My place of work backs onto Portsmouth harbour and I was browsing Google maps, which has very nice clear imagery of the shallow estuary - including some interesting small features that I might investigate at low tide.
Anyway, I noticed that around the points where boats are anchored, there is often a radial pattern - here is a good example.
What are these and what mechanism is creating them? My best guess at this point is that they represent either small furrows where an anchor has been dragged up, or that perhaps they are caused by a rope/chain laying down into the silt and being pulled up again as the tide rises and falls.
I think you have it with your last guess. The giveaway is that the marks associated with the multihulls (all four of the landward most line of vessels) they are like long slits, correlating with the shape of multhull hulls. The sloop to the left of the large multihull is associated with much blunter, rounder marks, correlating with its more beamy hull.
That was my guess as well, since the marks correspond in length and general dimension to the boats that are presently there. But it’s a little strange all the same, because you’d think the incoming tide would wash away the indentations. I’m also a little surprised the owners of the boats permit them to get beached in this way.
The water in this harbour is very calm - as you can see from zooming out to this level, there’s only a fairly small opening to the sea, so although tide affects the levels, it doesn’t do so at all violently, plus the effect is somewhat buffered by the constant fresh water input from the river. There are bits of quite old glass on the shore that are only minimally abraded because the water is so calm and flat.
It seems quite common practice around here to anchor boats in such a way that they end up partially or fully aground when the tide is out - I imagine the hulls are designed with this in mind, or it would be a big problem for everyone.
It has already been pointed out that they are almost all multihulls. Multihulls in most cases do not have keels, they may have lee boards that are retracted and as such would sit on the bottom much easier and not stick in the mud so to speak.
You will also notice that the monohulls are all in deeper water evident by the daker color there are one or two smaller monohulls right at where the transition happens, pretty cool picture where you can see this transition area go from light green to dark blue.
There are monohulls with keels that are designed to be “on the hard” on a regular basis for areas where there are not deep enough waters for a permenant marinas and where mooring out is the norm. The bottom of the keels have a blunt wing protruding from the port and starboard sides, like a upside down T on profile.
As for why these features are still visible and not “washed away with the tide” In most cases the tide does not create much current on the bottom. It is not like a bathtub draining to a centeral drain on the bottom pulling water in. It would be closer to two tubs linked by a surface connection and the water adjusting to a common level. Now out at the entrance to the harbor the bottom is more likely to get scoured by the tidal current do to the restriction created there.
Yes, this is a somewhat common design for sailboats. Many monohulls designed this way will have a short, stubby keel on each side sticking out about 1/3 of the way up the underwater portions of their hulls, rather than one large one on their centerline.
Seeing as how each circle has the shadow of the associated boat right in the middle of it, I would guess the rings are refractions caused by disruptions in the water around the boats.
It seems like all it would take would be minor swells on the water surface to cause each boat to send out circular ripples from bobbing up and down.
Actually, it doesn’t. the boat shadows are almost directly underneath the boats from our point of view, as you can see if you follow the mast shadows down on the boat near the exact centre of the linked picture (and even more clearly on the two at the very bottom centre of the image).