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Fuel Tank Replacement

Murre, Mariner 31

Hull 150, 1972

Winter, 2006

San Francisco, CA


I replaced Murre’s fuel tank in 2006 because I was rebuilding the cockpit well and so had the area opened up. 


The mild steel tank hadn’t given me any real trouble—it didn’t leak or seep—even though by the time I bought the boat it was already over 30 years old.  But its exterior suggested better days were a memory, and its interior hinted things could go badly soon and without much notice.  Large rust blooms were visible in places that had already been repainted at least once, the bottom of the dip stick tended to come up rusty at the tip, and brown rust flakes swam around in small, mean looking schools in the fuel filter bowl. 


And a way to resuscitate the old codger was not obvious, either.  Water that had collected under the fuel over the decades (little as it may have been) could not be got out because the drain valve was not located at the exact bottom of the tank; nor was the fill fitting large enough to allow a professional fuel polishing hose. 


Since the cockpit had been removed, the hour had arrived, so out came the tank.




Finding a replacement required choosing one of several standard fuel tank materials and deciding between custom fabrication or a factory made, off-the-shelf solution.  


Off-the-shelf tanks were easiest to come by and inexpensive, but finding the right size and shape was not.  This was an unfortunate discovery as my tank sourcing began after the cockpit project was well underway but not particularly well in hand, so I didn’t feel up to the task of customizing the area as other Mariner owners have done. 


I did find that many a local boat yard would do custom tank work, but the quotes (to a one) served to remind that, contrary to what we were taught in school, the French Revolution didn’t really eliminate snobbish, aristocratic, and expensive tendencies in yachting.  “It’s just a tank!” I’d gasp as the yard boss presented me with his corpulent quote.  “Oh, well, lots of overhead”, he’d say, suggesting by his wink that if I were worried about such a thing as mere price I’d chosen the wrong pursuit.  Murre had taught me this lesson long ago, and it didn’t need any reaffirming.  “What’s that?” I’d say, attempting to deflect attention from my ashen face by pointing to a 75 foot, carbon fiber spar so light it was nearly blowing off its stand in the afternoon breeze.  “Oh, that’s a new spinnaker pole for Larry Ellison,” would say the boss, “100K. Please excuse me while I take this call.”


So I pressed on, and after some searching found a lone, mostly-retired fabricator way up in Eureka who was happy to do custom work at a factory price. 

Next came the choice of metals.  Stainless steel and aluminum are now the standard, but I opted for the original mild steel for several reasons: 1) it was cheaper—though not by enough to warrant the choice; 2) both stainless and aluminum are said to resent standing salt water and given the fuel tank’s location, I was hard pressed to guarantee a entirely dry life, even with a new cockpit overhead; 3) my ground tackle in the bows weighs 150lbs, which means I don’t need weight savings at the stern.   The original material proved its worth by lasting 30 years.  Certainly it’s as fine a choice now as then.





Tank Specifications



Judging by its weight, the original tank was constructed of 12 gage mild steel, and that is the gage I ordered for its replacement.  I only made three small changes to the design: 1) an access hatch was added where the old fill fitting had been; 2) the fill fitting was moved slightly aft; 3) the drain valve fitting was moved to the dead forward part of the bottom panel (as opposed to near, but not at, the bottom of the front panel).  The reason for these changes was to provide easy cleaning and draining options as their lack was the death of the elder tank.


In case the measurements in the above drawing are not clear, they are as follows. 



Front panel

24” wide

17 ½” high


Top Panel

24” wide

32” long


Side Panel

17 ½” tall (front)

32” long (top)

33 ½” long (bottom)

7 ½” tall (back)


Description of Fittings

Fill = threaded female fitting to take 2” brass pipe

Access = 6” diameter opening with bolted covering plate and rubber gasket         


All other fittings are 3/8” threaded female, welded to tank

A = Out to primary filter, centered and roughly 2 ½” above bottom of panel

B = Drain valve, at extreme forward of bottom panel, roughly 6” in from the starboard side

C = Return to engine, 1” below top and 2” in from the starboard side

D = Vent, about 2 ½” from the back and 1 ½” from starboard side


One baffle athwart ships about half way back (I had two on the original drawing, which made the fabricator chuckle. “I don’t think the Coast Guard requires it in such a little piece a’ tank.” he said.)


Paint: powder coated white.


Capacity: 41.5 gallons


I don’t mean to play lightly with the issue of weight.  According to my calculations, this tank’s 23.4 square feet (including baffle) of 12 gage mild steel weighs just over 102 lbs.  A tank of equivalent dimensions but of much thinner 16 gage stainless steel would weigh 59 lbs and of aluminum, 17 lbs.  If mild steel’s corrosion resistance is driven by its thickness, it’s paid for in weight.  But to me the weight savings seemed nominal (43 lbs in the case of stainless and 85 lbs in the case of aluminum) when judged against other weights.  At 7.1 lbs per gallon, the fuel in a full-up tank will weigh just shy of 300 lbs; the engine must weigh well over 500 lbs, and then there’s the stuff in the lockers, etc. 





Top of new tank showing the access hatch forward, the fill moved aft, and the vent fitting aft to starboard (covered in blue masking tape to keep gunk out).  I have no electronic fuel gage attached, so when dipping via the fill, its location must be taken into account.





Front of tank showing placement of out to engine and engine return fittings.



Tank Placement and Support



Weight was a big issue, however, when trying to get the new tank into the boat.  Flopping the ripe, old tank onto the dock like a dead halibut was one thing, but with the new iron and its new paint, white gloves please.  I was doing the job single handed, and found that a double purchase and a little luck was more than enough to swing it in place. 


That’s not to say I didn’t get a special thrill when the harness slipped and the new tank was suddenly on its head and dangling precariously over my new cockpit. 


But that’s the kind of thrill single handing is all about, right?





The tank is held out of the bilge by three athwart ships frames that are glassed into the hull.  Two fore and aft braces atop the frames keep the tank from moving sideways.  On Murre the starboard of these braces is glassed into the frames while the one on the port side can be removed so that the tank can be slid to port and then out through the cockpit floor.  The frames are cushioned with foam rubber pipe insulation.  I spent a lot of time standing on these frames while building the cockpit, so, before replacing the tank, I doubled the most forward one from underneath with a frame of equal size, this because it is the longest, takes the most weight, and felt a little springy.






When properly in place, the tank sits to starboard of center.  It is slightly narrower than the area between the two cockpit sides, so to remove the tank, one needs only remove the cockpit floor and frames, remove the port brace, slide the cockpit a few inches to port and lift straight up.  Which is not to suggest it’s easy. 






As far as I could tell, the 2” brass fill pipe where it pierced the cockpit floor was all that kept the old tank from slipping forward.  It’s hard to complain too much as the tank never budged, but golly!  Though I used the fill pipe as anchor again, I also added a knee made of 1 ¾ inch timber (Santa Maria) that’s backed on both sides with 2 layers of 18 ounce roving.  It’s fastened to the cockpit side with four #14 wood screws and epoxy.  The leading edge is covered with clear hose cut to fit as cushion and paint protector.





Holding the tank in place from above are two frames, one that runs the length of the starboard cockpit side (not pictured) and one that runs across the tank amidships (seen here).  Prior to installing these braces, I filled the tank full of diesel and let it sit for a few days to compress the new foam rubber pads on the underside.


The end, a beginning…



The old tank sat on the dock next to the boat for weeks because I didn’t know how to dispose of it properly and had other things to worry about anyway.  Then one day Jake, a local who was restoring a Knarr in the yard adjacent to the marina, asked if he could buy it.  It seemed he needed a boiling tank for the steam box he was building, and my tank looked just the right size.  The offer of payment was protocol, wouldn’t have extended much past a beer or two, and was promptly declined, but I was happy to see the tank off to another phase of a long, useful life.