Tank Design

[vermin]

I am about to makea new tank for my 5m Hooker (project boat). There is an open deck area I have chosen for the tank.
I originally thought of building one above deck and covering with a casting platform (which I am going to build anyway).
After pondering about a 1000 different options I have decided to go with the following.

Suspending tank.
1. Cut the deck between the stringers and bulkheads in the area I have chosen (approx 650 x 650mm).
2. Make the bottom of the tank using 3mm aluminium to form the shape of the hull (approximately and not touching the hull).
3. Weld the bottom to a sheet of 4mm aluminium approx 700 x 700mm so that the overhang will sit "on" the deck.
4. Bed between the deck and the 4mm with sika (or something alike) and screw it in.

Can anyone see problems with this??

Of course there is the glassing of the edge of the deck, proper tank construction with baffles etc ...

The reason this appeals to me is I don't have to make a fiberglass lid and I get the most out of the space provided.

The forward (bow) part of the tank will be covered by the casting deck for access to breather and pickup. Filler will be in the middle for ease of filling :).

Can anyone see problems with this??

I have attached a crude picture of what I'm trying to do ...

Look forward to any comments.

Geoff.

[vermin]

This is an old picture of the area I am talking about. Boat look different now but still usefull picture..

Geoff.

- DELETED PICTURE - new location

[Any_Weather]

Mate looks good but one small bit of advise is that one breather is definatly not enough. You need one at least on each side so when the tank is full and you park on a hill it doesnt block the breather and syphon the fuel out. You cant have too many breathers on a fuel tank.

[peterbo3]

Geoff,
With a filler on the top of the tank you can probably dispense with a fuel level sender & go for a dipstick. More accurate, cheaper & with NO moving or electrical parts definitely more reliable.

[vermin]

Mate looks good but one small bit of advise is that one breather is definatly not enough. You need one at least on each side so when the tank is full and you park on a hill it doesnt block the breather and syphon the fuel out. You cant have too many breathers on a fuel tank.


Good point ... will definately look at the number and positions of breathers. Slight change of plans, the tank will now go in the space just behind the console. The space I thought I would get has a bulkhead there.

[vermin]

Geoff,
With a filler on the top of the tank you can probably dispense with a fuel level sender & go for a dipstick. More accurate, cheaper & with NO moving or electrical parts definitely more reliable.


Thanks peterbo3, it was an option which I thought wouldn't happen. I'm pretty keen for one of those Navman fuel guages anyway ...

Cheers,

Geoff.

[dicko1980]

With with my Hooker there are two filler necks for the one tank. The second neck is there to act as a big vent, so you can fuel up quicker and not worry about splash back. This may not be a option for you if your going to install a filler neck in the middle of the floor.

Cheers,
Rob

[Spaniard_King]

Vermin,

I am a littlle concerned with the 4mm flange and not supporting the tank beneath it. How much traffic is this tank going to have?

With all the weight of the fuel and a persons weight plus the forces applied by waves ect, is the 4mm flange going to give in?

I would be thinking of some sort of support beneath the tank such as 1/2 inch of rubber to take some weight off the flange.

also, how will you know if the tank has a leak?? will it drain out or is it a sealed compartment

cheers

Garry

[vermin]

With with my Hooker there are two filler necks for the one tank. The second neck is there to act as a big vent, so you can fuel up quicker and not worry about splash back. This may not be a option for you if your going to install a filler neck in the middle of the floor.

Cheers,
Rob

Rob, Since the tank is now going just behind the console I won't use a filler neck, just a filler cap at the top of the tank. I'll make sure I have plenty of breathers ...

Thanks,

Geoff.

[vermin]

Vermin,

I am a littlle concerned with the 4mm flange and not supporting the tank beneath it. How much traffic is this tank going to have?

With all the weight of the fuel and a persons weight plus the forces applied by waves ect, is the 4mm flange going to give in?

I would be thinking of some sort of support beneath the tank such as 1/2 inch of rubber to take some weight off the flange.

also, how will you know if the tank has a leak?? will it drain out or is it a sealed compartment

cheers

Garry


Garry, I was a bit concerned as well. The local guy I use showed me two examples which both used 4mm. I couldn't see any signs of giving way. One boat in particular looked like it had had a hard working life. He said that some people use 5mm for the top sheet ... maybe an option? 5mm may be a bit high but I have a bevelling machine and I'm probably going to carpet the floor anyway.
The bottom of the tank will end up about 15 - 20mm from the hull where I will squirt a good amount of silicon (the natural curing stuff) to act as a bit of support and adhesion.

The space isn't seeled. The floor is self draining. Under floor in bunged. If I have a leaking problem I will know once I pull the boat up on the trailer. At the moment the under floor area is powder dry.

Appreciate your advice ...


Geoff.

[vermin]

Another "old" picture showing new position ...

Approx 700 x 700

[88fishframe]

I have a mate with a 6.0m Hooker. On your second pic, where you have drawn in the GREEN lines, he had a second aluminium tank made in just that shape and had it carpeted over the top - end result is a fuel tank and extended casting deck. It works real well. Other benefit is the filler is above floor level [I have two fillers on my floor] - trust me, much better if you need to top up out at sea from a jerry can and there is water sloshing about the floor.

[FNQCairns]

Hi Vermin, these tanks are the hardest part to design in a boat I recon. I have been stewing over it for months now. Great tip above on the second breather, will include that now in my tank builds.

Here are a few thoughts that may or may not apply to you.

Be careful building a tank into a boat where if there is a leak the only place for it to go is toward the bung. That area may house electrically run units like bilge pumps etc. If you will be adding ANY electrics back there the tank need to be within a box again that can contain the leaked fuel against gravity until it can be dealt with safely.
If something did happen the courts in a civil suit would eat you alive as anything less is below the international abyc boat building code also dont forget the earth strap.

When cool fuel is pumped into the tank at the servo it very often cools the tank metal down to below the dew point and water will form on the tanks outside anywhere air is in contact this will shorten the life of the tank a great deal. Also if you live in a climate where the water could be 14 deg but the ambient temperature is in the 20s and above the inside hull bottom could condense water often. Consider coating the tank in an epoxy sealer a couple of times before fitting.

If you rubber the underside with anything make sure it has a very low or no carbon content as this will react with the bare aluminum and cause accelerated problems in the presence of water (even if the inside is bone dry to the touch there is still water working in there)

Consider the addition of many broken beads of 3m 4200 marine urethane running parallel with the boat to act as a cushion and to give some air circulation between the hull and tank to combat water and condensation. If an epoxy coating or similar is applied first the tank will last a long time it's the best option to fight pitting/corrosion.

Aluminium is not a good fuel tank material all things considered but it is cheap/light and easily worked. SS helps to avoid some of the corrosion problems related to wetness but is heavier more expensive and is more crack prone than aluminium when built commonly.

The best material is FRP, as the correct grade is impervious to water and fuel and can be sealed in completely.

With aluminum tanks there must be some way for air to circulate then it can last 10s of years compared to sometimes only a few.

Just a few thoughts, good luck with it.

cheers fnq

[vermin]

Fnq,

Thanks for you thoughts. These forums are great for tips you would never think of ...

Although there is nothing electrical under floor, I will need to think of something to prevent damage if there is a leak. Not good to have fuel splashing around at all in my books. There are cables (Steering, power etc) under floor but they are seperated from the tank by a stringer.
Will definately paint the tank with epoxy. Got Jotun Penguard set aside for this purpose.
As far as rubber for the underside goes, I got a tip from a couple of local boat builders to use a particular silicon (can't remember the number) which doesn't corrode aluminium as you mentioned. They personally prefer it because it doesn't seem to tear as easily as the respective sikaflex product. I can't comment on the 3m product as I've never used it.
I have actually scored a sheet of 316 stainless from a mate but I have decided to go aluminium. It is a shame not to use it as it is worth heaps but I have been scared off by several horror stories of cracks at welds ... although other people can't recomend it enough. I can get a sheet of 3mm and the required 4mm for around $200. Pretty cheap really.
I did think of using FRP. Decided against it as my boiler maker mate is much more skilled at boiler making then my skills at fiberglassing ... :) I think if I pull the tank out once a year and give it a good cleanup I should get a pretty good life out of it. Even if I get 10 years, that's not bad for $250.

Thanks again for your input.

Good luck with your tank.

Geoff.

PS. I had a look at your project ... great work.

[vermin]

I have a mate with a 6.0m Hooker. On your second pic, where you have drawn in the GREEN lines, he had a second aluminium tank made in just that shape and had it carpeted over the top - end result is a fuel tank and extended casting deck. It works real well. Other benefit is the filler is above floor level [I have two fillers on my floor] - trust me, much better if you need to top up out at sea from a jerry can and there is water sloshing about the floor.


Still plan to build a casting platform. Just that now I have extra room for storage ;D
Could even turn it into a built in cold box ..... the mind boggles ::)

[Duyz72]

4 things I like to see built under floor (as well as storage)

Fuel
Esky 1 Food
Esky 2 Fish
Kill Tank

but that's just me

[bazzacuda]

Fibreglass fuel tank... hmmmm that's an interesting idea I hadn't even thought of, thanks FNQ. #I've been thinking about an aluminium one - but that means getting someone else to make it. I wonder what the difficulties might be ? Might be a bit heavy ? #I'd be grateful for any experience / ideas.

regards

Barry

[vermin]

Fibreglass fuel tank... hmmmm that's an interesting idea I hadn't even thought of, thanks FNQ. I've been thinking about an aluminium one - but that means getting someone else to make it. I wonder what the difficulties might be ? Might be a bit heavy ? I'd be grateful for any experience / ideas.

regards

Barry


There are heaps of DIY fiberglass fuel tank web sites out there. I considered it for a while but got scared. Probably worth talking to your nearest boat builder.
Good mate is a boilermaker so I'm sticking with Aluminium.
Geoff.

[familyman]

Vermin dont forget the weight of the fuel slopping back and forth or worse up and down every 5 seconds on a rough trip back to port.Make sure it is well supported under the tank.
cheers jon

[vermin]

Vermin dont forget the weight of the fuel slopping back and forth or worse up and down every 5 seconds on a rough trip back to port.Make sure it is well supported under the tank.
cheers jon


Thanks Jon. Looks like I might have scored a 6mm plate for the top. Might have to call the boat Alibaba. Most of the weight will be held by this but there will be some support underneith..

Geoff.

[FNQCairns]

Barry Fibreglass looks to be the way I will be going but I might swap to aluminium at the last moment.

If you email me (link below) I have a pdf article from the net on building one.

The hardest part with the glass tanks is the fittings, apart from that the building is childs play if the right grade is used.

cheers fnq

[Mojo]

Barry, I must agree with FNQ FRP is by far the best material, It either leaks on the first day or never leaks at all, very long lasting. It will take a good pounding and flexing and not fail unlike Ally or Stainless tanks. I have had some experience with this and I can assure you you will not be sorry building a FRP tank. Not real hard to do either. Believe it or not the best material is ordinary old Polyester resin, better that epoxy and as good as vinyl ester, it's not real expensive either so you can make the tank a bit thicker. If you want to email to me your drawing with dimensions, I'll work out your materials and give you a bit of a run down on how to do it - or you could post your drawing with dimensions up here on the Forum if you are interested. It,s up to you Mojo

[bazzacuda]

Mojo,
thanks for the offer, I missed your post until just now but I will follow up when I have some info I need.
I need to do some homework to decide what the design will look like. I dragged the boat around to some tank makers on Monday to see what they might have in mind. Dropping the tank between the cross supports of the hull seems the best option but the capacity may be too small. To enlarge the tank reguires cutting a large section out of at least one of the cross supports and welding in a box section for strengthening, #and while two of the three tank makers seemed unconcerned about that, I'm not so sure. With a new boat it might be a go but this ally is 25years old now and that with the very light structure of the boat it sets the scene for any new welding cracking out before too long IMO.
So I am looking now at how I can maximize the space between the two cross supports.
At the same time I need to do some on water trials to see what my fuel usage is. So far I have run it on a standard 25litre outboard tank and still had fuel when I got home but I need to do some long runs to see whether a small tank might be ok anyway.
thanks again, I'll get back to you when I know where I'm at
Barry