Fuel Line Size vs. Fitting size vs Restriction question

[1969Roamer46]

I re-read Tony’s excellent article Understanding Fuel Line Sizes vs. Fuel Supply Restriction, and I was wondering about fuel line vs fuel fitting sizing when it comes to fuel line restrictions.

I’m finishing the fuel system on my major Chris Craft Roamer 46 refit. This is an aluminum hulled boat, and one of the few places where the plating showed any pitting at all was under the OEM copper fuel and water line runs. Hull plating is precious, so I de-coppered the boat to the extent possible, including getting rid of the OEM copper lines and using 1/2″ stainless tubing instead for the fuel supply and returns.

The boat originally came with 427 FE Ford gas engines, which were repowered in 1972 to 400hp twin turbo and intercooled (gas) 534ci Super SeaMasters, but they still used the OEM 3/8″ fuel lines. All of my research at the time (2012) indicated that for diesel, I should upgrade to 1/2″ supply lines, at least.

As I’m assembling the stainless tube, distribution manifold, and installing SMX FS1000/FS19513 fuel filters, I happened upon Tony’s article. It occurred to me that perhaps I should have gone with 5/8″ tube, valves, and Swagelok fittings instead of 1/2″.  But then I noticed that the OEM fuel fittings at the manual lift pump on my 1996 420/430hp 6CTAs appear to be 1/2″ NPT elbow to size 8 flare (3/8″ ID at the fitting) and the return line is a size 5 flare (3/16″ ID fitting, but is commonly used for 5/16″ ID hose).

So I was wondering what effect the (essentially) 3/8″ restricter right before the lift pump might be to a 1/2″ or 5/8″ ID supply line, especially when the OEM return necks down to a measly 3/16″? I suspect my system will be fine for my relatively low-powered 6CTAs, but I’m interested in any practical engineering-oriented take on this.

Cheers,<br />Q

Topic

[1969Roamer46]

Thanks Dan!

The tanks are new and were made for diesel, so we’re GTG on the submerged returns…I think. You’ve got me curious now, so I’ll confirm that when I’m on the boat next. If the manufacturer didn’t weld them in, I’ve got spare pickup tubes I can use for the returns. That’s a very good point about aeration.

Cheers,<br />Q

[Dan]

Have a look here for some info on fuel supply restriction, don’t forget to add in the length of the fuel line run length, lift height, elbows and filtration, all contribute to restriction.

http://www.designatedengineer.com/FuelLineSizing.html

With your conversion from gas to diesel check for non return valves in the supply (required on gas but will mess up the flow rate on diesel)) and confirm the return line in the tank is submerged as the diesel will foam if delivered to the top of the tank = aeration = power loss ( gas boats don’t require a submerged return)

Your refit is impressive, great attention to detail.

 

[Author]

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