Preventing and removing diesel algae

What is diesel fuel algae ?

There are actually different types of diesel algae, aka diesel bacteria. The most common species happily nest right in the space between water and the diesel fuel in your diesel tank. It is not advisable to have water in your diesel tank. However, a lot of boaters encounter water in their tank at some point in their boating careers. Water can enter your tank in different ways and when it does, you run the risk of diesel tank algae wandering in and doing their thing. Diesel algae reproduce by multiplying themselves, so they can rapidly grow in number.

Once they grow in number, you will notice that your diesel fuel will start looking misty instead of bright yellow. When your personal bacteria population is left to grow even more, you will even find brown sludge, flakes or slimy strands in your diesel, e.g. in your primary filter. Often, boaters only notice that they have diesel fuel algae, once they see their engine smoke producing more smoke, dropping in rpm or even just plain stalling (because their filters are clogged). In some cases you might even notice a rotten egg smell coming from your diesel. When in doubt, you can buy bacteria tests, in order to examine if you have diesel algae in your diesel fuel.

Below, you see an example of fairly contaminated diesel fuel. The algae seen here is referred to as ‘biomass’ and ‘bacteria sludge’.





How the heck did that diesel algae get into my tank ?

Some diesel bacteria live in between the diesel and the layer of air in your tank, but most of them do so in between the diesel and water layer in your tank. The bacteria themselves are present in the air all around us. Most of the time they get into your tank through the tank vent. So whenever you not only have diesel, but also water in your tank, you present the diesel bacteria with a nice habitat and so you run the risk of getting a thriving diesel bacteria colony. In addition, water in your diesel causes reduced lubrication of your engine, which results in more wear and tear, as well as your engine running irregularly and therefore reduced performance.

Water can get in your tank in a number of different ways. Here are the most common ones:

– By condensation: a tank is fitted inside the boat. At night (especially in the winter) the tank cools down, while during the day the tank warms up again. The air in the tank also contains a very small amount of water, which then condenses and thus ends up as droplets on the sides of the tank. This water trickles down and then sinks to the bottom of the tank, underneath the diesel  (diesel has a lower density than water).

– A badly installed tank vent, e.g. a tank vent that lets rainwater seep through to the tank, or a vent that allows waves splashing against the stern to press water into the tank vent.

– A badly sealing diesel tank filler cap, that lets rainwater or seawater seep through the filler opening.

Diesel algae like hot and humid weather to grow rapidly, but also during the winter they can run go crazy inside your tank.



What are the consequences of diesel bacteria for your marine engine ?

Having diesel bacteria in your tank can be very dangerous for your safety at sea. The bacteria can clog your filters and that, in turn, can cause a drop in rpm or stalling of the engine. And exactly at a time when you’ve had rough weather on the water, the contents of your tank will have been shaken quite a bit. As a result, the sludge from the bacteria will have gotten separated from the bottom of the tank and fom the layer between the water and the diesel fuel. That sludge will have been mixed with the diesel itself, pumped through the diesel filters, thus clogging them and causing your engine to stall, right when you need it the most to make that difficult harbour entrance


Zeilboot die vastgelopen is voor de kust


The diesel bacteria can not only cause damage to your diesel filters (which you will usually need to replace), but possibly also to the injectors. It can also cause corrosion of the tank itself and it can affect rubber parts of your fuel system (gaskets, fuel hoses, etc.).


Has diesel changed over the years ? And what about biodiesel ?

Yes, the diesel fuel of today is different from the the diesel fuel of the past:

  • In a lot of countries, by law, diesel fuel nowadays should contain less sulfur than before. Burning sulfur is indeed quite damaging to the environment. But on the other hand, sulfur lubricates your engine and diesel bacteria don’t really like a sulfury environment. Darn it !
  • Moreover, nowadays, more and more biodiesel is added to the diesel fuel we use. Biodiesel is made from vegetable products and is therefore interesting, because the oil reserves in the world are diminishing. But biodiesel has the chemical property of attracting water, which in turn attracts the diesel bacteria. Darn it again !

Biodiesel met bloem



What can I do about diesel algae ?

A. Prevention

It is essential to keep your tank clean ! You can do this as follows

1. Get high quality diesel in your tank

The diesel you get in western countries is usually of good quality, but beware if you are travelling, more southbound for example (eg in the Mediterranean area, Africa, …). When you buy diesel fuel, try to get an idea of how the diesel is stored at the place where you are getting your diesel. When you can see for example that the gas station stores the diesel in a large diesel tank and you suspect that the tank is rarely used, you run the risk that the diesel is infected. When in doubt, you can always get a little bit of diesel fuel in a separate (see-through) container and take a sample from it. Or you can carry a diesel testing kit on board to be sure.

The following two points are your most important weapons against diesel bacteria:

2. Make sure that as little water as possible gets into your tank

  • Check whether your diesel filler closes properly. Make sure you check the rubber seal in the cap !
  • Maybe your cap is in a place that systematicallly collects water (eg. in a certain spot in the gangway). In that case, a solution could be to change the location of the filler cap.
  • What about the hole of the tank vent? Try to imagine what happens to your vent in different conditions (rising waves, if your boat is heeling towards port or starboard, in rough weather, with waves coming in sideways, …). Is it possible that water is coming in through the vent opening? Maybe the vent is mounted in such a way that rain can just leak in? Or maybe rain only leaks in while heeled towards port? If you think that is the cause, try to find another place for the vent whole. Sometimes it also helps to have a nice gooseneck in the piping of the vent to the tank, so rain can no longer get into the tank itself.
  • Always fill your tank completely whey you plan to stop using the boat for a while (eg for the winter). After all, the less space there is at the top or sides of the tank where condense can form, the better.


3. Find ways to detect water in your tank and drain it.

  • Install a drain valve or drain plug at the lowest point of your diesel tank.
  • Install a primary fuel filter in between your tank and the secondary filter (mounted on the engine itself), make sure that that primary filter has a tap at the lowest point of the filter. An inspection glass at the bottom of the filter is also very  useful to see if there is water or sludge in the filter.
  • Every few boating trips, you should drain some diesel from the tank and fuel filter and check whether there is water in it. If so, find the cause. When the diesel is cloudy or looks dirty, you’ll obviously know that something is wrong.


B. The cure: when it all went wrong

There are many products on the market to remove diesel algae (different types of additives for diesel, magnets, …). The only thing at has proven its effectiveness convincingly at this time are biocides. Biocides are chemical substances that specialize in killing bacteria and other organisms. In hospitals for example, biocides are used for disinfection. Some antifoulings contain biocides as well, which obviously kill the fouling on your hull. There is a multitude of biocides available on the market to add to your diesel.

Most of the products you simply need to add to the diesel after your tank has been emptied and thoroughly cleaned. The bacteria often stick firmly to the sides of your tank. Here you see the importance of having an inspection hole in your tank: it gives you the possibility to clean your tank completely by hand when it is necessary. Additionally, you can add the biocides as a precautionary measure each time you refuel.

As an emergency solution you could always have a fresh set of filters on board and a separate container with diesel fuel. If you notice that your filters are very dirty or infested by diesel bactia, you can often still reach the port unharmed with that emergency hose and jerry can.


Want to share your own diesel bacteria story? Do you have other tips? Please add it in the comment section below!


Did you find this article interesting and want more? Like & share it ! Thanks !


Preparing your boat engine for spring

In another article we were talking about winterizing your engine. In this one you can find more information on prepairing your marine engine for spring. The good news: if you did a good job when going into the winter, then this job will not take quite as long.




Time to check

First of all check all hoses and hose clamps, fittings, etc., for frost or other damage. If any of your through-hull openings or hoses have been damaged due to frost, you will want to know about it before going into the water again. Similarly, if you closed op all openings in your engine for winter (air intake, fuel tank vent, …), then you should of course open them up again.


Fuel Tank

Drain a bit of fuel from your tank (and possibly your coarse filter) and check that sample on water and any sludge or pollution. If the sample doesn’t look all that good, read Bob’s article on diesel algae.



Check the anodes in your engine, if your engine has them. Especially older marine engines have anodes in spefic parts of the engine that come into contact with seawater (eg heat exchangers, …). These anodes are small rods made of zinc or other types of metal, fixed on a screw so that they can be screwed into your engine. These rods make sure the inside of your engine is not affected by galvanic corrosion. After disassembling the anode, you can remove the metal that has already corroded on the anode by tapping it gently with a hammer. However, if more than half of the anode is eaten away, the time has come to get yourself a new one.

Of course, anodes can also be found on the outside of your boat, e.g. on your hull, on your saildrive, on your prop shaft, etc. Check if these anodes haven’t corroded for more than 50%. If this is the case, replacing them is the most sensible thing to do.



Wet sand the hull below the waterline, so that you get the surface clean and rough. First find out just where the area in your marina is where you are allowed to sand. In most marinas it is not permitted to sand antifouling just anywhere. Do not sand your hull with dry sandpaper, but use abrasive pads you can wet. Sanding dust particles of antifouling paint are very toxic, so you really don’t want to inhale them or get too much of them on your skin/eyes/… Wear gloves and a mouth mask (and really preferably a respirator) when you’re working with antifouling. I myself always use vinyl or latex gloves while sanding/painting/… : after finishing your job, you won’t need to put thinner or some other solvent on your hands to get off the paint, sand dust, … Just washing your hands after the job is usualy enough. Quick, easy and safe …

After sanding, paint the hull with antifouling. Make sure the antifouling you’re using adheres well to the previous layer. The easiest way is just using the same brand and type of antifouling as last time. If you plan on switching brands or types, read the antifouling user guide to see what other types of antifouling the new antifouling adheres to well. Make sure not to paint your anodes with antifouling, because it drastically reduces the effectiveness of the anodes. If you have a saildrive, beware as well; the aluminum saildrives are made of doesn’t react well to (copper-containing) antifouling. So in order to prevent your saildrive from seriously corroding, treat it with a protective paint before applying antifouling to it. Most antifouling paint producers also have this protective paint in their product range, so you’ll find more information in their product brochures.


Cooling system

Install the impeller after you’ve greased it with waterproof grease (such as Vaseline). Fill your seawater system with water (e.g. by pouring water into the raw water strainer untill it is full), to ensure the impeller doesn’t run dry for too long. This is especially important if you have drained your cooling system before winter. If you have winterized your cooling system by flushing antifreeze through it, try to collect the antifreeze coming out your exhaust hole by hanging a bucket under it when you restart your engine for the first time after winter. Note: I have actually never really succeeded in doing this properly, since the exhaust hole on my boat is very difficult to reach once the boat is in the water (…and a lot of cursing echoed through the marina 🙂 …). It is therefore more advisable to use a non-toxic antifreeze (propylene glycol) so that you don’t harm the environment with your first spring start (see the article on winterizing your engine).



Check the voltage on your battery. If it’s less than 12 to 12.5 V, give it a good charge before you crank your engine, since cranking the engine asks a significant electrical current from your battery.


Fan belt

Check the tension of your V-belt, tighten it if necessary.


It’s now time to go and get that boat into the water !

Oil System

You’ve probably changed the oil before winter and the new oil has circulated thoroughly through the engine. The oil has protected your engine nicely all winter long. Now, in spring, check the oil level just to be sure. After this, the most important thing is to first build up oil pressure in the engine before you start the engine. You can build up the oil pressure by putting the contact switch in the ‘on’ position (which in most recent engines starts the oil pump within the engine). Watch the oil pressure gauge closely. If your oil pressure still doesn’t rise as expected, you can ignite the engine with the stop lever pulled out until you see the oil pressure gauge rise.


Let’s go!

Then finally you can start the engine and let it heat up (don’t forget the bucket under your exhaust to collect the antifreeze). Keep a close eye on your gauges (oil pressure, temperature, …). Also check if there is enough water coming out of the exhaust and finally check in the engine room for oil, fuel, or coolant leaks.


Do you have a tip to add? Leave a comment below !


Why BootCoachBob ?

Learn all about the maintenance of your boat with

Hi, I’m Meindert Giessen, the founder of


Since 2008 I have a private boat (a Hurley 750 cabin sailing yacht). I soon realized how difficult it is to find good and unambiguous information on the internet about maintaining my boat.

In books on yacht maintenance I found good information, but the photographs and drawings in these books still were unclear to me.

There must be another way“, I thought. So I started You will find articles, video courses, useful links, etc. that will, in plain-spoken word, tell you how you can perform a number of boat jobs according to the rules of art.

I have a crew of experts who guide me to insure that the provided information makes sense (or at least, as much as possible).

Nose around and take a look on my site!




The cooling of your boat engine: direct or indirect?

In order to keep your diesel engine from jamming or overheating you have to cool it. A boat engine is usually not cooled by air (like a car) but by using external, fresh water for cooling. This can be done in 2 ways:


Difference between direct and indirect cooling

  • Direct cooling in which fresh water is pumped through cooling channels inside the engine. 
  • Indirect cooling through a heat exchanger: The fresh water is not pumped directly through the engine, but through a heat exchanger. This heat exchanger is a casing with tubes inside of it. The fresh water flows through the inside tubes. The hot coolant from the engine circulates outside of these tubes. This is the manner in which the heat exchanger exchanges heat with the coolant that flows through the engine. A disassembled heat exchanger looks like this: 

Heat exchanger of a marine engine

And when the heat exchange element is slid back into the coolant tank it looks like this (left side view of the previous picture):

heat exchanger of a boat engine side view


Pros and cons of direct and indirect cooling 

Direct cooling systems are less complicated than indirect cooling systems, since you just have fresh water running through your engine. In an indirect cooling system, two types of fluid must exchange heat which makes the system more complex, of course.

One drawback of direct cooling is that seawater is running through cooling channels inside your engine. This has 2 adverse consequences:

  • The salt in seawater more rapidly causes corrosion (rust) than cooling fluid. Coolant often contains corrosion inhibitors to protect the inside of your engine. Not all coolants contain corrosion inhibitors, but almost all coolants from the most common manufacturers (Volvo Penta, Yanmar, Vetus, …) contain these substances. It is recommended that you change the coolant in your engine every 2 years, because the coolant will still cool the engine, but the corrosion inhibitor substance within the coolant will disappear.
  • The engine heat causes a buildup of calcium and lime inside the cooling channels. This tends to clog the cooling channels and results in overheating. In a direct cooled engine external water will only be in the heat exchanger and the buildup will thus only occur in the heat exchanger element. Of course this element is much easier to clean than the cooling channels inside your engine.

In a direct cooled engine the risk of salt and lime buildup is more prevalent, therefore the operating temperature of direct cooled engines (55°-70° C or 131°-158° F) is kept lower than that of indirect cooled engines (70°-85° C or 158°-185° F). This operating temperature is controlled by the use of different thermostats that open up at different temperatures. At a lower operating temperature less buildup will occur in your direct cooled engine, which is a good thing. However an engine feels more comfortable at a higher operating temperature (70°-85° C or 158°-185° F). If your engine is continuously running at a lower operating temperature, that will reduce the lifespan of the engine.

An added benefit of indirect cooled engines is the winter resistance: coolant is usually resistant to frost (frost resistance can be measured with an antifreeze scale for example). If something has goes wrong while you were doing the winter layup of your engine, and as a consequence fresh water has frozen inside your cooling system, it could be that only the heat exchanger element is damaged and not the cooling channels in your engine. Of course the heat exchanger element can be expensive, but certainly not as expensive as a whole new engine…



An indirect cooled engine has in most cases more pros than cons. Most new boat diesel engines are indirect cooled engines. If you still have a direct cooled diesel engine, it can be converted into an indirect cooled engine. But doing this (or having this done) obviously depends on your boat budget and/or interest in doing some major tinkering.


Enjoy cooling off!



Winterizing your boat engine

Bob’s Ultimate Guide to winterizing your boat engine

Many Bob-fans have asked me how they should winterize their marine engine. Below you’ll find the result of my own experience (and a good amount of research ! 🙂 ).




Help me to make this list even better!

  • Do you have questions about certain paragraphs ?
  • Do you usually do certain tasks differently ?
  • Do you do other things to winterize your engine ?

Let me know! Leave a comment at the bottom of the page or send me an email at meindert*at*


Oil system

Drain the oil or suck it out of the engine with an oil pump. Replace the oil filter and fill the engine with new oil.

Oil must be changed at the end of the season because there are acids and other substances in used oil that can damage the parts inside your engine. Not something you want going on during the entire winter, so it’s best to change the oil before the start of winter. Be sure to also change the oil in the transmission gearbox.

Start the engine, let it warm up a bit and put it in forward and reverse for a few seconds so the fresh oil can thoroughly spread throughout the entire engine. Recheck the oil level and top it off if necessary.


Fuel system

Check the fuel filter for water and pollution. Is there water or dirt (sludge) in the inspection glass of the filter? Then there might be a lot more water, diesel bacteria or dirt at the bottom of your tank, and that can be very bad for your engine. Drain the bottom layer of diesel from your tank (Hopefully you have a drain valve, if so; check to make sure it is situated at the lowest point of your tank). Otherwise you can pump a few liters of diesel fuel from the bottom of your tank with a hose (inserted through the fuel filler tube) that reaches the bottom of the tank. If there is also sludge (diesel bacteria) or water in this diesel fuel, then you can try to completely drain the lower part of fuel from your tank until you don’t encounter any more sludge or water. When you don’t find any more sludge or water and the diesel fuel is clear (not dark in color), you can normally sail on.

If however, your diesel fuel is dark in color, the only thing that can save you is draining and cleaning the entire fuel tank with a special product. If you encounter water/sludge it is always a good idea to replace your fuel filters, since chances are the sludge has also made its way into your filters. If you often have water in your diesel fuel, it is a good idea to check the air vent of your tank. Sometimes the vent outlet is installed in the hull in such a way that rainwater can easily get in, or strong waves can easily push water inside the vent.

Be sure to replace the primary and secondary filter of your engine’s fuel system every autumn. Bleed the air from the fuel system. Fill the tank completely to the top with diesel. Due to temperature differences inside and outside of the boat, condensation can form on the inside of the tank (on the topside). Part of the condensed water then sinks in and under the diesel fuel. Diesel bacteria really like hanging out in between the water (at the bottom) and the diesel (above the water), and that is not a good thing.

Now is a good time to haul the boat out of the water.



For engines with a Saildrive: replace the oil in the saildrive leg when the boat is on shore.


Cooling system

Water can freeze, thus expanding and damaging parts of your engine in the process. For directly cooled engines (in which seawater cools the entire engine through water channels running through the engine) this poses a bigger risk than for indirectly cooled engines (where seawater cools coolant which in turn runs through the entire engine).

If you have an indirectly cooled engine, you have to make sure the coolant in your engine can’t freeze. You’ll need to check whether the coolant still protects your engine sufficiently from frost (this is only the case with indirectly cooled engines of course). You can perform this check with a simple tool called a hydrometer (for sale at car part shops). If the coolant does not offer enough protection against frost you can fill it up or drain all of the the coolant and replace it with new coolant.

As for the raw water system, it is best to start out by rinsing the entire raw water system with fresh water: open the raw water valve, disconnect the raw water hose and dip it in a bucket of fresh water. Start your engine and let it run until the bucket is empty. Now you can protect your raw water system against frost. There are two options.

  1. Filling the raw water system with antifreeze: dip the disconnected raw water hose in a bucket with a mixture of water and antifreeze. Preferably don’t use antifreeze with ethylene glycol (the substance that is usually found in antifreeze), because this is very harmful to the environment. It is usually hard to intercept the antifreeze that comes out of your exhaust during the first engine start in spring . So every spring a lot of toxic antifreeze gets poured into our waters. Instead, use antifreeze based on propylene glycol, which is not toxic and is also used for the frost protection of drinking water systems on board.
  2. A second option is simply draining the raw water system. This entails draining all raw water canals in the engine. Be sure to also drain the lowest parts of the engine. Then you remove the impeller. Lubricate the impeller with waterproof grease (e.g. Vaseline) and put it back. Then lightly screw on the pump cover (don’t screw it back tightly) to prevent the impeller from getting stuck during the winter. An alternative is to keep the impeller inside a sealed plastic bag. Have the engine running for a few seconds to blow remaining water out of the outlet hose. If you have an exhaust with a water-noise damper or a waterlock, you’ll have to suck the remaining water out of the exhaust with a small pump.


Raw water filter (or raw water strainer)

Clean the raw water filter (or ‘strainer’): unscrew the filter cover from the filter and clean the filter insert with water or if necessary with a mild detergent solution (preferably a biodegradable detergent, like Ecover). Also check the rubber gasket of the filter for cracks and other damage, replace it if necessary and lubricate it with waterproof grease (eg. Vaseline) before you reassemble the filter.


Cleaning of engine exterior

Check the exterior of the engine for remains dirt/oil/diesel. Be sure to also check the joints where different parts of the engine are mounted onto each other together (eg. where the heat exchanger is mounted onto the rest of the engine). If you see small (old) oil leaks or a whitish residue (salt crystals) in certain places, this can be a sign that a gasket has decayed, so that oil or water can leak from the engine. Depending on where the gasket is located you should replace the gasket if you can do it yourself or have the engine checked by a mechanic.


Rust protection

Check the control cables of the engine for damage and rust (sometimes visible if the cable sleeve is a bit swollen). Replace the cable if necessary and grease the metal parts of the cables, e.g. with WD40. Spray the alternator and the starter lightly with WD40 to counteract moisture (and to lubricate the alternator). You can also do this with the other electrical components on the engine (cables, connectors …) to counteract moisture.



If your engine has grease points, grease ‘em ! Check your manual for the specific grease points on your engine.



Make sure the battery/batteries are fully charged. A battery can discharge itself in a few months and a discharged battery can freeze and get damaged in the process. Recharge the battery fully at least once a month, or install a trickle charger. A small solar panel or small wind generator is also well suited for this purpose. You could also disconnect the battery from the rest of your electrical system to minimize discharging during the winter.


Hoses and rubber parts

Check hoses for damage: soft spots, holes, cracks … Also check underneath the hose clamps: unscrew them a little so you can move them over a bit and you can inspect the hose. And while you’re at it, why not also check the hose clamps themselves for wear. Don’t forget to screw them back on tightly once you’re done. Check all rubber parts of the engine for wear and weak spots. Your rubber engine mounts for example can degenerate by diesel that has been spilled onto them.


Antisiphon Valve

Check if the antisiphon valve is still working properly. In the case of an antisiphon valve with a rubber valve, you can wash the valve with warm water (or a solution of water and a mild detergent). In doing this you wash off the salt , that can otherwise cause the valve to get stuck, thus rendering the antisiphon valve useless. Rinse and dry well and reassemble.



Remove the propeller. Grease the propeller shaft (or the small shaft on your saildrive leg) with waterproof grease (eg Vaseline). Check the propeller for wear. If the propeller is corroded, there might be a problem with the anodes on the bottom of the hull, or something else causing galvanic corrosion. If you have a folding prop, disassemble it, degrease it (e.g. with brake cleaner), grease it back up and store it in a dry place. Check the rubber seal between the propeller and the shaft or saildrive for wear.


Openings to the inside of the engine

Plug the openings to the inside of the engine (air intake, exhaust, fuel tank vent), for example with a piece of cloth. This way you avoid ending up with unwanted visitors (spiders, moths) and dirt in your engine. Finally you can cover the engine with a tarp or a thick piece of plastic sheet, so that if there might be leak someplace in your boat just above the engine, water won’t seep onto the engine itself.


Last but not least: people forget more than they think…

Make sure you leave clear notes for yourself (e.g. on a sheet of paper on your chart table) with the things on it that you need to do before you start your engine back up in the summer (e.g. putting the impeller back, opening up the air intake back again, …).


I wish you the best of luck winterizing your boat ! Don’t forget to share this article & leave a comment below!



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