Fuel tank design / pick up tube

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Boxboy
Posts: 11
Joined: Wed Mar 27, 2019 8:16 pm

Fuel tank design / pick up tube

#1

Post by Boxboy »

Looking around I don't see many options ( off the shelf) for fuel pick up tubes. Mainly only see the moeller tubes ( aluminium 90 degree head and plastic pick up tube). But reading reviews people complain about the degredation / life span. And if I were to use one would it be ok to use brass male barb npt fitting, or should I find an aluminum one ( or the marine store suggested nylon one but unsure if it would be ok for gas).

Regards p.
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kmorin
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Posts: 1530
Joined: Mon Aug 18, 2008 1:37 am
Location: Kenai, Alaska

Re: Fuel tank design / pick up tube

#2

Post by kmorin »

Boxboy,

couple of remarks, hopefully of some help? First, unless a plastic draw tube (downcomer) is secured in some way? just a little slosh/wave/roll or the hull will allow the fuel to push it back and forth (side to side or fore and aft?) and then decouple at the fitting/top.

So, if you're using this type of fitting/draw tube I'd consider putting it inside a concentric pipe welded in to the tank top at the female pipe fitting, and bottom with some strut/bracket to locate to the bottom of the tank. This allows the plastic to be secured and not move- eliminating the failure point at the plastic-to-aluminum joint.

Next, aluminum-to-aluminum threads are a poor joint! But, if all you can find is these fittings and your tank is welded aluminum? you'll need to take great care to thread the male alum. fitting into the female welded to the tank. Goop, like TefGel or White Locktite or other lube and isolation thread compounds go a long way to helping the alum. threads "live" longer.

Using any copper alloy (brass) in aluminum is poor practice. A better choice is SS, if you can find the fittings? King Nipples, as you show them above, even in SS have a large galvanic differential from aluminum so... if you use SS, it's best practice to passivate the SS to reduce surface reactivity to aluminum.

I usually use 3/8" pipe as the fuel draw/downcomer/fuel piping inside the tank, unless the engine is inboard diesel where a 1/2" pipe will not over burden those fuel pumps. If the outboard is less than 35 hp you might get away with 1/4" pipe as a rigid downcomer, and not burden the fuel pump.

Of course, larger diameter (than your plastic tube, shown) metal pipe as a support for a plastic draw tube as you show will not burden the fuel pump pulling/lifting/drawing up the plastic's dia.

Cheers,
Kevin Morin
Kenai, AK
kmorin

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