Excess Swinging at Anchor

[morgan3d]

Last night I was anchored in 25-30 kts, with full protection from waves due to a breakwater but only partial protection from the wind. There were two other sailboats and we were all in about 12' of water.

We were on an almost unladen Jeanneau 389 40' charter, with a very short single snubber (what was available) and about 90' of rode out. The rudders were locked in the center position at the wheel. During the gusts we were slowly swinging like a fish in a 20' arc, first to one side and then the other, like a sailing dinghy being towed with centerboard down. We were also rolling more than desirable as it fished back and forth, despite almost no waves. Fin keel, dual spade rudders. We didn't drag at all on the anchor overnight.

The other boats looked like liveaboard cruisers, both 40'. One had a full bridle snubber that looked like at least 20' x2, and the other had a single snubber that was about 10'. Both had much higher windage than us, including large stack packs, biminies, and wind generators, but were sitting lower in the water and probably a lot heavier. I don't know what their keels were. They barely swung in the gusts and did not appear to be rocking significantly.

Was the difference due to the weight? The bridle? The keel? Is there something we could have done to stop the swinging and rolling? I didn't want to set a stern anchor because I was concerned about the wind backing in the night and wanted the big swing to match the boats around us.

[Lodesman]

Most people think a boat points directly into the wind, but it's rare that they do. The hull shape is essentially an aerofoil, on its side. As it gains a positive angle of attack, lift is generated which pushes it out to the end of its tether (anchor), where AoA changes direction, it goes the other direction, and so forth. The typical anchored boat scribes a sloppy figure-8. I like to use an offset, or side bridle to put the port bow (or stbd, depending on the situation) into the wind. This eliminates the oscillation.

[GordMay]

Originally Posted by Lodesman

Most people think a boat points directly into the wind, but it's rare that they do. The hull shape is essentially an aerofoil, on its side. As it gains a positive angle of attack, lift is generated which pushes it out to the end of its tether (anchor), where AoA changes direction, it goes the other direction, and so forth. The typical anchored boat scribes a sloppy figure-8. I like to use an offset, or side bridle to put the port bow (or stbd, depending on the situation) into the wind. This eliminates the oscillation.

Indeed.



See also: Wind Against Tide: “ANCHOR SPRING LINES”
➥ https://www.cruisersforum.com/forums...ml#post2917149


and: “Sailing at Anchor” ➥ https://www.cruisersforum.com/forums...or-211012.html

and “Ways to Reduce Extreme Swing at Anchor”
➥ https://www.cruisersforum.com/forums...or-268185.html

and: “Weaving Back & Forth at Anchor” ➥ https://www.cruisersforum.com/forums...r-32665-2.html

[JBP]

Stack packs, Biminis, and other windage aft of the keel help reduce sailing at anchor. This is how an anchor sail works. Windage forward, like a tender on deck, makes it worse. Also try locking the rudders to one side, this helps a lot on a tiller rudder.

[WatchingWaves]

My Hunter sailboat has the same problem. It swings way more than most other boats once the wind speed is over about 18 kts.


I spent my first winter on anchor in the Bahamas last season where this happened a lot. So after a lot of research, I improvised a simple riding sail by cutting a corner off an old tarp. It worked surprisingly well.



With out it, the side to side distance of my swing would be about however much rode I had out. With the riding sail, it went down to less than 40 feet and felt much more comfortable. No more hard jerk when the boat changed direction and less rolling.


This worked really well until one night the winds gusted too high and shredded my tarp.


First thing I did when I got back to the states was buy the Sailrite riding sail kit. Can't wait to try it out.


https://www.practical-sailor.com/sai...-a-riding-sail


https://www.sailrite.com/Anchor-Ridi...-20-25-Sq-Feet

[capn_billl]

I've had the exact same issue.

Turning the rudder, running the boom to one side, or even a stern anchor.

Sometimes they help, sometimes not.

[CarlF]

A Jeneau 389 will swing more than most designs. 1) Mast relatively far forward, 2) Fin keel, 3) almost flush cabin reduces windage aft. 4) Light 5) canoe bottom

A riding sail would help

[TrentePieds]

Read posts #2 and #3 carefully :-)!

Lodesman is dead on! The boat thinks it an airfoil. What you have to do is "stall" the airfoil by increasing the "angle of attack" so the flow over the "airfoil" breaks, goes turbulent and no longer produces "lift". If you do that to an airplane, you'll fall out of the sky. When you do it to a boat, she'll stop "hunting" from side to side and become far better behaved.

The way you increase the angle of attack is by increasing the "angle of incidence". You do that with a bridle as GordMay pictures in Post #3.

If the boat is "hunting", it's telling you that you haven't done you job of anchoring right. But now, thanks to Lodesman and GordMay you'll know how to do it right in the future :-)!

TrentePieds

[s/v Jedi]

Well, no. What Gord shows is the bridle to lay at a different angle to the wind so that you get an incoming swell on the bow instead of rolling to death.

What the post before that says is what we do as well: bring the snubber not over the anchor roller but through a mooring chock on the port or starboard side of the bow.

If that is not enough, you need a riding sail or more windage aft. The riding sail is the final solution if the snubber off center doesn’t help (enough)

[Lepke]

When I snub the anchor chain, I use 2 heavy rubber snubbers from the same point on the chain to points about 15' back from the bow on each side. Snubbers are close to the chain so they don't rub on the boat. Nylon line is used from the snubber to the attachment point on the side.I adjust the chain and snubbers so the snubbers are taking some of the chain weight by letting out a little more chain after rigged.
When the wind pushes the bow to one side the snubber on that side is stretched more than the opposite side so pulls the bow back before it goes far. In lighter winds I use nylon line alone the same way.
The snubbers are heavier than the ones usually found on yacht dock lines. Although dock line snubbers would probably work on smaller boats.

[Chotu]

I used to have those rubber snubbers. Current boat just chews them up and spits them out in pieces. Lol

Ripped up the big ones to fit 3/4” line just through how hard it pulls on the line.

They used to work quite well with my other boats however

[Tedd]

Originally Posted by JBP

Also try locking the rudders to one side, this helps a lot on a tiller rudder.

D'oh! Why didn't I think of that?

[Eder]

A trick I used often in the Sea of Cortez was to put a 20' snubber on the chain led back to a cleat. Then let out at least another 80' of chain to hog down the bow a bit.
Tamed those Coromuel winds a bit.

[JPA Cate]

Almost any boat lies more quietly to chain than to rope rode, so that is a possible change you could make. With all chain, you mostly are safe on 3:1 chain to depth ratio, so for 12' plus the height of the topsides, maybe 4 more ft., 3 x 16 = 48' rode.

If it is your boat, and you really don't want her dancing at anchor, there are other approaches, including dropping a small sea anchor over one side of the bow. Some people run really long snubbers, all the way back to the midships cleats. I'd try the latter first, because anything you do that makes it more steps or more complicated to get the hook up, makes it harder to leave quickly. You mostly don't have to leave quickly, but when you do, like dragging onto a lee shore, you REALLY do.

Ann

[SV Coronado]

Did you have an all-chain rode? Were you anchored someplace with a strong current? Both will cause boats at anchor to swing differently.