Chapter 17:Sea Suction
You can see how these self-cleaning sea
strainers are configured in Figure 17-10.
Note that this schematic shows the emer-
gency bilge suction leading to the side port
built into this type of seacock. Note also that
this sea suction is set up as a common-rail
manifold (see Chapter 4). The entire sea-
suction intake line—in this configuration—
must be large enough in diameter to serve
all the water demands of the equipment it
serves.Pumps Sharing a Single
Suction or Source
In the reasonable desire to reduce the number
of holes in a boat’s hull, it’s common to
arrange a single seacock to serve as sea suc-
tion for several functions, as in Figure 17-10.
You might have an air-conditioner-condenser
pump, a refrigeration-condenser pump, a
galley raw-water pump, and a deck-washdown
pump all sharing suction (water intake) from a
single seacock. This is not a bad idea, but it
must be approached with caution. Particularly
if one pump is significantly more powerful
than others in the group (such as a wash-
down), it may starve the other pumps of water.
This can cause the other pumps to lose prime
(especially if they are centrifugal). The result-
ing problems can be intermittent and hard to
track down in service.
If you have multiple pumps sharing a sin-
gle sea suction, be sure the cross-section area
of the through-hull ID is at least 1.1 times the
total cross-section area of all the individual
takeoff lines to each pump, and there is at
Figure 17-8. Sea strainer (Courtesy Groco) least 2 feet (0.6 m) of head at the through-hull
Figure 17-9.
Duplex sea
strainer
(Courtesy Groco)Sea strainers are simply filters designed to
remove sediment, seaweed, and similar impu-
rities from the raw water (Figure 17-8). They
should have a clear bowl so you can easily see
if there’s a buildup of gunk. Though many suc-
cessful vessels are fitted with single sea strain-
ers on the sea-suction line, it’s a good idea to
use switchable duplex sea strainers for critical
systems such as the main engine (Figure 17-9).
As with switchable duplex fuel filters, this
allows you to change instantly from a fouled
sea strainer to a clear one and then clean the
fouled filter without missing a beat.
A relatively new development is self-
cleaning sea strainers. These units come with
a built-in macerator pump that flushes the fil-
ter clean and grinds up any gunk and debris,
expelling the waste as a slurry through a
smaller dedicated seacock and through-hull.
This is a nice feature for a large yacht or
commercial vessel, but in general, I prefer
duplex sea strainers. The macerator pump,
its related fittings, and the necessary wiring
involve more systems and more complexity
and mean more to maintain.