Horsepower rating?

[coolbri70]

Re: Horsepower rating?

so if i repower my 1968 boat with a new 120hp motor(the max listed on my transom tag) i would be 7% overpowered

 

[Slide]

Re: Horsepower rating?

so if i repower my 1968 boat with a new 120hp motor(the max listed on my transom tag) i would be 7% overpowered

The motor will never deliver more power than it's rated for. "Technically", assuming that the manufacturer tag was applied with the assumption that the 120hp is a crank rated, then you will be slightly overpowered, but not enough to be unsafe, nor for it to be an insurance issue. To Geico, a 120hp outboard is a 120hp outboard.

 

[QC]

Re: Horsepower rating?

Comparing Apples to Apples is difficult.

Agree that. I think it is somewhere between my assessment and Ricks. Parasitic loss is the (current) topic, so same drive to same drive is most appropriate even if it is an unlikely pairing. I used the 3.0 to emphasize the point. Merc ratings suggest that it is not simply a percentage loss for all ratings. I'm looking up examples, but even that will not be scientific...

 

[HT32BSX115]

Re: Horsepower rating?

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.

The above of course, was my primary point..........

More HP/torque applied to any drive would produce higher (friction) losses.... The percentage loss with an Alpha powered by a 3.0L engine would be significantly lower than the "same" Alpha with a 454 in front of it.

Makes sense when you see stock drives with ridiculously huge engines that REQUIRE a "shower" just to stay cool enough to stay together ...........

 

[emilsr]

Re: Horsepower rating?

The above of course, was my primary point..........

More HP/torque applied to any drive would produce higher (friction) losses.... The percentage loss with an Alpha powered by a 3.0L engine would be significantly lower than the "same" Alpha with a 454 in front of it.

Makes sense when you see stock drives with ridiculously huge engines that REQUIRE a "shower" just to stay cool enough to stay together ...........

Exactly. It isn't just rpm or hp or even frictional losses....there's some combination of all of the above at work.

"Ridiculously huge" of course depends on your point of view. Somewhere around 600-700hp would be just about right in my boat, but unfortunately my wife (and my insurance company) don't agree. Perhaps it's time to shop for a new wife and insurance company.

Without delving too deeply into a different subject; all drives have limitations. The Bravo is "maxed out" with stock Mercruiser power so it's relatively easy to exceed it's capability now that you can buy "off the shelf" engines from Mercury racing producing up to 1,350hp. In these cases, drive showers are roughly equivalent to putting a bandaid on a sucking chest wound. They do keep the drive cleaner, but that doesn't mean you can over power the drive and expect it to survive. B-max has an internal cooling system in their drives (versus an external shower), however that's just part of what they've done to enable their drives to handle more power. IMCO has also developed stronger drives to handle the increased power of today's high performance marine engines and of course there's the venerable dry sump SSM-6 drive from Mercury Racing (and it's newer, bigger brother the M8). The right tool for the right job, and somewhere just north of 500hp, even the stronger iterations of the Bravo drive (like the XR) aren't the right tool anymore.

 

[HT32BSX115]

Re: Horsepower rating?

"Ridiculously huge" of course depends on your point of view. Somewhere around 600-700hp would be just about right in my boat, but unfortunately my wife (and my insurance company) don't agree. Perhaps it's time to shop for a new wife and insurance company.

You know, it's funny you mention that. I called Geico to get insurance for my Liberator (which isn't really a fast or "hot-rod" boat) They flatly told me that they would NOT insure a basically stock Four Winns Liberator...........

And all I have is liability.

 

[Frank Acampora]

Re: Horsepower rating?

What no one has yet addressed is this: Four cycle engines (IO) produce one power pulse per cylinder for every two revolutions. Two cycle engines (OB) produce one power pulse per cylinder for every revolution. Disregarding power loss and weight, The two cycle engine will produce MORE torque than the same number of cylinder four cycle. The net effect is a higher AVERAGE horsepower from hole shot to top speed. Thus, all things equal, the two cycle has an advantage over the four.

As commentary: Why do you think that Mercury supercharged the four cycle outboard Verado engines? By packing more burnable mix into the cylinders at all speeds, the supercharger effectively increases torque, thus offsetting the relative weakness and hopefully equalling the two cycle performance.

 

[QC]

Re: Horsepower rating?

While I agree with what you say 100%, Frank, it doesn't change horsepower loss at the drive. I guess it does mean there is more usable horsepower with a 2 cycle, but unless hole shot is your goal it doesn't help much. I want to repeat, yes, helps with hole shot. Got that, but what else you gonna do with it? Do they deserve a higher rating system?

For the other little drive train loss discussion, here is some info that will muck it up some more. I am not sure where I got this, but it is from Merc. It shows the ratings in 1990 when they were transitioning from crank to propshsaft. They list both. What a mess :facepalm: . . .

 

[smoedog]

Re: Horsepower rating?

I'm going to bring my experience with cars into this, and yes I know they are to seperate worlds....

Now first, how much boost are the Verado's putting out. Do they have a bypass system put into them to bleed of that boost as engine load goes down?

Second, I know going from idle to WOT will put 100% load on the motor, BUT, once on plane at WOT are you still operating at 100% load and full boost?

The whole 2 stroke vs 4 stroke is purely based on use. I believe that 2 strokes perform better of the line due to their narrow power band. But once up to speed I believe a 4 stroke will have better response due to their wider power band. But a 115, be it a 4 stroke or 2 stroke will have similar top speed figures, but it depends on what you do between wot and idle is what the deciding factor should be.

Another example is R/C cars. When I raced nitro cars back in the day, they came out with a 4 stroke nitro motor. The 2 strokes had better throttle response but it took a little longer to get the momentum up and start building speed. The 4 stroke motors had a lot of initial torque, but getting up to speed took longer.

So in simple terms...

Racing on a tight technical track, the 4 stroke's performed better since they would accelerate out of a corner better. But on long tracks with big straights, the 2 strokes were better since they were a little weak coming out of the corners but once they started winding out, they would blow past the 4 strokes.


Now since this is a post based off of my experince with cars and dirt bikes, mixed with a little Cruzan rum.... All I'm doing is rambling.

To sum it up is I guess you would have to take your boats weight and what you do into account.

Lighter boat...2 Stroke

Heavier boat...4 stroke

On that note...time to go drink more...

 

[HT32BSX115]

Re: Horsepower rating?

On that note...time to go drink more...

Yeah baby!!

 

[QC]

Re: Horsepower rating?

I am answering in the spirit of understanding this stuff. No offense intended whatsoever.

Now first, how much boost are the Verado's putting out. Do they have a bypass system put into them to bleed of that boost as engine load goes down?

Why do you care?

Second, I know going from idle to WOT will put 100% load on the motor, BUT, once on plane at WOT are you still operating at 100% load and full boost?

Once on plane and at WOT ALL boats are at full load providing they are propped correctly. Other than under acceleration or when towing or climbing on plane, at no other time are they at full load. Another way to look at it is they are only at full load when the throttle(s) is mashed. Why do you care about boost?

The whole 2 stroke vs 4 stroke is purely based on use. I believe that 2 strokes perform better of the line due to their narrow power band. But once up to speed I believe a 4 stroke will have better response due to their wider power band. But a 115, be it a 4 stroke or 2 stroke will have similar top speed figures, but it depends on what you do between wot and idle is what the deciding factor should be.

I almost agree with the top speed thing, but the rest is too much of a generality. Define "wider power band" and "narrow power band".

Lighter boat...2 Stroke

Heavier boat...4 stroke

That's backwards to the two stroke having more low end torque for the same horsepower rating, which I believe is pretty well established.

On that note...time to go drink more...

Roger that!

Edit: Rick took the high road :redface:

 

[glenn property of pam]

Re: Horsepower rating?

ok thinking out of the box. apples to apples the outboard with the same max hp and set up as a inboard would provide more thrust in the water due to the exhaust gas thrust not being counted on a dyno. this might only be a few hp worth of thrust on a large motor but its thrust that can add to speed so it counts "i think"

 

[QC]

Re: Horsepower rating?

Seems like a reach. Maybe worth a 1/10 of a horsepower. Maybe more, but I doubt you could even count the contribution. And when you get to big horsepower thru prop eats a teeny tiny bit of horsepower.

Exhaust does drive a turbine (turbo) to drive a compressor and subsequently deliver a significant amount of boost, so there is definitely lost power there, but I would struggle to see it as thrust.

 

[HT32BSX115]

Re: Horsepower rating?

Maybe worth a 1/10 of a horsepower.

Maybe even less..........

In the 40's and 50's, The (2-stroke) Diesel Napier Nomad was developed as a "Compound" engine

It recovered enough exhaust energy to produce 320 LBS of "thrust" in addition to the 3000HP it produced.