A boat passes by in the bay.
It leaves a discernable disturbance in the water. It is not the wash though.
That disturbance lasts for a while afterwards, leaving a trail about as wide as the boat’s displacement.
Why does this happen? Is it to do with oils from the boat?
I was going to post a full reply, but then I found this link which explains this, and wakes in general, far better than I could:
http://www.madsci.org/posts/archives/jul2001/994688283.Ph.r.html
From there:
“…the long streaks of calm water that reflect the sunlight better
than the surrounding water are most probably a complicated non-linear
interaction of the narrow-V, the vortices, and some internal wakes that
depend on the exact structure of the water column.”
I have wondered the same thing, and I figured it was just all those tiny microbubbles that formed from cavitation and from the disturbance of the water. On big ships that draw 40 feet of water, these could take quite a while to reach the surface. But the trails can be miles long, which still is hard to explain even with the explanation given in Kymodoce’s link. I tend to think that a small amount of leaked oils also helps make the trail. That’s because a small amount of oil can make a sheen over a very large area given the right conditions.Here’s a picture I took a couple weeks ago, it shows what the OP is talking about in the upper part of the picture.
Does the phenomenon also occur behind craft not powered by a motor that might leak oils?
That’s an interesting question. Of course, not many boats powered by anything other than traditional “oily” engines go as fast as those that are, and this could affect the amount of disturbance in the water column.
I tend to think that oil is a red herring, as it were. Is it likely that any powered craft would leave behind sufficient oil to have this effect? Even at very small concentrations that’s going to amount to a lot of oil.
Whilst bubbles in the wake may rise and dissipate too quickly to form the length of trail witnessed, how about the swirling vortices from the stern? It seems to me that the disturbance in the water from these currents could last much longer as they are not turbulence per se.
I think vortices are probably the answer; if you look closely at the trails, they seem to consist of a smooth middle, a pair of ‘rough’ looking edges and then normal ocean surface; I wonder if the edges of the boat create a parallel set of nearly-static vortices that form some kind of ‘curtain’, sheltering the zone between them from wavelets.
