[Putershark]

My best most cost effective recommendation is to run down to West Marine and pick up one of these http://www.propulse.se/english/index.html and work with the pitch adjustments till you find what works the best for your boat.

Even if you to not use it as your main screw, once you dial in the pitch that works the best, you can then have a good idea as to which one you need in a Stainless Steel model. These are never hard to take off the output shaft (no metal to rust weld on) and weight next to nothing. They can also (once you elect to upgrade if you do) get left in the boat in a bag as a back up unit that will never rust.

Some Mercury dealers on also part of the demo prop program that allows you to switch out a few till you find the correct combo http://sites.mercurymarine.com/porta..._schema=PORTAL
I would use the posted Triumph Yamaha boat test data as your best pitch and diameter starting point.