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my new prop...

Thunder I may need one LOL the dents are adding up in my grass rake and the engine aint even broke in yet. I maybe could go with one of the hydaulic cylinder clutches and hide the mechanism where no one would notice it and keep them guessing. that would be a trip.
 
So, If you had an 80" 2-blade prop,(with a lot of pitch) and went to an 80" 3-blade prop, (and took some pitch out)
you would gain trust? (or un stuck power)
 
robert, i think leo would say yes, if you could still reach your peak power rpms. i changed from a 72" 2 blade signature to a 3 blade narrow sensenich 78" and it created a noticeable difference in thrust. honestly, it seems to spin us faster too but that's probably just the louder prop playing games with my head. although, i've never seen an A/C engine that wasn't responsive with a standard size and pitched prop.
 
So what you are saying is every time I stumble across a few extra ponies I should spend 2000 to 3000 bucks on a new prop so I am able to JUSTIFY LOWERING PITCH! Man you have to be kidding! Maybe your pocket book is that big but to the average airboater you better be able to prove some serious results before spending that kind of money.

The obvious solution in most cases when extra power is found is to INCREASE PITCH TO REDUCE RPM's to a suitable level and 99% of the time INCREASING thrust.


That’s just common since, not engineering




Read this post by Waterthunder Leo:

http://www.southernairboat.com/phpBB2/v ... php?t=2934



It contradicts just about everything you are saying about lowering pitch to achieve more thrust, but yet he has proved it time and time again by swinging the prop slower with tons of pitch it out-performs a faster prop speed with less pitch.

Kevin
 
Your prop isn't that sensitive to 20 HP. But getting your entire combo to match is the wrong prop or gearbox can make something a terd but changing 20HP or 1/2 degree wouldn't turn something into or out of a terd. Me and Felber have done things that we know makes 25 plus more HP and not even pick up a hundred RPM. A humid day or cool night will make over a hundred RPM difference. If your prop is close you can adjust the pitch and make it right, however if you go from a 2to1 to a 2.68to1 you may need a new prop if you motor makes good power.
 
What's up Kevin? Man I got my boat from Ronnie Thibodaux and love it. He's about to change my 2:1 to 2.38:1 so I can maintain peak hp/torque but be able to get more pitch. This should make for a lot more "un-stuck" power and lower cruising rpm's.

Ronnie's gonna try it on one of his smaller personal boats and call me Friday and let me know what type of increase he attained.

Have you been catching any frogs?
 
I get what Leo's point is that less pitch will give you more thrust. That is because there is more surface area cutting into the wind. But, I think it is better to say that a prop will be more efficient with less pitch. Does not mean you can't attain more thrust with a prop with more pitch applied. Depends what kind of set up you have.
 
You guys are forgeting about slippage. Every combination has a different slippage rate or percentage. Slippage is a big part of the efficency equation. I do know you must have some degree of slippage to produce thrust but I'm not sure what the optimal rate is. If anyone knows please chime in.

Josh
 
Slippage:

Taking a run at the airboat ramp and when you are 75% up it finding you are going forward 1 foot and backward 3 feet ! Slippage.

Scotty :D
 
Not trying to tick any one off but if you talk to Sensenich they will tell you they do not put any weight in static thrust tests. Static looks good but when a prop is being fed it all goes out the window. Truth is it only matters how it performs in the water, if you are happy with how your boat runs thats all that matters. If you are not satisfied tinker with it till you get what works best for you, no two instances are the same. There is no doubt that there are a lot of people here and a welth of knowledge to pull from but even that will only get you so far.

Static thrust testing has a lot of inconsistancies, for example try a test with same prop and engine @ 98 degrees and do the test @ 90 degrees and you will get considerably different readings. There are to many variables temperature, elevation, and humidity just to mention a few.

Trial and error, no two boats are the same but there is enough help here to hopefully save you a few trials and errors to get you where you need to be quicker.

Great forum guys, this one is getting a lot of attention rather quickly.
 
Air density ..... it changes everything.

The guys in Nebraska, or our buddy Ron in Michigan can probably run with a 1/4 tick less pitch in their props most of the time than we do and still get very similar performance just because the air their prop is biting is 'thicker'.

Remember that air is a gas, not a liquid, only because of temperature.

olf
 
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