Keel Cooling Calculator & Tank Design

Overview

In a typical modern diesel engine of the heat that is generated by the combustion process, approximately 30% is used for power, 30% is expelled through the exhaust, 10% is naturally radiated leaving 30% which is transferred into the engine cooling system and needs to be dissipated.

There is a direct correlation between engine power, speed, load and the subsequent heat generated (in kW) which needs to be dissipated from the engine cooling system. This allows a calculation to be made which dictates the minimum surface area of the hull that is required to be exposed to the cool canal, river or sea water.

This correlation varies with the material the hull and cooling tank are constructed from due to deferring thermal conductivity efficiency. Although a Narrowboat, Wide Beam or Dutch Barge engine may not always be used at full speed (RPM) which as a consequence will generate less heat to be dissipated, the tank surface area should always be capable of cooling the engine based on its maximum rated power.

For hulls manufactured in steel the calculation is;

Engine bhp ÷ 4 = area in square feet

Engine kW ÷ 32 = area square metres

For hulls manufactured in aluminium the calculation is;

Engine bhp ÷ 5 = area in square feet

Engine kW ÷ 40 = area square metres

Tank Design – Vertical

Depending on hull design and layout, vertical tanks are preferable to base tanks as they are the most efficient at maintaining the maximum amount of contact of the incoming hot engine water with the cooling outer surface area. As a consequence engine cooling water expansion is kept to the minimum.

For maximum efficiency they must be located below the waterline and should preferably be built into the swim area of the hull. The tanks profile should be slim 30 < 40 mm with an inlet placed at the top of the tank at one end and an outlet placed at the bottom of the tank on the same end. A bleed screw must be fitted at the highest point of the tank.

To ensure an efficient circuit flow through the tank is maintained maximising cooling efficiency, the tank must have a baffle placed horizontally within, effectively dividing the tank into two distinct areas through which the water must flow around in a U shape fashion.

The baffle should be as simple as possible so that no restriction is placed upon the engine circulation pump and be continuously welded to the inside of the hulls outer plating and as tight as fit as possible to the inner side of the tank to ensure maximum thermal conductivity.

Tank Design – Expansion

When water heats up its density drops and as a consequence its volume increases;

A typical water antifreeze mix of 30% at 10°C has a density of 1043 kg/m³

At typical engine running temperature of 80°C 80 this falls to 1005 kg/m³

This equates to almost a 4% increase in coolant volume, for a 45.5 litre (10 Gallon) system the expansion equates to almost 1.7 additional litre (3 Pints), so a provision is required for this expansion by adding into the circuit an appropriately sized remote coolant header tank. If this is not done water will lost through the engine coolant cap overflow and need to be replaced each time the engine has cooled down.

Engine Coolant Cap Overflow

Remote Coolant Header Tank

Tank Design – Hydraulic Propulsion

A total hydraulic drive system delivers a robust method of propulsion with the benefit of allowing installation within any convenient location in a Narrowboat, Wide Beam or Dutch Barge. The compromise it that full hydraulic propulsion is not as efficient as mechanical propulsion and generates additional heat. As a consequence the cooling surface of the tank should be increased by a minimum of 30%.