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Better Balance of Ships in Wavy Seas

09/30/2017 12:22 PM

If ships were made conical, with the broader side at the bottom off course, less volume would've been 'available' for additional bayoncy forces to act, in wavy seas, on one side, when water on one side of the ship is higher (crest) than the other side. (valley) Off course the same is true for ship rocking along the longitudinal axis when the bow or stern is in the water.

Considering the sphere as a floating vehicle is very interesting, in regards to its stability versus resistance in propulsion. Is there a better 3d floating shape to negotiate the wavy seas, than a sphere.

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#1

Re: Better balance of ships in wavy seas.

09/30/2017 12:29 PM

The USN is already aware & using the "Tumbledown" narrow@top / wider@bottom sloped hull shape on some of its newer ships. It's a hull design that's been around nearly 100 years.

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#3
In reply to #1

Re: Better balance of ships in wavy seas.

09/30/2017 7:22 PM

I believe "tumbledown" or "tumblehome" refers to narrowing of the ship above the water line.

https://en.wikipedia.org/wiki/Tumblehome

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#9
In reply to #1

Re: Better balance of ships in wavy seas.

10/01/2017 10:38 AM

"It's a hull design that's been around nearly 100 years."

WHERE ? Why I have not seen any for 50 years. Half of my close relatives are seamen. In any way it's nice to talk about things as new if you haven't see anything about them or most importantly if there's nothing to study about it. Or there is ?

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#2

Re: Better balance of ships in wavy seas.

09/30/2017 1:54 PM

It's a tradeoff, to make a ship aerodynamic for travel you need a certain design to minimize friction and also to enhance stability...The details become clear when the purpose of the vessel is defined....cargo, stealth, passenger, speed, the hull is designed for purpose....

...A lot of cruise ships have hull-mounted wings that are used for stabilization...

...here bubbles are generated to lubricate flow, and enhance speed....

http://www.innovationtoronto.com/2012/01/mitsubishi-reduces-friction-on-ship-hulls-by-blowing-bubbles/

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#4

Re: Better Balance of Ships in Wavy Seas

09/30/2017 8:22 PM

I'm thinking it might be a little more complicated. There are two vectors involved in the stability of a ship. The ship's weight is a vector acting through the ship's center of gravity (the centroid of the ship's mass). The weight of the ship is supported by an equal and opposite vector, the buoyancy vector which acts through the ship's center of buoyancy (the centroid of the displaced water).

If the ship is resting quietly in the water, the weight vector and buoyancy vector are in line, equal, and in opposite directions.

https://www.marineinsight.com/naval-architecture/ship-stability-introduction-hydrostatics-stability-surface-ships/

If the ship lists to one side, the buoyancy vector shifts to the new centroid of the displaced water, no longer aligned with the weight vector. If the ship is stable, the buoyancy vector shifts further to the lower side of the ship than the weight vector. The weight and buoyancy vectors form a "couple" which applies a torque to restore the ship upright.

The further the buoyancy vector shifts, the stronger the restoring torque and the higher the stability of the ship. A catamaran is an example of a strongly stable boat. A canoe is an example of a weakly stable boat.

The restoring couple that keeps the ship upright is the same that makes it susceptible to wave action, so it's a trade-off between stability and susceptibility to wave action.

Just my thoughts...

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#12
In reply to #4

Re: Better Balance of Ships in Wavy Seas

10/01/2017 10:56 AM

Your thoughts are welcomed. Thank you

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#5

Re: Better Balance of Ships in Wavy Seas

09/30/2017 11:06 PM

There is a potentially fatal flaw with this concept and that occurs when the center of gravity is above the center of buoyancy. If you can guarantee that the center of gravity will always be below the center of buoyancy, then you won't re-enact the Poseidon Adventure. LtCdr McHale, may he rest in peace.

(yep, showing my again. )

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#7
In reply to #5

Re: Better Balance of Ships in Wavy Seas

09/30/2017 11:53 PM

age . . .

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#8
In reply to #5

Re: Better Balance of Ships in Wavy Seas

10/01/2017 7:12 AM

You are correct, it would be more stable with the center of gravity below the center of buoyancy. But it can actually be stable with the center of gravity above the center of buoyancy. When the ship lists to the port side, for example, more water is displaced on that side which means that the center of buoyancy is shifted to that side, applying a restoring torque to right the ship.

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#10
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Re: Better Balance of Ships in Wavy Seas

10/01/2017 10:39 AM

Here is an example I was thinking of. Take a piece of styrofoam, 6" x 6" x 1" high. It will clearly float stably with maybe the bottom 1/8" submerged in the water. So the center of gravity (CG) is 1/2" from the bottom and center of buoyancy (CB) is under the CG, about 1/16" from the bottom.

If you add a weight on one side, the CG shifts and the weight and buoyancy vectors are no longer aligned. This causes a torque which rotates the piece so that it is deeper in the water on the weighted side. The CB moves to a new point underneath the new CG (more displaced water on that side), the weight and buoyancy vectors are again equal and opposite, and stability is regained.

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#11
In reply to #5

Re: Better Balance of Ships in Wavy Seas

10/01/2017 10:51 AM

I can't see at what angle the center of gravity is above the center of buoyancy. If you could please show it with a quick sketch.

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#6

Re: Better Balance of Ships in Wavy Seas

09/30/2017 11:50 PM

https://en.m.wikipedia.org/wiki/Small-waterplane-area_twin_hull

Minimizing the water plane area greatly enhances stability of a vessel in a given seastate.

The SWATH hull form is highly stable although relatively payload sensitive.

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#13

Re: Better Balance of Ships in Wavy Seas

10/01/2017 5:06 PM

BS. Use subfoils and get a propulsion to get it above the waterline.

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#14
In reply to #13

Re: Better Balance of Ships in Wavy Seas

10/01/2017 9:58 PM

Ah, yes, that works too, lift instead of buoyancy!

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#30
In reply to #13

Re: Better Balance of Ships in Wavy Seas

10/03/2017 6:43 AM

Please, we're talking big commercial ships.

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#34
In reply to #30

Re: Better Balance of Ships in Wavy Seas

10/03/2017 12:01 PM

Are you stating unequivocally that no large commercial ships use any form of hydrofoil? Just asking...

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#15

Re: Better Balance of Ships in Wavy Seas

10/02/2017 10:26 AM

To best understand hull cross sections and how they effect the boat a canoe is invaluable. Canoes come in almost every imaginable hull shape all though bow line configuration is more limited. The concepts of stability and hull shape become very really and first person in a canoe as does loading and center of gravity. LoL

It's an enjoyable lesson as long as you don't mind getting wet.

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#16
In reply to #15

Re: Better Balance of Ships in Wavy Seas

10/02/2017 11:28 AM

I only got to the lesson where the guy with the smallest boat in the lake learns not to make waves.

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#17

Re: Better Balance of Ships in Wavy Seas

10/02/2017 12:17 PM

You are proposing a solution to which there is no problem. Ships are already stable or they would sink. Passenger ships where "roll" could be a problem are fitted with hydraulically operated fins as already mentioned in a previous post. Further, your solution introduces a whole raft of new problems.

1 You can't tie up a cone alongside a wharf to ease loading. The wider base gets in the way.

2 Cargo ships are predominately container vessels. The reverse angled sides would prevent top loading with a crane. Too much space would be lost if these reverse angle areas were not used. Even in passenger ships there is little point in having wider lower decks where the cheap accommodation is situated. Operators want the width in the upper decks where they can charge a premium for the extra space. Bulk carriers and tankers wouldn't suffer from either of the above objections but they only have crews of a dozen seasoned sailors so who cares if they roll a bit.

3 Cones are round. No pointy bit at the front to cut through the water and reduce drag.

4 Once you add a pointy bit it takes very little more energy to push a long hull through the water than pushing a short hull. Ships maximum hull lengths are limited by wharf space and navigation considerations. If the only design criteria was running economics they would be much longer than a present.

5 Having arrived at an optimum design that is long, the side of a ship presents a solid slab wall to oncoming waves. The difference in roll characteristic made by widening the base would be minimal.

6 To accommodate more containers modern super-container ships are much wider. This significantly reduces the tendency for roll so they can also be stacked higher, a double whammy for the ship builders and operators, but also another reason why your solution is looking for a problem to solve.

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#31
In reply to #17

Re: Better Balance of Ships in Wavy Seas

10/03/2017 6:58 AM

If you haven't been in ships watch a few videos on youtube of huge commercial ships in wavy seas, then you may feel to rewrite your post. IT'S DANGEROUS.

Fins is a funny way to stop the rocking of a ship in wavy ships. You must reconsider also other points: added cone ?? etc.

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#39
In reply to #31

Re: Better Balance of Ships in Wavy Seas

10/03/2017 2:49 PM

I am not a mariner but I do hold an RYA Yachtmaster Ocean certificate of competence and own a 34 foot sloop. Regulars to this forum will have noticed my absence recently as I was sailing from the UK to the Azores and back. I have commissioned Water Cannon on an Oil Rig Disaster Rapid Recovery Vessel during a force 8 in the North Sea, bilge and ballast actuators on several 100,000 tonne OBOs and all manner of engine monitoring and hydraulic equipment between, including ships hydraulically powered stabilizers to reduce roll in heavy weather and I can assure you that they do work. If you are or were a professional seaman you will know that ship owners do not spend cash on kit that does not perform. So explain why so many passenger vessels are fitted with stabilizers or fins. I have survived an all be it small typhoon and more dangerously a crew drinks party in Kingston Jamaica. I write serious posts with valid points. Constructive criticism of my input is always welcome.

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#46
In reply to #39

Re: Better Balance of Ships in Wavy Seas

10/04/2017 1:54 PM

Constructive criticism of my input is always welcomed. Congratulations, me too, civilized manners above all. The fact of the matter is though that, you enlist roll reducing methods used, but you have said this is a proposal for a problem that does not exist. It exists then, has it been solved, in my opinion no, what's cheaper, to start with.

P.S. Beetwen me and you, now that nobory listens, the effactiveness of stabilizers (in high risers too) look accesories like cosmetics to me, too slow to do the work. Ship owners usually don't know what the ships have or have not, they are only interested what's the cost of the insurance and crew payroll, some times fuel too.

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#18

Re: Better Balance of Ships in Wavy Seas

10/02/2017 1:16 PM

Obviously very few marine engineers on this group.

On nearly all ships, the center of gravity is above the center of buoyancy. By itself, this would lead most ships to prefer an inverted orientation. What prevents this is a feature of marine engineering referred to as metacentric height. Look it up in Wikpedia. As a ship inclines to one side or the other, the shape of the hull causes the center of buoyancy to shift in the direction of the tilt and move farther laterally than the center of gravity. The distance between the center of buoyancy and the center of gravity is called the righting arm and the weight (displacenment) of the ship times the length of the righting arm generates the forces needed to pop the ship vertical again. If you draw a line vertically from the center of buoyancy of an inclined hull, the metacentric height is the elevation where that vertical line intersects the tilted centerline of the hull. A big metacentric height means a very stable ship with a strong tendency to return to the upright position. A low metacentric height means the ship will be slow to return to upright.

The idea of a conical hull cross section could work as long as the metacentric height is positive.

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#19
In reply to #18

Re: Better Balance of Ships in Wavy Seas

10/02/2017 1:18 PM

BTW, submarines are the only ships built with the center of gravity below the center of buoyancy.

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#20
In reply to #19

Re: Better Balance of Ships in Wavy Seas

10/02/2017 1:40 PM

Yes, but technically, they are still boats. LOL

How does the righting arm work on a sub? The same way?

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#21
In reply to #20

Re: Better Balance of Ships in Wavy Seas

10/02/2017 1:54 PM

On a submarine it works like a pendulum since the center of gravity is below the center of buoyancy. Same idea as a hot air balloon.

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#22
In reply to #21

Re: Better Balance of Ships in Wavy Seas

10/02/2017 2:29 PM

That works for me, especially when she is designed to be and remain under water.

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#24
In reply to #21

Re: Better Balance of Ships in Wavy Seas

10/02/2017 2:38 PM

Very cool, it will be interesting to see how well the Zumwalt handles heavy seas.

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#25
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Re: Better Balance of Ships in Wavy Seas

10/02/2017 2:50 PM

It seems the Wikipedia entry for Zumwalt class destroyer with the tumblehome hull, and reverse bow profile may present stability problems in heavy seas when the waves are overtaking the ship from the stern, possible roll-over issue.

Tumblehome hull has not been used since about 1905.

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#29
In reply to #19

Re: Better Balance of Ships in Wavy Seas

10/03/2017 4:55 AM

Well, not the only ships built with COG below COB.

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#33
In reply to #29

Re: Better Balance of Ships in Wavy Seas

10/03/2017 12:00 PM

Square rigger? Obviously, the moments induced by the action of sails in the wind are very important factors related to stability in this case. We wouldn't want to haul her over.

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#37
In reply to #33

Re: Better Balance of Ships in Wavy Seas

10/03/2017 2:06 PM

I needed at least 3 square rigged mast to make sure no one could claim it was just a boat.

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#40
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Re: Better Balance of Ships in Wavy Seas

10/03/2017 3:14 PM

I thought that all ships of the line, and freighters used stone ballast.

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#35
In reply to #29

Re: Better Balance of Ships in Wavy Seas

10/03/2017 12:43 PM

Sorry, TTINAC, The center of gravity on a clipper ship was several feet above the center of buoyancy and the hull shape, curvature and lofting was very carefully shaped to provide good metacentric height.

On most ships, if the ship heels to one side more than about 45 degrees, the ship will roll upside down.

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#36
In reply to #35

Re: Better Balance of Ships in Wavy Seas

10/03/2017 2:05 PM

Hmmm. And the gigantic lead weight in the keel...typically on par with the weight of the remainder of the hull?

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#38
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Re: Better Balance of Ships in Wavy Seas

10/03/2017 2:28 PM

The large square riggers didn't use a lead keel. That's something peculiar to the smaller sailboats.

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#41
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Re: Better Balance of Ships in Wavy Seas

10/03/2017 7:17 PM

Actually smaller sailboats rely typically exclusively on form stability. Centerboards are the norn....no ballast there to speak of.

Consider the Americas Cup defender Puritan around the turn of the previous to last century. Water line a little over 81 ft, beam 22.5 ft. Displacement 105 tons of which 48 tons was a lead keel almost 9 ft below the water line.

If we consider a rectangle 80ft by 20 ft, half of that displacement would occur in the first ft. Given the upper-half-of-an-hour-glass longitudinal section of a sailing vessel, most of the other half of that displacement will occur near the surface as well. The CoB should be not more than 1.5 ft below the waterline.

With half of the mast being in the ballast 7ft below the likely CoB, for the CoG to be above the CoB, the center of mass for the remainder of the vessel (excluding lead ballast) would have to be more than 7ft above the CoB which would put it at least 5ft above the waterline....which is probably above the freeboard. The mast and rigging might be heavy by todays standards, but not that heavy.

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#42
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Re: Better Balance of Ships in Wavy Seas

10/04/2017 10:17 AM

Puritan was a racing sailboat, not a displacement type ship hull with very limited freeboard. Doesn't really apply to the discussion.

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#43
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Re: Better Balance of Ships in Wavy Seas

10/04/2017 11:28 AM

As a former submariner, I can tell you definitively that submarines are also not ships.

.

I guess the information probably doesn't sound as interesting if you have to phrase it as,

'... No large watercraft, being a ship or something big enough people might call a ship (even if incorrect) except for those meant for racing on wind power alone, has a center of gravity below its center of bouyancy, except submarines and some sunken ships....".

.

I'm not sure dessiminating this information is of sufficient value to warrant 'don't let truth get in the way of good story' merely for the sake of being concise.

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#44
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Re: Better Balance of Ships in Wavy Seas

10/04/2017 1:35 PM

Having worked for Adm Rickover for five years, I can definitively say that you can carry a boat on a ship, but you can't carry a ship on a boat.

Defining a submarine as a boat pertains specifically to the Royal Navy. In general a ship is a watergoing vessel with at least one watertight deck extending from bow to stern.

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#45
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Re: Better Balance of Ships in Wavy Seas

10/04/2017 1:39 PM

Winner, winner, chicken dinner!

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#47
In reply to #44

Re: Better Balance of Ships in Wavy Seas

10/04/2017 2:09 PM

No submarine I've been aboard has had one deck extending from bow to stern.

....so 'a watergoing vessel with at least one watertight deck extending from bow to stern' is a 'ship', huh?

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#49
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Re: Better Balance of Ships in Wavy Seas

10/04/2017 3:51 PM

I'd say a ship is a blue water vessel with two or more significant above-water-line decks of roughly similar size arranged with one directly above the other. I'd even be happy to throw in that to be a ship CoG must be at greater height than CoB when functioning as intended.

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#23
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Re: Better Balance of Ships in Wavy Seas

10/02/2017 2:37 PM

Umm...actually it's a naval architecture issue, not marine engineering. Marine engineering concerns the mechanical plant and systems onboard the vessel. Hull form, stability, resistance and powering are all naval arch topic space

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#26
In reply to #23

Re: Better Balance of Ships in Wavy Seas

10/02/2017 4:24 PM

All I can say is it was part of the curriculum.

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#27
In reply to #26

Re: Better Balance of Ships in Wavy Seas

10/02/2017 4:40 PM

They do quite hand in hand, indeed although I had to buy separate textbooks to cover both...$$$$ LoL!!

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#32
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Re: Better Balance of Ships in Wavy Seas

10/03/2017 7:14 AM

You did very well saying this though.

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#28

Re: Better Balance of Ships in Wavy Seas

10/03/2017 2:24 AM

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#48

Re: Better Balance of Ships in Wavy Seas

10/04/2017 2:31 PM

It seems that the centers of gravity and bayoncy monopolized the comments. I think that in conical hull there will be no problem in following the proven stability rules, so the ship will not be overturned in heavy wavy seas even if damaged. The center of gravity will never point out of the bottom any way (ships travel loaded all the time, usefull load or ballast) But since nobody said anything directly against it, it must mean that in big waves it's going to roll less in all possible directions. That's what we're looking for, you ?

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#50
In reply to #48

Re: Better Balance of Ships in Wavy Seas

10/04/2017 3:56 PM

Have you made models and tested them in wave machines?

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#51
In reply to #50

Re: Better Balance of Ships in Wavy Seas

10/04/2017 5:51 PM

No need for that...

"....nobody said anything directly against it, it must mean that in big waves it's going to roll less in all possible directions..."

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#52
In reply to #51

Re: Better Balance of Ships in Wavy Seas

10/05/2017 10:46 AM

That quote represents, "the new physics". No one said anything against it, your honor!

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#53
In reply to #52

Re: Better Balance of Ships in Wavy Seas

10/05/2017 5:07 PM

"Now 100% Empiricism Free!

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