Old vs. New HP ratings

[shinola]

Someone once told me that at some point in the past, they measured hp output for an outboard at the prop, and now they measure that the fly wheel. The result being that an old outboard at say 25 HP, was stronger than a modern 25. So I have a couple of questions. First, is there any truth to this? Second, what year did they change the method, and (3) What's the rough conversion. For instance, the person who told me this suggested an older 18hp was more equivalent to a modern 25.

Just a curiosity. Thanks...

 

[fdrgator]

There might be a shred of truth in that old tale. But the rest of the truth is that you will never be able to tell the actual difference unless you are running it in a lab. Motors are generally used on a boat on a lake. not in a lab. JMHO.

 

[racerone]

Shinola---Nonsense.-----Your information is backwards.-------The old motors were rated at the power head.-----In about 1984 they started rating motors at the prop.-----Power is lost running the water pump and churning up the oil in the gearcase.--------So an older 18 HP is more like a modern 15 HP !!!

 

[vics]

As Racerone says the change came about in 1984 after the publication of the ICOMIA standard 28-83

You can read all about it here: http://continuouswave.com/whaler/reference/ICOMIA28-83.html

 

[timguy]

The difference may be made up with torque, depending on model. Some older designs utilized engineering that created more torque but less horsepower. So horsepower ratings can SOMETIMES be misleading. Seems to me like Mercury was pretty cool with underating their horsepower....or at least misleading us a bit by categorizing motor size based on cubic inch.
For instance, my Mark 30 could easily run with Dad's Lark 40, and both boats were 16' aluminum. We raced them for years and years.