My diesel fuel tank does not have a fuel gauge or sender installed, well not that I can find anyway. The tank is mounted behind the engine under the cockpit floor, so access is extremely limited. At the moment I calculate fuel remaining from the hour meter, but that is not very accurate when offshore in a sea.
The filler opening is on the top with a right angle bend to the port side, photo attached to show the space available.
What would be my options for fitting a sender? Otherwise fit a sight glass at the outlet that flows back to the vent hose?
You could fit a 6 in inspection port in the cockpit floor then fit a sender in the top of the tank.
An electrical sender will fail sooner or later. Reinforced plastic hose from the drain plug is as good as any. Use a tap and only open to check. The plastic will become brittle and opaque after awhile but you will get a few years from it. Probably the only time you should have clear plastic pipes in a boat.
I made a sight gauge for my fuel tank with a T junction and used Teflon tubing immune to diesel... I think. I got the Teflon tube from Ebay for $10. The sight gauge is marked off in litres remaining which was measured when filling the tank one time.
What I really want is to know how much water is in the keel tank. Apart from measuring how much goes in and calculating it I dont have a way to do it. Its inlet/outlet is a 1/2" pipe.
www.westmarine.com/sending-units
On the above link you will see a few different designs the one below would be my choice why it had no moving parts except the float moves up and down the rod
The style below could be used on other fluid as well
The style below under wave movement will rock and roll far more than the design above
the choice is yours
My fuel tank gauge at the moment is the original 1/2 dowel with marks on it indicating 1/4 to 1/2 to 3/4 to full
To use it I have to remove and rubber plug.
I will be removing the tank and replacing it with some thing more substantial than whats there and Ill be making sure every battery, water ,fuel and what ever stays in place If the boat does a 360 they will stay where they are
I have always used a dip stick with calibration marks on it. That or a sight glass are the simplest and most reliable ways of knowing how much fuel you have left.
With the limited space you have, fitting sight glass, dip stick or sender unit are equally difficult.
Keoghs Marine sell a flow meter type gauge that is quite cheap and seems reliable and it's easy to install. It is OK for fresh water Trek.
what I don't like about my dip stick for measuring fuel is the plug to me If the boat did a 360 the plug would fall out and diesel every where inside the boat.
Our water tank gauge is very unreliable due to the shape of the water tank and a linear gauge. Just got one of these to install tomorrow. Good reports from camper and RV users.
www.topargee.com.au/h2f-fm-water-tank-gauge.html
They also do models for fuel flow. Probably much easier than installing an in-tank sender.
www.topargee.com.au/fuel-flow-meters.html
Select to expand quoteHG02 said..
what I don't like about my dip stick for measuring fuel is the plug to me If the boat did a 360 the plug would fall out and diesel every where inside the boat.
A 360 in your boat HG?? I think you are a bit overly worried, besides it is a sustained 180 that is going to spill all the fuel. 
Select to expand quotecisco said..HG02 said..
what I don't like about my dip stick for measuring fuel is the plug to me If the boat did a 360 the plug would fall out and diesel every where inside the boat.
A 360 in your boat HG?? I think you are a bit overly worried, besides it is a sustained 180 that is going to spill all the fuel.
it might be the least of my worries hey Cisco 
