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Old 02-19-2014, 07:56 AM   #1
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What makes a boat swing at anchor?

When anchored, most, if not all boats swing back and forth. Some swing quite a bit. Why? What causes it, and what, if anything, can be done to reduce it?
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Old 02-19-2014, 08:15 AM   #2
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Like a BAD windvane...wind will eventually load a side and it will sail off until the force on the anchor rode overcomes and now it's weight drags the bow back through the windline and the opposite direction takes over...repeat till a new force overcomes the tendency.

A steadying sail way aft works to a degree, stern anchor adjust deck windage (usually not realistic).....

Lobster boats have riding sails similar to a sail vessel with a mizzen mast that will use them in the same way...
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Old 02-19-2014, 08:37 AM   #3
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A bad design, or a nature of many non-sailing boats, including mine. It swings in excess of 30 degree either way ... bad for anchor holding power which thends to be yanked from the ground and dragged.
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Old 02-19-2014, 08:42 AM   #4
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On some boats with widely separated bow cleats...a bridle may reduce the amount of swing...others rig their snubber led aft to a cleat so the boat sits at an angle to the wind to begin with...also useful to point the bow more towards a prevailing wave pattern to lessen roll.
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Old 02-19-2014, 09:36 AM   #5
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Good points ... need to experiment more.

So far, I have used dinghy as a poor man's storm drogue to reduce swinging with mixed results. Mizzen mast and sail would be an excellent solution ... worked well on a sailboat.
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Old 02-19-2014, 09:55 AM   #6
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The bigger and heavier the boat, the less it will move. Buy a bigger boat!
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Old 02-19-2014, 09:57 AM   #7
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My boat swims at anchor too. Annoying, but I got used to it. I have a bottom designed for efficient planing, so there are trade-offs, and swimming is one.
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Old 02-19-2014, 10:29 AM   #8
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Flopper stoppers are useful.
I have installed the Magma ones, and reduces the rolling at anchor by 80%, particularly using 2 sets.
My wife states that is the best "invention" (improvement) I have made to the boat
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Old 02-19-2014, 10:35 AM   #9
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The bigger and heavier the boat, the less it will move. Buy a bigger boat!
At first I thought this was a question designed to stimulate the moribund geezers amongst us into action. Now I know, Twisted wants a bigger Nordhavn and needs justification to convince the admiral. Yup, that is the answer, a bigger boat, much bigger.

And Ski, my boat nor occupants don't swim at anchor in the PNW at all, the water is too cold.
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Old 02-19-2014, 11:22 AM   #10
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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.

Mostly a matter of where the windage and draft is.

My Willard sails a lot and I haven't rigged a bridal yet. Willy is short so even w her substantial keel along most of her length she sails. With Willy I think it has a lot to do w the bow. Big and tall.

It may have a lot to do w where the attach point is on the bow. The bow eyes on small boats are frequently at or near the WL and they frequently tow well. Same boat from the top of the stem and the're all over the place.

Also I suspect that w a boat that has equal windage and draft along it's length will still sail. The wind seems to have more effect on the bow than the stern. By the time the wind/air gets to the stern it's significantly aligned to the hull.

Not claiming any of this to be true. More like thoughts, hunches and suspicions. But the cure seems to be a bridal. We were at an anchorage sailing back and forth a lot. There was one other boat there at anchor ... w a bridal rigged and he wasn't swing'in AT ALL.
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Old 02-19-2014, 11:37 AM   #11
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A boat yaws at anchor because it is an unstable system trying to find equilibrium. Generally speaking the distributed forces applied by the wind and water change relative to the center of gravity of the boat.

Steadying or riding sails aft or using an anchor bridle to a side cleat forward changes the centroids of these forces and lessens the effect.
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Old 02-19-2014, 12:07 PM   #12
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A boat yaws at anchor because it is an unstable system trying to find equilibrium. Generally speaking the distributed forces applied by the wind and water change relative to the center of gravity of the boat.

Steadying or riding sails aft or using an anchor bridle to a side cleat forward changes the centroids of these forces and lessens the effect.
I agree.

I have found little difference using bridle or not in any given circumstance.

For dauntless, it does seem to be the current versus the wind and since the wind is usually less steady in a short period (seconds), the boat swings.

The stronger the current, the less the swing.
Last Summer in Maine, I was in an anchorage, with about 8 other boats: 5 sailboats, 1 Nordhavn and a KK42 and KK48. I noticed that the two Krogens moved pretty much in lockstep, and for more than an hour, the two Krogens were perpendicular to the other 6 boats.

Don't really know if that was a function of current or wind, but clearly we were affected differently, at least for a while. and since, all the boats eventually came back into alignment, I thought it was a current (tidal) issue.

Here is a video I took at the time

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Old 02-19-2014, 12:17 PM   #13
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A full displacment heavy boat does notr swing as much. I also read that if the house/super was back like the explorer/expidtion the house acts like a vane and does not swing as much. The Eagle house is about the middle of the boat and full displacement so it does not swing to much. As mentioned be for a steading sail bakc by the stern helps reduce the swing.

At our first moorage we where close to the speed marked area so we would be some big waves and rocking, so what I did was hang rocker stoppers. they as plastic cones you can buy West marine for 10 bucks. I had 4 on each side, at the bow and stern, they really helped. They nestled nicely when stored and quick to deploy. I have thought about just hanging them while we are under way with a heavier mush room anchor. I mean under 200 bucks and you have stabilizers and help reduce the roll and swing?
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Old 02-19-2014, 12:35 PM   #14
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Greetings,
P/F. "...hanging them while we are under way...". You CAN'T be serious!
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Old 02-19-2014, 01:10 PM   #15
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Boats that tend to sail around usually have lots of freeboard forward or a large pilothouse/cabin forward. Boats that dont tend to swing are full displacement sailboats or boats with relatively low even freeboard. A good deep keel helps.

Next time you are in an anchorage, observe other boats at anchor. Depending on wind and current conditions, you will notice sailboats tend to swing with the current and power boats tend to swing with the wind. Generally, powerboats swing a lot more than power boats as power boats have more surface area exposed to the winds and sailboats have large deep keels that have mroe exposed to current.

At crowded harbors, you will find the harbormaster keeps all the power boats moorings together and all the sailboat moorings together. He is not segregating the boating community because of the whole power vs sail 'thing'. Harbormasters know that sailboat (in general) swing alike, and powerboats swing alike.
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Old 02-19-2014, 01:13 PM   #16
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Greetings,
P/F. "...hanging them while we are under way...". You CAN'T be serious!
Why not! Hey, once you have a ugly old slow trawler a little more does not hurt. What is the difference does it make if they are plastic or steal. They worked at the dock/anchor so why not when under way.
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Old 02-19-2014, 01:28 PM   #17
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A boat yaws at anchor because it is an unstable system trying to find equilibrium. Generally speaking the distributed forces applied by the wind and water change relative to the center of gravity of the boat. .....
That's it in a nutshell. What to do about it? Try to anchor out of the current and protected from the wind.

In many places, the current direction and strength will change during the time you are anchored. The same applies to the wind.

My boat doesn't seem to do this much and in any case, I ignore it since there's nothing I can do about it.
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Old 02-19-2014, 01:38 PM   #18
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Fortunately the Mrs is totally on-board with the Nordhavn.

People mention greater displacement as being more stable, but what got me wondering about this again is a very experienced Nordhavn delivery/training captain who commented on how much they sail at anchor.

Sure, a shift of wind here or there will cause a boat to move around, but many seem to just constantly oscillate no mater what.

As for sail boats vs power boats, what you say about different windage etc makes sense, yet 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.
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Old 02-19-2014, 01:51 PM   #19
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Fortunately the Mrs is totally on-board with the Nordhavn.

People mention greater displacement as being more stable, but what got me wondering about this again is a very experienced Nordhavn delivery/training captain who commented on how much they sail at anchor.

Sure, a shift of wind here or there will cause a boat to move around, but many seem to just constantly oscillate no mater what.

As for sail boats vs power boats, what you say about different windage etc makes sense, yet 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.
Like Northern Spy said and I posted in #2...it's about unequal forces and how/why the boat sails back and keeps oscillating....not mater what size or shape boat...obviously the better weather vane or the boat weight that only a hurricane can get to move is going to sail less....and underwater shape with current will also have an effect.

So what works for one boat may not another..like I said about bridles...unless you have a broad bow with wide points of attachment...less help ...no better than just a single snubber through the pulpit.

But also as NS and I posted..if not having much luck and conditions are still reasonable...taking a snubber to a cleat/bridle off the side does work for many boats not matter size shape, etc..etc as now you are preventing the oscillation because you are never allowing wind pressure to the opposite side.

In mild conditions...a good reason for all chain anchor rode as it really limits the swing.
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Old 02-19-2014, 02:21 PM   #20
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Yes, I get the unequal forces and all that. Just trying to dig deeper. For example, most boats are symmetrical side to side, or very close to it. So why doesn't it tail straight into the wind and remain stable? And when gusts or whatever yaw it to one side, the counteracting forces should kick in immediately, so why doesn't it damp right out?

I've heard the single line vs bridle argument, and you would think it would make a significant difference, but I've heard lots of people say it makes little to no difference.

Some claim that an attachment point closer to the water line is better than one up on the bow, but again I've heard a number of people say it made no difference when they tried it.

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......
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