The overheating issue on my Volvo 2003 28 HP engine has been perplexing me for a while --I was pretty sure it wasn't any obvious suspects --impeller, plugged heat exchanger, plugged hoses, etc. Bottom of the boat was painted in March and boat run all spring/summer (so prop is clean too). We predictably overheat at 1900 RPMs. Engine is rated to 3200 max, but we max out at 2900 b/c we're probably slightly overpropped (but have been advised that getting within 10% of the engine's rated max. HP is sufficient, and 2900 is right at the 10% boundary).

I had a brain flash the other day that I would remove the vented loop on the raw water system, in effect, simplifying and shortening the raw water flow by sending it directly from the heat exchanger to the exhaust elbow, instead of up to the vented loop before it goes to the exhaust elbow. Sure enough, running the boat today, we were able to get up to 2300 RPMs without overheating. Shutting the engine down and re-hooking the vented loop results in the engine eventually overheating at a lower RPM. Seems pretty clear that the vented loop is providing too much raw water flow restriction and the heat exchanger is not being cooled enough as a result.

Problem is I need a vented loop, and the only way to install it, to mount a vented loop above the water line, is to run the hoses in a gentle bend up from the engine, then make them do a sharp 90* bend (fittings, not bending the hose which could collapse it) where they go up to the vented loop on the bulkhead. The hoses, like the piping on the engine, is 1/2". Should I upsize this vented loop hose run considerably (3/4"-1") to help ensure adequate raw water flow with this "extra" piece in the raw water system? I can't think of what else to do.

(What got me thinking about this was, first, the experiment we did while running the boat today, taking off, then putting back on the vented loop in the system and seeing how it affected engine cooling; and, second, finding this opinion on a forum this evening: "Rubber impeller pumps are not particularly good at suction and any reduction through a seacock, skin fitting, hose length, bends in the hose and so on will all help restrict the volume of water the pump will uptake. Then bends in the system rob you of a tremendous amount of flow. I forget the actual number, but I think it is as high as 10% for every bend due to just friction loss. Certainly elbows and other fittings have big problems with restricting flow because of the inside reduction in diameter. If you have lots of fittings and elbows, you almost want to go the next size up in hoseing throughout the system to cope with the losses." From: http://www.cruisersforum.com/forums/f55/yanmar-overheating-4572-2.html#post39041)

Anyone have any experience with this? Long cooling water hoses, or multiple bends in them, or too skinny hoses leading to overheating problems due to lack of flow? Is there anything else I can do besides upsize hoses? All this being said, we now overheat at 2400 RPMs, instead of 1900. An improvement, or sure, but it still seems like we should be able to run higher...? No?