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Old 27-11-2012, 16:02   #1
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Take the load off the bow

I posted this in a previous thread on the effects of anchor chain weight, and I am curious if anyone has tried it and can report back. Robert Danforth Ogg, co-developer of the Danforth anchor, did a lot of research in developing anchors for the Navy and it is worth it to find and read some of the old brochures and papers published by the Danforth company. The weight of chain produces the catenary, which adds some shock absorption in light to moderate winds, but Danforth wrote, "...we see the value of using a buoy and pennant where possible instead of connecting the boat and anchor directly." He believed, based on his testing, that keeping the weight of the chain off the bow of the boat was very important in order to allow the boat "freedom" to respond to wave action in rough conditions. "Allowance for this freedom is a major consideration in the proper design of a mooring or selection of anchor rode. Failure to provide this freedom may result in tremendous and destructive surge loads." In another spot he wrote: "NOTE: For safest mooring or anchoring, a buoy should be used to carry the bulk of the vertical load, leaving only a horizontal load on the bow of the boat as well as on the anchor."

My understanding is that this is a technique used by West Coast fishermen when anchored.

I edited this later to add that it seems this might be of use in deep anchorages particularly, when you have a lot of chain hanging down from the bow, but might not mean anything if anchored in typical East Coast 6-20 foot depths.
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Old 27-11-2012, 17:20   #2
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Re: Take the load off the bow

Is this like mounting a bow eye near the waterline for leading the rode to the deck? I know that this is a good idea, but I've never actually done the deed.
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Old 27-11-2012, 17:25   #3
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Re: Take the load off the bow

My old mooring had this system, with a big bouy taking all the weight of the mooring chain and a long pendant to the bow. In a good blow it used to get dragged well underwater and I am sure the drag helped to pull the bow into the wind and reduce the yawing. The lack of weight on the bow seemed to help it lift nicely.

Another idea I have toyed with is tieing a couple of car tyres onto the chain or onto a big snubber, the drag of them being pulled through the water should do a lot to reduce the yawing and dampen the loads on the chain. In addition if they are tied or shackled in with a bight of chain or rope they have a good bit of stretch. The drag of the tyre or bouy has the big advantage over stretchy snubbers and chain catenery of not storing the energy and then using the energy to pull the boat forward in a lull.

A single car tyre added to the middle of a towing warp made a huge difference once when towing a 65 footer of a reef. Without it the rope snapped as soon as load was starting to be applied. With the tire in the warp the yacht was eventually pulled free.
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Old 27-11-2012, 17:31   #4
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Re: Take the load off the bow

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Is this like mounting a bow eye near the waterline for leading the rode to the deck? I know that this is a good idea, but I've never actually done the deed.
Did you see my article in Ocean Navigator on this? I have used a bow eye near the waterline for years. I attach a snubber line to it, using just an eye splice so it is quiet, and in a heavy blow I use that to attach the chain to the boat. Reduces the angle on the anchor chain just like letting out more scope, and also seems to reduce yawing at anchor.

However, this float idea is different. What you would do is lower the correct amount of chain, and then attach something like a 24-inch float to the chain, then let out another 20 feet or so. Then most of the chain weight would be supported by the float, while your boat was free to bob up and down on the waves, reducing the snatch load on the anchor chain. Also, in heavy gusts, as your boat pulled back it would be trying to pull that big float under water, which is hard to do, introducing a lot of elasticity and cushioning into the system.
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Old 27-11-2012, 17:33   #5
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Re: Take the load off the bow

We have used this technique from time-to-time, but not on a daily basis. Usually in surgey conditions (whether wind or wave/current induced). Not so much to take the weight off the bow, but to provide shock absorption on top of that provided by a snubber. The pennant has to be long enough that if the entire system is pulled taut the bouy will be pulled underwater, and the bouy has to have sufficient flotation to make this difficult. The combination of the two makes are pretty effective shock absorber (spring damper ) on both the anchor side and the bow side.
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Old 27-11-2012, 17:40   #6
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Re: Take the load off the bow

I'm not certain why the rode's weight would be a concern. All that weight was on the bow before the anchor was deployed. If anything, it weighs less once deployed, not only according to the Archimedes Principle, but because significant weight is now on the bottom. If the bow was free to respond to surge while underway, why would it be less free under the influence of the rode's catenary?
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Old 27-11-2012, 17:51   #7
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Re: Take the load off the bow

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If the bow was free to respond to surge while underway, why would it be less free under the influence of the rode's catenary?
My initial post was somewhat misleading because I used the term "weight." We're talking about high winds when you will be straightening out that chain--in essence you are chained to the bottom, which everyone seems to agree begins to happen at pretty low wind speeds. The float puts a big additional bend in that system and allows the boat to lift to the waves--or so goes the theory. But, when you do deploy the chain you also get a lot of that weight off the bow of the boat, and it should be able to respond better to waves.
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Old 27-11-2012, 17:56   #8
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Re: Take the load off the bow

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Originally Posted by Kettlewell View Post
My initial post was somewhat misleading because I used the term "weight." We're talking about high winds when you will be straightening out that chain--in essence you are chained to the bottom, which everyone seems to agree begins to happen at pretty low wind speeds. The float puts a big additional bend in that system and allows the boat to lift to the waves--or so goes the theory. But, when you do deploy the chain you also get a lot of that weight off the bow of the boat, and it should be able to respond better to waves.
Thanks. That makes sense.
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Old 27-11-2012, 17:59   #9
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Re: Take the load off the bow

If you use the bouy approach along with an eye at/near the waterline then the pull on the bow is strictly horizontal, allowing the bow to move up and down more freely. With the anchor rode pulled taut there is some component of downward pull on the bow that restricts that movement. If the rode is chain then the weight of that chain further restricts that movement (and the total weight on the bow may be less, but the weight is as far forward as it can get, extending lever arm over where it is in the chain locker).

Is that freedom of movement better for the anchor/system? I think Danforth believed it was, and our experience is that even though the bow may move up and down more (some may not like that) it puts less strain on things in doing so. Now I need to go set up a test rig to measure if that is true
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Old 28-11-2012, 00:43   #10
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Re: Take the load off the bow

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Originally Posted by Kettlewell View Post
....We're talking about high winds when you will be straightening out that chain--in essence you are chained to the bottom, which everyone seems to agree begins to happen at pretty low wind speeds....
my underlining

Not quite everyone: this 'agreement', it seems to me, is largely a recent www artefact.

Except in shallow water, my view is that if rode straightening is a problem at what qualifies -- for any given sailor -- as 'pretty low wind speeds', then their chain is way too light.

I reckon the key criterion is whether the last few links, at the anchor end of the rode, lift off the bottom. If they don't, then the anchor is 'seeing' a scope of infinity to one. It's not possible to determine this from on the boat.

What's more, the calculations which purport to prove (inter alia) that a kellet near the anchor will not help with this are frankly fanciful.

For really windy situations, my preference is to carry three-link shots of 25mm stud-link chain as optional kellets on say a 13mm anchor chain: easily handled (they find their way onboard over the bow roller with a bit of encouragement), they provide massive frictional drag on the sea floor to resist sideways perturbations of the anchor shank ... and my feeling is that any gust which could lift the lowest one off the bottom pretty much deserves to win the boat. (Bottom shot should be clipped to the chain at both ends for best results.)

To give an idea of scale, I'm thinking of these in connection with an 8T sailboat with a 30kg anchor.

Quote:
Originally Posted by Kettlewell View Post
....The float puts a big additional bend in that system and allows the boat to lift to the waves--or so goes the theory. But, when you do deploy the chain you also get a lot of that weight off the bow of the boat, and it should be able to respond better to waves.
Two interesting ideas in this thread. The use of an eyebolt on the stem at the waterline has always seemed to me the ideal place to terminate a snubber, except for the logistics (eg when it's necessary to increase scope)

Advantages include water cooling in severe snubbing scenario, lack of chafe if the snubber has a hard thimble and a strong shackle to the eyebolt, lack of strain on the deck (the stem should be really strong at the waterline), and the geometry provides an effective increase in scope with no increase in swinging room.

Having said that, it's one of those great ideas I've not "yet" got around to trying.

The other idea, in the OP, seems to me to be mainly about providing a very effective snubber for a heavy chain. The snubbing action arises from trying to submerge the buoy.

I can see that, particularly if combined with a waterline connection to the stem as above, there's also a degree of freedom for the bow to kick up without unduly loading up the rode or the strongpoint, which it seems to me could be worth quite a lot in short, steep chop (such as you get in a confined anchorage in storm force winds)
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Old 28-11-2012, 01:27   #11
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Re: Take the load off the bow

As for this being a technique of west coast (USA) commercial fisherman I suspect this may be a localized thing. In my 6 yrs comm fishing out of central and southern CA I recall the only times we used this was a few times on the backside of the channel islands (where the wind is born). And that was rare. Most often when anchoring we wouldn't bother with bouying the rode. I do not recall seeing anyone anchored out in that fashion. Perhaps I did but thought they were in a mooring field, as sparse as those were.

I know this technique is popular with the inshore fishermen or surfers who boat to isolated sections of an unprotected coast along California. These boats are typically hard chined trailer boats of 15-35'.

However, in Hawaii in anchoring light boats in deep water (minimum 15 fathoms) it was quite common to use this technique. When not bouying the rode we'd use shots of heavier chain as Andrew describes.
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Old 28-11-2012, 02:31   #12
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Re: Take the load off the bow

Another benefit of the buoy idea is relevant if you have to anchor in a situation where you would ideally leave someone on board but are not able to (eg you're shorthanded), and you want to keep tabs on how things are going.

I was first alerted to this by a guy who used this when anchored in the open roadstead off Pitcairn.

He could monitor how much load was on the rode, from anywhere on that side of the island, by seeing how deeply the (big) buoy was being submerged.
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Old 28-11-2012, 03:54   #13
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Re: Take the load off the bow

I can see a real problem with vessels passing close to the bow of the anchored boat and catching on the chain.
If you have ever seen this happen its often an ugly collision.
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Old 28-11-2012, 05:19   #14
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Re: Take the load off the bow

Quote:
Originally Posted by Richard5 View Post
I know this technique is popular with the inshore fishermen or surfers who boat to isolated sections of an unprotected coast along California. These boats are typically hard chined trailer boats of 15-35'.

However, in Hawaii in anchoring light boats in deep water (minimum 15 fathoms) it was quite common to use this technique.
For me both those approaches sound like the technique is (also?) used to reduce the length of chain / rode needed onboard (as well as any benefits from being "better")......any thoughts?

An interesting idea and one which I had never considered (nor seen, at least not knowingly!) - maybe a(nother) reason to have a giant fender onboard (or simply an inflateable?).

Funnily enough I was pondering over the car tyre as snubber idea for an anchor in recent times (historically I have always carried a couple onboard anyway!). I am presently moored to the dock / harbour wall - bow on......and the bow lines use tyres as snubbers (chained to the top of the dock - above water) and they work very well.....never thought about dropping the tyre / snubber into the water though........
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Old 28-11-2012, 09:51   #15
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Re: Take the load off the bow

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Originally Posted by Roy M View Post
Is this like mounting a bow eye near the waterline for leading the rode to the deck? I know that this is a good idea, but I've never actually done the deed.
It's a similar idea, yes. A snubber here means the chain rode is jerked with boat movement more back and forth than up and down.

You attach the snubber to the rode at the desired length to provide adequate catenary, and the remainder to the deck is left slack.
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