I was using a 2:1 reduction with a 14x10 prop with no overheating issues, but I may have been using a bigger controller. I had no active cooling on the controller, just a ribbed case and an aluminum plate in lieu of a heat sink. I don't think your prop is to blame. I think the Millipak controller just doesn't disappate heat very well so I am also in the camp recommending a cooling fan. A big heat sink with a fan blowing on both heat sink and controller case is what I would do. Use short standoffs to allow unimpeded circulation between it and whatever you mounted it to. Reducing prop size should be a last resort IMHO.
Reducing prop PITCH slightly could improve overall performance, though. A lot of props just picked at random salvaged from ICE powered motorboats are IMHO overpitched for boats that move more slowly through the water. My feeling is that the pitch number should be significantly less than the diameter when top speed is in the mid single digit range. Overpitched props on slow boats expend a lot of energy just beating the water. This extra energy results in needless heat losses in motor and controller. Just from comparing notes with other folks with small displacement boats, a pitch of 2/3 to 3/4 of the diameter seems to work well. Props that originally were fitted to cabin cruiser type boats want higher pitch to compensate for the increased water flow over the prop.
Most people who really think this out, end up with a prop that is of a diameter that just leaves decent clearance with hull or skeg. The larger swept area of the prop reduce the relative impact of the strut, hub, etc drag. There are a lot of things to balance out, but leaning toward the larger diameter prop seems to be a good startting point, and reducing it seems not to be the thing to do if it can be helped.
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