The majority of engines fitted to canal boats are water cooled. Water has a high specific heat capacity. In other words, it is an excellent substance for removing heat from a body such as a hot engine.
Typically water cooling systems are of two distinct types.
Keel cooling/Skin Tank
Engines in our cars have an enclosed, pressurised cooling system that relies on the passage of air across the fins of a radiator to cool the liquid in the system.
In a canal boat the radiator is replaced by what is known as a skin tank. It is simply a tank, which is usually welded to the swim of the hull, which relies on the external canal or river water to cool the engine coolant which is circulated through it.
In canal boats, true keel tanks are welded flat to the base plate. This is not as practical in reality as hot spots can form in the tank due to the nature of fluid circulation so the preferred option is a steel tank or tanks built onto the swim.
A slim tank in terms of depth, say 40mm, is preferable as a slim tank reduces the amount of liquid in the system and thus the amount of expansion as the temperature rises. It is standard practise to introduce hot coolant from the engine into the top of the tank, allowing cooled coolant to circulated out of a bottom fitting. Inside the tank may be welded one or two equally spaced baffle plates which force the coolant into a “C” or “S” shaped path through the tank. For larger engines it is acceptable to have two tanks, vertically mounted on either side of the swim.
A common rule of thumb for calculating the surface area of one side of the cooling tank is maximum engine power at full RPM divided by 4. So, an engine rated at 35 HP will warrant a tank with one side of 8.75 ft² or 1260 in².
In reality this may work out at a tank which is 20 in x 63 in x 1.5 in.
You may say that most canal boat engines will never run at their maximum rated RPM and this is true. Generally canal boats are over-propped and this enables them to make progress at much lower RPM’s whilst still being able to take advantage of the engines torque. The engine may only be producing 15 HP at 1000 RPM cruising speed so why the big tank?
The reason why it is recommended to calculate surface area by using the maximum rated RPM for a particular engine is to allow for extreme circumstances such as prolonged river cruising. Cruising along the River Trent between Newark and Keadby for example will require you to work the engine higher up the rev range, especially if the tide turns against you on the final approach!
For hydraulic propulsion systems, one manufacturer recommends increasing the size of the tank by 30% to account for the inefficiencies of hydraulic drives when compared to mechanical.
Raw water cooled
The second common method of water cooling a canal boat engine is by using water from the canal or river directly by pumping it through the engine block or a manifold mounted heat exchanger and out through the exhaust or a separate outlet in the hull.
Water is sucked into the system by an engine driven externally mounted impeller pump. It passes through a filter or mud box, into the impeller where it is pressurised and sent round the system. A thermostat will control the passage of water, as it does on a sealed tank system, allowing cooling water to dissipate the heat once the engine reaches operating temperature.
Basic systems will circulate the water through the engine block, but other systems may use a manifold mounted heat exchanger. Here the “raw” canal water is pumped into a tank on the exhaust manifold which contains a series of pipes. In these pipes runs the engine coolant, which is circulated around the engine block by a second, often internal, water pump. The raw water passes over the tubes, thus cooling the internal engine coolant, before being expelled out through the exhaust.
Calorifiers can be plumbed into both types of water cooled canal boat systems thereby using the heated water from the engine to heat water for domestic use.