That is interesting. I am assuming that you have the copper nipples going right into the exhaust ports on the back of the motor.

I didn't do it that way. I built a small plenum (about 3"long and the width of the shaft housing) so that the exhaust is perhaps cooled in this plenum. The exhaust exits from this plenum. The inside of the plenum is a formed piece of thin aluminum, shaped so it will stay attached. It is then firmly held in place by the fiberglass which covers it and extends over the shaft housing.

Perhaps this aluminum sheet, in contact with the shaft housing, is providing sufficient heat dissipation so that the exiting exhaust gas is not particularly hot. The total contact area between the lower unit housing and the formed piect of sheet aluminum is perhaps 4 square inches.

Of course, the exiting exhaust might not be as hot as well.