I picked up on most of that already. It was the thickness that had me
flustered. You are right that .125" would be much better. Easier to
get good welds on too. Baffels are required every 30" and rosette welds
are what those filled holes are called. Fillers must be a minimum of 1
1/2" and vents 5/8". Those things I expected.

Other things I found that I was not expecting: 46 CFR 183.518 for small
passenter vessels says ALL fittings must be on top. 46 CFR 58.10-10(a4)
allows cleanout plugs and supply outlets on the bottom of the tank.

What has me really confused is the relationship between 33 CFR 181 - 183
and 46 CFR. They are not easy to figure out in the first place and the
way they are presented on the web makes it a lot worse.

SAFETY STANDARDS FOR BACKYARD BOAT BUILDERS has nothing in it that
applies to diesel powered vessels over 20'. I guess I will have to
spring for the ABYC guides but from what I have seen so far they don't
track exactly with the CFRs either.

Brian D wrote:
By now you may have found the online articles about tank failures and 0.090"
walls, right? Big improvement if you go to 0.125". Number one reason for
failure on thin tanks is metal fatigue, especially at or near weldments.
Number one reason for failure on heavier tanks is corrosion. Number one
reason for corrosion is tank immersion (in water) or standing water on the
tank. If the tank is kept dry, ventilated, and built with 0.125" or thicker
walls and lid, then you've nixed 99% of the tank failure problems. Add
baffles, I think 20" to 30" apart? Stephen? What's the spec here? A
friend that builds tanks drills holes in the lid and sides, makes baffles
(lots of holes and limbers) with a bent over tab on top and both ends. He
welds the bent tabs to the tank through the holes, then fills the holes with
fill material. Grinds flush. Having baffles welded to lid and sides means
that regardless of tank mounting, that the lid-to-tank weldment is prevented
from fatigue failures since the baffles are doing double duty and
transferring load to the tank sides. He either glass beads or etches (can't
remember which) the whole tank, primers and paints. He doesn't use a
special anti-corrosion primer such as zinc chromate, but you could if you
want. The paint is some kind of epoxy paint. Ameron Devoe 229h I think
...industrial grade epoxy paint.

He puts the pick-up at the aft end, an inspection plate above that point,
sometimes a well (water and sludge collection) below that point. He uses
1-1/2" filler and a vent ...what was it? 5/8" or so? Can't remember. He
doesn't put sending units in the tank...FloScan instead. When my boat is
nearer being ready, I'll probably let him build me a tank ...

Brian


--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com