Who Needs a Snubber, Anyway?

[conachair]

Originally Posted by Dockhead

This would allow a stouter line to be used for the snubber itself, providing more total energy absorption, but giving some soft initial compliance for the smaller impacts. I like the looks of Kenomac's system.

Certainly looks pretty

But from Newtons point of view when things really get bad maybe not as good as a longer snubber. Total energy absorption is more of a factor in snubber sizing, you want thinner to lessen the deceleration, but be tough enough for the job as well.

Like everything in a boat, a compromise

[Dockhead]

Originally Posted by conachair

Maybe I'm reading that incorrectly, but it sounds just plain wrong. It takes much more force to lift the chain off the seabed in deep water than it does in shallow.

Try playing around here,
Cable Sag Error (Catenary Curve Effect) Calculator

A 1000N (about 100Kg) horizontal force will just lift the chain in 26m of water, in 12m of water 200Kg is needed.

It has everything to do with depth of water.

You are confusing length of the chain with depth of the water. There is no variable for water depth in that calculator at all.

Naturally, in shallow water you have a "head start" at lifting the chain. So in deeper water, catenary will start working sooner. But once enough force has been applied to approach lifting the chain off the bottom, the depth of the water plays almost no role at all.

I say "almost", because different depth of water implies different vertical displacement between the two ends of the chain. A larger vertical displacement between the ends of the chain -- caused by greater depth -- will actually REDUCE the catenary effect, for a given length of chain.

[thinwater]

Originally Posted by Dockhead

I don't think dyneema is overused at all. It's perfect for this kind of use because it's a snap to splice, like 10x easier to splice than polyester double braid, and because it's highly resistant to chafe, two huge advantages over other kinds of rope for a use like this.

Dyneema has few downsides, not even really cost if you downsize the rope, and by next year, my boat will have almost no polyester left on her, other than dock lines. I will not really be using polyester rope anymore except where some stretch is specifically needed, like for anchor rode, dock lines, etc.

Yes, neat stuff, I use it a number of places, including halyards, soft shackles and as a leader several places where chafe is an issue. It is not about cost, and a child can splice it.

Curiously, a knot seldom functionally weakens a rope. If the rope is sized for good fatigue strength (10x SWL) it will fail somewhere else due to chafe. I've broken many knots and splices in testing, but never in the real world. Thus, much, perhaps most splicing is just for looks.

In this case it was being used to secure the bitter end in the locker. This should be an easy-to-cut fuse. A lashing is traditional and probably best for that reason. If there is chafe, something is very wrong.

For a racer Dyneema may make sense for all running rigging, but for a cruiser I think you are wrong for many applications. Who on earth needs a zero-stretch mainsheet or traveler when it comes time to jibe? Polyester is simply better. I had a Dyneema traveler line and removed it. Like jibing against a brick.

There are many places where downsizing is not possible. A neighbor used it for davit tackle; polyester will last as long and feels better. Neither stretches in a tackle, BS hardly matters, and both will last until the UV eats the cover.

Zero stretch is NOT always better, therefore it is not always the best material.

Sail Delmarva: Line Selection--Is Low Stretch More Myth Than Function?

[Kenomac]

Originally Posted by conachair

Certainly looks pretty

But from Newtons point of view when things really get bad maybe not as good as a longer snubber. Total energy absorption is more of a factor in snubber sizing, you want thinner to lessen the deceleration, but be tough enough for the job as well.

Like everything in a boat, a compromise

We tried the traditional long rope with a chain hook on the end method for our first year, however... Our Supper snubber wins hands down for performance and long term durability. We get plenty of stretch out of the two rubber shocks surrounded by several rope coils, much more than plain rope alone, which is why we do it this way now. The total length of rope is over 15ft for each one.

But maybe you need to see it in action once again to believe me.

https://m.youtube.com/watch?v=9lyypubKVrU

[thinwater]

Originally Posted by Dockhead

I agree with the first statement -- the point where catenary comes out has nothing to do with the depth of the water, contrary to what someone posted above. It is a function of load vs length and weight of the chain. Simple catenary function; can be worked out mathematically.[/SERMON MODE OFF]

In the sense that deeper water generally means more chain length, there is certainly a relationship. I did not add that qualifier because it seemed obvious. There are also dynamic factors that go beyond a simple equation; in deeper water there is more damping due to friction and momentum of the chain, waves are not as steep, and the wave may pass before the chain gets tight. In the shallows, the reactions are quicker. You saw this damping at work.

I based my comments on testing, and others have done the same.

[Dockhead]

Originally Posted by conachair

Certainly looks pretty

But from Newtons point of view when things really get bad maybe not as good as a longer snubber. Total energy absorption is more of a factor in snubber sizing, you want thinner to lessen the deceleration, but be tough enough for the job as well.

Like everything in a boat, a compromise

Indeed, and putting in a rubber buffer will help you design a snubber to do exactly that -- it will give you the initial softness while allowing you to use a more robust rope, which has more energy absorbing capacity but less compliance. Total energy absorbing capacity of the snubber can be increased by increasing the diameter, but that will reduce the compliance unless you increase the length. Increasing the length gets impractical (or at least difficult) fast. So much so, that some cruisers (Evans Starzinger I believe) run their snubbers along their side decks to an aft cleat.

It's a bit like variable rate springs on cars -- springs which are wound lightly at the bottom then progressively tighter. For the same reason -- to give initial compliance without reducing total energy absorption capacity.

We could do the same thing by splicing light and heavy nylon rope together in parallel, with the heavier rope being a bit longer. Initial shocks would be taken by the lighter line, but when this stretches out to the max capacity of that line, the heavier line takes over. Or just by putting on two snubbers at once -- one heavy, one light -- something I actually do sometimes.

Would be pretty cool, but the rubber buffer is much simpler.

[Dockhead]

Originally Posted by thinwater

In the sense that deeper water generally means more chain length, there is certainly a relationship. I did not add that qualifier because it seemed obvious. There are also dynamic factors that go beyond a simple equation; in deeper water there is more damping due to friction and momentum of the chain, waves are not as steep, and the wave may pass before the chain gets tight. In the shallows, the reactions are quicker. You saw this damping at work.

I based my comments on testing, and others have done the same.

Now I see.

But it would have been much more accurate to say "more chain" rather than "greater depth". It's a function of chain length, not water depth.

But concerning the other effects of deep water -- I completely agree. Anchoring in deep water with a lot of chain out is MUCH more comfortable, for the reasons you state. PROVIDED you are able to find decent shelter. The positive effect of deep water can be cancelled by worse sea condition caused by increased fetch resulting from anchoring further from the shore.

[conachair]

Originally Posted by Dockhead

You are confusing length of the chain with depth of the water. There is no variable for water depth in that calculator at all.

Nope, as we're talking of just lifting the last chain link here the cable sag is equivalent to water depth.


But looking at it more the force isn't the horizontal force, it's the chain tension so yes, it's not applicable here.

And I got it the wrong way round


It's quite well known that in deeper water you need less scope than shallow. Get very shallow the catinary helps less and less.

Naturally, in shallow water you have a "head start" at lifting the chain. So in deeper water, catenary will start working sooner. But once enough force has been applied to approach lifting the chain off the bottom, the depth of the water plays almost no role at all.

I say "almost", because different depth of water implies different vertical displacement between the two ends of the chain. A larger vertical displacement between the ends of the chain -- caused by greater depth -- will actually REDUCE the catenary effect, for a given length of chain.

Nope. Have a read here..

Water depth plays a big role.
Catenary & Scope In Anchor Rode: Anchor Systems For Small Boats





EDIT:

Knew it was stashed away in a bookmark somewhere..

ANCHORWATCH- THE THEORY OF ANCHORING

Catenary Force in kg = Specific chain weight x (chain length)^2/(2 x Depth)

[Dockhead]

Originally Posted by conachair

Nope, as we're talking of just lifting the last chain link here the cable sag is equivalent to water depth.


But looking at it more the force isn't the horizontal force, it's the chain tension so yes, it's not applicable here.

And I got it the wrong way round


It's quite well known that in deeper water you need less scope than shallow. Get very shallow the catinary helps less and less.



Nope. Have a read here..

Water depth plays a big role.
Catenary & Scope In Anchor Rode: Anchor Systems For Small Boats

That's a great article, in my opinion the seminal article on anchor rodes. I learned a lot of what I know from this, and re-read it periodically.

However, the diagram presented is for CONSTANT SCOPE, so more and more chain. Obviously more chain gives more effect, but that's a function of more chain, not of more depth. In fact nothing in Peter's article contradicts what I said. Let me try to restate the arguments more clearly:

1. More chain -- more length, and/or more weight -- gives more catenary effect.

2. More depth allows the catenary effect of a given length and weight of chain to start working sooner. That's because more of the chain will be lifted off the bottom sooner. When chain is lying on the bottom, it's not giving any catenary effect. This is an effect of geometry in two different ways: vertical space needed for the catenary sag, and separately, the angle of pull on the chain in deeper water. That's what Peter Smith is talking about when he says that catenary effect doesn't do much good in shallow water -- he means that it gets going later.

3. HOWEVER, more depth, for the same length and weight of chain, REDUCES catenary effect, as a matter of geometry, due to increased vertical displacement of the ends of the chain, as long as the chain is substantially off the bottom. A simple thought-experiment to prove this: imagine so much depth that our anchor chain is now hanging vertically. NO catenary effect at all -- zero. The force of the weight of the chain has no horizontal component.

4. To say another way, if you are comparing shallow water and deep water, and a given length and weight of chain, once the chain is nearly all off the bottom, the catenary effect will be GREATER in shallow water because of the reduced vertical displacement between the ends of the chain.

5. Despite all this, the PRACTICAL benefit of catenary is greater in deeper water, because you are getting useful catenary effect over a wider range. But catenary effect is not the only benefit of chain -- it also works, just in a different way, while lying on the bottom in shallow water.

6. In all of this, I'm talking about HORIZONTAL FORCE created by the sag in the chain, NOT change of angle of pull on the anchor. Peter Smith talks about both at various times, which is confusing. I'm talking about the "catenary effect" as the term is used by engineers. The angle of pull issue is actually a bit more complicated than you might gather from Peter's article. In shallow water, catenary starts to work later, because significant parts of the chain stay on the bottom, for longer. But so long as significant amounts of chain are lying on the bottom, any force imparted to the anchor will be HORIZONTAL anyway, so no improvement to the angle of pull is needed. So although the catenary is not doing anything directly qua catenary, the length and weight of the chain is having an equally beneficial effect, by keeping itself on the bottom. So it is not true that this is less beneficial, than the same chain would be in deeper water. There's less of a problem to solve, too, because in shallower water, the angle of pull is better in the first place.


All this is out the window if you let out more chain in deeper water. But you can let out more chain in shallow water, too. One shouldn't confuse the effects produced by more chain, with the effects produced by more depth. More chain and heavier chain is ALWAYS better*, even if it's just lying on the bottom. In fact it's even better just lying on the bottom in shallow water, than it is hanging in catenary in deep water, because it is interacting mechanically with the seabed and adding mechanical holding force.

Peter was trying to show why you need less SCOPE in deep water. But he didn't say you need less CHAIN. On the contrary, you have a bigger problem to solve in deep water (angle of pull), and need MORE chain, even if the scope can be less.



* This might seem to contradict what Peter was saying about bar tight chains, but I stand by it. It's true that once the chain is bar tight it is no longer improving the angle of pull -- Peter's main point. But improving the angle of pull is not the only beneficial effect of heavy chain.

[Kenomac]

Dockhead,

If someone is having diffficulty understanding this concept, the lightbulb above their head will probably turn on the next time they attempt to raise their anchor after anchoring in 80ft depth.

[valhalla360]

Originally Posted by Cadence

A trip line always makes sense as does a snubber with a chain. So does an anchor sally with rope.

A small effort for saving a possilbe disastrous result . JMHO

I have mixed feelings on the trip line. In an isolated anchorage they may be OK but I've seen a few run over in busy anchorages as people are trying to use up any available space.

You may know what you are doing but no guarantee the next guy is smart enough to realize what the float is and that he shouldn't run it over.

[Kenomac]

Originally Posted by valhalla360

I have mixed feelings on the trip line. In an isolated anchorage they may be OK but I've seen a few run over in busy anchorages as people are trying to use up any available space.

You may know what you are doing but no guarantee the next guy is smart enough to realize what the float is and that he shouldn't run it over.

I've seen people tie onto the float thinking it was a mooring.

[Mike OReilly]

I used hefty double-braid for our snubbers/bridle (tied on with rolling hitches). With the loads on our boat I find the triple braid to sometimes be too stretchy, which as Ann said, can produce a mighty slingshot effect.

Even with our bridle I sometimes get chain noise as we shift around. Depends on the bottom and whether we are moving over top of our rode. Since our chain locker opens into our V-berth, I can usually hear and assess what is going on from bed.

I'm not a big fan of anchor floats. I occasionally set one if I'm anchoring in an area where fouling is a real possibility. So far I've never had to deal with a hurricane (although we did manage through a tornado this past season). Sounds like a good idea to set one then. Problem with floats in busy anchorages is that they occupy swing space that other boats could use (kinda like a mooring), and they risk getting grabbed by passing boats.

[conachair]

Originally Posted by Dockhead

That's a great article, in my opinion the seminal article on anchor rodes. I learned a lot of what I know from this, and re-read it periodically......................

I'm loosing the plot here about what you're actually arguing, sorry

So my point - the catenary curve changes as the water depth gets greater for a given scope.

Maybe you missed the edit, but from this equation..

Catenary Force in kg = Specific chain weight x (chain length)^2/(2 x Depth)

So for 10mm chain (about 2kg/m) in 4m water at 5:1 scope to just lift the last link:
F=(2*20*20)/(4*2)=100Kg

Same in 10m water:
F=(2*50*50)/(10*2)=250Kg

Or a scope of 3.16:1 for a horizontal force of 100Kg.
Water depth makes a big difference, you need less scope for the same horizontal force compared to shallower water.

In very shallow water forget the catenery effect , it hardly exists unless you have a ridiculous scope out. Though the angle to the anchor is so shallow that it doesn't matter much with our lovely new gen anchors

Some maths:
http://www.awelina.co.uk/anchor_rode/rode_length.htm
http://www.anchorwatch.co.uk/index-page04.htm

[S/V Alchemy]

Originally Posted by Dockhead

I don't think dyneema is overused at all. It's perfect for this kind of use because it's a snap to splice, like 10x easier to splice than polyester double braid, and because it's highly resistant to chafe, two huge advantages over other kinds of rope for a use like this.

Dyneema has few downsides, not even really cost if you downsize the rope, and by next year, my boat will have almost no polyester left on her, other than dock lines. I will not really be using polyester rope anymore except where some stretch is specifically needed, like for anchor rode, dock lines, etc.

I bought about 400 feet of Dyneema core 1/2" Dacron cover for three halyards and a topping lift and I'm very pleased with it. I don't feel the need to change over my sheets, but lower-stretch halyards are very desirable.