Difference between rating at prop or engine?

[JoeMan]

I'm still a boating cadet. Not sure if you'd actually call this a "prop" question, but here goes. I'm moving this week from an older Merc outboard to newer. My older one is 60 hp, rated 'at the motor', it's an '87. The newer '96 Merc 60 I'm told is rated at the prop, and that this means the motor will be a bit more powerful. What is the real difference there? Is the newer motor itself simply more horse power?

 

[Crownie2]

Re: Difference between rating at prop or engine?

It was decided, I think around 1990 - but I could be off a little, that measuring at the propshaft would be more accurate.

This would include the losses going through the gears, so, bottom line is the newer engines rated at the same level do have more power than the older ones did.

As I recall the difference ranges from about 10 hp on the smaller engines to about 25 hp on the larger ones.

Bob

 

[Texasmark]

Re: Difference between rating at prop or engine?

I "think" the Merc 150 I 6 dounsized to the 115 when they changed. Both engines were 99 cu in and to get 35 hp with just bigger jets is stretching it so there may be some truth to it.

HTH

Mark

 

[Rancherlee]

Re: Difference between rating at prop or engine?

I'm still scratching my head on how they can loose so much power through only 1 gear set going from the crank to the prop. Most automotive rear ends (which basically is the same thing with a ring a pinion) only cause a 2-4% loss in power running 80-90w gear lube with a ford 9" being one of the worst @ 4% loss. I keep hearing numbers in the 15-20% range from crank to prop loss. I'm inclined to believe that the "crank" ratings were over stated in the first place and the prop rating totally leveled the playing field and cut out the lies.

 

[walleyehed]

Re: Difference between rating at prop or engine?

The change came in 1985, when all manufacturers were required to use the same std of measurement. The 1987 IS prop-shaft rated as is the newer one.

The 1976-1984 V6 200HP Johnson/Evinrude puts out about 155HP at the prop at best.

 

[JoeMan]

Re: Difference between rating at prop or engine?

The change came in 1985, when all manufacturers were required to use the same std of measurement. The 1987 IS prop-shaft rated as is the newer one.

The 1976-1984 V6 200HP Johnson/Evinrude puts out about 155HP at the prop at best.

I was told by a dealer that my old '87 was not prop rated.

 

[Ron G]

Re: Difference between rating at prop or engine?

I think i'll trust kenny any day over a dealer!if ya want more power why not get a bigger motor?

 

[Texasmark]

Re: Difference between rating at prop or engine?

Other things are anything handing on the engine that requires power to drive it.

What's interesting is to go to the old OMC site and look at how they rated them back in the 50's. Called it "brake hp". Well brake hp is at the crank output with nothing connected to the engine; not prop rated. There was a lot of hype at the time that Merc was overrating their engines cause they were crank rated.

Also they (OMC) rated their engines at 4000 rpm while the recommended oper range was 4000 to 5000 for some engines. Interesting......but look at the cubic inches on the old 25 hp.....like 35 cu in. No wonder that sucker could plane out a 16' loaded fishing boat and run her 25 mph, or pull a couple of skiiers on a light (wood) semi-V boat.

Mark

 

[JoeMan]

Re: Difference between rating at prop or engine?

I think i'll trust kenny any day over a dealer!if ya want more power why not get a bigger motor?

Ron G, I agree with you on that! The advice and help I've gotten here has been great, and I know that sometimes, sales people will say what they need to. :^

I'ts really not more power that I need at all. It's a 16 foot aluminum runabout with 2 back-to-backs, so the Merc 60 should get her going at least 40 or more mph. Just wanted to know if I should expect a difference in power. From what you're saying though, the prop rating came the same year as my '87, so it should be about the same, especially if the prop pitch is the same. That's cool.

Thanks again for the help guys.

 

[walleyehed]

Re: Difference between rating at prop or engine?

New technology may gain you some torque, a bit better fuel economy, and likely a little quicker throttle response.
Top-speed difference would be subtle in the same HP.

 

[jimmbo]

Re: Difference between rating at prop or engine?

Mercury prop rated the inline sixes and V6s starting in 1982.
During the 50s, Mercury under rated their motors considerably.

Those old 'rated @ 4000' OMC engines had an operating range of 3500 to 4500.

 

[Texasmark]

Re: Difference between rating at prop or engine?

jimbo, some do some don't. The spec sheets on old-omc.de show those that do and those that don't. That's why I used the numbers I used.
-----------------------
On new technology, a guy showed pics of his 20 hp Merc on his alum boat. Had a really nice wake telling me that that little (recent) 20 was doing it's job.

Mark