Smoother sea in a boat's wake - why?

dogjones

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dogjones
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I've noticed that the trail left by a boat consists of sea far smoother than the sea outside the wake.

I reckon this is because the propellers/hull etc churn up so many waves that they have a greater chance of canceling out. But interestingly enough the trail lasts quite a long time, longer than I would have thought if this were the case.

Anyone have any thoughts?
 
dogjones said:
I've noticed that the trail left by a boat consists of sea far smoother than the sea outside the wake.

I reckon this is because the propellers/hull etc churn up so many waves that they have a greater chance of canceling out. But interestingly enough the trail lasts quite a long time, longer than I would have thought if this were the case.

Anyone have any thoughts?
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I think waves are caused by wind, and the waves before the boat passes are an accumulation of lots of wind/water interactions going back for some period of time.

My guess would be that the bow of the boat "smooths" this (in a cone that expands behind the boat), the same way a straight edge over a cup of flour smooths that. It takes some time for new wind/water events to take that new smoothness and make it look as random and chaotic as the surrounding water.
 
Ah, but (in powered boats anyway) the first 15 or so feet from the stern is a seething mass of foam, eddies, waves etc. After which it smooths out.
 
A brief search turns up this explanation -- in essence, the wake also includes a higher concentration of surfactant that retards wave transmission.

However, I've also seen this effect in ultra-clean water tunnels, so this is only part of the story. The wake also includes bubbles that have a similar effect, even if there is no oil, plankton, or anything.

There is also residual vorticity in the wake, as the ship essentially sets up a shear layer. This will contribute to mixing and generation of such bubbles long after the ship has passed, and will also confuse incident waves, helping to average them out.

The effect will be of different magnitude for shallow versus deep-water waves, obviously.
 
I think you guys are making it more complicated and mysterious than it really is. The hull of the boat smooths the waves sort of like a knife smoothing the frosting on a cake or a trowel smoothing wet concrete. It's as simple as that.
 
I thought it stretched the water flat.




.
:D

ETA: But only downhill. Uphill boats make ripples.
 
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Towlie said:
I think you guys are making it more complicated and mysterious than it really is. The hull of the boat smooths the waves sort of like a knife smoothing the frosting on a cake or a trowel smoothing wet concrete. It's as simple as that.
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The wedge-shaped bow deflects the wave energy outward, so the boat's wake has less energy and less wave.

Notice that the bow wave that moves out in a chevron shape is quite turbulent. (this is because some energy was added by the boat's engine, in addition to the baseline wave energy on the surface being deflected).
 
A boat's wake (if it's traveling faster than the speed waves travel) is a shock wave, just like an ultrasonic shock wave in the air. Most waves in a lake/sea pass all directions, adding pretty much linearly, to make a choppy surface. But the bow shock is highly non-linear, rolling over the surface and acting like a limiter for those other waves to get past. Ergo, the surface inside those bow shocks is smoother. When the wake has gone far enough out that it's not cresting anymore and becomes just a rolling wave, this is linear and lets the other waves in once more.

So the water will be smoother behind a boat to a width that's as wide as the sharp part of the wake.
 
CurtC said:
A boat's wake (if it's traveling faster than the speed waves travel) is a shock wave, just like an ultrasonic shock wave in the air. Most waves in a lake/sea pass all directions, adding pretty much linearly, to make a choppy surface. But the bow shock is highly non-linear, rolling over the surface and acting like a limiter for those other waves to get past. Ergo, the surface inside those bow shocks is smoother. When the wake has gone far enough out that it's not cresting anymore and becomes just a rolling wave, this is linear and lets the other waves in once more.

So the water will be smoother behind a boat to a width that's as wide as the sharp part of the wake.
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This is correct -- but only in the near field. In the far field, the surfactant effect dominates. The characteristic length scale is L ~ u2/g where u is the velocity of the craft and g is gravitational acceleration. At distances much longer than this, the ship wave is negligible.

This is how Kelvin originally approached the problem...
 
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