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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation > current theory
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Topic Review (Newest First)
05-21-2011 09:59 PM
Waltthesalt This appears to be one of those "Well it works in practice but does it work in therory?" Situations. I think the issue here is the preception of a more tender helm. The boat may be turning at a rate consistent with its speed thru the water but since by your visual refernces to land you're going more slowly when heading into the current the perception is that you're turning more quickly for the speed. According to this theory then your boat's turning should appear to be more sluggish when heading downstream.
05-21-2011 08:25 PM
davidpm
Quote:
Originally Posted by AdamLein View Post
Unless there's a very strong lateral force on the boat (like leeway), this current is always balanced on the two sides of the bow.
This makes sense if your boat is point directly up current.

Just a slight drift to one side or the other and this effect kicks in yes?

"It doesn't really matter as to where you center of effort or propulsion is at... It is the fact that when you are stemming the tide or current (amounts to the same thing), when your bow falls off the course the bow up stream will have pressure on it and the bow section down stream will have much less pressure. THUS your bow will be pushed around if you are not applying correcting rudder."

The point I'm making is that it is not, as far as I can tell, an illusion due to watching the shore.
Next time I'll through a blanket over my head and the pedestal so I can only see the compass. I'm pretty sure I'll still be able to tell by the feel alone that I'm fighting the current.
Do you agree. It is easy enough to test.
05-21-2011 03:06 PM
Boasun
Quote:
Originally Posted by AdamLein View Post
You're talking about the current that moves past the boat as a result of the boat's motion? Unless there's a very strong lateral force on the boat (like leeway), this current is always balanced on the two sides of the bow.
If you're talking about ground-relative current, as plenty of posters have already pointed out, you don't experience this current directly.
The subject is going into a strong current as stated in post #1. Currents from the side are compensated differently...

In post #14 I stated the facts as simply as possible. It doesn't really matter as to where you center of effort or propulsion is at... It is the fact that when you are stemming the tide or current (amounts to the same thing), when your bow falls off the course the bow up stream will have pressure on it and the bow section down stream will have much less pressure. THUS your bow will be pushed around if you are not applying correcting rudder. No need for higher math to explain it.
This is one of the reasons that a ship will take a wild sheer when going up river. The other reason is that the ship sniffed the bottom or the side of the channel. One or all of those reasons can, will and have caused many a ship, vessel and boat to run aground.
So pay strick attention to your steering.

But seeing how I've captained large supply vessels, large factory trawlers and sailed as mates on ships, under Post #1's conditions I do have an excellent Idea of what is happening...
But then there are people who will argue their point of view just for the fun of it... At least I hope that is their reason.
05-20-2011 06:55 PM
Bene505 This one is going downwind (like going down current on a boat), but it shows how the view is different.


This one shows going upwind (lower left picture), but it's a really small picture.


Note that the effect feels dramatic as the plane is really going fast downwind. Even turning broadside to the wind, you get pushed downwind. And after straightening out you end up back behind where you would have been otherwise.

As pilots we had to learn to make perfect circles over the ground on a windy day.

Regards,
Brad
05-20-2011 06:44 PM
Bene505 As I understand it, the only differences when going upstream (e.g. through Plum Gut against the tide on a rainy Sunday morning) are 1) a difference in apparant wind due to the boat's decelleration relative to the headwind we had, and 2) turbulence in the water.

Everything else is perception. It looks strange, as anyone who paddles up a slow moving river can attest. You feel "caught up" in the current when you turn away from due upstream, and it moves you quickly sideways relatively to the shore.

The shape of a plane circling in the sky (maintaining constant heading change) shows this very well by the shape over ground. If I can find that, I'll post it.

Regards,
Brad
05-20-2011 06:22 PM
AdamLein
Quote:
Originally Posted by Boasun View Post
In post #14 I stated the facts as simply as possible. It doesn't really matter as to where you center of effort or propulsion is at... It is the fact that when you are stemming the tide or current (amounts to the same thing), when your bow falls off the course the bow up stream will have pressure on it and the bow section down stream will have much less pressure.
You're talking about the current that moves past the boat as a result of the boat's motion? Unless there's a very strong lateral force on the boat (like leeway), this current is always balanced on the two sides of the bow.

If you're talking about ground-relative current, as plenty of posters have already pointed out, you don't experience this current directly.
05-20-2011 05:28 PM
Boasun In post #14 I stated the facts as simply as possible. It doesn't really matter as to where you center of effort or propulsion is at... It is the fact that when you are stemming the tide or current (amounts to the same thing), when your bow falls off the course the bow up stream will have pressure on it and the bow section down stream will have much less pressure. THUS your bow will be pushed around if you are not applying correcting rudder. No need for higher math to explain it.
This is one of the reasons that a ship will take a wild sheer when going up river. The other reason is that the ship sniffed the bottom or the side of the channel. One or all of those reasons can, will and have caused many a ship, vessel and boat to run aground.
So pay strick attention to your steering.
05-20-2011 04:29 PM
pdqaltair ... and the surface of the earth is NOT a nonaccelerating frame of reference. The Coriolis force is one artifact, though none of the corrections are relevant to the discussion.
05-20-2011 03:49 PM
pdqaltair
Quote:
Originally Posted by AdamLein View Post
If it were, imagine running due west at 10 mph. Because of the rotation of the earth, you're actually moving less than 10 mph through space. If you suddenly turn north, "the vectors would change rapidly" and you'd be thrown around to the east or something like that.
From a point of view, we always sail in a 900 nmph current, not counting velocity through space (which is either un-knowable, irrelavant, or undefined, depending on your aproach). All of the flow arguments are off the point.

Variable curents around piers and bulkheads are a whole nuther matter: I've made more than a few aborted aproaches, when I misunderstood what was going on! Even a relativly steady current can feel pretty weird when the relative acceleration hurls some obstical at you at double speed!
05-20-2011 03:38 PM
AdamLein I am with pdqaltair on this one. Assuming the current is uniform near your boat, the bit about

Quote:
Originally Posted by davidpm
the vectors change rapidly
isn't accurate. If it were, imagine running due west at 10 mph. Because of the rotation of the earth, you're actually moving less than 10 mph through space. If you suddenly turn north, "the vectors would change rapidly" and you'd be thrown around to the east or something like that.

That said, it has been observed in this thread that currents are not uniform. The current is slower closer to the bottom of the channel due to friction. This creates a gradient in current speed along the depth of your keel and between your boat and the shore. For example, if your boat is angled a bit towards the near shore, I would expect stronger current at the stern than the bow. But even this assumes that you are not in an eddy, which can reverse the direction of current near your boat.
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