Separation Anxiety

Ever find yourself wondering if the fuel you're taking on is of good quality, or whether it might be contaminated by dirt, water, rust, or worse? Well, you should, especially if you travel to third-world countries, which can be notorious for selling sub-standard fuel, especially marine diesel.

Since your fuel options are usually quite limited when you're in the middle of nowhere, your first line of defense is a supplemental fuel filtration system installed upstream of the engine-mounted factory filter. For the vast majority of boaters, this means one or more fuel-water separators.

Why not just a particulate filter? Because the most common contaminant found in diesel is water, thanks mainly to poor handling between the refinery and the pump, but also due to condensation once it's in your tank. Water is a problem for a number of reasons. First it creates oxidation, which leads to rust, a particulate that can quickly clog your engine's filters. Secondly, in displacing diesel fuel, water robs injectors and pumps of crucial lubrication, accelerating wear. Third, water does not compress when it's in the combustion chamber so something will have to give when the piston rises, namely a crucial (and expensive) engine component.

While most factory-installed fuel filters are effective at removing particulates, they may not be as good at getting water out of fuel. But more important, these filters typically do not have clear settling bowls where an owner can see an accumulation of water and drain it before it becomes problematic. With most factory filters you either must guess when to drain or wait for the engine to quit.

The other problem with standard engine-mounted filters is just that— they're on the engine where they're hard to see and get at. Supplementary fuel-water separators can be mounted anywhere, usually up on a bulkhead where they are visible and accessible.

The two big names in marine fuel-water separators are Racor and Separ, with Racor being considerably more popular. Both are two-stage units, meaning they contain a particulate filter and a centrifuging chamber in which water is removed. Generally speaking, the primary filter is rated at 30 microns to trap large contaminants, leaving the very fine contaminants to be caught by the engine-mounted filter. A two-micron filter is the finest available, and for this reason, it is usually reserved for the secondary filter nearest the engine. Both Separ and Racor offer optional filter elements that are rated for finer filtration.

Regardless of which brand separator you choose, consider a duplex system, two or more filters connected by a manifold and valve that allows the engine to continue running on one filter while you service the other. That way a clogged or contaminated fuel-water separator will never leave you dead in the water. Just make sure to keep lots of spare elements onboard.

Professor Diesel Q&A


Q: Every time my captain brings my sportfish back, he gooses the engines just before he shuts them down. My neighbor says this will damage the engines but my captain says everyone does it and it's no problem. Who's right? —R. Turnbull via e-mail

A: Your neighbor. Although that little shot of throttle probably won't kill your engines immediately, it isn't doing them any good. Of particular concern are the turbochargers. When you rev up the engine you also spool up the turbos, perhaps as high as 10,000 rpm. Turbochargers are made to operate at that speed, but only as long as they have a ready supply of cool oil to their bearings. When your engines stop turning, they stop pumping oil to the turbos, even though the turbines keep spinning. This can result in the overheating of the bearings and baking or "coking" of whatever oil is left on them. Eventually that will render the bearings useless.

The same thing can happen (although to a lesser extent) to hot oil that sits in various oil galleys, particularly in the cylinder heads. For this and other reasons, it's always wise to let the engines idle for a minute or two before you shut them down. If your boat is equipped with engine oil (as opposed to gear oil) temperature gauges, they can provide a helpful guide as to when to shut the engines off. When the needle drops 20 to 25 degrees, you can usually assume the engines have cooled enough to shut them off. Don't rely on the coolant temperature gauges as they're affected by the thermostat.

This article originally appeared in the February 2009 issue of Power & Motoryacht magazine.