When did horsepower start being rated at the prop?

[BWR1953]

I know that back in the olden days that HP was rated at the powerhead crank, just like cars. Then the rating switched to being rated at the prop, just as cars started rating power with accessories, etc. instead of at the crank.

So about when did this all happen? Late 70s? Early 80s? I remember when it happened for cars back in the 70s.

My 1976 boat is rated for 50hp. So a 50hp motor of that same year would produce less thrust than a brand new 50hp motor of today. Understood. Now I just need to know about when the changeover happened. Would the boat to be a little zippier with modern prop rated HP instead of crank rated. Ya know?

Been looking at quite a few motors for sale and a 1984 Chrysler 55 with TNT just popped up for sale for cheap. Is that year the old rating system or the new? Could be just the engine I need!

 

[alldodge]

It varies a bit by manufacture but all were switching between 82 to 85. In 1985 was when all had to meet the requirement to my understanding

 

[BWR1953]

Thanks for the info. Will keep that in mind as I continue my search.

Missed out on the Chrsyler. It's already gone. It was only $200 for a running motor with TNT and good prop. Arghh!

 

[boobie]

'85-'86 was when OMC made the change.

 

[BWR1953]

Okay, so as I continue with my engine search, is there a general rule of thumb on the thrust performance difference between the two ratings?

Say I found a 1973 Johnson 50hp motor and also a 1989 Johnson 50hp motor. Would the newer motor have 10% more thrust using the exact same prop? Or would it be 5% or 15% or 7% or something else? Just a ballpark figure would be nice to know.

I'm going to go look at a 50hp Evinrude this morning. All I know about it is that it's newer than the antique 15hp motor that was on my boat before. Will be bringing along a model year lookup link to help ID the age.

For my application, I'd like to get something at least from the 80s or better still from the 90s. But cost is always a consideration. The motor I'm checking this morning is part of the deal I made yesterday when I sold the old motor. I got cash plus trade. The 50hp motor has good spark and compression and TNT I'm told, so if it's a working motor even from the 70s, I'll take it in the deal!

 

[roscoe]

The difference is roughly 5%.
The power loss is 2 bearings is not much.

To further confuse the situation, consider how the outboard engine industry rates the engines.
To allow for manufacturing variances, the motor only has to be within 10% of the rated hp.
So a 50 hp motor may actually produce 55 hp, or as little as 45 hp. Now factor in the crankshaft rating, and your old motor may be as little as 42 hp.
Subtract another gob of hp for the engine being old and worn out, weak coils, gummed up carb, or low compression, and you could be down to 30 or even 20 hp.

So, powerhead rated, or prop rated, is the least of your worries.

A properly maintained and running engine, in good mechanical condition, is what you need to look for.

 

[BWR1953]

The difference is roughly 5%.
The power loss is 2 bearings is not much.

To further confuse the situation, consider how the outboard engine industry rates the engines.
To allow for manufacturing variances, the motor only has to be within 10% of the rated hp.
So a 50 hp motor may actually produce 55 hp, or as little as 45 hp. Now factor in the crankshaft rating, and your old motor may be as little as 42 hp.
Subtract another gob of hp for the engine being old and worn out, weak coils, gummed up carb, or low compression, and you could be down to 30 or even 20 hp.

So, powerhead rated, or prop rated, is the least of your worries.

A properly maintained and running engine, in good mechanical condition, is what you need to look for.

Thanks for that info. 5% certainly isn't much of a rating difference, especially given the 10% manufacturing tolerances you mention. I just didn't know if the difference was going to be 15% or something like that.

And yes, I'm looking for an engine which has been well maintained, runs well and in good mechanical shape. I'll be looking for good compression as one part of that overall picture.

The ideal engine for me will be a 50hp motor with power trim and tilt. I'll be concentrating on something like that but will also keep lower HP engines as options if I just can't find what I want in the next week or two.

Thanks again!

 

[Frank Acampora]

Well here! I'll confuse you with the facts a little bit.

The old Chrysler 105 was rated at the crank shaft as 105 horsepower. When the change in rating method was mandated by NMMA in the early 1980s, the 105 was simply down-rated to 90 with no other changes. THUS: In this case we can imply that the gears and water pump were absorbing 15 horsepower or about 14 1/4 per cent.

Add to this the common beliefs that Mercury engines were always rated at the prop, were under-rated, and were rated at 3/4 throttle and you have a situation where you just don't know!

 

[BWR1953]

Well here! I'll confuse you with the facts a little bit.

The old Chrysler 105 was rated at the crank shaft as 105 horsepower. When the change in rating method was mandated by NMMA in the early 1980s, the 105 was simply down-rated to 90 with no other changes. THUS: In this case we can imply that the gears and water pump were absorbing 15 horsepower or about 14 1/4 per cent.

Add to this the common beliefs that Mercury engines were always rated at the prop, were under-rated, and were rated at 3/4 throttle and you have a situation where you just don't know!

Ah jeez. :facepalm:

Well, that certainly explains why I was always happiest on my prior boats when there was a Mercury sitting on the stern.

My current boat is #11 that I've owned over the years. I've had Chrysler, Mariner, Evinrude, Sea King, Johnson and Mercury engines in the past that I can remember. I'm sure there were other makes but it's been so long that the other names have faded back into the sea mists of time. I also used a 7.5hp Honda back in the early 80s on a boat that wasn't mine. Great little motor. Used that boat a lot. Got me seriously, truly interested in fishing.

Were the Suzuki and Yamaha motors similarly rated/under-rated? The Yamaha seems to have a pretty good reputation around here.