Well, will they? and if so, why?
(I did a search, but couldn’t find this topic addressed anywhere)
Sure they will, if the water between them is moving faster than the water to their other sides; in a river, the water in the center of the channel at the surface is the fastest because it faces the least resistance (just other water and air), so if two vessels are heading in the same direction on either side of this fast-flowing region, they will be drawn together by the lower pressure created perpendicular to the dirction of the flow of the faster water, or, conversely, pushed together by the greater pressure of the slower moving water towards the banks.
Absolutely true.
All the Nato fleets routinely carry out Replenishment At Sea(RAS).
This involves the supplier vessel and the supplied vessels steaming on a parallel course someting between 50 - 150 yards apart, it depends on the sea conditions, speed of the ships such as in convoy escort excercises.
The larger vessel holds its course, normally it is the supply ship of upwards of 20k tons displacement whereas the supllied ship is a warship and is often between 4000tons to around 15000tons.
The smaller vessel is the one that makes all the adjustments to keep at the specified distance between the two.
You get oilers, which only carry fuel, so its a case of attatching a cable between them and sending a heavy duty hose on cable runners along it, the supplied end has a large metal male connector that locks into a female connector which then automatically operates the shut-off lugs on the male.Then the fuel is pumped along the line at a fairly high pressure so that transfer is as as fast as possible.
When you load stores such as food and spares a cable is run between the two and at the supplied ship end it is secured to the top of a collapsible stump mast, you then send your store over on a cable trolley, this is pulled backwards using a winch from the supply ship. The cable is sloped downwards to the supplied ship so that the trolley rolls downhill under the control of the winch.
What you have then is up to a possible 50k tons or more of metal in two separate lumps joined together by a steel cable of many tons breaking strain which against the actuals weights involved is like a strand of cotton.
If there should be any problem with maintaining a parallel course that cable will go bar taut, will vibrate or ‘sing’, and snap. The cable will fly back at immense speed, supersonic in fact, anyone standing in the way will likely die, this happened to one Chief Petty Officer on HMS Alacrity(I think thats the ship but it was a long time ago) who was trying to clear the area when the steering gear had failed. The ship was emergnecy manually steered away from the supplier vessel, cable snapped, the recoiling cable whipped back and destroyed his head.
When two ships sail clse together on a parallel course they displace water in front of them down either side of each.
Since both ships are displacing water into the space between them it follows that it has to go somewhere, it speeds up, this causes a reduction in water pressure in the inside compared to the outside and so a force is generated to push the two together.The only way to keep the two vessels apart is to put opposite rudder angle on them, which is why when the steering gear fails you have to pull away as quickly as you can, any delay could cause a serious incident.
Whenever ships are doing this they close all the watertight doors and the damage control teams are on alert.
You can demonstrate a similar effect with a couple of ping-pong balls, you may well have done this in early science classes, hang the two of them up on cotton about half an inch apart, then blow between them and intuition might say they move further away, but you’d be wrong, they pull together.
Even if the ships are not moving, the exclusion of waves from the region between them will tend to force them together as more wave energy is dissipated against the outward facing hull surfaces than against the inner surfaces. A similar thing happens with closely spaced plates of metal in a vauum: http://math.ucr.edu/home/baez/physics/casimir.html
You’re mostly correct, casdave.
First of all, to answer the OP: Yes. This phenomenon is called the Venturi effect.
I was a surface warfare officer on a cruiser in the Pacific, and had the deck and/or the conn for many underway replenishments.
One thing you had wrong, casdave, is that it’s the receiving ship that controls the maneuver, regardless of its size. And both ships “hold their course;” the first cable sent over is always a sound-powered phone line, to put both bridges in constant contact throughout the evolution. Course corrections are on the order of one degree at a time; speed corrections are on the order of fractions of knots.
They are indeed performed at a minimum distance of 50 yards or so. In addition to the danger of the Venturi effect, the weight of the lines themselves tend to pull the ships toward each other. And the churning water between the ships is lethal to any seaman who should happen to fall in. This is why UNREPs are the most dangerous activity Naval ships engage in routinely. But it was also the most exciting.
Once, while preparing for an UNREP, my captain told me that when he had been an ensign, UNREPs were performed at only 80 feet of separation, and that “eventually the Navy decided life didn’t need to be that interesting.”
Also if the ships are at drift the wind will push one into the other as the downwind ship is shilded. This effect could work underpower too but requires certain wind angles so one ship gets shielded
** Fiver**
Yes I know all about the sound powered phone lines that came across attatched to the distance line, I was that man on the fo’c’sle getting drenched through trying to hide behind the breakwater.
I was the one responsible for all the phones, intercoms, broadcast systems, recreational sound network, flight deck mag loop system and inter systems operator communications.
For those not in the know the distance line is a rope with coloured markers at measured intervals(or luminous divers lights at night), it’s set up in front of the bridge to give the Officer On Watch visible indication of the distance between the ships.
The way it was done was that the supplier vessel would hold its course and we would come in alongside, when the loading was complete we would do the breakaway, if any emergency arose - such as steering gear failure we would do the breakaway, it was never the supplier vessel.
If there was any changes in course then we would direct the supplier to the required direction change and we would then steer accordingly to hold station.
Speed changes the same, the supplied would notify the speed change, the supplier would carry out the manoeuver and we would take the necessary action to keep station.
[quote]
I was a surface warfare officer on a cruiser in the Pacific, and had the deck and/or the conn for many underway replenishments
…But it was also the most exciting.
**
Excitement!?!
So it was your lot that had me freezing my knackers off,in the Atlantic drudge, piss wet through, holding a poxy, rough arsed, chafing line without gloves on (safety-they ain’t allowed) yelling at me to keep the lines taut, and while you’re at it, why could your ops lads never wear a headset without busting it after a few hours use ?
[sup]don’t worry it’s a joke I’m sure he’ll understand[/sup]
Of course they attract each other. All masses do.
in fact the attraction can be calculated with:
Gm[sub]1[/sub]m[sub]2[/sub]/d[sup]2[/sup]
Thank you and have a nice day. 
You can’t possibly believe that gravity between the two boats is stronger than the aforementioned phenomena. It’s like a fart in a hurricane in comparison.
Actually I asked if they would be drawn together (i.e. would they perceptibly move toward each other), not whether there would be some minuscule so-small-as-to-be-immeasurable force acting between them; I can’t believe that the gravitational attraction of the two ships upon each other would ever have any kind of effect that was measurable against the noise of even very small ocean waves.
But it was a clever answer, so thanks.
Will the two ships be drawn together? Well, I suppose that depends on the artist drawing them.
How? An artist who can draw with both hands?
I’ve heard stories of an attempted ship-to-ship cargo transfers between destroyers (one was HMCS Annapolis; the other was a USN vessel, but I don’t know the name) in the North Sea. In the end, apparently, they resorted to a jack-stay transfer with a handy oil tanker (as a stable platform between the two much smaller ships).
I was sorry I missed that one…