how close is to close

[c chardt]

should you try to keep the prop above the transom, and how far away from transom should it be.

 

[Daddy Dave]

From what I've been told, most of the "push" from the new composites come from very near the hub. Most important prop consideration is cage clearance. If you plan to run dry a lot, the advice I've been given says: "run the longest stick that will clear your cage by at least 3 inches all the way around."

My cage is not symetrical, so I had to settle for a 72" when several friends who hunt Kissimmee hard and dry prefer the 76" Terminator. My boat pushes just fine with the 72".

 

[c chardt]

I run a 350 with a 2.12 rotator and a 6 blade turbo warp drive at 80" have not played with the pitch yet,going to try that this weekend. take some out and see how well it runs dry. wright now it dose not like running up hill very easy.but will go if i keep turning back and forth.

 

[jim]

I copied this out of a prop design paper:

"The outer portions of the blade are more efficient than the inboard portion. Most of the thrust is made between about 30% and 90% of the blade radius. Closer to the root the tangential velocity is too low, and the required thickness of the blade for structural requirements is too high for that portion of the blade to do very much. Out near the tip the lift drops off, reaching exactly zero at the tip for the same reason the lift at the tip of a wing is zero. "

The emphasis is on "most", not all the thrust is made there.

I think the question would be best answered by a prop company.

Jim

 

[marshmaster pat]

CC - I think this one will have many different opinions.

I will just tell you what the boats around here have.

Covered back deck has about 3 inches clearance between covered deck and prop, about three inches between cage and prop. (Billy Kline built)

Covered back deck has about 2 inches clearance between prop and deck and 4 between prop and cage. (Boyd Stossel built)

Panther (Classic 14' with 350 gearbox) have about 4 inches clearance between prop and transom, and has about 12 inches of a 76 power shift below the back of transom.

Mark's said about 2 inches was fine between rudders and prop, cage alittle more.

If you want I will be around 4 to 6 boats tomorrow and can get measurements. I check if you want, post a note tonight or in the morning.

I agree with Big Daddy. Alittle extra clearance is nice if you are running dry alot. Have watched a few things pass through that clearance unharmed, so the larger the gap is nice (within limits of course).

 

[Daddy Dave]

Here is someting else to consider. Warp is the only synthetic prop manufacturer using thin, narrow blades. All others are adding blade surface and cup to create push at lower rpm. Some airboaters here in Florida are concerned about warps making the new sound requirement "grade".

 

[harvey]

Here is something interesting to try. Kinda goes with what cracker said. Put a piece of flagging on a stick---with the prop turning put it near the tip---from the rear of the boat it will actually suck the flagging to the prop from the tip to about 3-4 inches----so I would say 3-4 below the transom would be a good starting place. I think each application would vary somewhat.

 

[jim]

Harvey,

I would be interested in knowing what kind of prop you did that test on, and where was the pitch set? (if adjustable)

Jim

 

[marshmaster pat]

Big Daddy - Those warp create a different pitch of noise that just drives me crazy. Just doesn't sound right. You can hear them from a good distance away, just not the classic airboat sound.

I think those paddle props noise is like the bass all the kids have, just low frequency and it carrys through everything. I would love to have a descibe meter to test several types.

Good Luck over there, sounds like regulations may be coming.

 

[Daddy Dave]

Marshmaster,

make plans now to come to River ranch resort on SR 60 about 25 miles east of Lake wales for the second annual Florida Airboat Rodeo. dates are April 29, 30 & may 1. Promises to be the best Airboat Roundup ever staged. Plenty of sound testing and everything else you might imagine.

 

[c chardt]

My prop is 4" from top of stringer 3"away from transom and 12"below the top of the transom.the regular warp drive is very noisey and the turbo is quieter than the wood prop i use to run.I did notice that when there is a little water in the hull and I hit the gas the water goes up to the front of the boat.

 

[harvey]

I was told that the prop was too far below the transom if it blew the sand in the bottom of your boat to the front. I would think this would apply to blowing water or anything to the front. But don't go movin things on what I say, but 12 inches sounds too far to me.
Harvey

 

[c chardt]

I'll build new riggin when i get a new hull and make it as high as the transom.

 

[harvey]

Don't put it as high as the transom---that's a little too high. Why don't you call Paul Dixon @ 229 246 5760--- I've never had anyone explain it to me better---however when I try to explain what he said I sound like Tim the tool man repeating what Wilson his neighbor said.

Thanks,
Harvey

 

[c chardt]

I'm going to take some pitch out of the prop and see what it will do I was told that a warp drive needs to spin around 2900 rpm and by my calculation the prop is only turning 2100 rpm the way it is.