Fuel tank. Above or below deck?

[Autolux]

Hi.
I have an Atlantic 21/22. I had pretty much commited to a new stainless under deck tank until I saw this on the website of XS Ribs:

1) Under deck tanks do not lower the CoG as the tubes on the craft negate any effect that moving the fuel 50 cm vertically has.



2) There is normally less capacity under the deck than in the console.



3) Due to the greater foot print of the tank the amount of unusable fuel compared to a smaller foot print and taller on deck tank is MUCH greater.



4) Under deck tanks will suffer more condensation than in console ones



5) Fuel senders, which are electrical, do not last as long in a damp under deck installation when compared to a well ventilated in console tank.



6) When a boat is completely swamped there is a greater risk of water ingress in to the tank compared to an above deck installation.


So should I just go for a tank inside the double console I am installing on the deck?

[jwalker]

I'm surprised you found all that on the XS rib website, IMHO it's almost entirely incorrect.

The one disadvantage of underdeck tanking is that if you do get a serious problem you may need to lift the deck to fix it. However, if the tank is well designed and properly fitted it can be a lifelong installation.

[GuyC]

Originally Posted by Autolux

Hi.
I have an Atlantic 21/22. I had pretty much commited to a new stainless under deck tank until I saw this on the website of XS Ribs:

1) Under deck tanks do not lower the CoG as the tubes on the craft negate any effect that moving the fuel 50 cm vertically has.

This is just not true. Having a full tank of 180 litres (135kg excluding the tank) lower in the boat will lower the VCG. The tubes are generally not in contact with the water when the boat is on the plane.

2) There is normally less capacity under the deck than in the console.

Obviously this is dependent on the size of the console and the under deck space. The A22 has a lot of space below the deck so I would say you can get more below the deck than you could in a console particularly as the console also potentially contains batteries, electronics and your packed lunch.

3) Due to the greater foot print of the tank the amount of unusable fuel compared to a smaller foot print and taller on deck tank is MUCH greater.

If your fuel pick up pipe is at the lowest point of the tank, the difference in unpumpable fuel will be negligible.

4) Under deck tanks will suffer more condensation than in console ones

I don't have any proof but I wouldn't have thought this was correct. If it is variations in temperature that cause condensation, I would have thought a console tank may be more prone to condensation.

5) Fuel senders, which are electrical, do not last as long in a damp under deck installation when compared to a well ventilated in console tank.

I think this is debatable. A small hatch in the deck will provide access to the sender and pickup tube etc whereas the same access to c console tank may be vying for the same space as the electronics.

6) When a boat is completely swamped there is a greater risk of water ingress in to the tank compared to an above deck installation.

If the filler pipe and vent pipe are sealed, then I don't think the risk is any greater.


So should I just go for a tank inside the double console I am installing on the deck?

No

(all IMO of course)

[Chanchan168]

I had a 300l under deck tank on my ballistic that split. Hell of a job lifting the deck, getting the old foamed in tank out and then having to re glass a new deck in.

Wouldn’t do that job ever again. I now have a 160l tank in my console as I can see it, it has less chance of condensation trouble and is easy to fix if something goes wrong.

[ribber]

Chanchan just out of interest how much fuel did that old tank hold. Looking at the fence panels I guess six feet long by 4 feet wide.

Curious, as if I did the same (if under tank leaked) I couldn't get the capacity of fuel into, say the jockey seat or back bench seat. How did you do it?

[Wightdiver]

Originally Posted by Chanchan168

I had a 300l under deck tank on my ballistic that split. Hell of a job lifting the deck, getting the old foamed in tank out and then having to re glass a new deck in.

Wouldn’t do that job ever again. I now have a 160l tank in my console as I can see it, it has less chance of condensation trouble and is easy to fix if something goes wrong.Attachment 132806Attachment 132807

I think the dimensions of this tank demonstrate point 3 above really well. Even with baffles you would have to have a significant amount of fuel in the tank to give enough depth consistently for the pick up to not suck in air.

[ribber]

Sorry Chanchan should have read your post properly.... Can you share what tank you used holding 160l, re its material construction and shape, as it's a great idea once under deck tank becomes a problem.

An upright rectangle tank in the jockey seat would work, but getting anything in the console for me would be tight for space.

[ribber]

The more I look into it there are quite a few options in plastic fuel tanks of various shapes and sizes.

http://www.floatyourboat.co.uk/fuel-...ies/fuel-tanks

[jwalker]

Originally Posted by Wightdiver

I think the dimensions of this tank demonstrate point 3 above really well. Even with baffles you would have to have a significant amount of fuel in the tank to give enough depth consistently for the pick up to not suck in air.

This is not correct.

A properly designed tank will use horizontal baffles, it will have a small chamber at the fuel feed point which, on a rib, is likely to be at the rear lowest point. The chamber holds splash fuel which cannot quickly drain back into the main tank so the supply of fuel is uninterrupted and drainable almost to the last drop.

A drain is not needed because all condensation will fall to the fuel feed point and be pulled along to the water separator where it can be dealt with. This is not normally the case with a deck tank because it is flat bottomed so it's possible to build up a considerable quantity of water which can be lifted in one gloop when the feed condition becomes favourable. Unless the fuel feed system is designed to facilitate it, there may be no way to be sure all condensation or contamination is removed.

[Autolux]

Originally Posted by GuyC

(all IMO of course)

Thanks for the reply.
So these photos show my rib and the console I will be using.

There is plenty of room for a fuel tank in the consol but the under deck fuel tank will have to be custom made (316l stainless) as it is narrow and will be a nightmare to remove if there is an issue down the line.

What are the disadvantages to me placing a fuel tank above deck in the console?

[Nico501]

Hello, for the calculation of the center of gravity, I did it quickly for my Yamaha xs600 and 130hp with 110 petrol and 2 batteries.
between putting it under the bridge or on the bridge with 2 people on board the center of gravity changed by 10cm vertically which is little knowing that during navigation the mass of petrol will decrease, hence a displacement of the center of gravity in the case of the tank on the deck the CDG will drop (improvement) in the case of the tank under the deck the CDG will take off! (less good!)
sorry for my english I'm french!

[Chanchan168]

Originally Posted by ribber

Sorry Chanchan should have read your post properly.... Can you share what tank you used holding 160l, re its material construction and shape, as it's a great idea once under deck tank becomes a problem.

An upright rectangle tank in the jockey seat would work, but getting anything in the console for me would be tight for space.

I used two tanks. A plastic 105l for my main and a further 55l for my reserve.both tanks I got from www.shop4tanks.co.uk. Jenny there is really helpful

[Iron Dials]

I’ve had different ribs with tanks above and below deck.
Height of C of G has never been remarkable either way.

I would however be very careful about any fore/aft change to the position of the tank, the trim could potentially be adversely affected (or improved!).
Atlantics are well thought out boats, the balance of the boat would have been well considered at design stage. I know you probably won’t have as much gear on deck though.

Underfloor tank frees up your console for storage.

As Chanchan mentioned shop4tanks are very good.
They also sell proper fuel lines, filler hose, vents etc.

Looking forward to some pics of the finished boat.

[Locozodiac]

A center console which has 2 independent chambers is intended to install a 6 gal fuel tank and a battery, usually aft bat with front tank inside console.

There are large center consoles that can take two 6 gal tanks at front and a baterry aft.

Happy Boating

[diver 1]

Highly recommend shop 4 tanks. They made me a custom made 2mm thick 316 grade stainless steel tank for my ribcraft to my drawings / dimensions. Perfect job and will undoubtedly last longer than the boat now.

[Iankristy]

Originally Posted by Autolux

Thanks for the reply.

So these photos show my rib and the console I will be using.



There is plenty of room for a fuel tank in the consol but the under deck fuel tank will have to be custom made (316l stainless) as it is narrow and will be a nightmare to remove if there is an issue down the line.



What are the disadvantages to me placing a fuel tank above deck in the console?

Number one disadvantage for me would be putting fuel and electricity together. Second issue would be you’ve lost the cooling effect of the water on the fuel in an under deck. You’ll lose much more in evaporation assuming you’ll have a vented filler on outside of console. No reason why a decent quality under deck tank properly installed would have an issue imho

[Autolux]

I could install a plastic fuel tank in the console and then my 2 batteries aft below deck accessible via a deck hatch.
This would mean the above deck fuel tank holding 140 liters will weigh 105kg.
The batteries @ 24kg each will weigh 48kg below deck.

The more I think about it I am leaning towards the fuel in the console

[Autolux]

I could install a plastic fuel tank in the console and then my 2 batteries aft below deck accessible via a deck hatch.
This would mean the above deck fuel tank holding 140 liters will weight 105kg.
The batteries @ 24kg each will weigh 48kg below deck.

The more I think about it I am leaning towards the fuel in the console

[Iron Dials]

If batteries below deck the water will always want to find them, particularly as the weather gets wetter.

Lots of ribs have underfloor tanks without issue, we only read about the ones that went wrong.

You have the advantage of a zero time installation with a properly made and installed tank below deck and batteries on/above deck, per original design.

*the next person will be along shortly with the polar opposite opinion

[Allye]

Just do what you want, both methods will work and you won’t notice any difference in handling or COG.