Cooling
Running WOT is a good test of the cooling system. The temperature should not rise much above whatever it is at lower speeds. Salt water does nasty things, especially where heat is involved. One of the more commonly discussed problems in previous posts is at the mixing elbow, where the cooling sea water enters the exhaust piping. You can experience blockage on both the water and exhaust surfaces. I would start out by carefully checking the internal water level. It doesn't take much air to affect the efficiency of the circulating pump. I would assure that the sea water pump is in good shape, with a good impeller and no wear on the end plates. I would then check both passages at the mixing elbow. Previous posts indicate that 4 to 6 years is time to get in trouble. If all this doesn't find the overheating cause, the heat exchanger could be getting fouled. These can be best cleaned by pulling the tubing bundle and taking it to a good automotive radiator repair shop for cleaning. If you don't want to dump the antifreeze, you can mix a radiator cleaner in a bucket and suck it into the heat exchanger and let it stay there for a day or two. You should also make sure that the hoses are properly sized. Water inlet fittings can have their effective area greatly reduced by a barnical or two in the sea cock. Exhaust hoses sometimes get shut down by a misplaced anchor or the like. Sometimes undersized hoses are installed by the boat builder. You should be able to run WOT without overheating.
