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Old 11-08-2014, 03:54   #1
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Fuel return

Hi all, refitting my Ketch with a new Nanni 4.50 engine and the fit instructions state that the fuel return should be 'Returned to below the minimum fuel level'. Now I am running a 30ltr day tank and the existing return line finishes just below the top of the main tank (500ltrs) in the keel. So is there any real reason why it should go through the tank to below the minimum fuel level? I would need to cut the top of the tank open and refit a longer tube to facilitate this?

Cheers

Cal
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Old 11-08-2014, 05:03   #2
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Re: Fuel return

So that the returned fuel does not create bubbles when injected back into the tank. These bubbles become potential problems when fuel levels are low.
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Old 11-08-2014, 05:07   #3
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Re: Fuel return

I am fitting one myself and wondered exactly the same thing. The only ideas I had where that the non return valve in the return line might dry out if the fuel escaped from the return line. If it leaked slightly then air might get into the fuel supply line.

Or alternatively the siphon would be broken and any fuel left in the return line might flow back towards the engine through a leaky non return valve and leak someplace it shouldn't ?
These are just guesses on my partů Hopefully someone with some real knowledge will chime in. Cheers
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Old 11-08-2014, 05:33   #4
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Re: Fuel return

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Originally Posted by Blue Stocking View Post
So that the returned fuel does not create bubbles when injected back into the tank. These bubbles become potential problems when fuel levels are low.
Indeed.
Returning the fuel below the level of the fuel minimizes the potential for fuel foaming.
Think drawing beer from a tap.
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Old 11-08-2014, 05:38   #5
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Re: Fuel return

Thanks Blue Stocking & GordMay I new there had to be a reason, looks like I will be cutting into the tank

Cheers

Cal
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Old 11-08-2014, 05:54   #6
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Re: Fuel return

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So that the returned fuel does not create bubbles when injected back into the tank. These bubbles become potential problems when fuel levels are low.
You bet me too it by a few minutes, didn't see your post, cheers. It does beg the question as to just how low is a problem, and if the return goes to the other side of the tank, with a decent baffle between it and the supply, is it still going to be a problem. Given 90% of boats have the return led to the top of the tank and seem to get by just fine?

Another idea is a small inline tank that the return runs into. This Might be easier than modifying an existing tank?
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Old 11-08-2014, 06:15   #7
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Re: Fuel return

Snowpetrel a reply from Baeckmo from Boat design forums also said;

You have two issues here:

A/ The tank end of the return should always be submerged, particularly if the tank is lower than the engine. If open to air, the line may slowly be drained when there is no flow. Finally, this can lead to the draining of the injection pump itself, with starting problems as a result.

B/ Depending on the actual design of the injection system, the return flow may, or may not be a considerable share of the total flow. Now if the return leads back to the main tank, you can reduce the operating time on the day tank, since it will be quickly emptied by the return flow to the main tank.

I would take the return to the day tank and make certain that the end always is submerged. The idea with a day tank is to isolate the main supply from the operationally necessary volume for safety and maintenance reasons.

So I plan on taking the fuel return to the base of my 30 ltr day tank as it is above the engine.
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