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Why Boats Yaw At Anchor

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Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Initially Posted - 01/11/2004 : 18:18:04

[url="http://www.stewartfam.net/arlyn/anchor_yaw.html"]Why Boats Yaw At Anchor[/url]

Arlyn C-250 W/B #224

N/E Texas and Great Lakes
Arlyn's Sailing Site

Edited by - on

Sea Trac
Master Marine Consultant

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Vanuatu
1357 Posts

Response Posted - 01/11/2004 : 19:19:10

Nicely written conclusion to all the previous discussions, Arlyn. I'm also convinced that hull foil is the cause of sailing at anchor. Personally, I believe that the high freeboard at the bow (i.e., in relation to the stern), however, is the cause of the non-tack in higher wind speeds. My opinion is that the increased force on the bow overcomes the momentum force exerted by the rode and the stern. Thoughts?

A couple of additional questions:

First, conventional wisdom has it that sails and running rigging should NOT be completely stowed when anchoring or at anchor, so as to offer an emergency fail safe. Since you mention that you were glad that you had covered the sails and belayed the halyards, what are your thoughts here?

Second, how do you plan on overcoming the complication of driving rain as it relates to the stern anchor scenario? Given that stern anchoring would be most beneficial during a squall, one is likely to get a cabin full of water; even with crib boards in place.

Edited by - on

jsummerfield
1st Mate

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USA
96 Posts

Response Posted - 01/11/2004 : 21:57:04

Look into Von Karman Vortices or Vortex Shedding
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1985ATJFE.107..298G
http://www.eng.vt.edu/fluids/msc/gallery/vortex/re200.htm
http://www.mecaconsulting.com/vortex_shedding.htm
http://www.itsc.com/movvkv.htm

With vortex shedding, continuous vortices form downstream of a solid object in a gas or liquid stream. The vortices are carried downstream by the flow. A vortex swirles on oposite sides of the obstruction. This oscillation is used for flow measurement and is similar to the phenomenon at anchor.

Edited by - jsummerfield on 01/11/2004 21:58:35

Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 01/11/2004 : 21:58:15

The recent discussions caused me write the results of that incident that I'd planned but put off for a long time.

I found it quite interesting that while I had believed from shortly after the incident that hull lift was the cause of anchor hunt, (Don Lucier and I traded ideas on this a year or so ago) the thought never came to me that the same dynamic was involved in adding leeway to a weather course until today when finishing up the piece.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">My opinion is that the increased force on the bow overcomes the momentum force exerted by the rode and the stern<hr height="1" noshade id="quote"></blockquote id="quote">
I don't know... it seems to me that the bow will come up on the rode and the stern will be both pushed and pulled through to force the tack. My guess is that the only reason it didnt when I was dragging was the latteral resistance held that process at bay... and of course the high wind kept pushing the bow off with the anchor dragging... so that the bow never came up in check. I gotta believe that the only reason it didn't tack in the high wind was the anchor was dragging.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">conventional wisdom has it that sails and running rigging should NOT be completely stowed when anchoring or at anchor, so as to offer an emergency fail safe.<hr height="1" noshade id="quote"></blockquote id="quote">
I'm not a great subscriber to that wisdom. There have been a great many furled headsails destroyed when microburst got a hold on them. The best prevention for that is the jib halyard well wrapping it. We've had several reports of headsails lost to them reported to this forum.

As to the main... some creativity could keep the halyard from slapping and allow the main to be reaady to go... but it would take a beating by the bugs if not covered.

<blockquote id="quote">quote:<hr height="1" noshade id="quote"> overcoming the complication of driving rain as it relates to the stern anchor scenario?<hr height="1" noshade id="quote"></blockquote id="quote">
The c250 has an all weather pop top cover which allows for standing headroom and a water tight companionway in foul weather.

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Oscar
Master Marine Consultant

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USA
2030 Posts

Response Posted - 01/12/2004 : 09:10:25

Question: Would steering the boat, possibly with motor assistance, have stabilized your dragging situation, ie. allowed you to keep the bow into the wind untill it all calmed down?

Oscar

Lady Kay 250 WB #618
Sunrise on the Neuse River...

Edited by - on

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 01/12/2004 : 09:52:47

I read an amazing eyewitness account of extreme yawing behavior from a cruising couple caught in one of the major hurricanes in the Carribean. They took a direct hit while they were in what was regarded as a 'safe' hurricane hole... along with a lot of other boats.

They described the anchored boats sailing wildly from port to starboard and sometimes running directly into the wind and sailing right over their anchor rodes. Many of the vessels in the anchorage were beached and destroyed when they pulled their anchors out, parted their gear, or were run down by boats that had broken loose. From their account, it was apparent that in extreme winds the boats literally became a 'kite on a string'... and a very unstable kite at that.

Their boat was saved when the skipper started the engine and 'drove' the boat while still on the anchor, avoiding the wild yawing effect and also steering to avoid boats being driven downwind.

Edited by - on

JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 01/13/2004 : 00:22:42

I don't know why boats yaw at anchor, it could be due to random forces (slight wind variations). I do know that, once your boat is beam on to the wind the force of the wind on your boat goes up by many orders (as you are presenting more surface area to the wind). Perhaps a slight random variation could get you turning in one direction, then the force of the wind on the boat increases as it turns. This causes more turning and pushes the boat to leeward. When the boat comes up hard on the rode, this produces a turning force which pulls the bow back into the wind. This actually over corrects and turns you the other direction so you sail that way.

Notice how, in very light air, every boat in the anchorage is pointing in different directions? This shows the importance of random forces. Once the wind comes up a little they all line up.

You didn't mention WHERE this occured (I assume Great Lakes?) or mention what ground tackle and rode you deployed. I am curious. Do you have water ballast and a centerboard? Was the board down? What was the bottom? I am sure you know that (in moderate winds) sailboats tend to lie to the current while power boats tend to lie to the wind. Was there any current? All those questions are vital to understanding why you dragged beam on to the wind. I suspect that the anchor did not break loose and was providing a force, as was the wind, and the vector sum of the forces indicated the direction you dragged. However, a power boat will come beam on to the wind and then drift down wind. If your board was not down you would have less underbody and act more like a power boat.

While I can't comment too much on the theory, I am interested in the art and prtactice of anchoring. I like to have lots of weight on the bottom, which is why I use 50' of chain on my primary anchor plus lots of nylon rode. Here in CA I use a 13 lb Danforth as primary. I like to use an anchor bouy because you can see at a glance if you are dragging (the bouy gets dragged under). You can also see where the anchor is when you randomly wander all around the anchorage. If any kind of storm is expected I almost always get two anchors down, off the bow, about 90 degrees apart. In a big blow in the Bahamas I had 3 bow anchors down! In the Bahamas we often used the "Bahamian Moor" in which you deploy your bow anchor, let out twice as much scope as you need, take a second bow anchor to your stern, drop it in, and take up on rode 1 while letting out on rode 2. You will end up in the center and will not sail on the anchor or move at all, simply pivoting about the center of the 2 rodes. The boat will not drift. This is needed there because you are anchoring at high water in a very narrow channel and can't afford to drift onto a sand bar before low water.

In my C25 I am also a huge beliver in those orange cone roll stabilizers with 8 lb mushroom anchors. I hang 3 on each side off the cockpit winchs and find rolling reduced by 50% or more.

Edited by - JimB517 on 01/13/2004 13:27:34

Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 01/13/2004 : 21:14:50

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Once the wind comes up a little they all line up.<hr height="1" noshade id="quote"></blockquote id="quote">

Actually, as Bruce points out... when the wind really pipes up... they don't all line up. Its then that two can easily get together while hunting the rode on opposite tacks.

Great Lakes, no current, muddy bottom, 12lb danforth, 100 feet of 1/2 inch rode, 20 ft of 1/4 inch chain, 13 ft of water. Board was still down.

As you and Bruce pointed out in his story, which I have read as well... things can get out of hand quickly in high winds if the boat presents a hull side to the wind. In my instance it wasn't broadside... the wind remained on the fore quarter... get even that of course would increase loading greatly.

This instance was my first ever anchor drag... er... I mean after an anchor had been backed down and had dug in and held under reverse motor thrust. I've of course experienced very many failed attempts to get an anchor to bite. In this regard, the Horizon Claw has a much better percentage record for me in getting a bite. This of course is because of the sailing venue. The North Channel can offer a wide variety of bottoms.

Last summer I succesfully anchored on two inches of hard limestone bottom. It of course took diving on the anchors and using a number of thrust to punch the flukes and plow through the limestone layer.

I now have a great respect for the power of winds from a micro burst. While they aren't sustained very long... usually just a few minutes, they can have the strength of hurricane winds and can uproot trees, knock over buildings, and create a potential for real problems.

Several years ago... the local yacht club was totally wiped out in less than five minutes. The docks were destroyed and many boats very seriously damaged. When they rebuilt... the found a location across the lake protected better from the typical SW direction these storms travel.

Jim, an interesting read is the story of the sinking of the Grampa Woo...

http://www.lakesuperior.com/online/191/191jrnl.html

As you have powered through the Great Lakes area... you will relate.

Edited by - Arlyn Stewart on 01/14/2004 07:09:19

JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 01/14/2004 : 00:29:12

When the wind really pipes up - agreed they don't line up. Light air (5 - 10 knots) in my experience the boats start to line up. As you know the Danforth is of little use in mud, but premium in sand. My power boat didn't sail at anchor but most windage was near the center or lateral resistance or aft.

Don't know what I would have done in your situation - microbursts are terrifying. I know that I can't really "back down" on the anchor with my low thrust Honda 8 HP (as compared to twin 210 HP Cat diesels with huge props). I've never had a danforth hold in deep mud - I always used the CQR for that. Even so, the plows need TIME and a gentle pull to really settle in (which you didn't have).

We survived an extreme thunderstorm in the Benjamins with 2 bow anchors in clay (best holding) and a stern line to shore (ultimate holding). We also survived a BIG thunderstorm on the Erie canal firmly tied to a lock wall plus the afore mentioned big blow in the Bahamas at Norman's Cay (about 30 knots, gusting 35, for 3 days, all 3 anchors set).

In super extreme survival conditions I always figured I'd drive the boat on the softest shore possible (similar to the rescue boats in the Grampa Woo story).

Thanks for the link by the way. Nothing as terrifying as the Great Lakes in November!

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