El Mar came in first and both of you are correct.
Maiden voyage on my new Sisu and that is what happened to me. Engine started running hot after about 20 mins of cruising at about 3000 rpm so I stopped it. Checked the seacock, groco, heat exchanger and found these pretty well pushed into the water passages so I went and took the raw water pump apart to find that it had a nice soft new impeller. Who the hell changes an impeller and doesnt clean the heat exchanger? Maybe routine maintenance but you would still be missing a blade or 2 and maybe, just maybe you would know what happens when those go down stream?
Which leads me to my next thought as I sweated in the bilge of my mostly unknown boat for about an hour and a half.
Anyone ever mount a heat exchanger somewhere really accessible with a filter like a groco between it and the raw water pump? How about an electric raw water pump with metal centrifugal type impeller (mount a couple in parallel with valves for that matter)? Is there a better way?