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Water Worries

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As more states mandate E10 ethanol-blended gasoline for off-road use, the problem of water accumulation in fuel tanks grows. And nothing can ruin your day faster than your engine dying from a lethal gulp of H2O. If you’ve been burning E10 for a while, you might already have what techies call “water bottom.”

Water bottom is simply water under the fuel in a tank, and it’s unfortunately become more common with E10 because ethanol attracts and mixes with water. At low concentrations ethanol and water stay mixed, but too much water (more than 0.03 percent by volume, or about four ounces per ten gallons) causes both the alcohol and water to separate from the gasoline and sink to the bottom of the tank. Losing ethanol from the gasoline lowers its octane, even if the water bottom isn’t high enough to be drawn into your boat’s engine.

Here’s how to find out if you have water in your tanks and how deep it is. Smear a thin film of water-finding paste onto a measuring device—about anything you can stick into the tank will work for measuring, like a clean dowel or yardstick. If there’s limited clearance above the tank, try a folding carpenter’s ruler. If necessary, remove the fuel-gauge sending unit to dip the tank. Look for a water-finding paste that’s formulated for alcohol; two examples are Kolor Kut’s Modified Water Finding Paste and MDR’s 566 Water Probe Indicator. Water will make the paste turn color (usually red); the height of the color on the stick tells you the depth of the water in your tank.


Here’s how to find out if you have water in your tanks and how deep it is.

Many gas stations use Kolor Kut to check their tanks for water. Maybe the manager of your station will sell you a tube (usually for under $10) or steer you toward a local retailer. MDR 566 ($8.29) is easier to find. If your local chandlery doesn’t carry either, you can order them directly from either MDR or Channel Supplies, Kolor Kut’s distributor.

Or go high-tech: If you have the appropriate electronics, replace the fuel-gauge sending unit with one that can sense water. England-based Offshore Systems manufactures NMEA 2000-compatible fuel-level sender units with lights that flash repeatedly (from empty to full) in the presence of water. The senders will fit any standard SAE five-bolt mount, operate off 12 volts, and work for diesel as well. They’re sold by direct order only via the company’s Web site.

If you find water, talk to your mechanic about the best remedy. He might recommend filtering the gasoline to remove the water and then returning it to the tanks. But unless you have a serious water problem, you won’t have to go that far—there are fuel treatments, like MDR’s E-Zorb, that will emulsify water and ethanol, remixing both with the gasoline so the water burns away during normal operation. Once the tank is water-free, regular use of E-Zorb will keep it so. Other products do the same job—just be sure that the product you choose is alcohol-free.


I’ll be adding a baitwell this year. Before I launch my boat, I want to install a through-hull and seacock for the water intake. How do I do this?—G. Sacks, via e-mail

Check the intake size on the pump; usually it’s 3/4 inch. Buy an appropriately sized bronze through-hull fitting, a seacock, and a short nipple to attach the pump. Make sure all the threads match exactly. (Some pumps have male threads on the intake, so you won’t need the nipple.) Get a hole saw matching the outside diameter of the through-hull, some polyurethane sealant, and, if your hull is cored below the waterline, epoxy resin and thickener to make a putty.

Pick a spot for the through-hull that’s easy to reach and has a clean water flow at all speeds. Cut from the outside. Just before the hole saw breaks through, go inside the hull and finish from there to make a neat hole. If there’s coring, dig out about an inch of it around the hole and fill with the epoxy putty. This keeps water out of the core and is a good practice whenever drilling through cored laminate.

When the putty’s hard, dry-fit everything. Trim the through-hull so the seacock flange seats tightly against the inside of the hull, but leave as much thread as you can. Once it all fits, coat the through-hull and seacock flanges with sealant and assemble. Don’t tighten too much, or all the sealant will squeeze out. Let it harden according to the directions, and then attach the pump, using the nipple if necessary. Use Teflon tape on the threads. Close the seacock. When the boat’s launched, check for leaks everywhere.

Note to readers: Unless you’re a skilled do-it-yourselfer, you should consider leaving this project for the yard, as putting a hole in your hull improperly can sink your boat.

This article originally appeared in the May 2007 issue of Power & Motoryacht magazine.