How deep would you need to descend in a submarine or other submersible craft before you stopped feeling the effects of the waves at the surface? I’m not talking about deep ocean currents, but the surface wave motion. I have heard guesses that typically, the depth of the descent would need to be about five times larger than the wave height – in other words, if the seas are churning with five foot waves, things would get relatively calm below 25 feet. (That, in fact, has been my experience in my only sub trip to date). Using this 5x principle, a “Perfect Storm” scenario, where killer 80 foot waves were reported,would require going down about 400 feet to reach relative calm. Anyone have a good idea about this?
Never been in an ocean sub. From scuba diving in the ocean w/ swells, I can say that around 15 ft you can’t feel much of anything, and this was with 6 ft swells.
I do some occasional diving, and my observation is that it depends on how deep the water is. In deeper water, I’d agree that at 15 feet you wouldn’t notice a thing in 6-ft swells. But when I was lobster diving off the beach in San Diego a while back, my buddy and I were getting tossed around pretty well at that depth, since that was close to the bottom. Things calmed down some when the bottom went down to 25, but it was still noticeable. Waves on the surface were probably about 3 feet max.
I had some experience aboard a Navy sub. In one instance, there was a MAJOR storm going on above us in the North Atlantic. As we ascended to PD (periscope depth), the boat started making increasingly scary rolls. Eventually the skipper decided there was no way we were going up into that mess. As I recall, we descended past 250 ft (keel depth) before the rolls subsided. (Normally the rolls subside immediately upon submerging.)
I saw a formula that addressed this, but I don’t recall exactly what it was. As spirtle surmised, it was not a linear relationship. As I recall, it was an exponential decrease that was proportional to the distance between wave crests and the distance from the crest to the trough.
As a side note, I once dived on the wreck of the Eagle off the coast of Florida. It lies at a depth of around 120 feet. The ship is steel, is about 287 feet long and was broken in half by one the hurricanes that came through a few years ago. I would hate to have been on the water in that storm!
Gravity Waves 101 - or as close as I’m going to get without digging out my textbooks:
Diver’s on the right track - the motion of a particle of water in the water column beneath a wave is related to several things: Amplitude (height) of the wave, length of the wave, speed of the wave, depth of the overall water column, and depth of the particle in question. It’s actually a logarithmic function, where the velocity of the water particle is proportional to (among other things mentioned above) e^(kz), where z = depth below the surface. Wave equations can get pretty complex, but for your average wave on the surface, things get pretty close to zero movement somewhere around where the depth is equal to the wavelength, i.e., if you had 6’ waves that were 100’ between crests, you’re going to be hard pressed to measure any movement 100’ below the surface. What you can “feel” in real life would be considerably less, which any diver can attest to.
In a perfect wave (which could only be formed by a perfect storm of course!) water particles in a wave actually move in a circle as opposed to what you might think, though it has to be in deep water, which is considered at least twice the distance between wave crests. In shallower water, the circles begin to flatten to ellipses until in very shallow water, the particles are pretty much moving back and forth, something else anyone who’s been to the beach has experienced near shore, i.e., “surge”.
If you really want to hurt your brain, go to this site (I’ve never tried putting in a url, but here goes…) http://www.dnv.no/ocean/nbt/Wind/zFrame.htm ;j ← I did learn a few emoticons though!
I was so bored today, I actually plugged the wave equations into a spreadsheet, so if you want to see a plot of how the movement drops off with depth (no pun intended), let me know. Though it’ll have to wait till after the holidays…
[I fixed the link – Chronos]
[Edited by Chronos on 12-22-2000 at 11:54 PM]
PeeWee, if you want to do links the easy way, just leave off the tags entirely, and the board software will (usually) put them in automatically. If you want to get fancy, then the format is
[[sup][/sup]url=“http://www.straightdope.com”]The Straight Dope[[sup][/sup]/url]
which will become The Straight Dope.
By the way, what does this have to do with gravity waves?
Thanks for the help Chronos!
It doesn’t have anything at all to do with THOSE gravity waves!
“Gravity Waves” is a common oceanography term for your basic semi-sinusoidal ocean wave, as opposed to “trochoidal waves” or “breaking waves” or others. There’s at least a few different mathematical models for predicting wave surface profiles as well as individual particle velocities and directions (as I was referring to above.)
;j 
← I did learn a few emoticons though!