Manufacturers of direct-cooled marine engines design them carefully to minimise corrosion in the cooling passages in the engine block. It is possible to convert ('marinise') a standard car engine using direct cooling, and many small craft owners do it successfully.
To keep the engine running efficiently at its optimum temperature, the rate of water flow through the cylinder block is regulated manually or automatically.
Manual Temperature Control
In the discharge line from the raw water pump, fit a branch, controlled by a hand valve, to allow some of the cooling water to by-pass the engine. Start the engine with the by-pass valve open, then close it gradually as the engine warms up, until the optimum temperature is maintained.
Do not regulate the pump flow by recycling water from the pump discharge back into the inlet line, or by restricting either the pump discharge or suction pipes.
Fit a marine thermostat in the discharge line from the cylinder block. When the engine is cold, the thermostat restricts the flow of cooling water through the engine, diverting most of it to the exhaust via a spring-loaded valve. As the engine temperature increases, the thermostat automatically increases the flow of cooling water through the engine, diverting less through the valve, until the required operating temperature is maintained.
A pump recirculates fresh water in a closed circuit (the Primary Cooling Circuit) through the cylinder block via a thermostat and a heat exchanger. A second pump delivers river water or seawater ('raw water') to the heat exchanger, maintaining the water in the primary circuit at about 80-90ºC.
Keel cooling is a form of indirect cooling with part of the closed primary cooling circuit in the form of pipes, or sometimes a double skin, in the boat's bottom. The heat generated by the engine is dissipated directly to the river water or sea water flowing around the hull. Manual or automatic temperature control is achieved with a suitable by-pass and valve. In principle a single pump can recirculate the coolant, though in practice an additional pump is often required to maintain an adequate flow rate against the extra resistance of the extended pipework.
Raw Water Cooling Pumps
What cooling water flow rate does an engine need?
On average, petrol engines and fast-running diesel engines with direct cooling require a raw water flow of about 36-39 litres (8 - 8.5 gallons) per minute for each 75kW (100 bhp) of engine power. Engines with indirect cooling require more flow: about 65-70 litres (14.5 - 15.5 gallons) per minute for each 75kW (100bhp) of engine power.
To provide reliable performance under seagoing conditions, a raw water cooling pump must operate in a correctly-designed cooling system. It is essential that cooling water should reach the pump without having to overcome excessive resistance or restrictions.
The following general rules are given for guidance:
- Suction pipe bore must not be smaller than the pump inlet port. If the suction pipe exceeds 3 metres in length, its bore should be one size larger – particularly if the pump is operated at high speeds. Use pipe of the same bore throughout. Avoid sudden changes in pipe bore. Use long, tapered sections of pipe at any change in pipe bore.
- The suction pipe run should be as straight as possible – avoid unnecessary bends. Do not use square or standard elbows, but fit long swept bends.
- DO NOT fit gearbox or engine oil coolers in the pump suction system. Always install them after the pump.
- Seacocks should be of the ball or plug type and of the same nominal bore as the suction pipework, giving full through-bore in the open position. The handle position should clearly indicate whether the seacock is open or closed.
- Seawater inlet strainers should have a hole or mesh size no smaller than 3mm in diameter – or 4-5mm for larger pumps. The hole or mesh size should always be smaller than the heat exchanger tube bore.
- Check frequently that the inlet strainer is not clogged. If in doubt, clean it thoroughly.
- Fast boats (over 12-15 knots) and planing craft must be fitted with inlet scoops positioned in a permanently-wetted area of the boat’s hull, to create sufficient inlet pressure at high boat speed. Flush inlet fittings are not suitable for fast boats.