Pick up pipe/Tank Gap

[Nasher]

I’ve realised I have an issue with the installation of the 225Opti on my Ocean RIB which I’m very near to completion after many delays and false starts.

Mercury recommend a minimum of 5/16in (8mm) ID Fuel hose, but I’ve discovered the pick-up pipe welded in to my fitted 120l under-deck Stainless tank is only @5mm ID.
I never did notice a fuel starvation issue with my old DT200EFI, but then didn’t ever hold it at WOT for more than a few seconds at a time.
The boat was originally rigged with a Johnson 150VRO before I brought it, which drank petrol at an alarming rate, so I’m surprised.

I know for my use of the boat the existing pick-up pipe will probably be fine, but whilst I’m replacing all hoses and doing lots of other work I may as well put something bigger in.

However, the tank is obviously under the deck, with very little of it accessible, and I don’t really want to start drilling holes in and welding a tank 1/3 full of fuel, let alone cut up any of the deck to reach it, even within the jockey seats.

So I have a plan, but am wondering what sort of gap I should leave between the end of the pick-up pipe and the bottom of the tank?

NOTE:- The following are all really old images, but I don’t want to uncover the boat and take new ones in the pouring rain we have today.

The deck is laid out as below:


The tank stretches across under the deck almost the complete width of both Jockey seats, and the small round hatch between the jockeys covers where the fuel gauge sender once lived:


Note I gave up using fuel gauges years ago, so there is currently a plain round stainless plate screwed down to the tank over the hole.
I now use a Navman fuel computer flow sender and gauge with careful measurement of what goes in the tank volume wise.

Inside the port Jockey seat are the various connections to the tank.
Since this image was taken I’ve removed the rectangular plate revealing a hole in the deck just a little smaller than the plate, and have Flowcoated the inside of the jockey. Did it years ago.
From top to bottom:
Pick-up Pipe
Breather
Filler
Conduit to Transom area.


My plan is to cap off the existing pick-up pipe, and put a larger one in using the old sender mounting hole.
I’d need to have a right-angle fitting on top of the new plate that I could attach a new fuel hose to, which would then pass across the top of the tank and into the Jockey seat alongside the original pick-up pipe.
An added advantage is that being removable I’ll be able to add some kind of first stage filter to the bottom of the new pick-up pipe.
I hope this makes sense, new pipe/hose in Red:


But as mentioned, my question, after all of the above, is what gap to leave between the bottom of the new pipe and the bottom of the tank?

I obviously don’t want to suck up any crap in the tank, but also don’t want to lose too much useable volume.

[Matt]

I’d probably aim for 10mm. Suspect it’d be fine at 8mm too.

[Peter_C]

If it were mine I would run it as is and see how it does. If needed a low pressure pre-pump could be added along with a secondary tank which is often how inboard V8's are set up. Fuel injection often will recirculate the fuel unless it is pulse width modulated which is more common today. A test gauge on the inlet line would register if there is a larger than normal negative pressure at longer full throttle runs.

If installing a new intake I would cut it at an angle and keep it reasonably close to the bottom. Not using a tank sock as a canister water/fuel filter should be used. I've seen too many tank socks in vehicles plug up. Easier to address debris in the fuel filter by simply changing it. Cars use tank socks because the fuel pump is in the tank pre-main filter be it a pre-pump or high pressure pump.

[Nasher]

Thanks Matt and Peter C.

The Main fuel hose will go via a 2 Micron water separating canister filter at the transom.
The first stage filter I'm considering would be a very course one.

Not sure why I'd want to go to all the trouble of secondary tanks, additional pumps etc, that's the beauty of outboards and their self contained nature.
And there is obviously no fuel recirculation back to the tank.

I'm not sure that cutting the end at an angle helps too much in this application as when the top of the slash is exposed all suction is lost anyway.
I suppose it does give a bigger area of opening in the pipe which would be harder to block with debris inside the tank.

[Matt]

TBH, for the time being on my inboard boat I just have a single pump and feed/return to the tank - no high and low pressure circuits. But I am mindful I may need to add that in future.

As you say Nasher, outboards are designed to take just the single feed and usually have a "holding tank" on them that the high pressure pump feeds from (At least, my old xs-200 did).

[GuyC]

If you had a 5mm gap at the bottom, you could never suck anything up that wouldn't pass through the pipe.

[gtflash]

I had a 20mm gap. Ran out of fuel in the rough. Lowered it to 5mm. Crapped up my filter and picked up water. Bunged the inline filter and engine filter. Now set at 10mm on main and auxiliary which is a pre made sits curved around the bottom. I like a corse sock filter but if you can’t get to it it’s not worth it.

[Pikey Dave]

Originally Posted by Nasher

I’ve realised I have an issue with the installation of the 225Opti on my Ocean RIB which I’m very near to completion after many delays and false starts.



Mercury recommend a minimum of 5/16in (8mm) ID Fuel hose, but I’ve discovered the pick-up pipe welded in to my fitted 120l under-deck Stainless tank is only @5mm ID.

I never did notice a fuel starvation issue with my old DT200EFI, but then didn’t ever hold it at WOT for more than a few seconds at a time.

The boat was originally rigged with a Johnson 150VRO before I brought it, which drank petrol at an alarming rate, so I’m surprised.



I know for my use of the boat the existing pick-up pipe will probably be fine, but whilst I’m replacing all hoses and doing lots of other work I may as well put something bigger in.



However, the tank is obviously under the deck, with very little of it accessible, and I don’t really want to start drilling holes in and welding a tank 1/3 full of fuel, let alone cut up any of the deck to reach it, even within the jockey seats.



So I have a plan, but am wondering what sort of gap I should leave between the end of the pick-up pipe and the bottom of the tank?



NOTE:- The following are all really old images, but I don’t want to uncover the boat and take new ones in the pouring rain we have today.



The deck is laid out as below:





The tank stretches across under the deck almost the complete width of both Jockey seats, and the small round hatch between the jockeys covers where the fuel gauge sender once lived:





Note I gave up using fuel gauges years ago, so there is currently a plain round stainless plate screwed down to the tank over the hole.

I now use a Navman fuel computer flow sender and gauge with careful measurement of what goes in the tank volume wise.



Inside the port Jockey seat are the various connections to the tank.

Since this image was taken I’ve removed the rectangular plate revealing a hole in the deck just a little smaller than the plate, and have Flowcoated the inside of the jockey. Did it years ago.

From top to bottom:

Pick-up Pipe

Breather

Filler

Conduit to Transom area.





My plan is to cap off the existing pick-up pipe, and put a larger one in using the old sender mounting hole.

I’d need to have a right-angle fitting on top of the new plate that I could attach a new fuel hose to, which would then pass across the top of the tank and into the Jockey seat alongside the original pick-up pipe.

An added advantage is that being removable I’ll be able to add some kind of first stage filter to the bottom of the new pick-up pipe.

I hope this makes sense, new pipe/hose in Red:





But as mentioned, my question, after all of the above, is what gap to leave between the bottom of the new pipe and the bottom of the tank?



I obviously don’t want to suck up any crap in the tank, but also don’t want to lose too much useable volume.

Generally, flexible hoses present a higher resistance to flow than rigid. That’s why manufacturers spec larger hoses than required, it’s no different to speccing electrical cables. The pickup will have a slight resistance at high flow rates, by “going large” on the hose, you aren’t adding any additional resistance to the fuel line, assuming the hoses aren’t silly lengths, which they won’t be. Personally I think the existing pickup be fine.