Which Fuel Filter

[Searanger50]

My tank is tall, a little over a metre, and I have problems bleeding the system if the tank is less than half full. The primary filter is low, close to the base of the tank and the engine lift pump is also low so the lift pump is not strong enough to pull the fuel out of the top of the tank to bleed it. Would it cause any problems if I took a feed from halfway down the tank or even at the base?

[noelex 77]

The important distance is from top of the fuel level to lift pump (or primary filter depending which part of the system you are bleeding)

Changing the fuel pickup point will make little difference if the above criteria is unchanged. A pickup just below the fuel level and one at the bottom of the tank will require the same lift height from the pump.

If the fuel level can be substantially lower you will need a fuel bulb (like on an outboard) or an electric fuel pump. These may be needed only to prime the system, or they may be needed full time when the engine is running, in which case only the later option will work.
The bulb option is commonly used and is simple but does not meet some marine codes.

[Searanger50]

I think you've misunderstood my idea. I was looking at the possibility of taking an outlet from halfway down the tank so for the first half it would be gravity fed and the second half the pick-up would be a lot shorter.

[noelex 77]

Originally Posted by Searanger50

I think you've misunderstood my idea. I was looking at the possibility of taking an outlet from halfway down the tank so for the first half it would be gravity fed and the second half the pick-up would be a lot shorter.

Perhaps I am misunderstanding it often difficult to convey ideas typing.

However, if the fuel pipe is full of fuel it makes no difference where the pickup is on the tank.
With a pickup on the top, middle, or bottom the fuel will rise to same level. The suction required by the lift pump is the same no matter where the pickup is.

Try it with some water and a syphon transferring water from one bucket to another. Moving the pickup from the bottom to the top of the donating bucket will make no difference to the flow rate. (providing it stays below the water level)
Moving the pickup to the top of the bucket does not help the flow even though the drop in height between the ends of hose is now greater.
Water can be even made to flow "uphill" from a pickup that is below the level of the exit point of the pipe, providing the water level in the donating bucket is above the level in receiving bucket.
It is the fluid level that is important.

The lift pump has to lift the fluid from the top of fluid level in the tank to the engine. The tank can be 1 foot or 10 foot deep with a pickup at the bottom and the suction required is the same, if the fuel level is the same. (ignoring minor frictional effects)

[Ziggy]

Originally Posted by Searanger50

My tank is tall, a little over a metre, and I have problems bleeding the system if the tank is less than half full. The primary filter is low, close to the base of the tank and the engine lift pump is also low so the lift pump is not strong enough to pull the fuel out of the top of the tank to bleed it. Would it cause any problems if I took a feed from halfway down the tank or even at the base?

I don't know what ABYC has to say about tank fittings below the top. The principal problem I see is the possibility of leaks.

Lowering the feed would make it easier to prime and bleed the system, but if your fuel filter is close to the same level as the bottom of the tank, adding a shut-off valve just before the filter could make your system very easy to prime and bleed without using the lift pump. Simply shut the valve off before you open the filter to change the element. After the element is replaced, you can open the valve slowly and fuel will start flowing due to syphon action and will allow you to prime and bleed the system without using the lift pump.

All this assumes that the pick-up tube for the fuel feed hose extends almost to the bottom of your tank (as it should!)

[Searanger50]

thanks. all great ideas.

[CaptainRoss76]

From a very experienced worldly-wise marine engineer and delivery captain.
Are you going to sail far and wide?
If yes;
CAV - every time! - you will find replacement parts literally at the end of the world and everywhere in between! Watch flow requirements - spec carefully. In my experience anything over 20hp requires 2 x units in parallel.
Racor - If you know where your replacement elements / spares are coming from they win the race on ease of serviceability.

[Searanger50]

Thank you.