Diesel in the bilge!!

[Set2sea]

It could be tranny fluid?

[Roland Gendreau]

I thought it might be tranny fluid also except that he said there was fluid in the depression aft of the shaft tube.  I can't envision a scenario where tranny fluid could accumulate in that depression and also accumulate against the wall at the rear of the engine.
Look forward to learning what the fluid leak was.

[Noah]

Not that much tranny fluid avaiable. I would think that much "missing" would be obvious when/if you checked tranny dipstick. My guess is if you take a sample from the bilge, put it in Tupperware or some sealed container and open it up at home—-it will smell like diesel.

[Analgesic]

So, Noah was right.  When I returned yesterday, there was more  diesel only in the hollow above the shaft log and the paper towel between the tank and this area was soaked.  On close inspection, the aft inboard corner of the tank had signs of rust and the board underneath was moist, clearly a pinhole leak.  I was able to siphon off all 15 gallons into jerry cans.  With some online research I found Luthers Welding in Bristol, RI reporting they specialize in custom aluminum marine fuel tanks and have quoted me a price for about $200 less than Catalina Direct (and I can save $90 shipping by  driving down to pick it up an hour from hear).  The order is placed and I should be able to launch on time in May.  I'm very happy to have an answer, a plan,  and grateful for all the advice. 

[Noah]

I take no joy in "being right" in this case. If you are buying/building custom, be sure to have them add a couple of inspection/cleanout ports on either side of the baffle.

[Jon W]

On my 1987 MK1 the only hull depression is aft of the prop strut bolts. Is that the depression where your diesel accumulated after leaking from your tank?

[Noah]

I believe that was what he meant. Must be leaking aft?

[Jon W]

Trying to understand how it leaks from the tank to that depression. His boat must be on the hard with the bow angled up quite a bit, otherwise the leak would have to travel uphill?

[Analgesic]

Yes to the last three.  I will have the right inspection ports in the new tank.  The leak was aft, gently sliding down from the  inboard corner of the tank  to land in the depression above the shaft log.  The first time I saw it,  enough accumulated to overflow the first hollow, drip to the wall behind the engine, build up there, then leak around to the bilge mid-ships, never enough to reach the mast area.  I feel lucky.  If the 15 gallons let loose, I can't imagine what my problem (and stress level) might be. 

[Roland Gendreau]

Are there any recommended maintenance practices to either delay or eliminate the risk of pinhole leaks from the tank?  Do the tanks corrode from the outside or inside, or both?

[Noah]

I believe mine corroded from the inside. As far as prevention steps; keep the tank full to inhibit water? As a general rule (not so much an issue in our tank placement) isolate the bottom outside surface from sitting in any standing water?  But, all that said, it seems like 30 yrs. is the average before old age disease impacts our aluminum tanks. Also, perhaps using the engine more,  i.e. recirculating/burning more fuel through the system would help? I haven't used my engine very mich over the past few years so the fuel just sat in the tank—albeit with Biobor additive in it. But never had any indication of water in my Racor turbine filter however, so who knows?

[Patches]

Roland:

I think the bet way to prevent the corrosion is to apply the neoprene strips as described in my post above.  Was easy for me because I did the work on my new tank before installation.  A bit of a hassle if trying to do it on your existing tank.  It is a one person job to empty and remove the tank until you have to get the tank from the aft cabin and up the stairs into the cockpit.  Then a second set of hands is needed.

After looking at the inside of my old tank, it was pretty "chunky/dirty" inside.  If it hasn't been done, I would probably remove the tank to clean it and to check the condition of the bottom for corrosion.  If the tank is fine, you could apply the neoprene strips after cleaning the inside.  If it looks suspect or a leak is detected, then replace the tank and apply the neoprene strips then.

I'd give it a "6" on a scale of 1-10 for boat yoga.

Patches

[KWKloeber]

Brian

I know owners who have coated the bottom of a new or repaired tank with pickup truck bed coating. Not only does it seal out any moisture, it wears like iron.  I used some on my van step that bumper plastic cover and it is like having a steel covering. It also works great to protect the oil pan from rusting out.

https://www.harborfreight.com/catalogsearch/result/index/?q=bed+coating

-ken

[Noah]

I wouldn't do that on bare aluminum. It will cause you trouble. Bond will fail and trap moisture. You will need to etch/prime first.

[KWKloeber]

Good point. ABSOLUTELY.
On "the other" forum I put a link to the self-etching primer for akum.
Sand to get rid ofvoxidation, wash or tack rag, self-etchingvprimer.