Keeping your engine’s cool
We often get queries about skin tank and cooling here at usually if there’s an over-heating problem, so here are some facts you might like to know
Most narrowboats on English canals have some form of keel cooling, as do some boats that use estuaries and ports where fine silt and mud can block more normal seagoing heat-exchanger cooling systems.
‘Keel coolers’ or ‘skin tanks’ are usually just a double skin of steel boxed onto the side of the hull. That might sound simple, but the actual design is very important due to the amount of heat that needs to be dissipated.
Generally, a typical modern diesel engine is roughly 30 percent efficient which means that fuel, when combusted in the engine, only uses 30 percent of the heat for power and the rest is not utilised; a second 30 percent goes through the exhaust, 30 percent goes into the coolant and the final ten percent is radiated from the engine itself.
When put into context, a typical 50 or 60ft narrowboat fitted with, say, a Beta 43 engine produces 43hp/32kW at full load and speed of 2,800rpm.
At the same time, this engine also produces 32kW of heat into the cooling system. This heat flows to the keel cooling (skin) tank and is transferred through the steel hull and the ‘insulated’ thick painted surface, into the canal or river. To lose such a large amount of heat through what is a thick painted surface means it requires a large area. Generally, though, most boats cruise the canals and rivers at around about 1,400rpm, creating correspondingly less heat that needs to be lost.
The most important factors to consider when designing a keel cooling tank for a canal boat are: The surface area of the tank in contact with the cold water outside the boat. The ability of the tank design to ensure that all the water passing through it is forced to make contact with the cold surface and cannot take a ‘short cut’ – a baffle is normally needed. The total volume of the system and the effect on expansion.
Based on calculations and experience, the best keel cooling tank for a canal boat should be vertical and built into the swim. For engines up to 100hp, the tank should be slim preferably 30- 40mm, with the inlet at the top one end, and the outlet at the same end but at the bottom, making sure that there is a bleed screw at the highest point.
The tank should have a baffle dividing it into two, forcing the water to flow around in a U shape. This baffle should be continuously welded to the outer plating of the hull to give good thermal conductivity and as tight a fit as possible to the inner side of the tank. A simple baffle is preferable to keep the restriction placed upon the engine circulating pump
And that’s where the skin
tank gets its cooling