N38143 Chapter 4 Raw water systems A completely separate sea water system should be provided for each engine to prevent a blockage resulting in the need to shut down more than one engine. A typical system is shown in figure (A). The water intake fitting (A6), situated below the water line (A1), should not project appreciably below the bottom of the hull and it should be situated well clear of other components such as shafts, logs, rudders to prevent flow problems at high speeds. The intake fittings and pipework should have a minimum bore of 25mm (1”) (A2). Inboard of the intake fitting a 1 inch BSP or NPT sea cock (A3) must be provided.
This should be of the full flow type giving unobstructed passage to the water in the open position, with a minimum bore of 25mm (1”). Between the intake fitting and the sea water pump (A4) on the engine, there should be a strainer (A5) which should be easily accessible for routine examination, and should be easily removable. Note: Ensure a separate feed for each engine. A shared supply is not recommended. From the sea water strainer a pipe should be run to the sea water pump inlet connection on the engine (see figure A). The pipe may either be mainly rigid, of for example copper or cupro-nickel, or flexible, but only flexible hose which is reinforced to prevent collapse should be used.
The system must be sufficiently flexible to permit the engine to move on its flexible mountings. The sea water pump connection is for hose with a 32mm (1.25”) bore. Care should be taken to use compatible materials in the sea water systems, to prevent excessive galvanic corrosion. Systems incorporating copper, cupro-nickel, stainless steel Type 316, gun-metal, silver solder, and aluminium brass will generally be satisfactory. Components made from lead, iron, steel, aluminium or its alloys, zinc or magnesium, should be generally avoided. Page 15