Cobra rib fueling

Rusty nail · 25 December 2018, 10:03

Hi I've recently bought an older 6.5m Cobra rib, the internal fuel tank was disconnected years ago as there were fuelling problems under acceleration??? I suspect the fuel pick up may have been too far toward the bow and when on the plane the fuel may have been too far down the tank to be picked up. The tank runs to about 300mm from transom. Has anyone heard of this problem on Cobra or any other ribs? Can anyone offer a solution?

jeffstevens763@g · 25 December 2018, 12:56

I Had similar on a fast fisher I just lengthened the pickup hose to the stern

Xk59D · 25 December 2018, 12:59

Are you sure tank isn't cracked? Pressure test it would be worth doing to confirm.

Can you get access to the fuel sender or anything on the tank?

Rusty nail · 25 December 2018, 13:00

Quote:

Originally Posted by jeffstevens763@g

I Had similar on a fast fisher I just lengthened the pickup hose to the stern

Thanks, I was hoping to do that, might be a bit expensive though. Submersible fuel line £22.00 per ft. need at least 8ft. What did you use to extend? Thanks for reply.

Rusty nail · 25 December 2018, 13:06

Quote:

Originally Posted by Xk59D

Are you sure tank isn't cracked? Pressure test it would be worth doing to confirm.

Can you get access to the fuel sender or anything on the tank?

Hi, yes, the sender unit is on the deck 300mm behind the jockey seat also a hole cut by previous owner under the seat. I filled it with hot water last week and it was still full and hot the next day. Can you suggest a pressure testing method and what pressure to test to? Thanks for reply.

ShinyShoe · 25 December 2018, 13:27

Quote:

Originally Posted by Rusty nail

Hi, yes, the sender unit is on the deck 300mm behind the jockey seat also a hole cut by previous owner under the seat. I filled it with hot water last week and it was still full and hot the next day. Can you suggest a pressure testing method and what pressure to test to? Thanks for reply.

How are you now planning to dry that?!

Not sure why Xk59D thinks tank may be cracked.

The normal approach to pressure testing it to seal all exit points. Then to one exit connect an air pump. For safety - a hand powered pump that you might use for tubes is better than electric (not sure if there is any possibility of fuel in your tank). Maybe needs to be same metal as tank?

Ideally you have some means of pressure measurement - a gauge or a piece of tube with some coloured liquid in it will do just fine. Once pumped up. Leave it at a known pressure (mark position of coloured liquid in tube). If no leak it shouldn't move after a few hours...

I guess Xk59D is saying there could be a crack when the position changes so perhaps repeat under strain.

If it's the sender... Does the amount of fuel you are carrying mitigate it?

Any reason the submersible tube can't be "brass" (someone will be along shortly to tell us it shouldn't be for some reason... ).

Xk59D · 25 December 2018, 13:37

I'm saying it is possible tank is cracked at weld along top at back and when planning fuel gets out into bilge. It is not unheard of for tanks to crack, 2 friends ribs have abandoned tanks for that very reason.

I find it hard to believe a tank was abandoned for a pickup issue which should be serviceable via inspection port, I find it very easy to believe you would abandon the tank for a leak though as I assume it means ripping the deck up and I assume RIB has a bit of age.

So to be sure pressure test it, easy to do before putting a lot of fuel in it as it is a new to OP boat, If it is just the pickup then happy days. You won't need a lot of pressure in the tank, need to do some research but I would think it will be 1-2psi max...DOUBLE CHECK NUMBER!!!!

Rusty nail · 25 December 2018, 14:00

Quote:

Originally Posted by ShinyShoe

How are you now planning to dry that?!

Not sure why Xk59D thinks tank may be cracked.

The normal approach to pressure testing it to seal all exit points. Then to one exit connect an air pump. For safety - a hand powered pump that you might use for tubes is better than electric (not sure if there is any possibility of fuel in your tank). Maybe needs to be same metal as tank?

Ideally you have some means of pressure measurement - a gauge or a piece of tube with some coloured liquid in it will do just fine. Once pumped up. Leave it at a known pressure (mark position of coloured liquid in tube). If no leak it shouldn't move after a few hours...

I guess Xk59D is saying there could be a crack when the position changes so perhaps repeat under strain.

If it's the sender... Does the amount of fuel you are carrying mitigate it?

Any reason the submersible tube can't be "brass" (someone will be along shortly to tell us it shouldn't be for some reason... ).

Hi, I'll worry about drying it later!! At the bow under the console I have a 40mm fill pipe, a 3/8 bsp threaded pick up line and a 3/8 vent line. I've had an endoscope in the tank and it has baffles every 12" along its length. The new pick up would have to bypass the baffles which have 45 degree corners cut out to allow fuel to pass. So it needs to flex past these. I've been trying to upload photos, with no success. Thanks for replies, appreciate the help.

Rusty nail · 25 December 2018, 14:04

Quote:

Originally Posted by Xk59D

I'm saying it is possible tank is cracked at weld along top at back and when planning fuel gets out into bilge. It is not unheard of for tanks to crack, 2 friends ribs have abandoned tanks for that very reason.

I find it hard to believe a tank was abandoned for a pickup issue which should be serviceable via inspection port, I find it very easy to believe you would abandon the tank for a leak though as I assume it means ripping the deck up and I assume RIB has a bit of age.

So to be sure pressure test it, easy to do before putting a lot of fuel in it as it is a new to OP boat, If it is just the pickup then happy days. You won't need a lot of pressure in the tank, need to do some research but I would think it will be 1-2psi max...DOUBLE CHECK NUMBER!!!!

I've been struggling too with the reason given for abandoning of the tank tbh. Someone has attempted to fit an alternative pick up at some time but it was a short length of copper pipe which would have made little difference in my opinion. I've spoke to Cobra and they have never heard of the problem but it is an older boat. I'll set up a pressure test as suggested. Thanks.

jeffstevens763@g · 25 December 2018, 14:13

Quote:

Originally Posted by Rusty nail

Thanks, I was hoping to do that, might be a bit expensive though. Submersible fuel line £22.00 per ft. need at least 8ft. What did you use to extend? Thanks for reply.

Just used plastic fuel grade tube

Xk59D · 25 December 2018, 14:15

It is entirely possible the engine fuel pump wasn't happy with drawing fuel up through it due to poor pickup, internally degraded fuel lines which clogged up filters etc

Good news is you are now addressing it and if the tank holds pressure then you are onto a winner as everything else is fixable without ripping the deck up.

I would be tempted to renew pickup, all fuel line and filters while you are at it if tank tests out.

Rusty nail · 25 December 2018, 14:17

Quote:

Originally Posted by Xk59D

It is entirely possible the engine fuel pump wasn't happy with drawing fuel up through it due to poor pickup, internally degraded fuel lines which clogged up filters etc

Good news is you are now addressing it and if the tank holds pressure then you are onto a winner as everything else is fixable without ripping the deck up.

I would be tempted to renew pickup, all fuel line and filters while you are at it if tank tests out.

I will do Xk59D, thanks a lot.

Rusty nail · 30 December 2018, 12:49

Quote:

Originally Posted by Rusty nail

I will do Xk59D, thanks a lot.

Xk59D, it has held 2psi for 10mins now, thinking it should be good to go?

GuyC · 30 December 2018, 13:40

That sounds good news but I'd be tempted to leave it on for an hour.

Rusty nail · 30 December 2018, 14:04

Quote:

Originally Posted by GuyC

That sounds good news but I'd be tempted to leave it on for an hour.

Hi, surely if it was split, I would see an immediate pressure drop? I have left it and all is ok. cheers.

GuyC · 30 December 2018, 19:02

The hour is a generally accepted length of time for air tests to be left 'on'.

It just tries to cover the base of a very small leak.

On the test pressure - 2 psi is about 0.14 bar or 1.4m head of water. Just be careful that you don't over pressurise the tank. I'd suggest the maximum pressure should be equivalent to the vertical distance from the bottom of the tank to the overflow.