Preventing Fuel Algae


All diesel fuel carries the ingredients for a nice bloom of algae. This yucky goop can clog filters, and if it works its way into the fuel injection pump, do serious damage. Over the years we have seen lots of cruising yachts with algae problems. But so far, we have avoided this fate.

Algae requires two important items to bloom, water and heat. The heat comes from the excess fuel which the diesel injector pump returns to the tank (it is warmed running through the engine). The moisture is a byproduct of the temperature differential between the tank surfaces and ambient air.

Here are the system and maintenance items that have to date kept us clean:

  • Fuel magnet, as shown in the photo above: these were developed in New Zealand 25 years ago, and are used in a variety of commercial applications. They apparently kill algae. The Kiwi’s swear by them. We have no scientific evidence one way or the other, but have been using fuel magnets since 1988.
  • Day tank: the use of a day tank keeps warm fuel isolated from the main storage areas, reducing heat and moisture build up.
  • Tank vent control: a carefully plumbed breather system, with a shut off valve for use when the boat is stored helps control humidity inside the tanks.
  • Keeping tanks filled reduces the volume of air which contributes to moisture build up.
  • Fuel polishing system provides a method of cleaning fuel and an early warning that there are proems brewing.
  • Use a diesel fuel biocide on a regular basis.
  • Buy fuel from high volume commercial suppliers (where the fish boats go) whenever possible.

Posted by Steve Dashew  (February 3, 2010)

6 Responses to “Preventing Fuel Algae”

  1. david miller Says:

    Hi Steve

    Will you post the contact details for the manufacturers of the Bug Disrupter,please



  2. Steve Dashew Says:

    HI David:
    This was sourced at

  3. jeremy colles Says:

    Your blog is so great to follow because it combines travelogue, cruising info and lots of tested facts, backed up by stacks of quantitative data, especially when it comes to your rigorous testing for your new boats.

    It therefore seems out of character for you to promote a system for which there no scientific support. You seem to research everything so rigorously but in this case you say “… we have no scientific evidence one way or the other”.

    In fact there is plenty of scientific evidence showing that fuel magnet systems do not kill algae.

    Seafaring is full of traditional superstitions – this is the modern equivalent.

    For an engineer’s view try:

  4. Steve Dashew Says:

    Hi Jeremy:
    There are numerous commercial users of these magnetic devices in New Zealand who praise them. As stated in the blog, we are not sure if they work or not, but since they don’t hurt, except for the wallet, we figure they are worth a try. The fact that we have not had algae problems could easily be do to the use of a day tank, and or a biocide.

  5. David Power Says:

    Hi Steve, I have been using magnets on fuel lines for some 15 years. I was sceptical until I put some on a powerful motor boat (2 x 2,000hp engines). The engineer did not follow the instructions and only fitted them to one engine that drew and returned to its own fuel tank. After a summer season the tank tops were taken off for internal hull inspection. The fuel in the tank with the magnets fitted was clear. The fuel in the other tank was milky.

  6. Steve Dashew Says:

    Hi David:
    Thanks for the valuable feedback. I cannot think of a better test!