I plan on putting a Honda 2hp 4 stroke motor on my 15' square stern canoe. The stern transom is 17 1/4". Frequent shallow, rocky water in addition to lake fishing is the scenario. Would prefer minimal amount of motor hanging beneath canoe but still set for safe operation. Which would be best to put on canoe for use in these conditions: short shaft (15") or long shaft (20" with some sort of transom spacer)? I understand cavitation plate should be near equal to hull bottom, but . . . .? Thanks much for the help.