Inertia and Drag Effects in Ocean Currents


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Sep 20, 2000
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An empirical question here for you mariners and oceanographers.

My recollection of hydrology -- for streams at least -- is that the force exerted on an object by moving water, due to the velocity of the water itself, is proportional (but non-linearly) to its distance from the frictional surfaces of bottom and banks, which impede the flow. Thus, flow velocity towards the top middle of a stream channel is greater than that along the banks or bottom.

Inertia -- the fundamental resistance of an object to changes in momentum (in a vacuum, at least) -- is directly measured by its mass. Of course in fluid-filled, frictional environments, cross-sectional area perpendicular to a given force (draft, in the case of a ship versus currents) is a compounding factor to inertia.

So, my question is this: Would a small object of minimal draft drift further and faster in an OCEAN current than a much larger and more massive object of far deeper draft, assuming there were no "shadowing" interactions between the two?

That is, if I were to place a large ship (like Titanic) and a lifeboat next to each other -- perpendicular to the flow of the current, with no intervening obstructions or interactions -- in, say, the Labrador Current, then wait a sufficient amount of time, would the lifeboat ultimately drift measurably further than the ship? (Assume both remain afloat, of course.) And could the difference be significant on a scale of nautical miles in, say, four to eight hours if the current were gaged at about 1 Knot? What if the ship were a smaller one, say, the Californian -- would the difference, if any, still be significant?

Thanks!
John Feeney

(Note: cross-posted to both a.h.o.t. and the ET Message Board.)
 
Nov 5, 2000
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Hi John,

just a few fast ideas to answer your question

As long there is no wind, big and small objects drift with the same speed. When you put two boats in the water, one with it's bow heading to west and another one with it's bow to south, presumed a southerly current, the boat which is cross to the current needs less time to accomodate to the current than the boat which is in line with the current. In any case the final speed will be identical to the current.
This consideration is applicable when these boats are launched by an e.g. helicopter which is NOT moving while launching the boats. But the boats are launched by a ship which is already moving in the current. So the lifeboats can not "distinguish" whether they are lauchend into silent water by a "silent" helicopter or into a current water by a ship which has already the momentum of the current. The lifeboats already have a momentum according to the drift before they are launched.

I suppose you ask this question because the boats were found on 50° west, whereas the wreckege lies on 49°57' west, which is about 2.3 miles east of the boats. Robert Ballard assumed, a wind might have driven the boats to west in the morning.
In don't know whether this lifeboat position is confirmed by stellar observation of Carpathia or Californian, which was possible not before 4.30 a.m. in the morning. When this position is just the result of calculation of speed and course of Carpathia, who made her last stellar observations probably at the same time as Lightoller did, about 7.30 p.m., i would allow a difference of 2..4 miles between the estimated and the real position.


Markus
 
Sep 20, 2000
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Hi, Markus!

Thanks! I hadn't considered that relativistic aspect you brought up about the boats being launched from an already-drifting ship into the current. (Sort of like the classic "ball tossed up on a moving train" analogy, eh? It goes straight "up" but only from the perspective of someone on the train.)

Actually I'm not sure I had any one particular thing in mind. It just struck me to wonder whether drift could be differential, depending on the objects involved. (I wondered whether various dissimilar "stationary" objects could simply drift *apart* with time in the same current, disregarding wind.) The most concrete example was "big boat, little boat", though I did also wonder about the bodies, which were non-existent to Rostron, but turned up later miles away. (Of course, that's compounded by Blackmar's "inferior cork" theory, which suggests that they sank briefly then rose again later due to "natural causes" -- "nuff said" there!)

Much of this, including my inertia musings, was a flashback to Newtonian physics (F=m*a). The possible influence of draft (or is that "draught"?) came to mind initially with my stream hydrology springboard. In streams, at least, the deeper an object is, the less propelled it would tend to be, since the flow velocity dimishes toward the bottom and banks due to the adhesive and cohesive friction induced. Based on that, I wondered if a vessel of greater draft would achieve a lower terminal velocity in ocean currents because of the lower layers moving more slowly.

(Boy, is this hard to explain!)
happy.gif


Anyway, after that I came upon the opposite notion -- as you discussed in the cross-current example -- that a larger draft might work in current the way a bigger sail does in wind. And at that point, I was totally perplexed (to say the least). From what you're saying above, though, I assume that a slight separation might occur due to initial orientation to the current, but that it certainly wouldn't increase ad infinitum.

Thanks for your input!

Cheers!
John Feeney
 
Nov 5, 2000
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Hi John,

I am neighther a meteorologist nor an oceanographer, so i try to judge from my feeling: Currents like gulf stream or Labrador current travel hundreds or even thousands of miles. So i would exspect that the layers at the surface move with the same speed. I cannot tell whether the gulf stream has still the same strenth in a depth of 100 or 200 yards, but i would exspect not any perceptable difference after 1 or 10 yards.

I have a read a booklet about tidal streams. Sailors have to look very carefully after these streams, but i could not find a remark that they have to take in account the draught of their vessel to determine the drift. 2 knots 3 hours after highwater in Dover are 2 knots. It doesn't matter whether your vessel has a draught of 4 feet or 30 feet. Of course one has to care that the depth is sufficient during low water.

Markus
 
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