Seawater pump priming issue – Thursday 14 April 2022

Every time when Bubble is launched after maintenance I’ve noticed that the cooling water takes longer than I would like to emerge from the exhaust. Once it has emerged, though, there is never an issue with priming or flow-rate. I am careful to change the impeller each season and I change the water seal on the shaft when I spot drops coming from the weep-hole.

This year on launch day, it took even longer for the water to appear at the exhaust but, as usual, the flow emerged eventually. It was fine for a couple of sails but on the third sail I had very weak flow when starting the engine to come into the marina. The overheat alarm didn’t sound so there was sufficient flow to avoid an overheat situation. After a great deal of anxiety and checking of the strainer basket, strainer basket seal, strainer inlet seals. strainer lid, anti-siphon valve, heat exchanger seals and the water seal on the water-injection elbow into the exhaust – all of which were fine – I replaced my slightly worn Speedseal cover with the old, worn, original Volvo Penta one to see if that made a difference. The pump primed at once. Unfortunately, after another sail I felt that the flow was still too weak, so I splashed out on the absurdly over-priced original Volvo Penta wear kit. This included a shiny new cover and a new cam. When comparing the old cam with the new one it was immediately obvious what the problem almost certainly was: my cam was worn to about 50% the thickness of the new one. In my excitement I forgot to take a photo but this excellent one from Cox Engineering’s website shows a situation very much like my own.

My cam was worn as much as the one on the left. The pump still worked but it struggled to self-prime. Photo from Cox Engineering.

Cox Engineering quote the following stats:

Old pump inner chamber diameter: 50.2 mm. Diameter to cam: 48.25 mm
New pump inner chamber diameter: 50.0 mm. Diameter to cam: 46.00 mm

Old pump flow rate ~180L/h at 1000 rpm
New pump flow rate ~420L/h at 1000 rpm –
[over twice as much]
New pump flow rate ~630L/h at 2000 rpm

So the cam thickness really does make a big difference. I had spent a few anxious days searching the various posts about poor priming with the MD2020D Johnson pump but hardly any made reference to the cam. I have learned that the cam can wear significantly over the years. Unlike the cover, wear on the cam is much more difficult to notice as most of us won’t have a spare new one in stock. I think I will make a point of changing the cover and the cam every few years. I would like to change the pump completely but on the MD2020D removal is extremely difficult; I might leave it until I lift the engine to replace the sail drive seal … but that is not due just yet, thank God!

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