Dave Gerr H.M. Pope iii

Same basic equation, different fudge factor. In Schleicher's paper, it looks like the factor of 326 is to convert from lb*kt/hp to (lb*ft/s)/(lbf*ft/s). The result still has units of 1/acceleration, ie. s^2/ft in this definition (1/g, from the relation between lb mass and lb force).

If we're going to talk transport efficiency, let's find some fudge factors for each combination of units that produce results that can actually be compared. I'll take a closer look at this later when I have time.

 

This is so cool.

 

Matt,

I'm quite comfortable with the Et scale running between 1 and 100, that's because for the past 17 years I've been using the early Blount method outlined in post #36. Will's numbers in the 1000's appalled me...

I agree that a single Et number is pretty meaningless, but once you graph it against the Volume Froud Number (as post #36) it (IMO) starts to seem worthwhile.

 

I agree with Tom that none of these transport efficiency equations is very useful. They do not evaluate the efficiency to transport the payload. A 6 ton boat that transports a payload of 1 ton will get the same number for transport efficiency as a 3 ton boat carrying the same payload, even though the 6 ton boat burns twice as much fuel as the 3 ton boat.

Unfortunately, defining the payload is difficult for a leasure boat. The passengers are definitely payload, but what about fridge, shower cabin, tables etc? The comfort of standing height and large volume, is that payload? If two boats could transport the same number of people with equal comfort at the same speed, the boat using the least fuel to do so would in my opinion be the one with the highest transport efficiency. The weight of the empty boat doesn't matter.

The problem with dividing speed with engine power in the equations is that only fuel consumption per distance unit is evaluated. The equations don't consider the time it takes to get to the goal, only the amount of fuel burnt to get there. Usually time is of essence.

I'm sorry to only be negative, but I don't have a good idea how to compare transport efficiency myself. I hope Matt comes up with something useful.

Erik

 

In just less than two weeks there will be the Chesapeake Powerboat Symposium in Annapolis. The first speaker is Daniel Savitski on "Efficient powerboat concepts". Perhaps some light will be shed there. I will be there and hope to learn something of value from a talented list of speakers.

http://www.powerboatsymposium.com/home.html

In the end, this discussion of Te or Et or whatever we call it, has to be somewhat subjective. In the special case of cargo carriers or other similar craft, it seems a more straightforward and useful tool. For pleasure boats, not so much.

 

I disagree completely with your conclusions....

See Dean Schleicher's paper (posted above) for definition and equation for ULF, Useful Load Fraction. Load is what you are discussing. Again when ULF is graphed against FNv it creates a useful matrix of data.

If your 6 ton boat burns twice the fuel, it will require twice the horsepower, thus it's Et number will be very different, not "the same" as you state.

I don't think defining the payload has anything to do with this problem. If a boat of a given weight and power takes so long to get from A to B, and I can define that action with one number, that's handy info. If I do the same calculation for two boats of similar hull form, and my data is precise, I'll get two different numbers....thus I may learn something...that's really handy.

Transport Efficiency (Et) is not calculating how much a given boat can move, it is calculating what it takes to move a given boat....nothing more.

Speed is part of our equation, thus the time involved certainly is being evaluated.

 

I need to read through Dean Schleicher's paper a few more times to understand the proper use of Transport Efficiency, Et. My somewhat flawed understanding of the concept came from "The Nature of Boats". In "The Nature of Boats" a few different boats are each attributed with an Et number and compared. From Dean Schleicher's paper, Et is a function of speed (figure 2 in the paper), and I still don't see how a hull can be described with a single Et number.

So far my understanding is that the Et vs. FnV graphs for state of the art hulls can be used as a performance reference for new hull designs. I'm struggling to see if Et can improve my understanding, but that problem might be with the receiver and not the sender of the information.

Erik

 

Not my numbers... I was just calculating Et based on the formulae presented. I think there's little doubt that Et can be a useful tool, particularly for commercial or long-range recreational craft. Probably even for designers of a wide variety of boats. I can't see it catching on widely in the consumer arena, however, so it will probably remain a somewhat obscure term - especially as there are so many variations on the formula in use.

 

It took me some time, some rereading of previous posts and enclosed documents plus some thinking, but now I see the point with an Et graph plotted against FnV. Et plotted against FnV makes it easy to compare hull shape movement efficiency for hulls scaled to the same displacement. The key is to have a database with power vs speed for a variety of hulls. Et vs FnV is one way to look at that data.

Erik