How good is the Rocna?

[conachair]

Originally Posted by Dockhead

Sum of the mass of the object and the mass of displaced water . the sum of those masses is the WEIGHT of the object in water.

You mean, the difference of those masses don't you? Otherwise things would be heavier in water than in air.

[Dockhead]

Originally Posted by conachair

You mean, the difference of those masses don't you? Otherwise things would be heavier in water than in air.

Yes, but that part of my post is misleading in another way. I'll correct it when I'm near a computer. Thanks.

Sent from my D6633 using Cruisers Sailing Forum mobile app

[Kenomac]

Originally Posted by Dockhead

No -- a submarine's propeller and "stabilizer flaps" (you mean the control surfaces) most certainly do not control buoyancy. Submarines have buoyancy and will rise and sink without making way at all. Buoyancy is controlled separately by changing the weight of the vessel by pumping water into and out of the buoyancy tanks.

I wrote motion and balance, not buoyancy. I know that buoyancy is controlled by air, I watched "Voyage to the Bottom of the sea"

[uncle stinkybob]

Originally Posted by Kenomac

I wrote motion and balance, not buoyancy. I know that buoyancy is controlled by air, I watched "Voyage to the Bottom of the sea"

Is it "controlled" by air? How about the h20? a given object has more buoyancy in salt water than it does in fresh water so......and how does that affect all this minutiae of anchor balance and buoyancy?

[four winds]

I think Ken is referring to air in the ballast tanks, instead of water.

[Tomaz473]

Lodesman, try to understand Dockhead's posts, because he is correct.

If you prefer practice over theory then make two experiments:

One is with your two balls on 10m depth. As you are diver you can observe the balls going down from surface to 10m depth.

For the second experiment take a transparent ball and glue some weight on one side of the ball (on the inside). Let is fall. Observe how the ball rotates with the weighted end down. Unequal wigth distribution provides righting moment to it. You can do this experiment in water or in air. Same is with anchors (either some air in the shank or some lead in the fluke.

Nevertheless - I think chain (braking) and different shapes of fluke makes a lot more difference then some trapped air.

[Lodesman]

Originally Posted by Tomaz473

Lodesman, try to understand Dockhead's posts, because he is correct.

If you prefer practice over theory then make two experiments:

One is with your two balls on 10m depth. As you are diver you can observe the balls going down from surface to 10m depth.

For the second experiment take a transparent ball and glue some weight on one side of the ball (on the inside). Let is fall. Observe how the ball rotates with the weighted end down. Unequal wigth distribution provides righting moment to it. You can do this experiment in water or in air. Same is with anchors (either some air in the shank or some lead in the fluke.

Nevertheless - I think chain (braking) and different shapes of fluke makes a lot more difference then some trapped air.

Why don't you go do the experiments and report the results back to us. I did an experiment as suggested by Dockhead - I dropped a hammer, well actually a rubber mallet, but the rubber head was by far heavier than the handle. I dropped it several times in various orientations, from eye-height, about 1.7 m. Everytime the mallet hit the floor in the same orientation that it had left my hand. Perhaps if it had a long enough fall for the aerodynamic drag to have an effect, there would have been a noticeable difference, but as it was there was no observable tendency for the heavy part to spin to the bottom. I'd like to hear other's findings.

[conachair]

Originally Posted by Lodesman

I'd like to hear other's findings.

Newton got there before you

[chris95040]

Originally Posted by Lodesman

With due respect you are talking nonsense. Gravitational acceleration is equal for the two objects, as they are roughly the same distance from the centre of the gravitational mass, ie. the Earth - 9.8 m/s/s. Acting against them is hydrodynamic drag, which you agree is the same for both balls as they are identical in size and frictional coefficient. Also acting against them is, what I will concede to, is called the force of buoyancy. Both balls displace the same volume, roughly 1.3 litres, which is not applied proportionately, but equally reduces the mass force of both balls - making the steel ball effectively 8.7 kg, and the tungsten ball 23.7 kg. Both balls will accelerate at the same rate, but increased speed equals increased drag (both form and frictional), and at a certain speed the force of that drag will match the force of mass and the object will cease to accelerate - it is said to have reached terminal velocity.

Back to the woodshed! ( LOL- what does that even mean? )

You don't need to consider drag to get an intuitive understanding of why the heavier ball gets ahead. Its just a result of the turning moments on the object when the forces of buoyancy and gravity are considered, as in Dockheads example.

You seem hung up on the fact that acceleration due to gravity is constant in a vaccuum. Remember, F=ma. Its important to keep in mind *force* is /not/ constant from one 'm' to the next, as anyone who has lifted heavy things can attest. So the heavier ball has more gravitational force.

The corresponding decrease in this downward force from the opposing buoyancy force (which we've established is the same for both balls) will represent a higher proportional decrease for the lighter ball. So it will indeed accelerate slower than the heavy ball, until their vertical forces
are right on top of each other and no longer causing the object to rotate.

But regardless of what happens when the anchor is falling, I believe the shank design's impact on the COB and the COM affect its stabilty, its likelihood to remain upright, while lying on the seabed trying to get a bite. Significantly, or meaningfully, in todays anchors? I dunno.

[Dockhead]

Originally Posted by Lodesman

Why don't you go do the experiments and report the results back to us. I did an experiment as suggested by Dockhead - I dropped a hammer, well actually a rubber mallet, but the rubber head was by far heavier than the handle. I dropped it several times in various orientations, from eye-height, about 1.7 m. Everytime the mallet hit the floor in the same orientation that it had left my hand. Perhaps if it had a long enough fall for the aerodynamic drag to have an effect, there would have been a noticeable difference, but as it was there was no observable tendency for the heavy part to spin to the bottom. I'd like to hear other's findings.

To observe clearly what balance does to an object in motion, you would really need to be dropping a steel-headed hammer. The balance of a rubber hammer will not be displaced enough, to make the effect easy to observe.

I cited the principles of physics which cause objects to orient themselves towards their CG -- it has to do with torque.


But are we clear now on your two balls, and how buoyancy works?

Your steel and tungsten balls wouldn't land at the same time, even if dropped off the Tower of Pisa. I think the root of all of this is that you've misinterpreted Galileo's experiment, which only works in a vacuum. Aerodynamic drag is a force opposing gravity, just like buoyancy, just like hydrodynamic drag. So dropped off the Tower of Pisa, your two balls would experience the same aerodynamic drag, but the tungsten one will be subject to 2.45x as much gravitational force. That means that aerodynamic drag will reduce acceleration of both balls proportionate to the amount of force opposing gravity (Newton's Second Law), so the steel ball will see a greater reduction of acceleration, than the tungsten one, and will land later. The tungsten ball will also have a higher terminal velocity, than the steel one.

This is the formula for terminal velocity, which should make the last point clear:



where:

Vt Terminal velocity (m/s)
g Standard gravity
d Diameter of the object (m)
Cd Drag coefficient (dimensionless)
ρs Density of the falling object (kg/m3)
ρ Density of the fluid through which the object is falling (kg/m3)


Note that density of the falling object is an operator, and buoyancy force is also considered, even in air. Buoyancy will affect the terminal velocity even of a tungsten ball falling through air, although the effect will be extremely small. But the point is that gravity fights buoyancy + drag, for any object falling through a fluid, that is, any falling object other than one falling in a vacuum. A falling object will stop accelerating (but keep moving at a constant speed) when gravity = buoyancy + drag (has reached terminal velocity). It will stop MOVING, when gravity = buoyancy (which is a state of neutral buoyancy).


Acceleration comes from force (Newton's second law!), and not just from the force of gravity. When some other force is acting on an object besides gravity, you have to do a vector sum of all the different forces, in order to know how it will accelerate and in what direction. If different forces are acting on different centers of the object, then you get angular acceleration (torque) as well as linear acceleration. And that's where Righting Moment comes from!

[TheThunderbird]

Galilei is rolling in his grave, as much as your free dropping anchors, gents

[Dockhead]

Here is a great description of Galileo's Experiment:

https://en.wikipedia.org/wiki/Galile...isa_experiment


He used (or proposed to use) two lead balls of different sizes. Balls of the same DENSITY.

He was trying to disprove the idea that heavier objects per se fall faster than lighter ones, and he did.

However, the physics of falling objects is quite a bit more complicated than that. Even two lead balls of different sizes won't fall at exactly the same rate. Close enough for Renaissance folk watching the results with no instruments, probably, but still not exactly the same.

The reason is the REYNOLDS NUMBER, which explains why objects of the same shape but different sizes may not have the same drag coefficients. It's because the proportion between turbulent and laminar flow does not scale.

Because of this: Galileo's Experiment works perfectly in a vacuum. It sort of works with objects of the same density, shape, and texture, and of at least similar size. It does not work at all with objects of different densities, or greatly different size.

[Lodesman]

Originally Posted by chris95040

You seem hung up on the fact that acceleration due to gravity is constant in a vaccuum. Remember, F=ma. Its important to keep in mind *force* is /not/ constant from one 'm' to the next, as anyone who has lifted heavy things can attest. So the heavier ball has more gravitational force.

You forget the heavier ball also has more inertia - which is why it doesn't fall faster than a lighter object (minus other factors). Here's a hammer dropping experiment, reminding you the hammer has more gravitational force:

https://www.youtube.com/watch?v=-4_rceVPVSY

Originally Posted by chris95040

The corresponding decrease in this downward force from the opposing buoyancy force (which we've established is the same for both balls) will represent a higher proportional decrease for the lighter ball. So it will indeed accelerate slower than the heavy ball, until their vertical forces
are right on top of each other and no longer causing the object to rotate.

Why would you think acceleration decreases proportionately? There is a net decrease in force equivalent to that required to lift the displaced water up into the space vacated by the sinking ball (or other object). Each mass falling under water still needs to overcome its equivalent inertia, so acceleration should be the same regardless of mass.

[noelex 77]

Let's have a look at a practical example. This is a Shark. Not one of my favourite anchors.

You can see here how the weight distribution on this anchor is poor. The toe is not even close to the substrate. Hopefully it will eventually fall over so the fluke can start to dig in, but better anchors are unstable in this position. You cannot get them to balance in this manner.

If this Shark is stable in this position, even if it falls over how much pressure will there be on the toe to start it digging into the substrate?

Lodesman, if the toe was heavier and/or the shank lighter, do you think there would be no improvement?

[Dockhead]

Originally Posted by Lodesman

Why would you think acceleration decreases proportionately?

Because acceleration is directly proportionate to force.

In black and white:

"The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object."

Newton's Second Law of Motion

Note the word "net". Where different forces are exerted at the same time, you have to net them out, to determine what the acceleration of the object will be.


In very, very simple terms:

Double the net force, double the acceleration, and so forth.

Double the mass, halve the acceleration, and so forth.

If you have an object which weighs 2kg, and it has a volume of 1 liter (so producing 1kg of buoyancy), it will accelerate when you drop it at half the standard gravitational rate (18 feet per second per second, instead of 36), until the force of hydrodynamic drag starts to reduce acceleration even more.

If the object weighs 3kg, and has the same volume of 1 liter, it will accelerate at 2/3 of the standard gravitational rate.


I'm not sure why you're not getting this -- do you not understand that gravity is just one force, among many? How do you think anything floats, or flies? Only because buoyancy, or lift, counteracts the force of gravity. A hovering helicopter is producing lift which equals the force of gravity, and will start to gradually accelerate upwards as the lift exceeds the force of gravity.