I managed to unmount the pump for the seawater cooling system for the diesel today. I say manged to, because the way the engine is mounted makes it difficult to do, at least the first time around.
Anyway, I disassembled the pump, and saw what looked to me like a perfectly fine impeller. So why did the pump not pump water very well ?
The answer came after consulting with the owner of our local boat gear shop: The central part of the impeller, which is driven by the engine is probably loose so the impeller blades are not turning (or only slowly turning). (This was ofcourse after I had disassembled the pump, cleansed the tube going from the pump to the engine, and remounted the pump while making sure the (danish: kilrem) was properly tightened (my first conclusion was that the kilrem probably was not tight enough, so the pump was not being driven properly), so I unmounted and disassembled the pump once again, and found that he was indeed right, and inserted the old impeller which was worn, but did not have this particular defect. After remounting the pump the pump worked much better than before. I am still going to buy a new impeller (and probably one more in reserve) before the sailing season starts again.
But very nice to know I have an engine I can trust once again.