Prop Demand or Full Power Fuel Curves?

[Deleted User YDKXO]

Following on from DAKA's post below I've got a question concerning calculation of theoretical mpg and range using the engine manufacturer's own fuel curves. I've spent some time recently on my boat collecting accurate speed data at various engine rpm under various load conditions. Using the engine manufacturers own fuel curve giving fuel consumption in lph or gph, I can calculate a theoretical mpg and range at any given rpm
The trouble is that there are generally 2 fuel curves given by a manufacturer for any particular engine, one for the engine operating at full power throught the rpm range and one called a prop load or prop demand curve which reflects the fact that a fixed pitch prop (as fitted to all of our boats) is only ever 100% efficient at one particular rpm, usually designed to be max rpm.

However, as I understand it, a prop demand curve is generally only accurate for displacement speed boats and not planing boats, as confirmed by Cat on this data

So, a question for the learned panel. Which curve do I use, full power or prop demand fuel curves or somewhere in between?

 

[MapisM]

Prop demand, no doubt about it.
The reason why they specify "displacement hull" is because that's closer to ideal conditions, with constant load increase.
With planing hulls, the prop demand would have a peak when the hull is in the transition phase (though still remaining below the max power).
But of course the engine manufacturer doesn't know at which rpm that peak is, and what's its shape, because that depends on the hull.
And btw, the reason for the two curves is not because the prop max efficiency is at max rpm. In fact, on one hand that's not necessarily true, and even when it's true (usually for extreme performance boats), that's not the reason anyway.
But that's another story.

 

[Deleted User YDKXO]

In the case of the Cat 3208 curve, why do the prop demand and full power curves coincide at max rpm if not because the assumption is that the prop is designed for max efficiency at max rpm? If not max rpm, what rpm do you assume when sizing pitch/dia of a prop?

 

[Deleted User YDKXO]

The problem with using the prop demand curve is that, using this data, my fuel consumption at 20knots is calculated at around 1.0mpg and I know for sure this isn't true because I generally average about 0.75mpg and, in any case, 1.0mpg is too optimistic for any 46ft boat let alone a heavy Ferretti.
On the other hand, using the full power curve predicts too low a mpg so that's why I say that the true prop demand curve for my boat must lie somewhere in between these 2 curves

 

[MapisM]

Two separate matters here.
[ QUOTE ]
why do the prop demand and full power curves coincide at max rpm if not because the assumption is that the prop is designed for max efficiency at max rpm?

[/ QUOTE ]Simply because that's the way any prop demand curve is built.
The underlying assumption (which is not always true btw, and not just because of wrong prop choices) is that a boat hull, its propulsion and its engines are designed in such way that at max rpm you're squeezing all the power the block can deliver, getting the max possible speed. And at anything below, the prop load decrease is "faster" than the decrease in the max power the block can deliver.
With a longer prop, the engine would not be powerful enough to reach its max rpm, and the prop load curve would reach the max power curve before the max rpm.
With a shorter prop, the engine would overrev, and the prop load curve would stay below the max power curve at the max rated rpm.
But even a "right" prop, which allows to reach exactly the max rpm, could be designed (blades shape, etc. - not just a matter of pitch/dia) to be more efficient at lower speeds - unless, as I said before, max absolute speed is the primary target.
[ QUOTE ]
If not max rpm, what rpm do you assume when sizing pitch/dia of a prop?

[/ QUOTE ]Max rpm it is, but! (see above).

 

[MapisM]

Whoops, I missed this post of yours while writing the previous reply.
Well, of course since the prop demand is theorical, any boat has its own at the end of the day.
You might also have a boat where the actual consumption curves are better compared to the official prop demand curves.
My short initial reply "Prop demand, no doubt about it" was obviously a tad simplistic in this respect.

PS: in fact, actually a lower exponent factor is sometimes used for planing hulls (e.g. 2.5 or 2.7 instead of the typical 3.0), but that's an assumption anyway. Also, any constant factor can't consider the "peak" of load required by the hull to get over the hump.

 

[Deleted User YDKXO]

Umm, that looks like you agree with me then!

 

[MapisM]

Yup, if you mean on the "somewhere in between" bit - though in my experience the prop load curve is much nearer to the reality than the max power, and not just on displacement boats.
But not on the max prop efficiency at max rpm, anyhow... /forums/images/graemlins/smile.gif

 

[Latestarter1]

'Yep somewhere in between' sounds good to me.

Propellers drive boats NOT engines.

Simple answer to the question regarding why the prop curve HAS to pass though the max Hp curve is real simple props are only efficient at one speed and simple logic is that it must pass though engine Rated Speed. In practical terms it should actually pass just above rated speed to allow a diesel engine to go out on the governor ‘droop’ with full tanks and a clean bottom. This precaution is to allow for weed growth and extra gear.

The problem for the engine manufacturer is that they do not know how a vessel will be propped so they have to give a purely mathematical guessitimate useful for visualising the relationship between engine power and propeller power which actually drives the boat.

Over the years Naval Architects have considered exponents between 2.2 and 3. An exponent of 2.7 is good for almost all medium to high speed vessels. For heavy displacement vessels with high thrust ratio props 3 is good.

However whatever the figure it is still a mathematically educated guess, and it is still good practice to draw a REAL consumption curve from accurate sea trial data.

Data sheets are produced to give an indication of performance for comparing engines however they are a less than perfect tool for calculating fuel consumption as it is still only an educated guess as to how much of the available engine power is being absorbed by the prop .

The demand curve certainly tells you more than the power curve because that is the potential power the engine can provide at a given RPM and not the ACTUAL power being absorbed by the prop.

It is worth remembering that if you overload an engine it will be by the propeller exponent and not as a direct ratio to the rpm. Therefore a propeller preventing an engine to reach Rated Speed by just a few hundred rpm could reflect an engine overload of near 100%.

 

[Deleted User YDKXO]

Lateboater, thanks for your interesting comments. Last night, I looked again at some speed and fuel consumption data given in the manual for my particular Ferretti boat. What I thought was a full power fuel curve for my boat's engines actually turns out to be not so but neither it is the prop demand fuel curve published by the engine manufacturer, Cat. It is a curve that falls somewhere between these 2 curves and, subject to confirmation, I believe that this curve may be the actual prop demand curve for my particular boat. Certainly, using this curve, the calculated mpg figures are much closer to what I actually am achieving

 

[Latestarter1]

Sounds like Ferretti have done the job properly.

We spent hours up and down the Solent generating our own fuel consumption curve when preparing for the RB 2008, however it really paid off on the long legs, getting the speed/range strategy right.