Accuracy of outboards claimed hp ratings?

[Jason123]

Does anyone know exactly how accurate a manufacture has to be when statind hp ratings for a particular engine? I read somewhere that<br />there was a 10 percent leyway. anyone if this is true for both japanesa and american manufactures?

 

[steelespike]

Re: Accuracy of outboards claimed hp ratings?

Dont know for a fact ratings are accurate but I<br />believe they are I think the 10% you are<br />talking about is perhaps the difference that occured with the change from a powerhead rating to<br />prop rating in the early 80s.Actual prop rating on older motors would be about 10% less at the prop.A powerhead rated 20 hp would be approximately 18 at the prop.

 

[Forktail]

Re: Accuracy of outboards claimed hp ratings?

Most outboard manufacturers are certified as rating their horsepower by N.M.M.A. (National Marine Manufacturer Association) standards. The N.M.M.A has adopted the International Council of Marine Industry Association (ICOMIA) standard 28/83. And this standard is in addition to the ISO (International Organization of Standards) marine test methods.<br /><br />"Section 7: Manufacturing Tolerance<br /><br />The corrected power at rated speed of any individual marine propulsion engine or propulsion system must not deviate more than +-10% or .45 kW, whichever is greater, from its declared power, except that for governed engines or systems of more than 100 kW the tolerance shall be +- 5%."<br /><br />So bigger engines in the 100 kW and greater range would be limited to 5%. These are the 135 hp and up models. A .45 kW deviation is about a .6 hp deviation. So outboards in the small 6 hp range and under can only deviate the .6 hp. That leaves the models between 6 and 135 hp to deviate +-10%.<br /><br />On top of that, the hp is based on a declared crankshaft speed. That speed must be the mid-point of the full throttle speed range recommended by the manufacturer for a particular propeller selection. So for an outboard with an operating speed range of 5000 to 6000 rpm, the outboard must produce its maximum power at 5500 rpm. Furthermore, and very importanty, the highest power within the range can not exceed the declared power by more than 6%. If it does exceed the 6%, both powers must be stated.<br /><br />So to make a point, a 130 hp outboard with an operating range of 5000-6000 rpm is allowed a +-10% deviation. The 130 hp would be at 5500 rpm. So the outboard could be 143 hp at that 5500 rpm, utilizing the top end of the 10% hp deviation. Additionally the outboard is allowed another 6% for the top of the rpm range. This could be another 7.8 hp. Theoretically this 130 hp could actually be over 150 hp at full rpm. Or...if we are a smart manufacturer, we could market an additional 150 hp outboard, using the exact same engine, by just increasing the rpm operating range. Sound familiar?<br /><br />IMO a 10% drop in power between the powerhead and the prop is too much. It's probably more like 3-6%, only due to the small number of gears and bearings.