Steve
Nice thinking! That's a great thing to do.
I personally would avoid ANY brass or bronze fittings and go to all-stainless if at all possible - any buildup of
salt on the outside of the sump/fitting interface (not even visibly thick) is hygroscopic, so it can create a conductive path between the fitting and the aluminium, happily crossing any thread tape or dope. Imagine a wee trail of tiny invisible ants marching 24/7/365, each one taking a subatomic nibble.
However you may well consider it to be in a location easy enough to keep it clean and dry, in which case I'm sure you won't have a problem if you just interpose a stainless nipple. (well doped, of course - for the benefit of others who might read this)
As an aside:
I did the consulting decades ago for a shallow
stainless steel fuel tank under the
cockpit floor of a small but very highly specced
offshore sailboat.
The tank was rectangular in plan view, level top, but sloping bottom (deepest forrard) and mindful that access to any clean-out
ports would be impossible, we came up with a pair of sumps just like yours (except for the material, naturally, and in our case they were not separated from the tank, but integral) at the forrard
outboard corners.
I think the entire bottom screws off for major cleanout, the "sump" being a large bore stainless internally threaded nipple, but the hex boss of the
plug is threaded for a ball valve just like your drain valve, with a smaller plug for the exact same reason!
From there we ran the fuel forrard separately from a takeoff near the top of either sump, with the two lines joining at a two-way selector valve, in case it was ever necessary to motorsail at a heavy angle of heel with a near empty tank, or to buy time if one of the sumps or takeoff lines become corrupted. VERY similar thinking to yours.
In a belt and braces move, I built a second sump like yours at the high point of the subsequent run, but it was much smaller. It had a clear section at the bottom (for water separation) and a longer one at the top, for air to separate out if the fuel were to come frothy from the tank.
The upper chamber can also be used as a gravity feed, for bleeding (or even to keep the
motor running by refilling it with a
turkey baster or
battery hydrometer from a spare jerry, if the main tank developed a major issue)
The first time I made it, it was bored (from both ends) in a solid round billet of Perspex (US=Plexiglass) - with all sorts of internal
ports and passages all visible at the midpoint, even when I was machining it ... great fun! and if I do say so it was a
work of art ... large diameter integral "sight glass" sections gleaming at top and bottom, single-point internal threads and O ring grooves top and bottom and SAE O-ring ports for the lab quality Swagelok stainless pipe fittings ... a bit like looking at a 3D CAD solid model of a hydraulic manifold with "transparency" turned on ...
but within weeks a fine pattern of cracks started to propagate outwards from the "sight glass" bores, caused by residual stresses interacting with the hydrocarbons, either in the kerosene I used as a cutting lubricant, or the
diesel fuel, or both .... so I had to remake the central section in aluminium (anodised) with large-bore
beer hose (soft clear plastic) chambers top and bottom, with custom caps.
I hope someday someone will read this and be saved having to learn this wee lesson for themselves, the hard way ...
Another innovation was an overflow tank in the breather line, lower than the
deck fill, consisting of a translucent window washer fluid tank, which, as well as gurgling and warning you the tank is now full (even if you forget to open the
cockpit cave locker so you can see it start to fill up) also acts a bit like an airlock used for brewing wine, to keep atmospheric
condensation at bay by greatly reducing the surface area in contact with ambient air. (Unlike petrol, the vapour pressure of
diesel is insufficient to displace all the air in the tank.) This tank, being shallow, had a very large surface area (so of course it had to have multiple baffles)
The breather from that overflow tank ram into and part way up a pushpit stanchion, and there was either a porous plastic silencer (from a pneumatic valve) to slow down any loss of fuel (or water ingress) in the case of a
capsize, or it might just have been a very fine-bored piece of tubing - almost capillary - I forget now.
I know we used ultra-fine bored
beer hose for the breather from the (close-fitting) sealed
battery box to vent any hydrogen (wet cells)
It's one of those cases where you take ALL the precautions, and 25 years on, there has never been any fuel issue on that vessel, whether water, air or diesel bacteria. The sumps get drained once a year, but it's a formality.
And the
mast has never been in the water, let alone a 'proper'
capsize.
Whereas IF we had omitted to take any precautions, you can bet we would have had unending problems ... at least, if John Vigor's "Black Box" theory has any merit.
(PS- I'm warming to your 'brow'!)
.
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