HP vs RPM's Question

[Texasmark]

Weather is horrible. Non-home boy is home locked. :''( Surfed the www to pass the time. Thought about upgrading my 90 to a 4 cyl. 115.

If you are interested in the midrange higher hp engines you would be amazed at the offerings this year as compared to previous (maybe 5 years ago) years.

Seems I only have 2 options (2 stroke) on a 115: Rude or Yammy, to get a 4 cyl and they come as a V4 at that, and loop charged. Not bad. Merc is definitely out as they souped up what I have now (90 hp 3 cyl) and made a 115 out of it.....more of a bad thing (#1 nuisance clutch dog rattle has to be worse) I guess to rid themselves of their I4. Must have been a disaster with the 2-4 cylinder thing and rpm upper limit or the crank goes (so I hear). I thought Cadillac taught the industry that the 2-4-8 cylinder thing was a joke.....never had one, but if it was a success you'd still be seeing them in production wouldn't you think?

Besides, I liked my I6 Merc Tower of Power, but it was always in the way. The V4 is a nice low profile; better than my current 3 cyl which is a vast improvement over the I6.
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Anyway back to the plot. While surfing yesterday, I noticed some verbiage from Johnny/Rude about max HP ratings. They showed a plot (graph) of HP vs RPM for a particular engine, and in this graph HP peaked at 5000 RPM. It went on to say that the engine would probably be rated 4500 to 5500 for the WOT range.

Since HP = torque + RPM + a constant, that tells me that torque starts falling off causing HP to fall faster than RPM's increase. No surprise as I have seen a lot of automotive charts and they do that.

So following along the thought process I'd think that if I ran this particular engine at 5000 RPM, I ought to get max MPH and if I wasn't propped right and ran on up to say 6000 my speed should fall off cause the HP (that turns the prop) fell off.......but you gotta ask yourself how'd I get to 6000 if that were the case?

Well, that's not the case. My current engine is rated 5000-5500. But If I am running a prop that will allow me to go to 6000 (or more with another prop of less pitch) the boat speed keeps increasing as the RPM's increase.

Since the load on the prop (from the boat's water resistance and weight) should be about equal at those speeds (45-50 MPH), if the engine started having breathing problems and the torque fell off above the peak, then the boat should start slowing down, wouldn't you think?

Just Curious.

Mark

 

[Silvertip]

Re: HP vs RPM's Question

You might be interested to know that the Cadillac 4-6-8 concept is not dead (although) it was for some years. GM and Dodge both currently use the technology called Displacement On Demand. However now its 4 or 8 cylinders. Chevy's Impala with the 303 HP V8 uses it as does the pickup and Dodge uses it on their Hemi.

 

[Texasmark]

Re: HP vs RPM's Question

Yeah, now that you mention it I do recall seeing a Ram advertisement about it....but here you are talking about a 4 stroke; the other is 2 stroke. I see that as a major lubrication/flooding problem and have read articles about getting the 4 back up and running after being on 2.
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While you're here Silver, I figured you, of all people, would kick in some reasoning in answering my question.

Mark

 

[Silvertip]

Re: HP vs RPM's Question

As with any engine design, there are limits so the 20% RPM gain from 5000 - 6000 RPM is simply a little extra reving ability even tthough torque and HP may be down at that point. With no load you can run an engine to the point where it goes to engine heaven. Porting and other performance mods can certainly raise HP and torque but it moves those values up or down the RPM and changes the shape of the curves. Unfortunately marine engines don't have multi-speed transmissions so some pretty serious dyno work must be done. I'd love to follow a new engine development program sometime.

 

[Texasmark]

Re: HP vs RPM's Question

Well you can do just that. On www go to allpar.com/moopar/47html and eat your heart out. I have one in my 2005 Ram 1500 with their 6 speed tranny. Gets the 16 mpg they advertise day in and day out and pulls my boat just fine. Course the tranny won't let the engine lug and any indication of that and it grabs a gear or two.

I like the part where they talk about the oil pump was designed specifically for 5W-30 motor oil. I'm a heavy duty oil nut and it took some discipline for me to comply, but I did and the oil pressure is always rock solid, even when hot. I'm impressed.

Mark

 

[QC]

Re: HP vs RPM's Question

You're making a fundamental mistake Mark. If the RPM is increasing then the propeller pitch is such that it does not reach full bhp load until above it's max rating. Basically, lets say the horsepower falls off to 98 at 6000. The load created by your lower unit in the water, and your boat over the water, and against the air at the speed she reaches at 6000 RPM is apparently equal to that same 98 bhp. If the engine truly makes more horsepower at 5500, then she will be faster if you prop her to do so . . . If you mash the throttle with even less pitch she'll go even higher in RPM unless she blows up. Boat will probably go slower because the max power available at that RPM is less, but she'll still go up in RPM until the engine reaches an equilibrium between horsepower required (load) and horsepower available.

BTW, what you have discovered is why high low-end torque numbers are almost meaningless to marine engines. Yes, higher peak torque will improve hole shot, but a high peak torque engine will not be faster than a lower peak torque engine with the same max bhp rating . . . Both will merrily rev past that peak torque point until, again, the equilibrium is reached or you back out of the throttle. Moral of long story? You need more pitch to cause that equilibrium to be reached at Max bhp capability, to take advantage of your engine's available bhp . . .

Also, the calculation is bhp = torque X RPM / 5252(constant)

 

[Texasmark]

Re: HP vs RPM's Question

That makes a lot of sense when you talk about loading; and that's what the hp is up against and that would explain the phenomenon; shifting the pitch, shifts the torque curve and moves the peak hp. Since torque would go up, with lower pitch, the curve would move out, the hp peak would move out and the speed would continue to increase as rpm's increased.

I got it QC. Nice job man. 8)

So, if I could see the different rpm/torque curves for the different props it probably would be crystal clear as to the relationship.

I am in total agreement that torque makes the hole shot....case in point Merc going from a 2000cc in their 150 to a 2500cc for just that purpose and the old 25 hp OMC with 35 cu in, that would nicely plane out a 16' boat with 3 people and all their gear.

Thanks,

Mark

 

[QC]

Re: HP vs RPM's Question

So, if I could see the different rpm/torque curves for the different props it probably would be crystal clear as to the relationship.

Propeller load "curves" are really not curved at all. They rise diagonally up relatively straight with RPM. It takes the same torque to spin it once as it does to spin it 5500 times in a minute, so the load increases linearly . . . There are no gears or hills to increase the load against the propeller at fixed RPM points. The only exception is going from displacement to planing speeds and that "hill" is only there for a second if you have decent power. Try bumping your throttle up 100 RPM at a time and you'll see what I mean, the only way you are using full torque capability at any RPM is with the throttle hammered all of the way (WOT), so if you are making little tiny bumps 100 at a time, then it only develops max torque when you can't push it any farther . . .

I am in total agreement that torque makes the hole shot....case in point Merc going from a 2000cc in their 150 to a 2500cc for just that purpose and the old 25 hp OMC with 35 cu in, that would nicely plane out a 16' boat with 3 people and all their gear.

If you used the 100 RPM bump method on both of those engines, there would be no difference in performance as long as they were both truly 150 bhp engines at the same WOT RPM, same gear ratio, same prop, same trim, same boat, same weight.

 

[Texasmark]

Re: HP vs RPM's Question

Thanks QC. Nice job. Hope others gained insight as I have. Good things to keep in mind when you are pondering upgrading your rig.

Mark

 

[waterinthefuel]

Re: HP vs RPM's Question

Huh?

 

[WillyBWright]

Re: HP vs RPM's Question

Back in the olden days (1950s), OMC's motors were typically rated at 4500rpm. So Merc rated their's there too. Their Hurricane was advertised as an 11Hp. But they could easily crank out six grand and put out 18Hp. That's the start of Mercury's racing reputation. (Or so I've been told) Nobody else could compete because they couldn't match six grand.

 

[itstippy]

Re: HP vs RPM's Question

"That makes a lot of sense when you talk about loading; and that's what the hp is up against and that would explain the phenomenon; shifting the pitch, shifts the torque curve and moves the peak hp. "

Sigh.

No. Shifting the pitch has no impact on the torque curve. The powerhead has a torque curve and that's that. You change the prop pitch to take advantage of the powerhead's torque curve, not to change it. Big fat heavy slow corn barge you use a low pitch prop and ballsy gear ratio to take advantage of the torque curve. Skinny sleek light speed boat you use a high pitch prop and screamin' gear ratio to take advantage of the torque curve. The torque curve doesn't change.

You get yourself a boat with a planing hull and it takes some grunt to get it up on plane. You need HP - torque - grunt - to get 'er up to speed. Once it's on plane it has less resistance and if you can continue to increase the RPM's you will keep going faster, even with slightly less HP output. At 6000 RPM you are going faster than you are at 5500 RPM, even though you are producing slightly less torque. You ARE going faster, so you are producing more measurable horsepower if we're calculating horsepower by the weight of the boat and how fast it's going (work done). But you are producing LESS torque, or potential measurable horsepower.

 

[eatropefish]

Re: HP vs RPM's Question

Interesting reading! How feasable is it to change cu in by increasing the bore and stroke in an outboard. Say for instance a 45 Mariner with a cu in of 43.8. Is there enough cylinder wall to safely bore .015-.030 over and change rods to increase the stroke. How would this affect the performance of the engine? Is it even possible? It's probably cheaper just to buy a bigger engine. But say you're O/Hing and want to beef up your existing engine. Just curious. What say you?

 

[WillyBWright]

Re: HP vs RPM's Question

Punching to the max oversize piston only gives a fraction of one HP. Stroking it would make substantial gains, but no kits available. If we were talking MerCruiser 5.7L (350ci), that would be a different story. Those can be stroked to 6.2L (383ci).

 

[eatropefish]

Re: HP vs RPM's Question

Kinda what I thought, Willy. Thanks for taking time to answer. I'm not interested in tearing apart a perfectly good motor to undertake such an endeavor. If it were possible, then........maybe. Thanks again for your time.

 

[Texasmark]

Re: HP vs RPM's Question

Could start a new thread for clarity but we are supposed to stay with the same one.
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Well, I thought I had it, but I guess not.
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Torque x rpm (+ constant....whatever the formula for that is) = hp. Hp (kilowatts) is a measurement of the rate of consumption of energy; hence work. I think that has been adequately recorded in reference text.

I have 2 situations.

I have 2 boats of different configurations that are operating at certain speeds in the same environment using identical engines and props.

One is light and fast, the other has more of a load and is slower.

The engine on the heavier boat is operating 1000 rpm's below the engine on the other. Does that mean that the work required of the heavier boat is greater than the other and that it can't produce the amount of work required? Why?

If torque is a function of the engine design, and torque and rpm's are a measurement of hp, then the heavier boat is not using it's available hp. Why?

Because the load of (placed on) the heavier boat is not allowing it's engine to reach max rpm's......but it may be sitting on the peak of it's torque curve.
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So here I guess we get into the question as to what are the shapes of the curves and does torque fall off at the same rate as rpm's increase? Well what if it doesn't?....hp falls off. What if it does?.....hp is constant. Okayeeeeeee.
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So, on the heavier boat, I change the pitch of the prop to a lower pitch....all other things constant.
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This reduces the load that the engine sees (I guess) and allows the rpm's to move up 1000 rpm's. Now they are both at the same rpm. Am I assuming something here that I shouldn't be?

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What has happened (by changing pitch) and what is happening with the new pitch?
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I don't know about you guys, but I am on here for an education as well as sharing experiences, attempting to help those in need...that I think I can, to have something to do, and to keep my typing skills in order.

From the respondents, their responses, and frequency of them, it appears that others feel that way too, at least partly, and would rather be doing this than something else....or you would be doing it, right?

So, in the interest of educating me, and whomever else reads this, and to clarify something that you may have known or thought you knew......as I, I am keenly interested in answers to these question.......because it takes an educated mind to make educated decisions and knowing things like this can help one to make the right decisions when the time comes.......just like (one of ) the questions/statements within.....geez, if you double the hp you would at least expect some speed increase.

Thanks,

Mark

 

[itstippy]

Re: HP vs RPM's Question

In our example we failed to say at what RPM the identical motors reach their maximum horsepower output. We want to prop them so that at Wide Open Throttle they reach the RPM where they produce their maximum horsepower output. Let's say the maximum horsepower output is at 5500 RPM. Both boats are at WOT.

Prop #1 - high pitch
Big Barge - 4500 RPM; Speedy Boat 5500 RPM (1000 RPM difference).
Big Barge is way overpropped. It's unable to get close to 5500 RPM so it's not reaching its peak HP output. The Big Barge has a hull speed above which the amount of effort required to move faster is too great. We're lugging the motor and we're gonna coke it up.
Speedy Boat is ideally propped. It maxes out at 5500 RPM, right at the peak of its HP output. We're getting all it has to offer.

Prop #2 - medium pitch
Big Barge 5000 RPM; Speedy Boat 6000 RPM (1000 RPM difference).
Big Barge is still overpropped. It still can't reach its peak HP RPM. Better, but not ideal.
Speedy Boat is now underpropped. It's climbed past its peak HP output and HP is tailing off. We were going faster when we were using prop #1 and maxed out at 5500 RPM where the peak HP is.

Prop #3 - low pitch
Big Barge 5500 RPM; Speedy Boat 6500 RPM (1000 RPM difference).
Big Barge is ideally propped. It maxes out at 5500 RPM, right at the peak of its HP output. We're getting all it has to offer.
Speedy Boat is way underpropped. It's running dangerously past its maximum HP output RPM's. We're gonna blow it up doing this.

Actually, we will not find this nice linear curve if we run tests comparing these two boats with identical motors because of the different hull designs.
The Big Barge is not a planing hull. The faster it goes through the water the more resistance the hull makes. In a geometric progression, not liner. It take 4 times the horsepower to move it at 20 miles per hour than it did to move it at 10 miles per hour. That why the gas mileage on our pickup truck drops to pathetic levels when we go 75 MPH instead of 65 MPH. The increase in wind resistance is a geometric progression, not a linear one. We can save a LOT more gas by backing down to 55MPH. WAY less wind resistance.
Speedy Boat has a planing hull. Resistance actually drops off once she's up to speed. Get her up on the pad and the water resistance all but disappears. It's literally flying; nothing in the water but a foot of pad and a prop.

Since we can't shift gears in a boat we're always compromising with what (fixed) prop pitch we use. Go for the utmost top end and we lose holeshot. Go for a great holeshot and we lose top end. If only we could get Speedy Boat up on the pad, then shift to a higher pitched prop. We would kill ourselves!

 

[itstippy]

Re: HP vs RPM's Question

A bit more on hull characteristics and how they affect our choices in Horsepower and Prop Pitch. Let's look at two extreme examples.

Back in WWII the Kaiser Boat Yards built "Liberty Ships". They were the ultimate displacement hull - "Big Barge". They could haul an unbelievable amount of war material across the pond. But they were only going to go so fast with that hull. It did not matter how much horsepower you put on them or how you fiddled with the screw pitch. They hit their designed hull speed and that was IT. Quadruple the horsepower and you might see a speed increase of 1 MPH. And at terrible fuel costs. So they lumbered across, doing what they were designed to do.

On this forum, right now, we have a guy who soups up inflatable boats for maximum speed. His inflatables are the ultimate "Speedy Boat". Even at rest the boat displaces hardly any water and just skims the surface. Almost no water resistance at any speed. This guy has figured out that with the hull characteristics of an inflatable he can use radically pitched props and still "get on plane" - the thing's practically "on plane" sitting still! And horsepower increases result in speed increases. One of these days this speed enthusiast will either break the sound barrier or break his neck. So his boats scream across the water at 100+MPH, doing what they are designed to do.

 

[Texasmark]

Re: HP vs RPM's Question

I just punched the wrong key and my whole dissertation just went up in smoke. :/
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In short.

Let's go back to the prop/gearbox as a torque multiplier.

I realize that an (any) engine has it's torque curve and it's hp curve.

Just as a transmission in a truck multiplies the engine's torque for it, to start a 10,000 # trailer rolling and then go down the interstate with it at 60 mph, I see the things mentioned as torque multipliers for OB's.......agree?

I realize that you can't shift gears in an OB but there are modifiers of sorts to level the playing field somewhat. Ports in the prop for one, smart tabs another, as you mentined, hull design .

Also there is the water which can be forced out of the way under heavy loads (slip) whereas the truck is locked to a solid surface.

I spend a lot of time comparing different mfgrs specs for the same hp engine. They all seem to have their own ideas as to how to achieve the desired results. I also reminiss a lot and frequent the old-omc.de site comparing hp-rpm-cu in- and gear ratio's for different engines over the years.......wondering what drove the designers to do what they did when they did.......what seemed to work and what didn't.

So, when I take my 90 Merc at 85 cu in and 2.3 gearbox rated at 5000-5500 rpm's and compare it to a 90 Yammie with 70 cu in's, 2:1 gearbox and rpm rating at 4500-5500 rpm's It looks to me like something is terribly wrong. Shooting from the hip I'd expect the torque of the Merc to be superior (cubes + gear ratio) to the Yam giving me a better hole shot and the higher rpm's should give me more top speed once up and on top.

So as a perspective buyer, with a certain application in mind, which engine am I going to select for my task at hand.

To me, torque is what gets the load rolling and rpm's just determine how fast you are applying the torque (rotating moment of inertia).

So if you take two potential buyers, one with an aluminum boat, and one with a FG deep V, which engine are you going to select and why.

Since OEM's own boat building facilities nowadays and put their engines on their boats, like was mentioned a couple of days ago, if you don't like a Yammie and want an e-tech, but want a G3 alum boat, you have to deal with two different dealers.
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My 2.3 gearbox is causing me to run a very large, high pitched prop, on my light boat, which I know is responsible for my very annoying "clutch dog rattle" at idle speeds. Not sure what I'm going to do about it but I am looking around. Want to make the right decisions if I choose something else and this subject is surely part of it.

Again, thanks for your time. Hope others have profited by this exchange of information.

Mark

 

[smiley2007]

Re: HP vs RPM's Question

Hi,

Very interesting reading and one that I have wondered about for some time.

Most sites I've visited say to prop your engine to meet the max revs at top of the engine range.

i.e.

E-tecs have a range of 4500 - 5600 revs.

Max power generated at 5100 revs

So why prop a boat any higher than 5100 revs ??

Surely 5100 is the best revs to aim for and not 5600 , which would have a drop off in power and thus a slightly lower top speed with the lower pitch needed to hit the 5600 revs.

I can never see the reason to hit max revs as most suggest

or and i off the mark :%:%:%