Prop pitch vs length

[Papee]

I'm done, well almost. I have to put a starter/gen on it yet then get it repainted. It's time to start with the new one.

[Papee]

Papee,

Somehow my reply got lost when Airhead was also replying.

Oh well. I hope to be able to hear the valve train over the mag. I was warned that the mag was noisy, so I was not surprised. When it's running, all I hear is exhaust, so far.

One of MY goals is to have a relatively QUIET airboat.

Thanks,

Duane

Yea, that's one of the reasons I wanted to go back to a carbon fiber. I like the sound when you mash the gas but quiet is better. I've already had a couple dozen people flag me down and ask what I was running because it runs so good and it's not as loud as other boats in the area.

[Flathead]

Hi all,

Just a note to let you know I just talked to Bob at competition and ordered up a 3 blade Ultra prop. Turns out Bob grew up just a few miles away from where I live, so we had a bit to chat about. I will say he went out his way to be sure I had everything I needed. He's sending me a 59 inch prop and a variety of shims. I'll cut them to length as needed.....

All the talk about prop pitch and length will be invaluable as I begin to set up my boat. A question about clearances, How much tip clearance from the cage do I want to be safe.(measured from blade tip to cage) My boat had a 52" on it, I think I could go up to 54" safely and possibly 56" with a little cage modification. My goal is to have a boat that comes out of the hole snappy and has about a 30 mph top end. Do you think I'd get better results with a smaller prop at a higher pitch or vise-versa? I'm running a 1600cc VW motor rated at 48 hp on a 10.5 x5.5 Combee Tomcat. I know, car motors are not allowed on mini's, but I have to explore the possibilities before I consider a re-power.

I'm looking forward to hearing that valve train sound. By the way I was amazed at how quiet my engine ran at 3000rpm's without a prop in it.

I would be lost without the knowledge and experience you have all offered so freely...

Bruce

[airhead]

Flathead,

I just sent you a pm but something did not look right. If you didn't get it you can pm me about the vw. Don't claim to know it all but have built many engines and still remember most of it.

Jim

[Papee]

Yea Vdubs are stuck in their own world, not enough motor for the
big Guys" and being a car motor puts it out of the mini class. They are always welcome here.

I hope I can get this out so it's understandable, my brain goes 150 different ways when I think about your questions. First, I prefer 2" of clearance from prop tip to the closest part of the cage. Some use less, if your cage is really secure and there in no movement or twist you could go less.

The performance of a boat depends on so many different things and with these low horse power setups all these things are magnified. Every little thing you do to get things right makes a difference unlike the huge HP setups you have enough power there to overcome the small stuff.

The hull is a big part of it, The hull has to be matched to the motor setup that will be used to get the boat moving. It's all about "floating your boat", getting your boat to rest in the water a light as possible. Along with this you have the surface area of the boat, the amount of area that contacts the water. At some point the surface area would cause an over amount of drag and would not be good for a low power setup.The hull must be in good shape and as free of defects as can be.

We've talked about the importance matching the prop to the motor to get the most performance from both the prop and the motor, you want to get the prop set up so it's most efficient at the props recommended rpm while making sure that you are taking full advantage of the motors torque curve. Props are as different as snowflakes, the different designs of the blades, length, pitch, etc. Choice of props by hobbyists and builders are also as different as the snowflakes.

Then there is engine tilt from front to back either pushing down on the front or lifting it as needed. As I said above this is another one of those adjustments that are very critical on a low HP setup. One washer can make a difference in the way your boat runs on top of the water.

Then there is weight distribution, spreading the weight out that is in your boat so it floats even and generally distributing the weight over the area of the boat while still having room to move around the boat. A boat will only do so much, it's up to us tinkerers to find out what that is.

When I think about this my thoughts just start racing and my head hurts I'm sure I missed a bunch, every time I try to sit and write something like this I sit here and repeat (now where was I and where am I going with this) to myself.

Disclaimer: These are my opinions and my wife says I have an opinion but it's hers. I've been tinkering for a long time but I have learned a bunch from Chuck at Airboat Pros. I've also learned a lot but reading up on prop thrust, pitch and what it all means.

[Flathead]

Papee,

If I ever get to point where I know a tenth of what you do about Airboats, then I'll know 10 times as much as I do now. I'm looking forward to tinkering and getting it all right..... thanks for the direction

Bruce