I was told today by a salesman that the reason they had primarily 40 HP Yamahas on most of their 16 foot boats was because there is not much difference between the 40, 50 and 60 HP two-stroke models. He told me that given identical boats there would only be a 2 mph difference between the 40HP and the 60HP. Can anyone confirm or deny this. I am getting ready to buy my first boat and I could really use the advice. I have decided to buy a 16 foot duck boat for hunting and fishing. So far I have looked at Express and G3.
First I would think that 40 hp is plenty on a 16 foot duck boat. Make sure you check out the boat's maximum hp rating.Yamaha's 40 and 50 2-stroke are basically the same outboard. Same 3 cylinder 698 cc block, same compression ratio, same rpm range, same gearing, same prop selection, same weight. Given the fact that the hp is rated by the factory according to the NMMA (which uses the ICOMIA and ISO standards) which allows a +-10% deviation in posted ratings from actual, these outboards probably don't have much mechanical difference...if any. I'd have to look at the parts book. The seat-of-the-pants difference would be hardly noticeable.The 60 hp on the other hand is a completely different outboard. Bigger 849 cc block, higher compression ratio, different ignition system, higher gearing, larger pitch props available, and 34 lbs heavier. Probably too much for your boat. Even though it has more HP, it is geared much lower, so speed won't increase that much with it, but it will push a heavier load faster. Your dealer is probably right about the similar speeds. But the 60 will give way more Torque and better overall performance (and drink more fuel doing it)
Here's some info to help decide in these situations. You can try plugging in different pitch sizes to see what the speed does, but notice the Torque jump with the 60 HP.Prop RPM = engine RPM/Gear Ratio(Torque X Prop RPM)/5252 = Prop HP[(Prop Pitch in inches/12 in/ft) X (RPM/Gear Ratio) X (60 min/hr)]/5280 ft/mile = Speed (theoretical) in MPH.So, for the 40 hp Yamaha:Prop RPM = 5500/1.85 = 2973 RPMProp Torque = (40 HP X 5252)/2973 RPM = 70.7 ftlbs TorqueSpeed (for 13 pitch example) = [(13/12) X (5500/1.85) X 60]/5280 = 36.6 MPHNow for the 50 hp:Prop RPM = 5500/1.85 = 2973 RPMProp Torque = (50 HP X 5252)/2973 RPM = 88.3 ftlbs TorqueSpeed (for 13 pitch example) = [(13/12) X (5500/1.85) X 60]/5280 = 36.6 MPHBut you supposidly have a little more Torque with the 50, so we'll give it another pitch jump on the prop. Without increasing pitch speed won't increase, but the ability to push a heavier load faster over the 40 will.Speed (for 14 pitch prop example) = [(14/12) X (5500/1.85) X 60]/5280 = 39.4 MPHBut for the 60 hp:Prop RPM = 5500/2.33 = 2361 RPMProp Torque = (60 HP X 5252)/2361 RPM = 133.5 ftlbs TorqueThat's a lot more torque (almost double the 40 hp), so we can really add prop pitch and push a bigger load.Speed (for a 17" pitch example) = [(17/12) X (5500/2.33) X 60]/5280 = 38.0 MPHKind of fun.