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Boating – What does B7 diesel contain and why wonder?

Does your pleasure boat, whether an inboard boat or a sailboat, have a diesel engine??

Since 2009, diesel, renamed B7 or B10, includes a share of biofuel. What is it, what are the compositions and shelf life of this fuel?

The long shelf life of diesel in the tanks of pleasure craft leads to a degradation of the fuel which requires more precise filtration than that traditionally put in place.

What is the composition of B7

B7 designates, in the European nomenclature of fuels, a diesel fuel composed of 97% petrodiesel, 7% FAME and additives.

In the resort, there is also B10, at 10% FAME.


It is light fuel oil, a product made 100% from petroleum by refining.

Petrodiesel consists of 75% saturated hydrocarbons and 25% aromatic hydrocarbons. It freezes at -8.1°C. It’s the diesel of yesteryear, more than 15 years ago.


FAME (fatty acid methyl ester), FAME in English, are produced using a chemical process (transesterification) using vegetable (palm) and animal (tallow) oils as raw materials, as well as used frying oils. FAME freezes at much lower temperatures than petrodiesel.

The vegetable oils present in the FAME of the B7 represent 76% of the national consumption of palm oil for 400,000 tons annually!

300,000 tons of palm oil in diesel each year in France, that’s 50 kg per inhabitant.

Faced with this figure, we can continue to consume Nutella…


Additives are mixed in small quantities with the fuel to optimize the cleanliness of the fuel supply system, limit the appearance of corrosion phenomena and allow easier filling of the fuel tank thanks to an antifoam product.

The FAME problem

B7 diesel has a significant cetane rate, a component of volatility, which makes it possible to compare it, in terms of volatility, with SP98 E5.

A lot of cetanes is good.

In contrast, FAMEs are known to oxidize much faster than petrodiesel.

What does not pose a problem on a car, generates others on boats or the full are very spaced.

The phenomenon of cetane oxidation induces fundamental changes in the chemical and physical structures of the fuel, leading to oxidation products which are responsible for the formation of solids.

These solids form deposits present from the tank to the combustion chamber, degrade performance, pollutant emissions and damage engines.

Diesel bacteria

Diesel bacteria

Wintering and protection of diesel circuits

Automobiles are used far more frequently than recreational vessels. In the tanks of the latter, the diesel stagnates and degrades. Condensation does its work and the fuel suffers from pollution.
What can you do to protect your engine and filters?

We can decide to winter, empty tanks.

Although it is customary to completely empty the tanks during wintering to prevent condensation, this does not solve the problem of the rapid oxidation of FAME.
Transferring half a 200 L tank into cans does not look like relaxation.

Fuel can be further filtered
What do professionals do on board yachts or merchant ships that are faced with fuels of variable quality?
They always have a particularly efficient diesel filtration system.
Such a system consists of several filters capable of retaining bacteria, but also suspended solids and a secondary tank, a day tank, in which the right amount of fuel necessary for 24 or 36 hours of navigation is stored. A pump operates the whole.

Yacht fuel filtration
Yacht fuel filtration

Yacht fuel filtration

Optimize fuel filtration on board

But we are not going to install such a system on a small boat!

No, on the other hand, apart from the day-tank, these systems consist of three different filters, unlike the equipment of most pleasure boats, which only has one.

Most fuel filters contain a screen made of paper or non-woven material capable of retaining solids greater than 4 microns in diameter (4/1000th of a millimeter).
These filters stop solids but do not catch condensation water or the much smaller suspended bacteria.

Boating - What does B7 diesel contain and why wonder?

Without going so far as to use these systems as powerful as the one we described above, and if your filter does not include a separator, the installation of a second filter, a debug, and a separator filter, both mounted upstream of the cartridge filter, are such as to guarantee the purity of the fuel supplied to the injection pump.

Debug, splitter prefilter, filter.

Debug, splitter prefilter, filter.

The role of the separator pre-filter is to separate the fuel from the water. Thus, condensation or not, the water will not reach the injection system. Some of these filters play two roles, separator and 4 micron filtration.

A separator

A separator

The debug, on the other hand, is a filter capable of neutralizing the bacteria that pass through it via a fairly powerful magnetic field produced by its magnets.

A debug does not require maintenance and costs less than 200 €.

Boating - What does B7 diesel contain and why wonder?

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