Water in a Diesel Fuel Tank

Someone removed the fuel cap on our boat and there was enough rain that got into the tank to push some diesel out the vent. The tank had recently been topped up, ( 400 US Gallons) so I suspect that there might be as much as 4 gallons of water in the tank.

I will of course pump out the water through the engine feed fuel line by disconnecting it then try to get a hose into the bottom of the tank and pump what ever I can get out.

The issue is the remaining water that will be in the tank that cannot be pumped.
If it was a gas engine, I would run methyl hydrate through it and call it a day.
There are Racor 500 filters before the engines.

There is not drain as I can see.

So the questions are:
1) Can I use methyl hydrate to take out the rest of the water
2) Some posts on diesel forums say it is ok and others say that it is not
3) How many quarts per 100 gallons, or litres per hundred litres, etc of methyl can I add

 

On the water-fuel boundary bacteria will thrive and build gossamer structures that will slowly clog your filters. Check the water separators regularly, most of the water will collect there. A tell-tale for the presence of bacteria is a distinct acrid smell. The little bit remaining on the bottom of the tank can be dissolved in methanol or ethanol. Use small amounts, less than 1% and make sure it is well mixed.

 

This is one case where "fuel polishing" might work.

At the lowest fitting on the tank attach a fuel grade hose a Diesel fuel filter DC fuel pump and suck the suspect fuel thru the filter, return the cleaner fuel to the tank.

Watch the filter bowl and drain it each time its half full of water.

You suck thru the filter to not have the pump mix the water and fuel,

An old style glass bowl with fabric filter will do as well as a Raycor 1000.

No chemicals will help much but a DE EMULSIFIER , that causes ghe water to fall out of suspension will make it easier to find at the tank bottom.

It sounds like you have a box of fuel, not a fuel tank.

A genuine fuel tank would have a sump, and all you would have to do is drain it a few times.