Pontoon Boat & Deck Boat Forum

I have an older (1985) Evinrude 25 hp outboard on my small pontoon. It runs great and I'm satisfied with the performance but I don't like the noise and am thinking about getting a new 4 stroke.

Long story short, I keep reading that they rated hp differently back then compared to how they rate it now. Does anyone have any idea what I would need in a new 4 stroke to equal the performance/hp rating of my '85 two stroke.

Thanks guys.

KPC

The difference you're speaking of is "crank rated horsepower" vs. "prop rated horsepower". Back in the day, outboards were labelled based on the power they made at the engine. The problem here is that it doesn't account for drivetrain loss when running through the shaft and gearcase to the prop. This meant that, depending on the motor, somewhere between 10-20% of power was lost so the consumer had really no way of knowing exactly how much power he was putting on his boat. Today, outboards are labelled based on the power they put out at the prop. This means that a 20 horse Merc will give you the same (peak) power as a 20 horse Evinrude, Yamaha, etc.


I've never been able to narrow down the years exactly, but it seems that OMC began prop rating around 1981-82. Mercury apparently officially did the same around then too. That's when the companies often just changed stickers on their motors. For example, their 140 was now a 125, and the 125 now the oddball numbered 110. (Why OMC had an "88" and a "48" I don't know...).

As a fan of vintage Mercs, I've learned that Mercury in particular was notorious for being all over the map for power ratings. In the late '40s they had a 10 hp (advertised at 4000 rpm) that really put out 18 hp at full throttle. Then everyone who's ever owned a late '80s Merc 35 (I had two) knew it ran like it was a 20.

After that long story, aside from power lost due to wear & tear, I would suspect your '85 Evinrude 25 would run similar to a modern 25. (Except the new 25 won't shake the hell out of your boat and make you deaf like your current 'Rude).



LC.

Thanks LC, the info is MUCH appreciated.

KPC

Just as an example of HP ratings, the Mercury 40 HP jet drive engine (4 cyl) has the exact same engine specs as the Mercury 60 HP prop engine.

It takes 20 more engine HP (as well as an extra cylinder) for the jet to get the same drive HP as a prop drive.

The 40 HP Prop has a 45.6" displacement (3 cyl) engine while the 40 HP jet drive uses a 60.8" displacement (4 cyl) engine.

The 81-82 timeframe stated by LC sounds about right to me. The change in rating was caused by the Japanese entering the mainstream outboard market in the US. They used the prop rating method which gave them a competitive advantage over Merc and OMC. I remember that the first year that Yamaha was in the market that their 10 hp would run with Merc's 15 hp outboard. After that, everyone went to prop ratings and you saw some strange marketing like the Mercury "Classic 50" which carried 45 hp stickers.

The Japanese also brought in their motorcycle tech experience and introduced oil injection at the same time. The US manufacturers were playing catchup for the next five years but finally got there but frankly they had gotten a bit complacent in the late 70s. The competition made them get off their duffs and get back on their A game.