Outboard Physical Size - 50 HP

[Bartableman7]

Why is there such a huge difference between the physical size of the entire motor within the same HP?  My 1980 50 HP Merc is really big compared to a Johnson 2003 50 HP...both 2-stroke.  Are they really the same HP and will they both push with the same force?

[Hez]

They are probably not the same horsepower, due to around mid 1980's outboard motor manufacturers changed how they rated the HP. 

 

Prior to the change, they always rated HP at the powerhead.  After the mid 1980's, manufacturers (at different times, not all at once) started basing their HP ratings off the prop.

 

It is discretionary, but most people say the difference in HP rating at the powerhead vs the prop is approximately 10%.  

 

So...in 1980...your 50 HP motor would probably be equivalent to a modern 45 HP motor, rated at the prop.

 

That, along with other technological advances enabling denser and more efficient motors - probably is the reason for the difference in size you are questioning.  

Edited May 26, 2017 by Hez
had another thought...

[Bartableman7]

So the smaller 2 cylinder Johnson with the small prop is going to push my boat the same speed as my 50 HP Mercury with 4 cylinders and the big prop? Just seems odd and impossible...unless the Johnson prop spins a ton faster.

[Way2slow]

Depending on the size difference in CC's, if your older Merc still has good compression and in good tune, it may have a better hole shot than the new motor but top end speed would probably be close to the same.

As for size difference, that's improved technology.  Back in 1980 a 350 chevy was a bad a** at 400hp, today, 600hp is nothing.

[XpressJeff]

They similarly changed automobile engine power ratings from the crankshaft to the rear wheel.

 

As Way2slow stated we used to be challenged getting our race cars to 1 hp per cu.in. Today the 440 in my Dart makes nearly 2 hp per cu.in. without anything special. Guys with little Suburu WRXs are getting almost 4 hp per cu.in..

 

Scary fast!