In spring, I put an accumulator on the boat water supply. This is a tank positioned between the water pump and the taps; there is a head of air in the tank, whose pressure is adjusted through a Schrader valve on the top of the tank (a Schrader valve is like the ones you get on car tyres). The pressure in the tank with the pump switched off should be the same as the cut-in pressure of the pump. Then, when the pump is operating, it pushes water into the tank until it reaches the cut-out pressure. When you open a tap, water flows smoothly out until the pressure has dropped to the point that the pump starts again, and refills the tank.
The pump had begun stopping and starting much more frequently and for shorter periods than usual; so I decided to add more air.
I switched off the pump (top picture) then drained off the water in the tank.
Then I connected my bicycle pump to the valve on top of the tank
...and pumped it up to about 15 psi. My bicycle pump has a useful gauge on it.
...and now it is working as well as it used to. My friend Philip, who knows a lot about pump systems, suggested that the diaphragm in the tank may be ruptured, and suggested replacing the tank under warranty. But for the moment, I'll see how long this recharging will last....