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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #11  
Old 12-23-2011
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Quote:
Originally Posted by davidpm View Post
You are sailing down a river with a sharp bend. Is the current strongest on the outside of the bend or the inside?
Current strongest? Generally on the outside.

Fastest path? Generally outside going downstream, inside going upstream.

That's due to more erosion/deeper on the outside. But I could envision a deeper channel on the inside, such as a river going through swamps (Where exactly is the outside?) or a man-made river where the channel isn't necessarily on the outside.

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  #12  
Old 12-23-2011
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I can tell you, from running many whitewater rivers, creeks, and sea kayaking places with lots of current that there is no rule saying that the deepest, swiftest part will be on the outside of a turn. In a sailboat, most of the rivers you run will have been altered by dredging or boat traffic. Local commercial boat traffic is often the major determiner of where the "channel" might be. You really have to take a moment to look carefully at the surface. In observing moving water, a chain of regular waves, not always very obvious, will indicate where the deepest, swiftest part is. Just moving to the outside of a turn will very often bring you into shallow water because you are simply closer to shore. It may look faster because of turbulence caused by rocks. Rock-strewn, ledgy river beds are not readily shaped by current and you just can't use a rule of thumb.
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  #13  
Old 12-23-2011
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Quote:
Originally Posted by davidpm View Post
You are sailing down a river with a sharp bend. Is the current strongest on the outside of the bend or the inside?
Consider a rotating wheel. The angular speed is constant. Measuring the linear speed of the flow tangent to the circumference at any radius from the "axel", the speed must increase as the radius increases (preservation of angular momentum). This accounts for the fact that the deepest water is usually on the outside of a curved waterway; and, bank erosion greatest on the outside curve, due to hydraulic scouring. Further, growth of the shallows on the inside of a curve is caused when the flow of water slows, allowing sediment to settle out at a faster rate due to a reduction in the agitation of the water flow that otherwise holds solids in suspension.

FWIW...
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  #14  
Old 12-23-2011
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Seems like, as with any physical problem, the number of second-order variables that are unseen have first-order effects. Generally, in dealing with tidal currents in Puget Sound, the outside of the bends is the faster track when going with the current. However, local knowledge has saved us many times.

There is a locally well known passage, known as Colvos Passage, up the west side of Vashon Island, that always flows north, even when the flood is proceeding south. If you look at a chart of the area around Vashon, the main flood current proceeds along the much wider and deeper east passage, down the east side of the island, and the fluid dynamics and topography make for a 'downhill run' for the water northward in Colvos. Backeddies can be your friend.
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Old 12-23-2011
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For those of us sailing the bc coast, currents are a big , useful part of exciting cruising .Local knowledge can be expensively earned.We don't have to dodge log booms as frequently as back in the day but they can still add a new dimension to hull damage .Current tends to flow unimpeded under a moored boom and if this is in a bend in the channel or the Fraser River , you suddenly discover 'you not goin wher you point' .Just drifting or engine failure adds to the excitement. Done both so 'Told ya so'
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Old 12-24-2011
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great question for the bar - bet would generate huge discussion.
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Old 12-24-2011
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Quote:
Originally Posted by svHyLyte
Consider a rotating wheel. The angular speed is constant. Measuring the linear speed of the flow tangent to the circumference at any radius from the "axel", the speed must increase as the radius increases (preservation of angular momentum).
True, but a river is not very much like a wheel. The latter is rigid while the former flows. With rigid rotation you will get tangential speed proportional to radius, but with a liquid, not necessarily.

A naive application of the Bernoulli principle puts the fast water inside, since the inside of turns is associated with low pressure, which is associated with high speed.

Erosion could indicate fast water on the outside, or it could indicate that water is better at eroding soil that it crashes into than soil that it rushes past, ie. On the high pressure side of the curve.
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  #18  
Old 12-24-2011
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Quote:
Originally Posted by AdamLein View Post
True, but a river is not very much like a wheel. The latter is rigid while the former flows. With rigid rotation you will get tangential speed proportional to radius, but with a liquid, not necessarily.

A naive application of the Bernoulli principle puts the fast water inside, since the inside of turns is associated with low pressure, which is associated with high speed.

Erosion could indicate fast water on the outside, or it could indicate that water is better at eroding soil that it crashes into than soil that it rushes past, ie. On the high pressure side of the curve.
That's what the book said.
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Old 12-25-2011
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Quote:
Originally Posted by davidpm
That's what the book said.
That's good to hear, but I take what books say about fluid dynamics with a grain of salt, especially when Bernoulli is mentioned. But that was my intuition.
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  #20  
Old 12-28-2011
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This is a page from the book I was talking about.
So from a physics point of view the inner side of the curve will have faster water.
But from a practical point of view and what I have seen myself the movement of soil from the outer to the inner sides means that the inner side shoals making the outer side deeper and the deeper water is usually faster.
The end result is that due to draft requirements it seems like it is most likely the outer side of the curve is more likely to have navigable water.
At least that is what I have seen.
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Last edited by davidpm; 12-28-2011 at 07:40 PM.
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