HOT WATER CYLINDER
RAW WATER SYSTEM
The Nav Station...
Yanmar Pleasure Boat Marine Engine Help
RAW WATER SYSTEM
Water Flow Schematic for a 'Fresh Water' Cooled Engine
Scoop: dont fit an exterior scoop unless you have an overheating problem caused by the position of the thru-hull fitting on the hull. When installing a scoop on a slower vessel (below 11 knots) install a slotted scoop facing aft. This helps reduce the possibility of a leaf, stray plastic bag, etc., blocking the intake and wont force water into the engine while sailing.
With a multiple engine installation and forward facing scoops (planning hull craft) the raw water cock on a non-operating engine must be turned off while motoring or the engine doctor will make lots of money again!
Thru-hull fitting: make it a hose size larger than the waterpump intake. e.g., if the pump is .625inch / 16mm, install a .75inch or 19mm intake system. If it is 1.75inch - 45mm, install a 2.00 inch - 50mm intake system to the raw water pump. Everything must be the larger size, cock, strainer, hose. Step the hose down at the pump pump intake. A piece of thin hose of the correct inside diameter works well. There is no need to glue it all together as the hose clamp(s) will hold it safely in place.
Install the thru-hull in a position that is always deep in the water under ALL motoring conditions.
In my experience the best raw water components are made of Marelon®, from FORESPAR ®, rather than bronze fittngs that are much more expensive and subject to electrolysis .
Hose clamps must be 316 stainless steel, check that the tightening screw is also 316SS as the cheap ones are steel which will corrode and the hoseclamp will fail. Note that 304SS, 18/8 fittings will oxidize in salt air. Yes, 316SS is more expensive but it is cheap insurance!
Hose: Remember that the intake system is subject to SUCTION so the hose must be re-enforced against collapsing. It must also be diesel resistant. Clear, fuel resistant, re-enforced plastic hose is a good choice and avialable from chandlers and various industrial suppliers.
Water strainer: should have a window or clear top that you can see through from your normal access point to the engine room. Preferably installed just below the waterline so that air pockets dont form.