Thread: Motor Horsepower....Vintage vs. Modern

Well that's a tough one to answer, in the old days it was measured at the power head, then it changed when CG got involved it changed to prop shaft HP. In the really early days it was just what the manufacturer decided to call it. That old 20 hp merc was probably pushing close to 30. Today's motors are getting more HP out of smaller displacement engines as a result of better tuning internally, more efficient electronic ingnition systems and better more effect fuel delivery systems. Beter prop designs and gear ratio's. Just like your truck with a V6 getting much better fuel mileage, higher torque and HP than the older big block engines. Your old motor is probably close to the 15 rating as far as performance. Still be better than a new 10. If you get into the smaller motors under 25 and in 4 stroke you will see some performance drop off compared to a 2 stroke. So if I had a 15 and wanted to upgrade to a 4 stroke I would go to a 20 or 25.

It wasn't the coast guard that made the change it was the manufacturers so the rating would be standardized throughout the industry. This happened in the early 80's. Search the forum on iBoats.com, Lots of info on the subject. As I read it there is only 3-4% difference between the crank shaft and the prop. Not really enough to be concerned about. Also the Coast Guard doesn't care what size motor you put on but they do insist the proper tag be put on giving max recommended people capacity and HP of the motor.

Last year I bought a new 4 stroke 25 hp electric start short shaft Yamaha. Last week, I got a advertisement from Yamaha, that they re made the 2017 electric start short shaft 25 hp,4 stroke, 50 lbs lighter, total weight at 125 lbs, smaller in size, with more take off power and faster, than the 2016. Yamaha is calling their 2017 portable and that is a vast improvement to me the way they are engineering and making these motors and how they keep improving them. If using a motor that is rated for the boat, I personally think there is no comparison between the old vs new. The comparison that can be argued is 2 stoke gives alittle more power than 4 stroke and that is where you will have the greatest discussion.

If I went to a dealer and they suggested over powering the rating plate I would leave. That was put there by the manufacture of that boat for a reason. There are several things that factor into the hp rating. In today's motors you get more bang with less weight for sure. Just because it's lighter doesn't mean you can hang more hp. Today's 25hp compared to the 80's 25hp, todays will out perform because of weight savings

Power to weight ratio is really what you are looking at.

I had a conversation with a boat manufacturer a number of years ago (my sister is invovled in the industry). We specifically discussed power to weight ratio. Keep in mind that this was at the time when 4 stroke motors were being introduced and you still had a choice of purchasing a 2 stroke or a 4 stroke model from many of the companies as they offered both.

The specific question that I asked delt with power to weight. I asked him if anyone ever considered changing the inspection plates to read differences between a 4 stroke and 2 stroke. He scratched his head for a minute. I went on to say: This boat is rated for a 40hp. It doesn't say anything about 2 stroke verses 4 stroke. It is also rated for a specific weight (including persons, motor, and gear). IF by chance I purchase a 40hp two stroke, what would be the difference between that and a 4 stroke. You could see the light bulb click in his head. What I was getting at is... why would anyone purchase the heavier 4 stroke (remember they were in their infancy at this time) versus the tried and true 2 stroke. Everyone knows you will have more power with less weight on the two stroke.

Basically put... he said that, at the time, the boats were being rated for 2 stroke motors. No one really knew what to do with the 4 strokes. They knew they weighted a substancial amount more but no one really knew what could be done in the form of the inspection plates.

Fast forward to now: 4 stroke motors have shaved a ton of weight compared to their counterparts of yesteryear. Sure, people still opt for the 2 strokes in some instances but the weight issues are pretty negligable now compared to the time I am speaking of.

As bill alluded to, many of the companies were doing whatever they could. A ton of motors came with the exact same block (still do). Many of them simply came with smaller carburators and a different sticker on the hood. It was very easy to purchase a 9.9hp and convert it to a 15 hp with little work (back then).

Here is what I would look at in your shoes. Look at what the MAX HP rating is for your boat. Keep in mind that that took in to consideration the weights of motors of that ERA. Now, look at what that motor (from that era) would have weighted. Next, take a look at todays model (say a 4 stroke of the same HP). Look at the weight of that motor. You may find that there isn't that much of a difference. This would go back to the power to weight ratio. This is what I would do to compare.

In reading the responses, I notice some accurate and some not so much in regards to the way I understand the facts to be. I believe the best way to answer your question and possibly shed some light on some of the myths is to simply state the facts as I know them.

Prior to the mid 80's, all the american outboard manufacturers rated their engines' horsepower at the crankshaft. Through a cooperative effort put forth by the NMMA (BIA), a standardized 'standard' was adopted. In regards to horsepower, the manufacturers were to be within a 10% variance of rated power. In those days, most opted to take the true average rated power and round up while remaining in the 10% variance. In those days the consumer market was coming around to wanting more power. It was easier for them to over rate their motors than it was to actually produce more power.
The changes that came in the 80's were somewhat small but had a major impact on the way we now view the motors of that era. In terms of technology, Mercury Marine partnered with a Japanese company for various reasons. And at nearly the same time, Yamaha outboards (which were being produced by that same company) were new to the U.S. Market. Here's the big outcomes.... The Japanese outboards were being rated at the propshaft which yielded a more powerful side by side comparison motor to the U.S. motors. (This is due to the power lose that occurs from running through a gearbox.)
To add to this, the Japanese motors horsepower rating tended to be rounded down from true horsepower but still remain in the 10% variance common to the U.S. market. This resulted in a 15hp Japanese motor putting out a minimum of 15hp at the propshaft running against a U.S. manufactured 20hp outboard that was less than 20hp at the crank with an even further reduction of power at the propshaft. Basically, the U.S. motors were getting outrun by smaller, "lesser hp rated" Japanese motors. The U.S. manufacturers quickly adjusted to this as they began seeing sales decline. I believe 1985 was the year that most U.S. motors were made and rated to the new standard.
Another big change was that partnership with the Japanese company that I mentioned earlier. Mercury Marine quickly found out that the Japanese engineers had made enormous strides in their 2 stroke engineering. The result was a smaller package, lighter weighing motor that produced more power throughout the entire RPM range when compared to what the U.S. was doing. And Mercury quickly adapted to a better way of thinking. Many of the models that Mercury produced since that time were either a product of a partnered effort or the use of what was learned from the Japanese.

In regards to a boat's hp rating. What began by some boat manufacturer's as a form of a liability disclaimer has evolved into Coast Guard mandated regulation. The hp rating is the sum of a mathematical formula that involves the boats length, width, method of steering and the such. The only thing I'll comment about this is that if your boat is operated on a body of water controlled by the CG, it needs to be within their regulation. Some state enforcement divisions are more strict than others but the general thought is that most all have adopted regulation to match that of the CG. If you go over that rating, you are open to major liability should an accident occur with injury.

Not mentioned in the original post but later commented was power to weight ratio. I think many folks view a narrow spectrum of this topic. What I mean by that is I hear many speak of how heavy their 4 stroke outboard is and how it's slow and sluggish. All because it weighs 60lbs more than other, faster models. There can be a hint of truth in their statements because PTW has a real and noticeable impact. But to concentrate soley on the weight of the motor is missing the mark. It's the total weight of the boat and it's placement along the axis that has the most impact in terms of speed and handling. The more accurate reason for the older heavy 4 strokes not being as 'peppy' is not near so much their weight. But more so the lack of advanced technology that's needed to produce the torque across the RPM spectrum. I explain this regularly like this. "25hp is 25hp is 25hp." But two motors can have the same rated hp at WOT and still be completely opposite in terms of what they do between the in-gear idle and WOT. The motor that builds torque faster and at lower rpm is the one that everyone will prefer.

Having said all of that, to answer the original question. If your 1980 model 15hp Evinrude were to be rated by today's standards, it would probably be rated a 10. If you're looking to buy a new outboard, I would suggest buying as much motor as you can afford toward the maximum rating of your boat. Anything less will leave you wishing you had at a later time.