# Does a floating object always rotate about its centre of mass?

If a large, stable floating object is placed in the ocean, free floating, and the tides and currents act on that object causing it to rotate, will it always rotate about its center of mass?

jbriggs444
Homework Helper
Imagine a boat which is tethered to a sea anchor. This assembly will rotate around the sea anchor, not about the center of mass of the assembly. Of course, any rotation would also be damped unless the water itself is rotating.

http://en.wikipedia.org/wiki/Sea_anchor

Okay, so it won't necessarily rotate around the center of the rigid object, it really depends on the distribution of the mass (the shape) of the object? One part of the object may be shaped in such a way that it generates more drag, etc.?

Okay, so it won't necessarily rotate around the center of the rigid object, it really depends on the distribution of the mass (the shape) of the object? One part of the object may be shaped in such a way that it generates more drag, etc.?

It also will depend on the forces acting upon the body and their directions and magnitudes.

sophiecentaur
Gold Member
2020 Award
I think it's the centre of pressure and not the centre of mass that a boat will rotate around when an external force is added (if, by 'rotate', you mean about a vertical axis). Moving the (light) centre board of a sailing dinghy makes a huge difference to where it will point.
But it would be the detailed motion of the turbulent currents (the 'Curl' of the flow) that would affect the rotation point. Any bulk (tidal) flow would not be expected to affect the heading of a free floating boat with no wind.

Imagine a boat which is tethered to a sea anchor. This assembly will rotate around the sea anchor, not about the center of mass of the assembly. Of course, any rotation would also be damped unless the water itself is rotating.

http://en.wikipedia.org/wiki/Sea_anchor
Won't adding a sea anchor just extend the assembly.The earth and the centre of it's mass would in effect become part of the floating object.So it won't be rotating around the sea anchor it will be rotating around the centre of mass of the earth.

jbriggs444
Homework Helper
I'm not sure what point you are trying to make here.

We are not asked about how the Earth is rotating about its axis. We are asked about how a boat is rotating. That boat is not floating weightless in a vacuum. It is floating in and surrounded by an ocean.

The point that a number of folks here have made is that the way the boat rotates will depend not just on its mass distribution but on how it interacts with the water -- whether that involves sea anchors, centerboards, rudders, barnacles or crewmen dangling their hands in the water.

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I think it partly depends upon what you mean by rotate.

Take the Moon. It keeps one face towards the earth so it rotates on its own axis. It also descibes (somewhat) circular motion around the earth.

Place your boat in a similar situation at the whirlpool of Scilla and Charybdis. The boat will describe two motions similar to the moon about the earth.

jbriggs, do you not think that the sea anchor removes the free floating condition in the original post?

It is floating in and surrounded by an ocean.
That is the point I am trying to make, the OP's question was about a free floating object.
Attaching an anchor, then it's no longer free floating or have I not read the OP question properly and put my own wrong interpretation upon it.
I don't see that other folk introduced anchors and the question was specificaly about a stable floating object.
I am just trying to keep the thread to OP's topic.

jbriggs444
Homework Helper
A sea anchor is not made of metal and does not go to the bottom of the ocean.

A sea anchor is not made of metal and does not go to the bottom of the ocean.
Not allways a traditional sea anchor is made of metal and can go to the bottom of the sea.
Your Wiki type don't but if you google images of sea anchors you will see the type that is usualy known as a sea anchor along with a plethora of other less known types.
Never the less by attaching an anchor that provides drag from the sea you are still by extension shifting the COM towards the earths centre.
Imagine attaching a hydrogen balloon to someone on a scale you would jush shift some of the mass to the atmosphere etc.

A sea anchor is not made of metal and does not go to the bottom of the ocean.

I don't see how either statement is relevant to the free floating condition.

jbriggs444
Homework Helper
A traditional anchor is not a "sea anchor". Google it. But that's irrelevant to the physics of the situation. It's just terminology. Let's call it a drogue instead.

Buckleymanor points out that extending a drogue into the water cause drag from the water. Yes, that's the point.

He also points out that extending a drogue into the water will shift the center of mass downward toward the center of the earth. That true, but...

1. I believe that we are discussing rotation around a vertical axis. Assuming that this axis aligns with the center of mass, displacing that center downward would not change the axis of rotation.

2. The displacement of the center of mass will not, in general, match the displacement of the "center of drag".

With all that said, Sophie's suggestion to consider a movable centerboard is much more apt than consideration of a drogue.

sophiecentaur
Gold Member
2020 Award
A sea anchor is not made of metal and does not go to the bottom of the ocean.
Precisely. A sea anchor is a device, consisting of a bucket, drogue or anything else of neutral density (it stays just below the surface)) that will produce a drag effect when hung over the bow or stern of a boat in heavy weather. It moves the centre of pressure (underwater) well to the front or the back (depending on what you want) to keep either the bow or stern into the wind because the centre of effort ( that is the effect of wind pressure of the parts of the boat above the water) will be on the superstructure / sails and will be downwind of the sea anchor.

Introducing centre of mass into the analysis is missing the point of what is happening. If you really are concerned with centres of mass then you really need to be considering, also, the moving / swirling mass of the water that the boat is floating in. This makes life far too complicated, which is why 'they' don't do it that way. Centre of pressure is the point on the hull (below the water) which all the lateral forces can be considered as acting upon. It is a bit like the CM, in that it can be considered as the point of rotation. BUT this point is not fixed because the lateral pressure is not uniform and can change (for instance with the rudder position or the heel of the boat - or moving the centre board).

If a large, stable floating object is placed in the ocean, free floating, and the tides and currents act on that object causing it to rotate, will it always rotate about its center of mass?

I know what a sea anchor is.

But you still haven't shown how that is addressing the question.

Is a boat with a sea anchor, or any other appendage, large, stable, free floating and subject only to the forces of water movement?

If you allow a sea anchor, why not a ballon drogue? Why not a tether to another boat? Where do you draw the line?

I prefer to think that the OP meant something like an aircraft carrier or oil tanker that is not under power.

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Imagine a boat which is tethered to a sea anchor. This assembly will rotate around the sea anchor, not about the center of mass of the assembly. Of course, any rotation would also be damped unless the water itself is rotating.

http://en.wikipedia.org/wiki/Sea_anchor

This thread is a great example of what's wrong with Physics Forums.
A guy posts a legitimate, thoughtful question, which is met by a reply that is either:
a) "Whazzamatta, can't you try GOOGLE?," or
b) "I will now post some completely oblique thing that has nothing to do with the guy's question, but is written in a style that is mysterious, fatherly and wise."

Screw that. Answer the guy's question.

DaveC426913
Gold Member
Screw that. Answer the guy's question.
This is an textbook example of committing the very crime you accuse others of doing. You criticize others for not answering, but then do not contribute yourself.

There are many ways to interpret the OP's question, which is why there are so many answers. But the upshot is that there are certainly examples involving drag that will cause a floating object to rotate about an axis other than its CoM.

This is an textbook example of committing the very crime you accuse others of doing. You criticize others for not answering, but then do not contribute yourself.

There are many ways to interpret the OP's question, which is why there are so many answers. But the upshot is that there are certainly examples involving drag that will cause a floating object to rotate about an axis other than its CoM.

Dave,

Thanks for the reply. However I think my point is valid. Let's have a look. The OP said,
If a large, stable floating object is placed in the ocean, free floating, and the tides and currents act on that object causing it to rotate, will it always rotate about its center of mass?

The guy in question starts off with,
Imagine a boat which is tethered to a sea anchor...

Imagine? I imagined what the guy said, which was a "large, stable floating object placed in the ocean, free floating." Where is the sea anchor in the guy's question? He didn't say "damping," he didn't say "asymmetric," he didn't say "tethered."

I personally find it to be a pet peeve when people act wise and imperious, and write in a manner that seems like talking down. "Imagine, little person, a situation in which..." the guy didn't say "help me imagine," he said "what's the answer."

Anyways that's my 2 cents. Peace

sophiecentaur
Gold Member
2020 Award
Glad you know about nautical.matters. The contents of some of the posts on the thread implied that not everyone knew.
My point is that the CM is not relevant to how the boat interacts with the water in this case. A drogue or a towed tender need hardly change the CM at all but can affect the pivot point position greatly.

Now that we have started to seriously engage our physics brains about the subject perhaps we can discuss how the forces imposed by water movement can cause rotation.

Can pressure forces, which are normal forces, cause rotation under any circumstances?
The pressure on a surface can be repleced by a single force acting on the centre of pressure. It is clear that the moment of this force must be zero about its own line of action.

Is there any condition on the flow regime of the water required. For instance can a parallel velocity field lead to rotation?
Even the whirlpool example I offered earlier has rotationless flow.

It is a good question, worthy of treating with respect.

jbriggs444
Homework Helper
Imagine a hose sprayed onto the bowsprit of a boat from the starboard side.

Yes, pressure forces can cause rotation under some circumstances.

Yes, the moment of that force is zero along its own line of action. But that line of action need not intersect any particular axis of rotation.

Imagine a hose sprayed onto the bowsprit of a boat from the starboard side.

Yes indeed there are many mechanisms that could force rotation about any desired axis, including the finger of God.

But the OP specified that the rotation was to be effected by the tides and currents.

When you were on the Titanic did you see the icebergs go spinning gaily past?

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jbriggs444
Homework Helper
The stream from a hose is a current.

The stream from a hose is a current.

That's just dodging the issue.

Let us see your proof that natural water movements can rotate a floating rigid body about a vertical axis other than the COM of the body.

Bear in mind that we don't know if this body has any motive power or is under way.
Also that a boat plus sea anchor is not a rigid body.