fuel tank options

[bonneville]

OK, don't know whether this has been done before !!!!
as per a previous post, I have 2 s/s tanks under floor, probably 160 odd litres, that are badly vented. Can be re- vented, but there's also the problem that both have no fuel level senders, hence running one tank dry, lets me know what's left ! not really ideal when venturing out wide. Want to repower eventually with a 4 stroke, so am mindfull of weight.
The issue I have is the cavity under floor is full length but only 185mm high, so plastic is not an option as ive been unable to find any plastic tanks under this, aluminium tanks, ive been told, need at least 15mm underneath to be blocked up and about 20mm clearance on top to allow for the sender unit, which would leave me with a tank height of around 150, so a custom made aluminium tank taking up the full length would give me a capacity of around 150 litres. Im trying to take a bit of weight off the back with the impending re-power and trying and get a kill tank in as well, so it doesn't work....
My solution, is the question I want to put forward.... presently I have the old pedestal seats, so I was considering getting seat boxes made up, which instead of using them for storage compartments inside, getting fuel tanks made up ( aluminium), to the shape of the seat pods, which would get fixed to the floor, with the fibreglass seat pods slipped over the top and fixed !
This would certainly save weight on the back, with the fuel tanks weight then mid drift, and certainly give me options for kill tanks etc in the centre area. Ive calculated it would take about 40 odd kilo's off the transom area which would effectively mean with the new 4 stroke bolted on, I would have 20-30 less weight hanging off the transom than I currently do at present.
haven't worked out what fuel volumes I would have yet, but should be able to get around the same fuel ( 160 litres ) with this method.
very interested in comments and thoughts on the above ? silly / plausible ????
bonneville

[Spaniard_King]

Buy a 4 stroke that has fuel management so that it counts the fuel you have used... ie Honda .. They have fuel management as standard with their engines;D

No need for a gauge

[bonneville]

cheers Spaniard, the fuel management side of things im not to concerned about, the lowrance fuel flow meters with the nmea will hook up to the sounder and work no prob.. that's if I go the way of get a custom under floor full length tank made up...
but still prefer to look at seat pod idea simply on the weight issue.........

bonneville

[Secret Spot]

Bonneville, not sure of your location but Ralph of Compass Alloy Boats at the Bribie Island turnoff would be a good option to talk to he is a wealth of knowledge with this type of thing. Here is his number if you want 0417212694 cheers

[bonneville]

thanks secret spot, appreciated .................

bonneville

[tunaticer]

Just be mindful of weight concrentrated in a rather small area (pedestal size) may need some fairly serious underfloor reinforcements to carry that weight and the pounding involved.
Also worth considering is the higher centre of gavity of the fuel and how that will affect the boat.

My choice would be underground fuel cell with minimal clearances and install fuel metres instead of a fuel level guage.

I have seen tanks before where the top plate is actually the floor of the boat. A simple deck mat covers that.

[Duckfish]

Had a similar problem with my Haines. I removed the filler plug in the middle of the tank and fitted a new plug with a vent extension plus a suitable length of reinforced pvc tubing to the vent outside the splash well.
Duck.

[bonneville]

Hi tunaticer,
I hear what your saying ! as mentioned this was only a thought, hence putting it out there for comment.
I had thoughts of around 80 litres per side, with a base dimension of 400mm x 400mm and 500 in height in the seating position on both sides. Then a dummy fibreglass seat box slipped over the top....
that's 80 kilo's each. I would have thought the said tank would have to be custom made, and obviously heavily baffled. If you look at a pedestal seat with an approx. 300 diam base, then a seat on a pole, then the average 90 kg bloke swingin off it, a tank made with a 20mm flange screwed around its square sides would probably be more secure. IN fact I regularly have a deckie that comes in at about 140 kg LOL
I seem to remember, somewhere, I read a bloke that did this, cant recall where.
but I do take on what your saying.... that's why I put it out there for comment on its viability...

cheers
bonneville

[Crocodile]

Hi Bonneville,

some thoughts;
Weight;
Petrol has a density of 717 kg/m3 which means that 80 litres will weight about 58kg + the weight of the tank ???, I would expect the floor to be able to cope with that extra weight.
Height of fuel;
Floor + 500mm+ planning angle, may bring the height of the petrol above the level of the bottom carbie of the motor with the risk of siphoning if the float needle sticks in the carbie.

I think that it is a great idea.
I would not expect the height to affect stability.
Gets the weight forward.
Tanks are more accessible for maint/cleaning.
Fuel management is simpler with two tanks.
May even be possible to get custom poly tanks that would allow visual inspection of the fuel levels.

Maybe try it first with 20 litre drums of water and see what effects it has.

[bonneville]

thanks for your input crocodile,
I always thought petrol was 1 litre = 1kg, wasn't aware of your density equation.
because I would have the exit point pretty well adjacient to the floor, then routed under floor ( where the current tanks are) it would then exit as it currently does, the fuel/water filter would be higher than the exit point.
again, im only theorising at the minute, hoping to get measured responses, like yours to make a decision to try it out.
im pretty sure a heavily baffled tank would illuminate petrol surge or slop, which wouldn't effect things and the share base of 400x 400 would spread any loading around that area, regarding weight. Access for maintenance would be an advantage, fuel levels would be easy to set up, with the simple method of clear tubing running down the height of the tank, and with the addition of a fuel flow meter. I was thinking the fibreglass pod, with seat, could be hinged for access for filling. hmmmm have I been watching to many sci fi films or what LOL
will ponder on this for a while im picking.
regards
bonneville

[docaster]

Hi tunaticer,
I hear what your saying ! as mentioned this was only a thought, hence putting it out there for comment.
I had thoughts of around 80 litres per side, with a base dimension of 400mm x 400mm and 500 in height in the seating position on both sides. Then a dummy fibreglass seat box slipped over the top....
that's 80 kilo's each. I would have thought the said tank would have to be custom made, and obviously heavily baffled. If you look at a pedestal seat with an approx. 300 diam base, then a seat on a pole, then the average 90 kg bloke swingin off it, a tank made with a 20mm flange screwed around its square sides would probably be more secure. IN fact I regularly have a deckie that comes in at about 140 kg LOL
I seem to remember, somewhere, I read a bloke that did this, cant recall where.
but I do take on what your saying.... that's why I put it out there for comment on its viability...

cheers
bonneville

Hi Bonneville, i did exactly that many many years ago. Much the same size as your talking about. Worked well. I did a triangle shape in the bow as well, it was a runabout.
Considering a bloke can put 100 odd kilos in a single footprint, weight is not a problem.
Make sure they weld in baffles and keep the pickup up at the top via a tube to the back of the tank in the tank.
I had the same thread as a 44 gal drum bung to make it simple. again vent etc at the front.
cheers John

[bonneville]

gidday john,
just to clarify what you did, the proposed tank would be 400x400 base and app 500 high, so you saying to have the pickup not at the bottom as per a normal set up ? and vent from the front ? would it not be better to vent it at the top ?
sorry maybe I got confused on your description.
what size, shape did you have for the seat tanks.
cheers in advance
bonneville

[docaster]

gidday john,
just to clarify what you did, the proposed tank would be 400x400 base and app 500 high, so you saying to have the pickup not at the bottom as per a normal set up ? and vent from the front ? would it not be better to vent it at the top ?
sorry maybe I got confused on your description.
what size, shape did you have for the seat tanks.
cheers in advance
bonneville

I think from memory about 700 long x 400 wide x 200 high roughly. not as high as your proposing.
I had the filler, vent pipe and pick up all at the front at the top. The pick up tube went from that position to the bottom but at the back.
I like to be able to replace hoses without spilling fuel.
Having the height bigger than the width might cause a problem or not.
What's your boat details?

[bonneville]

its a cruise craft reef finder 5.4 mtr. so its the 2 front pedestal seat areas that I was looking at setting up the fuel tanks.
in thinking about it, your dimensions would add up to about 56 litres. I suppose the shape of the tanks could vary, with the end aim for them to be encapsulated in a fibreglass seat box arrangement ( hidden ) but special moulds could be difficult to produce, and im picking having aluminium seat pods custom made, accommodating a shorter (in height tank inside) would add to much weight, and that's presuming there was a single fibreglass seat pod on the market that would accommodate the 500 high tank I was looking at.
ive opened up a can of worms here LOL
still like the principle of the idea tho


bonneville

[chris69]

Have you had a look at santmarine fuel tanks they might have something that fits.

[bonneville]

Have you had a look at santmarine fuel tanks they might have something that fits.

yeah, nothing as narrow as I need to make it fit unfortunately. If I don't design something under pedestal, it will have to be a custom ali or s/s

bonneville

[tunaticer]

Hi Bonneville,

some thoughts;
Weight;
Petrol has a density of 717 kg/m3 which means that 80 litres will weight about 58kg + the weight of the tank ???, I would expect the floor to be able to cope with that extra weight.
Height of fuel;
Floor + 500mm+ planning angle, may bring the height of the petrol above the level of the bottom carbie of the motor with the risk of siphoning if the float needle sticks in the carbie.

I think that it is a great idea.
I would not expect the height to affect stability.
Gets the weight forward.
Tanks are more accessible for maint/cleaning.
Fuel management is simpler with two tanks.
May even be possible to get custom poly tanks that would allow visual inspection of the fuel levels.

Maybe try it first with 20 litre drums of water and see what effects it has.

Try catching 58kg plus tank weight freefalling off the back of a wave..........even if you try to avoid falling off the back of a wave it is inevitable that you will find a sudden halt in a hole.......do not think it will ever be a static weight in a static environment.

[bonneville]

Try catching 58kg plus tank weight freefalling off the back of a wave..........even if you try to avoid falling off the back of a wave it is inevitable that you will find a sudden halt in a hole.......do not think it will ever be a static weight in a static environment.

oh o.k,
I was planning to have someone standing there ready to catch it should the situation occur
he's a pretty big bloke, bench presses 350lb's
LOL

im figuring it will stay put the same way any other fuel tank does, static weight or static environment included !
by being bolted down securely, and with a heavily baffled tank inside and a fibreglass or aluminium seat box bolted over the top of that.
considering you'd also be sitting on the said seat, I recon that's not an issue.
I recon ive had an escy with nearly 50 kilo's of ice and fish in it, un bolted and been in some nasty stuff out Portland and no ones needed to make a diving catch !
but I take your point, hence, why the actual size or shape of said tank is set in stone yet...

bonneville