Horsepower rating?

[jsimms724]

Is outboard h/p calculated differently I/0 hp.?the reason I ask is there seems to be huge performance difference between the two with simular boats with the same h/p rating. I understand there is a wieght difference Thanks in advance

 

[JB]

Re: Horsepower rating?

Prior to the mid 80s outboard engines were rated at the crank. After that most are rated at the prop. All I/Os are rated at the prop since about 1990.

There are too many speed affecting differences between boats and their rigging to explain them as differences in HP rating of the powerplants.

 

[jsimms724]

Re: Horsepower rating?

Prior to the mid 80s outboard engines were rated at the crank. After that most are rated at the prop. All I/Os are rated at the prop since about 1990.

There are too many speed affecting differences between boats and their rigging to explain them as differences in HP rating of the powerplants.

thanks j.b. for you fast response My interest is for future reference but what is the difference between rating at the prop and rating at the crank . in your experience is one method more accurate than other or maybe a better indicator of the actual h/p . thanks in advance

 

[QC]

Re: Horsepower rating?

Horsepower at the flywheel (or crank) is the power output of the engine itself. It can drive a wheeled vehicle, a boat, a plane, a pump, a generator etc. It really doesn't "know" and it really doesn't care. All of these applications lose some of that available power by driving something else. It takes horsepower to turn shafts and gears, and for gear reduction, and then also a significant amount for a 90 degree change of direction. In an OB I would guess that it is around 4 - 7 % loss from crank to prop (propshaft actually). And in an I/O it is about 10 - 15%. Fairly significant. A low horsepower engine through a large drive system would lose the most as a percent of it's "available" crankshaft power. A true IB without any gear reduction (1:1 ratio) would theoretically have the least loss, but then there is the problem of the prop not pushing straight back, so you can't win . . .

There is also horsepower "lost" by driving oil pumps, alternators, water pumps etc. etc. So where and how you "rate" an engine's power is often determined by some sort of governing body. With wheeled vehicles it is often SAE. With boats, the NMMA. But these are not legally binding agencies, so it is somewhat left up to honesty and agreement. You could invent your own horsepower rating system if you'd like and then you could sell what Evinrude might call a 9.9 as say a 1722. Well, you could try anyway

 

[stspinner]

Re: Horsepower rating?

I have an 1987 boat with an 1986 OMC 5.7 L rated at 260 HP is this at the crank or prop ?

 

[QC]

Re: Horsepower rating?

Funny you asked here I answered that yesterday in your thread Probably around 235 at the prop and 260 at the crank.

 

[mark in new jersey]

Re: Horsepower rating?

Horsepower at the flywheel (or crank) is the power output of the engine itself. It can drive a wheeled vehicle, a boat, a plane, a pump, a generator etc. It really doesn't "know" and it really doesn't care. All of these applications lose some of that available power by driving something else. It takes horsepower to turn shafts and gears, and for gear reduction, and then also a significant amount for a 90 degree change of direction. In an OB I would guess that it is around 4 - 7 % loss from crank to prop (propshaft actually). And in an I/O it is about 10 - 15%. Fairly significant. A low horsepower engine through a large drive system would lose the most as a percent of it's "available" crankshaft power. A true IB without any gear reduction (1:1 ratio) would theoretically have the least loss, but then there is the problem of the prop not pushing straight back, so you can't win . . .

There is also horsepower "lost" by driving oil pumps, alternators, water pumps etc. etc. So where and how you "rate" an engine's power is often determined by some sort of governing body. With wheeled vehicles it is often SAE. With boats, the NMMA. But these are not legally binding agencies, so it is somewhat left up to honesty and agreement. You could invent your own horsepower rating system if you'd like and then you could sell what Evinrude might call a 9.9 as say a 1722. Well, you could try anyway

Awesome post....

I'm reminded of an engineering class many, many years ago. We were tasked with the calculations of watt loss through the complete transmission of power from a car's 300HP engine crank, all the way through to where the tires hit the pavement.

Part of the calculations included the engine's other tasks, too: alternator, power steering pump, AC, etc.

As I recall, the calculated HP, at the wheels, was under 100HP. The watt-loss through the trans & diff was astounding.

Boat engines: I'd imagine the losses to be less, but conceptually: same thing....

 

[QC]

Re: Horsepower rating?

My experience is primarily heavy-duty truck, but we typically looked for 80% of flywheel horsepower at the wheels. That was through a 9 - 18 speed gearbox, but in direct (1:1) they all draw pretty similar amounts. There was also tandem diffs and 8 tires. That combo pulled out about 100 hp, so I doubt that you could make a case for 200 hp loss on a single tire drive axle, and smaller trans. All of this is so important to understand, and also so misunderstood. We used to calculate wheel loss and crank that back into our wheeled dyno reports (Caterpillar system) so we reported "flywheel horsepower" from a wheel dyno. I am sure this messed up a lot of people even on the CB. "I gots 550 out of my big CAT here, come baacck". "Hmmmmm, I only got fo hunded, my Cummins must be sick." Endless.

 

[Oshkosh1]

Re: Horsepower rating?

Awesome post....
As I recall, the calculated HP, at the wheels, was under 100HP. The watt-loss through the trans & diff was astounding.

Not unless you had some SERIOUS problems with the drivetrain...

You should expect to see somewhere around 250 Hp(180kW) or so at the wheels under normal circumstances in a single axle street driven car.

 

[Don S]

Re: Horsepower rating?

I'm moving this over to the Boat topics forum, then both the IO and the OB people can join in. No sense just asking one side. Both sides visit Boat Topics.

 

[H20Rat]

Re: Horsepower rating?

I'm reminded of an engineering class many, many years ago.

As I recall, the calculated HP, at the wheels, was under 100HP. The watt-loss through the trans & diff was astounding.

Might have been off in your calculations...

At least on my car, the engine from the factory dynos around 270 horse. On a chassis dyno, it will show anywhere from 185 to 200 horse, so its losing 25-30% in the drivetrain. Keep in mind this is for a full time AWD vehicle that never disengages any wheels. A 2 wheel drive vehicle will have loses that are a little lower.

The tricky part about drivetrain loses is that it doesn't scale with horsepower. If I added another 100 hp to the engine, drivetrain loses aren't going to change, its the same drivetrain overall.

 

[glenn property of pam]

Re: Horsepower rating?

another little point is max hp verses tourqe. at high rpm both inboard/outboard are about the same but its the tourqe curve that gets you to high rpm when theres no gears to assist.

 

[spoilsofwar]

Re: Horsepower rating?

Standard drivetrain loss at the wheels for a rwd vehicle is usually in the vicinity of 15%. Of course, the only way to know for sure is to engine dyno, and then chassis dyno (and have the chassis dyno calibrated correctly - which is rare). Mustang/Dyno dynamics/and dynojet machines will all result in widely differing numbers depending on setup (correction factor). My turbo 350z puts down 421rwhp and 458rwtq @14psi with 50/50 meth/water injection on a dyno dynamics dynometer.

What does this mean on a dynojet or mustang?? nothing But, it means I gained ~80rwhp and ~80rwtq over my previous run @8psi without meth/water injection on the same dyno - the only relevant numbers.

AWD/4WD vehicles obviously incur greater drivetrain loss, since power is transmitted through a transfer case and then an additional diff.

Sooo... What were we talking about, again? Oh right, boats.

 

[QC]

Re: Horsepower rating?

but its the tourqe curve that gets you to high rpm when theres no gears to assist.

The torque curve IS a horsepower curve. Can't help it if RPM is involved

 

[UncleWillie]

Re: Horsepower rating?

... The watt-loss through the trans & diff was astounding.
Boat engines: I'd imagine the losses to be less, but conceptually: same thing....

For all the doubters; Consider that a truck transmission's temperature can get into the upper 100 degree range even with an aftermarket radiator cooler in series with the stock radiator cooler.
All that heat is wasted energy, HorsePower, and all the energy is coming from the engine.

There are aftermarket upper unit coolers, Drive Showers, available for the same reason.
They get HOT! And all that heat can only come from the losses in the upper unit.
You may have noticed that your upper unit get a white chalky film on it.
This is the remains of the water after it has boiled off the outdrive.
If you were to have the guts, and be dumb enough, to climb back there while under way for a while, you would burn your hand.

 

[HT32BSX115]

Re: Horsepower rating?

Is outboard h/p calculated differently I/0 hp.?the reason I ask is there seems to be huge performance difference between the two with simular boats with the same h/p rating. I understand there is a wieght difference Thanks in advance

In 1987, OMC posted 340 (crankshaft) HP for the original (Ford/OMG 460) and 330HP (GM 7.4L) in my Four Winns 211 Liberator.


A similar comparison is the Mercruiser 7,4L (330 crankshaft HP) engine with a Bravo drive on the back that is rated at 300 Propshaft HP.

The boat performs approx the same with either engine and drive (although the Bravo III produces FAR better hole shot....which is not relevant here)

In view of the above rating, you probably "lose" around 11% of the power (to friction/heat) through any stern drive.



Outboard engines might lose slightly less since the engines are vertical type engines so you only have to transfer power though one set of gears instead of 2.

And yes, the loss is usually in the form of friction/heat dissipated in the huge heat sink below (the LAKE!!)

( The 2nd law of thermodynamics can sometimes be neglected, it cannot be ignored!)


Cheers,


Rick

 

[QC]

Re: Horsepower rating?

In view of the above rating, you probably "lose" around 11% of the power (to friction/heat) through any stern drive.

I see this differently, Rick, and posted something like that earlier here. So did Smokingcrater. The drive "eats" so much horsepower to turn it so many RPM. In the case of your example, 30 horsepower. wouldn't it also eat 30 horsepower from a 140 horse 3.0? Hence netting 11O at the propshaft and a much higher percentage loss?

 

[UncleWillie]

Re: Horsepower rating?

I... wouldn't it also eat 30 horsepower from a 140 horse 3.0? Hence netting 11O at the prop-shaft and a much higher percentage loss?

That would be true if it was the same out-drive on different motors.

The a 3L engine will likely be in front of an Alpha1 with a 2:1 out-drive.
The 7.4L on the Bravo3 may be geared much lower. 1.5:1?

The prop is generally turning slower than the engine on a recreational boat.

Comparing Apples to Apples is difficult.

 

[HT32BSX115]

Re: Horsepower rating?

I see this differently, Rick, and posted something like that earlier here. So did Smokingcrater. The drive "eats" so much horsepower to turn it so many RPM. In the case of your example, 30 horsepower. wouldn't it also eat 30 horsepower from a 140 horse 3.0? Hence netting 11O at the propshaft and a much higher percentage loss?

No, not at all......... it would be HP/RPM AND torque related.

(since the 4.3L V-6 and the BB are found in front Bravo III's ) ..............For example, running a Bravo III behind a 330hp 454 you would "lose" more to friction vs running the same drive behind a 4.3L V-8 because the 454 would be producing far more torque (and friction) at around the SAME RPM........... so, more losses due to friction and heat...............

It isn't JUST RPM related.......

 

[emilsr]

Re: Horsepower rating?

In 1987, OMC posted 340 (crankshaft) HP for the original (Ford/OMG 460) and 330HP (GM 7.4L) in my Four Winns 211 Liberator.


A similar comparison is the Mercruiser 7,4L (330 crankshaft HP) engine with a Bravo drive on the back that is rated at 300 Propshaft HP.

The boat performs approx the same with either engine and drive (although the Bravo III produces FAR better hole shot....which is not relevant here)

In view of the above rating, you probably "lose" around 11% of the power (to friction/heat) through any stern drive.



Outboard engines might lose slightly less since the engines are vertical type engines so you only have to transfer power though one set of gears instead of 2.

And yes, the loss is usually in the form of friction/heat dissipated in the huge heat sink below (the LAKE!!)

( The 2nd law of thermodynamics can sometimes be neglected, it cannot be ignored!)


Cheers,


Rick

The go-fast guys and racers have reams of data on this as some of them would sell their mother for 1 mph.....but they still don't all agree.

My take on it; both you and QC are correct. Yes, you lose 10-11% through a Bravo drive (this is pretty well documented on the dyno), however the 330hp engine is producing that power at 4,600rpm while a 425hp 496 is spinning 5,000 rpm (and loses about 40hp through the drive) and a 525hp Mercury racing engine turning 5,400rpm loses closer to 50hp through a Bravo drive. It would appear the power lost is not a linear equation and COULD be better defined as hp loss at a particular rpm, but your 11% theory (at least for Bravo drives) is a pretty close approximation given the dyno data. In any case, higher horsepower usually comes with higher rpm, hp loss goes up and the percentage of power remains relatively constant.....more or less.

I don't have any such data on alpha (or pre-alpha) drives so not sure if the 140hp 3.0 analogy is appropriate.

Back to the OP's original question; weight matters as does balance. In one example, a single 454/bravo combination weighs about 1,300 lbs. If we use the 330hp variant for comparison purposes, 300hp at the prop shaft, a single Yamaha 300 OB weighs less than half as much. 600+ lbs is a significant difference and will result in much different performance.

Move from a 330hp 454 to a 425hp 496, same weight, and the weight of 2 x 200+hp outboards negates some of that performance advantage the outboard had in the first example.

Also, the larger drive will produce more parasitic drag. Obviously the faster the boat the more this will have an impact, but even at 40mph there is a measurable difference. Stick a second drive in the water (as in example #2) and that performance advantage can disappear completly.

You could probably distill all the pertinent factors into a handy, dandy formula but you'd need a REALLY sharp pencil.