LK and others pose some interesting questions. Such as: "if a 15 and a 20hp kicker weigh the same, unless cost is prohibitive, why not go with the 20?"
Logically it seems to make sense, but it really is not that simple.
I have been playing around with small engines since '46 on motorcycles and '57 on boats. One thing I have found to be pretty consistent through all of the years is that there is no free lunch.
Translated, that means that if a manufacturer makes two engines whose weight, and more importantly displacement are the same, and one motor has a higher horsepower rating than the other, that apparent superiority usually is not found in all conditions and at all rpm's.
It's usually a matter of trade-offs. An engine can be designed and tuned to be most efficient at high rpm's or low, but rarely both.
High horsepower from a given displacement usually comes at the expense of losing some "grunt" down low. Conversely, the same engine design tuned for pushing heavy loads at low speeds will not make as much peak(high rpm)horsepower, but may actually have MORE horsepower and torque at the lower rpm's which it was designed for.
The Honda 15 and the 20 are in fact based on the same 350 cc block. The 20 is tuned more for high rpm, and makes its 20 hp at 5500 rpm. The 15 is tuned more for low end and makes peak hp at 5000 rpm; 500 rpm lower.(this is according to the official '05 Honda Marine booklet)
Essentially, Honda has moved the power peak 500 rpm lower, which also has the effect of having the engine start making usable power 500 rpm sooner on the low end.
I don't have the charts to back me up, but I would wager a fist full of Benjamins that the 15 makes more power at low and medium rpm's than the 20. The 20 would be faster on the top end, on a light boat, but the 15 would be more efficient and more powerful at the lower speeds and rpm's typical of a kicker.
You would need to run the 20 above the 5000 rpm peak of the 15(by using an unusually low-pitched prop) in order to benefit from its higher theoretical horsepower. I'm not prepared to do that.
That's why I chose the 15 over the 20 as my kicker.
The major exceptions to this are engines with variable valve timing, but neither the Honda 15 nor 20 make use of that technology.
Logically it seems to make sense, but it really is not that simple.
I have been playing around with small engines since '46 on motorcycles and '57 on boats. One thing I have found to be pretty consistent through all of the years is that there is no free lunch.
Translated, that means that if a manufacturer makes two engines whose weight, and more importantly displacement are the same, and one motor has a higher horsepower rating than the other, that apparent superiority usually is not found in all conditions and at all rpm's.
It's usually a matter of trade-offs. An engine can be designed and tuned to be most efficient at high rpm's or low, but rarely both.
High horsepower from a given displacement usually comes at the expense of losing some "grunt" down low. Conversely, the same engine design tuned for pushing heavy loads at low speeds will not make as much peak(high rpm)horsepower, but may actually have MORE horsepower and torque at the lower rpm's which it was designed for.
The Honda 15 and the 20 are in fact based on the same 350 cc block. The 20 is tuned more for high rpm, and makes its 20 hp at 5500 rpm. The 15 is tuned more for low end and makes peak hp at 5000 rpm; 500 rpm lower.(this is according to the official '05 Honda Marine booklet)
Essentially, Honda has moved the power peak 500 rpm lower, which also has the effect of having the engine start making usable power 500 rpm sooner on the low end.
I don't have the charts to back me up, but I would wager a fist full of Benjamins that the 15 makes more power at low and medium rpm's than the 20. The 20 would be faster on the top end, on a light boat, but the 15 would be more efficient and more powerful at the lower speeds and rpm's typical of a kicker.
You would need to run the 20 above the 5000 rpm peak of the 15(by using an unusually low-pitched prop) in order to benefit from its higher theoretical horsepower. I'm not prepared to do that.
That's why I chose the 15 over the 20 as my kicker.
The major exceptions to this are engines with variable valve timing, but neither the Honda 15 nor 20 make use of that technology.