Winter Lay up, To fuel or not to fuel?

2 February 2022

Should you fill your tank or leave it empty during winter?

Winter Layup


Don’t top up your fuel tank….


So it’s come around to Autumn and everyone is thinking of laying up their vessel and what you are  going to do about all that expensive diesel in the fuel tank?


All the oil majors collectively agree you should not store diesel longer than 6 months but lets face it that is totally impractical on a boat especially given the time it gets to you, the end user, it is often up to 2months old anyway.

So for decades the general consensus was to fill the fuel tank100% full in an effort to reduce as much as possible the exposed tank area above the level of fuel which gave rise to condensation that would lead to free water on the bottom of the tank and the very real potential for diesel bug to proliferate.

That made a lot of sense given that diesel bug live in water and like all bacteria, double their number every twenty minutes. You could return in the spring and fire up the engine and head out for a days cruising only then to find all the dead diesel bug and their excrement stirring up from the bottom of the tank the minute you hit the breakwater, blocking the filters, stopping the engine and certainly ruining your day. Even more so if you have to call out the RNLI, we hear this scenario a lot.

So why don’t we recommend topping the tank up any more?

Modern diesel is highly unstable, it’s been refined to within an inch of it’s life to remove the sulphur and now it has 7% plant oil along with animal and cooking fats. It degrades very quickly producing sludge that ends upon the bottom of your tank. The stability improver and antioxidant we blend into Marine 16 Diesel Fuel Complete (DFC) helps slow some of the degradation, but it cannot be fully prevented or reversed, would you get a blackened banana in the fruit bowl and try and recover it? No? you can’t, it’s same with diesel, it just doesn’t go as black.

So here at Marine 16 we recommend running the fuel tank down to as empty as possible. Get as much water out as you can, if you can, and put in a kill dose of Diesel Bug treatment (DBT) to kill any bug over the lay up that might try and take refuge in your tank. You can stop the moisture entering the tank by fitting a desiccant breather on the vent. This will dry the air as it is drawn into the tank so it cannot condense on the sides. Available on our website soon.

Additionally add a dose of Diesel Fuel Complete (DFC), even if you have been using it. From our research with fuel testing laboratories throughout this year we have improved DFC with an additional antioxidant in the blend in an attempt to mitigate the continued degradation of the FAME (The bio bit in the Diesel) which leads to carboxylate acid. We can’t stop the degradation, see above about the banana, but along with the dispersant in the blend this will help reduce deposit formation on the bottom of the tank.

When you return in the spring, drain any water from the drain cock or push a pipe down the filler to the bottom to try and draw off any accumulated water, or fit a diesel dipper over layup that can do it for you. If you invested in a desiccant breather there be should be very little anyway.

You should now be able to fill up the tank with clean fresh diesel but don’t forget to add, and keep adding, Diesel Fuel Complete (DFC) which will help mitigate the problems of storing it longer than 6 months, which is inevitable as you enjoy the summer, especially if you’re sailing.


You can purchase all products online at or at your nearest chandlery.

If you do have any questions, please contact the Marine 16 team, you can email or call 01686 622213


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PowerBoat & Rib
2 February 2022

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