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SOT: Naval Architects of Lore May Have Been Right After All

 
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SOT: Naval Architects of Lore May Have Been Right After... - 6/1/2015 3:16:00 AM   
witpqs


Posts: 26087
Joined: 10/4/2004
From: Argleton
Status: offline 		Nice pic of the USS Texas, too.

Naval Architects of Lore May Have Been Right After All

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RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/1/2015 4:11:39 AM   
jmalter

 

Posts: 1673
Joined: 10/12/2010
Status: offline 		Innerstin', I'd thought that the bulb-bow idea was adopted to improve propulsion performance, but obvs it increases bow buoyancy (to even out the stress along the ship-beam), while adding room for nose-mounted sonar, or mebbe a front-end steering motor. But concepts like 'hull prismatic coefficient' are a bit above my (unpaid) pay-grade, I'm happy enough if the boat stays dry-side upright.

'Tis a nice photo of USS Texas - those fighting tops on the tripod foremast are 'pure dreadnought'.

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Post #: 2
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/1/2015 6:16:05 AM   
obvert


Posts: 14050
Joined: 1/17/2011
From: PDX (and now) London, UK
Status: offline 		Very cool. I always thought they looked odd, but kind of proud, leading with a protruding bow.

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Post #: 3
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/1/2015 2:51:11 PM   
Symon


Posts: 1928
Joined: 11/24/2012
From: De Eye-lands, Mon
Status: offline 		I’m a bit skeptical. And a bit annoyed, but more at the idiot who wrote the article. There are no 100 year old myths to debunk, here. Software is a nice tool, but overated.

I certainly agree that there’s a great deal we don’t fully understand about their functional hydrodynamic qualities. But many permutations of bulbous fore-bodies have been extensively evaluated in empirical model/tank testing for decades, all over the world: Taylor Basin, Stevens Institute, NJ; Maritime Research Institute (MARIN), Netherlands; CEHIPAR, Spain; Bassin d'essai des Carènes, France; HSVA, Germany; SSPA, Sweden, etc..

The empirical test results are unambiguously clear and have been verified in actual ships. There was a test performed on identical C2 cargo ships, some with various bulb configurations, some with a stock bow, by Todd Shipyards (now Vigor) in the mid ‘60s under Ralph Anselmi. Once again, the results were unambiguously positive. The different bulb configurations helped promote an understanding of just what the heck was going on. Many, many things are happening, and many, many things are still in dispute (the math/understanding just isn’t there).

“Roughly”, and I do mean roughly, some things are “known”. One thing, certain, is that the “known” effects relate to the reduction of wave-making resistance, and are most pronounced at the first Froude limit.
1) a bulb adds an arbitrary volume which allows for a consequent reduction in volume in the remaining bow sections, in turn, allowing a finer entry with reduced wave-making resistance while maintaining coefficients (@jmalter, there’s where your prismatics come to play pal).
2) depending on shape, length, etc.. a properly designed bulb sets up an interference wave pattern that destructively interferes with W-1 at Fr-1 and reduces wave-making resistance by “spoofing” a larger [Sqrt]L.

Obviously, if one is below the Froude limit, wetted surface area is the prime determinant of hull resistance. A bulbous bow at low speeds don’t hep ya none, might even hurt ya some (but even here, the volumetric compensations limit the effects to negligent quantities). But for straight line tracking, at-speed, it demonstrably works. It works best at that break point in the power curve at Fr-1. For stuff like this, the math is simple, just plug in a slightly higher [Root]L and you get either lower Power for the given V, or a higher V for the given Power.

Sorry for the pedantic nonsense. It's just that this is very interesting to me and is probably more important to me than my profession.

Ciao. JWE

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RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/1/2015 3:56:52 PM   
witpqs


Posts: 26087
Joined: 10/4/2004
From: Argleton
Status: offline 		Not nonsense, John, your skepticism is justified. I found the article along with a comment thread on it and one of the comments made some similar points, but he cited his source of info (he did not claim to have a background in the material) as a Wikipedia article. I thought about including a link to the comments page but didn't.

I noticed that the design currently under consideration is a hybrid, not the old design. I further noted that they still have a long way to go with actual testing as opposed to just computer modeling. That made me consider not even posting the link here, but they also claim to have one cruise ship in service with an earlier version of the design that is performing well and that caused me to include the article.

As far as the presentation, I find it (sadly) what I consider standard 'journalism'. What the researchers are looking at is not debunking a 100-year old myth. One of the comments (that I didn't link) pointed out that a contemporary design in competition with the Texas' design was an early form of bulbous bow, so naval architects have been working to suss out the stubborn details for a while. But that's not catchy. Headlines a little closer to "Alien Artifacts Made Ancient Ships 10x More Efficient Than Today" seem to be more in vogue!

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RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/1/2015 4:04:15 PM   
Symon


Posts: 1928
Joined: 11/24/2012
From: De Eye-lands, Mon
Status: offline
quote:

ORIGINAL: jmalter
Innerstin', I'd thought that the bulb-bow idea was adopted to improve propulsion performance, but obvs it increases bow buoyancy (to even out the stress along the ship-beam), while adding room for nose-mounted sonar, or mebbe a front-end steering motor. But concepts like 'hull prismatic coefficient' are a bit above my (unpaid) pay-grade, I'm happy enough if the boat stays dry-side upright.

First, thanks to @witpqs for his support.
Uh, nuh .. nuh .. n .. n .. no, not really. The bulb is immersed, so it is buoyancy neutral. Yes, if you arbitrarily stuck a bulb on a ship (think in terms of a “camel”) it would float different. But ships designed with bulbs float on their design trim lines and don’t have any ‘special’ buoyancy characteristics. I know this sounds a bit counter-intuitive, but it really isn’t if you think about it. There are many interesting counter-intuitive things in life, the universe, and everything.
Can say a lot more, but … Ciao. JWE
@witpqs, thanks. Would ya'll really like to know more?

< Message edited by Symon -- 6/1/2015 5:20:16 PM >

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RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/1/2015 4:20:10 PM   
Andav

 

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I thought it was a beautiful picture of the Texas surrounded by a bunch of words.

Wa

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Post #: 7
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/1/2015 4:29:02 PM   
Symon


Posts: 1928
Joined: 11/24/2012
From: De Eye-lands, Mon
Status: offline
quote:

ORIGINAL: Andav
I thought it was a beautiful picture of the Texas surrounded by a bunch of words.
Wa

Dude !!! Utter frikkin gloss !!! USN Texas still here amongst us !!! I'll let it go here. Ciao. JWE

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Post #: 8
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/1/2015 5:05:22 PM   
tiemanjw

 

Posts: 580
Joined: 12/6/2008
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quote:

ORIGINAL: Symon

Would ya'll really like to know more?



I'm not a naval architect, but I find the subject fascinating. If you go on, I'll keep reading. I appreciate all you've said already.

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Post #: 9
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/2/2015 12:48:08 AM   
pontiouspilot


Posts: 1127
Joined: 7/27/2012
Status: offline 		While we have the attention of PEngs or architects I have a question about the decks of US carriers. What was the rationale for wooden flight decks? What was 1st US CV with non-wooded deck? Why didn't they start at least plating the wood during the kamikaze heyday?

Final question: I assume the Japanese also used wooden decks...perhaps balsa!!

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Post #: 10
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/2/2015 1:15:36 AM   
wdolson

 

Posts: 10398
Joined: 6/28/2006
From: Near Portland, OR
Status: offline 		Balsa would have been too soft for a carrier deck.

Wood does make a better traction surface than steel, so they was probably a factor. Wood also flexes more, so it would absorb more of the force of impact and save wear and tear on airframes.

When the kamikaze attacks started, the US didn't have the time to spare to take their carriers out of service to put new decks on them. Plus the carriers were already top heavy and putting more steel that far up on the ship would have made it a lot worse. Additionally, wood decks, while damaged from kamikaze impacts were also fairly easy to repair, compared to steel. A lot of kamikaze impacts were repaired fairly quickly. If they didn't start a fire, or impact something important and fragile, the decks could be repaired in a fairly short stint in the yards. Sometimes even at sea.

On the other hand a penetrating hit on a steel deck would require cutting out the damaged steel and replacing it with new sheets of steel. The RN carriers survived kamikazes better because the flight deck was the armor deck (US carriers had the armor deck the base of the hanger deck). This did prevent damage, but at a high cost. Because the carriers had armor higher up on the ship, sacrifices had to be made, most important of these was the size of the air group and hangers. This limited the usefulness of RN carriers in the Pacific where a large air group was very important. Additionally, hits to the armor deck could warp the structure of the entire ship to a point where it could continue operating, but would limit the life of the ship. I know at least one RN carrier was retired immediately after the war because the structure of the ship was too badly warped to continue operating her effectively and it would have cost too much to tear apart the ship and repair it.

Steel decks for US carriers came with the jet age. Planes got heavier and the wooden decks couldn't take the weight of the new aircraft.

Bill

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RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/2/2015 3:40:15 AM   
jmalter

 

Posts: 1673
Joined: 10/12/2010
Status: offline 		I appreciate the contributions of Symon & wdolson to this thread.

And didn't CV Yorktown receive significant damage to several 'frames' (cross-wise supporting structure) at Coral Sea, that were just barely patched up at Pearl, allowing her to add her airgroup's weight at Midway?

Any info about naval architecture & design would be really interesting. I'm down w/ hogging & sagging, but not clear on Center of Buoyancy vs. Center of Gravity, & which is supposed to be above t'other.

I think the orig link's headline should've been 'Naval Architects of Yore', if it referenced historical designs / designers. Those guys would've used 'lore' to inform their designs, as well as new ideas. I blame the headline-writer.

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Post #: 12
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/2/2015 7:15:06 AM   
CaptBeefheart


Posts: 2301
Joined: 7/4/2003
From: Seoul, Korea
Status: offline 		Yes, "yore" is a much better word. It's the sad state of internet news editing, or lack thereof, on display.

Good discussion; it reminds me of seeing the wave tank in action in the basement at the East Engin building when I was an undergrad at U-M.

Cheers,
CC

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RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/2/2015 12:09:31 PM   
wegman58

 

Posts: 460
Joined: 12/28/2013
From: Edina, MN (FROM the Bronx)
Status: offline
quote:

ORIGINAL: pontiouspilot

While we have the attention of PEngs or architects I have a question about the decks of US carriers. What was the rationale for wooden flight decks? What was 1st US CV with non-wooded deck? Why didn't they start at least plating the wood during the kamikaze heyday?

Final question: I assume the Japanese also used wooden decks...perhaps balsa!!


A lot of it had to do with making the ship too top heavy.

Two articles for your reading pleasure:

http://www.navweaps.com/index_tech/tech-030.htm

http://www.navweaps.com/index_tech/tech-042.htm

(I'm a big fan of the www.navweaps.com)

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Post #: 14
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/2/2015 1:40:41 PM   
Symon


Posts: 1928
Joined: 11/24/2012
From: De Eye-lands, Mon
Status: offline
quote:

ORIGINAL: jmalter
Any info about naval architecture & design would be really interesting. I'm down w/ hogging & sagging, but not clear on Center of Buoyancy vs. Center of Gravity, & which is supposed to be above t'other.

Center of mass (CG) and center of buoyancy (CB) are pretty straight forward. Two different kinds of stability relations, though; load-stability (top), and roll-stability (bottom).

Generally, deeper you float, the more stable. The deeper you are, the lower the CG point. That’s why you read about ships taking water ballast into fuel tanks in rough seas. And why counter flooding (adding ‘more’ water inside) works. The CB point is dynamic. It depends on immersed hull form, so it moves up and down (and left and right) with hull displacement and roll aspect.

Roll-stability is a bit more of a witch because it gets into the metacenter, but once again, it’s simply that the CB moves left and right. Roll a ‘little’ and you get a restoring torque because the plane of the CB is on the ‘good’ side of the plane of the CG. It’s points and vectors, actually, but visualizing it as planes I think helps understanding. Roll a ‘lot’ and the metacenter goes all to heck, the plane of CB shifts to the ‘bad’ side of the plane of the CG, causing the dreaded ‘death roll’ torque.

So, jmalter, it really doesn't matter much which is 'above' the other, although if CG is above CB, it shouldn't be by a whole lot because that will have implications for roll-stability (it mucks up the metacenter). What matters most is keeping CB on the 'good' side of CG and, of course, keeping CB below the level of the waterline .

That’s really all there is to it. Tons of stuff on the net from stick figures to the vector calculus if ya google center of mass and center of buoyancy. Ciao. JWE


I should mention that I don't have practical experience with warships or large commercial vessels. I have the book learnin' but my practical experience is with hull section and keel/rudder redesign/optimization for hi-performance racing sailboats, and more for the fun of it than for the (negligible) profit. Still, a vessel, is a vessel, is a vessel, and the same concepts and math applies. Glad you find my contributions helpful. JWE

Attachment (1)

< Message edited by Symon -- 6/2/2015 3:11:59 PM >

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RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/2/2015 6:29:35 PM   
pws1225

 

Posts: 1166
Joined: 8/9/2010
From: Tate's Hell, Florida
Status: offline 		And just what the frig is a Froude limit? I'd check Wikipedia but that source seems discredited now.

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Post #: 16
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/2/2015 7:20:57 PM   
geofflambert


Posts: 14863
Joined: 12/23/2010
From: St. Louis
Status: offline
quote:

ORIGINAL: pontiouspilot

While we have the attention of PEngs or architects I have a question about the decks of US carriers. What was the rationale for wooden flight decks? What was 1st US CV with non-wooded deck? Why didn't they start at least plating the wood during the kamikaze heyday?

Final question: I assume the Japanese also used wooden decks...perhaps balsa!!



Remember, most (I think) BBs had optional teak decking.

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Post #: 17
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/2/2015 7:26:13 PM   
geofflambert


Posts: 14863
Joined: 12/23/2010
From: St. Louis
Status: offline 		The Texas is not "afloat", it's resting on the bottom with the lower reaches of the ship filled with concrete.

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Post #: 18
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/2/2015 9:35:09 PM   
Symon


Posts: 1928
Joined: 11/24/2012
From: De Eye-lands, Mon
Status: offline
quote:

ORIGINAL: pws1225
And just what the frig is a Froude limit? I'd check Wikipedia but that source seems discredited now.

It's the area just past the second gator outside the city limits of Tate's Hell J

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Post #: 19
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/3/2015 1:10:26 AM   
pws1225

 

Posts: 1166
Joined: 8/9/2010
From: Tate's Hell, Florida
Status: offline
quote:

ORIGINAL: Symon


quote:

ORIGINAL: pws1225
And just what the frig is a Froude limit? I'd check Wikipedia but that source seems discredited now.

It's the area just past the second gator outside the city limits of Tate's Hell J


Well frig me! I shoulda knowed that.

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Post #: 20
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/3/2015 4:57:55 PM   
Symon


Posts: 1928
Joined: 11/24/2012
From: De Eye-lands, Mon
Status: offline
quote:

ORIGINAL: pws1225
quote:

ORIGINAL: Symon
quote:

ORIGINAL: pws1225
And just what the frig is a Froude limit? I'd check Wikipedia but that source seems discredited now.

It's the area just past the second gator outside the city limits of Tate's Hell J

Well frig me! I shoulda knowed that.


It’s kinda funny because I’m visiting my sister, who’s a designer at GD Electric Boat in Groton. I asked her the question .. “so .. just what the frik is a Froude limit?” You would not believe the “look” I got. Anyway, this is our collective take.

Froude number is ‘jargon’ that’s used as a way of talking about hull speed and wave resistance (drag). The energy to propel a hull through water is transferred to the water, creating standing waves (bow and stern waves). As speed varies, these waves change shape, magnitude and angle (wavelength, phase speed, etc. for the tech heads); they constructively and destructively interfere with one another. Points of interference have a pronounced effect on drag. Drag is relatively proportionally linear to power (speed) up to a first interaction point where it increases dramatically because of a shift in the wave train characteristic.

A simplistic way of looking at it, but one suitable for game terms, is this is the ‘economic’ cruise speed point. A second interaction point is where the ship kinda, sorta, ‘outruns’ the bow wave, thereby effectively constructively converging it with the stern wave, causing a huge shift in the wave train characteristic. Simplistically, this is the vessel’s effective hull speed, because power to make another half knot would be prohibitive (if it even exists) against the added drag.

The Froude number is the characteristic hydrodynamic flow velocity divided by the square root of a characteristic length. Because all vessels have different lengths, hull forms, etc., the Froude number permits comparison between heterogeneous vessels, because standing wave pattern generation is similar at similar Froude numbers for dissimilar vessels.

The Froude limits are (very simplistically) those points where the wavelength of the characteristic standing wave pattern is approximately ½ and 1 of the characteristic length of the vessel (tech heads can think of pi and 2pi superposition). It wouldn’t be too far off to think of Froude numbers (limits) as the hydrodynamic equivalent of Mach numbers.

Hope this helps. JWE


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Post #: 21
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/3/2015 6:44:22 PM   
pws1225

 

Posts: 1166
Joined: 8/9/2010
From: Tate's Hell, Florida
Status: offline 		Hory crap! I better just stick with hull speed.

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Post #: 22
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/3/2015 8:20:20 PM   
JockJimmy

 

Posts: 37
Joined: 5/17/2013
Status: offline 		Thanks for all that. Just one more question - Does the pointy end go at the front or back?

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Post #: 23
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/3/2015 8:24:31 PM   
warspite1


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From: England
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quote:

ORIGINAL: JockJimmy

Thanks for all that. Just one more question - Does the pointy end go at the front or back?
warspite1

Neither, the front bit of the boat is called Starbord.


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Post #: 24
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/3/2015 9:00:02 PM   
witpqs


Posts: 26087
Joined: 10/4/2004
From: Argleton
Status: offline
quote:

ORIGINAL: warspite1


quote:

ORIGINAL: JockJimmy

Thanks for all that. Just one more question - Does the pointy end go at the front or back?
warspite1

Neither, the front bit of the boat is called Starbord.


I didn't know that boats have bits.

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Post #: 25
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/3/2015 9:06:11 PM   
warspite1


Posts: 41353
Joined: 2/2/2008
From: England
Status: offline
quote:

ORIGINAL: witpqs


quote:

ORIGINAL: warspite1


quote:

ORIGINAL: JockJimmy

Thanks for all that. Just one more question - Does the pointy end go at the front or back?
warspite1

Neither, the front bit of the boat is called Starbord.


I didn't know that boats have bits.
warspite1

Well to be fair witpqs I am not surprised. Some of the ignorance of boaty things round here is staggering. If you need to know more about stuff just let me know.


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Post #: 26
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/3/2015 9:42:32 PM   
Sardaukar


Posts: 9847
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From: Finland/Israel
Status: offline
quote:

ORIGINAL: warspite1


quote:

ORIGINAL: JockJimmy

Thanks for all that. Just one more question - Does the pointy end go at the front or back?
warspite1

Neither, the front bit of the boat is called Starbord.



These paramilitary yacht clubs have their own language. You know..port is where they drink port, starboard is where they gaze stars, bow is where they do archery and stern is the..stern place where guys shoveling coal are...

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Post #: 27
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/3/2015 11:08:23 PM   
witpqs


Posts: 26087
Joined: 10/4/2004
From: Argleton
Status: offline
quote:

ORIGINAL: warspite1


quote:

ORIGINAL: witpqs


quote:

ORIGINAL: warspite1


quote:

ORIGINAL: JockJimmy

Thanks for all that. Just one more question - Does the pointy end go at the front or back?
warspite1

Neither, the front bit of the boat is called Starbord.


I didn't know that boats have bits.
warspite1

Well to be fair witpqs I am not surprised. Some of the ignorance of boaty things round here is staggering. If you need to know more about stuff just let me know.


But, but, but I always boats were referred to as 'she'!

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Post #: 28
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/4/2015 3:18:52 AM   
geofflambert


Posts: 14863
Joined: 12/23/2010
From: St. Louis
Status: offline
quote:

ORIGINAL: Symon


quote:

ORIGINAL: pws1225
quote:

ORIGINAL: Symon
quote:

ORIGINAL: pws1225
And just what the frig is a Froude limit? I'd check Wikipedia but that source seems discredited now.

It's the area just past the second gator outside the city limits of Tate's Hell J

Well frig me! I shoulda knowed that.


It’s kinda funny because I’m visiting my sister, who’s a designer at GD Electric Boat in Groton. I asked her the question .. “so .. just what the frik is a Froude limit?” You would not believe the “look” I got. Anyway, this is our collective take.

Froude number is ‘jargon’ that’s used as a way of talking about hull speed and wave resistance (drag). The energy to propel a hull through water is transferred to the water, creating standing waves (bow and stern waves). As speed varies, these waves change shape, magnitude and angle (wavelength, phase speed, etc. for the tech heads); they constructively and destructively interfere with one another. Points of interference have a pronounced effect on drag. Drag is relatively proportionally linear to power (speed) up to a first interaction point where it increases dramatically because of a shift in the wave train characteristic.

A simplistic way of looking at it, but one suitable for game terms, is this is the ‘economic’ cruise speed point. A second interaction point is where the ship kinda, sorta, ‘outruns’ the bow wave, thereby effectively constructively converging it with the stern wave, causing a huge shift in the wave train characteristic. Simplistically, this is the vessel’s effective hull speed, because power to make another half knot would be prohibitive (if it even exists) against the added drag.

The Froude number is the characteristic hydrodynamic flow velocity divided by the square root of a characteristic length. Because all vessels have different lengths, hull forms, etc., the Froude number permits comparison between heterogeneous vessels, because standing wave pattern generation is similar at similar Froude numbers for dissimilar vessels.

The Froude limits are (very simplistically) those points where the wavelength of the characteristic standing wave pattern is approximately ½ and 1 of the characteristic length of the vessel (tech heads can think of pi and 2pi superposition). It wouldn’t be too far off to think of Froude numbers (limits) as the hydrodynamic equivalent of Mach numbers.

Hope this helps. JWE



JWE, this is probably not your best post, but I'm thinking it's the best I've seen, at least for a while. Thank you for this instructive post. Let me digress. It's likely not the best I've seen from you, in part because I've noticed you craft excellent posts frequently. You also know a lot of stuff most of the rest of us don't know. I don't know what the WITPAE gamers would do without your input. You are a more important asset to this forum than me, without question. That's all.

(in reply to Symon)
Post #: 29
RE: SOT: Naval Architects of Lore May Have Been Right A... - 6/4/2015 12:20:58 PM   
AW1Steve


Posts: 14507
Joined: 3/10/2007
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quote:

ORIGINAL: geofflambert


quote:

ORIGINAL: Symon


quote:

ORIGINAL: pws1225
quote:

ORIGINAL: Symon
quote:

ORIGINAL: pws1225
And just what the frig is a Froude limit? I'd check Wikipedia but that source seems discredited now.

It's the area just past the second gator outside the city limits of Tate's Hell J

Well frig me! I shoulda knowed that.


It’s kinda funny because I’m visiting my sister, who’s a designer at GD Electric Boat in Groton. I asked her the question .. “so .. just what the frik is a Froude limit?” You would not believe the “look” I got. Anyway, this is our collective take.

Froude number is ‘jargon’ that’s used as a way of talking about hull speed and wave resistance (drag). The energy to propel a hull through water is transferred to the water, creating standing waves (bow and stern waves). As speed varies, these waves change shape, magnitude and angle (wavelength, phase speed, etc. for the tech heads); they constructively and destructively interfere with one another. Points of interference have a pronounced effect on drag. Drag is relatively proportionally linear to power (speed) up to a first interaction point where it increases dramatically because of a shift in the wave train characteristic.

A simplistic way of looking at it, but one suitable for game terms, is this is the ‘economic’ cruise speed point. A second interaction point is where the ship kinda, sorta, ‘outruns’ the bow wave, thereby effectively constructively converging it with the stern wave, causing a huge shift in the wave train characteristic. Simplistically, this is the vessel’s effective hull speed, because power to make another half knot would be prohibitive (if it even exists) against the added drag.

The Froude number is the characteristic hydrodynamic flow velocity divided by the square root of a characteristic length. Because all vessels have different lengths, hull forms, etc., the Froude number permits comparison between heterogeneous vessels, because standing wave pattern generation is similar at similar Froude numbers for dissimilar vessels.

The Froude limits are (very simplistically) those points where the wavelength of the characteristic standing wave pattern is approximately ½ and 1 of the characteristic length of the vessel (tech heads can think of pi and 2pi superposition). It wouldn’t be too far off to think of Froude numbers (limits) as the hydrodynamic equivalent of Mach numbers.

Hope this helps. JWE



JWE, this is probably not your best post, but I'm thinking it's the best I've seen, at least for a while. Thank you for this instructive post. Let me digress. It's likely not the best I've seen from you, in part because I've noticed you craft excellent posts frequently. You also know a lot of stuff most of the rest of us don't know. I don't know what the WITPAE gamers would do without your input. You are a more important asset to this forum than me, without question. That's all.

+1.

_____________________________


(in reply to geofflambert)
Post #: 30
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