Moeller fuel tank plumbing suggestions

Hello All, I am cutting out my old pitted and leaking 200 gallon mild steel saddle fuel tanks. I am planning on replacing them with three Moeller plastic tanks per side. I am interested in your suggestions for plumbing them. The boat is a twin.
Specifically, How would I plumb the fueling manifold? Directly from the deck fitting to one tank and connecting that tank to the other two at the bottom of the tanks by cutting holes in them near the bottom or teeing the fill manifold above the tanks? (possible slow fueling rate)
Would I tee all the vent lines and use the existing vent through hull or drill four more holes through the hull?
Would I run one return line per tank? I guess that would depend on how the fueling manifold is plumbed.
Would you suggest installing valves at the bottom of the tanks to drain off water?
Where would I install ball valves?
So many questions. I appreciate any experience or wisdom you can pass on.

Your solution sounds complicated for a relatively small amount of fuel (<50 gal per tank). Our boat has 5 main fuel tanks by design and each tank has its own fill, vent, return, and drain line. The drains all go to a common sump in a day tank. The returns come from a return manifold. There is also a transfer manifold which moves fuel from tank to tank through a racor filter. Did your tanks rust from the outside in or inside out? Six little moellers I think would create more problems than 2 repaired or replaced tanks. Lots of coatings and sealants are available to repair old steel tanks from Flamemaster Corp.


Spell check via iPhone.

I replaced my 2-200 gallon saddle tanks with 2 - 58 gallon Moelers (1 each side).

I cruise slow enough that at 3.3 NMPG, the 2 small tanks give me a 300 mile range with comfy reserve.....and LOTS more storage space in the engine room.

I may add a 20 gallon day tank at some point and could always add a stray tank here or there later and transfer when necessary.

I feel the 300 NM range is plenty for my current and future expeditions...and figuring out complicated fill issues were not necessary.

The tanks are rusted from the inside all along the low point, inboard from forward to aft. (seven feet) At some point in its life there must have been a fair amount of water sitting in there. Initially I was going to repair the tanks but after cutting an inspection port and having a good look inside I decided I would sleep better if I replaced them. Once the old tanks are out I will have to replace them with smaller ones unless I want to start pulling engines etcetera, which I don't.
A question I forgot to ask on my previous post for skippers that have gone this route, are you able to draw fuel from all the tanks simultaneously or do you have to draw from one tank at a time.

GordC said:

The tanks are rusted from the inside all along the low point, inboard from forward to aft. (seven feet) At some point in its life there must have been a fair amount of water sitting in there. Initially I was going to repair the tanks but after cutting an inspection port and having a good look inside I decided I would sleep better if I replaced them. Once the old tanks are out I will have to replace them with smaller ones unless I want to start pulling engines etcetera, which I don't.
A question I forgot to ask on my previous post for skippers that have gone this route, are you able to draw fuel from all the tanks simultaneously or do you have to draw from one tank at a time.

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you can draw from multiple tanks till one is (near) empty...then I believe you will induce air into your system and wish you hadn't.

As long as the tank bottoms of a "bank" are all tee'd together and the top vents are tee'd together, they will all fill and drain together as one. Given that, you can treat the tanks as one and make the plumbing as simple as one overboard vent, one fuel fill pipe, one fuel outlet, one fuel return, and one sightglass.
I recall the problem with plastic tanks is that they expand significantly, causing a tie-down problem.

m8 gallon tanks hardly expanded worth mentioning...I left a half inch and a 1/4 would have been more than enough.


The plastic tanks generally only have top feeds and finding aluminum hose barbs without anti-siphon halves was nearly impossible (but I am sure someone will now post a link).

So equalization without pumping is a another whole engineering issue.


The nice thing about poly tanks is no sight glass...you can see the exact level at anytime. I marked mine in 5 gallon increments and now am not afraid to take it down to just about sucking air (in clam conditions only).

I have two Moeller tanks. I only have a fill on the port tank. I transfer fuel to the stbd tank by setting my return to the port tank while drawing fuel from the port tank. When there is less room in the stbd tank than fuel in the port tank I draw the stbd tank down. If you run this way you do have to keep an eye on your tank levels so you don't run out of fuel in the drawing tank or overfill the other tank via the return. It works well for me and assures me that I have a tank with filtered fuel available. I have dual racors dedicated to the tanks so when I am drawing from the stbd tank I have a filter that has only seen pre-filtered fuel.

The only problem with Moeller tanks is the size of the return line. The fitting they sell only has a 1/4" hose barb. On a larger engine that puts a lot fuel into the return that may not be enough. in that case i would drill and tap for a larger barb.

Thanks for your posts guys. Nice to have just one tank psneeld and all that extra space in your engine room. My boat is not as efficient as yours and I would like to carry more fuel. High Wire, teeing the tanks at the bottom certainly does simplify things. A shame though to drill holes in the bottom of a tank that only has top fittings. As usual a compromise decision has to be made. Why do the barbs have to be aluminium? Is bronze not compatible with diesel?

GordC said:

Thanks for your posts guys. Nice to have just one tank psneeld and all that extra space in your engine room. My boat is not as efficient as yours and I would like to carry more fuel. High Wire, teeing the tanks at the bottom certainly does simplify things. A shame though to drill holes in the bottom of a tank that only has top fittings. As usual a compromise decision has to be made. Why do the barbs have to be aluminium? Is bronze not compatible with diesel?

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I have seen tanks cross connected at the bottom using plastic bulkhead fittings. For example:



The two forward tanks are diesel and the aft tanks fresh water. They're cross connected with no valves using tank bulkhead fittings. This will allow fueling/water on either side of the boat and fuel return to either side. The fellow that built this boat owns(ed) a plumbing supply co. so I assume he knew what he was doing.

My own research has show similar results with the exception of a valve in each line to isolate tanks when required. Your best bet would be to go to a local plastics supply co and see what they have to recommend. There are fittings that can be 'rotationally welded' to poly tanks but I don't think I'd trust those over bulkhead fittings.

GordC said:

Thanks for your posts guys. Nice to have just one tank psneeld and all that extra space in your engine room. My boat is not as efficient as yours and I would like to carry more fuel. High Wire, teeing the tanks at the bottom certainly does simplify things. A shame though to drill holes in the bottom of a tank that only has top fittings. As usual a compromise decision has to be made. Why do the barbs have to be aluminium? Is bronze not compatible with diesel?

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My tanks had aluminum fuel posts on the tank....brass barbs and the aluminum fitting is a big no no.

If not aluminum such as stainless then fine...just double check.

That's a good idea TDunn. I don't think it would work for me though. I have twin Lehman 120's and from what I have read on this forum the amount of return fuel is insignificant.
Thanks for the pic She-Kon, that is really helpful.
I have a couple of other questions that have popped up but I think I will start a new post.