New F9.9 vs F15

[MrBigStuff]

I'm considering buying a 4 stroke for my new 14' rowboat (Lowe 1457) despite their increased weight and need to be transported in a particular position.

The boat is intended mainly for small pond and river fishing. Lots of weeds so water cooling is somewhat of a minus.

I have the Yamaha brochure and it shows the 9.9 and 15 having the same specifications (weight, displacement, gearing etc). How can this be? If the mechanical specs are the same, what makes the extra 5 hp? Must be a typo...

Anyone have experience with either of these motors? Advice? Big difference in pushing power?

My other choice would be a 2 stroke 8hp motor that weighs much less but will be louder and have to mix fuel.

Hoping you have some advice or experience to lend.

 

[Silvertip]

Re: New F9.9 vs F15

This age old question has a very simple answer. Look at the engine line ups of any manufacturer be car, truck, boat, airplane, etc. and you will find displacement (bore and stroke) will be the same for at least two, three, and even four engines of different horsepower. In four strokes, cam profiles, compression, carburetiion (or fuel injection) and ignition advance curves determine horsepower output. In two strokes it amounts to intake and exhaust porting, compression, carburetion (or fuel injection) and ignition timing. If you look at Chevy car engines, you'll find the 350 engine had horsepower ratings from as little as 155 to well over 350 over the years.

 

[MrBigStuff]

Re: New F9.9 vs F15

Well, guess what, I logged on to the official Yamaha site and lo and behold, there are significant differences between the two motors. Displacement, compression ratio and weight. That's what I get for trusting the brochure. Too bad because I was already chomping at the bit for getting the 9.9 and putting in the go fast goodies of the 15. Alas, the nearly free lunch has vaporized d

I agree with your comments but have one tidbit to add; at one point, they changed the method of measuring hp on car engines. I forget now if it was bhp or rwhp or some such change but it resulted in a significant shift in rating for essentially the same motor.

Thanks x 10^6 for your input! It's much appreciated.

I'm still keenly interested in any feedback on the remaining questions...

 

[Silvertip]

Re: New F9.9 vs F15

There were actually two periods when auto engine HP ratings were changed. In the early 1970's HP took a significant drop primarily due the effects of adding all the emissions junk (air pumps, catalytic converters, retarded ignition timing, etc.). Fact is, there were still several engines with the same displacement but different HP ratings. Generally two barrel VS four barrel carbs, higher vs lower compression. Currently engines are rated with all accessories. As for choice of outboard motors in your case, I would definitely choose the two stroke. They are lighter, can be stored/hauled in any position, need far less maintenance, and yes, may be a tad noisier and thirstier at mid and low speeds, but if you need to worry about fuel economy with an 8 HP engine, there are other financial issues to consider.