Question: I thoroughly enjoyed your 1997 Handbook of Offshore Sailing. I noticed the advertisement for the Nordhavn 62 (page 26) mentioned a “keel cooling system and dry exhaust.” I am unfamiliar with this system, but assume that it involves closed-loop freshwater coolant circulated inside the keel.
I am curious to learn if such a system has been applied in sailboat keels. It would seem relatively easy to attach a circulating pipe grid to the keel bolts before the keel is poured. Lead is an excellent thermal conductor and such a system might avoid the problems associated with using salt water for cooling (blocked intakes and corrosion). Roger Kingsland, Pittsburgh, Pa.
Answer: As far as I could find, no one has built a sailboat in which the external ballast keel is used as a heat sink for the cooling system. The system Mr. Kingsland describes may or may not be feasible.
Most important, however, lead is not an excellent heat conductor. It has a relatively low heat-transfer coefficientabout half that of iron. Fairing compound and bottom paint on the outside of the keel would further inhibit heat transfer. Don Chatmuck of Walter Keel Coolers thought the heat transfer problems would probably be insurmountable. According to Chatmuck, even a coat of bottom paint reduces the heat transfer of a standard type of keel cooler used on small commercial power boats (these units typically consist of a pipe grid outside the hull) by at least 30%. Put the same cooler inside a lead keel and the heat transfer goes to almost nothing.
As Mr. Kingsland points out, keel cooling eliminates one of the boat’s seawater systems, a big advantage: no raw water pump, no engine heat exchanger, no blocked intakes, etc.
Like all other features of boat design, there is a price to pay. The cooler creates additional drag, though Chatmuck contends that it is not noticeable at sailboat or trawler speeds. The drag is somewhat reduced if the keel cooler is recessed into the hull, but this creates problems of its own. The other problem with keel coolers is biological fouling. Any type of growth obviously reduces the heat transfer characteristics of the keel cooler.
Boats with keel coolers are almost always fitted with dry exhaust systems to avoid the complications of installing a seawater system just to provide seawater for the exhaust.
Dry exhaust systems eliminate the possibility of seawater backflooding into the engine through the exhaust system, so many aspects of boat design are simplified. Muffler location and height are no longer critical. Risers can be eliminated. The exhaust lines can be routed almost anywhere, since height above or below the waterline is not a consideration.
Because wet exhaust systems must push water out the exhaust, they usually run with a higher back pressure than would a dry system.
Dry systems, however, are typically louder. Water injection dampens exhaust noise considerably.
And dry exhaust lines require more space. Since the gases are never cooled by water injection, they occupy more volume and require a bigger exhaust line. Also, the outside of the exhaust lines must be insulated, further increasing the space required. Because of exhaust gas temperatures, it is not feasible to use exhaust hose. Steel pipe is normally used.
Like all things tried and true, there’s a reason why almost all yachts are built with heat exchangers and a wet exhaust. They work, and the alternatives don’t present any real advantages.
Steve Knox is president of Knox Marine Consultants, a marine survey firm in Norfolk, Va.
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