New Fuel Tank?

[jpendoley]

Since I have the engine out for a rebuild, I pulled the tank today and will bead blast, measure pitting and make a call to  replace with new, plate or possibly somehow coat the outside. there is extensive rusting, but unclear how deep it goes. How have others approached this challenge? How expensive would a new tank be if I had one fabbed up? Does anyone have experience using an exterior type of coating to fill the pitting due to rusting?
Feedback much appreciated!

[S/V AMITY]

   If it's the original steel tank, best policy would be to replace the tank, likely there is pitting on the inside too.  Pulling the engine is a major job and it wouldn't be good to have to pull it again in a year or two.  Yep, it's a bite on the wallet but the return on the investment will peace of mind. 

[SVJourney]

New tank custom fabbed from .125 aluminum was ~$600 USD including a large clean out port and fuel level sending unit.  This was in New Zealand, but can't imagine it would be too awful much more here.  Hardest part was getting it out (eng and cabinet work)   I could not be happier after it was done.  I gave the old tank to the fabricator as a blueprint.

Old tank had intergranular corrosion plus pitting.  I could remove the pits, but the rust was running between the grain on the top of the tank.  Dana's blog of the process: http://www.galleywenchtales.com/2016/02/heave-ho-out-she-goes.html

[S/V AMITY]

  One other thing:  If you do go with an aluminum tank, for cushioning or hold-down strap anti chafe, do not use black rubber.  The formula for black rubber most times contains carbon.  In contact with aluminum the rubber can cause electrolysis.  Had an old Aquasport 22 inboard where the aluminum tank pitted & leaked where it sat on black rubber.  Rest of the tank was perfect.

[P69]

I fixed the old photobucket link in my fuel tank replacement post from a few years ago.

https://pearson365.com/forum/index.php?topic=1369.msg7583#msg7583

The only mental to non-metal contact is with the brackets, that are bolted to epoxy/cloth sealed plywood cleats. The tank brackets bolt to the aluminum brackets, so there is no mental-on-wood or on-rubber or on-dissimilar metal contact.

I also changed dimensions to give more space between engine and tank.

[S/V AMITY]

  While we're on fuel tanks, anyone have info. whether regular use of Seafoam will reduce/eliminate tank sludge? 

[jpendoley]

Love Seafoam but have not heard of it being used for sludge prevention.  Regarding construction material-aluminum would be nice because the steel tank is heavy to move-what are the downside of going with aluminum?

[jpendoley]

Tank is definitely salvageable. Pitting is localized and seems about 50% deep. This occurred on the forward deep face-my take is it pressed up against the wood retainer and stayed damp. The interior looked just about clean as a whistle-I was really surprised.  I'm taking it to my local welding shop for an estimate to plate the forward section AND a quote for new in aluminum.
If I go new in aluminum I really like P69s design and may duplicate. Question about the design is why a return line for every pickup tube. I understand the fuel return for the engine, but why for the heater?

[jpendoley]

Also, what are the negatives of going with a fibreglass and epoxy tank?  No corrosion is a very attractive quality and my water tanks are still as good as new plus its a very easy build-essentially a box with a lid. Build a male mold and wrap with mat and biax. Leave an inward turning flange and put a top on-bolt or bed in some adhesive-I  l know they make specialty adhesives for the purpose.  Need a baffle or two, but thats easy. Has to lighter than steel and close to aluminum....flammabilty is the big drawback, but then our boats are made of fibreglass.
What am I missing?

[P69]

I've read that there are no compatibility issues between diesel and fiberglass (epoxy or polyester/vinylester). The issue will be design, build, attaching fittings, and that actual cost-difference between the two materials. You will probably end up with no savings by going with fiberglass, once you have factored in the cost of materials for mold and final part. When I replaced my tank I did think of using fiberglass. 

No comment on the question about fire.  I don't know enough about that to offer any advice.

You need  a core material because fiberglass panels in those dimensions will be too flexible.  In order for the interior to be smooth make a mold. Male mold for each half with no lid. Don't use gelcoat on the mold, just spray tinted resin to get the interior surface smooth. Do not lay glass up without spraying up a layer  of resin and letting it cure.  If you omit the spray step, you will have air pockets that will show up as cavities and imperfections in the interior surface. One role of gelcoat is to hide the imperfections of the material layup. Gelcoat is porous and you can't use epoxy over poly gel coat.  I suggest you use vinlylester that is tinted white and thickened or thinned to the consistency of gelcoat so you can can spray the first layer on, just as if you were to spray the gelcoat layer on a male mold.

Vinylester is probably good enough. Its plenty strong, cheap, easily repaired, and has a high temperature resistance.

Epoxy is good, but its weakness is cure time and when temps get around 180 degrees F, it will soften. It actually will liquify when grinding with a cutoff blade on  4.5" grinder. I use that property to when I have to remove metallic parts that I used for a mold. apply torch flame to metal part, it heats the surrounding epoxy to soften it, then gently unscrew or pound out the metal mold. When epoxy gets back to standard temperature, it hardens.

Just off-the-cuff, if I were to do this, here is what I think I would do. 
Make a male mold of the tank with no lid, so just the sides and bottom
Spray tinted vinylester resin on the mold to get the perfect, smooth interior sufrace
lay in some mat, a layer of 1708, some 1/2" core (Coosa board?), then lay in some  layers of 1708 (maybe 3 or 4?).
Th inward flange might be good, but you have to add that later. You can't make a mold with an inward flange.
Glass in the internal baffles (1708, mat, coosa, mat, 1708 with vinly ester resin), make sure the corners are cut off so fuel can flow around. Don't need circles cut out of center, just cut the corners so all 4 corners of the baffle are cut (dog eared)

Make the lid with 2 layers of 1708, 1 layer of mat, 1/2" core, mat, 2  layers of 1708). Omit the core where you will put the connection penetrations. fill with 1708 to same thickness as rest of lid.

Connections:  Only way I would do this is to make 1/2" thick blocks of 1708 and epoxy resin, say about 2" x 2" each. One block for each threaded fitting. If all fittings are along one edge, make a 1/2" block 2" wide by whatever length.

Epoxy this (these) blocks to the part of the tank where you will thread into the tank.  Make sure they are inboard enough so you can lay in an adequate amount of galss when you seal the lid to the tank.  Drill holes and tap for tapered threads through the epoxy block and tank lid.

With an aluminum tank, we welded threaded nipples to the tank for the penetrations and used a white permatex for thread sealant. Don't use teflon tape or paste, diesel will dissolve that material.

You have to seal the threads and I don't know if the same white permatex will have reactions to the epoxy.  I suppose you could, instead of tapping, just glass fiberglass tube with OD of ID of your hose, but you have to file/mold a barb on the fiberglass tube to get a barb for the hose to clamp to.  That's probably not a good idea. Go with threaded holes, you will likely need 90 degree barbs.

Once lid and fittings penetration block(s) are done, glass the lid to the tank. you will only be able to glass the exterior, so use epoxy for this to ensure maximum strength. You don't need any special adhesive just thickened epoxy if you have a flange, just thick enough to not drip everywhere and set the lid down until next day. Ten round edges and epoxy/cloth the edge seam all round to permanently fasten the lid.

I did not mention cleanouts, which you should have. I'm not sure how to make cleanouts that will seal the diesel in.
Also, you need to add pickup tubes and a return tube for the fuel polishing system. This return tube runs to the bottom of the tank opposite the polishing pickup tube so it circulates the fuel towards the lowest part.  That tube has to be securely fastened in several points so the vibration of the pump does not cause that tube to rattle against the tank.

You also need the 1.5" OD fuel input barb and some way to fasten the tank fuel level sender if you elect to have one.

Lastly, you have to design how you will fasten the tank. Make sure you leave room under the tanks so water can flow to the bilge and leave enough room so you can reach under the tank to sweep debris and dropped tools forward toward the engine for retrieval.

It's possible that you can make the tank without core if you add stiffeners to the unsupported panel widths to prevent flexing.  With one baffle, and a few stiffeners, you might be able to stiffen it up sufficiently to eliminate flexing.

Here is a tread the might help your decision.
https://www.cruisersforum.com/forums/f55/fiberglass-diesel-fuel-tank-89627-2.html

[jpendoley]

P69 (Pete?),
I hoped you would chime in, your thoughts are always methodical and meticulous.  One of the reasons I want to use glass (isopthalic) is I can keep the same stringers and box enclosure that held it in place.  The timbers are  in good shape and held the steel tank very securely so I can save some work if I reuse that structure.  The steel tank barely made it out of the engine bay-with perhaps 1/16" clearance, so replicating the dimensions precisely will be important-to allow it to slide back in and to reuse the mounting logs and enclosure. That said, I've never sprayed resin-though I do have a decent compressor that could probably do it.   Using the tank for a female mold would replicate the dimensions precisely though I do think I'd need to a lot of fillets and popping it out might be a bear.

I saw a good thread by someone who did the same but made a male mold of MDF, routed radii on the outside and essentially wrapped it without spraying. The MDF of course provides a pretty smooth surface. I did wonder whether to core with coosa or just go with multiple laminates. Strong baffles might impart enough strength on their own-after all our water tanks are not cored. 
Lots to think about and you have given me some great tips-thank you. Have not ruled out steel or aluminum either, just at the noodling stage presently.  Do need to get the engine back together first-spring is coming! Will keep the group updated on the tank as it evolves.
Thank you again for the write up-I'd be at a loss without the input of the group.
Jim

[Sandy]

geez
Im starting to think steel wasnt such a bad idea in the first place, I could do the thing in 3 days and most likely it would out survive me anyways.
The only advantage I can think of for a plywood epoxy tank is that I could put it in in two halves and bond the halves together in the boat, which saves pulling the engine.
Of course each half would have to be designed to fit through the sail locker so probably not the original shape.

[S/V AMITY]

   Why is the fuel tank where it is on these boats?  Why not substitute one of the water tank locations for fuel and put that 50 gallons of water where the fuel tank was?  Make for much easier monitoring of the fuel tank condition and would allow for convenient inspection plate locations for fuel tank cleaning.

[Sandy]

servicing anything around the engine fuel tank space is crazy difficult
but it still has to be done
I dont have any real answers other than the most robust designs in the first place

[jpendoley]

Just dropped the tank off for an estimate at a welding shop that does a lot of boat work. They assumed I wanted it in aluminum.  When I mentioned that I wanted to keep the original plywood mini bulwarks that held the original tank and I was concerned aluminum would corrode against the wood-he shrugged and said to paint it. He also confessed his own tank on his boat only lasted ten years because it got.. wet.  I'm hoping for a twenty year tank not a ten year tank.  Also, I assumed my tank was original, but I don't know that for a fact-it could have been a replacement tank as well...
So now I am thinking of going with P69s design and taking those prints to another shop for an estimate.  Glassing in new supports is not a big enough task to warrant keeping steel or avoiding aluminum. And the weight savings would be welcome to this aging mariner.