Any explanation for this strange behavior at anchor?
Posted by dmootzler@reddit | sailing | View on Reddit | 33 comments
I was anchored in a big bay last night during a tide change. Initially, the current and wind were from the same direction, so the boat lay exactly as you would expect: facing directly into the wind with the anchor at 12 o’clock off the bow.
A few hours later, though, the tide changed, and things got weird: the boat swung with the current, keeping it off the bow, and the wind direction remained the same (ie current from 12 o’clock, wind from 6) **BUT THE ANCHOR WAS ALMOST DUE STARBOARD**
How is this possible??? I don’t see how a stable configuration could ever be reached with the anchor 90 degrees off the bow — the forces stretching the rode tight *must* swing the boat in line with the anchor….but they didn’t. And we sat like that for at least an hour with almost no change in position or angle.
The only thing I can think of is some interaction between the keel and the current (which was probably pushing 2 knots) creating lift that pulled the boat off to one side, but even in that situation I think I would still expect the anchor to end up dead ahead of the boat...any ideas?? Anybody experienced this before?
And to rule out the obvious explanations:
- We weren’t dragging (the boat was stationary)
- We weren’t aground (this was in 44ft of water)
- The rode was not tangled around anything (I pulled it right up, and it was off to starboard the whole time)
boatslut@reddit
Vectors & moment arms
Current acts on keel & underbody with a center of effort (COE) ~midpoint of a modern spade keel
Wind acts on topsides, mast etc. If the sail was hoisted & trimmed COE would be above the underbody COE. The anchored COR is more complicated as it depends on how much & where you have longitudinal area eg topsides, cabin, dodger, solar panels etc.
Wind pushes topsides in one direction, current pushes in another, waves in a 3rd. How the boat sits is at the balancing point of all the forces acting on it. Analogous to apparent wind.
BranchLatter4294@reddit
Seems normal from my experience, especially with sailboats where the current acting on the keel can be vastly different from the wind acting on the hull. Since water is much heavier than air, a slight current acting on the keel can put it at a very different orientation to the wind. Nothing strange about physics.
dmootzler@reddit (OP)
Yeah, the boat might not face into the wind, but it should still face into its own anchor, no?
BranchLatter4294@reddit
Not based on my experience.
MadYarpen@reddit
I understand that it is the chain that holds the boat the most, not the anchor. So i assume it can naturally rotate around the chain.
pervader@reddit
Yes
pervader@reddit
No
jfinkpottery@reddit
The boat is almost always going to face into the current, the forces from the keel and rudder are absolutely massive if the boat is anything off of what it wants to be in the current. But the boat will drift in a direction influenced by both the wind and current. Sounds like the direction your boat was pushed by the combination was 90 degrees off of the current.
OldBowDude@reddit
Mermaids! 🧜♀️ They love messing with sailors at anchor. ⚓️
Switch-in-MD@reddit
Still doing bow? How old? What boats?
OldBowDude@reddit
Not on the bow any longer, I let the youngsters do it now. I'm too old for the bow. I started at 13. Had to buy myself a boat to get off the bow! Lots of boats, one designs, IOR 3/4 to 2 tons, IMS, IRC, lots of J-boats, lots that were well known. I don't want to dox myself so I'm not going to say which ones. 😄
Switch-in-MD@reddit
Jerry Kirby - my superhero?
I’m not a well known bowman, yet still up there at (mid fifties). I own a stinkpot, but don’t own a blow boat.
Switch-in-MD@reddit
Rudder position?
Otherwise agree with Old Bow Dude. Mermaids.
dmootzler@reddit (OP)
It wasn’t locked off, so I don’t think it should be a factor
Switch-in-MD@reddit
Ok. Now follow up questions.
Full keep or Shorter?
Is rudder hung from the transom or from a rudder post under the wheel?
Regardless of keel length, if it’s a rudder post / forward of transom, it’s possible to have it failed that way and self set without you locking it.
chrisxls@reddit
Good question
TopCobbler8985@reddit
perfectly normal, particularly in rias/rivers and estuaries
Senior-Jellyfish-452@reddit
As other have said it is due to the different effects of the wind and tide. The boat is sailing due to the wind behind. No need for a sail to be up, the rig etc will provide the drive with the wind behind. So you are starting to sail downwind but the tide / current is against you. So the yacht positioning will be the combination of the 2 factors. It is not uncommon to have the anchor angled off at 90 degrees to the the yacht in these conditions.
Cos at that moment the wind from behind is not strong enough to overcome the tide otherwise the anchor line would be streaming out behind the yacht. And the current is not strong enough to overcome the effect of the yacht trying to sail downwind if it was then the anchor line would be streaming out from the bow.
Normally you see the forces in action and the difference it makes by playing around with the tiller / wheel.
M37841@reddit
Here’s a slightly handwavy, but true,physics explanation. Imagine a force acting on the stern of the boat.That causes a torque which wants to rotate the boat around the anchor. Now put a bigger force opposite that, but much further forward. That creates an opposing torque which holds the boat steady. Still the anchor is forward of the boat.
But now look at a point on the boat between the two forces: you’ve got two opposing forces acting to spin the boat around some axis that is between th two forces. The anchor plays no role in this as the bow is freely able to rotate around the anchor chain. The boat will not rotate around the anchor because the two torques are equal at the anchor, so it rotates further back.
Because torque depends not just on force but on the distance between where that force acts and the pivot point, you’ve got two different equations to solve: the pivot around the anchor and the pivot of the boat itself. The right combination of strong enough forces can give them very different solutions, so the boat angle and anchor angle are visibly different.
diekthx-@reddit
It’s really common where the current is strong. Boats in our mooring field will even face different directions from each other depending on the interaction of current, windage, and wind direction.
dmootzler@reddit (OP)
Yeah boats facing different directions from each other makes sense, but I’d expect each boat to eventually end up oriented in line with its own anchor
flyingron@reddit
You assume that the current and the wind operate on the same 'center' point
If the current is stronger than the wind, the boat will be positioned relative to the anchor in line with the current, but the wind can turn the boat relative to that position.
dmootzler@reddit (OP)
Mmm yes that whole center of effort vs center of lateral resistance thing.
flyingron@reddit
It could go either way depending on the current and the wind loading on the boat. The boat is desigend to be balanced with the sails up. The windage on the boat at mooring is less of a concerned and could be way off the center of resistance.
desertrumpet@reddit
seems like a university physics problem on vector math
dmootzler@reddit (OP)
No there’s definitely more to it than just wind vector + current vector
stumanchu3@reddit
The real fun begins when your anchor is dragging at 3:36 AM and you’re about to slam into a much nicer boat that’s anchored there before you set yours. Good times!
e1p1@reddit
Not sure if this helps, but generally speaking wind affects the bow more than the stern. I'm sure you've noticed when drifting that the bow will get pushed off downwind before the stern.
So think back to the situation and I'm wondering if the force of the current was pushing the stern in one direction to starboard and the force of the wind pushing the bow to port and the anchor was keeping the bow from swinging fully into the current direction.
I can tell you from experience that while anchored in Thailand with the current running, don't remember which direction, when a Squall came in with 30 to 40 knot winds , the Ericson 32 I was living on would tack at anchor back and forth.
dtdowntime@reddit
Ive had this but with around 10-15 knots of wind, anchored in a semi protected shelter so virtually no current, but without much wind protection, and we kept swinging around like crazy, would hardly stay in the same location for more than a few seconds
jonathanrdt@reddit
You want some real fun: anchor a bunch of different keel types with changing wind and tides, and watch how the different boats behave totally differently: full keel are more affected by crosswind currents. Wake up in the middle of the night w new neighbors with your anchor somewhere to your port stern.
H0LD_FAST@reddit
The more surface area of the keel the more it’s effected by the current, and the current usually wins even in fairly strong winds. With a long deep keel in current, my boat will sometimes drive forward on a mooring ball or at anchor.
Anchoring in channels with strong current and strong wind (20+) I’ll see the anchor out to the side of the boat, where the boat mostly points into the current and the wind is trying to push the boat sideways away from the anchor.
caeru1ean@reddit
If you’ve ever anchored in La Paz BCS this is super common and called “the la Paz waltz”, where all the boats face different directions and swing around. There is a strong tidal current twice a day and 20+ knot northerlies that make things very interesting
Westar-35@reddit
This checks out. The boat can end up in any manner of odd positions relative to the anchor. I’ve had current shifts against the wind that caused my anchor buoy to foul the prop…