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Driver’s Education, US Navy Edition
There was very little mainstream media coverage but the United States Navy proved again this week that no matter the size of the ocean, they will take a ship and hit something with it. This time, it was another one of our ships.
On Tuesday the USS Leyte Gulf (CG-55), a guided-missle cruiser and the USNS Robert E. Peary (T-AKE-5), a supply ship, collided during a training exercise off the coast of Florida. No one was injured. The Peary came away with an 8-inch gash above its waterline. The two ships were practicing an “underway replenishment” in which supplies are passed from ship to ship via rigging traveling together at speed.
The Leyte Gulf is currently assigned to carrier group of the USS Abraham Lincoln (CVN-72).
Hat tip to the US Naval Institute News.
Published in Military
Well, in a connected replenishment the vessels aren’t at an angle, they’re parallel throughout the evolution.
Although the vessels are parallel, the shape of warship hulls does broaden as one moves back from the bow and that might, I suppose, result in some degree of greater flow velocity at midships and stern than at the bow. I don’t know how great the effect would be if any, but ships are rigid bodies that pivot at the stern where the drive and rudder are, so steering can overcome any such affect. That’s why ships do this at 12 knots or so, so they can steer.
Everywhere outside the venturi the pressure will be greater, of course. Nothing special pressure-wise at the stern, no bumblebee turbulence as far as I’ve ever heard.
You quote a comment saying the receiving ship has to steer away from the course a bit to fight the venturi. Actually, both ships feel its effect, but @cqness must be assuming a common case – one of the ships is usually a great deal larger than the other. Aircraft carriers dwarf destroyers, and even a fleet oiler is huge next to a frigate. In those cases the big ship won’t have to do much steering away if any.
With no actual knowledge whatsoever, it would seem to me that it would make sense to have one ship maintain a ‘straight line’ while the other (probably smaller) one does any adjustments needed to come closer or move farther away. If both are adjusting, wouldn’t it be more likely that there would be a series of over-corrections?
I’m pretty sure they pivot at their center of mass, though I imagine it might appear to a helmsman otherwise. I’ve never conned a ship during unrep.
The pivot point of most naval ships is a little forward of amidships. I’m not sure whether or not it is a center of mass, we were only concerned with the bow swinging one direction and the stern the other as the rudder is put over.
Thanks, but this was not completely clear to me. Whether the ships are the same size or not, do they need to orient themselves at some small angle in order to counter a Venturi effect, or not?
It has to be center of mass. That’s how physics works. I was correcting an earlier statement saying the ship pivots at the stern.
IIRC the protocol is that the refueling ship maintains course and speed and the refuels ship does the adjustments.
I’m not too sure about this. I don’t think there’s any physical law requiring the pivot point to be at the center of mass. That’s assuming that the pivot point is defined as any point whose projection onto the centerline is moving in the direction of the centerline.
I think it’s like a car. The point on the centerline that is moving in the direction of the centerline is on the rear axle, not near the center of mass. If a car is turning left, and a coffee cup in the center console is at the center of mass, its direction of motion is left of the orientation of the car.
I confess I don’t understand what you’re saying here, but I’ll submit that a car is different from a waterborne craft in that a car has tires that use friction to hold it on the road. A ship is in water, a fluid. Fluids are defined as a substance that cannot withstand a shear force. The ship will rotate and translate through the water. Any force applied to the ship, such as a steering input, will cause a translation and a moment. The moment causes a rotation and the rotation will be about the center of mass.
I was curious so I googled it. It turns out that the pivot point in a ship making headway is not at the center of mass, but at about the 25% point. Here is a link
My guess about the meaning of “pivot point” was correct:
This is an interesting subject! I had never learned about it.
Ok, sorry I mentioned the pivot point. Not relevant, everybody forget about that.
As for the angle: the ships both steer the exact same heading. Both ships experience a force toward the other. If one (or both) is small enough to be driven off course by that force, it compensates by turning its rudder just a little. To stay on course.
Well, Barfly, it seems we are steaming in circles and are in danger of cutting our towline. End of replenishment exercise, resume previous course and speed. Also, under @judgemental‘s interrogation, Fred MacMurray admitted that HE stole the five pounds of strawberries.
Generally speaking the pivot point is at the center of resistance which is really more about something like friction rather than mass and has to do with the shape of the hull as it is being rotated. Both the sterns and the bows of boats have many conformations from flat bowed landing ships to pointy bowed faster ships with a fine entry. The stern also can be pushed side to side, which is pretty much how they’re steered, and they pivot on center of resistance, essentially, against the mass of water they must displace, and this also is dependent upon speed. Depending on speed and the angle and strength of the thrust the boat turns faster or slower with more or less side to side motion.
Good thing, too. I was about to shiver me barnacles.
I’m waiting for @judgemental‘s next installment – I might have a bone to pick with that guy over brains.
Fantastic!
Thanks, Mark.
My brain is boneless.
I wish we could all say that.
Forgive me, but what does that mean? Boneheaded?