Curious how often most change their main engine impellers. I have always done them annually. I typically see 125hrs a year on my main engines for usage. Right now on my common rail MAN's we are finishing a complete cooling system service and it got me thinking about next items on my annual maintenance list. I do oil and fuel service myself in late spring. The impeller on the Port engine is such a PIA I am tempted to consider every other year but not sure if that would be a short sighted decision or not. Generator gets impeller annually no question as it runs quite a bit more than mains. Wondering if others are religious about annual impeller changes or if they sometimes go every other year. Port engine water pump was fully serviced last winter 2018 and stbd pump was gone through in 2017 and fully serviced.