Bruce In Bangkok wrote:

On Sun, 21 Dec 2008 22:45:50 -0800 (PST),
wrote:

This installation diagram from Vetus on their water strainer
specifically shows that the strainer be at least 15cm / 6" ABOVE the
waterline.

http://www.vetusweb.com/manuals/file...01%2007-07.pdf

Yes it does but I can see no reason for it. To clean the filter you
turn the sea cock off so that can't be the reason

Yes it can be, but it probably isn't the only reason.

Doing it this way means you don't *have to* close the seacock when
cleaning the filter, although it is of course good practice to do so
anyway. If the strainer is mounted above the waterline, then it's
easier to see whether it even needs cleaning, without having to bend
down and put your head and an inspection lamp into a difficult to get
at space.

A few more reasons:

(1) If the strainer lid leaks slightly (this should never happen, but
it could), then if it's below the waterline you will get water coming
into the boat unless you close the seacock every time you stop the
engine (and if you do that, you have to remember to open it every time
you start the engine). Most people don't do that, they only close
seacocks when leaving the boat unattended for a prolonged period (more
than a few hours).

(2) If the strainer is mounted above the waterline, it's likely to be
in a more visible position, and will have some air in the top. This makes
it easy to tell visually how well the pump is sucking, from the gush of
water coming up the feed pipe and splashing against the underside of the
strainer lid. This is sometimes easier than looking for water coming out
of the exhaust.

(3) If it's ever necessary to prime the pump manually, this is more easily
done if it's above the pump: just open the strainer lid and pour water in.

and the way it is
specified every engine start is with a dry impeller in the pump.

This is not actually true. Typically the hose from strainer to pump is
always full of water, and so is part (maybe half) of the strainer body.