Thin blades, cupping, high rake for bow lift (for boats that can benefit) and ports for good hole shot and high pitch for top end!
+1.
Let me also add the Uber important progressive pitch and highly sophisticated and more specialized blade geometry for a specific application, which also allows for higher, more efficient engine mounting.
In this application, blade flex has zero to do with it.
To put it in simple/general terms, everyone wants more power. A decently matched stainless prop can add very noticeable increases in overall performance, which is "effective" HP, the only number that really matters. Better fuel mileage is also a nice benefit and can more than pay for a prop over its lifespan.
Lots of used merc props out their in the ~<=17 pitch range. 40mph is reasonable with lifting engine the one hole that's available. A used Turbo 1 would do it. Lots of other options. New can be had for ~300.
The 90's ran 2.0/1 gear ratios and a 19" pitch is too high. Does the motor have power trim (it may have been mentioned)?
In all fairness, there are some pretty good aluminum props, but since you need a prop and you need a fairly common prop, may be the time to go stainless.
