Pemanant flexible bladders

My new-to-me boat was built as a launch, and has ~190 gallons of fuel capacity in two saddle tanks. I've been examining options for increasing tankage, which leads me to flexible bladders.

Here's a picture facing aft from an access hatch in the ER. The boat was built with tall stringers and a floor that's fully tabbed in, and all systems and fuel tanks above the floor. This box is approximately 30" wide, 22" high and at least 15' between bulkheads. That's a lifting point in the picture.


I'm getting ready to engage with ATL to spec out a pair of bladders. Each would have a dedicated fill and transfer pump to the existing tank, which would be directly above. My basic game plan would be to line the bottom and sides of the space with rubber mat, then squeeze the bladders in without major disassembly of the boat My intention is to keep them full normally. The weight won't hurt me.

I've read lots here about bladders. The consistent objection seems to be with longevity. Baffling is a nagging concern I know that ATL provides foam baffles, but am ignorant of how suitable their system is. The tank will be fully supported on all sides, long and skinny fore and aft.

I appreciate the benefits of fixed tanks, but am not going to cut things open enough to get them in this space.

Comments? Suggestions? Thanks in advance. Jeff

I would just do it the correct way and install permanent tanks. The boat is fiberglass so you can cut almost anything and then repair it. You will have endless issues with the bladder tanks. Especially if you are using them as long term tanks. One time trip purposes, then they are fine but not for long term.

Forgot to link to ATL. http://atlinc.com/marine.html

They'll build to fit.

I know pillow tanks have issues, but this seems similar to lining an old fuel tank. Wondering how folks have fared in that situation.

Following with interest. The old steel tanks were built into my wood former crabber/gillnetter, then a rear cabin built over that area of deck when she was converted to pleasure. No way to get them out in one piece nor slide solid replacements back in.

ATL is the company you want to buy from. Their quality is the best.

I sold a bunch of their pillow tanks to people I assume were drug smugglers back in the seventies and eighties. They always paid in twenties. They must have been happy with the tanks as they never came back to kill me.

If you talk to them ask if Peter Regna is still with the company. He was the president of the company back then. He must be retired by now.

I wouldn’t use a pillow tank in a permanent situation but the type of semi rigid tanks you’re talking about should be fine. I think they fill the tank with a very porous foam to prevent sloshing. If I remember correctly the foam is also used in the tanks they build for race cars. It prevents explosions by slowing the flame front.

HopCar said:

I wouldn’t use a pillow tank in a permanent situation but the type of semi rigid tanks you’re talking about should be fine. I think they fill the tank with a very porous foam to prevent sloshing. If I remember correctly the foam is also used in the tanks they build for race cars. It prevents explosions by slowing the flame front.

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Yes, I've been reading a lot of their material. I'm quite satisfied that the foam will be effective as baffling in my case.

Jeff, to give our boat a reserve water tank, I installed a triangular shaped bladder tank in under the for'd berth floor. I had a tap system that allowed me to access it if needed - seldom was as it happened, as the two food grade plastic rigid tanks I installed in the lazaret of 250L each proved plenty most times.

I took care to line the space with old carpet, to prevent movement-related chafe, and we never had an issue.

I see your last post is nearly a month old, but just wondered what you did in the end..? I see no issue with what you were planning, but would add that lining the space with something as padding, for the reason mentioned above, would be wise.

Thanks Peter. My plan is to fully line the compartment with some sort of foam or neoprene sheeting.

I've got a glass guy coming to do some other work in a couple of weeks. I'm hoping I can him to install a bulkhead that will give me support on 4 sides for the tanks. The rest is easy as far as I can see.

Will post updates here. I'm going cruising in ten weeks and juggling a few other upgrades, so this one might sit on the shelf for a while.

Had the same 50ft utility. Have no problem with bladders , but if I were doing the job I would install plastic tanks by cutting the access hole in the cabin sole larger.

When not required for a transit individual tanks could be kept empty .

DD 6-71 With a 3-1 T-D and 32x32 prop 7K was under 3 GPH.

FF said:

Had the same 50ft utility. Have no problem with bladders , but if I were doing the job I would install plastic tanks by cutting the access hole in the cabin sole larger.

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Thanks. The interior build doesn't allow me to easily open up access. I thought about several smaller rigid tanks, but larger flexible tanks built to fit the space can be installed with minimal cutting.

I've got a JD4045 and don't suffer too much from range anxiety, but also have diesel heat and would like to be able to keep it running through a Canadian winter without having to refuel.

"also have diesel heat and would like to be able to keep it running through a Canadian winter without having to refuel."

Diesel heat will burn 1 to 4 GPD , and heat is still required in spring when days get warmer but the sea temp is low.

Your Monel tanks of almost 200G do a great job of cleaning the fuel if you use the drain to feed the heater.

Use a Ba Ha style filter on refilling as house fuel or drum diesel can be dirty.

Fuel Drums in the cockpit stored on shore in the summer work fine.


I suggest you look down the fuel fills to examine the fill hose , they die slowly.

So about a year later now, did you do it?
I have the same vessel, and would very much like to have more range without drums on deck.
There’s quite a lot of useable space below the floor, but precious little access.

kapnd said:

So about a year later now, did you do it?
I have the same vessel, and would very much like to have more range without drums on deck.
There’s quite a lot of useable space below the floor, but precious little access.

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I didn't. I've been off the dock cruising for the last 8 months, and this has moved down on my priority list during that time. Maybe if I change my boating habits or travel plans it'll get done, but for now I don't feel I'm constrained by the existing tankage. Minimizing fuel consumption is important to me, and at my customary slow eco-cruising speed I'm comfortably getting 500-600 NM between fillups.

I still think it's a good solution. IIRC the tanks collapse to less than a third of their filled size for installation, and the existing floor and stringers provide a ready made box once they're in.

The tanks are smaller than I thought, BTW. I've run them dry, and mine only hold 80 gallons each.

What are you using your boat for? Have you put a deck house on it? Got pics?

My tanks measure out at over 90 gallons, but the deep fill neck limits them to more like 75.
Looking for an internal pipe cutter to remedy that, but most available models are designed to cut PVC or ABS, and would definitely be useless against the Monel filler pipe.
Another option may be to cut a new hole in the top of the tank and bolt on a threaded flange for a new fill tube.
As for the boat’s use, I do a little sports and commercial fishing and some inter island cruising. I’m in the tropics, so no diesel heater, and shelter is primarily for respite from the sun.
I have not finished out the interior, just slung a couple of hammocks under the forward canvas shelter for sleeping accommodations.
The boats original motor (Cummins 6BT 180 hp) and drivetrain are still in great shape, cruises about 7.5 knots at below 2 gph in rough open ocean conditions.
It’s a very capable offshore boat, she can roll pretty steeply in certain conditions, but behaves better when heavily loaded.

Pics from recent haulout

kapnd said:

My tanks measure out at over 90 gallons, but the deep fill neck limits them to more like 75.

Looking for an internal pipe cutter to remedy that, but most available models are designed to cut PVC or ABS, and would definitely be useless against the Monel filler pipe.

Another option may be to cut a new hole in the top of the tank and bolt on a threaded flange for a new fill tube.

As for the boat’s use, I do a little sports and commercial fishing and some inter island cruising. I’m in the tropics, so no diesel heater, and shelter is primarily for respite from the sun.

I have not finished out the interior, just slung a couple of hammocks under the forward canvas shelter for sleeping accommodations.

The boats original motor (Cummins 6BT 180 hp) and drivetrain are still in great shape, cruises about 7.5 knots at below 2 gph in rough open ocean conditions.

It’s a very capable offshore boat, she can roll pretty steeply in certain conditions, but behaves better when heavily loaded.

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Just a observation...... I do not know if cutting the fill tube or to add another fill hole is a good idea. It is there to help with expansion. Fuel oil expands as it warms up. Just like transmission fluid in an automatic car.

Air is more expansive than most liquids, and reacts more quickly to temperature changes, so excessive airspace is more likely to push fuel out than a tank full of fuel.
The tanks are properly vented.
My locale is blessed with very stable temps year round, the biggest temp swings in the fuel tanks occur when the fuel heats up during prolonged voyages.
My boat has some rather strange features built in, explainable only by the fact that it was built to some very stringent military specs, including the ability to survive being dropped into the water from an aircraft carriers deck, and drive away!
I have no qualms about decreasing the fuel tanks airspace.
Sorry for pulling this thread so far off topic!