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


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Topic Review (Newest First)
12-31-2011 07:49 AM
smurphny As Len has noted, there is no rule of thumb. Sediment does certainly deposit on the very inside of turns except when it doesn't. Learning to read the water is really the only last resort to take a shot at where the channel runs. Where commercial, every day traffic transits waterways, the dredging from their wheels forms the channel. In many places the ONLY way channels are kept open is by boats working in the area. That's why, all else failing, following one of them in is probably a good idea, especially when it comes to shifting inlets.
12-31-2011 02:39 AM
Capt Len If you ever traveled a highly transited river ;like the Rhine, you might notice the variances that river captains follow. Every turn has it's own solution.This might have to do with the geological differences like soft sediments, hard sediments and rocks It may not be obvious but local knowledge and experience trumps a scholarly discussion every time. I've done the Rhine , Mackenzie and Fraser and I still am not qualified to call 'By the mark ,twain'
12-29-2011 10:19 PM
MarkSF For some reason the ONLY thing I can remember from high school geography classes is the method of formation of an ox-bow lake, which is the end result of meandering.
12-29-2011 12:23 PM
svHyLyte
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.
Adam--

With all due respect, the development of sedimentation banks on the inside of the curves of channels is common knowledge and is explained by the fact that the water flow rate slows (relative to the overall flow rate), reducing agitation within the water column, allowing heavier sediments to settle out and at a progressively faster rate. As this sedimentation continues, to maintain the overall mass flow rate which must be constant from up-stream to down-stream in the channel on either side of a curve, water must shift to the outside of the curve, which is logical intuitively as well as mathematically. To change the direction of the flow, i.e. to curve the water-race, there must be a side force applied to the water flow--e.g. the resistence of the banks on the outside of the curve to resist/counter the water's linear inertia. This hydraulic pressure of course eventually dissolves the outside banks--even if they are solid rock--aided and abetted by the scouring action of the sediment suspended in the water itself, not unlike wet sand-blasting a surface such as a hull's bottom. The foregoing accounts for the meanders of rivers over time which one can easily observe from satellite photos of land forms which reveal the evolution of river beds.

For the mariner, the importance of the foregoing is that the deepest water will generally be on the outside of a curved channel tho' one is also likely to find that if "going with the flow", one is likely to get a faster ride than one migh like or, if opposing the flow, little at all.

Here Homer nods...
12-28-2011 07:35 PM
davidpm 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.
12-25-2011 02:14 AM
AdamLein
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.
12-24-2011 10:23 PM
davidpm
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.
12-24-2011 07:39 PM
AdamLein
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.
12-24-2011 07:08 PM
WDS123 great question for the bar - bet would generate huge discussion.
12-23-2011 09:18 PM
Capt Len 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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