Average Range of Yawing at Anchor in Storms

[thinwater]

How much does your boat yaw at anchor when it's really blowing, enough to get the chain just off the bottom (at least 35 knots sustained)? Have you looked at the compass, perhaps trying to separate wind shifts? I've watched my compass and I have observed others, but I never really focused on others.


I've seen light boats with dinghies on the bow sail though more than 120 degrees. I've seen lots of boats sail through over 90 degrees. My PDQ, with a long bridle was about 35 degrees, but without it she was all over the place, more than 100 degrees. My tri is about 30 degrees with bridle and rudder up, but as much as 130 degrees anchored by the bow alone. My impression is that in strong conditions, 50-80 degrees is pretty common among monohulls, although I don't think they all realize it ("it's the the other guy"). These same boats may sit still when they have some chain on the bottom.


Please only comment if you have studied the compass bearings during strong conditions, or if you anchor using rope. In lighter winds, with chain on the bottom, you won't yaw much.


(I'm testing three riding sail designs on a nervous boat)

[billknny]

Thinwater,

Here is some data that might help you that I collected as part of a design project for a riding sail on my boat. The boat is a 52 foot ketch, anchored on 3/8" chain, with a 40 foot 3/4" nylon snubber in 18 feet of water. I was trying to determine (before sewing!) what the best riding sail setup would be.

Most of the ones I see are sheeted down the centerline of the boat, which is NOT the way I was taught they work best. So here is the experiment:

Using the mizzen in lieu of a riding sail, I recorded the AWA as the boat tacked back and forth at anchor. First data is with the mizzen sail furled.



Second data set is with the mizzen sail unfurled, and sheeted tight on the centerline of the boat. The amplitude of the swing is reduced, and the period shortened, but the boat still tacks back and forth, and the sail luffs annoyingly as the boat comes through the wind.



Finally, I moved the mizzen traveler off to the starboard side, giving an angle to the centerline of about 35º. Set up like this, tacking stops, and the pull on the anchor no longer surges as the boat tacks, but is steady.



My conclusion, that the the riding sail needs to be sheeted off center to hold the boat at a consistent angle to the wind. to eliminate tacking and greatly reduce shock loading on the anchor.

The one thing I wish I had had was a strain gauge on the rode. it would have been interesting to see quantitatively how the average and peak loading was effected.

Now, what happens in storm conditions: In 50 knots of wind the boat tacks through an apparent wind angle of 90 degrees, both part and starboard. I don't have any data with the mizzen set under those conditions.

[Singularity]

FWIW there are some general angle references in this 3 page thread on the subject of riding sails (along with some related theory):

http://www.cruisersforum.com/forums/...ork-16696.html

[Cheechako]

I've been lucky I guess, not had a boat that yawed at anchor. But I dont buy light boats.

[wsmurdoch]

From an earlier thread see post 107 for measured data at (sorry) lower wind speed. At 35+kt I would be chicken to set and douse the riding sail on the backstay for comparison testing. My riding sail dropped the variation in apparent wind direction from +/- 30 degrees to less than +/- 15 degrees. I think that a riding sail should minimize variation in apparent wind direction not variation in boat heading. In other words, watch the wind not the compass. Also, I have had better results with the sail set on the centerline of the boat rather than set to one side.

http://www.cruisersforum.com/forums/...ml#post2407522

Bill

[thinwater]

Originally Posted by billknny

Thinwater,

Here is some data that might help you that I collected as part of a design project for a riding sail on my boat. The boat is a 52 foot ketch, anchored on 3/8" chain, with a 40 foot 3/4" nylon snubber in 18 feet of water. I was trying to determine (before sewing!) what the best riding sail setup would be.

Most of the ones I see are sheeted down the centerline of the boat, which is NOT the way I was taught they work best. So here is the experiment:

Using the mizzen in lieu of a riding sail, I recorded the AWA as the boat tacked back and forth at anchor. First data is with the mizzen sail furled.

Second data set is with the mizzen sail unfurled, and sheeted tight on the centerline of the boat. The amplitude of the swing is reduced, and the period shortened, but the boat still tacks back and forth, and the sail luffs annoyingly as the boat comes through the wind.

Finally, I moved the mizzen traveler off to the starboard side, giving an angle to the centerline of about 35º. Set up like this, tacking stops, and the pull on the anchor no longer surges as the boat tacks, but is steady.

My conclusion, that the the riding sail needs to be sheeted off center to hold the boat at a consistent angle to the wind. to eliminate tacking and greatly reduce shock loading on the anchor.

The one thing I wish I had had was a strain gauge on the rode. it would have been interesting to see quantitatively how the average and peak loading was effected.

Now, what happens in storm conditions: In 50 knots of wind the boat tacks through an apparent wind angle of 90 degrees, both part and starboard. I don't have any data with the mizzen set under those conditions.

Fantastic data!


I would be willing to bet the tension on the rode nearly doubles when you are yawing 90 degrees (it does on my boat). Thus, reducing that should be worth leaving some canvass up, even though that is against our first reflex. In your case, perhaps a mizen with 2 reefs would be enough. And YES, sheeted off the one side, as you pointed out.


A corollary, to me, is that any valid riding sail must work at >60 knots. Most of the free-flying designs can't do that, so they fail EXACTLY when you need them most. A good riding sail reduces rode tension and anchor wiggling and thus should be up in a storm. No? If not, why not?

[thinwater]

Originally Posted by Cheechako

I've been lucky I guess, not had a boat that yawed at anchor. But I dont buy light boats.

It is about balance, not weight. In fact, yawing through large angles is a common problem with >50,000 ton ships! Billknny's boat isn't light.

https://www.piclub.or.jp/wp-content/...l.25-Light.pdf

[newhaul]

My islander would yaw about 30° in light winds but the stronger the wind the less the yaw .
My columbia defender is about the same . But like others have said heavy boats with full keels.
Try two riding sails set to about 20° out each way from centerline . Should calm your boat down considerably.

[billknny]

Originally Posted by thinwater

[FONT=arial][SIZE=2][COLOR=black][SIZE=2]It is about balance, not weight. In fact, yawing through large angles is a common problem with >50,000 ton ships! Billknny's boat isn't light.

Not light, but not especially heavy either. With a D/L ratio of 240 at full (measured) cruising weight, she comes pretty much on the lighter side of "moderate" in most ratings. Certainly a long way from the traditional heavy cruiser, but by no means a ULDB sled either...

A riding sail that stops tacking in 20 knots of wind, is not really worth much if you have to take it down in 50 knots when the peak loads from uncontrolled tacking are REALLY high. Tacking back and forth at 20 knots or even 30 knots is really just an aesthetic issue, not one that matters a wit to my anchor.

A riding sail that had no significant effect on the boat's behavior at 15 to 20 knots but reduced peak loads at 50 knots is far more valuable.

Thinking out loud...
In 15 knots of wind my 205 ft^2 mizen is just about perfect for controlling tacking. Using a really simple (actually "simplistic" would be the better term..) analysis says that a riding sail of about 15 ft^2 would do the same job at 55 knots.

More thinking out loud...
The way the riding sail works is it balances the windage of the forward part of the boat. As the wind picks up, the forces on the front of the boat increase, in (very roughly) the same proportion as the force on the riding sail. So it's not at all clear that a significantly smaller riding sail would help in stronger winds. But again, that's a really simple analysis.

[thinwater]

As I hinted, I'm testing three riding sail designs (I've tested some of these before, but not so formally). One is the conventional sort, sheeted to one side. The others are V-styles, and are more stable at high wind speeds.


I have always believed, based on experience and testing, that yawing is one of the greatest threats to anchoring. This is true of most "anchor types, including construction pilings and rock climbing anchors. If you want to pull them out, wiggle them. I largely quit worrying about it when I started sailing multihulls with bridles that didn't yaw much. But now I find myself thinking the whole dynamic is underappreciated.


There are numerous ways to reduce yawing. Riding sails are only one.

[Hydra]

Originally Posted by thinwater

It is about balance, not weight. In fact, yawing through large angles is a common problem with >50,000 ton ships! Billknny's boat isn't light.

https://www.piclub.or.jp/wp-content/...l.25-Light.pdf

I have been on a 600-tonne warship anchored on chain (of course!) that yawed through approximately 60° in Beaufort 4.


My own boat (30', 4t, fin keel, hank-on genoa), anchored on 30m chain and 20m rope in 5m depth, yawed through 90° in a gale, so quickly that it was unpleasant to be sitting in the V-berth.



I am considering making a V-type riding sail and researching guidance about the dimensions.

Alain

[wsmurdoch]

My triangular flat riding sail is different from a mizzen.

When attached to the backstay, with its unsupported edges forward, and sheeted tightly on the centerline of my boat, my riding sail does not luff. That is, it does not have a position where the sail flaps like a flag. As the wind direction changes from port to starboard, my riding sail goes from one stable position with the sail to starboard instantly to a second stable position with the sail to port. Looking up at my riding sail and the Windex, as soon as the wind shifts to a new side of the boat, the riding sail immediately snaps over to the opposite side and begins working to return the boat back toward head-to-wind.

When used as a riding sail. a mizzen sail has a central range of wind angles where the sail luffs. In that range of angles, it contributes little to the yaw stability of the boat. Only outside of the luffing range does it contribute to keeping the boat in alignment with the wind.

I wonder if that is why billknny finds an off-center alignment with the wind always on one side of his mizzen sail to be advantageous while I find an on-center alignment of my riding sail with the wind alternating from one side of the sail to the other to be better. Or, maybe this is just one of life's imponderables.

Bill

[wsmurdoch]

Originally Posted by thinwater

... One [riding sail] is the conventional sort, sheeted to one side. The others are V-styles, and are more stable at high wind speeds.

One of the things I do not like about my flat riding sail is raising or lowering it in high wind. When the halyard is tight but the sheet still loose, the sail and its sheet flog about like a monster. It is much worse than a foresail both because the stern rail contains more targets for the extra long sheet to tangle with and because the sail must be reversed through 180 degrees as the sail is set. We have had to raise the riding sail with wind blowing when we did not raise it earlier, and we have had to lower it in howling wind when leaving an anchorage that was becoming untenable. They are not pleasant experiences.

I suspect that a V-style or Y-style riding sail would be easier to deal with as its forward edge could be set first. But, I have no experience.

Bill

[AndyEss]

Excellent discussion.
Thank you billknny for your data set.
As you noted, getting measurements of rode loading in addition to the data set you acquired is actually what we need to know. Staying anchored with no dragging is the performance we critically need, not necessarily how our boats yaw. I use a 25 ft snubber run low on my bow through my bobstay hull fitting which does substantial duty in keeping the rode angle as horizontal as possible. I'm not sure if it does anything about yawing.
I hope to add data logging to my nav system so I can research how to optimize my cruising experience.
The riding sail discussion is very useful as while I have one (someplace in all my equipment) I have yet to try it.

[contrail]

For what it is worth, my 45 foot Leopard cat yaws no more than about 30 degrees, by the compass, to either side in winds up to about 40 -45 knots. Haven't had more than that, at anchor. The bridle is only of moderate length, but is always in place, as the anchor roller is just forward of the bridge deck. If the bridle were not there, it wouldn't take that much of a yaw to bring the chain under a hull.



In the BVI, we use moorings a lot, and my set up is almost identical. On rare occasions, I dispense with the bridle, and bring the mooring line straight up over the crossbeam, using a roller a couple of feet off centerline. Oddly, the yaw does not increase, but I have never used this set up higher than normal anchoring winds, let's say about 20 - 30, so it doesn't answer your question. But, I did find it unexpectedly interesting.


My old 33 monohull, a skittish fin keeler with spade rudder, was very lively at anchor, sailing at anchor quite a bit, but never close to 90 degrees, either side. It's a guess, but I would say 60 would be the limit. I did sit out hurricane Marty, however, whose eye passed over us and in which we hit 80 -90 knots. I had always thought that a huge part of the anchor loads would come from yawing back and forth, so my preparation included stripping everything forward, such as the jib and the main. The mainsail cover had a big "throat" at the mast. But, I deliberately left quite a bit up aft increasing windage. This included a very sturdy dodger, under which I could sit, the outboard and an anchor on the stern pulpit, some man overboard gear and a solar panel. All were very strongly secured, but there was quite a bit of windage. I anchored with chain, a very stretchy snubber (light better than heavy, so it can stretch), and a kellet, just above the seabed. My theory was that the additional wind resistance of the stuff aft would be more than compensated for by the yawing it would prevent, and that is exactly what happened.



Nothing blew away, the snubber did not break and yawing was well under control. The kellet kept the angle of the snubber so that it was never less than 45 degrees from the horizontal. The boat rode well and I had no damage, and the wind did reach a measured 90. The only time I had trouble was when I experimented with the engine. No matter what throttle setting, I had trouble keeping the boat into the wind, and the way we shot back and forth was very illustrative of what might have been happening had the boat been yawing a lot, on its own. No thanks. It was actually very alarming.



My conclusion was that if I were in the same situation again, I would do the same. I would keep stuff on deck if it was aft, increasing windage, but very well lashed down. I would totally strip the forward part of the boat. I would count on the steadier movement, with little yawing, to make up for the loads imposed by the stuff aft. All in all, something like a riding sail.