Merc HP ratings change in 70s?

[sho305]

I read here Merc went to prop hp ratings and the 140 became a 115. Does that make my 1973 150 inline6 more like a 120hp? If I run my boat on Mercurys prop calculator with the size & weight of my boat, it comes out to about what a new 120-125hp would do speed and prop-wise. <br /><br />The owner says it went faster, but this would explain 52mph on a 17' Checkmate BR@5400rpm with a LaserII 20p or 21p Merc; just what the Merc site says it would do. It seems to run fine and not be able to do 60mph no way after 6 props tried@ 5200 to 5700rpm, SS and not, and about every mechanical check done including a dealer checkup.

 

[sho305]

Re: Merc HP ratings change in 70s?

Any takers in this?

 

[grandx]

Re: Merc HP ratings change in 70s?

Don't know exactly when Merc quit rating HP at the power head and started rating it at the prop shaft (where it should be), but I was told it was in the mid 80's. So your older motors have less actual prop shaft HP than the newer ones. 15-20 % less is what I was told, thats about what you have stated. Hope this helps.

 

[sho305]

Re: Merc HP ratings change in 70s?

Thank for the reply Grandx. Thinking this is the case, and 20% would be 30hp so 120-125 comes out right. Factory says newer little heavier 17' model does 55mph with a 125hp-and 67 with a 150. Seems to run nice and has great compression as well, it will do for a while.