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Old 20-06-2023, 17:40   #1
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Fuel Tank Vent Routing Question

I am in the process of converting a water tank to a fuel tank. The two existing fuel tanks are on the port side and vent to the stern. The vent hose goes up to the hull deck joint and then to the stern of the boat.

The water tank being converted is on the port side, and the current vent goes to the galley sink... It is difficult to route the vent hose along the starboard side, so I am planning to route the hose just above the tank under the cabin sole and then run it parallel to the port side vent hoses.

It has been suggested that the vent hose needs to go vertical ASAP, and that if I run it across the boat there is a risk of a vacuum forming. I can't see why, and if so how bad the risk is. Explanations gratefully received. Also would a loop first help?
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Old 20-06-2023, 18:30   #2
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Re: Fuel Tank Vent Routing Question

Indirectly related is there a label on the fill that indicates water? Or anything other than fuel?
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Old 20-06-2023, 18:30   #3
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Re: Fuel Tank Vent Routing Question

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Originally Posted by Tday01 View Post
I am in the process of converting a water tank to a fuel tank. The two existing fuel tanks are on the port side and vent to the stern. The vent hose goes up to the hull deck joint and then to the stern of the boat.

The water tank being converted is on the port side, and the current vent goes to the galley sink... It is difficult to route the vent hose along the starboard side, so I am planning to route the hose just above the tank under the cabin sole and then run it parallel to the port side vent hoses.

It has been suggested that the vent hose needs to go vertical ASAP, and that if I run it across the boat there is a risk of a vacuum forming. I can't see why, and if so how bad the risk is. Explanations gratefully received. Also would a loop first help?
I'm not sure about a vacuum, but from experience, if any fuel gets in the short horizontal run of vent hose, then when you fill the tank that fuel will spit out the stern of your boat, and the fuel dock will get really pissed.

I would think that any "vacuum" would simply pull the fuel back down into the tank.
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Old 20-06-2023, 19:58   #4
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Re: Fuel Tank Vent Routing Question

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Originally Posted by AKA-None View Post
Indirectly related is there a label on the fill that indicates water? Or anything other than fuel?
I'm converting a water tank to a fuel tank, so I'll be replacing the deck fill and rerouting the vent.
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Old 20-06-2023, 20:30   #5
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Re: Fuel Tank Vent Routing Question

The length of the hose does not much matter in and of itself.

What you must avoid are any low spots where fuel or water could puddle. The longer the length of hose, and the more convoluted its routing, the harder it becomes to be SURE that the hose is continuously sloped either to the tank or to the outlet.
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Old 21-06-2023, 02:19   #6
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Re: Fuel Tank Vent Routing Question

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Originally Posted by SailingHarmonie View Post
The length of the hose does not much matter in and of itself.

What you must avoid are any low spots where fuel or water could puddle. The longer the length of hose, and the more convoluted its routing, the harder it becomes to be SURE that the hose is continuously sloped either to the tank or to the outlet.
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“... No part of the vent system shall trap liquid ...”
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