No, I am not sure it is not a metric shaft. The boat is a 1988 38' Marine
with a single
135HP Ford Lehman
. With about a 1 1/6" opening in the nut visible in my pictures, it doesn't seem likely that the thread size is 1 1/8", which would make the threads only about 1/32" proud on the shaft. If the threads were 3/32" tall, which seems much more likely, that would give me an outside shaft thread diameter of 1 1/4".
I think the angle in my first shaft picture makes the thread pitch
deceptive. In the picture below, I do count 7 tpi. This picture also seems to verify the SAE prop shaft specs for a 1 3/4" shaft. The specs list 5" of taper, 2 1/4" from end of taper to end of thread, and 2 3/4" from end of taper to end of shaft. From this information I think I have a 1 1/4" x 7 tpi nut. Does that look right?