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I have read that older engines with a lot of hours loose compression over time. It is not uncommon for a "tired" motor with a factory cylinder compression rating of 115 to be at 95 or 100 after 15 years.

As for props, there has been extensive discussion on the web regarding the classic Montauk and a E-TEC 90.

One of the most discussed and tested props by whaler owners that has good performance is the 13.25x19 Stiletto ADVANTAGE 1. WOT speed is ~ 44 mph @ 5200 with a light load.

A 15" version resulted in 43 mph at 5700 rpm's.

A site search will show the other good props.

An ETEC 90 on a classic Montauk should be mounted at it's highest setting.