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I recommend you go to the Boston Whaler website, look up the performance report on the $50,000+ new 170 Montauk with a Mercury 90, and see what prop line has been selected for the factory package. Whaler does extensive testing of their boat/engine combinations in conjunction with Mercury, so you can assume they use the best suited prop.

I think the regular Enertia line is what you want, although maybe higher pitch of 20, 21 or 22 inches. I can't image that the huge Eco prop would be correct for a lightweight boat like a Montauk. They are for heavy multi-engine boats with high HP, and as a more economical alternative to the Revolution 4, also for the same applications.

Edited note to PHil: The three props you mentioned above are for a standard mid-sized gear case on a Mercury 90.
The Command Thrust gearcase uses the large size props of the higher HP engines. As an example, the large hub Vengeance props are 14" x 19". Laser II props in the large size gearcase were never made in 20" pitch, only 19, 21, 23 and 25, and they are now discontinued. The 20" Laser II props are for a mid-sized gear case.

Ever since the Montauk was introduced, BW recommended the engine be mounted up one hole. With an Enertia prop you can go up one more hole, using the middle (3rd) hole of the five, so that recommendation is correct. This whole situation should give you top speed in the range of 45-46 mph, if you are actually getting 44 with the 21 Aluminum. One of Mercury's old tricks of the last 50 years is to design the engines with a slightly shorter mid section than the OMC or Yamaha competitors, for the obvious reason of an automatic slight rise in running height. This is one of the reasons Mercs have always been faster then the same HP competitors engine, and only need to be up in the 3rd holes.

Finally, with an Enertia prop, be sure you are given the new "Flo-Torq SSR" quiet shift hub kit.

https://www.mercurymarine.com/en/us/d...the-clunk/

This is good advice - don't ignore it.