Tech: Installing a seawater pump

After running out of freshwater on one of my sailing trips last summer, I decided to install a seawater pump in the galley. Here’s the step-by-step installation process in pictures:

The first part of the installation has to be done when your vessel is out of the water, namely installing a thru-hull and seacock. I could have beached Arriba and worked at low tide but I decided to wait until her regular haul-out (top photo).

Personally, I don’t like drilling holes through hulls, so I got a professional to do it. The thru-hull is directly beneath the galley, in Arriba’s starboard hull. 

Spot the thru-hull (just above the mini keel)

This location ensures a short run up to the sink while also positioning the inlet well forward of the engine’s seawater cooling outlet.

Thru-hull installation (L to R) before, with seacock and connected to hose.

It was interesting to see that the floor of the galley hull is 10cm (4") below water level.

The plumbing is straightforward and you don’t need a professional to do it. Here’s what you’ll need.

Hand pump, raw water strainer, hose, connectors and clamps.

The seacock outlet is connected via ¾" hose to a raw water strainer, which is a good idea to prevent flotsam from choking the pump. I added a 90° curved hose adapter to the seacock to minimize bends in the hose. Also, to minimize the chance of leaks, there is only one connection below the waterline and, as is good practice, it has two hose clamps. (Above the water line I’m less worried). The raw water strainer is located directly under the sink, for ease of inspection and access.

Raw water strainer.

The outlet of the strainer steps down to ½" and connects to the new pump, which is manual, not electric. Since this is a low-pressure plumbing application, high-pressure tubing is not required. I chose to use inexpensive barb fittings with hose clamps, rather than quick-connect fittings.

Pump connection under the sink.

The new pump, a Whale V Mk 6, is installed behind the existing (freshwater) tap.

Originally I'd planned to install a non-return valve, but the pump creates sufficient suction that it is not necessary. The pump requires very little effort to use, and produces a flow rate of 8 liters (2.2 gallons) per minute, which fills half the sink in about a minute.

Parts list: hand pump, strainer, hose, clamps.

In this post I write about why I think having a seawater pump is so useful.

OVER.