Fuel sending unit is the fuel tank level sensor that's hanging in a fuel tank that has a remote reading fuel gage and has a tank penetration that Deadline says he had trouble with. You may or may not have one, my 76 h30 does not have one but what you are basically looking for is any opening in your fuel tank or tubing that will let a tiny amount of liquid diesel to escape- as Deadline pointed out, even normally dry openings that are not tightly sealed can let fuel splash out and you'd never know it unless you got right down there and looked, giggled, sniffed and touched. Empty the cockpit locker, move the companionway ladder out of the way and clean everything you can see and reach and rig a drop light or two above your motor/ fuel tank (cords away from belts) get a big bright flash light and run your motor and look for drips or wet spots to form. Good luck.