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Message: <blockquote data-quote="Gotem" data-source="post: 354330" data-attributes="member: 9901">Knowing your RPM's is important.  I experimented from the standard 9.25x10 prop to a 9.25x8 prop on a 70's Johnson 15 hp mounted on a heavy boat rated for a 40 hp (I fish restricted horsepower lakes).  The boat with a 9.9 carb ran 10 mph.  With a 15 hp carb it runs 13 mph (a little faster right after ice out from the denser water).  Switching to a 9.25x8 prop increased the mph to 15, but the increase in noise made me switch back to the 9.25x10 prop.  I do not remember the exact rpms but the 9.25x10 was around 5200 rpm and the 9.25x8 was about 6800 rpm.  All this information was done on a fully loaded 15 foot 1956 Arkansas Traveler which was converted to a bass boat layout.FYI - a 1974 Johnson 6hp pushed the boat at 6 mph.The point is the increase I experienced was marginal and the tradeoff in noise was not worth it.</blockquote>