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Old 12-02-2009, 18:04   #16
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Your keel is a sail

Your keel is acting like a sail and it is sailing against the current. This happens because your boat is acting like a keel in the wind and giving it something to push against. This should not surprise you since your keel is shaped like a wing and it has lift like a wing.

Interesting note - this is one of the main reasons modern anchors don't have two flukes. Your boat can sail over the anchor and when the tied goes out - bad day. It can also happen with just the tide or just the current.

Have fun take pictures. You have another good sailing story.
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Old 12-02-2009, 18:13   #17
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oh yeah, right at a river bend...interesting..
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Old 12-02-2009, 18:18   #18
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Originally Posted by peterroach View Post
Your keel is acting like a sail and it is sailing against the current. This happens because your boat is acting like a keel in the wind and giving it something to push against. This should not surprise you since your keel is shaped like a wing and it has lift like a wing.

Interesting note - this is one of the main reasons modern anchors don't have two flukes. Your boat can sail over the anchor and when the tied goes out - bad day. It can also happen with just the tide or just the current.

Have fun take pictures. You have another good sailing story.
Wait a minute......where are you guys getting this tuff? A keel has two convex symmetrical sides. Both sides have high pressure. A wing is shaped like a sail. One side is convex (causing a low, high pressure) the other concave (causing a higher, low pressure) thus resulting in lift. That is what drives a sailboat forward when going to windward.
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Old 12-02-2009, 18:23   #19
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Interesting conundrum. However, I'd put it down to global warming. Most things seem to be attributed to global warming these days.
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Old 12-02-2009, 18:32   #20
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Wait a minute......where are you guys getting this tuff? A keel has two convex symmetrical sides. Both sides have high pressure. A wing is shaped like a sail. One side is convex (causing a low, high pressure) the other concave (causing a higher, low pressure) thus resulting in lift. That is what drives a sailboat forward when going to windward.
You are perfectly correct provided that the current is coming from perfectly in front of the boat and the anchor rode is perfectly straight out in front of the boat. Also the wind has to be perfectly in front of the boat. If you have a slight angle on any of these then the boat will sail in the current.

Also, if you look at a lot of WWI biplanes they had a symmetrical wings. Tilt the nose up slightly and the air has farther to go over the top of the wing when compared to the bottom of the wing. You will notice that boats with fixed sails (wings) are symmetrical as well. It works on the same principle.

Finally why do you think your keel is shaped that way? The keel actually sails your boat closer to the wind when you are heeled over. Otherwise all keels would be a flat piece of board. They are a wing.
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Old 12-02-2009, 18:43   #21
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Think Thermocline

Depending upon where you're located, say in a river but close to an opening to the sea for example; and, particularly when there is a good difference in water temperatures between the relatively lighter, warmer, fresh water of the river; and, denser colder sea-water of the incoming tide, the water will stratify with the fresh water continuing its way down stream to the sea near the surface while the tide "floods" up-river below this layer. This phenomena was well understood in the old days of sail around the Chesapeake and in the Hudson River, when sail born river lighters with retractable center boards, would lower or raise their centerboards to take advantage of whichever water layer favored their course. Needing to go "up-stream", they'd lower the board to hook-up to the inbound stream below and go merrily one their way even though floatsome on the surface proceeded down stream. I suspect you're experiencing the same. Your keel and rudder tips are being aligned by the flood, but the yacht is headed in the opposit direction. Pitch a piece of wadded up tissue paper off-board and it will drift "down-stream", into the supposed incoming tide.

FWIW...

s/v HyLyte

PS: Are you sure you really spent time in the Coast Guard?
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Old 12-02-2009, 18:53   #22
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You are perfectly correct provided that the current is coming from perfectly in front of the boat and the anchor rode is perfectly straight out in front of the boat. Also the wind has to be perfectly in front of the boat. If you have a slight angle on any of these then the boat will sail in the current.

Also, if you look at a lot of WWI biplanes they had a symmetrical wings. Tilt the nose up slightly and the air has farther to go over the top of the wing when compared to the bottom of the wing. You will notice that boats with fixed sails (wings) are symmetrical as well. It works on the same principle.

Finally why do you think your keel is shaped that way? The keel actually sails your boat closer to the wind when you are heeled over. Otherwise all keels would be a flat piece of board. They are a wing.
When you are heeled over, the keel has a high pressure side from the weight of the keel trying to force the keel upright (gravity) and a low pressure side for the same reason. The dynamics are completely different from that of a wing. However, it does add a very small amount of forward momentum because the boat is being driven forward by the sails (and additional force). However the keel is acting more like an oar than a wing.

If you were to pull a boat down by the top of the mast (in calm conditions), then let the mast go, the righting movement of the keel would force the boat slightly forward, like an oar.

If this vessel were being driven forward by the motion of the water coming at an angle to the keel, it would give the boat some forward motion but it would also give it sideways motion and the anchor rode would be off to one side and. If you let the anchor rode go, the bow would fall off (to the opposite direction of the anchor rode) from the motion of the water and the boat would travel down stream with the current.

The fact that the rode is streaming straight back is what is so perplexing for the skipper of this boat, I'm sure.
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Old 12-02-2009, 18:54   #23
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OK....you are sitting right on the bend of a river. You are in an area of extreme tidal flow on the surface but the current below could actually be going the opposite direction. This is common in the bend of a river. The other possibility is that your keel is in the mud .

If you look straight across the river, you will see a sand bank. That is caused by the water turbulence picking up sand from the side you are on and placing it over there.

Google earth actually shows you on land. Maybe an aligator has your keel....

Creative spot to anchor. I've never anchored up there, all the times I've done the ditch.

Is this your position:
30 41 06.1n 81 28 02.99 W

If you're at 30 41 06.1n 81 28 29.9 W......you're in real trouble.....
Wait, what? trouble?

I dropped the hook at near low tide in 11 feet and I've got 16 now and it's slack high. I'm a couple hundred yards from shore. There's plenty of other boats anchored out here, some closer to the shore but I'm the furthest toward the mouth of the river. I think its a good spot.
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Old 12-02-2009, 18:56   #24
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In estuaries, the scientists I work with call it a salt water wedge. The previous descriptions of this are pretty much correct. We use a bottom tracking Acoustic Doppler Current Profiler (ADCP) to look at the wedge on a computer monitor. I have seen the top of the wedge as close to the surface as 2 meters and as far down as a few meters off the bottom. On occasion, there can be over a 2 meter per second (four knot) difference between the two different currents. They also rarely run at 180 degrees from one another. These currents can also run in nearly the same direction as well with one current running faster than the other.

Certain types of plankton ride these salt wedges to transport themselves. They have the ability to move up or down in the water column in order to get a ride in the direction they want to go... downriver towards more saline water or upriver towards less saline water. The transport of this these types of plankton are studied quite a bit because they are food for larger critters.

The difference in density of the water is caused much more by salinity than by temperature.
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Old 12-02-2009, 18:58   #25
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What position was the wheel when you got her to sit down current??

Try getting down current again, and instead of locking the wheel...put a bungy cord on it...let the rudder have some give. Beyond that you could have cross currents as Kanani indicates?

How much scope do you have out?
I don't recall wheel position and the current is gone now. I've got about 75' out. Plus chain(24')
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Old 12-02-2009, 19:08   #26
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Depending upon where you're located, say in a river but close to an opening to the sea for example; and, particularly when there is a good difference in water temperatures between the relatively lighter, warmer, fresh water of the river; and, denser colder sea-water of the incoming tide, the water will stratify with the fresh water continuing its way down stream to the sea near the surface while the tide "floods" up-river below this layer. This phenomena was well understood in the old days of sail around the Chesapeake and in the Hudson River, when sail born river lighters with retractable center boards, would lower or raise their centerboards to take advantage of whichever water layer favored their course. Needing to go "up-stream", they'd lower the board to hook-up to the inbound stream below and go merrily one their way even though floatsome on the surface proceeded down stream. I suspect you're experiencing the same. Your keel and rudder tips are being aligned by the flood, but the yacht is headed in the opposit direction. Pitch a piece of wadded up tissue paper off-board and it will drift "down-stream", into the supposed incoming tide.

FWIW...

s/v HyLyte

PS: Are you sure you really spent time in the Coast Guard?
Brilliant! I hadn't even thought of the fresh/salt factor. Good info.
And yes, I really was, but hydrodynamics principals weren't discussed at great length in the electronics lab.
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Old 12-02-2009, 19:09   #27
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Try rocking your fin keel sailboat side to side. Let us know what happens. The keel is definitely a wing if you have a standard keel. Full keels and wing keels act a little different but they all have wing qualities which I have learned to appreciate when my engine quits and I need to get into a slip. "Just rock the boat baby"
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Old 12-02-2009, 19:12   #28
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Try rocking your fin keel sailboat side to side. Let us know what happens. The keel is definitely a wing if you have a standard keel. Full keels and wing keels act a little different but they all have wing qualities which I have learned to appreciate when my engine quits and I need to get into a slip. "Just rock the boat baby"
I have a wing keel.
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Old 12-02-2009, 19:17   #29
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The oscillation you are getting is a pretty common phenomena. I have seen this before. Its much like a pendulum but energy is added each time your boat is partially broadside to the current. When the pendulum gets far to one side, the tension on the anchor line gets to the point where it whips the bow back the other way....past the point where your bow is head to current. Your boat gains energy going the other way, gaining energy again. Try putting your rudder hard over and this may help stop the oscillation.
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Old 13-02-2009, 23:30   #30
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Symmetrical wings and even flat plates create lift. All you need is fluid flowing and an angle of attack.

Here is one of a series of articles by Arvel Gentry.
http://www.arvelgentry.com/magaz/How...Gives_Lift.pdf

Who is he?
Who Is Arvel Gentry?

Lift, force on the object perpendicular to the fluid flow, is always associated with drag, force on the object in the direction of the fluid flow, so the total force, vector sum of lift and drag is always greater than 90 degrees to the fluid flow. Take the kite example. How far over your head can the kite go? It will never reach directly overhead. Apply this to a keel and anchor rode in current with no wind. The boat will never reach 90 degrees to the anchor. Current sailing can't account for the boat sailing upcurrent of the anchor.

The keel is a wing. It requires an angle of attack, boat crabbing or being pulled slightly sideways, to produce lift. This is the leeway. In normal sailing this is caused by the sails, which most of the time are pulling at an angle to the direction of travel. This works whether the boat is heeled or not.

If you don't think keels produce lift, try sailing a short sailboard. When I was learning to point higher the board suddenly took off going sideways downwind, I was absolutely convinced that the skeg had snapped off. When I flipped the board over the skeg was there and fine. Did that two more times before I figured out that I had increased the angle of attack to the point that the skeg was stalling out (and/or ventilating). Pushing the skeg sideways through the water at 15 knots gave no feeling of resistance whatever, whereas when it was producing lift it felt like your feet are pushing against a wall.

John



More Gentry articles:

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