We must be following the same plan, though I bring the jug down below and pour it directly into the tank, as my marina frowns on filling up/topping off in your slip. And the roadside diesel - while not exactly cheap anymore - always costs quite less than marina diesel.
where in the tank do you pour the diesel? Do you remove the "full/empty" sight gauge" or other port?
Tank has an inspection port on top. (Came that way when we bought the boat.) Just open it with a spanner plate key wrench and fill.
Yeah, I would hesitate in hoisting a five gallon jug in the slip in broad daylight. The rattle siphon is very easy, discreet and efficient. By having a towel handy when removing the siphon from either tank or jug, it is easy to do so without a single drop of fuel on boat or water. Of course, you wouldn't want to "top off" if there is the least chance of over-filling, but if the jug is mostly lower than the fuel fill opening during transfer operations, it will stop when levels match inside and outside. If I needed 100% tank capacity before a long passage, I would probably stop at a fuel dock, or carry a full fuel jug for transfer later.
I actually bought one a few years back based on a similar discussion here, but have never used it. Five years doing it this way, never a spilled drop (knock wood), easy to see how the diesel looks, and its level compared to my sight gauge - since I don't have a working fuel gauge on the engine panel. Just takes two minutes to remove the Q berth cushions and lay down a towel. Plus I try to keep her full or close to full, and never really need more than five gallons, so it's not like I'm lugging 20 gallons up and down. I tend to stick with what works, even at the risk of being unseamanlike. Sorry Christian, guess I'm guilty as charged in this arena. I promise it won't lead to sailing with the fenders deployed though.
