What's more meaningful, HP or Torque

[Big Al]

What is the most meaningful way to measure engine numbers for a boat, HP or torque figures. I tend to compare a boat(cruiser) to a truck. In a truck I believe engine torque numbers are the best way to evaluate the engine capacity to move the truck and its load. In a boat the engine has to provide high torque at low RPMs to get the boat on plane fast and then keep provide that torque at medium RPM to keep it on plane.

I know torque numbers are generallly not provided by most popular engine brands (except GM). I don't know why.

ref: http://www.gm.com/explore/technology/gmpowertrain/engines/specialized/marine/marine_engines.jsp

http://www.gm.com/explore/technology/gmpowertrain/engines/specialized/marine/2008_8100_HP_Marine.pdf

 

[wickeral]

Torque vs Horsepower! Egg or Chicken? Linked forever. Torque generally is thought of in the "Brute Force" available created by a power producing machine. Horse Power is the highest amount of force available during an engines power cycle or curve. They are inseparable because they mean just about the same thing.
Now, a large Heat generating furnace only has Horsepower without a specified Torque. Take a Steam Locomotive. It has a Boiler that creates Horsepower in the form of Steam. No Rotating shaft to create Torque with.
In contrast to a torque curve , horsepower rises rapidly with rpm, especially when torque values are also climbing. Horsepower will continue to climb, however, until well past the torque peak, and will continue to rise as engine speed climbs, until the torque curve really begins to plummet, faster than engine rpm is rising.
Bottom line, Torque is an engines most efficient Horsepower Point.

 

[sbw1]

In a truck or car torque gets you off the line quickly and HP gives you passing speed. For example an Audi TT with a tiny 1.8 liter engine makes 225 HP. It is slow off the line and has little torque but is a screamer on the highway when you keep the revs up where the 225 HP is made. Boats are somewhat the same. Diesels produce a lot of twisting power at the shaft which allows a big prop to move you around at a good pace around the dock and in big following seas. The top speed of boats is determined by hull shape and HP, not torque .

 

[jrcinnh]

horse power is just torque times engine speed. So there's two ways to make more HP, make more torque or make the engine spin faster. Engine makers usually sell HP, they may give you a torque number but the big seller is HP. The peak HP is usually near wide open throttle, how often do you really operate a boat at WOT.

 

[comsnark]

Both numbers are misleading in my mind.

Torque is nice. . .if the peak torque is available at the right point in boats power curve. Diesel locamotives -and bulldozers- are designed to deliver torque (and horse power) at low speeds. The Audi is designed to deliver it at high speed.

And good luck getting any real info from boat manufactures. Hard enough to get a reliable RPM/speed curve.

 

[Sorrento 25]

HP = (RPM * Torque) / 5252. They are directly related. A graph of both will show the lines crossing at 5252 RPM - the point where they are equal on any engine capable of 5252 RPM. A .21 cubic inch model car engine can make 3HP at 40,000 RPM but can't mow your lawn.

 

[keokie]

A flatter torque/horsepower curve will always help a planing boat. Compare a 350 Mag and old 454 MPI, both have the same horsepower, but the much flatter horsepower/torque curve of the 454 makes it feel like a much stronger engine in a cruiser.

 

[Dave M.]

Both numbers are misleading in my mind.
Diesel locamotives -and bulldozers- are designed to deliver torque (and horse power) at low speeds. The Audi is designed to deliver it at high speed.

I believe diesel locomotives are designed to deliver tractive effort at low speeds. Most diesels, and many steam locomotives, have enough torque to spin the wheels when they start moving. So they are limited by the wheel adhesion to the rail when starting a train. Modern diesels use axle speed sensing and calculate SOG sensing to detect and eliminate wheel slip.

Steam engines have enough torque to spin the wheels at standstill. Assume a typical boiler pressure of 200 psi, and a cylinder diameter of 25 1/4 inches. The end area of the cylinder is pi*(25.25/2)^2 or 500 sq inches. Then 200 psi times 500 sq inches is 100,000 lb. Apply that at the driving wheel driver pin to axle center distance of say 2 feet, and you have 200,000 ft-lb of torque. But what is applied at the wheel to rail interface is force. You have to divide the 200,000 ft-lb by the wheel radius to get the force. Say the radius is 4 feet, then the force that could be applied is 50,000 lb.

A large steam engine such as a Mikado might have 220,000-300,000 lb on the drivers. The adhesion, or coefficient of friction, is only about 25%, so the wheels would slip at about 50,000 - 70,000 lb. So if my calculation is correct, and who knows if it is or not, then the loco may slip it drivers when starting. Oh oh, better get back on track. :smt101

And good luck getting any real info from boat manufactures. Hard enough to get a reliable RPM/speed curve.

True. I think the HP curves shown by GM are an artists rendition, as are most manufacturers engine HP curves. If you have a real plot of either torque or HP vs RPM, you can calculate the other.

HP = (RPM * Torque) / 5252. They are directly related. A graph of both will show the lines crossing at 5252 RPM - the point where they are equal on any engine capable of 5252 RPM

Interesting point. I measured the torque curves for the two engines at various RPMs, and put them in a spreadsheet. Then I calculated the HP, so I am assuming the torque curves are correct. Here is what I came up with, after making up fake data to get the regular 8.1 up to 5250 rpm. It sure shows what Sorrento is talking about.

If you look at the HP charts for the two engines on the GM site, I think you won't find any HP difference in the two engines below 3500 RPM. Thus my comment is it an artists rendition, and likely intended to not be accurate enough to show that the lower HP engine puts out more HP in the normal operating range. But in fact, if the torque curves are correct, the lower HP rated engine puts out almost 10% more HP at the slower speeds. It has to if the torque at those speeds is nearly 10% higher. This characteristic has been mentioned before on the forum.

 

[Big Al]

Thank you for the information, quite useful. The comparison between the two GM's 8.1L engines is very interesting. If you assume that you normally operate your boat below 3700 rpm, you would be better with the "standard" 8.1 L engine (more HP and torque for less $$) ? That could be the case of someone who owns a cruiser but not the case of a speedboat owner.

 

[comsnark]

Release your inner geek!!

BTW: The area of a 25.25" pistion would be 500 in2. (Pi R^2).

Horsepower-torque curves CAN be useful. But. . .how does that translate into boat performance? That information is never published. If it is, it is only published for ONE boat/engine combination -> and so you must guess how a different powerplant will affect the numbers -> and you question the integrity of the first curve.

And from those curves, you can only infer numbers like "plowing speed" and "minimum planing speed".

In my case, I am pretty sure my top end speed is limited by prop cavitation. At the top end, more RPM does does not result in higher speed. Right now, I am pretty sure I have fouled props (Barnecles: happened last year at this time) My peak RPM and peak speed ain't what it was at Labor day.

 

[Sorrento 25]

Interesting comparison on the GM 8.1s. The only benefit to the HO is well above 3500 RPM - a range I only operate in when testing a prop or being chased by pirates. The non-HO has much more area under the curve up to 3500 RPM, would be the better cruiser engine, and should have better performance onto plane.

This is a classic example of the trade-off of low RPM power for high RPM power. If you move the torque peak up in the powerband the result is more peak HP based on the math, but a boat/truck/RV application is one where you do not want to sacrifice low RPM torque for high RPM HP.