In the ideal installation illustrated in the drawing below, the cooling water enters
the vessel via it's own through-hull located deep on the hull. The incoming water then rises to the sea strainer and
continues smoothly uphill to the inlet of the water pump. (The through-hull, sea
strainer and pump are positioned so as to remain under the water line and continue this
uphill sloping attitude at all angles of heel.) The discharge outlet of the pump
points upwards with the connecting hose continuing to rise smoothly into the Condenser
Unit inlet. From the Condenser Unit outlet, the connecting hose then continues to a discharge through-hull located above the water line and above
the level of the water pump.
Centrifugal Pump Q&A
The following answers provided to commonly asked questions may further clarify what
factors ensure a good centrifugal pump installation.
Q: If I mount my pump under the water line will the it self-prime?
A: Not necessarily. A location below the water line is just one of the requirements for a
centrifugal pump to prime. You must also be sure there are no air traps in the intake line
feeding the pump. A good rule of thumb is to remember that if an air bubble was to enter
the through-hull, it must rise smoothly into the pump head unhindered. If it stops at any
point along the way the pumps will not reliably self-prime.
Q: Do I need a "speed scoop" on my through-hull when using a centrifugal
pump?
A: Speed scoops are a good idea but they are not always necessary. On fast monohulls and
catamarans they are often required to ensure that the pump continues to operate at high
speed. On full-keel cruising boats with very deep intake through-hulls, they can often be
eliminated.
Q: Do I have to discharge water above the water line?
A: No. As long as the pump head is below the discharge port and, the discharge hose
does not "dip" below the level of the water pump.
Q: Can I share a through-hull with another pump?
A: On merchant vessels a common through-hull feeding multiple pumps is used all the time.
Problems are prevented by using very large sea-strainers and a one-way 'check"
valve on the pump discharge line (or discharging below the waterline). The potential for
problems stems from the fact that a centrifugal pump at rest permits water to flow in both
directions. This can cause the other pumps on the circuit to suck in air thus loosing
their own prime. Proper installation can prevent this from occurring. In all cases, only
low-demand through-hulls such as toilet intakes, cockpit drains or the galley sea water
pump should be considered as a multiple intake source.
Q: What causes a pump to work well at the dock but not under way?
A: Sometimes boats prime well when they are upright but not on a heel. This is usually
caused by the change in relationship of the water line to the pump and other components as
the boat heels over. Additionally, some hull shapes induce a suction (ie. Ventura effect)
as the boat reaches speed, thus taking the prime from the pump. This problem is alleviated
by using a "speed scoop" type through-hull.
