pro27

Not the boat, the engine !

Any one have experiences running the motor inverted (head on the bottom for you challenged folk)

Makes sense to allow a lower CG. With more of the engine weight closer to the hull. Also would allow the tank to be positioned lower to be in-line with the carb, which would also move more weight closer to the hull.[8D]

Usually a bit harder to start as they tend to flood easier.
Problems with water spray getting into the carb?

LtDoc

I think that pretty well covers the 'bad' points. Not sure how much weight shift you'd really have, could be enough to make some difference.
- Doc

pro27

Granted, the overall weight is still there, but maybe a better term would be 'inertial weight'.
Airboats ARE top heavy by design of the beast.

EXAMPLE.......
From hull bottom to top of engine head, the distance is 10". If we invert the engine and lower the fuel tank, we can lower the component height by say 2", so now the hull bottom to top of engine (actually bottom of engine since we are inverted) is only 8". This should make a change in how top heavy the boat is. How HIGH you have the above deck components has a great effect on how the hull handles in turns.

I design my airboats with above deck height in mind. I will run smaller props (more rpm) with a larger pitch vs larger props (more torque) with less pitch just to be able to have a lower height above the deck.

After running rc cars for many years, it is amazing how big a difference a small change or adjustment can make.
Applying the same phylosophy here should also produce some change for the better.

Mr Cajun Gator

yes you would be lowering the CG. Got a even better idea of lowering it. mount your engine as low as it can go with th prop off. engine on the bottom of the hull. Have a shaft fabricated that would hold the prop and some drive pulleys. mount this on the stand with a set of bearings. install a set of pulleys on the engine, install large rubber O-rings for drive belts. with this you can even make it like a reducion gear. I will be fabing some up for my hulls, and I see what it cost if any one interested. Most important you will nolonger have all of that weight up high, you would beable to place the fuel tanks anywhere in the hull to set the hull cg. You could even install a starter cone on the prop shaft.
But installing the engine upside down would help a lot, just lay the hull on the side to start it, to help from flooding the piston with fuel.

TERBObob

My Feedback: (21)

HALJMAC ,
You asked if anyone might be interested ..

Well ....


INTERESTED , INTERESTED

Oh ... and PRO ... not really sure how much difference there would be with a nitro motor , but with a gasser/weedie , there is a BIG difference ! ( and thats how I have my RYOBI mounted - inverted )

pro27

haljmac......... look into the KSJ mfg Pacer Airboat. Can't remember who distributes it. I think it is still available. It uses a Heli .32 engine that if I remember right, uses a belt(s) to drive the prop simular to the fashion you describe. The engine is mounted lower to the hull on the pylon ( not as low as you describe). It used a wide, 3/8" ish toothed belt and toothed pulleys.
Not to discourage you, but the PAcer is , was, been known to go thru belts (not cheap either) rather quickly.
IMO, I still think it is an idea worthy of more attention, and would be interested in hearing how your setup turns out !

Ya know, I never thought of your simple solution of turning the boat on its side to start it. Even after many years of flying UC and RC airplanes w/inverted engines, it just never dawned on me ! [sm=sleeping.gif]

pro27

TERBO...... thinking of my gas banger yard tools, they are all inverted as compared to the position the tool is used in. I have no clue why but they start and run with no problems. Is it because they start with a choke? Maybe the amount of gas being forced into the cylinder as compared to a nitro motor. Compression ratio?

From experience, I know how easy it is to flood a nitro engine, and having it inverted multiplies those chances.
What I resorted to when starting them inverted was not to prime it beforehand, but while turning it over to start it, placing a finger over the carb/venturi, much as a choke, and when it fired up immediately remove the finger from the carb. Now, this was many years ago, and a lot of times limited because of obstructions or safe access to the carb.

LtDoc

...I wonder, has anyone tried using a ducted fan for propulsion on an airboat? I'm assuming that a smaller than usual prop could be used (showing my ignorance here), and that would also lower the 'moment arm' and the CG. Not much, granted, but some.
- 'Doc

pro27

LtDoc.......... you're really trying to test my memory here [X(]

Waaaayyyy back in the late 70's or sometime in the 80's, an article in FLYING MODELS magazine by Bob Aberle had plans/or at least an article on using one of the early fan units (I beleive a Midwest 40 unit or Turbax unit) mounted on a foam swim board. It did work, but it was not up to speed of a typical airboat. But it was very simple and an interesting project.

Taking into consideration the advancements in DF technology as well as engine HP increases over the past 20 some years, I'm sure a modern version would do quite well in the speed catagory.
A lot would also depend on the hull it was placed on. The Aberle design, like I said, was just a foam swim board type. Lots of drag as it was on the large side. (but cheap to make). If put to use on a tunnel type hull, I think the performance would be pretty good, especially with a tuned pipe sticking out the back.

Anyone out there wishing to donate a unit and engine for experimenting with for this purpose, can contact me with your very generous donation

pro27

Just dug up the details...........

Flying Models, January 1979
Aquiduct 40
48" length by 24" beam
Midwest RK40 DF Unit
K&B 6.5
plan # CB011, $9.00, copy of article available

http://www.flying-models.com

TERBObob

My Feedback: (21)

Hey PRO ,

Is this when they used those great BIG servo motors and had radio tubes in the Rx and Tx's?

( only kidding ... )

Wasn't there a fellow here on the AIRBOAT forum section that DID make a ducted fan airboat ?
I forgot what his results were ..

And for the weedie engines ... not really sure why they fire up so well when inverted .
I DO know that BEFORE I did any mods on my RYOBI that she did exactly that . choke , and fire right up . But since I modded the transfers and manifolds ( and carb ) , and lowered the jug , she NOW CAN get flooded if I don;t catch her on the first or second pull ( when choked ).
Got her pushing some air too . 19 X 14 four blader .

JJeter1954

My Feedback: (21)

To those thinking of the weed-eater airboat, I'm working on a 45" Deep V made into a Tri-Hull with a pair of 36" Goldberg floats with swing up water rudders and 4" rear wind rudders. A Zenoah 260 from an FG on road car, She swings a 16-10 Prop. Havn't put her in the water yet but she sincerely goes on land!

jasperrc

Pics man, we want to see some pics!

bign

yeah !!!! we like pics and Vids !!!!