Actual horsepower ?

[Lonnie]

Something I have wondered and am curious about is this. Are the manufacturers advertised horsepower numbers for an outboard engine actually what it produces in real on the water conditions, or something different than what it really produces? How do manufactures figure horsepower ratings? Finally, do some brands or motors appear to be overrated and some underrated in regards to actual power they produce compared to their advertised output?

Thanks, Lonnie

[boat nut]

They are allowed plus or minus 10%. Typically, they will go to the plus side, to make their engine "a little better than" the "other guy's". Seing as they all do it, they all make similar power at WOT.

[MarkG]

I am not actually sure how the math is done,but the measurement is actually at the propshaft,not the crankshaft. I think it is measured initially with a dyno at the propshaft,probably during the R+D phases,as I sort of doubt every motor ,or for that matter any motor coming off an assembly line gets put on a dyno. Once a design makes it to manufacture, unless there is a problem off the assembly line ,it should produce it's rated power at rated RPM,within a certain allowable percentage variance. In earlier years, I believe up to the early 1980s, outboards were crankshaft rated,which would result in little higher number than if propshaft rated as today. If you look at ratings and specs ,you will notice multiple outboards of the same specs,that is..number of cylinders, displacement in CCs or cubic inches,number of carbs,same lower unit and gear ratio, same weight etc,,,motors that their specs are virtually identical EXCEPT for horsepower rating. These are in fact the same motors with minor differences ,possibly different carb size,intake and port design,etc,to have the same engine run at different HP ratings. This is actually more a marketing strategy than anything,to allow virtually the same motors to be sold on different rated boats, at different pricing points ,without having to have a separate motor for every HP rating. I have been told by more than one mechanic that the lower rated motor of the same size and design is likly to be putting out HP somewhat in excess of its rating,where the highest rated was pretty much maxed out.