'Diesel Bug' & Inland Boaters

 Any boat will collect water in the bottom of the fuel tank and this will cause problems. Over recent years it has become apparent that leisure boaters in particular may suffer the growth of microbes, yeasts and moulds at the interface between this water and fuel. This is collectively known as 'Diesel Bug'. If left untreated it will eventually get into the fuel system and block water traps, filters and eventually pipes and pumps.

Diesel Bug growth in Narrowboat Diesel Fuel Tanks

I am convinced that this is the main area where problems will occur, especially on narrowboats that in general are NOT fitted with adequate fuel tank drain taps.
Boaters should strive to remove the water from the bottom of the tank. In most narrowboats the only practical way is to use a pump and length of pipe through the fuel filler. Causing the boat to list towards filler will help to concentrate the water in an area you can get at.
Do not just pump out water, also pump out and dispose of any fuel that appears cloudy. This is likely to be bug before it develops into filter blocking jelly. This should be an annual job.
At the very least use a fuel additive that removes water from the fuel and on older boats with fuel tanks in an unknown condition it might be a good idea to use one that contains a biocide as well like Marine 16.

Choosing your Diesel Fuel Supplier

The problems with EN590 B7 on forecourts indicates that many marinas will not be able to look after their fuel well enough and will not have a fast enough turn over. It would be a very good idea to only buy fuel from suppliers who:-
1.  Have the highest turnover you can find.
2.  Would find it very expensive to supply contaminated fuel.
This probably means hire fleets.
Try to be present when your fuel filters are changed so you can inspect the fuel that remains in them and their parts for signs of cloudy fuel or jelly like substances. If you find any immediately use a high dose of biocide additive.
All of this sounds alarming but I do not anticipate any major problems for well-maintained boats where the fuel tanks are kept as water free as possible. I do expect problems with old boats that have not been well maintained – especially in respect of fuel bug. 

Why De-emulsifiers may not be enough to prevent Diesel Bug

It is for this reason that I have long advocated the use of an emulsifying fuel additive so the water is safely passed through the fuel system and thus preventing the bug from thriving. I have been using Fuelset for over 11 years now.
At the 2011 show a representative of one of the major forced draught heater manufacturers told me that they had found problems with such products but as my experiments had not showed this and I had received no other reports I could not justify changing my advice. However over the last year a company I work closely with, River Canal Rescue Ltd, has found some rather worrying trends.
It seems that the combination of the FAME in the fuel, which has good emulsifying properties, plus the additive causes a reaction that we have not seen before.
It seems a "mousse" forms at the interface of the water and fuel and a "waxy" substance is produced that blocks filters while leaving them looking perfectly clean.
This has caused me to revise the advice I give and also will follow as soon as my boat is suitably modified
First of all you can not assume that your fuel will be FAME free even if the vendor says it is. The fuel may well be cross-contaminated at any point between refinery and the vendor so assume the worst and you will minimise the chances of problems. (Some refineries are now claiming that they are supplying bio free gas oil.)
My advice now is that you use a combination DE-EMULSIFIER and BIOCIDE such as Marine16 or Grotamar. The biocide should take care of any bug growth at the interface of fuel and water while the de-emulsifier will force the water out of the FAME. However that leaves the problem of a slow build-up of water in the tank.
Most boats should have a tank drain at the tanks lowest point and if so it has always been good practise to use this to drain a sample of whatever is lurking in the bottom of the tank a couple of times a year. However some production boats and far too many narrowboats have no such drain so in these cases it will be necessary to get access through something like the fuel filler or fuel gauge sender (tank unit) hole. Inset a length of pipe connected to some sort of pump or vacuum device. This can then be used to "vacuum clean" the bottom of the tank. Keep going until all the cloudy fuel has been removed.
Ideally do this once a year.
Alternatively you can employ a yard to "polish" the fuel. This uses an electric pump and successively finer filters to filter the water and such like from the fuel.

Please note that I have no connection with any product named and they are only named as illustrations of type.

Copyright Tony Brooks  2010


Thanks to TB-Training

This information is kindly provided by Tony Brooks, an expert on Marine Diesel Engine & Boat Maintenance matters. Thanks to him and TB-Training for permission to reproduce these materials.

The images below are reproduced with permission from River and Canal Rescue Ltd.


Diesel bug separatingFuel, contaminated fuel & mousse

Contaminated diesel filterA 'clean' but actually blocked filter.


All materials and images © Canal Junction Ltd. No unauthorised reproduction. About | Terms | Privacy