Here is a pretty good discussion of old vs new
https://forums.iboats.com/showthread.php?t=10336
Old vs New Horsepower Rating
It seems I recall that outboards used to be rated for horsepower at thecrank, and now they are rated at the prop shaft. The net difference beingthat a newer motor actually develops more power for the same rating than anolder motor.Is that true?If so, at about what time did motors start being rated at the prop shaft?The reason I ask is that I just got my Whacko (Waco) on the water.My Waco Link They way it is configured it is rated for a 50HP outboard. I have an olderForce 50 on it that I picked up from Ebay. It runs great, but I waswondering if shopping for a newer motor might get me a little better push.
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Re: Old vs New Horsepower Rating
Prior to 1983 most outboard manufacturers were declaring their HP at the flywheel. There was really no defined industry standard, or enforcement of a standard. So prior to 1983 a few outboard makers decided to take advantage of the declared ratings by using propshaft HP. This gave them the competitive edge of a few more HP, especially in racing. I think Merc was one.Other manufacturers cried out and in 1983 along came ICOMIA Standard 28-83 (Section 3). This standardized the industry's rating and declaration method.
About a year or so ago I did an analysis here of the HP losses between the engine and propshaft of an outboard using basic marine engineering practices.There's very few bearings and only one gear reduction between the flywheel and prop of an outboard.
So if I remember, doing the math, the difference in flywheel and propshaft ratings came out to be 3-4%.You'll hear a 10-15% difference thrown around a lot, but I've never seen any justification.