A White Paper on “Direct Drive Automotive Airboats”

A general, non-powerplant specific, discussion on airboat technology, ie., hulls, rigging, polymer, etc..
Uhairball
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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by Uhairball » Tue May 15, 2012 6:12 pm

So please explain to me why you can't buy a prop that is capable of
turning 4000+ rpm's??

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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by todd glover » Tue May 15, 2012 6:59 pm

Not to start a war I run 3a I'll run against a caddy on the big end for fun and I'm only a 4 banger

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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by WaterWalker » Tue May 15, 2012 7:29 pm

Uhairball wrote:So please explain to me why you can't buy a prop that is capable of
turning 4000+ rpm's??
Uhairball:

Let me take a shot at explaining an answer to your question. I passed over it earlier on this post but lets give it a try.

An O-540 would typically swing a 72" prop and I would assume an automotive direct drive competetor would try to swing a similar sized propeller.

The speed of sound and the 'sound barrier" are considered the top end limiting factor in airboat propeller prop tip speed. The top limit should be even less than the speed of sound so that the airfoils do not go in and out of the sound barrier creating extreme stress on the propeller strructurally and the sound produced would rumble through the swamp like a low flying fighter jet. So lets do the math;

72 inch diameter prop disc times Pi (3.1415927) equals 226.19 inches diameter of the circle that a tip would go around. 226.19 divided by 12 inches equals an 18.85 foot diameter circle. Multiply the 18.85 feet by say the 4000 RPMs that you mention for an answer of 75,398.22 feet that the tip would travel in a minute around that cicrle. Divide that number by 5280 feet in a mile and you find that each tip has traveled 14.28 miles miles in one minute. Multiply that by 60 minutes in an hour and you find the tips have traveled 856.80 miles in one hour. That means the tips are traveling at 856.80 Miles per Hour (MPH).

The speed of sound at sea level on a regular day is about 720 Miles per hour. Your propeller would be way above that if it held together.

Further the formulas for computing centrifugal forces that grow exponentially indicate that this blade would have trouble staying together going in and out of the sound barrier. and so forth.

At 2800 RPM, that an O-540's horsepower is typically rated at, a 72" prop has tip speeds of 599.76 Miles per hour which while still loud is well below the sound barrier, etc.

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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by :) » Tue May 15, 2012 7:43 pm

Then we need a adjustable on the fly DD prop, like the concept boat!
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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by emmitt Kelly » Tue May 15, 2012 8:35 pm

Image

Here is your adjustable prop on the fly...on an airboat it had better be big.
A constant speed engine...Maintained constant rpm by fuel and prop pitch...with reverse..you can back up a C130
the engine tach is in Percent of RPM ...100% = 13,820 engine rpm = 1080 prop RPM anything over 103% rpm .immediate engine
shutdown.

those that are watching Ice Pilots on the Weather Channel..they are using Lockheed Electras ( P3 Orion Sub chaser same plane)
It is the same engine turned upside down from C-130 engine ...T56 Allison

" IF " Prop tip speed ever exceeded the sound barrier it would have been a disaster..

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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by fl cracker » Tue May 15, 2012 8:37 pm

It feels like the government is trying to shove this DD down our throats :lol: I feel either power plant will never compare to each other and one will always out perform the other in certain situations. If you like having big boats to carry 3-4 more people and all the baggage that comes with it then a C/M is the best way to go. I have owned C/M in the past and enjoyed all of them, but like BigDaddy posted I have never seen a direct drive C/M SBC or caddie in a bone dry gardeners marsh running with me. My buddy always said he would go where I would with his aluminum panther/ 472 caddie/ 74" whisper tip, and that was true in the water :lol: that was over in one trip a 1/4 way through the marsh on one Saturday evening. Run the power plant that best suits you and your budget. I am no mechanic but I know a DD carmotor is going to need around 500 HP to atleast have around 300 hp for the props operating range. As far as reliability, i know which one is still used in planes today and that's good enough for me. :wink: I just wish y'all could have see the aluminum diamondback / Waterthunder hung up at the bus with 2 guys in the easy grass :lol:
Last edited by fl cracker on Sun May 20, 2012 8:51 pm, edited 1 time in total.
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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by Uhairball » Tue May 15, 2012 9:32 pm

WaterWalker wrote:
Uhairball wrote:So please explain to me why you can't buy a prop that is capable of
turning 4000+ rpm's??
Uhairball:

Let me take a shot at explaining an answer to your question. I passed over it earlier on this post but lets give it a try.

An O-540 would typically swing a 72" prop and I would assume an automotive direct drive competetor would try to swing a similar sized propeller.

The speed of sound and the 'sound barrier" are considered the top end limiting factor in airboat propeller prop tip speed. The top limit should be even less than the speed of sound so that the airfoils do not go in and out of the sound barrier creating extreme stress on the propeller strructurally and the sound produced would rumble through the swamp like a low flying fighter jet. So lets do the math;

72 inch diameter prop disc times Pi (3.1415927) equals 226.19 inches diameter of the circle that a tip would go around. 226.19 divided by 12 inches equals an 18.85 foot diameter circle. Multiply the 18.85 feet by say the 4000 RPMs that you mention for an answer of 75,398.22 feet that the tip would travel in a minute around that cicrle. Divide that number by 5280 feet in a mile and you find that each tip has traveled 14.28 miles miles in one minute. Multiply that by 60 minutes in an hour and you find the tips have traveled 856.80 miles in one hour. That means the tips are traveling at 856.80 Miles per Hour (MPH).

The speed of sound at sea level on a regular day is about 720 Miles per hour. Your propeller would be way above that if it held together.

Further the formulas for computing centrifugal forces that grow exponentially indicate that this blade would have trouble staying together going in and out of the sound barrier. and so forth.

At 2800 RPM, that an O-540's horsepower is typically rated at, a 72" prop has tip speeds of 599.76 Miles per hour which while still loud is well below the sound barrier, etc.

David Wine

David, thanks for the explanation. What I don't understand is why the prop would self destruct at those rpm's, but a turbine engine can have the blades spinning in excess of 10K rpm with no harm to the engine.. Thanks again. Dean

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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by Larry » Tue May 15, 2012 9:40 pm

It is all about the diameter of the prop and its relation to tip speed. the longer the prop the higher the tip speed even at the same engine rpm. They have already had wood props that could turn 4500-4600rpm over 20 years ago called speed props. They were usually under 60" diameter and thats what allows them to see such high rpms. I remember a 250cu 4.1l all aluminum caddy that was running one out at my camp in the glades around the later 80's. Sounded strange hearing a boat revving that high but it ran pretty good.

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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by Larry » Tue May 15, 2012 9:57 pm

Here is a link to a tip speed calculator that is on warp drives website that allows you to see the rpm vs tip speed of the different diameter props. using a 60" prop for example it states that you are safe tip speed wise up to 4000rpm.

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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by aircraftman01 » Tue May 15, 2012 10:17 pm

DD c/m will never hang with A/C Until they build an auto engine that produces aircraft engine torque at comperable RPM and weight.

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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by Olf Art » Tue May 15, 2012 10:26 pm

Uhairball wrote:David, thanks for the explanation. What I don't understand is why the prop would self destruct at those rpm's, but a turbine engine can have the blades spinning in excess of 10K rpm with no harm to the engine.. Thanks again. Dean
Dean, the other factor acting on a prop is centrifugal force. When you spin one up the weight of each blade compounds exponentially, putting huge stresses on the prop hub and on the blade itself structurally.
An extreme example would be the rotor blades on a helicopter ..... the rotor head may only be turning in the vicinity of 350 rpm, but at that speed the tips of those long blades are traveling at several hundred miles per hour. On a UH-1B Huey at that RPM, centrifugal force will have multiplied the weight of it's two rotor blades to in excess of 90 tons.

We've all seen pictures of what happens on an airboat when it looses part of one of it's blades ..... it usually rips the engine stand out and trashes the rest of the boat. That's centrifugal force working in harmony with a sudden imbalance.
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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by plumcrazy » Tue May 15, 2012 10:35 pm

its all about a happy medium , i love a 4 cylinder airboat built right easy on the fuel and will go where you want , 540 medium sized boat will get er done but eat fuel. the concept boat has impressed me with its fuel consuption, it burns less pump gas per hr than my 348 ci continital 4 cylinder burns 100ll , the air assist hull and the pitch on the fly prop make the boat run free and easy in deep water the dry wt of the concept is over 2500 lbs. testing the boat has shown me that airboats can be better. the main thing that will effect future airboats is federal regulations on engine emmissions and we will all have to imbrace new technology.
i would call myself a a/c kinda guy, but i feel the wind of change blowin

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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by Gangreen » Sun May 20, 2012 7:05 pm

There is a prop that can spin 4000+. Warp drive can. And something else to think about, Aircraft or a car motor was not built for the water.

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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by emmitt Kelly » Sun May 20, 2012 7:21 pm

you can stick your pickup in the mud and spin the tires 80 mph...but..what does it get you..smoke and noise
is all I know of
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Re: A White Paper on “Direct Drive Automotive Airboats”

Post by aherr520 » Mon May 21, 2012 9:38 pm

If someone can build a DD cm to last as long as an AC and not get stuck everywhere I will be impressed, if I ever get a cm it will have gear reduction.
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