It seems that I have resolved an overheating problem on my Volvo Penta AQ 125 A (270 stern). Brief description of the problem I had: engine start overheating on the high (max) speed especially if there were waves. My theory is that when boat was jumping waves or high speed and smaller waves, the hull would create "foam" water with too many bubbles, and than when overheating would happen. I did the following (professionals, no kicking please! 
<br />1. New alternator belt, in addition I have made a simple bracked that is attached to the alternator bracket and to the area where dip stick bracket is mounted. I noticed that during the high speed or waves, alternator was moving a lot (very flimsy bracket that is not thick/sturdy enough) thus not turning water pump at all times.<br />2. Copper pipe for fresh water was replaced and rerouted. I removed the original copper pipe that is connected to the pipe that comes from the stern drive (thicker pipe connected with the rubber hose to the copper pipe that goes into the water pump). From that pipe that is connected to the stern I ran a plastic pipe (same diameter) that I bought at the local boat store. Plastic pipe that I bought is flexible, white in color, with plastic spirol reinforcement inside (very hard, cannot deform/flatten with hand). To attach plastic pipe, I cut the original copper pipe ends (about 2 inches) and attached the plastic pipe.To attach, I had boil plastic pipe for few minutes and than I was able to put it onto the copper ends. Used some silicon, the grade that used for faucets. Plastic pipe came up to be about 3 1/2 feet long, mounted with the brackets to the wall of the engine compartment, where pipe have to be below the water pump.<br />3. Read a lot about flange washer that goes into the stern drive, the one that allows air inside when worn. So, my options were to purchase new washers/gaskets and disassemble/assemble stern drive or do something else, I chose something else I took plastic clip (the one that electricians are using to bunch wires together, type of belt 3/16 wide, about 6 inches long) put it over where the worn flange washer is and tight it (all the way to the flange washer). Then I filled area all around it with the silicon that I mentioned above, and placed another clip over that, without much squeezing. Did not touched anything for few days, letting silicon to dry. Tried few days later, no air was coming through there.<br />I understand that par. 3 is very unprofessional, however this is something I chose to do and it worked for me. I took boat out saturday/sunday and went at the full throttle over the lake back and forth. Waves were about 1 1/2 - 2 feet, jumping waves, etc did not changed temperature, which was between 180 - 190 deg at all times. I also changed manifold few weeks ago.
<br />1. New alternator belt, in addition I have made a simple bracked that is attached to the alternator bracket and to the area where dip stick bracket is mounted. I noticed that during the high speed or waves, alternator was moving a lot (very flimsy bracket that is not thick/sturdy enough) thus not turning water pump at all times.<br />2. Copper pipe for fresh water was replaced and rerouted. I removed the original copper pipe that is connected to the pipe that comes from the stern drive (thicker pipe connected with the rubber hose to the copper pipe that goes into the water pump). From that pipe that is connected to the stern I ran a plastic pipe (same diameter) that I bought at the local boat store. Plastic pipe that I bought is flexible, white in color, with plastic spirol reinforcement inside (very hard, cannot deform/flatten with hand). To attach plastic pipe, I cut the original copper pipe ends (about 2 inches) and attached the plastic pipe.To attach, I had boil plastic pipe for few minutes and than I was able to put it onto the copper ends. Used some silicon, the grade that used for faucets. Plastic pipe came up to be about 3 1/2 feet long, mounted with the brackets to the wall of the engine compartment, where pipe have to be below the water pump.<br />3. Read a lot about flange washer that goes into the stern drive, the one that allows air inside when worn. So, my options were to purchase new washers/gaskets and disassemble/assemble stern drive or do something else, I chose something else I took plastic clip (the one that electricians are using to bunch wires together, type of belt 3/16 wide, about 6 inches long) put it over where the worn flange washer is and tight it (all the way to the flange washer). Then I filled area all around it with the silicon that I mentioned above, and placed another clip over that, without much squeezing. Did not touched anything for few days, letting silicon to dry. Tried few days later, no air was coming through there.<br />I understand that par. 3 is very unprofessional, however this is something I chose to do and it worked for me. I took boat out saturday/sunday and went at the full throttle over the lake back and forth. Waves were about 1 1/2 - 2 feet, jumping waves, etc did not changed temperature, which was between 180 - 190 deg at all times. I also changed manifold few weeks ago.