fuel probs

Hi all i am building a nitro air boat and i have noticed that a lot of the builds have the fuel tank level / in line with the carb
but on the life size boats the fuel tanks are lower down

what i would like to know is are there any low voltage nitro fuel pumps out there that i could use
as i don't have enough back pressure on the manifold to get the pressure that i need
i am using a tiger 40 engine which is 4" above the fuel tank
help please

I've run nitro engines on many cars in my younger days, most of the cars have the fuel tanks lower to keep the 'cog' right for the model, now you do not need a fuel pump.....


What you need to do is attach the fuel line to he carb as normal, the top of your fuel tank needs a fuel line nipple added to it and your exhaust/silencer should also have a nipple attached, you simply connect the nipple of the silencer/exhaust to the nipple on top of the fuel tank....

This when the engine starts to run builds pressure in the silencer/exhaust which is passed to the fuel tank via the fuel line, this then pressures the fuel forcing it up to the carb...

Easy pressure from the engine making it a self pumping unit,

To prime the carb I usually blow down the exhaust tube into the tank to force fuel to the carb reattach he line back to the exhaust and fire it up as normal.

Now if your not getting enough pressure you should have the exhaust nipple on the exhaust about and inch from the exhaust exit not on the manifold, but also pre blowing the tank during priming will give you enough pressure to keep fuel going.

Other methods are to shorten the tank distance to the engine.

If you can find a fuel pump the biggest problem you can face is flooding and hydrolic lock and blowing the piston and its supercharger internal fan. 

Hence why you can never find fuel pumps for nitro!

There are nitro primer pumps but that's the same as blowing down the pipe just using a winder, and some are electric pumps, honestly I wouldn't recommend converting one to internal usage!

Have you tried firing up the engine and putting your finger over he exhaust hole to help build pressure for about 30 seconds?

http://www.modellbauuk.com/jp-deluxe-hand-fuel-pump-2993-p.asp

When building a model you sometimes have to sacrifice a position of an item for practicality. 

Photo of your setup would help me identifie an issues, sometimes it's not always a tank issues it could be your tuning for idle and throttle open, choking with to much fuel, idle should be 1-1/2 turns from dead lock out and you need to probably have 2 full turns on your high speed needle for the right mixture, that was my settings on .21 and .28 30% buggy and racing trucks.
It also could be your plug running either to hot or too cold you should try a med plug until the engine has decent break In time, and then swap plugs to how he engine runs, hot engine cool plug, FYI your engine should get hot but not so hot you can't touch it after a run, a good tuned engine should run warm, and if water cooled header then it should be bone cold and run on a hot plug. 

Do you have a clutch on the shaft? That could be another issue for halting the engine.

To many things to list, I can only thank the councils for stopping ic engines In boats I forgot how much I hated messing around for the right setting and trouble shooting when things ain't right!!

thanks for the info
sorry when i said manifold i meant the exhaust
the pipes are set up the standard way
1st from fuel tank to carb
2nd from fuel tank to exhaust
3rd filler pipe on fuel tank
the problem was i was not getting enough pressure from the exhaust to pressurize the fuel tank
even when the engine was running at a high rpm

so i have taken the plunge and moved the tank up
so that the carb is about 1" from the top of the fuel tank and 2 1/2" from the bottom of the tank
i looked at the various pumps and the thought of drilling and tapping the end of the engine to put in a nipple was just too risky (for my luck)
i am just waiting for some epoxy to go off on the new fuel tank mount then i will be able to do an engine test
i will be posting some pictures of the build in the next couple of days

Looking forward to seeing those, might inspire me to do a 4 stroke fishing cruiser ( much quieter ) 

RR

hi i was going to post some pics of the build but the forums way of up loading pics is too dam complicated for me i tried following the tutorial but i got no where with it and also lost all the description of the build as well
sorry folks
i will try again another day

Do you have a Photobucket account or the like ? if so copy the bottom of the four links (IMG) and paste where you want it.

Did you try the updated method of uploading photos on the forum?

I've updated the photo tutorial to make it easier to understand... linked below

https://www.thercmodelboatforum.com/t39-uploading-photo-s-tutorial#95

Roadrunner wrote:

Did you try the updated method of uploading photos on the forum?

I've updated the photo tutorial to make it easier to understand... linked below

https://www.thercmodelboatforum.com/t39-uploading-photo-s-tutorial#95

hi roadrunner sorry no i dont have any thing like that

Ok here we go again


Hi all i finally finished my air boat after much head scratching and throwing things in the bin
i started with some plans i down loaded from Google images
cut up an old electric fan for a prop guard and hay presto one air boat well it wasn't quite that simple
here are some pictures of the build




















yahooooo it worked thanks for the new upload instructions

Haha! that looks like a monster air boat, excellent!


Look forward to see some photos of it working!

me too as it weighs in at a massive 10lb 12oz with battery and a full tank of fuel
it dose float but its a bit tail heavy

i expect you'll need a bit of weight in the bow to level it, when it pushes forwards its going to lean back anyway, but you dont want it to lean with thrust when already stern down, that's how they flip over.

i would have thought that it would lift at the back the same as life size air boats

The only way to find out is to get it on the water.

I do believe some air boats achieve lift with the fitting of an aileron on the rudders, others have the prop angle higher. 

Problems you face when making something to scale is that the elements can't be scaled and in this case water, the model may/will behave differently compared to its full size counterpart.

The best things known for being toss about a lot are scale warships, they have a serious tendency to roll like they are in force 8 storms on a mild chop or a breeze with the high superstructures, i expect that's why so many are built to a much larger scale then in the 3/4 foot range.

Obviously you won't need this on the air boat, but it dose explain why a lot of models sometimes get fitted with false keels to keep them more stable.

The same is said for scale air craft. a light wind can knock a plane about quite drastically.

My advice is during test to take some weigh with you and add it as you need, the same can be said that the model may require less weight which is much harder to achieve in a model.

Test, test, test and hope for the best!

yep i have just got to tune up the engine on Sunday and hopefully get it on the water Monday
i will take the camera with me to video and photograph the boat as it moves and see if i need to add weight
to it i have done a balance check and the tipping point is 1/3rd from the back which i understand is about the right place
i am not sure how i will be able to remove weight though
the body is built from 3mm ply the spaces under the decking are filed with expanding foam so no weight there
the fuel tank (red barrel ) comes in at 140g plus 100ml of fuel
and the engine mount and fan guard cant weigh much being just a wire frame
so i am not sure where i will loose the lbs

loosing weight isn't something easily achieved if you need to, not saying you will, the simple solution if weight loss is required is to counteract it adding more weight in another section, ( they usually call that trimming up) 

Be aware expanding foam can be heavy once its fully cured, floats like no bodys business but it can have some serious weight if a lot is used.

I expect you'll just need some weight in the bow to keep the nose from flying up and catching air for a flip other then that i'm pretty sure you'll have a reasonable first .. umm voyage...

oh on another note, do you have a fail safe fitted to the throttle on the IC? usually it jams the servo to close the carb in case of signal failure.

If not GET ONE  

https://www.google.co.uk/search?q=RC+fail+safe&oq=RC+fail+safe&aqs=chrome..69i57j0l5.1734j0j9&sourceid=chrome&ie=UTF-8#tbm=shop&q=RCfail+safe&spd=1661989026931547698

i used to run all my cars and other engine projects with one, and as another back up in case the on board battery suddenly dies, i fitted a counter acting spring to the throttle leaver, should the on board batt die then the spring was powerful enough to pull the carb closed with the servo still attached!

Used to wear out the servos quicker but i've had and see some serious accidents with throttle sticking, power failure, signal failure on cars, and in this case your spinning a prop... those things are deadly if they shatter at full RPm off the engine if you loose control and something flips or catches it, when close to others. least with air craft props you tend to be at distance when they hit the deck.

better safe then sorry.

i never new tuning an engine could be such a pain in the a@#@s
hvaeing a heck of a prob just getting it started let alone tuned
i will figure it out eventually lol
unless there is some one out there that can give me an idiots guide to tuning

Idiot guide to tuning....

Its always tricky to try to type out what to do... i've always just given the engine a fair few pulls to see if i'm getting 'flame on' and go from there, far to many things to look for or explain typing.

could just be a stiff liner ( needs break in time) to plug issues, the battery for the plug primer not being fully charged, flooded plug, tuning settings incorrect, not enough fuel pressure, fuel mix wrong, fuel mix to lean ( 0ver heating/ smoking engines) to rich, choking itself. the list is bloody endless!.

The manufacture should have provided you with some optimal startup settings for the fuel mix and the carb position which leaves you to only tune for high speed. those settings are basic but they will give u a rich mix and not kill the engine. my first suggestion is to refer back to those original instructions before progressing.

when setting up ensure to treat your engine as a model aircraft as your spinning a prop, this will help mentally.

If you don't have those, this is the standard setting for your engine!

''Turn the needle valve clockwise until you begin to feel resistance. This the fully closed position. Do not force the needle valve or you may damage your carburetor! Now turn the needle valve counter-clockwise about 2 - 2 1/2 turns. This will be a good place to start. (Turn the needle valve clockwise to “close” for leaner mixture, or counter-clockwise to “open” for richer mixture.) ''



Richer mix is always recommended on new engines until the liner has broken in!!!!!!

lean it out after about a full gallon of nitro has been run through this was the recommended instruction for when i raced cars, be sure to also look at after run oils (google!!!) this will help prevent bad startups after being idle for long periods as well as protecting the inner workings from corrosion and fuel oil build ups! 

further original instructions for your engine are here

http://mms.tiger.tw/upload/2012-5/20125303712.pdf

and you should exhaust this before looking at 'other sites so called settings'

that said

not all engines behave the way they should and you'll have to tweek with it... 

so i'll post some links with decent guides how to get engines going. i'ld be more help but like i said its hard to run through it all without playing with the bloody thing!

http://www.flyrc.com/glow-engine-tuning-setup/

( this ones a few diffrent answered FAQ's )

http://www.supertigre.com/faq/product-faq.html

thanks i will have a good read

well i have had a good read that you sent
i will have another go later on this week
something i did learn was that the spray bar is upside down which wont help things
i have now put it the correct way up with the spray  spraying into the carb instead of out of the carb lol
also i think i may have the rudders too far apart so with that and being back end heavy
i might end up redesigning the whole back end
ah well such is life
we will see how it goes once running

Good luck getting it going still look forward to a few shots running, as for redesigning the back wait until u have it running and see how it performs before committing to redesign, if you do go that far, a good suggestion may be to rebuild the entire model as a mk2 this way you can take what you have learnt and improve.

Wouldn't be the first time many of us have rebuilt a model or two and I'm the worst for it!!

thanks good point though a MK2 hmmmm thinks

When I built my brushless motor powered airboats the consensus on USA forums was to balance at 1/3rd the overall length from the stern I sat it on some 1/2" dowel and shifted batteries & RC gear plus a little lead until it balanced.

mine is balanced at 1/3rd from the rear but it still sits back end down we will have to see how it handles once it is water bound

ps it sat back end down when i put it in the bath just to see if it would float

I gues the only way to be sure on the balance will be to run it and trim accordingly, but at least it floats and doesn't seam top heavy enough to tip which is a good thing

well finally let it see water
first attempt it was sitting way down on its bum and flipped over on a slow turn
so moved the engine and frame work forward by 1 1/2 " and added some lead to the bow
and took it out again and hey we have an air boat that moves at a reasonable speed
but it just sits too low in the water at low speed and at high speed the bow lifts nicely but it still sticks its bum under water
and with moving things forward it isn't as water tight as it was so after about 4mins the control box is full of water and it now chugs along like a tug
so i think a MK 2 is called for similar design but much much lighter and a bigger engine (.61)
carbon fiber for the fan guard and no expanding foam just good seals and an air frame type hull

Lighter boat yes, bigger engine doubtful you'll need it, remember power to weight ratio, lighter it is faster it will go, look at changing prop size and pitch to maximises engine use, and tune settings leaning out to increase rpm first.

Sometimes little changes make huge differences.

Remember the internets free plans may not alway be the best ones to follow, and that maybe a reason why they are free!

It may be worth looking at one of the Dumas air boat kits, they are not unreasonably priced,most are designed for engine fitting and will supply you with a decent hull design and lightweight strong material to build it.

http://www.cornwallmodelboats.co.uk/acatalog/Dumas-Air-Boats.html

The prop guard is a big issue as you recycled a fan case, good idea but that maybe a cause to your back end issues. 

If your paranoid about protecting the prop and fingers, try a more simple guard just a simple frame covered with some wire mesh, would be good enough, or you could try using some styrene rod material, which can be plastic welded together easy enough. It will be strong enough for a decent scale guard as well as being very lightweight.

the reason for the bigger engine is that at the moment i have the biggest pitch and prop combination i can for the size of engine
that i have (40) and it isnt enough to move it on short grass
i tuned the engine to the best that i could and then took it to a RC flying club and asked them to fine tune it for me
so i have got the most out of the engine and prop
on the MK2 i am going to use 3mm and 1mm carbon fiber for the prop guard and only use 2mm steel bar for the engine mount
also i will be moving the fuel tank down onto the deck and forward and i will fit a Perry pump to the engine
(a chap at the flying club has a machine shop where he can tap and thread the back of the crank case)
as at the moment having it that high it makes it too top heavy on a slow turn and causes it to capsize
also this time the reinforcing under the deck will be the same as in an airplane wing (full of holes)
and use balsa wood for the rudders or stiff plastic
all in all it should be lighter and a lot more stable on the turns
just need funds (end of month lol)

Your welcome to do a build blog of your mk2 on the forum, I'm sure we would all take interest!

Roadrunner wrote:

Your welcome to do a build blog of your mk2 on the forum, I'm sure we would all take interest!

.
I would love to see one of these boats built from start to finish

i will be doing a build log when i have sorted out funds and dimetions

Great stuff, I'm looking forward to you starting

well i finally managed to get a couple of short vids onto you tube of the first runs on the pond

Looks great, will look forward to your build log and the run of your next machine 

Nice I want one 

thanks for the comments