Vertical tanks are preferable because they maintain the maximum amount of contact with the outer surface. Hull base tanks are less efficient due to the fact that the hot water remains at the top of the tank away from the cold base/ outer hull. They can, however, be made more efficient if they are kept to a minimum depth of 30- 40mm utilising the same baffle system as the vertical tank. Welding ‘fins’ to the base before manufacture of this type of tank can be even more efficient. Engine kW ÷ 32 = area square metres
for aluminium boats you can use the following rule because it has a higher thermal conductivity, the cooler size may be smaller:
Engine bhp ÷ 5 = area in square feet Engine kW ÷ 40 = area square metres. This surface area is a minimum for an engine developing its maximum continuous power at full engine rpm.
In practice, much smaller areas have been used without overheating due to a number of factors that affect the engine – the power used by most boaters when cruising on the canal is considerably less than maximum and many canal boat engines are over-propped and incapable of reaching their maximum rpm/power, even on a river. expansion. When water heats up its density drops thus increasing its volume, a typical water antifreeze mix of 50 percent at 10C has a density of 1043 kg/ m3, this falls to 1005 kg/m3 at 80C (a typical engine running temperature).
This is approximately four percent difference in volume, and so for a ten-gallon system, the expansion is around three pints which means that there must be provision for expansion of three pints. If there isn’t, the water will be lost through the overflow and have to be replaced each time the engine cools down.
So the larger the cooling system, the larger the expansion; the objective must be to keep the volume of the total system as low as possible using a slimline tank.