Free horsepower, why isn't somebody doing this?

[John Brown]

Why not tap into the thrust generated by the prop to make some free boost?

According to a thrust comparison test in US Airboat magazine a 4 blade Powershift Maximus makes 1100 lbs. of thrust @ 2000 rpm, prop length was not given.

Assuming an 82" prop that would be 5281 square inches of area generating 1100 lbs of pressure, or about .2 lbs of thrust per sq. in.

If the thrust was proportional from the center of the hub to the blade tip that wouldn't be enough to make much difference... But it is not, most of the thrust is generated somewhere towards the tip of the props, with little or none at the center. Guessing that 75% of the thrust is generated in the half of the prop nearest the tip I calculated an average of 1.5 psi generated in that half, I would also guess somewhere along that half significantly higher pressures would be found.

Given a manometer you should be able to measure static pressure in different locations behind the prop to find the optimum spot to generate some free boost.

Assuming an engine with a compression ratio of around 9:1, horsepower will essentially double at 14.7 lbs of boost. Or, 6.8% hp for every pound of boost. A 500 hp engine would gain 34 hp if you could achieve 1 psi of boost.

It seems to me it would be simple to run a duct from the air intake to a location behind the prop to take advantage of it.

 

[under pressure]

Hey John, You like calculators dont you! I get your point, so you would have to build a duct system large enough to flow all the air needed for the motor . Easily enough done. OK, now the problems with the idea. The whole idea behind supercharging is to make your cylinder heads flow more air and achieve a volumetric efficiency that exceeds 100%. You would therefore need to maintain constant pressure in the duct system to realize any gain. The figures you refered to are at full thrust which is rarely achieved except at launching. For cruising speed I feel there might not be enough pressure to create boost. If it would work it would act like turbo lag,as rpm comes up boost would come up. The idea is like air scoops on drag cars,faster they go the more air pressure is generated on top of the carb. I wonder if any of the aircraft guys have seen this on airplanes where the prop is so close to the carb? If a small scoop was installed it would give a similar result as ducting. That might be a start on the experiment :idea:. Build it and tell us about it . Be sure to install a boost sensor in the intake where it counts! Or if you really want to add boost try a 6-71GMC and a couple of carbs and really have some fun :shock:.

 

[Anonymous]

sounds like it would suck up a lot of s#!t in the tube so it wouldn't work the air filter would always be cloged

 

[Gbenzx]

Guys please excuse my stupity,

but why don't we see more turbo's on the gm auto engines? Am I missing something here?
Seems like it would be an easy power boost & also a good hush kit too. Your thrust pressure could then be ducted into a special type of intake & use very little power for final boost.
Some of the older aircraft with wind driven prop generators are still around & produce abt as much juice as is needed.
A very small prop behind the big prop? Or is that perpetual?

Thanx Gben

 

[Anonymous]

ok,,,ill need 2 roll's of duct tape...3 packages of J b weld, some 6'' pvc....couple of elbows....heck ital work.....

 

[under pressure]

Great! We will have airboats on the Red-Green Show next!!!!!!!!

 

[John Brown]

Hi guys, nice to meet you.

Gben, thanks for the welcome why is it the Fort? is there a story there? I don't know why there aren't more turbos, other than turbo lag which under pressure mentioned, I think they are a pretty good way to make torque. Maybe the cost or the weight? Where I live I don't see airboats period so I really can't speak to it. Nearly every time I stop at a gas station here some guy comes up and wants to ask question about the boat for 15 minutes.

I got nothing against PVC and duct tape, I can fix about anything with JB weld, duct tape and tie wire.

I would use about anything for a prototype that wouldn't break up and get sucked in the engine or the prop. I was actually considering mocking one up out of duct board, I think carbon fiber might be pretty cool for the real thing though, these guys might be worth talking to - http://www.crawfordcomposites.com/products.htm or these guys might be able to help as well - http://www.ramairbox.com/home_page.html

Under pressure, yea, I like putting a pencil to stuff, I have been thinking about it for awhile, it's kind of like mental pushups.

And you are absolutely right, the only time this idea would make a lot of difference is when running up over 4000 engine rpm's, not necessarily a bad thing.

I know if I want real boost a blower is the answer, I might not spend a lot more money than I will have in this deal by the time I am done, and I don't even know if it will work.

My primary objective is to build a high torque motor that won't break. One of my reasons for considering this is I don't really need a lot more power, a little when I am at high rpm's would be nice, but not critical. I'm running a 502/502 with flowed and ported heads, and a trunnion shaft aluminum high ratio rocker arm system. Check these guys out - http://www.dreracing.com/ that and the head work is where I put my money.

Some extra torque from boost that's made when I am at high rpm's and cool clean air is what I hope to get.

I think the best way to make power is with cubic inches, timing and flow. I think balancing the motor and blueprinting the lubrication system is essential if you expect an engine to hold up long at high rpm's, I've proven that to myself several times. I really don't like to subject a motor to the stress big blowers, nitrous, high compression and high rpm's put on an engine if I can get around it. I used think different, I even tried running straight oxygen in a motor once...it got a little hot, ran real good for a while though. As I've gotten a little older I got tired of rebuilding them.

Just like you I've thought about the scoop idea too, had them on cars, and I'm not buying it. There is some interesting reading on the subject here - http://www.snowgoercanada.com/tech_ram_air.shtml and here - http://www.vetteguru.com/ramair/

What I'm talking about is not the same thing as "ram air". You have to be doing 300 mph for ram air to begin to compress air and have any effect. Which, by the way, is about what a prop will turn at high rpm's, I think, I haven't done the math on that one yet :wink:

As I understand it the reduction units are necessary because without them at high engine rpm's the props can turn in excess of the speed of sound (over 600 mph) which disrupts airflow and thrust is lost.

With a reduction unit, at high rpm's the prop is still turning and pushing air over 300 mph and in doing so compressing it. In order to make boost the air must be compressed to a higher pressure than the ambient atmospheric pressure. That is usable boost if you could deliver it to the engine. I don't think it makes any difference whether the boat is at cruise speed or coming up on plane, if it is pushing air 300+ mph it's compressing it. The times when the prop is at high rpm's are exactly when you need the boost and resultant torque such as when running ground, launching, running from the game warden etc... The rest of the time all it would do is supply plenty of cleaner cooler air than is available over the engine. Another advantage is no parasitic load on the engine when its not needed, or the extra moving parts and things to break associated with running blowers.

I think you also have a point about the constant pressure in the duct, but if it can hold pressure then I'm not sure it would be an issue. There would certainly be more air volume in the duct than the displacement of the engine,so it would never starve for air, unlike a lot of air filter boxes. And if the duct flowed the same or more than the air intake on the engine with at least 1-2 psi of pressure being applied to the intake end of the duct I would think it would stay relatively constant as long as the rpm's stayed up. It probably would have some fluctuation or resonance due to the big blades popping by it, but I would think the volume of air in the duct would buffer that some. I'm not sure it would be much different than a blower or turbo other than the amount of boost and when you got it. There would be some lag compared to a blower, but a turbo isn't instant power either, and it wouldn't take air long to travel the 6-7 feet down the duct at 300+ mph.

Anyway, it helps me think to put it all down in writing, sorry to torture you guys with it. If it works well enough to be worthwhile maybe one of you can build the second one, these kind of deals always come out better the second time.

-John

 

[Larry]

About 2 years ago I was experimenting with my 250 inline 6 chevy and turbo'ed it. I put a junkyard garettt t3 turbo from a volvo that I got for free and was blowing through a holley 600dp. The thing was running awesome, had boost starting at 1500-1700 rpm, and the boat jumped on a plane within its own length. Only problem I could find with my setup was after about 30 seconds wide open the power would taper off badly, I figured due to no intercooler the intake air temp would get too hot. I overproped the motor before the turbo and it would only spin 2400 rpm, after turbo it was hitting 3100 rpm at 12psi of boost. Unfortunatly the bottom end was already worn out on this motor and it let loose after about a month, blowing the tops off of 4 pistons, but it served my experment well and I still am planning a good turbo boat project in the future.

ps. the exhaust was also super quiet as well.

Larry

 

[Waterthunder]

No offense but that will never possibly work it’s called the Bernoulli effect. Where should I start (1) It can’t be done!!!!!!! The only way that would work would be to positively pressurize the air entering your carb if you did that then air can’t go any where but the carb and when you do this the prop doesn’t get any air. If in any way you try to use the air going to the prop you will create a low pressure area (Bernoulli effect) at the carb and your prop will suck air and fuel out of the carb.(2) you could never properly keep it jetted to the constant air velocity fluctuations. If you have ever built an air-pan for a hood scoop in a drag race car you will learn of the hundreds of issues that prevent this from being a possibility. Unless all of the moving air is directed into and only the carb it will never work. If your run a hood scoop and leave any area open to the rear you will run into ton’s of problems from a Bernoulli effect this will cause you to loose horsepower not make it. Trust me I speak from years of experience on this one.

 

[T-REX]

cant he steal alittle air from the back of the prop ?

 

[Waterthunder]

In theory it will work but the practicality is not there. A carb must be jetted to available oxygen and you could never jet it correctly. I know this can be debated but to actually have it operate on an airboat is not feasible. A blow thru Turbo is next to imposable to correctly jet utilizing a carb. Now a DFI will run like a beast with a turbo but that’s $4k more and you need several information inputs to allow the DFI to remain correctly jetted to the ever changing inlet pressure. It’s like anything an IF YOU THROW ENOUGH MONEY AND TIME AT IT may work. But it will take a lot of booth and to run off a prop pressure that changes ever so constantly it would be much easier to make gold from water.

 

[under pressure]

Hey Thunder, I'm confused. I thought he meant putting like scoops on the BACK side of the prop and then piping it to the carb not the front. Definately there is a low pressure spot on the engine side of the prop. I thought he meant checking for low pressure areas on the front side of the prop and that would be the highest velocity of aif coming off the BACK side in order to locate the scoops. I think that was Johns intent. Please help me so I dont go in the wrong direction. My reference to scoops was simply to put air on top of the carb. Also i had a DD cadillac and while tuning the carb on the trailer one day I actually saw fuel being pulled OUT of the venturies. 75% of the fuel got sucked back into the carb but the rest went thru the prop!! I was amazed . The prop was competing with the carb. I immediately built an air cleaner with full enclosure except in the front. We sure are getting deep in just a simple idea! :shock: This is fun, keep it up all.

 

[Gbenzx]

John ,

There is no telling why I said Fort sub thimkin I said Port.

Thimk I was still on the phone with the folks in NC. I kept waiting & waiting for them to qiut & finally just started pecking.

Fort Airboat Florida? Port Airboat N C?

Gben

 

[marshmaster pat]

Under - but if you were tuning the carb, you have the breather cover off, could that have allowed the negative pressure to suck fuel out.

 

[under pressure]

Yes Marshmaster, it was off. I had been running a regular round air cleaner . I felt if that much suction was coming from the prop there had to be a negative effect on the carbs incoming air due to the open nature of my filter. The fuel could not be taken away with the filter on but just seeing how the air acted around the carb was very interesting. The fuel was actually being pulled from the primary venturies and then sucked down the secondary barrels. It was a holley DP. I see guys with the flat Edelbrock foam filters and can only imagine whats going on inside the carb.