The wiper arm on the boat is flat at the end with a hole to take the bolt you can see in the pic up top in my first post. The wiper moves in an arc with and in line with the arm, so if I can fix it rigidly it should be ok, I think...
Go with the two at 16" but purchase a third blade and chop off 4" and superglue to the end.
make sure that the glued end of the now 20" wiper blade is at the end of least movement when refitted.
Back in the 1970's O-rings came in a kit of long black round rubber lengths of various diameters and you just cut and made up the ring with superglue. Breath on the area to be glued before applying the glue for a better joint, like putting steam (condensation) on a mirror. /forums/images/graemlins/wink.gif
If you make the join by cutting at an angle, you will get a stronger joint. /forums/images/graemlins/smile.gif
I can remember my early days with motor cars in the early 60's. Our Austin A35 used to have wiper rubbers not too dissimilar to the one in your photograph.
It was always the case of just sliding the old rubber out and replacing it with new and often you could not get an exact length so you just bought a longer piece and chopped a bit off.
I can't remember ever buying a complete arm, just the rubber insert. /forums/images/graemlins/smile.gif