I've installed a dual battery system, 2 new batteries. Took the boat out
this morning, and when I run at over 2500rpm, the voltage creeps up to
16.6V. This voltage is confirmed by an electronic multimeter and Furuno
depth sounder. Reading google has led me to believe that the rectifier must
be fried.

Can anyone give me more information on the electronics side of my engine?
Its a Suzuki DT75 (75HP) 2-stroke oil injected, late 70's AFAIK, electric
Trim & tilt. What size is the alternator output? Would I be correct in
assuming the rectifier is dead? And if you're in Australia, any idea what
sort of money is the replacement?