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All Merc 90's after 1986 (last year of the in-line-6 90) use a 2.33 gear ratio.

If you want stainless, your fastest accelerating and top speed will be the new 4 bladed Spitfire X7 in 19" pitch, or maybe 21" pitch.

https://www.mercurymarine.com/en/us/p...itfire-x7/

Another option is the 20" Laser II.

If you want aluminum, the Spitfire 4 blade aluminum is your best choice, 19" or 21" pitch. The 3 bladed Black Max 21" pitch is also a decent prop, probably the best aluminum 3 blade you can buy.

In all cases, mount the engine in the third (middle) set of bolt holes.

Except for the Spitfire X7, the other three props are also available under Mercury's Quicksilver branding, at slightly less cost and widely available on the web from discounters.

What I would recommend in order to determine correct pitch is buy the bottom line 3 blade alum Black max in 21" pitch (about $100) and see what RPM you get. Then you can accurately spend up for a stainless model, keeping the alum for a spare.

jacosmarine.com is your best pricing for any Mercury SS prop. When using SS, be sure to use the Flo-torq III hub kit to eliminate clutch rattle at low RPM.

The Montauk should run 42-45 MPH with Opti 90.