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Old 02-19-2014, 02:36 PM   #21
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Night and day difference at anchor or mooring with the sail up.


The swing arc is dramatically reduced.


Would be even better if the mast were further aft and the sail larger.

my boat is fairly light at 20k#'s.

The jury is still out on whether the hullform is true displacement or not.(Eric will decide).
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Old 02-19-2014, 02:44 PM   #22
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All this suggests to me that we don't REALLY know what the detailed dynamics are. Where are the rocket scientest when you need them......
I cannot mathematically express this but I know what makes my boat a bad a$$ boat at anchor. It's not much about center of gravity, or center of buoyancy, as much as about combined center of effort above water and combined center of resistance under water ... very much like with a sailing vessel. More info here: Applied Sailing - Technical Support For Sailors - ASA 106-2: Force diagrams

My bad a$$ boat has center of gravity and center of resistance close to its heavy stern, and center of effort close to its high windage and light bow. This is what makes its bow swinging away from wind at anchor, and when its stern catches with the bow the pendulum starts swinging the other way.

The design is so bad that I cannot swing the bow into the wind stronger than 5 knots while going in reverse at idle RPMs with a single engine. Once I manage to force the bow to cross the wind (higher RPMs ... not good while docking), it falls off the wind to other side at rapid rate and is impossible to control. This makes for a very entertaining docking ... not!
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Old 02-19-2014, 03:00 PM   #23
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the worst boat I ever saw sailing at anchor was actually a sail boat. At times I though it was going to hit it's neighboring boats it was so bad. The dock guy said that boat always does it.
This may be due Bernoulli's principle on the fin keel acting like a sideways underwater wing as pulls it one way, angle of attack changes, then it stalls, the anchor rode pulls the bow around on a different tack, until it stalls....

... or maybe it's just ornery.
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Old 02-19-2014, 03:07 PM   #24
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I just found this Rode Dynamic Behavior 3 which help illustrate and explain whats going on. From it, you can see where all our intuitive ideas come into play.

Here's my attempt to summarize:

There are three significant elements at play:

1) The bow attachment point

2) The center of windage when the boat is at a slight yaw

3) The center of gravity of the boat.

The closer the center of windage is to the bow attachment point and further from the center of gravity, the more the boat will sail back and forth. This creates an unstable dynamic.

The further the center of windage is from the bow attachment point and the closer to the center of gravity, the less it will sail.

Sailing happens because, with a forward center of windage, a slight yaw causes the bow to push off of the wind and yaw more and more. As the boat becomes more and more broadside to the wind, more of the boat's side is exposed to the wind and the windage center moves back. Plus, the anchor rode pull is more and more off to the side helping to counteract the yaw. Eventually the process reverses, but just keeps on going.

Intuitive remedies work as follows:

Bridles: These have the effect of moving the bow attachment point further forward, but it's effectiveness depends on how far apart the side attachment points are on the boat. It works particularly well for catamarans, and will be less effective for boats where the two sides of the bridle are close to the center line.

Anchor Sails: These move the windage center further back.

Cool stuff. Now I think I understand.
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Old 02-19-2014, 03:13 PM   #25
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The further the center of windage is from the bow attachment point and the closer to the center of gravity, the less it will sail.
That is the best take away from this discussion as the cause of swinging.



How to counter it when the center of windage and effort is on the opposite end from center of gravity and resistance ... and a back sail is not an option? Bridles? What else?
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Old 02-19-2014, 03:15 PM   #26
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Nordhavn delivery/training captain w
Is his name John?
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Old 02-19-2014, 03:16 PM   #27
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I just found this Rode Dynamic Behavior 3 which help illustrate and explain whats going on.
It took forever (in computer terms, about 20 seconds) to for me to update and download shockwave (and not install McAfee), to view his graphic, and then it didn't even show the vectors. But yeah, that's pretty much how it works.
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Old 02-19-2014, 03:32 PM   #28
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How to counter it when the center of windage and effort is on the opposite end from center of gravity and resistance ... and a back sail is not an option? Bridles? What else?
Perhaps other creative ways to alter the windage profile using canvas, etc. It would be very specific to a particular boat, but a curtain here or there might help.

Also a very long bowsprit/pulpit?

And a bridle with wide attachment points. But it seems there is a tradeoff with that since the side attachment points are typically further aft than the bow roller, so you are improving in one way, but worsening in another. Who knows where it would shake out.

By the way, this suggests that a low attachment point like a tow eye should not be any better than a high attachment point up on the bow. In fact, there is probably an argument that a tow eye would be worse since it is likely further aft than the bow roller......
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Old 02-19-2014, 05:04 PM   #29
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Stolen from an article showing CE and CLR effects on yawing by Greg Walsh...

Two important design factors are also involved in a typical anchoring situation: Center of Effort and Center of Lateral Resistance.

Center of Effort is the point through which all wind forces tend to act on a boat. It is different for all vessels and it varies with conditions of sail, rigging, deck gear, cargo, and deck structures. It is, in effect, the total average midpoint of everything taken together on the boat above the waterline. An invisible finger pushing on the CE would simply make a boat heel directly over to one side. It is an important factor to consider when a boat is anchored in rising wind conditions.
Center of Lateral Resistance is the geometric center of the bottom configuration. If one were to dive underwater and push on the CLR, the boat would theoretically move directly sideways. CLR can change as a vessel heels, but it generally doesnít move much.
In most situations the CE and CLR need to be reasonably close for a boat to be balanced and handle properly. Strategy for boat anchoring is also a function of these factors. The CE usually is forward of the CLR, even for an anchored boat. This is because of the traditional shape of a hull without sails - there is more mass up forward. Different styles of boats have more or less mass forward of the beam.
A vesselís tendency to yaw or swing is enhanced as the CE moves forward of the CLR. The more surface area there is forward of the CLR, or forward of amidships, the more yawing tendency will develop as winds and waves increase.
If the CE can be kept aft - or moved aft - a boatís tendency to ride properly at anchor will be much improved. That is one of the great advantages of a yawl or ketch rig, or of a vessel equipped with a small riding sail flown from the backstay. These small, flat sails tend to move the CE aft and keep the boat weather-cocked up into the wind. (The sail must be completely flat, however, so as not to drive the vessel ahead.) Considering the awesome power of a gale of wind, any such riding sail or mizzen sail must be tough and very small, perhaps just a few square feet, so as not to be overpowered. Just as often, however, even the smallest riding sail must eventually be doused as the wind reaches the upper ranges of intensity. What may save a boat at that point are efforts the crew has already made to move the CE aft by reducing windage as much as possible forward of amidships (stripping sails, booms, excess rigging and equipment, etc.).
Yaw, of course, is difficult to avoid, since the force of wind and waves almost always strike the boat at some point forward of the CLR The motions of yaw and swing seem almost inconsequential when anchored in light conditions. As wind increases, however, the tendency to yaw and swing can combine with catastrophic results. As soon as a boatís bow blows off the wind (the boat turning on its vertical axis) it will begin to swing off to one side because of the force of wind on its side. As more of the boatís side is exposed the tendency both to yaw and swing will increase. Before long, in the worst situations, a boat may be swinging back and forth like a pendulum at the end of its anchor rode, each time yawing so as to present a different side to the wind. The acceleration of swing in each direction can dramatically increase the forces involved, especially at the apex of each swing. All the work of stopping the swing, reversing the bow, and holding up to total wind force falls directly on the anchor and its gear. Total loading on the anchor gear consists of pure wind force per square foot, plus additional forces for acceleration or momentum of swing, surge, current, and wave action.
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Old 02-19-2014, 05:38 PM   #30
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Two important design factors are also involved in a typical anchoring situation: Center of Effort and Center of Lateral Resistance.
Right, this is what I said #22 ...
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Old 02-19-2014, 08:12 PM   #31
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I have tried to use reefed mainsail to position the boat so the bow is facing the swell.
One problem is the sail is too far forward; a mizzen sail would be much more effective.
Still, I can get it set to point the right way, and all is good.... for 10 minutes or so. Then the wind changes slightly, or the tidal flow increases/decreases, or the moon rises; changing the equilibrium ever so slightly.

So I just drop an aft anchor and all is good.
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Old 02-19-2014, 08:16 PM   #32
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Ah yes Twistedtree that's the part I was missing .. CG. I see that now and that explains why Willy swings a lot even though she has a big long keel, plenty of weight and a bow that's only semi-high. Her CG is unusually far aft. Mystery solved.

A similar boat (Fisher) has a much further fwd CG (judging from her slack cheeks aft) and I'll bet swings much less. Better make a bridal. To be most effective it could tie off or attach to the gunn'l amidships ...... what think?
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Old 02-19-2014, 08:36 PM   #33
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It's nice to have some science behind all this to guide our experiments to reduce swinging. Otherwise it's just poking in the dark, and guessing at the cause and effect. I'm too lazy for that.
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Old 02-20-2014, 03:03 AM   #34
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The cause is usually the owner demand for a roomeran ,

the cure is a second anchor out at about a 60 deg angle from the bow.
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Old 02-20-2014, 05:16 AM   #35
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Perhaps a study of the "pivot point" would help illuminate this discussion ... or cloud it even further.

Hint: For the purpose of this discussion, CG is irrelevant.
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Old 02-20-2014, 05:56 AM   #36
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I think design elements like a high bow and fwd cabin linked w little windage and keel aft. Not say'in that's the combo but consider a boat w a low bow and a huge cabin right at the stern. Probably make like an arrow and get stable ... especially if the boat is long.
Eric, I think you are right on the money there, and I'd wager the Nordhavn 62, rides very nicely and sedately at anchor, and probably without a bridle. There's one on our forum, who might chime in and corroborate this, but I suspect it is precisely because it is long, and has most of the windage aft. I noticed my Lotus sat much quieter at anchor, and sailed back and forth much less, after I had the extended flybridge and side deck canopy added, because it sort of acts like the fletching on an arrow.
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