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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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The moving pivot point

Is it intuitive for you when you sail or do you understand the principal and know how to use it to your advantage?

I'm not trying to say I understand it all, but when realizing it and studying more about it, many things about sailing fell into place one day to give that "AH HAH !" feeling.

What's your experience with it?

Andrew
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Re: The moving pivot point

You talking about Center of Effort, and/or Center of Lateral Resistance?
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Re: The moving pivot point

I have the idea in my head but I need a boat before i can even imagine the ah haa that
would go with the Idea. Would this be the feeling that goes with the term tender (not a dinghy)?
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Re: The moving pivot point

I found this article and thought is was thought provoking in a good way. This is what made me ask the question. Give it a read.

SHNOOL- I think its more towards center of lateral resistance, but actually a sum of all the forces involved in an manuever- be it wind, wave, current, any sail combination, or under engine power.

desert rat- not really, more like a Homer Simpson "Doh!" feeling when it all clicks for you and you realize where your pivot point is in any given manuever.


http://www.cnrs-scrn.org/northern_ma..._9_3_53-59.pdf

Cheers,
Andrew
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Re: The moving pivot point

Definitely center of lateral resistance. The article refers to tugs and such a decent amount, and the point (basically on the waterline) where a boat tends to turn.

If I might add, sailors become experts at this, become the best tackers!

If you ever watch a small boat do a PERFECT roll tack you'll see what I mean... the rudder NEVER gets touched.
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Re: The moving pivot point

Not only tacking, but sailing in and out of tight spots if your engine quits on you. That's where knowing this is necessary.

Cheers
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Re: The moving pivot point

Nice find! However, the "moving pivot point" is more apparent to ships and canoes using their form for lateral resistance. Most sail craft use a substantial fin, which has an outsize effect on the CLR.
Try maneuvering a canoe to see the effects discussed.
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Re: The moving pivot point

Me thinks you guys are talking about two different things. Or over complicating it with big words.
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Re: The moving pivot point

That's why I threw it out to the forum. To get other view points. They seem intertwined in a way.
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Re: The moving pivot point

Forget about the center of this or that your just over thinking it.
Usually itís a bit forward of midships. Exactly where varies with the shape of the vessel particularly the underwater shape.
Generally, the finer the lines, the faster the vessel the further forward. Length and Beam play a part as well.
Plaining vessels it may be aft of midships while planing and forward when displacing more at low speed.
For a typical fin keel sail boat just a bit forward of midships.

KISS
The Pivot Point is the point about which a vessel or boat turns
The Pivot Point moves.
When power is applied. The Pivot Point moves in the direction the power is applied.
Apply Ahead thrust or power the Pivot Point moves forward.
Apply Astern thrust or Power the Pivot Point moves Aft.

When the acceleration (or deceleration) is over. The Pivot Point returns back to its original position just forward of amidships.

When a vessel is not moving the Pivot Point doesnít move it will be close to midship.

Now I will complicate things a bit.
Think of a boat with its bow against a dock unable to move forward.
In this case the Pivot Point is right at the bow.
You can create the same situation with a spring line.
(A midship spring its mishap)

Same boat remove the dock and or spring boat stopped in water clear of any obstruction.
Apply a short burst of power. For a very brief moment until the vessel starts to move the Pivot Point will be right at the bow.
As the momentum increases the Pivot Point quickly moves back to roughly midships
This sudden movement of the Pivot Point happens when the vessel is moving. If an increase in power is applied.
You can use this phenomena to help you turn.

You can tighten a turn by applying a brief burst of power temporarily moving the Pivot Point forward.
This effect is most pronounced when the vessel is stopped or nearly stopped.

Often referred to as turning short round.
Vessel stopped or nearly stopped. Put Rudder hard Over, Apply brief burst of ahead trust.
Just as the vessel starts to move ahead. Stop motor and apply a brief burst of Astern
Most of the force will turn the vessel
Usually works better to Starboard as prop walk helps the turn when going astern.
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