Anchor chain SPRING.

[TRUNDLETRUC]

There are always threads on this forum about anchoring but nothing about reducing the strain on the chain. Our yacht is fairly light with high top sides and in a blow sails itself wildly from side to side. When the yacht changes direction and falls back on the other tack the "Angle of Dangle"of the chain becomes more horizontal than at any other time showing that there is a lot more of tension. I have tried many ways to try and reduce this but the only sure way to stop the swinging is to rig a spring from the stern of the boat to a point on the chain some way in front of the stem holding it permanently on one tack.
My question is, to the scientists/engineers amongst us (Viv?), with your strain gauges does a yacht, set at the minimum angle to hold it on one tack, but a constant load, generate more tension on the chain than the tension generated when falling onto the new tack if anchoring free?
Is there any research? It is quite an important consideration which depending on the result could help you make the most efficient use of the length of chain you have.
I look forward to some facts from the experts.

 

[sarabande]

have a look at the discussion and some reference sites I've put on JM's anchor chain thread.

http://www.ybw.com/forums/showthread.php?435044-Anchor-chain-length&p=5362504&highlight=#post5362504

 

[jimbaerselman]

There are always threads on this forum about anchoring but nothing about reducing the strain on the chain.

Peak forces are generated when inertia has to be overcome. That's the snatch when you "tack" while weaving back and forth, or the snatch when you're pitching due to waves. These inertia forces are unpredictable, but are always an addition to steady state forces caused (mainly by) wind drag. A look at chain angle downward from the bow (and the jerk which pulls you off balance) soon illustrate the point that inertia forces are much higher than steady state forces. They'll vary strongly from boat to boat, with high displacement and high yaw inertia (lots of weight in the ends) creating more extreme peaks of force.

Four answers:
1. Reduce yawing with rear end centre of pressure (a vee shaped canvas hoisted astern is best, a sort of bifurcated mizzen with about 30 degrees internal angle, sheeted to each quarter)
2. Your solution - lay partly beam to with a spring
3. Forked moor.
4. Line ashore.

2 and 3 above confuse some others anchoring nearby, who may have difficulty interpreting where your anchor/s lie! - if they actually think that far ahead when anchoring.

 

[NormanS]

Peak forces are generated when inertia has to be overcome. That's the snatch when you "tack" while weaving back and forth, or the snatch when you're pitching due to waves. These inertia forces are unpredictable, but are always an addition to steady state forces caused (mainly by) wind drag. A look at chain angle downward from the bow (and the jerk which pulls you off balance) soon illustrate the point that inertia forces are much higher than steady state forces. They'll vary strongly from boat to boat, with high displacement and high yaw inertia (lots of weight in the ends) creating more extreme peaks of force.

Four answers:
1. Reduce yawing with rear end centre of pressure (a vee shaped canvas hoisted astern is best, a sort of bifurcated mizzen with about 30 degrees internal angle, sheeted to each quarter)
2. Your solution - lay partly beam to with a spring
3. Forked moor.
4. Line ashore.

2 and 3 above confuse some others anchoring nearby, who may have difficulty interpreting where your anchor/s lie! - if they actually think that far ahead when anchoring.

I use your No 1. suggestion successfully. It's easy for me, with a ketch.

 

[jimbaerselman]

I use your No 1. suggestion successfully. It's easy for me, with a ketch.

You remind me. I left out the live aboard special:

5. Mount two fans in the stern to counter the yaw. Add solar panels to drive the fans by day. Add a dinghy hoisted on davits, and as many satellite dishes as you can afford. Additional canvas to taste.

 

[vyv_cox]

Other than Jim's inertia forces, which I would guess are almost always going to be greater than anything the wind can generate, it comes down to the projected area that the wind can act on. Holding the boat at a smaller angle to the wind using your method projects a smaller angle than the the boat does at its maximum angle of yaw, although there are other times when the angle is at a minimum of course. This is a bit theoretical because everywhere I anchor the wind is never constant in direction and the boat would almost certainly yaw even if its angle was offset.

We don't have any means of adding a steadying sail aft but we do use a fork moor whenever the wind is over about force six and gusting down various valleys, provided there are no other yachts around to confuse the situation. I find that yaw angle is typically reduced from about 140 degrees to about 90, which not only makes life aboard a lot more comfortable but reduces the amount of 'working' at the anchor by an appreciable amount. I think this is a significant factor in reducing the likelihood of drag.

When watching what is happening to the all chain rode on the Rocna and the almost all rope rode of the Fortress there is no significant difference. As the load comes on each one it will lift entirely off the bottom, even with very large scopes.

 

[B&M]

I have been told that yawing is caused by the flow of air around the hull, effective causing lift. The solution or reduction of this effect can be achieved by leaving your fenders out and placing them at different heights and spacing them along the length of the boat, this disrupts the flow of air around the hull. I have yet to try this, but would be interested in any comments.

 

[TRUNDLETRUC]

Tried putting out fenders but it had no effect on my boat.
I do know that a padded luff jib makes the yawing worse!

Thanks for all the comments I will come back later when I have more time.

 

[Sybarite]

Hh

There are always threads on this forum about anchoring but nothing about reducing the strain on the chain. Our yacht is fairly light with high top sides and in a blow sails itself wildly from side to side. When the yacht changes direction and falls back on the other tack the "Angle of Dangle"of the chain becomes more horizontal than at any other time showing that there is a lot more of tension. I have tried many ways to try and reduce this but the only sure way to stop the swinging is to rig a spring from the stern of the boat to a point on the chain some way in front of the stem holding it permanently on one tack.
My question is, to the scientists/engineers amongst us (Viv?), with your strain gauges does a yacht, set at the minimum angle to hold it on one tack, but a constant load, generate more tension on the chain than the tension generated when falling onto the new tack if anchoring free?
Is there any research? It is quite an important consideration which depending on the result could help you make the most efficient use of the length of chain you have.
I look forward to some facts from the experts.

Try hanging a bucket over the bow to dampen the sheering.

 

[KellysEye]

>Our yacht is fairly light with high top sides and in a blow sails itself wildly from side to side.

We used a triangular riding sail, pointy bit forward on out mizzen topping lift. On a sloop use the backstay and tie the pointy bit with two small ropes to a guard rail on both sides of the boat. It cuts down swing from 90 degrees or more to 35 to 40 and thus cuts down the snatch loads on the anchor. I've always been staggered how few boats use one and are happy to sit there swinging 90 degrees + and the chain going taught. With no catenary there is a reasonable chance of dragging. You can either make one they are flat so easy make, use 5oz cloth and seams for strength around it or Sailrite in America sell them http://www.sailrite.com/Anchor-Riding-Sail-Kit-12-5-Sq-Feet# We bought one from them and later we made all our canvas work with a Sailrite sewing machine,we ordered both at the same time.

 

[GrahamM376]

On a sloop use the backstay and tie the pointy bit with two small ropes to a guard rail on both sides of the boat.

At what height do you have the tack from the deck?

 

[geem]

Other than Jim's inertia forces, which I would guess are almost always going to be greater than anything the wind can generate, it comes down to the projected area that the wind can act on. Holding the boat at a smaller angle to the wind using your method projects a smaller angle than the the boat does at its maximum angle of yaw, although there are other times when the angle is at a minimum of course. This is a bit theoretical because everywhere I anchor the wind is never constant in direction and the boat would almost certainly yaw even if its angle was offset.

We don't have any means of adding a steadying sail aft but we do use a fork moor whenever the wind is over about force six and gusting down various valleys, provided there are no other yachts around to confuse the situation. I find that yaw angle is typically reduced from about 140 degrees to about 90, which not only makes life aboard a lot more comfortable but reduces the amount of 'working' at the anchor by an appreciable amount. I think this is a significant factor in reducing the likelihood of drag.

When watching what is happening to the all chain rode on the Rocna and the almost all rope rode of the Fortress there is no significant difference. As the load comes on each one it will lift entirely off the bottom, even with very large scopes.

We adopt a similar mooring solution when the wind is blowing hard. We add a second anchor (which is a very large aluminium anchor rather like a fortress thats been on steroids) using the rib. We pull the yacht forward with the anchor rode on the second anchor then drop it quickly and the yacht will then fall back and set the anchor. We then adjust the length on the second anchor rode to maintain good tension on it. We have successfully laid to this set up several times with wind gusting over 40 knots off the land. The wind can shear about but the boat will move minimally. I think this set up is the way to go if you have the room to do so. if we are in a crowded anchorage and we have these conditions we have often moved somewhere else so we can set up our two anchors and sleep soundly. I accept that this is not always possible.
interestingly we spent 5 months this winter cruising in company in the Caribbean with my pal. He has a very shallow draft lifting keel yacht. He likes to anchor close in to the beach so nobody can drag in to him. He finds a conventional snubber of no use on his boat as it just does nothing to stop him sailing around the anchor when his keel is up. He always lies to two anchors to hold the bow into the wind. He has lived on board boats most of his life in the Caribbean and Bahamas where squalls are frequent in the summer. He can’t understand why people only set one anchor! During summer months in the Bahamas when the risk of squall is high, he regularly has multiple anchors set.

 

[tabnab]

I had this made up, a copy from another member, works a treat.
I found I did not need the spar to keep the sail open as my guard rails were perfect to clip on to. Clip on all 4 lines in cockpit before pulling up into strong / gale wind with main halyard.

V-twin Riding Sail for Anchoring.
The key is the V-twin configuration: like two small jibs joined at their luffs, the clews spread apart by a spar. The clews are just forward of the stern-rail, with the 'sheets' doubled round it and looped over the ends of the spar. The tack has twin down-hauls with clips to stanchion-bases, with them all in place, the head is rapidly hoisted on the main halyard and that's it! The luff is near vertical, just clear of the boom end. Because the sail is not attached to any stay but set 'flying' and winched up bar taut there is no vibration, no flogging, no attrition, no chatter; it is silent!

Dimensions for a 32-footer.
Luff : 9ft (1.5in.)
Leech: 11ft. (1.75in.)
Foot: 4ft 6in (1.0in.)
Cloth: dacron/polyester 6oz.
Fittings: D-rings -- Must be of a size to accept the splice of a 6/8mm sheet And the tapered end of the spar.
Construction: Lay one sail on the other, turn over top one like a page of a book to leave the luffs edge-to-edge. Join luffs with 2" flat webbing to stiffen and 'round' the leading edge of the 'V' when set; to minimise turbulent eddies which cause attrition or noise.

 

[NormanS]

My anchor sail is very similar. For a 36' Ketch, the luff is 3.25m, the foot is 1.83m, and the leach is 3.9m. It's probably even easier to rig on a ketch, with slides on the luff, which go into the spring-loaded gate on the mizzen mast track. it's sheeted to each quarter, and can be rigged in a couple of minutes. I don't use it all the time, but if I feel that the boat is sheering about too much, I'll rig it, and it calms things down.

 

[Chris_Robb]

The simple solution - but which wont stop veering, is to use a 20 meter long nylon snubber rope with a chain hook (or rolling hitch) to your chain. Leave a great loop of chain hanging down between. This simply eliminates all snatch loads - you should see just how much nylon stretches on a veer.

A sail as described sounds a great addition - but the snubber solution is cheap and stops the snatching.

 

[jimbaerselman]

The simple solution - but which wont stop veering, is to use a 20 meter long nylon snubber rope with a chain hook (or rolling hitch) to your chain. Leave a great loop of chain hanging down between. This simply eliminates all snatch loads - you should see just how much nylon stretches on a veer.

Yes, should give you about 1m to 2m of "spring", depending on the weave and diameter of the rope.

To get most spring, use a light line - 10mm or 12mm - and ease the chain out by 1.5m per 10m of snubber. That prevents the snubber from being stretched to breaking point. Min breaking point is about 1,500 kgf for 10mm, about 2,500 for 12mm.

Of course, that is above the safe working load for nylon, so if you're using it as an extension of your rode, you would go 3 to 5 times the strength - 18 to 22mm. But then you won't get so much spring.

 

[dratsea]

A chum?

Classically you run a weight down the anchor cable to just reach the putty and this reduces snatch. I think for us (65lb CQR, 10mm chain, yacht of 20tons and very low in water) 10Kg weight was suggested. I had a length (couple of meters and a bit) of 1/2 inch chain in good nick that we put around rocks ashore for mooring. It weighed about the right amount so tried it. Running this down the chain on a shackle and small line attached until most was on the bottom reduced the snatch but also dramatically killed the sheer. Just a suggestion, could try with a bit of rusty mooring chain before splashing out on new purchase.

 

[jimbaerselman]

10kg - that's the weight of about 4m of 10mm chain.

Achieve the same effect (hmm, very little in my opinion) with less hassle by letting out 4mm more chain.