use my electric starter?

Would a 3" be big enough or do I need somehting closer to 4"?

You will need to use a Tru-Turn Big Bird spinner if you want to use a 3". You will have to do quite a bit of cutting and balancing on any other aluminum spinner. I don't think that there are any plastic spinners that will fit the large engines/props.