How do I change the watermaker filters on a Nimbus boat?

To change your Nimbus watermaker's prefilters, stop and isolate the unit, close its seawater inlet, release the pressure and replace the cartridges with the specified equivalents. Check the watermaker's make and model first: filter sizes, filtration order and restart procedure depend on the equipment fitted to your boat.

Identify the filters

Read the watermaker's identification plate, then check the labels on the filter housings and existing cartridges. The Nimbus model name or build year alone cannot identify the watermaker, particularly if equipment has been added or replaced.

There are three service items to distinguish:

• Seawater strainer: a coarse basket or stainless steel screen upstream of the feed pump, normally cleaned rather than replaced.
• Sediment prefilters: replaceable cartridges that remove suspended dirt before seawater reaches the membrane.
• Freshwater-flush carbon filter: fitted on some systems to remove chlorine from flushing water before it reaches the membrane.

A common arrangement has a 20-micron sediment cartridge followed by a 5-micron cartridge. Some units use a different rating or just one stage. Match the specified length, diameter, end fittings and micron rating; do not order by appearance alone. The long pressure vessel containing the reverse-osmosis membrane is not a routine cartridge-filter housing.

Isolate and open the housings

Have the correct cartridges, housing spanner, replacement O-rings, clean cloths and a shallow tray ready. Photograph the filter arrangement and valve positions before starting.

Switch off and electrically isolate the watermaker, prevent an automatic flush from starting, close the seawater inlet seacock and release pressure before opening any housing.

1. Stop the machine using its normal shutdown sequence, including any required freshwater flush.
2. Isolate its electrical supply and close the inlet seacock. If servicing the flush filter, isolate its freshwater supply too.
3. Release trapped pressure using the manufacturer's specified vent or bleed procedure. Confirm that any relevant pressure gauge reads zero.
4. Place the tray underneath and unscrew each filter bowl with its housing spanner, supporting the mounting bracket to prevent it twisting.
5. Remove the cartridges, noting which rating belongs in each housing.

If a bowl will not turn, check again for trapped pressure before applying more force. Do not use a pipe wrench or extension bar on a plastic housing.

Clean and fit the cartridges

With the seacock still closed, remove and clean the seawater strainer's element. Check its lid gasket and refit it correctly. A poor seal can admit air and reduce feed-water flow.