Outboard Physical Size Difference - 50 HP

Bassn7

Seaman
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May 25, 2015
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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?
 

ondarvr

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HP is HP, the size of the box doesn't matter. You're comparing a 4 cylinder to a 2 cylinder.
 

RCO

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Jun 15, 2016
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Actually the newer motor should have 10-12% more horsepower since the 1980 was rated at the crankshaft, and the 2003 will be rated at the prop. Compared to some of the new 4 strokes, that Merc isn't that big.
 

ondarvr

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That Merc didn't change in size even after the prop shaft hp ratings started in the mid 80s, and neither did the Johnson, they were the same physical size before and after.
 

jimmbo

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They are both about 44 cubic inches. Both have their charms and/or faults. Prop rated the Merc became a 45 or later a 40hp. The OMC, probably rebadged a crankrated 55 or 60 as the prop rated 50. Same physical size
 

Bassn7

Seaman
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May 25, 2015
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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.
 

ondarvr

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It's impossible, can't happen, the manufacturers are lying.

Merc also made a 2 cylinder two stroke 50 about the same size as the Johnson.
 

jimmbo

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It's impossible, can't happen, the manufacturers are lying.

Merc also made a 2 cylinder two stroke 50 about the same size as the Johnson.

I don't ever recall a 50hp 2 cylinder Mercury.
 

jimmbo

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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.

Actually a prop rated 50hp will probably push the boat faster than a crank rated 50. Both engines are almost the same displacement 44 cubic inches. The merc just has less displacement per cylinder than the Johnson, but there are more cylinders on the merc. RPM of the prop is dependent on the gear ratios which will also affect the pitch of the prop. The merc is 2:1 and the Johnson is 29:12
 

ondarvr

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I don't ever recall a 50hp 2 cylinder Mercury.

​Sorry, it was a 3 cylinder.

​The ELPTO 50, it was the newer next generation style of motor, but they continued to make the classic 50 four cylinder at the same time for many more years. I don't know how many different HP models they made with that block.

​The J/E two cylinder was made in many different HP ratings.
 
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jimmbo

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The Merc 44 cu in 4 cylinder was first made in 1958, it has been crank rated as a 40, 45, and a 50, later it was prop rated as a 45 and a 40.

The OMC 2 cylinder 50 started as 41.5 inch block in 1971, in 76 it became a 55hp with a 45 inch block. In 1980 it became a 60hp. In 1985 it was offered as a 40, a 50, and as a 60, all 3 using the 45 inch block. Looking at the HPs. it get iffy as to when OMC started rating the twins at the prop.
 
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