Comments about Fuel usage

OK...

Before I go off and point out where his simple assumption made him produce a very wrong result and piss someone off, do you know this author?
 
OK... I found out a little about this guy:

In the early '70s, Hargrave dipped his design pen into big-ship waters with an innovative catamaran tug and barge system. Dudley Dawson, Yachting's Technical Editor and a naval architect in his own right, joined Hargrave's design team in 1974 to work on this 625-foot tanker. The finished design was patented and two "catugs" were built initially: one to carry petroleum products and the other with tankage for a variety of chemical products. Eventually, an additional 18 ships were built from Hargrave's patented concept.

I'm really surprised that a guy who is a "naval architect in his own right" could make such a rookie mistake of essentially saying drag on a planing hull behaves like a displacement hull and then goes and gives calculations based on that assumption and distribute them to the world in a magazine... This problem and physics are essentially the difference between putting a wing vertical in the water and looking at drag at slow speeds (and not worrying about weight) versus putting a wing horizontal in the water and varying it's angle of attack to hold up the weight... big difference.

Maybe I'll write to the guy... See what he says.
 
Last edited:
The author probably figured that it sounded good, and was close enough for the average yachtsman. He probably never figured on guys with a working knowledge of physics or fluid dynamics reading it.

Yachtings bio...
DUDLEY DAWsOn
Technical Editor
A licensed professional engineer and naval architect, Dudley started out in the Merchant Marines as an active duty officer in the U.S. Coast Guard. For over 20 years, Dudley has headed his own design and consulting firm. He brings his expertise to Yachting’s reviews of new megayachts and more.
Click to expand...
 
Well... What he published for a planing hull is flat out wrong... I'm curious to see what his response is.
 
Although the math has been dumbed down, the author is correct for displacement hulls. Reduction in velocity will create fuel consumption savings on displacement hulls. The shipping industry has been practicing this for the past 18 months, calling it 'slow steaming', where they have dialed back to below 20 knots for containerships on the Asia-US and Asia- Europe trades.

Put the article in context. Yachting Magazine tends to cover topics about 60' and longer yachts and super-yachts, and these tend to have displacement hulls. The guy was writing to his readership.
 
Henry Boyd said:
Although the math has been dumbed down, the author is correct for displacement hulls. Reduction in velocity will create fuel consumption savings on displacement hulls. The shipping industry has been practicing this for the past 18 months, calling it 'slow steaming', where they have dialed back to below 20 knots for containerships on the Asia-US and Asia- Europe trades.

Put the article in context. Yachting Magazine tends to cover topics about 60' and longer yachts and super-yachts, and these tend to have displacement hulls. The guy was writing to his readership.
Click to expand...

I agree about the math for displacement hulls but his major error was he transferred this math and physics to planing hulls and directly discusses sport fishing boats, 18 knots vs. 20 knots, etc. per the articles. His target audience is not owners of boats that displacement hull speeds are 20 knots (that would be a 200+ foot boat). A 60-100 foot boat does not have a 20 knot displacement hull speed.
 
Last edited:
Pseudomind said:
How about this article? i think this may apply to larger boats, or it may apply to those around 40 foot or longer

Engine Performance Curve

:huh:
Click to expand...

Thanks for the replies. I for one had no intention to denigrate anyone, but posted as a learning tool. Apparently, I must of had the incorrect belief that some members on CSR are here to learn something.:huh:

I also will admit I was guessing about the 40 feet in my statement above, I did feel he was referring to larger boats. I just did not know the minimum size

I always strive to learn and try to continue to learn. I was not lucky enough to, nor had the privilege of being born with all of the knowledge of the world, and the luxury of a "Cleaver" home life during my upbringing.

But , there is one thing I always push, 'Learning," perhaps if more did so instead of beating their chest, half the crap happenig in the world today would not exist. Yes, I said, "Crap."

:thumbsup:
 
anything between 8mph and 22mph on my boat is very inefficient. Between 1200 and 1800 rpms produces almost NO speed increase, just wake size. right at my plane speed, a 7% drop in rpms will result in a 46% drop in speed. Don't know anything about fluid dynamics or hull designs, all I do is look at the percentages to determine where i'm going to run my boat if conditions allow.
 
Pseudomind said:
Thanks for the replies. I for one had no intention to denigrate anyone, but posted as a learning tool. Apparently, I must of had the incorrect belief that some members on CSR are here to learn something.:huh:

I also will admit I was guessing about the 40 feet in my statement above, I did feel he was referring to larger boats. I just did not know the minimum size

I always strive to learn and try to continue to learn. I was not lucky enough to, nor had the privilege of being born with all of the knowledge of the world, and the luxury of a "Cleaver" home life during my upbringing.

But , there is one thing I always push, 'Learning," perhaps if more did so instead of beating their chest, half the crap happenig in the world today would not exist. Yes, I said, "Crap."
Click to expand...


Ever hear of "Peer review?" This is pretty restrained compared to some peer reviews. There is an entire process as to how knowledge advances. The rebuttals here aren't that far off when faulty knowledge is proposed.

If the "authorities" were not challenged people would still believe the earth orbits the sun and there would be no theory of relativity, for two examples.

Best regards,
Frank
 
jg300da said:
I'm no authority but I'll assume this is a typo.
Click to expand...

Actually.... the earth does not orbit the sun... everything orbits around the center of mass of the solar system which is just outside the radius of the sun... :grin: That's how they find out if distant stars have planets by looking at the "wobble" of the star.
 
Just where is the faulty knowledge per se, maybe there needs to just be a clarification of what knowledge applies where?

And then there is the question(s) of who is actually right, maybe neither, both the writer of the article(s) and comments made here.

But then again, if no questions are asked, what does one ever learn?

Maybe neither of these articles apply at all for boats for CSR members. How many CSR members have boats longer than 60 feet?


Henry Boyd said:
Although the math has been dumbed down, the author is correct for displacement hulls. Reduction in velocity will create fuel consumption savings on displacement hulls. The shipping industry has been practicing this for the past 18 months, calling it 'slow steaming', where they have dialed back to below 20 knots for containerships on the Asia-US and Asia- Europe trades.

Put the article in context. Yachting Magazine tends to cover topics about 60' and longer yachts and super-yachts, and these tend to have displacement hulls. The guy was writing to his readership.
Click to expand...

fc3 said:
Ever hear of "Peer review?" This is pretty restrained compared to some peer reviews. There is an entire process as to how knowledge advances. The rebuttals here aren't that far off when faulty knowledge is proposed.

If the "authorities" were not challenged people would still believe the earth orbits the sun and there would be no theory of relativity, for two examples.

Best regards,
Frank
Click to expand...
 
The easiest way to look for planets around other stars is to notice their gravitational effect on the stars they orbit. One signature of a planet would be that the star would appear to wobble about as the star and the planet orbit a point situated between them, proportionally closer to the more massive star, called the center of mass. Our Sun wobbles because of the gravity of the planets orbiting it. Most of the wobble is due to Jupiter which contains more mass than all of the other planets combined. However, the wobble is tiny! Because the Sun is over a thousand times more massive than Jupiter, the center of mass is over a thousand times closer to the Sun, or about 47,000 kilometers above the surface of the Sun (this distance is less than 7% the radius of the Sun). Just keeping the train off the track!!!
 
Last edited:
Aarrhh & Aarrhh said:
or about 47,000 kilometers above the surface of the Sun (this distance is less than 7% the radius of the Sun). Just keeping the train off the track!!!
Click to expand...

How can something be above the surface of the Sun but yet be less than 7% of the radius of it? Is this a trick question?

Back on topic... I think it's good to bring up these articles here for debate. I hear people talking all the time about "back it down to save fuel" and it's just not always true... It's clear from reading both those articles there are targeted towards the planing hull crowd and the guy just didn't do a good job... but he'll be viewed by many as the "authority"...
 
Four Suns said:
Actually.... the earth does not orbit the sun... ......
Click to expand...

Hey...you mean I was right on that 6th grade science test!!!! I want a re-count....I could've gotten an 'A' on that exam..:smt089..Where's Sister Joseph Mary now ey?
 
Last edited:
osd9 said:
Hey...you mean I was right on that 6th grade science test!!!! I want a re-count....I could've gotten an 'A' on that exam..:smt089..Where's Sister Joseph Mary now ey?
Click to expand...

The reason you failed the test Dom was because you said "Sister? Uranus is so big it's causing the Sun to wobble... " although correct... you can't say that to Sister Joseph Mary.


I still think the world is flat... I've been off the Jersey coast and I really think you could fall off.
 
Last edited:
Just goes to show you gotta question everything. Like why would a Sister be named Joseph...
 
Four Suns said:
How can something be above the surface of the Sun but yet be less than 7% of the radius of it? Is this a trick question?

Back on topic... I think it's good to bring up these articles here for debate. I hear people talking all the time about "back it down to save fuel" and it's just not always true... It's clear from reading both those articles there are targeted towards the planing hull crowd and the guy just didn't do a good job... but he'll be viewed by many as the "authority"...
Click to expand...

Good point this debate has been worth reading, although some of which is way over my head.

The bottom line comes down to the question, does slowing down save you fuel? In all the crude tests I have run with my boat I can say the answer is no, but that’s me and my boat.

Besides if I was truly worried about the amount of fuel I use, I would not have bought a bigger boat.:thumbsup:
 
You must log in or register to reply here.

Forum statistics

Threads
113,196
Messages
1,428,323
Members
61,103
Latest member
Navymustng
Back
Top