Choice of Material for Snubbers

[Delfin]

Originally Posted by JonJo

The tests were conducted with no bridle, so no snubber. We will eventually repeat with a snubber, or snubbers and we are also aiming to repeat with a 50' yacht (limited by what we can have access to).

Jonathan

Jonathan, when your boat veered, I presume it was dancing pretty quickly. Care to guess how fast?

[Sandero]

I've been anchoring with an all rode for over 20 years on Shiva a 36' mono w/ high free board and a fractional rig (forward mast and lots of windage). She veers back and forth especially without a steadying sail at anchor.

The snubber I use with the 5/16 chain is a 1" braid on braid nylon line with a heavy rubber mooring compensator w/ 3 wraps. The snubber is 30' feet long and connected to the chain with a stainless steel reefing hook. The compensator stretches a lot more than the nylon snubber alone would and this really absorbs the shocks. The snubber is led over the bow roller and to a deck cleat. Seems to work fine. The the wind pipes up and the compensator is not stretching and unwinding a bit... the anchor is dragging.

[JonJo]

Originally Posted by Jim Cate

JonJo, when you set up your next test series, how about checking out Dockheads theories on peak loading from a moving boat? One could, under calm conditions, first motor up creating a large slack in the rode, and then motoring briskly in reverse until the slack takes up and jerks the boat to a stop. I suspect that the numbers thus generated would be far lower than his calculations suggest, but real data would be useful to test the theory.

Another query, slightly off thread: in calculating the frontal area of a catamaran, can one really ignore the "tunnel"? I believe that at higher velocities one might find that that apparently open area added significantly to the total drag... don't know about the kind of wind speeds that we (hopefully) will be experiencing.

Cheers,

Jim

Jim,

Last one first. Ignoring the tunnel, I know what you mean - but it is difficult to decide how much of the tunnel to include. Sometimes in very choppy conditions there is no tunnel (its got waves in it). There is the same problem when you look at the 'area' of a chain, do you include the holes in the links? Its not a question of ignoring it, more how do you include it? I suspect its, the tunnel, a much bigger issue at 50 knots (as most of these variables are) than at 10 knots.

Dynamic loads, can do, though might not go to very high loads (they are a bit daunting). The 'instantaneous' loading is interesting (its not instantaneous but certainly short and sharp). The only marine product tested this way is dynamic climbing rope (and its not tested that way for marine applications). Mention has been made of heat damaging cordage, I suspect this is 'time' dependent and maybe different if wet? Anchor chain would merit dynamic testing - but I'm not sure how it would be actually done, G7 being supposedly less sympathetic than say G3 (dynamic tests suggest high tensile steels withstand dynamic loads better than expected). Chain is tested with slowly increasing loads, not a sudden (dynamic) load. We have had blocks explode under dynamic loads, when I might have thought (subjective) they had previously withstood higher loads applied more gently. Given that there are lots of dynamic loads (on a yacht) it is surprising more has not been done (waves in an anchorage and knots being topical examples).

Jonathan

[Kettlewell]

Another query, slightly off thread: in calculating the frontal area of a catamaran, can one really ignore the "tunnel"? I believe that at higher velocities one might find that that apparently open area added significantly to the total drag... don't know about the kind of wind speeds that we (hopefully) will be experiencing.

I don't know the answer to that, except to say that on my 32-foot cat you could feel a significant speeding up of the wind through the tunnel in lighter airs. I suspect that it might create a pressure differential that would either pull the boat down or want to make it fly, depending on the configuration. I was worried about my cat taking off in Hurricane Bob, but strangely it felt even more "planted" than normal. That was purely a feeling--no evidence.

I will say that in general a cat of the same length of a mono will have greater windage than the mono, and will have an average higher pull on the anchor. However, with a bridle a cat points very steadily into the wind and doesn't experience peak loads like a mono veering all over the harbor would.

[JonJo]

Originally Posted by Delfin

Jonathan, when your boat veered, I presume it was dancing pretty quickly. Care to guess how fast?

At short scopes faster than long scopes, but actual speeds - no real idea. It would be nice to know how to measure it. Neither a through hull log or GPS have the accuracy.

Jonathan

[Kettlewell]

The tension on the anchor rodes is the important figure, more than the speed.

[Jim Cate]

Originally Posted by Kettlewell

The tension on the anchor rodes is the important figure, more than the speed.

But if we are to test the Dockhead theory we need to know the speed, and transverse velocities will be hard to do. That's why I was thinking of backing straight (if the test boat can do that!) as an experiment.

Cheers,

Jim

[Cotemar]

Originally Posted by estarzinger

This is my last thought for now on this topic, until I have more direct experience with and have fined tuned my 'new solution' . . . .

(1) Clearly an all nylon snubber works and is a perfectly adequate solution. It's the traditional and well proven solution. I don't think anyone will say that all nylon is 'unreasonable'.

(3) In all this, I think the 'soft shackle' is the most useful/practical development . . . . more so than the rode discussion. Both the rolling hitch and chain hook obviously generally work, but IMHO the soft shackle is just simply day to day "easier and better" than both with zero downside.

Points 1 & 3 are the most important points made in this whole thread.
Glade you circled around to those key points again.

I have a 20 foot Nylon Snubber and have just made a bunch of Soft Shackles to play with.

Now can we all go sailing?

[Dennis.G]

Without delving into the math, I think there are some issues in the OP’s calculations in reference to the boundary conditions assumed. In the simple “stopping distance” approach he has assumed the snubbing line to be in a straight line with the center of mass of the yacht, and has also assumed only stretch of chain in the case of chain only. I find the following problems with this:

· A boat will most usually come up tight to its rode with the rode at an angle to the centerline of the boat. The result will be a yawing of the boat to straighten it with the rode. This process will effectively increase the stopping distance (if measured say from anchor to center of mass of boat).

· Even if boat had centerline pointed inline with the anchor rode when coming up tight, there would still be a vertical pitch pulling bow down (this assumes that the force vector of rode would have its line of force well above the center of mass of the boat). This would also increase stopping distance similar to the yawing effect.

· Although the above cases might not contribute any large length to the stopping distance, the additional lengths would be significant when compared to the small stretch lengths of the polyester braid and chain.

· The removal of chain catenary from consideration in the stopping distance I think is a big mistake. The assumption that “that this effect works only up to a certain force” is just not true, as it would take an infinite tension to truly straighten out chain to a straight line. True, for practical purposes it may be deemed “bar tight”, the fact is that 50 m of 12 mm chain will still have significant catenary and resulting horizontal “give” in relation to the calculated 3.4 cm of chain stretch at 1.7 ton. Reality is that the exponential increase in tension of the chain as the catenary curve is straightened will be decelerating the boat before chain stretch even becomes significant.

I think that the wrong assumptions used in the calculations have exaggerated the ultimate forces involved, and that the exaggerations are especially evident the lower the elasticity of the material considered (i.e., making the polyester braid and chain look much much worse than reality).

[conachair]

Originally Posted by Dennis.G

Without delving into the math,

delve away

Cable Sag Error (Catenary Curve Effect) Calculator

[Dockhead]

Originally Posted by Dennis.G

Without delving into the math, I think there are some issues in the OP’s calculations in reference to the boundary conditions assumed. In the simple “stopping distance” approach he has assumed the snubbing line to be in a straight line with the center of mass of the yacht, and has also assumed only stretch of chain in the case of chain only. I find the following problems with this:

· A boat will most usually come up tight to its rode with the rode at an angle to the centerline of the boat. The result will be a yawing of the boat to straighten it with the rode. This process will effectively increase the stopping distance (if measured say from anchor to center of mass of boat).

· Even if boat had centerline pointed inline with the anchor rode when coming up tight, there would still be a vertical pitch pulling bow down (this assumes that the force vector of rode would have its line of force well above the center of mass of the boat). This would also increase stopping distance similar to the yawing effect.

· Although the above cases might not contribute any large length to the stopping distance, the additional lengths would be significant when compared to the small stretch lengths of the polyester braid and chain.

· The removal of chain catenary from consideration in the stopping distance I think is a big mistake. The assumption that “that this effect works only up to a certain force” is just not true, as it would take an infinite tension to truly straighten out chain to a straight line. True, for practical purposes it may be deemed “bar tight”, the fact is that 50 m of 12 mm chain will still have significant catenary and resulting horizontal “give” in relation to the calculated 3.4 cm of chain stretch at 1.7 ton. Reality is that the exponential increase in tension of the chain as the catenary curve is straightened will be decelerating the boat before chain stretch even becomes significant.

I think that the wrong assumptions used in the calculations have exaggerated the ultimate forces involved, and that the exaggerations are especially evident the lower the elasticity of the material considered (i.e., making the polyester braid and chain look much much worse than reality).

I think that all this is very valuable and true, and for all these reasons, and others which were mentioned, the real forces in a real situation will -- as I said several times -- usually be less than the theoretical calculations. For these reasons, on top of the fact that -- as I wrote several times -- there will be other flex in the system which adds to the "stopping distance".

The purpose of my calculations was not to predict what forces will necessarily be in all real life situations, and I never claimed that for them. I was trying to show the forces which, in theory, could be generated, and under some circumstances might actually be generated, and the dramatic differences in the ability of the different materials to handle these forces. The title of the thread is -- pay attention -- choice of material etc. I did not consider other softening factors -- merely the three materials in isolation and the raw force of the boat stopping, as if your boat was being stopped purely by the snubber. It is an artificial and unrealistic scenario, but the correct kind of scenario to use when you're trying to clearly understand the difference in materials.

It would be as if -- you want to understand the difference in acceleration performance of three cars. What do you do? You put them on a drag strip and see how they do. Someone comes along and says -- "but that's artificial and unrealistic -- in real life, you have red lights and speed limits, and curves". Well -- yes, of course. In real life the theoretical acceleration performance will never be used fully -- all kinds of other factors come into play. City traffic is not much like a drag strip. But you will still never understand that one quality in isolation of your three cars without the artificial and unrealistic drag strip, which allows you to isolate that quality. So that is exactly what my thing was intended to be -- a "drag strip" for snubbers where no other complicating factors are considered -- snubber versus 25 ton boat, one-on-one.

The value of the exercise is actually illustrated and underlined by the difficulty some people have of comprehending the very large forces involved in stopping a boat from a modest speed. Those calculations are straightforward -- high school level physics, and you don't even need that if you use this simple calculator: Online calculator: Kinematics. The Equations for Uniform Acceleration The difficulty some people have in accepting this shows that it is not intuitive that stopping a boat from two knots in a few inches will create many tons of potentially destructive force, but it is a simple fact of physics.

The purpose of thinking about that is to provide a reality check for anyone who considers using a snubber which only provides a few inches of stretch. Sure, you will get away with it in most ordinary conditions -- the conditions don't actually create motion which amounts to two knots; the boat yaws; the bow roller flexes, etc., etc., etc. All these things soften the blow, compared to the theoretical force. And the most important softening factor will be this -- the anchor moves in the seabed, or gets pulled out of the seabed altogether -- did anyone think about this? The other important purpose of stretch in your snubber is to keep the motion of your boat from pulling the anchor out, a purpose which might be no less important than preventing gear breakage.

Based on my own practical experience, which is limited like everyone else's, I don't think wind loads short of hurricanes create such big snatch loads (nothing like 2 knots), and I don't think wind loads or even 60 knot gusts are huge challenges for snubbers, unless we're talking about a hurricane which goes on for hours, yawing the boat back and forth (and then chafe becomes even more important than energy absorption). I think that for most sailors the real challenge to a snubber is going to be wave action, which can result when you lose your shelter due to a wind shift. My own personal, rude experience with many tons of destructive shock loads came from these situations. Again, the prudent thing to do is not to rely on your excellent snubber, but up anchor and move, but sometimes it's not possible. So here the ultimate performance of your snubber might save your a$$ some day, so surely it is not at all a bad thing to understand your materials and the amount of energy they can absorb.

[DumnMad]

Dockhead's figures are not his theory, they are science. e.g. ship hitting wharf. Slow speed doesn't save the wharf unless there are soft deep fenders.
Good guidance in getting your snubbing practice right and he is not claiming that the figures address all the facts.
A full chain (or Dyneema) snubber would be useless. Only thing that would save the boat from shock loading would be straightening the catenary, stretch in the rode and possibly more important, incremental movement of the anchor.
Under severe cyclic storm wave loading nylon needs to be kept cool. Here is where Evans hybrid comes in. Dyneema or chain for chafe long enough for the nylon to get plenty of wave wetting and cooling.

[Dockhead]

Originally Posted by Dennis.G

Without delving into the math, I think there are some issues in the OP’s calculations in reference to the boundary conditions assumed. In the simple “stopping distance” approach he has assumed the snubbing line to be in a straight line with the center of mass of the yacht, and has also assumed only stretch of chain in the case of chain only. I find the following problems with this:

· A boat will most usually come up tight to its rode with the rode at an angle to the centerline of the boat. The result will be a yawing of the boat to straighten it with the rode. This process will effectively increase the stopping distance (if measured say from anchor to center of mass of boat).

· Even if boat had centerline pointed inline with the anchor rode when coming up tight, there would still be a vertical pitch pulling bow down (this assumes that the force vector of rode would have its line of force well above the center of mass of the boat). This would also increase stopping distance similar to the yawing effect.

· Although the above cases might not contribute any large length to the stopping distance, the additional lengths would be significant when compared to the small stretch lengths of the polyester braid and chain.

· The removal of chain catenary from consideration in the stopping distance I think is a big mistake. The assumption that “that this effect works only up to a certain force” is just not true, as it would take an infinite tension to truly straighten out chain to a straight line. True, for practical purposes it may be deemed “bar tight”, the fact is that 50 m of 12 mm chain will still have significant catenary and resulting horizontal “give” in relation to the calculated 3.4 cm of chain stretch at 1.7 ton. Reality is that the exponential increase in tension of the chain as the catenary curve is straightened will be decelerating the boat before chain stretch even becomes significant.

I think that the wrong assumptions used in the calculations have exaggerated the ultimate forces involved, and that the exaggerations are especially evident the lower the elasticity of the material considered (i.e., making the polyester braid and chain look much much worse than reality).

This is a really insightful post which I found worth further study and thought.

I agree with 100% of it, except only:

1. As I said, I did not assume that there were no other forces involved. I ignored them intentionally so that we could consider the energy absorption properties of the different snubber materials in isolation. Those are entirely different things.

2. I don't think the paragraph about the effect of chain catenary at the end is correct. It is true that the force required to turn the chain into a straight line approaches infinity as the chain get "bar tight". But as a result of the exact same effect, the chain is no longer absorbing any energy at that point! So the energy absorbing character of the chain diminishes, rather than increases, as the chain draws tight. The less sag there is in the chain, the more the force acts not on the sagging loop, but directly on the anchor. Once there's no sagging loop, you might as well have a solid bar, as far as energy absorbing is concerned. "Reality is that the exponential increase in tension of the chain as the catenary curve is straightened will be decelerating the boat before chain stretch even becomes significant" -- absolutely right, but wrong conclusion -- the tension increases exponentionally and you are left with nothing but elasticity of the chain itself (and of your bow roller as it breaks ). So I think it is absolutely correct to say that once the chain is "bar tight" (which does not mean absolutely straight geometrically), the catenary is playing no role in absorbing the force of stopping the boat.


Otherwise, I agree with every word of this interesting post.

[Dockhead]

Originally Posted by Kettlewell

The tension on the anchor rodes is the important figure, more than the speed.

You need to know the kinetic energy which needs to be dissipated. It's a function of mass and velocity, so speed is essential.

Tension on the rode will be all different things depending on the elasticity of the rode. A given amount of kinetic energy will produce dramatically different tension on the rode depending on the "stopping distance" -- the elasticity of the rode (yes, the whole system including anchor in the seabed, pitching down of the bow of the boat, flexing or breaking of the bow roller, etc., etc. -- sorry for confusing everyone by focussing exclusively on the snubber).

[Dockhead]

Originally Posted by Cotemar

Now can we all go sailing?

Not me -- although the weather is gorgeous, I am presently 2000 miles from my boat Have to wait until next week