Anchor snubber design.

[Neeves]

Coincidentally this has just been released:

https://www.mantusmarine.com/snubbe...il&utm_term=0_b290768734-67979dda78-104676541

Something for Practical Sailor to get their teeth into.

Edit Another device, along the lines of what GHA proposes is a nylon line with dyneema spliced into it, or a moveable dyneema sleeve (with a method for securement when the location is decided) to provide an 'abrasion resistant' point. If there is a market for a strop (why not buy a soft shackle), both of which can be made easily at home - then there is a market for a snubber with built in abrasion resistant points.

close edit

 

[zoidberg]

Coincidentally this has just been released:

Isn't it remarkable how old techniques from the Age of Sail are being rediscovered and marketed as 'wunnerful new ideas'...?

 

[GHA]

Coincidentally this has just been released:

https://www.mantusmarine.com/snubbe...il&utm_term=0_b290768734-67979dda78-104676541

Something for Practical Sailor to get their teeth into.

Edit Another device, along the lines of what GHA proposes is a nylon line with dyneema spliced into it, or a moveable dyneema sleeve (with a method for securement when the location is decided) to provide an 'abrasion resistant' point. If there is a market for a strop (why not buy a soft shackle), both of which can be made easily at home - then there is a market for a snubber with built in abrasion resistant points.

close edit

Jasus!!! $55.00 – $103.00 for a bit of posh string!

 

[Neeves]

Jasus!!! $55.00 – $103.00 for a bit of posh string!

But it is endless.

GHA - you have a way with words, I need some lessons.

Jonathan

 

[tudorsailor]

Can I ask what people think is the primary purpose of a snubber is?

For me the main purpose is to take the load of the windlass as it is not designed to take the load of the anchor plus chain when the wind gets up. Indeed my Lewmar has two cones that "lock" with friction. So if the wind gets up, the chain could slowly pay out if the top nut us not tight enough (and has). If one then tightens the nut to stop the slippage, it can be difficult subsequently to get the nut undone to allow the chain to free run when deploying the anchor.

When there is no wind, the chain forms a caternary. As the wind strengthen the chain straightens and only when fully straight can there be a snatch load. With more scope the higher the wind strength required to straighten the chain. So although theoretically the snubber should be stretchy, does it really matter when most of the shock absorbing comes from the loss of caternary?

For a bit of thread drift, but most of the above does not apply when stern-to the quay against the anchor.


TudorSailor

 

[GHA]

Can I ask what people think is the primary purpose of a snubber is?

For me the main purpose is to take the load of the windlass as it is not designed to take the load of the anchor plus chain when the wind gets up. Indeed my Lewmar has two cones that "lock" with friction. So if the wind gets up, the chain could slowly pay out if the top nut us not tight enough (and has). If one then tightens the nut to stop the slippage, it can be difficult subsequently to get the nut undone to allow the chain to free run when deploying the anchor.

When there is no wind, the chain forms a caternary. As the wind strengthen the chain straightens and only when fully straight can there be a snatch load. With more scope the higher the wind strength required to straighten the chain. So although theoretically the snubber should be stretchy, does it really matter when most of the shock absorbing comes from the loss of caternary?

For a bit of thread drift, but most of the above does not apply when stern-to the quay against the anchor.


TudorSailor

Primarily to lessen the force on the anchor IMHO. Once the wind is up towards 25/30Kts the bulk of the catinary is gone with the static force of the wind, "most of the shock absorbing comes from the loss of caternary?" is no longer the case, to straighten the chain just a little bit more the force required goes through the roof.

So with any veering or when the bow rises & falls the energy has to go somewhere if the boat is to stay where you put it, with so little catenary left the boat deccelerates rapidly and the force on the anchor can be huge. A snubber still has stretch left in it so absorbs some of the energy along the way slowing the energy transfer so the boat deccelerates slower. Less force at the anchor. Bearing in mind it's additional to any shock absorption provided by the anchor chain catenary/stretch.

Sort of

Though not something which is actually needed very often in a lot of cruising grounds.

 

[Neeves]

Primarily to lessen the force on the anchor IMHO. Once the wind is up towards 25/30Kts the bulk of the catinary is gone with the static force of the wind, "most of the shock absorbing comes from the loss of caternary?" is no longer the case, to straighten the chain just a little bit more the force required goes through the roof.

So with any veering or when the bow rises & falls the energy has to go somewhere if the boat is to stay where you put it, with so little catenary left the boat deccelerates rapidly and the force on the anchor can be huge. A snubber still has stretch left in it so absorbs some of the energy along the way slowing the energy transfer so the boat deccelerates slower. Less force at the anchor. Bearing in mind it's additional to any shock absorption provided by the anchor chain catenary/stretch.

Sort of

Though not something which is actually needed very often in a lot of cruising grounds.

Not 'Sort of' but precisely.

Obviously if you have lots of chain you could deploy more chain but you might not have lots or there is no room to deploy more.

Its not one thing or the other - the chain and snubber work together.

The other factor for windlass is that they have seals to keep water out of the gearbox - it is unlikely (but possible) to bend a shaft but much more likely you damage a seal - so keeping the tension off the gear box is very advantageous.

As GHA suggests anchoring such that your yacht is fully exposed to 30 knots, or more, is not 'normal' most people will not have ventured out at all. Equally most 'defined' anchorages are protected from swell and seas. However if you are 'caught' having snubbers gives you options. Most of the time a short snubber, without elasticity, will be more than adequate but I'd strongly recommend having one with elasticity (anyway) especially if you are focussed on, say, NW Scotland.

Jonathan

Edit

You do not need a snubber to protect the windlass, you can use a chain lock (buy them off the shelf) or simply a chain hook attached with a short strop to the Samson post or a bow cleat. Having a chain lock, or something very simple to apply, means if your anchor is well dug in you can then 'isolate' the windlass when you try to break out at retrieval. You can also use it if you power set. Any mechanism that 'isolates' the windlass from higher tensions will help to prolong the windlass life. In fact I would not use an elastic snubber to power set - the energy goes into stretching the snubber not setting the anchor - seems daft to me.

Close edit

 

[BelleSerene]

You do not need a snubber to protect the windlass, you can use a chain lock (buy them off the shelf) or simply a chain hook attached with a short strop to the Samson post or a bow cleat. Having a chain lock, or something very simple to apply, means if your anchor is well dug in you can then 'isolate' the windlass when you try to break out at retrieval. You can also use it if you power set. Any mechanism that 'isolates' the windlass from higher tensions will help to prolong the windlass life. In fact I would not use an elastic snubber to power set - the energy goes into stretching the snubber not setting the anchor - seems daft to me.

I agree with what you say Jonathan, except the last statement. Seems to me it's about force not energy, and once the elastic stop stretching the force the boat the boat's reverse power applies to one end of the anchor snubber is the force being transmitted to the anchor at the other end. The elastic smooths out transient peaks in that force, but doesn't stop your engine pulling the anchor with all the force it would do with simply a length of chain. The result is that having a snubber in situ while power setting doesn't inhibit the set.

I agree with everything else you say.

 

[BelleSerene]

Am I the only one to find it satisfactory to use a shortish snubbing warp with rubber bungies to make up for length?

It avoids the need for a boat length of rope, which will as often as not sit on the bottom of the sea and defeat the point of having a snubber - and I personally find it gives me (12m boat, ~8 tonnes) quite enough snubbing effect.

I use a warp of just 4-5 metres (including length for OXO round a bow cleat) with a chain hook spliced into one end and a couple of short rubber bungies like this [ http://www.arolyth.se/txbueng.html ] towards the hook end. The hook is always just above the water line, I can see the catenary of the chain dipping from it, and the thing doesn't endanger the side decks.

I see that PBO reviewed them here FWIW: http://www.pbo.co.uk/gear/pbo-tested-12-mooring-snubbers-45131

 

[thinwater]

I agree with what you say Jonathan, except the last statement. Seems to me it's about force not energy, and once the elastic stop stretching the force the boat the boat's reverse power applies to one end of the anchor snubber is the force being transmitted to the anchor at the other end. The elastic smooths out transient peaks in that force, but doesn't stop your engine pulling the anchor with all the force it would do with simply a length of chain. The result is that having a snubber in situ while power setting doesn't inhibit the set.

I agree with everything else you say.

I would go farther and say that it is about time over a critical force. In mud, at least, the anchor is setting through a viscous medium, and simply hitting it with a hammer does not do the job. You need to lean on it until it moves, and that is where the elastic helps. Those of us from snow or mud country know there is a reason that vehicle recovery straps stretch--you need force and time.

 

[noelex]

If you set the anchor with gradually increasing reverse, as most people do, setting the anchor with or without a snubber normalły it makes little difference. I have tried both and observed the effect underwater. If there is swell or a soft substrate the elasticity of the snubber is if anything helpful.

The advantage of using the snubber is that it takes the load off the windlass (although there are other ways of doing this) and if you rig the snubber from the bow it allows setting of the anchor with the maximum scope.

The only drawback is if the anchor does not set the first time it is more effort to disconnect the snubber, but good modern anchors almost always set the first time so this is a minor effect.

The alternative way of setting the anchor is to use the momentum of the yacht to provide the setting force. Here the elasticity of the snubber helps.

 

[thinwater]

The alternative way of setting the anchor is to use the momentum of the yacht to provide the setting force. Here the elasticity of the snubber helps.

Bingo.

By extension, if you are setting the anchor with an anemic outboard, which has even less thrust in reverse than forward, this is something to consider. I doubt the typical 6hp small boat kicker has more than 35 pounds thrust in reverse, no more than just setting hard by hand. They must use momentum.

 

[BelleSerene]

BTW does anyone have an informed view of whether it's OK to have the snubber exerting a sideways force on the windlass? The direct line from my bow roller to either foredeck winch runs the snubber warp over the metal wall of the bow roller, and I don't want to chaff it. So I run the warp to one side of the the windlass instead and from there to the other bow roller. I guess there's a 30° bend in the snubber line on the winch. If one objective of a snubber line is to relieve the windlass of force while it's not weighing the anchor, you could say this somewhat defeats it. But with my setup I haven't found a better solution.

 

[thinwater]

BTW does anyone have an informed view of whether it's OK to have the snubber exerting a sideways force on the windlass? The direct line from my bow roller to either foredeck winch runs the snubber warp over the metal wall of the bow roller, and I don't want to chaff it. So I run the warp to one side of the the windlass instead and from there to the other bow roller. I guess there's a 30° bend in the snubber line on the winch. If one objective of a snubber line is to relieve the windlass of force while it's not weighing the anchor, you could say this somewhat defeats it. But with my setup I haven't found a better solution.

A picture would help, but since the chain should be slack, yeah, this is not doing the job completely. So far as I know, all of the windlass makers tell you to take the load off the windlass and to use it ONLY for hoisting the anchor, after it is broken loose.

A snubber does not need to run over the roller. It can run from a bow chock and it can be a bridle. You can install a new strong point (cleat, u-bolt, bolt hanger, or ring). For example, If I didn't use a separate bridle, the rode would run out of the roller at a 30 degree angle.

 

[GHA]

I agree with what you say Jonathan, except the last statement. Seems to me it's about force not energy, and once the elastic stop stretching the force the boat the boat's reverse power applies to one end of the anchor snubber is the force being transmitted to the anchor at the other end. The elastic smooths out transient peaks in that force, but doesn't stop your engine pulling the anchor with all the force it would do with simply a length of chain. The result is that having a snubber in situ while power setting doesn't inhibit the set.

I agree with everything else you say.

:encouragement:

This seems to come up a lot even in magazine articles, where the energy goes is only one part of the picture and pretty meaningless when looked at in isolation.
From my university of "google, youtube and lots of coffee with a keen interest" (so feel free any academics to correct any glaring erorrs) , it's the rate of energy transfer that is of interest, which is proportional to the force.

So ignoring static forces for a moment, with the boat sailing across the anchorage after a big gust from a different direction the boat gains kinetic energy, if you want to boat to stop the energy needs to go somewhere . Stop it with tight chain then the energy gets transferred very quickly from the boat, force = mass X acceleration so lots force and rapid change in velocity as the boat stops.
Stick snubber in the energy gets transferred much slower, much less force.

Some equations:
E = F * D, E = 1/2 M * V²
Or energy in joules = force in newtons times distance in metres, and also 1/2 times mass times speed squared.

So for a 10,000Kg boat moving at 2kts or about 1m/s the energy is 1/2 * 10,000 * 1² = 5.000J

Say we have a snubber which in this case stretches 2m, 5,000 = force x 2, force = 2,500 Newtons or about 250Kg.
(note that's the average force, if the snubber stretches linearly then peak load at the end will be double)

Reaally big gust , boat shoots off at 4Kts or about 2m/s, energy is now not double, but 4 times >
1/2 * 10,000 * 2² = 20,000J.
If the snubber stretches twice as much, 4m, then 20,000 = F x 4 = 5,000N or about 500Kg, twice the force to stop twice the speed in twice the distance.

So it really must pay to keep the boat speed down when it's really blowing.

As a quick check, newton says velocity² = 2 x Acceleration * distance, so 1²= 2 * A x 2, so A=0.25
Also force = mass * acceleration so F = 10,000 * 0.25 = 2,500N. So it seems to tally up.

Might even be along the right tracks
Though the real world has lots more going on; drag through the water etc but at least some numbers might help us get a handle on what's going on a bit better.

And off hand, I can't think of any magazine articles which get past the "snubber absorbs energy" which on it's own doesn't really say anything - why is that a good thing? Good as it increases the time / distance over which the boat kinetic energy goes somewhere else and so the anchor is a bit happier as the force is less.

So I wonder where the energy from the wind creating the static forces goes when the boat is static ? Noise? Heat?


PS: Agree with Jonathon, I'd never bother using a snubber digging the anchor in, it it needs finesse then use less revs.

 

[GHA]

The alternative way of setting the anchor is to use the momentum of the yacht to provide the setting force. Here the elasticity of the snubber helps.

Not so sure about that, if you want to get the hook well dug in then you want plenty of force, the snubber will reduce the force.

Must admit, in most anchorages I'm pretty lax about the "art of anchoring", drop the hook and make sure there isn't a pile of chain on top of it then let out a load and when the boat's got a fair chunk of the chain straightened out give it a load of reverse. When the bow spins round and dips right down then the holding is good and you can sleep well. (chain hook on a samson post of course)

Exceptions re rocky & soft bottoms where you seem to need to coax it in a bit more gentle, especially rocky, slowly, slowly..

 

[thinwater]

Not so sure about that, if you want to get the hook well dug in then you want plenty of force, the snubber will reduce the force.

No. Force depends on speed at impact, so the statement, taken in isolation as you say, is incorrect. The trick if using momentum, is to match the speed to the snubber to get the force right and maximize the duration of the force. Depending on the mass of he boat and the snubber, this will be between 1.5 and 2 knots. Additionally, if the setting is done slowly with the engine, the snubber is stretched into equilibrium and there is obviously no effect on force, greater or lesser.

As I said before, this only matters if the boat does not have enough reverse thrust (out board or failed engine) to use the engine.

As for the effects of yawing at anchor, that is really a separate problem that should and can be solved for each boat. Expecting the snubber to deal with that is like taking morphine for a broken bone--better to set the bone, fixing the problem.

 

[GHA]

No. Force depends on speed at impact, so the statement, taken in isolation as you say, is incorrect. The trick if using momentum, is to match the speed to the snubber to get the force right and maximize the duration of the force. Depending on the mass of he boat and the snubber, this will be between 1.5 and 2 knots. Additionally, if the setting is done slowly with the engine, the snubber is stretched into equilibrium and there is obviously no effect on force, greater or lesser.

As I said before, this only matters if the boat does not have enough reverse thrust (out board or failed engine) to use the engine.

As for the effects of yawing at anchor, that is really a separate problem that should and can be solved for each boat. Expecting the snubber to deal with that is like taking morphine for a broken bone--better to set the bone, fixing the problem.

Still disagree, I had a useless engine for years, want to dig the hook in then get a bit or reverse on and let the hook bite, a snubber reduces the force, just what you don't want. In most bottoms anyway.

Force depends not only on velocity but also distance/time of deceleration, snubber does a good job of increasing that and decreasing the force when you want it to

Easier to use less speed and keep the snubber for later.

Worked for me anyway, snubber just dilutes everything, if you want to bang a nail in you don't use a rubber hammer.

 

[BelleSerene]

A picture would help, but since the chain should be slack, yeah, this is not doing the job completely. So far as I know, all of the windlass makers tell you to take the load off the windlass and to use it ONLY for hoisting the anchor, after it is broken loose.

A snubber does not need to run over the roller. It can run from a bow chock and it can be a bridle. You can install a new strong point (cleat, u-bolt, bolt hanger, or ring). For example, If I didn't use a separate bridle, the rode would run out of the roller at a 30 degree angle.

I probably explained badly, I’m sorry: yes, the chain is slack which is why the snubber is tight. But I like your idea of putting the snubber outside the decks, and even better in a bridle. Can’t think why I didn’t think of that. Probably just the convenience of being able to clip the snubber hook onto the chain as the last 2-3m goes out and let it run out over the bow roller. But you have the more professional solution in a bridle. I think I’ll change to that, add a couple of rubber bungies on each side and one end fed through each fairlead to bowlines dropped over the bow cleats.

 

[thinwater]

Still disagree, I had a useless engine for years, want to dig the hook in then get a bit or reverse on and let the hook bite, a snubber reduces the force, just what you don't want. In most bottoms anyway.

Force depends not only on velocity but also distance/time of deceleration, snubber does a good job of increasing that and decreasing the force when you want it to

Easier to use less speed and keep the snubber for later.

Worked for me anyway, snubber just dilutes everything, if you want to bang a nail in you don't use a rubber hammer.

You just misunderstand. You could have backed up even faster with the snubber in place and applied the same force for a longer time period. The hammer analogy is false.; it assumes a substrate where friction drops with speed, like a straw through a potato, whereas in mud friction increases with speed. Just think about it; does a hard yank get the anchor out of the bottom, or a steady pull? Does a hard yank or a steady pull get you rubbers out of the mud? It is well known that a steady pull is what is required. You know this but have overlooked that application on your anchor, perhaps because you were not watching the anchor.

I'm not guessing. I charted the impact forces with a variety of speeds, snubbers, and boat sizes with a load cell, and measure anchor set performance for an article.