How a seawater pump works
Simple Boat Maintenance author Pat Manley gives Yachting Monthly his expert guide to how a seawater pump works
The water and oil seals on a water pump shaft will eventually wear with time, leading to pump-shaft corrosion or loss of engine oil. A daily check of the water pump’s ‘tell-tale’ hole will give advance warning of a seal failure. The water pump is often mounted on the front of the engine. However, on some engines it’s mounted on the rear or at the side, so check your handbook.
Leaks from the pump body
There’s a water seal on the drive shaft and possibly an oil seal as well. The water seal, especially, will start to leak after a while and if this isn’t replaced in time, shaft damage will occur. If there’s an oil seal it will usually be identical to the water seal, but fitted facing in the opposite direction.
If you have a ‘Speed Seal’ fitted as here, check the condition of the rubber O-ring regularly
The pump body has its tell-tale drip hole on its underside and any leakage past the seals will be detected here. A daily wipe of the underside of the pump body with a finger will quickly detect the first signs of a leak. You can continue to use the engine but the sooner you do something about it the cheaper the repair will be.
Replacement of the seals is beyond the scope of this advice but many pumps have a repair kit available.
You may need to enlist the help of your dealer to remove and replace the shaft bearings to allow the seals to be changed in the water pump.
Leaking from the faceplate
You won’t normally get a leak from the faceplate unless you’ve just put it back, although in time, weeping around the edges may become apparent.
Waterproof grease will keep a leak at bay for a while
Remove the faceplate and check the gasket. These are normally very thin and easily damaged. A smear of waterproof grease in conjunction with the gasket will probably keep the leak at bay for a while. If the gasket is too badly damaged and you haven’t got a spare onboard, an application of a silicone gasket sealant will do the trick.
If you use a sealant, fully tighten the screws straight away, without waiting for the sealant to set (as you would normally expect to do). This will ensure minimum gasket thickness and, in turn, maximum impeller compression. If you have a ‘Speed Seal’ fitted, check the rubber O-ring seal and replace it if necessary.
Professional tip
Some engines have no repair kit available so eventually a replacement seawater pump will be required. It’s often possible to get a different make of pump to replace your old one at half the cost of the original – and it may have spares available with it as well.
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