Prop power curve

When I look at published diesel motor power curves, they have two curves, one for shaft and one for prop. Can anyone explain the difference between the two?

If drag calculations indicate that my boat needs 160 hp to achieve top cruising speed, can an appropriate motor be selected from the published prop power curves? Or do I need to factor some other significant inefficiencies in there?

 

I have never seen prop and shaft HP published as different values. They are equivalent. If the boat needs 160 HP for cruising speed, the engine should be rated at about 175HP. Are you sure the two curves are not HP and torque?

 

Take a look at this spec sheet from Yanmar - look at the power chart:
http://www.yanmarmarine.com/products/pdf/JH/4JH3_DTE_TechData.pdf

It's typical of every marine engine power chart that I've looked at.

 

There are a couple of different things that need clarification when looking at a published engine curve like the one that you posted. Let me see if I can shed some light on what is published.

First, the engine is a power "generator", and you can think of the "maximum output" curves as "potential power". The two max output curves relate to the same potential engine power, just at different points on the power train. The "crankshaft" power is measured at the engine's flywheel, and is more commonly known as "brake power" (i.e., Pb, BHP). The torque curve represents the same energy as brake power, just in terms of torque rather than power. Attach a gear box with something like a 3% loss, and you have the "prop shaft" power at the output side of the gear box, which is known as the "shaft power" (i.e., Ps, SHP). Shaft power was the traditional engine output measure for most manufacturer's published spec sheets. It has only been in the last 10-15 years or so that brake power is universally also shown with shaft power.

(BTW - it appears that the labels on the Yanmar plot are backwards. Brake power is always a larger magnitude than shaft power.)

These curves represent the potential of the engine, but the actual generated power is really a function of what is attached to the engine. This is where the "propeller power curve" comes in.

Let me digress for a moment. One of the many misconceptions about engines is that if it is running at rated RPM, then it is also generating rated power. This is not necessarily true. For example, picture a boat in the water running at rated RPM, and consider the power that the engine will produce to spin the propeller. Now, take that boat out of the water. How much power does the engine produce to spin the propeller in air at rated RPM? Much, much less than in water. Same RPM, significantly different power generation. The point is that you need to look at what is attached to the engine (e.g., the propeller) to determine the power that the engine will be producing.

When you take a boat's drag a particular speed, and match a propeller thrust and torque to that point, you can derive the amount of power that an engine will produce to spin the propeller. The prop curve is a simplistic representation of what the power-RPM curve might look like if the propeller were sized exactly for rated power and RPM with no margins. In other words, the prop curve is a completely fictional, idealized curve that does not represent any particular boat, but is intended to generically represent all boats.

It is a reasonably close approximation for slow speed displacement hulls with conventional propellers. However, it is completely unsuitable if you have

a. waterjets, surface drives, or highly-cavitating propellers
b. semi-displacment boats, planing hulls or catamarans
c. boats under high thrust conditions, such as tugs or trawlers
d. dynamic loading such as acceleration
e. a propeller sized with a power margin (e.g., 85% MCR)

I have been trying to get engine manufacturers to change their naming of this "prop curve" for almost 15 years - and I've had some success with the companies that we work with. This curve is a simple function of RPM-cubed (a pump law relationship), and is found from a dyno test in the plant. Some companies are now calling it what is really is, a "cubic demand curve". It has very little real usefulness, with one exception - it does give a pretty close measure of fuel consumption for a particular operating power. Unfortunately, you cannot use this curve to find fuel rate from RPM (which is one thing that you can accurtely measure), only for power. We use it in our software to help improve the prediction of fuel consumption, but that is about all its good for.

The maximum output power curve is actually more important from a design perspective. The shape can often dictate how well a boat passes through the hump speed range, or how much towing thrust it can generate. There is a publication that you can grab from the Knowledge page on our website that goes into more detail about this. Look for "Special Performance Considerations for Boats with Electronic Control Engines" at http://www.hydrocompinc.com/knowledge/library.htm.

Regards,

Don MacPherson
HydroComp, Inc.

 

Don,

That shed a whole lot of light! I've read through about a thousand web sites trying to answer that question (I'm a bit slower than most...) - your response was the best I've seen. Thank you!

The article you referenced also was helpful. A bunch of the other articles you have listed on the Hydrocomp website looks interesting too - I've bookmarked you for later consumption.

So I'm still left with my original problem. I've developed a hull for a displacement catamaran (~45ftx3ft - 16,000 lbs disp). Targeted cruising speed is 20 kts. I've run it through Michlet and come up with a hull resistance curve all the way up to 30 kts. Ignoring wind and wave resistance for now, how do I take those resistance values and select an engine/prop combination (and thus determine fuel consumption)? I'm planning to use controllable pitch props.

Does Hydrocomp provide that type of consulting service? I might be interested in getting a professional involved to get this answer right.

Bob Deering

 

Bob:

Thanks for your kind words. We do our best...

We do indeed provide these types of consulting services. It's a large part of our business. Pop me an email through the web site, and we can start a communication on the subject.

Regards,

Don MacPherson
HydroComp, Inc.

 

One other observation: Crank HP is measured without any accesories like alternator, raw water pump, etc. Also, it is rated at 20C air temperature. You should derated to the expected engine room temperature.