Old vs New Horsepower Rating

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Bob La Londe

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It seems I recall that outboards used to be rated for horsepower at the<br />crank, and now they are rated at the prop shaft. The net difference being<br />that a newer motor actually develops more power for the same rating than an<br />older motor.<br /><br />Is that true?<br /><br />If so, at about what time did motors start being rated at the prop shaft?<br /><br />The reason I ask is that I just got my Whacko (Waco) on the water.<br /><br /> My Waco Link <br /><br />They way it is configured it is rated for a 50HP outboard. I have an older<br />Force 50 on it that I picked up from Ebay. It runs great, but I was<br />wondering if shopping for a newer motor might get me a little better push.
 

roscoe

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Re: Old vs New Horsepower Rating

refer to the post you left in the Force forum.
 

ZmOz

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Re: Old vs New Horsepower Rating

Yep, that's true. I believe it was some time in the '80s that they changed the rating system...I want to say 1983 but I don't know why. You will technically have more HP with a newer motor rated at the same HP.
 

roscoe

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Re: Old vs New Horsepower Rating

Not all manufacturers changed the ratings at the same time. Some in the 80's, others in the 90's. Some contend that Force motors were never rated at the prop.
 

wayne h

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Re: Old vs New Horsepower Rating

so the new HP ratings are acually more HP then the old 1ns ? and all this time i thought i had a 9.5 haha
 

Bob La Londe

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Re: Old vs New Horsepower Rating

Roscoe, In repsonse to your reply in the Force forum. That topic is marked closed. Anyway, I don't plan to go looking for a motor, but if a newer motor stumbled into my lap, or if I ran across a great dela on one I might get it. Then I might not. <br /><br />Over all I am not unhappy with the speed of this setup. I still have throttle leftover at 5200 too. Based on that I thinke ven when I get a full load in the boat I should be able to get 30MPH witht he increased drag of sitting lower int he water.
 
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DJ

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Re: Old vs New Horsepower Rating

Bob La Londe,<br /><br />THe prop HP ratings came into effect in 1983.
 

Forktail

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Re: Old vs New Horsepower Rating

Prior to 1983 most outboard manufacturers were declaring their HP at the flywheel. There was really no defined industry standard, or enforcement of a standard.<br /> <br />So prior to 1983 a few outboard makers decided to take advantage of the declared ratings by using propshaft HP. This gave them the competitive edge of a few more HP, especially in racing. I think Merc was one.<br /><br />Other manufacturers cried out and in 1983 along came ICOMIA Standard 28-83 (Section 3). This standardized the industry's rating and declaration method.<br /><br />About a year or so ago I did an analysis here of the HP losses between the engine and propshaft of an outboard using basic marine engineering practices.<br /><br />There's very few bearings and only one gear reduction between the flywheel and prop of an outboard. So if I remember, doing the math, the difference in flywheel and propshaft ratings came out to be 3-4%.<br /><br />You'll hear a 10-15% difference thrown around a lot, but I've never seen any justification. ;)
 

seahorse5

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Re: Old vs New Horsepower Rating

In the mid to late '60s the Boating Industry of America (BIA), the forerunner of the NMMA, set the standards and procedures for outboard horsepower ratings that each manufacturer abided by. Representatives of each company would visit and oversee the test results of 3 production powerheads. This was powerhead horsepower only and the motors were run on 20:1 mix, not 50:1.<br /><br />Forktail is pretty close on the gearcase loss figures. Actually, if I remember correctly, it was only a few percentage points more, allowing for the oil viscosity, but not near 10% on large outboards. On the small motors, the percentage of loss was higher.
 

Clams Canino

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Re: Old vs New Horsepower Rating

Originally posted by Forktail:<br />You'll hear a 10-15% difference thrown around a lot, but I've never seen any justification. ;)
How about that the Merc 115 became the 90? :D <br /><br />-W
 

Dhadley

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Re: Old vs New Horsepower Rating

But dont forget that the NMMA allows a 10% variation on actual vs advertised. And it depends on what rpm the motor is rated at. <br /><br />Dont ya love advertising?
 

Forktail

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Re: Old vs New Horsepower Rating

That would be a loss of 25 HP, or a 22% loss.<br /><br />That's just not going to happen between the crank and prop of a 115 HP outboard. A car maybe. ;) <br /><br />Like DHadley said, taking the top and bottom of the 10% variation, we could assume the 90 was a 99 and the 115 was a 104. This would be a 5 HP difference and more in line with the 3-4% loss.
 
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DJ

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Re: Old vs New Horsepower Rating

I have only had one outboard run on a dyno. It was a 1995 Suzuki DT-140.<br /><br />It came in at 144 HP at 5800 rpm. Obviously, well within the 10% range.
 

Dhadley

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Re: Old vs New Horsepower Rating

Most of the newer motors will be rated kinda conservative. Everybody wants to have the most powerful motor in a given class. Thats how OMC got yelled at when they brought out the 60 degree 150 in 1991. They averaged over the 10% deal on the dyno.
 

Cricket Too

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Re: Old vs New Horsepower Rating

Dhadley....Just a couple quick questions. What do you mean when you say it depends what rpm the motor is rated at? Does that mean that, for example, a Johnson 60 and 70 could be the exact same motor, but the 60 is rated at let's say 5200 and the 70 at 5800? Also what do you mean by the 60 degree 150 averaged over the 10% deal, does that mean it was actually higher than 150 HP on the dyno? Thanks.
 

Dhadley

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Re: Old vs New Horsepower Rating

Thats the general idea of how its done. On the motors with different ratings, but the same powerhead part number (therefore the same port timing and cylinder heads) they may also have slightly different carbs or reed stops or intakes etc. They are pretty much the same motor. They have the same bore and stroke, the same gear ratio and gearcase shape. The torque curves are almost if not exactly the same. How many times have you heard someone say they bought a larger horsepower motor of the same design and it was only slightly better at best, if any at all?<br /><br />Yes, the 1991 150 was over the 10% guideline. They changed the port timing in 92 for the 150 and 175 60 degree.
 

Forktail

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Re: Old vs New Horsepower Rating

The declared speed (for example: rated HP at 5500 RPM) shall be the mid-point of the full throttle speed range (for example 5000-6000 RPM) recommended by the manufacturer for propeller selection.<br /><br />The declared (rated) power of an engine shall be the full throttle power at the declared (rated) speed at the final output shaft of the engine as offered for sale by the manufacturer, based on the corrected power of one or more engines.<br /><br />The corrected power shall be the full throttle power of an engine measured in an engine dynomometer laboratory as specified, and corrected to the standard reference specified.<br /><br />So an outboard's power must be measured using a dynomometer and set standards. HP ratings must indicate not only HP in the full throttle range (not somewhere else in the power curve), but at the mid-point of that range. <br /><br />Depending on the engine, an outboard could produce more or less HP above or below the mid-point RPM. Many engines, especially 2-strokes, fall off their power dramatically after peak RPM. So exceeding the mid-point RPM may not be beneficial, depending on how the engine is calibrated. Other outboards may actually make more HP than their declared HP at the maximum end of the RPM range. These are usually engines that hold their torque (flat torque curve) within the total RPM range.
 

josip_voyager

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Re: Old vs New Horsepower Rating

I just want to add formula from mechanic science,and everything will become more clear.<br />P~M*ω^2<br />P is power (in kilo Watt), M is torque (in Newton meter) and ω is rotation speed (radian pro second). It means that power is proportional with torque and square of speed of rotation. So when you have two engines with indentical power head but different power,stronger engine will have more power at higher rotation speed.Problem is that when the torque is insufficient (for example when you overload your boat), stronger engine cannot reach rpm (rotation speed) to get more power,so you won't feel any difference between stronger and weaker engine. <br />Point is that torque is crucial in this discussion, and torque comes from displacement (measured in cubic centimeters or liter). For example: Honda BF 25 and BF 30 have identical power head (552 cubic centimeter displacement), but diferent power (BF25 at 5500 rpm and BF30 at 5950 rpm). On the other hand Honda BF20 (same model as BF15) have only 5 PS less than BF25, but BF25 is much better engine, while it has much better torque than BF20 which have only 350 ccm displacement against 552 ccm from BF25 model. It means that at entire rpm range you'll have much more power (or better torque) on disposal.
 

sdanville

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My 2010 Honda BF8 --- 8 HP seems to me to have more power than my 1985 Honda BF100 ( 10 HP )...
 

GA_Boater

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Really, sdanville? A ten year old topic! Please don't post in topics with no activity for 90 or more days. Thanks.
 
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