Prop for TQ or HP for top speed???
 

This should be easy. Where do you want your rpm for top speed? I would think right at peak torque. I read somewhere it should be after that but considerably before peak HP. TQ gains the MPH right? Another way to put it,,can you go any faster with 1 Ft/lb less torque but more hp?

Re: Prop for TQ or HP for top speed???
 

Top speed? At peak HP. Top speed is generlaly a short term thing and you have a motor that makes a certain amount of energy and you want to use it for peak performance.

Ideal criuise speed, peak torque. The motor shouldnt' be laboring as much and you should be able to cruise all day.

Somethiing like that. :evilb: :D

Re: Prop for TQ or HP for top speed???
 

well, we all prop to turn the motors has hard as we can for top speed, so i guess we ain't propping for torque.... cruise speed and best fuel mpg you would want to travel at peak torque or peak effiency.... but speed comes from rpm's.....

Re: Prop for TQ or HP for top speed???
 

Prop for hp peak,your torque peak will probably be close to your cruising speed anyways. If you had a motor that hp peaked at 5400 and pulled a 24 pitch prop and you tryed to turn a 30 pitch at 4200 your motor would be under a tremendous load,detonating,loading up etc. Plus if you were truly propped for "peak torque",you'd be at wot at that peak which again would make your motor act sick plus you'd have little acceleration and a real hard time getting boat on plane,Smitty

Re: Prop for TQ or HP for top speed???
 

Top speed will come when propped for top HP, I do know that. But, how would you know if you are propped best for peak torque and gain more mid-range speed while maintaining top end?

Re: Prop for TQ or HP for top speed???
 

Peak HP for top end, HP is a measure of the amount of work the motor can produce. TQ is the twisting force, acceleration. HP is speed and TQ is acceleration.

Propping for peak TQ will cause a variety of problems not only for the motor and drive train but will cause ill handling of the boat as well.

Matt

Re: Prop for TQ or HP for top speed???
 

If you look at the power curve the "sweet spot" or ideal cruising RPM is lower than the WOT RPM and will give better economy and less wear-and-tear for extended running periods.
Prop the boat to run at WOT in the recommended RPM range for that engine and throttle back to that ideal cruising RPM close to where the torque and horsepower lines cross. You'll feel the boat "settle-in" to its correct mode.

Re: Prop for TQ or HP for top speed???
 

It is all about compromise. If you want top end you want the tallest prop your motors can pull up to your max rpm. However this usually takes a while to get to and you will lose accelertion in the mid range. Most peopl will prop just slightly short of this 1-2 pitches which will give of a couple of mph on the top end but give you much better acceleration, which is what we use most IMO.
How many times have you really stretched you boat out, I mean way out like 10 plus miles WTO. Then how many times have you been running 50 -70 beside someone and hit the gas for a mile or so.

Re: Prop for TQ or HP for top speed???
 

try to prop it at least 1000rpm's above peak torque.

Re: Prop for TQ or HP for top speed???
 

Top speed is found when propped at peak HP (as installed with drive losses and marine exhausts).

Best and most efficient cruise is at torque peak, although a particular hull/power/setup combination may actually prefer a little more or less rpm for best cruise.

Torque and hp relationship is NOT nearly as confusing as many make it out to be.

Torque is twisting force. Simple enough. No more no less. Think of twisting force as pulling on a rope tied to a box sitting on the floor.. It may or may not be enough to drag the box (or turn the shaft) but it is still a measure of FORCE generated. No work is being done, however, until the box begins to move.

Power (horsepower, or other measure), is the amount of twisting force that is delivered in a certain period of time (or the distance you drag the box across the floor in a certain amunt of time).

Since torque is the twisting force on a prop, isn't more better? Yep. If all else remains the same.

But which of these is better?

400 ft-lbs at 100 rpm?
or
400 ft-lbs at 1000 rpm?

Obviously, the second one.

Okay, now a little twist.

How bout which one of these is "better"?

400 ft-lbs @ 5,000 rpm?
or
200 ft-lbs @ 10,000 rpm?

Answer? They are the same. You may have a reason for choosing one over the other, but they will both produce exactly the same amount of power at the listed rpm for each one.

You have to examine the torque curve of a motor to get feel for how to prop it for a particular boating application. Depends on usage and needs.

Re: Prop for TQ or HP for top speed???
 

^^^ what he said! HP= top end , torque gets you from point A-B faster

MC, you didnt tell us we would have a pop quiz...I didnt study.

Re: Prop for TQ or HP for top speed???
 

A question- It was stated to cruise at peak TQ but you cruise at 30-40% throttle(3000-3500rpm) and peak TQ is measure at WOT. What does the TQ curve look like at part throttle? Is it the same as WOT just lower or is it a different curve?

Re: Prop for TQ or HP for top speed???
 

This example may help:

The 455 inch Buick engine in a GS Stage 1 makes 360 hp @ 4600 RPM.

It makes 510 lbs. of torque @ 2600 RPM.

After 2600 RPM the torque drops off and the horsepower continues up.

Where they cross is the "sweet spot". If this was a boat engine it would be close to the ideal cruising speed depending, of course, on about a zillion variables.

Re: Prop for TQ or HP for top speed???
 

Torque and Horsepwer always cross at 5250 RPM in the power curve.

Wish that was my "cruse" RPM!!!!

Mark

Re: Prop for TQ or HP for top speed???
 

No, my point is that all dyno curves are done at WOT. You cruise at much less than WOT. What would a dyno curve look like at say 50% throttle. Would it have the same peak rpm points?

Re: Prop for TQ or HP for top speed???
 

When you guys say "most effecient cruise is at peak TQ" You don't really mean I'll get better engine life and milage at 4600rpm do you? I usually cruise around 2900 and 45Mph. Sometimes lower. My motors seem very happy from 2500 . The carbs run on the primaries up to around 3000 so I figure this is my best cruise. All the test I've seen show best mpg on most boats at around 2800-3000. I'n running 29 Hydros on a 29'fountain,twin 454s 1.5 gear. Haven't got the power to go more than 4700(79mph) with those props but I have a set of 27 bravo 3 blades I guess I'll go back to for more top end. peak TQ is right there but peak HP is around 5500. Guess I'll shoot for 5200 with the 27s.

Re: Prop for TQ or HP for top speed???
 

Good point.
No, the peak points would be lower in the rpm range, is my guess. Somebody run desktop dyno on an HP500 config, then run it again with a 325cfm carb. That'll tell us.

So yeah, I see what you're saying. You'd have to prop for WOT operation at peak torque to match the peak on the dyno sheet. And that would be a suck thing to do.

Torque peaks and BSFC numbers are an indication, though, of how efficiently the motor is converting fuel into power.

I guess if somebody was entering a "fuel economy range competition" with a given hull/motor combo, then the peaks at certain throttle openings would be necessary to baseline your optimal speed and proper prop selection, but in recreational boating, we almost always prop for WOT at peak hp, and the rest just ends up wherever it falls.

And cooltoys, your point is valid, too.
While the motor may INDEED be most efficient at 4200rpm, your planing hull requires an exponential increase in power to raise the speed. This makes it impossible to match cruise speed to peak torque.

But it does pretty much work that way in displacement hulls. My bad for crossing it over to planing hulls. Too much information is worse than not enough sometimes.

And engine life is directly related to increased rpm. As long as a motor isn't lugging, it will always last longer at lower rpms.

Re: Prop for TQ or HP for top speed???
 

That can't be right. What engine has a curve like that?

Re: Prop for TQ or HP for top speed???
 

Hp is not measured. It is a product of a mathematical equation, using torque (which is measured) and rpms. As part of this equation hp and torque always cross at 5250.

Re: Prop for TQ or HP for top speed???
 

it has to be equal at that point, or the universe will collapse.... :eek:

Re: Prop for TQ or HP for top speed???
 

Torque? Let me tell you Thats where it is . I recently put 2 540,s im my Formula . I have been dissatisfied with my dyno numbers , 575 hp and 630 ft lb of torque. I just compared my stats with that of the chief engines 540 . My torque is much higher . My old 454,s put out 535 hp but not nearly as much torque. I jumped up 5 prop sizes only went up 40 more hp. But the torque was almost 100 foot poiunds . Not totaly dialed in yet . I ran out of weather . But the torque gain from an 850 to a dominator was so impressive I alomst idled on plane on my last sea trial .So ther bottom line is torque will pick your as- up and hp will get your as- at the top end....

Re: Prop for TQ or HP for top speed???
 

Hey Boat .... what Formula and some numbers before and after ? Ballpark ...

Re: Prop for TQ or HP for top speed???
 

speaking of prop tuning ... in my 311/454mags I went to 23 Hydromotives ...I can get to 5000 but it takes some fine tuning ...triming ect ....should this be right or should I pretty much be able to spin right up to 5000 .... I have heard some like to underprop some so you will have throttle left ...then at heavier loads you would have better perfromance .... Matt at Throttle up calculated about a 22 prop but that was with very rough numbers ...these props are stock ....so there is room for matt to tune I suppose .....

Re: Prop for TQ or HP for top speed???
 

Only every engine on the plant that can spin 5250?
(Actually it's really 5252...)

ONE horsepower is the power needed to raise 550 pounds ONE foot vertically in ONE second. (Supposedly this "unit" was derived from the mining industry which used horses and mules to haul buckets of ore out of the mine shafts.)

So:

1hp = 550 pound-foot per second.
or
1hp = 33,000 pound-foot per minute.

In a rotating motor, you gotta sort of work backwards..

Each revolution of the motor, at a "1 foot" lever arm (or pulley with the same radius), you circumscribe a path of (2 x Pi x 1foot) 6.283 feet. (Perimeter=2 x Pi x Rad, and for this example Pi will be rounded to 3.1415)

So a motor with "T" lb-ft of torque can lift "T" pounds a distance of 6.283 feet per rev.
At 1 rpm, that motor will lift "T" pounds 6.283 feet per minute.
So, you can take that and rephrase that as:

"H" hp = ("T"/33,000 pounds) x (6.283 feet per rev) x (1 revs) per minute.
or
HP = Torque x (6.283/33000)RPM = Torque x (1/5,252)RPM
or finally
HP = Torque x RPM / 5252

mc

Re: Prop for TQ or HP for top speed???
 

OK, I am going to respectively disagree with a few of you here. I don't see any relationship between best cruise speed and peak torque, unless you define best cruise speed as the speed in which you feel the most surge in acceleration when you hammer the throttle.

The term "best cruise" is a bit nebulous anyway. I would venture to guess it would be the most economical running speed in terms of miles per gallon. That can only be determined through boat tests for your particular boat, engine, prop, etc.

As far as horsepower and torque peaks crossing - don't concern yourselves with this one either. In Europe, they measure in kilowatts, and Newton-Meters. Now what? The torque and horsepower curves are going to cross at a different RPM. That crossing is completely dependent on the graph scale, tells you it is bogus.

As far as peak horsepower, and best top speed, many of you got it correct. It is at the horsepower peak. Engine torque is one of those figures which is interesting on the dyno, but not important overall, because we have transmissions to change horsepower into drive torque. If it was important, we wouldn't have Formula 1 cars with their little 183 cu. in. engines doing 0-100 mph in 3.6 seconds.

Michael :D

Re: Prop for TQ or HP for top speed???
 

You may be right but if you pick the right conversion to Newt/mtrs and kilos I don't see how the equation could come out different. Trouble is I saw about 30 kinds of kilowatts and several NMs in my "pocket ref" so I'll leave it to MC to prove this.

Re: Prop for TQ or HP for top speed???
 

1 W = 1 N x m /sec
okay, what's a Newton? It's (1 Kg x m /sec^2).
so
1 W = 1 kg m^2 / s^3, but for Shts and Giggles, we'll leave Newton-meters as the unit of torque.

so

1 W = 1 N m /sec = 60 N m / min

So, a 1 meter radius pulley will lift a load 6.283 meters per rev.(we already did that math).

1 Watt = ("T"/60 Newtons) x (6.283 meter per rev) x (1 revs) per minute.
1Watt = T x (6.283/60) RPM = T x (1/9.55) RPM
1 Watt = T x RPM / 9.55
1 kW = T x RPM / 9550

Therefore, torque curve crosses the power curve at 9550 rpm when using Newton-meters and Kilowatts.

Let's see you try to prop for that......


mc

Re: Prop for TQ or HP for top speed???
 

That is great! :D :D :D Glad to see no one has lost their sense of humor during these technical discussions.

Michael

Re: Prop for TQ or HP for top speed???
 

Torque is measured, HP is calculated.
Torque ='s the amount of work that can be done
HP= how fast the work is done.

A boat needs a broad powerband since it is limited to a single gear ratio and limited by a narrow range of props sizes and pitches. In comparison a car has transmission gear choices, rear gear choices, and tire size choices. The broad power band is needed in the boat to plane, to cruise, and for top speed. We can not throw the throttle to 5K rpm and drop the shifter into gear.. . .a race car can.

So when building a marine engine, you better off to ask how broad is the power band then to ask what is the calculated HP.

Chris

Re: Prop for TQ or HP for top speed???
 

I'm just glad I ain't got no furrin motors. Can you imagine trying to find a 56" pitch Mirage,,much less controlling valvetrain float ;)
Yeah, I know, I know.

Re: Prop for TQ or HP for top speed???
 

Okay, if torque is not important overall. Lets build to boats to run from NYC to Key West. Identical hulls and drives in the 36ft range to take weather and other elements. We will put 500HP 540CID bulldogs in one boat and 650HP Turbo charged Chevy Ecotec 4 cylinder's in the other one. Each boat will be geared and propped accordingly to match engines max HP. Race will start with boats in neutral.

Anyone want to bet who will be sipping rum and coke at Sloppy Joe's?

Chris

Re: Prop for TQ or HP for top speed???
 

Thank you, I have been saying TQ for years. Perfect example above.

Re: Prop for TQ or HP for top speed???
 

Torque is king!

I raced my 28 Pantera in 2003 with a 540 CI motor turning 5800 RPM - Motor made 625 hp @ 5800 RPM . Turned a 26" prop - Boat 77-78 mph but took forever to get there.
Changed the cam so HP peak was now at 5000 RPM . Boat turned a 28 " prop @ 5000 RPM, midrange and acceleration were awesome . The tradeoff for the 2-3 mph in top speed was well worth it .

It comes down to hull design - Efficient hulls can use the hp to accelerate hard and run fast (cats , stepped hulls ) . Traditional deep vs' need the torque to accelerate hard and run good top speeds .
Bottom line - The boats' hull design dictates what works best . One engine recipe does not work for all !
Torque =acceleration , HP = speed