I know there is a thread related to this in the Airboat forum but I wanted to ask here since this forum gets far more attention.. I am building a Air-propped Outrigger airboat and I want to have the fuel tank under the engine. I am planning on running a Thunder Tiger Pro .46 on a Jett muffler and 10" prop. The engine will be mounted up front and mounted sideways (exhaust on the bottom). I expect the top of the tank to be 5.5-6.5" from the carb's spraybar. Whats the best (and most cost effective) way to accomplish proper fuel flow in this configuration?

I believe you'll need to use a pulse pump, that is operated off of the crankcase pressure pulses.
like a Perry Pump for example;
http://www3.towerhobbies.com/cgi-bin...XDG59&P=ML
Also hook up the muffler pressure too.

The Perry V-30 pump, as in Earl's pic, will allow you to put the tank just about anywhere.

I've used them several times...i.e. to allow the placing of the tank over the C.G. in a Pattern ship.

Okay.. where is the best place to put the pressure fitting on the crankcase? I figured in the middle of the backplate?

Yeah...that's probably the most convienient spot for it.

Hi!'
Must ask! Why have the tank so low????
Mount it behind the engine instead on the same 8mm plywood plate that the engine is mounted on...You use a plywood mount???

The boat will be set up similar to this boat using a similar radial mount.

(photo courtesy of Hooty301)

Okay, I have to revive this thread to get a little more advice from the veterans. I spoke with Darryl Cady yesterday about using a Tettra tank and no pump. (this was recommended by someone else on the forum awhile back) He said it just isnt going to work and said that using a hopper tank would work perfectly fine and negate the need for the fuel pump. Just in case, I did pick up a Hayes 2oz for the hopper and an 8oz slimline tank for the main tank below. Before I go through the whole hassle of setting it up, is this a viable option? Darryl seemed very definitive with this being the way to go (hopper tank). The guy at the plane/heli hobby shop I talked to yesterday seemed to think the hopper probably wont work because there wouldn't be enough pressure to push the fuel up to the hopper tank. (This guy I talked to has been flying planes for 50 years) I'm still a novice with the airplane engines but I'm trying to learn as best I can, so sorry for all of my million dollar questions.. Just want to know whats best.

Thanks for reading, and any info you may have to share.

Very nice boat!!

Glow engined ducted fan set-up has a hopper tank right up near the engine / fan unit...that works well and is a std. (typical ) set-up.

Darryl Cady knows his "stuff"...
Perhaps...if you already have the necessary fuel system components to try the hopper tank set-up, you could test that system out first...if it doesn't work, then the pump is an alternative...

Have you gotten a pump yet?

I haven't gotten the pump yet. I just grabbed the two fuel tanks since I needed the bigger one anyway. I just wanted to get a little more feedback. Some people say go with a pump and some people say go hopper tank. I do not doubt Darryl in any way - thats why I grabbed the hopper tank. If it ever warms up to at least freezing, I'll set the stand up and run the engine on the break-in stand (for comparison purposes, I'll use a tach to verify the performance of both setups) and on the engine stand/setup I'll use on the boat with the hopper.

On a side note: I picked up a bucket of methanol today. I'll order some nitro next week probably, and make a couple quarts of fuel to try out. I expect my fuel costs to be about half that of hobby store pricing.

That boat isn't mine, but it is very very nice. I think that boat is up to 80mph+ running a Jett 90lx on a pipe.

When using muffler pressure the hopper tank may not work. Actually it won't help as a matter of course. A hopper tank is used to keep a full tank of fuel that doesn't slosh around with air bubbles for aerobatic flying, especially aggressive aerobatics such as with helicopters. If however you use crankcase pressure then a regulator is all you really need.

Hopper tank or not, fuel still has to be either drawn or pushed to the carb.

You can also use a Cline or Iron Bay pressure regulator systems.  They use a check valve to retain pressure in the tank as developed from the exhaust system and then a demand regulator up close to the carb.  Takes a few turns on the starter to get the pressure to build up but then the engine is off and running.  Since it is a demand system, one only has to set the carb once and you are done.

Good luck on ordering a couple of quarts of Nitro.  It is going to cost you and arm and leg to get it shipped to you.  Probably requires a Hazmat license to ship and that is costly.  See if you can find a speed shop that sells it.  Drag races use a lot of it in their fuel systems.

Don't forget to get oil too.  You will need klotz and/or Castor oil for lubrication of the engine.  Were I you, I would use a blend of both oils and about 17 or 18% of the total fuel.

Chip

I don't think nitro requires a hasmat fee.  Not volitile enough.  Certainly not in quarts.  But expensive just the same.

If the engine is using a smaller bore carburetor with a venturi, you may be able to run it just fine with the fuel tank installed down low in the boat hull. I don't think you'll be doing loops or vertical climbs too much. But who knows, my old Dumas airboat used to get airborne if i wasn't careful.
But if the engine you have has a huge carb on it, you may have to go with a fuel pump to help get fuel up to the carb.
Now you could maybe use a bladder fuel tank. This is sort of like using a balloon and you force the fuel into the bladder expanding it. the bladder then forces the fuel into the carb. A number of racing and other competitions use bladders to good effect.

Another thought is to use crankcase pressure to pressurize the fuel tank. This was quite popular for racing, etc too. One way is to use a special pressure fitting off the backplate with a tapered rocket nozzle shape to the hole, of which the hole is smaller too. The pressure pulses from the crankcase are allowed out but the pressure doesn't get to bleed off quickly because of the hole or nozzle shape. Some engines had a place on the right side of the crankcase directly below the carb where you would drill and tap a hole to use timed crankcase pressure, in this case the pressure fitting doesn't need a special nozzle shape to it.
The Cox teeDee o49's have a small plastic fitting on engine right side just below the intake venturi that you could drill out for timed pressure purposes.


Here is a pic of a FOX .74 engine with timed pressure fitting location cast into the crankcase just for that purpose. You can see it below the carb just in front of the large part of the crankcase. Anyway more modern engines usually have enough crankcase material in that area to permit one to just drill and tap a hole in that area like for a Cox tee Dee engine. To drill the hole, you need to rotate the crankshaft so the intake cutout is open to where the drill will come out on the inside. You drill a small hole. Then carefully drill a slightly larger hole part way through., this you thread for the small pressure fitting, using a bottoming tap.
Timed pressure means you can hand flip the engine to start. But using a pressure backplate may require using a electric starter.

Hi!
If you intend to have a boat like that in picture you don't need a pump!
All glow engines can cope with having the tank 3-6 inches below the carb without trouble as long as you don't fly the boat, then it will not work!

I would you a Tettra "Bubble less" tank instead of a pump. Much more reliable!

Okay.. I read through all of the replies and I'm going to sort-of condense my response to everyone. I already have a checkvalve to use on the pressure line. I've used them on my r/c cars. Darryl didn't say anything about using one of these, but it certainly wouldn't hurt. He did say that a bladder tank isn't going to do what I need it to do. "It's just not gonna do it" I asked if muffler pressure alone would be enough pressure to push fuel up 6" to the hopper tank and he said absolutely.

As for the nitro, I already have a source and yes it is expensive. However - it is shipped in quarts so there isnt a hazmat fee tacked on. Shipping for 4qts was $13 and the nitro itself is 80% nitro/20% methanol and costs $13 per qt.. Its sent this way so hazmat isn't tacked on. If it were 100% nitro then it would. I had seen some websites selling nitro mixed with klotz oil to avoid the hazmat as well. I called every speedshop (most of them within 120 miles are closed down now), race track, etc. Nobody had it or knew where to get it. I was kinda stumped until someone found a few sites I hadnt found yet and called around.

As for the oil department. I'm going to run all castor in everything I have. Synthetics have their place, but not in model engines IMO. I have 3 engines that were designed to run on all-castor fuel and I intend to run them that way. The ball bearing engines I have can run either/or, but I think the synthetics are too expensive. For what they cost, and the fact they dont really protect the internal parts like castor oil does, its a no-brainer to me. It might make a bigger mess on some crafts, but on the models I run, it is a non-issue.

I have everything sourced that I'll need - and after crunching all the math, the final cost per gallon will end up being under $20. This is a bonus two ways - I will be able to run the kind of fuel my engines should be run on (that nobody will order for me locally) AND it'll be cheaper than buying it pre-mixed. In my area, a gallon of 25% nitro car fuel costs me $38 and a gallon of 10% nitro sig plane fuel costs me $36 a gallon (in quarts - they wont order any gallons of plane fuel). That doesn't help me enjoy the hobby when 1 trip to the hobby shop costs me $75 minimum and that doesnt include gas to get there and back... (that $75 is based on a gallon of car fuel, 2 or 3 glow plugs, and a few spare parts)


My boat will be similar to the one pictured, but smaller. It is NOT an airplane with sponsons on it. This is a boat meant for water only. No air. (unless I flip it)

As said above, based on advice from Darryl Cady... A Bladder tank isn't going to do what I want it to do. He suggested using Hayes or Dubro fuel tanks to run the Hopper tank setup.

I'm not dead set in one direction or the other, however I do trust what Darryl said to do should work. I'm more or less trying to find out if anyone has done this themselves and had it work or not work. Granted most people in this part of the forum fly planes, but I am sure there's someone thats done it.

The stock carburetor that came on my TT .46 has a 0.295" venturi diameter on the top side of the carb.

Whew.. sorry for that giant rant.

Doesn't sound like a giant rant to me....it sounds like you are on the right track[8D]

I think you you should do just fine...

Like I mentioned above...glow engined ducted fan set-ups typically have 3 tanks...2 saddle tanks that straddle the duct, and 1 header tank that sits right up top of the fan shroud, or as close to the engine as possable. Pipe pressure works well...but if you have problems going lean towards the end of the tank, then there's the pump option. (or perhaps the Cline regulator )

Another option could be a remotely operated needle valve assembly. That allows you to adjust the high speed needle "on the fly" via a servo on a seperate channel. (another bit of hardware frequently used on DF set-ups )

Good enough for me. Thanks for everyones input and help. I'll run the engine on the stand and post the results.. (if it ever gets remotely close to 32°F here in MN)

I don't know who Darryl Cady is but he is flat wrong.  Although I don't think a bladder tank is necessary, and wont help, it also won't hurt.  Also a hopper or header tank won't help, but won't hurt.  They are both used to insure a full flow of fuel is maintained for aerobatic manuvers, something you won't be doing.

See the attached link.

http://www.fraserker.com/heli/uniflo...flow_works.htm

Hi!
Darryl Caddy is pylon racer! Like him I have been active in pylonracing for decades.
I can assure you that all two strokes today can cope with a 4-6inches hight difference easily as long as you don't fly with the boat! Then it's another story.

Best tank is like I said a Tettra tank! Of course it would be best to mount a tank just behind the firewall , but if that isn't possible the next best is a bubbleless tank!

I don't see how a header tank or a bladder tank such as the Tetra will push fuel to the engine.  It may work without this, but it's getting close.  Muffler pressure is good for 8  inch's of fuel head and the tank is 6" below and 2 to 3 inchs deep.  You may run out.  Also with little additional head it is going to have a larger reliance on suction, this means it will lean out more.  Because it is well below the engine a uniflow setup will not help, nor will a header tank, nor a bladder.  The only option for consistant running is a pump, or crankcase pressure with a regulator. </p>

There was a post I came across awhile back that covered the same theory as the Uniflow link you posted. http://www.rcuniverse.com/forum/m_74...tm.htm#7430203

The only real drawback to setting it up this way (granted, the link I posted doesnt use a hopper tank as its in a r/c car and not a plane or boat) is because fuel flow stays consistent, you dont get much warning when you're almost out of fuel like you do with traditional fuel systems.

The problem I have with using bladder tanks is they cost 3-5 times as much as a standard tank plus shipping as none of my LHS's carry bladder tanks AND won't give me any different results than a standard tank will. Since the boat is going to ride almost dead level and not be running sideways or upside down, I see no added benefit to spending $20-40 on 2 fuel tanks that a set of $4-$6 tanks will do.

When I revived this thread, I went at it with the intent to get information from folks that have actually used the setup I outlined based on advice from Darryl. (I wasn't real clear on that when I started, so I apologize for that.) Being that I'm not real familiar with aircraft engines just yet, I'm trying to get the best advice/information as I can so I don't have to rip the hull apart if something doesnt work. Real-world experience trumps speculation any day of the week. Keep in mind, I don't say this to step on anyone's toes as any input people have on the subject I will take, learn from, and hopefully use (with success). I already have about 2 months and many many many hours or build time in on this project and I want to set it up right the first time.

Here is Darryl's website in case you'd like to read up on him and his experience. He was a very nice gentleman and was very concise with his advice. http://www.darrolcady.com/index.htm