Why is my boat watermaker producing water more slowly than usual?
My yacht's watermaker still makes fresh water, but filling the tank is taking noticeably longer than it did last season. The feed pump sounds slightly louder, and I haven't changed the prefilters recently. What should I check before paying for a new membrane?
Answer
Your watermaker is most likely producing less water because the seawater supply is restricted, the feed pump voltage is too low, or the seawater is colder than before. Check the intake, prefilters and voltage at the running pump first, then measure product and brine flow before suspecting the membrane.
Check the seawater supply
A blocked intake or dirty prefilter restricts flow to the pump, which often becomes noisier as output falls. Air drawn through a strainer lid, loose connection or freshwater flush assembly can cause similar symptoms, sometimes with bubbles or fluctuating pressure.
Stop the watermaker and close the intake seacock before opening the strainer or any seawater filter housing.
- Check that the intake seacock opens fully and that the external intake is clear of weed, plastic and marine growth.
- Clean the seawater strainer and check its lid seal for damage, grit or poor seating.
- Replace dirty prefilter cartridges with the micron ratings specified for your installation. Keep multiple stages in their specified order.
- Inspect suction hoses for kinks, collapse and leaking connections, then reopen the seacock and prime the system as directed in its manual.
A suction leak can draw air without visibly leaking seawater. If output improves immediately after changing filters, record the running pressure and flow as your new clean-filter baseline.
Measure voltage under load
Measure voltage directly across the feed pump's supply terminals while it runs. A healthy battery reading does not prove that the same voltage reaches the pump. Cable resistance, corroded terminals, fuse holders and switches can all cause voltage losses under load.
For the 12 V Spectra Cape Horn Extreme, the troubleshooting guidance lists motor voltage below 12.5 V as a possible cause of reduced output. A drop of 0.5 V can cost approximately 1.89 litres per hour of product water. Diagnostic testing is best done with at least 13.2 V reaching the system.
These figures are model-specific, so consult your manual for other watermakers. If output improves with the battery charger running, investigate battery condition and voltage drop before replacing pump parts. Isolate electrical power before cleaning or tightening connections.
Measure flow and operating conditions
Measure production with a graduated container and stopwatch rather than relying entirely on the panel flowmeter or tank gauge. Collect product water for a timed period: 500 ml in one minute equals 30 litres per hour. Follow the manufacturer's procedure for accessing discharge lines.