Centrifugal type water pumps offer several advantages over positive-displacement types. They typically run quieter, longer and consume less power. Unfortunately, centrifugal pumps are not very forgiving when it comes to poor installations. If you are installing one of these pumps (or having one installed by someone else) make sure you understand what is required to ensure a good self-priming installation.

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.


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