Practical experience using the keel as a cooler
From Ocean Navigator #91 July/August 1998
I think that we can, for the most part, agree that any engine will last considerably longer if it is freshwater cooled. The question is how to go about this, and to weigh the benefits, costs, and compromises involved in each method. In reading the answer by Mr. Knox, he seemed to heavily weigh the costs of the extra space needed to shield the dry exhaust, while giving much less weight to the need for a saltwater pump used with a typical heat exchanger and wet exhaust system.
Although I do not have a degree in thermal engineering, in my simple logic and simplistic experimentation it seemed to me that a big lead keel would be a great heat sink. As a boat builder I find it easier to buy a diesel engine with a heat exchanger installed by the engine manufacturer. As a boat captain/engineer I find that, as far as the propulsion system goes, the seawater pump, heat exchanger, and wet exhaust elbow or riser require more of my time for maintenance or repair than almost any other system on a boat.
The system I built was fairly simple. Prior to pouring the lead for a 14,000-pound keel, I laid close to 40 feet of soft copper tubing into the mold for the keel. I suspended it in the mold so that the tubing was stretched almost the full length of the keel, was not too close to a keel bolt, and was not too close to the bottom or to an outer edge of the lead. I left both ends of the copper tubing capped and held securely above the top of the mold. Later, while finishing the boat, I elected to use a simple system to regulate the flow of engine coolant water and hence the engine temperature. I plumbed crossover tubing between the outlet and return engine cooling tubes near the keel, with a -turn valve that I could secure in any position from completely open to completely closed. I had additional valves closer to the keel in both outflow and return lines.
After some slight experimentation I found settings for the valves that allowed 180° engine temperatures at various seawater temps. In general, the only adjustments I ever had to make were between summer Bahamas and fall Alaska ambient seawater temperatures.
That same engine, with only a valve job and very little work other than regular maintenance, is now 20 years old and still running every day. The original steel exhaust and muffler are also still in place and functional.
While being an advocate of copper tubing embedded in a lead keel, I question whether it could be used in a thin fin keel. I'm sure that the engineering could be worked out to do so, but I'm not sure it's worth the effort.
Personally I like the simplicity of this non-production keel cooler. It has never caused any problems, and I will use the same system on any lead keel boat I build. I see it as a positive selling point for any boat with the appropriately shaped keel. Imagine running a boat for 20 years and never changing an impeller, replacing the sea water pump, replacing the heat exchanger, pulling seaweed from the sea water strainer, or replacing corroded exhaust elbows.