i have been pondering this question for a while now.
i run gasser engines and some of you know that i have a 28cc dual ringes engine with a 11.1mm carb.
i have yet to get a tach reading and i am planning to buy one soon but for now we got a rough reading of over 10,000+ rpm on a MAS 14x7x3 pusher.

you see i run airboats but because they are basicly aircraft motors anyway and you get far more help here so this is where i posted my question.

so after hearing that i am getting around 10,000+ and most of you gasser guys like the 8000-9500 mark, i am afraid i might be getting tip flutter and exess noise from that high tip speed.

i want to get more static thrust and it does not matter if i lose at top end speed.
so in aircraft terms "i want to have extremly good vertical performance on a slow flyer".

i think that i will reverse the motor so i can use standard puller props in a pusher config (because of scale looks and all up lighter)
if i reverse the engine i am opening up a large range of props and to get maximum thrust i want large diameter and low pitch right?

both APC and MAS have 16x4 wide blade props for the 4-strokers but if i use that prop it will be spinning up in the 11,000's acording to a program called thrust HP.

so what do i do?
load up the engine more with a larger prop like a 16-18" with a pitch of 5-6"?
or do i get the APC 16x4 right up there in it's max safe rpm? remember that there is no unloading factor.

hope you guys can help me and i havent confused you too much[&:]

shaun

If your getting that kind of RPM you need to increase the load factor. Your load factor on a 16 figures out at 36. Your going to have to get that tach and try a few props and test it finding the prop that gives you a safe RPM depending on what motor you have. If you want more speed increase pitch. If your want more low end thrust increase prop length. double your prop length and add pitch. 2xlength + pitch. your running 36 now. Increasing the lentgh just 1 inch 17x4 will increase the load factor to 38. Doesn't take a whole lot to reduce the rpm. find what rpm the engine is rated for and size the prop to that rpm or even 10% less for safety and longer engine life.

Try an 18 x 6 APC prop. I think you will like the results.

AV8TOR

If you know any AMA members, there is an excellent article in the new "If It Flys" column about props, mostly relates to noise but theres a nunmch of good info!

tanks for your input guys
i had almost fogoten about hte load factor that i leared a long time ago hehehe

i thnik that i might go with av8tor's suggestion because with a 6" pitch it is less than what i have but gives me more load and according to thrust hp i will get around 8500rpm and 20lbs of thrust (giving me over a 1:1 thrust ot weight ratio[X(])

the original engine specs said that 8500 rpm was it's max but it is a full shaft engine so i think that if i did keep on running it on the 10,000 mark it would be ok but the prop is the problem being only 1000 rpm from the safe zone[:@]

so thanks for the input guys, i might have to try a few props (i have to make a new hull too because i have trashed the current one due to the punishment of my driving :P)
gotta go reverse the engine now for this to work!
shaun

Yup... but try the wide version. they make great thrust on 25cc toro's. I think actual measured thrust is between 12 and 13 Lbs.

Yeah, Thrust HP is fun to use to make comparisons and to easily get the pitch speed, but don't trust the thrust numbers as they are notoriously inaccurate.

AV8TOR

i don't trust any calculator programbe exclusively but currently it is the only thing i have because i don't have lots of expirience.
how do you messure thrust?
i have tryed putting the boat on the concrete floor sitting on rollers and hook it up to a spring scale.
same as getting your plane on a vertical right in front of you and hooking up a spring ballance, the weight it shows and add the weight of the plane and you have the thrust reading.
will this give and acurate reading? if not then how DO you messure thrust?

there is a wide blade 18x6? i must check that out.

shaun

Sounds fine to measure it that way. Any logical way you do it is fine, as long as you use the same method from test to test to keep your comparisons valid.

You are going to see a huge increase in static thrust going from your 14" high rpms setup to an 18" prop....

AV8TOR

thanks av8tor!
that is what i wanted to hear!!!
what i have now is borderline on enought thrust but this is fantastic if i can get a "huge increase" because it only needs at least another 2lbs to get better performance.
and with the 6" pitch i will not loose much speed either, but like i said...now i need to actually REVERSE the engine to get this to work!
you know, when that 3 blade is @ WOT the water droplets off tyhe boat hit the prop so hard that i am getting abrasion from the water LOL!!

thanks everyone!!
shaun

I have a stihl 28cc with an 11mmcarb.im running an APC 17x8.I tried a 18x8 carbon fiber from TBM but it seemed to make the engine sluggish.my tach broke so i dont have any figures untill i order my new one.Hope this helps a little.

when you chang the rotation of your engine remeber that your prop hub can and will unscrow of. I put set screews in mine to lock it down.

Paul

good old locktite.
but i have a few other tricks up my sleve, we have a lathe so we can machine left handed threads
shaun