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Re: How to Remove Diesel from Tank?
First, invert the boat.
Oh. Sucking the fuel all the way up the filler hose may be impossible. I also second the vacuum oil pump as being one of the best ways to suck stuff up, and you'd use it in the top of the diesel tank. There's either going to be an inspection plate, or a collar that allows the fuel gauge to be removed, accessing it from there.
A drain valve on a marine diesel tank is unlikely, on small craft apparently that's either an ABYC or USCG no-no.
You could also pick up a cheap fuel pump (as I understand it, a gasoline pump will work) and hook it up to the fuel feed on the tank, and just pump it out that way.
But if you are going to the trouble of cleaning out the tank, REALLY consider going the whole nine yards. Remove the tank, get back in there to clean and inspect, and while the tank is out have it steamed out to get rid of any crud, if you can find a radiator or tank shop that will do it for you.
Changing out old fuel, that's nice. But if there's crud in the fuel, odds are there's biomatter on the tank interior as well, and if you clean it out now you can make sure it doesn't come back.