Calculating "Load" - Need answer

[Howard W. Burns]

Guys often refer to a boat/engine(s) running at "X% of load." How is that "load" calculation made? Thanks for the help.

[JD5652]

% Load usually refers to the design WOT (Wide Open Throttle) RPM as compared to the "actual" engine RPM. This is calculated at load (meaning transmissions is in gear and engine running) versus unloaded which means transmissions in nuetral and engine running.

For example, a 671-TIB engine design WOT (Loaded ) is 2500 RPM. The book cruise on that engine is 2250 RPM or 90% load.

You get the 90% by dividing the 2250 by 2500 ....

Mathematically......2250/2500 = 0.9 or 90% Load

If you wanted to run this engine let's say at 80% load, then you multiply 0.80 (or 80%) x 2500 = 2000 RPM

I knew that engineering degree would pay off one day.....

[Fritz]

[QUOTE=JD5652;852735 I knew that engineering degree would pay off one day.....[/QUOTE]

Ah, but can you describe the decay chain of Uranium 238 and all its daughter products?

[Howard W. Burns]

Thank you for the explanation. Understood and much appreciated.

[Nomad]

% Load usually refers to the design WOT (Wide Open Throttle) RPM as compared to the "actual" engine RPM. This is calculated at load (meaning transmissions is in gear and engine running) versus unloaded which means transmissions in nuetral and engine running.

For example, a 671-TIB engine design WOT (Loaded ) is 2500 RPM. The book cruise on that engine is 2250 RPM or 90% load.

You get the 90% by dividing the 2250 by 2500 ....

Mathematically......2250/2500 = 0.9 or 90% Load

If you wanted to run this engine let's say at 80% load, then you multiply 0.80 (or 80%) x 2500 = 2000 RPM

I knew that engineering degree would pay off one day.....

With all respect, I don't think that is correct. It would imply that an engine running at 2500 rpm (in the example) is under 100% load, whether the boat is at the dock in neutral, or really straining with a dirty bottom and overpropped. Instead, load refers to how much horsepower the engine is putting out in relation to how much it is capable of putting out, at any given RPM. Usually, the output horsepower is determined by fuel consumption. If the engine is running right, there is a direct relationship between fuel in and horsepower out. In my example, an engine at the dock, but full rpm, would show little load (as it take very little fuel to run an unloaded engine up to full rpm). Conversely, even at much lower rpm, it is possible to "overload" an engine, by imposing a load that is too close (say 90% or more) of the max hp the engine is capable of producing at that rpm. Modern electronic diesels calculate load this way, and the information provided is very valuable to making sure that an engine is not overloaded at any point in its rpm range. Often times, the hp required to get a hull on plane imposes an unacceptably high load.
Hope this helps.

[Howard W. Burns]

Yes, I'm not ashamed to admit it -- I'm now confused. Does anyone have the definitive answer to the question?

[JD5652]

With all respect, I don't think that is correct. It would imply that an engine running at 2500 rpm (in the example) is under 100% load, whether the boat is at the dock in neutral, or really straining with a dirty bottom and overpropped. Instead, load refers to how much horsepower the engine is putting out in relation to how much it is capable of putting out, at any given RPM. Usually, the output horsepower is determined by fuel consumption. If the engine is running right, there is a direct relationship between fuel in and horsepower out. In my example, an engine at the dock, but full rpm, would show little load (as it take very little fuel to run an unloaded engine up to full rpm). Conversely, even at much lower rpm, it is possible to "overload" an engine, by imposing a load that is too close (say 90% or more) of the max hp the engine is capable of producing at that rpm. Modern electronic diesels calculate load this way, and the information provided is very valuable to making sure that an engine is not overloaded at any point in its rpm range. Often times, the hp required to get a hull on plane imposes an unacceptably high load.
Hope this helps.

Nomad-

Please take a closer look at my explanation, for load it states that the "transmission is in gear with the engine running".....unloaded = transmissions in neutral with engine running. A motor can not be loaded unless the transmission is in gear. Unloaded WOT RPM will be a different value than loaded RPM. In my example of the 671, design unloaded WOT RPM is 2650-2700. An engine can not create HP without a load attached.

Modern electronic diesels may calculate HP based upon fuel consumption based upon calculations performed by the engine's computer. However, HP is HP and the only way to "actually measure" HP is to put the engine on a dyno and measure the HP output at the motor crank. For marine engines, HP is generally rated in Shaft HP (SHP) which is the HP measured at the end of the prop end of shaft and accounts for losses in the running gear.

As you stated, it is true that a marine engine can be "overloaded" due to a number of things including dirty bottom, incorrectly sized props, etc. However, in the above example I was trying to make the explanation very simple assuming that the boat is correctly propped, bottom clean, etc.

I think we're saying the same thing in different words.

Howard, I think you can use those simple calculations for your needs. Attached is a sample performance curve for a 671-TIB that might make it clearer.

[JD5652]

Ah, but can you describe the decay chain of Uranium 238 and all its daughter products?

Fritz-

Wow!....maybe back in the day...but I retired by pocket protector a long time ago...what about "departure from nucleate boiling" (DNB)....

[Nomad]

Nomad-

Please take a closer look at my explanation, for load it states that the "transmission is in gear with the engine running".....unloaded = transmissions in neutral with engine running. A motor can not be loaded unless the transmission is in gear. Unloaded WOT RPM will be a different value than loaded RPM. In my example of the 671, design unloaded WOT RPM is 2650-2700.

Modern electronic diesels may calculate HP based upon fuel consumption based upon calculations performed by the engine's computer. However, HP is HP and the only way to "actually measure" HP is to put the engine on a dyno and measure the HP output at the motor crank. For marine engines, HP is generally rated in Shaft HP (SHP) which is the HP measured at the end of the prop end of shaft and accounts for losses in the running gear.

As you stated, it is true that a marine engine can be "overloaded" due to a number of things including dirty bottom, incorrectly sized props, etc. However, in the above example I was trying to make the explanation very simple assuming that the boat is correctly propped, bottom clean, etc.

I think we're saying the same thing in different words.

Howard, I think you can use those simple calculations for your needs.

We are not saying the same thing, notwithstanding your disclaimers about in-gear, etc. Your explanation would be correct only if two conditions are true 1) the WOT load is 100% -- ie, the engines make full rpm underload, but they are not underloaded -- this is true only if the boat is perfectly propped and the bottom (and sea) conditions never change, and 2) the engine's horsepower curve perfectly matches the prop curve -- that basically cannot happen.

If those were both true, then by coincidence the load would be proportionate to rpm. That is not the real world. In reality, a boat can be more than 90% loaded at well below 90% of max rpms.

Whether your formula is a close enough approximation, it really depends on what Howard needs the info for. If his concern is that the engines never be overloaded, then he is probably safe just by confirming that the boat can make full rpm (not at the dock, but under way). (The physics of horsepower required to move a boat through water, in combination with the hp/torque curves of modern marine diesels are such that the greatest load is experienced at full rpm.) After that, its just a matter of complying with the manufacturer's use restrictions, which are typically stated in terms of rpm's, not load.
On the other hand, if the load info is to be used as a diagnostic, then a more precise understanding and approach would be helpful.

[Nomad]

JD:
I didn't see the power curves before sending my earlier reply, but I think it demonstrates my point. It shows max rpm at 2500, and shaft hp of 400 at 2000 rpm. Using your formula (as I understand it), that would be 80% load (2000/2500). Yet, the prop curve shows a hp demand at 2000 rpm of only about 250 hp, which is only 63% of the hp the engine could produce at that rpm. Thus, in my opinion, load at that point is 63%, not 80%.

Interestingly, at 90% rpm (2250 rpm, which your formula equates to 90% load), the prop demand is about 350 hp while the engine is capable of producing 440 shaft hp at that rpm, indicating only an 80% load at that 90% rpm.

Hope this helps,
Rick