Dave Gerr H.M. Pope iii

Matt - I have to confess, I've never really given TE much thought, but surely if the nmpg remains constant for a given load, then the TE ought to do the same...?

 

Can you two guys expand on your thinking a little bit. You've taken this in an interesting direction.

 

Tolly - transport efficiency, as the name suggests, is an attempt to quantify how efficiently a given vessel moves. As I said, I've not given it much thought before, but my understanding is that it should be a means of comparing the efficiency of vessels of varying sizes. There are a number of threads that cover the subject, so maybe we both ought to have a read!

Just based on the few calcs I've made, I'm not so sure how well this particular incarnation of TE translates to boats of different sizes. For instance at 8 knots Graphite exhibits a TE of about 1870. At the other end of the spectrum, my parents just took delivery of an Offshore 64, which runs a pair of 700hp Cat C12's. According to the prop curve she produces about 60hp at 1050 rpm (per engine), which also equates to a speed of 8 knots. This would equate to a TE of 6000....
I know you are going to ask, so if you run her flat out at a fuel guzzling 22 knots, she consumes about 135 litres per hour per engine....
This calculates to a TE of 1400... better than both Graphite and Tom's BJ24 !!)

 

You're right, I need to go back, chase down those threads, and read them. It's a bit humbling that if this has been discussed in much detail I seem to have either overlooked it or glossed it over.

 

What is being overlooked in this simple discussion of TE is that the actual load must be considered. Graphite displaces about 7000lbs in light trim and Pope displaces about 19,500lbs in the same condition. Now, the job of both boats is to transport two to four people and their gear across a body of water. While the TE of both may calculate to a similar number, it should not take our rocket scientist to figure out which does the job with more realistic efficiency.

It reminds me of when our guv'ment decided to impose fuel mileage restrictions on cars back in the 1970's. They did it by rating cars on fuel use per ton-mile. Of course, the big and heavy iron got a break while the small and truly "efficient" cars got hammered even though they were both doing the same job of moving a couple passengers down the same road.

It's the same thing here. The TE, by the definition we are using, is only meaningful when compared to other boats of similar displacement doing a similar task. In an engineering sense, real efficiency is always the same. That is: (work out)/(power in).

 

This is some old writing (by me) on the subject of Et.......
The figure below is by Donald Blount....


There is a very useful comparison of speed/power/weight called transport efficiency (Et ). With this coefficient, plotted against a dimensionless speed, we can compare the efficiency of displacement, semi-displacement, and planning hulls.

Transport Efficiency takes into account not just the power required (in effect MPG), but also the weight (displacement in the case of a boat) being moved, and the time required to move that weight (speed).

Et = W * v / Pd * 550

W is operating displacement, in pounds.
v is speed in feet per second, (knots * 1.6889).
Pd is the horsepower required for the above v.

For Rambler (Harry Bryan design) I guessed at a cruising weight of 1600 pounds.
6 miles per hour is 5.2 knots is 8.798 feet per second.
I took the power as your stated 3.2 HP.

Et = 1600 * 8.798 / 3.2 * 550
Et = 7.998

Plot this against Volume Froude Number, (see WoodenBoat # 137)

FNv = v / (g * disp.^.666)^.5

v is speed in feet per second.
g is acceleration due to gravity, (32.2)
disp. Is displacement, this time in cubic feet.

FNv for Rambler is .53 . This is about mid-range for displacement boats. Displacement hulls operate at a FNv of 1.3 or less, semi-displacement hulls at FNv of from 1.0 to 3.0, and planning hulls at FNv of 2.3 or more.

Piquant, designed by Francis Herreshoff and W.J. Strawbridge in 1950 was close to the ultimate in performance for a displacement hull. She was 47’ by 6’6” and double ended, displacement was 6700 pounds. Twin 35 HP gas engines burned 3 gallons of gas per hour for a cruising speed of 16 knots. Top speed was 20 knots. I calculate her top speed Et at 5.88 and FNv at 1.26. A fairly high speed is possible if the boat is very light.

In the early 1970’s Bill Garden designed Clam, which he termed a doomsday cruiser. She is 42’ by 8’ and of very simple vee-bottom plywood form. Her power was a Gray 4-69 gas engine, developing 15 HP @ 1800 RPM. Cruising displacement would be about 9300 pounds and she has bunks for 4, an enclosed head, and galley. Mr. Garden calculated a cruising speed of 6 knots using 10 HP, 1575 RPM, using one gallon of gas per hour. I calculate an Et of 17.13 at a FNv of .34. A big boat going very slow can be very efficient.

Another efficient hull is Phil Bolger’s Slicer, 29’ by 5’2”, a plywood vee-bottom hull of very light construction. Her displacement is about 1700 pounds and she achieved 18 knots with a 15 HP outboard. This is an Et of 6.26 but at a FNv of 1.79, which is well up into semi-displacement territory.

Just for fun I ran the calculations on a typical SUV to see how it would come out. A 5000 pound SUV (might get?) 25 MPG on the highway, we’ll call that cruising speed. Working backwards I calculate about 56 HP is required to maintain 65 MPH. That gives me an Et of 13.9 at a FNv of 3.92. This is a transport efficiency far better than the best planning hulls.

Running through the published figures for the Hinckley 29 I come up with an Et of 1.81 at a FNv of 2.11. This is below the efficiency of the best planning boats and also below the performance of her big sister, the 37’ Picnic Boat. This probably reflects the smaller boat’s higher beam/length ratio.

Looking at performance figures (37 mph for 200 HP) and guessing a weight of 5000 pounds for the Parker 23 gives some interesting results. I get an Et of 2.46 at a FNv of 2.23. This is better than the Hinckley and is probably a reflection of the greater propulsive efficiency from the outboard as compared to the waterjet.

The ultimate Et is a moving target as improvements are made in hull form and propulsive efficiency. I’m sure all the vessels above could exhibit improved Et with better underbody fairing, different propellers/gearboxes, and optimized forms.

 

I have a bit of a perception problem here. On one hand I certainly understand that in a small boat crew weight is a significant part of the work accomplished moving the hull through the water. In a larger boat crew weight can become a much smaller percentage of the work accomplished, small enough in some cases to fade into insignificance.

I like the idea of a number that can be used to get a quick picture of the relative efficiency of different boats. I can see I still have a bit more studying to do. Hopefully others here will continue to expand on this while the kids at the back of the class struggle to get it.

I see Tad posted while I was writing. Thank you for fleshing this out a bit. Carry on.

 

"The ultimate Et is a moving target as improvements are made in hull form and propulsive efficiency. I’m sure all the vessels above could exhibit improved Et with better underbody fairing, different propellers/gearboxes, and optimized forms."

Indeed, optimization is one of the things that fascinate me. The easiest examples for me to see and grasp involve racing because the measurement of the goal is so easy to focus on.

I am quite pleased how this thread has evolved into a discussion over something that had bugged me for years, the definitions in Gerr's book. I mean no denigration of Dave Gerr. His writing has helped me understand a great deal more than I once did. I hope this doesn't sound too ingratiating but I must say the same for Tom Lathrop's writing on his website about planing, and the behavior of water particles. That gave me a real aha moment.

Gentlemen, by all means carry on.

 

WoodenBoat #206 (feb 2009) carries an article by Dave Gerr titled "The Efficient Powerboat".

All in all I think it's a poorly written piece that really adds nothing to the knowledge pool. Much unnecessarily confusion is added with his effort in "Normalizing" a series of very different designs. This is part of the articles downfall, but also he only talks in theoretical terms, with no reference to what really happens. He uses a bunch of his own designs but no real performance numbers or real weights. Why?

He does use a proper Et number though....even though he does not explain it or explain which units he is using. As it turns out he states....

Et = 7 * V (knts) * Disp / HP

V = speed in knots
Disp is in Long Tons
HP is as required to reach that speed.

This equation comes out very close to the one from Blount which I discussed in my post above.

 

If I'm understanding you correctly these two different equations are arriving at a similar place through different routes.

Et = 7 * V (knts) * Disp / HP
Et = W * v / Pd * 550

 

Neither is dimensionally consistent, though. Either the fudge factor (7 or 550) has to have units associated with it, or the quantity TE or Et has units- we can't use these formulae for comparisons unless we are completely consistent with units and fudge factors in our comparsions.

Will be back with more detail when I get a chance....

 

Yes, they both are essentially the same....

Speed multiplied by weight, divided by power.

 

Gentlemen,

I apologize for inserting my own formula for efficiency into the discussion. That came about after I read Gerr's (then) new book and stumbled on that chapter. I did at least come up with the proper organization of the elements that make up transport efficiency. I'm not sure that it makes any significant difference as long as everyone uses the same expression with the elements of speed, power and displacement in proper order.

I still don't think that TE tells much about real efficiency unless discussing similar boats.

 

It's 0700 here and after working on a computer model 'till after midnight, my ramblings this morning may lack some intelligent thought process...
At 0300 I had an epiphany and realised that I was not comparing like with like... in looking at TE of vastly differing vessels, we should be doing so not at similar speeds, but at similar SL's.
Of course, like many epiphany's I return here to discover that you guys have pipped me to the post

 

More clarification of Transport Efficiency (Et)

Paper by Dean Schleicher of D. Blount and Associates.

View attachment Transporteff.pdf

This one is different again.......Et = Vk * Disp (pounds) / 326 * HP