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PROPELLER AIRFLOW OBSTRUCTIONS

C

COLD

Well-known member
Just wondering! You have gas tanks and radiators blocking airflow, and propellers below transom height. That propeller blade hitting the point where there is not a clean airflow will back off, probably back off an inch for about three feet untill it picks up clean air again. I know how it works because at 40 below we run what many of us call "Ukrainian thermostats" on our vehicles. Chunk of cardboard with a hole to allow some air through, you can hear and see when things are not centered. It's just a 19" fan blade but the same thing would happen with a airboat prop. So how much more thrust could you get if you cleaned things up? I think the AC Deckover guys have the biggest advantage in this respect. How many airplanes you see with tons of stuff in the radius of the prop? I just think it should be a cosideration! 8)
 
It IS. It's current thought amongst many to think the bottom of the hull is everything when the top means a lot too. The packaging size of the powerplant also has to be a consideration. It's all developement, and why there are arguments fo all sides. If everyone can look at things objectively, everything gets better.

I once had a good discussion with a A/C guy about air quality, and his superior engine packaging. When I mentioned airflow over his front two cylinders, and rear two cylinders, with basically no airflow over the centers limiting his power potential, he quit talkin to me. Shame the discussion didn't continue. I know I'm faster now...Wonder if he is....

BTW Good post Cold

Felber
 
I agree! The AC guys should get over the good looks and shroud thier engines if they want to try and keep up. Since my engine is going to be in the bottom of the hull, I was thinking about putting the rad in front of the engine and keeping the prop as cleen as I can! I'm not saying i'm not creating headache's for myself for not doing the proven but you don't make gains by standing still, By the way Felber, I wish I would have pursued my dream and gone the hp route but your pics, Some of the coolest engines I have seen. Crank trigger with the belt dist. drive with belt drive cam. Waterthunder, yours are so cool too and you guys probably think my stuff is junk but I didn't do something where I can play with somebody elses money, What a great job you guys! I have to fix aircraft ground equipment :cry:
 
It has preety much been proven that cleaning up the air flow will increase thrust and reduce sound levels. Awhile back after discussing with cntry about air flow in front of his prop he moved his radiator forward from in back of the rear support to in front of the support and gained rpm on the engine with no other changes.also the sound scientist guy recomends that there be at least one blade cord lenght between the prop and any obstructions to reduce sound.example ; A wide blade prop 12" wide should be at least 12" from radiator ,cage,exhaust ect.A far as ac shrouds I've notice that most all army corp boats run a engine shroud ,though I don't know how good they work
 
excuse me but i'm an idiot. perhaps you could post a pic of a shrouded a/c engine so i know exactly what you mean. does this airflow have an affect on the prop, the engine or boat. cold-eh is talking about prop disturbance and felber is talking about airflow over cylinders. i'm kinda lost.
 
As soon as I can run across one again I,ll take a picture of it . I sits on top of the engine and extends down the sides .I assume it's placed there to help direct air to cool the cylinders as they do a lot of idleing when doing there herbicide applications , of course I could be mistaken.
 
We just got off on a tangent Duckluv. Aircraft engines in aircraft generally have a shroud. Cold air comes in the top and hot air goes out the bottom or visa versa. Just better cooling than the good looking airboat way. Did a search, Here's some stuff! 8)

Jeff,

I just put one on my 540 boat makes a world of difference. When you get a chance give me a call I can tell you who built mine. He actually has a mold for them. I don't know if he has one for a 220 but it make be worth checking into.

G. Padrick


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I've got one (a cooling cowl) on my 540 boat. I am very happy with it. Essentially it pulls air from below the motor and keeps the cylinder head temps down. I have not had it guaged with the cowl on it but before I had it Jr. Jurnigan was seeing in the high four hundreds. (Most aircraft people say the have to be below 360 I believe for the motor to have its longevity) However, for the record I am not a mechanic by any means and I am just repeating what Jr. told me when I had the motor rebuilt. I will try and get a picture of it tonight and post it. If anybody is interested though there are two people I know of who make and sell them. I will be happy to put you in touch. The one I have is fiberglass and cost about 450.00 on the boat and the other one is aluminum and runs about 600 (I think)
 
Well here goes. In 1968 three GO 425 Franklins were purchased from military surplus for airboat use. Rusty, a friend put one on a boat. Rusty had cylinder failures - exhaust valve seats dropping. The other two engines were having the same problems. I was making the valve seats and Randolph Aero was doing the seat installation. Mr. Randolph was a good friend of Rusty. Mr. Randolph finally got tired of use and told use "you need to make a cooling shroud for that D**** engine.� We installed cylinder temp. probes on all six cylinders and went to work. The shroud was very similar to what Airgator latter sold. I have built at least 30 shrouds since 1968 and won’t warranty an engine with out one. They reduce engine failures and valve train wear, even mag. problems are reduced-mags run cooler.
 
Strength is not what I intended, Just am making an observation. My cessna 182 has shrouds and baffles. They must have put them there for a reason. I know 100 MPH climb I would be full throttle, full rich and cowl flaps open and still have to watch CHT's. I do have boats, just no airboat yet! That is why I am in my second term of "University of Southern Airboat" 8) :wink: . I tell ya with the simplicity of hanging an AC engine (don't take me wrong again, no rad, gearbox, lighter etc) I would be in there if I could get an O470 or 520 for a price I could afford. This project is escelating in price and I hope I can build it in a year for around $10000.00
Ac engine just feels cost prohibitive to me, but then you take my 489 and put a gearbox on it and the prices are pretty comparable. I would probably be just as happy with a 520 but it looks like this first boat's going to be large. :wink:
 
COLD-EH'":2jydwjin said:
Isn't thaf funny how threads turn around! 8)
Click to expand...

Yeah, well, as one who is getting ready to re-do his motor stand and some
of the rigging, I was hoping for some tips, tricks, advice, and theories
about restricting the airflow to the prop.

As to the topic of the baffles that are in your aircraft, when Pete & I
recently picked up a box of 500 pistons, there were baffles in the box and
when I help up a baffle, asking why they throw them out instead of using
them on their motors, the asnwer was pretty clear: Prop is BEHIND the
motor - More crap to fall off and get into the prop. That little 4 ounce baffle
that goes between the cylinders would kill a prop if it came off at high rpm.

matt.
 
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