Fuel tank design considerations ..

[wind_magic]

The Diesel Polishing thread in the Diesel Engine forum made me curious but I don't want to side track that thread by asking questions.

My question is this - what considerations need to be taken into account when designing a fuel tank to put on the boat for a diesel engine ? I know some of the tanks have baffles in them to keep the fuel from sloshing around, and I think it is a good idea to have a drain plug at the bottom of the tank that the sides and bottom slope down to so you can empty the fuel out and hopefully catch the dirt too.

Are there other considerations ? What else can you do to make tank cleaning easier ?

[klem]

The biggest problem in my opinion is preventing sucking air bubbles when it gets rough and you don't have a lot of fuel in the tank. The two ways to do this are to use a deep sump or to have a tank shaped like a V or cone. The advantage of a sump is that you can make a square tank which is often easier to build and put in place and you can use a conventional fuel gauge on it. A tank with a sloped bottom can be fit into certain bilge areas better and can usually allow for more sediment buildup.

Baffles in some form are important. On a small boat, the fuel sloshing is actually noticeable. Regardless, it increases the chances that you will suck an air bubble and it increases the stress in everything.

I like having inspection ports in all tanks that allows them to be cleaned out.

One other thing to think about is whether the tank is built in place or is removable. If you can, it is very desireable to have a tank that can be removed without disassembling the entire interior and then removed through the companionway. Sooner or later, any tank needs to come out for some reason.

[sailingdog]

Having a sump area, that is lower than the fuel pickup, where the water will tend to concentrate, along with a sump pickup or drain of some sort, will make keeping the fuel tank clean and free of water a lot simpler.

Any tank larger than 20 gallons or so should probably be internally baffled. However, each section should have its own inspection port.

[wind_magic]

Thank you klem, that sounds like great advice.

[wind_magic]

Originally Posted by sailingdog

Having a sump area, that is lower than the fuel pickup, where the water will tend to concentrate, along with a sump pickup or drain of some sort, will make keeping the fuel tank clean and free of water a lot simpler.

Any tank larger than 20 gallons or so should probably be internally baffled. However, each section should have its own inspection port.

Is there any widely accepted standard for how many baffles to put in ?

[gtod25]

Click on my effort


This is good info;

David Pascoe

Except , do NOT use 5200, 3M recommends 2216 Scotch Weld.

A drain plug is not recommended, remember KISS.

Gerry

[klem]

30in is around the maximum of what I have run into for baffling. On really large tanks, the baffles are further apart but that does not apply to sailboats generally. The disadvantages to more baffles are needing more inspection ports, time, and money.

Regarding what to baffle with, there are many options. Two that I have used are taking a piece of sheet and cutting off the corners to allow fuel to flow there. In addition, I have cut a few holes in a sheet as well. The holes need to be large enough that they will not clog but the total hole area can be relatively low (think of the amount of fuel being pumped by your fuel pump).

[sailingdog]

I think a lot of how close the baffles need to be depends on the tank geometry. I seriously doubt a single answer is valid for all tanks.

I'd also point out that the primary reason for having the baffles is to prevent free surface effects of the fuel sloshing about in the tank, and that the shape of the tank and position of the tank have a lot to do with how likely the free surface effects are going to be with the tank.

[jrd22]

It's genreally accepted that any metal (SS,alum., iron) tank will eventually leak and need to be replaced. Integral fiberglass tanks (in a fiberglass boat) seem to be the bullet proof preferred way to go. If you have a sump in the tank it makes sense to have the inspection port directly over it so you can clean it out.

[wind_magic]

Originally Posted by jrd22

It's genreally accepted that any metal (SS,alum., iron) tank will eventually leak and need to be replaced. Integral fiberglass tanks (in a fiberglass boat) seem to be the bullet proof preferred way to go. If you have a sump in the tank it makes sense to have the inspection port directly over it so you can clean it out.

You know, from reading what you all have written, it seems like the baffles are essentially just creating a series of tanks that just happen to be connected together. If that is true then it seems like, from a design perspective, you could just as well have 10 tanks that are very small instead of 1 tank that has 9 baffles in it, and if that is true, why not just go ahead and make the 10 tanks, mount them each separately and securely, and then connect them all together with some kind of pipe tree so that you could remove each one of them and hand carry it when it needed maintenance ? Basically the same idea as using batteries that are all small enough to carry to build a battery bank. Seems like that could solve a lot of problems and only cause one problem, that problem being the same as with a battery bank, that you need a rather elaborate way to tie down and secure all of the batteries that make up the bank. In fact, it almost seems like what I'm talking about here is essentially just a jerry can that has two openings on it instead of just one, one at the top for filling / inspection, and the other at the bottom to connect to the fuel pipe tree, do they make little tanks like that ? They must, somewhere ...

If I was stuck in the middle of the ocean with dirty fuel I'd want a system like the above, because the parts would all be small enough that you could handle them and figure out some kind of a solution to your problem, one container at a time. It seems much more user friendly than standing there staring at this massive baffled fuel tank wondering what you are going to do. What advantage is there with one big tank, it is the same amount of fuel as a bunch of little tanks, right ? Especially if we're talking about dividing that one big tank up into 10 or however many little tanks with baffles, then you've essentially created a series of smaller tanks hooked together by some kind of fuel line, except in the case of the big tank the tree of pipes are holes in the baffles.

Edit, I like this thought above for another reason, if one of the smaller tanks ruptures then you don't lose all of your fuel if you catch it in time, you just remove the bad tank from the tree. Diesel has a lot of odor and there is a good chance you might find a leak before all of the fuel drained into the bilge, in fact, maybe the pipe tree used to connect the tanks together could have valves on it so that you only had one, or two, or three tanks "turned on" at a time, and the rest were reserves until you needed them. One big tank with baffles could never do that ...