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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 05-20-2011
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Quote:
Originally Posted by davidpm View Post
Actually the finger in my thought experiment was designed to simulate the propeller. I was visualizing the boat while motoring. Sorry I wasn't explicit.
The force while motoring is at the prop.
While sailing it is at the mast.
It is easier to visualize how tender the helm will be while motoring as the propulsive force is at the stern.

Less easy to visualize is when motoring with a fast current. Again the helm becomes very sloppy. But instead of tender it becomes sluggish.
I have experience this when running with the current through hell gate in nyc for example.
Prop in front of the rudder? Behind? Beside?? Never mind - perhaps it doesn't really matter.. This has been posted earlier, but maybe it wasn't clear:

Yacht rudders and keels have an aerofoil shape for a reason - to generate lift. This "lift" is generated by the flow of water around the foils and are shaped to work best when the flow is from the front to the back - like when you're happily sailing along.

If, for any reason you can think of, the water around you isn't moving, there's no lift and the helm will feel "dead"/"sluggish". If the water is flowing in reverse (ie. tidal flow from behind) the helm will feel really strange - "tender" perhaps? - because it isn't working as designed and is generating "lift" in a different direction and strength than it normally would. No water flow around a yacht moving-over-ground can be a dangerous thing - because, basically, the rudder now has little or no effect on where you are going.

Does it matter where the boat's forward (driving) force is coming from (sails or prop; forward or aft)? Nope. If it did, stinkpots wouldn't have the same prop/rudder orientation at their sterns that real yachts do. ..or vice versa.

Having posted all that, I'm now I'm not sure if I've cleared it up for you or not!
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Last edited by Classic30; 05-20-2011 at 04:25 AM.
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  #12  
Old 05-20-2011
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Really guys, this is simple.

It was Newton that pointed out that all non-accelerating frames of reference are equivalent, and if the current is steady, then nothing has changed...

... except it looks different, since you are basically on a conveyor belt. When you begin to turn away, you apear to accelerate, but only with refference to the shoreline. If you where in the Gulf Stream you would need instruments to tell you there was a current (except for wave action--the waves are often spawned outside the current and have a non-current adjusted spacing).

Just an optical illusion.

Eddys and variable currents are a different matter.
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Last edited by pdqaltair; 05-20-2011 at 03:51 PM.
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  #13  
Old 05-20-2011
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Hey,

IMHO, the difference is the due to the apparent speed of water over the keel and hull. It's just like the apparent wind speed.

When you are moving (motoring or sailing) directly into the current, the flow water past the foils (Speed Through W or STW) is maximized and the lift of the foils is maximized. As you start to turn, the STW drops quickly, and the lift generated by the foils drops quickly. So a small change in direction has a big impact on the force generated by the foil. As the rudder looses force, you need to react faster to stay on course.

Think of sailing downwind in strong, consistent wind. As you sail downwind, the apparent wind is minimized. If you start to turn upwind a little (carelessness by the helmsman, a wave pushes the stern, whatever), the apparent wind quickly increases and tries to pull the boat even more off course. You need to be quick and firm on the helm to stay on course.

It's the same sailing into a strong current.

At least that's how I see the physics working


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  #14  
Old 05-20-2011
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When you are pushing against a current dead on. You have equal current pressure on both sides of your stem. When you fall off one way or another you have the pressure only on one side and a lack there of, on the other and your vessel will fall off its course more readily. Thus you have to pay more attention on your steering when you are stemming a current.
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Last edited by Boasun; 05-20-2011 at 11:54 AM.
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  #15  
Old 05-20-2011
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Quote:
Originally Posted by davidpm View Post
Actually the finger in my thought experiment was designed to simulate the propeller. I was visualizing the boat while motoring. Sorry I wasn't explicit.
The force while motoring is at the prop.
While sailing it is at the mast.
It is easier to visualize how tender the helm will be while motoring as the propulsive force is at the stern.

Less easy to visualize is when motoring with a fast current. Again the helm becomes very sloppy. But instead of tender it becomes sluggish.
I have experience this when running with the current through hell gate in nyc for example.
F=MA... in this case, the mass is still fairly large, the force of the propeller pretty small.. the boat still moves around the center of mass, not the propeller, but the prop will of course generate some rotational force to port or starboard depending on the direction of rotation around the center of mass. Get your finger to work on that one! Of course your finger analogy might work for a Mac26 with it's 60 or whatever HP of love.
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Old 05-20-2011
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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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  #17  
Old 05-20-2011
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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!
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  #18  
Old 05-20-2011
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... 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.
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  #19  
Old 05-20-2011
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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.
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Last edited by Boasun; 05-20-2011 at 05:48 PM.
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Old 05-20-2011
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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.
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