How much snubber / bridle is too much?

[Pete7]

I guess the first question to ask is, does the fact the yacht is a trimaran have any effect on the design of the snubber?

I think not, what you actually have is a mono hull with two floats to stop it falling over, all be it with probably a very shallow draft, yes? I would also bet she sails all over the place at anchor without any real volume in the water to create resistance.

So your requirements are going to be similar to that of a motorboat with a planning hull shape, little below the water line lots above.

How does that sound?

Pete

[thinwater]

Originally Posted by MathiasW

Oh! If I didn't know climbing wasn't for me, I would know now...

I never took a long fall, but I once caught a 90' foot whipper. The impact force really was not that great, perhaps a few hundred pounds. The climber was young and didn't stop shaking for hours.

[thinwater]

Originally Posted by MathiasW

Yeah, I know that path! My question was more to figure out whether there are alternative, smarter ways of dealing with this...

Yes. Climbing rope, which is optimized for this (different weave). That was my point. But it only works for boats in the 28-43' range (different sizes are used), and the OPs may be a little too big. He might try 11mm rope with a bridle, but the thing is, a bridle does NOT reduce the load by half, or even 1/3 because of single leg loading.


Seriously, I doubt a snubber can stretch enough for this to be a factor. I did some experiments in gusty conditions, varying the elastic portion of a rode by using combinations of nylon and polyester, and monitoring tension vs. winds speed with a load cell. Only 6 feet of chain. Unless the nylon portion was more than 100 feet and relatively thin, the load was always less with nylon. Over 100 feet, yawing began to increase and the load would go up a little, but this also depends on how much the boat yaws. If it is well controlled, on a bridle, this won't happen. Simply stretching backwards won't increase the load, just give a weird ride.


Remember that the load is varying widely. It is hard to be precise in these conditions.



So I'm listening, waiting for someone to come up with a new, clever idea.


---


Regarding rope rodes, I'm a fan of slight oversizing. This controls the stretch and greatly increases life expectancy. For smaller boats, it makes it more hand-friendly. This is a problem for combination rope-chain gypsies.

[MathiasW]

So, to reformulate my original question in a nerdish way...

The ideal characteristics of a snubber / bridle would be to have an elasticity law of the form F = func(s) (where F is the force applied to its end, and s the elongation) such that the 2nd derivative of func(s) is always positive. And as positive as possible that is.

I am not a material scientist and I have not made the measurements of typical bridle / snubber materials, but my hunch is that it may be the opposite there, so the 2nd derivative is typically negative. This comes from the assumption that after some stretching of the bridle one comes to its limits where it won't stretch any more. F = func(s) would then go through the roof.

So, as a first step, when I cannot have a positive 2nd derivative, perhaps the nearest achievable target is to find materials for snubbers/bridles where the 2nd derivative is the least negative...

I know, it is nerdish, but this is what I like to do...

[MathiasW]

Originally Posted by Pete7

Given the work you did on catenary a while back, if I wanted the answer to that question, I would be asking you

Thanks, but I need the real-world input of experience from all you guys...

[MathiasW]

Originally Posted by Pete7

I guess the first question to ask is, does the fact the yacht is a trimaran have any effect on the design of the snubber?

I think not, what you actually have is a mono hull with two floats to stop it falling over, all be it with probably a very shallow draft, yes? I would also bet she sails all over the place at anchor without any real volume in the water to create resistance.

So your requirements are going to be similar to that of a motorboat with a planning hull shape, little below the water line lots above.

How does that sound?

Pete

Thanks Pete. Our trimaran is actually not sailing at all at anchor, or not that I notice this as an issue, at least. It may have to do, though, with me keeping one leg of the bridle a metre longer than the other, so she is always slightly at an angle to the wind. There have been a number of reports that not facing the wind heads-on but at an angle will stabilise the boat.

[MathiasW]

Originally Posted by thinwater

Yes. Climbing rope, which is optimized for this (different weave). That was my point. But it only works for boats in the 28-43' range (different sizes are used), and the OPs may be a little too big. He might try 11mm rope with a bridle, but the thing is, a bridle does NOT reduce the load by half, or even 1/3 because of single leg loading.


Seriously, I doubt a snubber can stretch enough for this to be a factor. I did some experiments in gusty conditions, varying the elastic portion of a rode by using combinations of nylon and polyester, and monitoring tension vs. winds speed with a load cell. Only 6 feet of chain. Unless the nylon portion was more than 100 feet and relatively thin, the load was always less with nylon. Over 100 feet, yawing began to increase and the load would go up a little, but this also depends on how much the boat yaws. If it is well controlled, on a bridle, this won't happen. Simply stretching backwards won't increase the load, just give a weird ride.


Remember that the load is varying widely. It is hard to be precise in these conditions.



So I'm listening, waiting for someone to come up with a new, clever idea.


---


Regarding rope rodes, I'm a fan of slight oversizing. This controls the stretch and greatly increases life expectancy. For smaller boats, it makes it more hand-friendly. This is a problem for combination rope-chain gypsies.

Yes, absolutely agreed, a snubber only approach is not the preferred way to absorb all the energy. I have a bridle (up to 13-ish metres) and trust that. My additional snubber is only there to make sure it is not my windlass taking the hit should my bridle fail.

And true: "Simply stretching backwards won't increase the load, just give a weird ride." No direct impact on load. But it is bad from an energy loading point of view, as it means all the energy released as wind strength times distance traveled backwards needs to go somewhere. And the only place it can go is the chain and the bridle / snubber. So, once their storage capacities for energy are exhausted, the loads will go up. But it is a race between these two phenomena, and as long as your nylon can cope with it and keeps stretching more and more, there will be no harm done, I agree.

[rslifkin]

Personally, I use the lazy loop of chain as a stretch limiter. And then you have to size your snubber accordingly. Ideally, the snubber will be stretching before you run out of chain catenary.

If you're hitting full stretch on the snubber too easily, it's time to go larger. If the larger snubber isn't providing enough stretch at a given load, make it longer. 5% stretch on a 50 foot line is the same as 10% on a 25 foot line.

So the worse the weather gets, the longer and larger the snubber required to properly absorb it. And as your chain gets lighter or shorter, the snubber needs to get stretchier (as the force required to remove useful catenary is lower, so the snubber needs to start absorbing significant energy at a lower force).

Excessive stretch can cause some issues in gusty winds though, especially if the wind directly is slightly shifty. If the boat stretches back too far in a gust, the rebound between gusts and shifts can lead to some pretty significant sailing. So if that starts to happen, you may want to shorten the snubber a bit, but not so much that you get excessive shock loading (which will make the ride unpleasant and possibly cause holding problems).

[Pete7]

Originally Posted by MathiasW

It may have to do, though, with me keeping one leg of the bridle a metre longer than the other, so she is always slightly at an angle to the wind. There have been a number of reports that not facing the wind heads-on but at an angle will stabilise the boat.

Interesting, the Pardey's had the same approach to using a para anchor in storm conditions. They would make off one line to the bow and another to one of the forward quarters so the yacht sat at an angle of 20 degrees or so rather than sailing back and forth.

Pete

[s/v Jedi]

I have never even tried to do the math as it’ll require a computer model I’m sure. That said, I think wind is a static load, stored as both snubber stretch and chain lift, while gusts and waves are the dynamic loads that are superponated with that... like ripple on a DC voltage.

I went the experiment route and think this may differ for each boat. I tried different lengths and different diameters 3-strand nylon. Both 5/8” and 3/4” nylon works for us but I have never seen improvements when the deployed part of the snubber is more than 20’. A 15’ snubber of 5/8” works about the same as a 20’ snubber of 3/4” for us. In early testing I did break a 1/2” diameter snubber, but during tests I deployed enough chain slack to make it possible to break it. For normal use, my chain does limit snubber stretch.

We recently went through hurricane Isaias with just the primary anchor with this 3/4” snubber deployed. We had some 70kts sustained wind periods but little to no waves and the snubber was good enough that I’m still using it now.

When you get squalls 35+ kts it’s the best time to test snubbers.

[MathiasW]

Originally Posted by MathiasW

So, to reformulate my original question in a nerdish way...

The ideal characteristics of a snubber / bridle would be to have an elasticity law of the form F = func(s) (where F is the force applied to its end, and s the elongation) such that the 2nd derivative of func(s) is always positive. And as positive as possible that is.

I am not a material scientist and I have not made the measurements of typical bridle / snubber materials, but my hunch is that it may be the opposite there, so the 2nd derivative is typically negative. This comes from the assumption that after some stretching of the bridle one comes to its limits where it won't stretch any more. F = func(s) would then go through the roof.

So, as a first step, when I cannot have a positive 2nd derivative, perhaps the nearest achievable target is to find materials for snubbers/bridles where the 2nd derivative is the least negative...

I know, it is nerdish, but this is what I like to do...

Ops, just noticed... Got my signs all wrong. Please change positive for negative and vice versa above... ^42

[thinwater]

Originally Posted by MathiasW

So, to reformulate my original question in a nerdish way...

The ideal characteristics of a snubber / bridle would be to have an elasticity law of the form F = func(s) (where F is the force applied to its end, and s the elongation) such that the 2nd derivative of func(s) is always positive. And as positive as possible that is.

I am not a material scientist and I have not made the measurements of typical bridle / snubber materials, but my hunch is that it may be the opposite there, so the 2nd derivative is typically negative. This comes from the assumption that after some stretching of the bridle one comes to its limits where it won't stretch any more. F = func(s) would then go through the roof.

So, as a first step, when I cannot have a positive 2nd derivative, perhaps the nearest achievable target is to find materials for snubbers/bridles where the 2nd derivative is the least negative...

I know, it is nerdish, but this is what I like to do...

In fact, it is more complicated than that. Rope experiences hysteresis when cycled, and the relationship varies with speed.

[SV__Grace]

I'm not a numbers guy and barely passed my math classes in school.

As a full time cruiser who lives on the hook, I'm a practical guy who cares about what works in the real world and am a nerd when it comes to anchors, ground tackle and anchoring, so you got my attention.

So the question of "how much" snubber length only matters in the real world and can only be answered by the user, given each boat and conditions are different.

For example, my boat, a 20 ton cutter/ketch, reacts very differently at anchor when I remove the headsail, a 110% jib. Without the windage and weight of that big, heavy jib I get much less pitching, yawing, and sailing at anchor when the wind kicks up. So in addition to the differences between boaters, there can be differences within each boat as above.

Snubbers can be individual lines or bridles. Either way, most boaters use one that is way too short. But how long should it be? Somewhere between 1/3 to 1/2 the boat length seems to be the sweet spot, but using a thicker line it would need to be longer to get the needed stretch, and a thinner line can be shorter to get the needed stretch.

And you would need the wind to kick up enough to overcome catenary and stretch the snubber enough to see if it's not enough, just enough, or too much, which requires real world experimentation and patience.

Working the numbers is a great place to start, then adjust based upon your real world conditions and boat.

If you're interested, my primary snubber is a 3/8 bridle, each leg is 22 feet with loops on the bitter ends, a soft shackle (with chafe protection) attached to dyneema line that is typically cleated midships. This arrangement can easily handle a gale up to 50 knots or so.

My waterline snubber for close quarters anchoring in settled conditions is 5/16, typically cleated on the windlass horn, with a 30 foot tail in case the wind kicks up and I need to increase scope.

My storm snubber is a 3/4 bridle, each leg is 30 feet, has 60 feet of dyneema at each bitter end that is run through the aft sheet turning blocks to the cockpit, and allows me to let out more scope from the cockpit in hairy conditions. I only drag it out when a strong gale over 50 knots is forecast.

[MathiasW]

Originally Posted by thinwater

In fact, it is more complicated than that. Rope experiences hysteresis when cycled, and the relationship varies with speed.

Yes, I know, and I reckon all these effects go in the wrong direction in terms of optimal elasticity characteristics...

[Pillroller]

I always thought the snubber was the shock absorber. The first line of resistance is raising the chain off the ground, the snubber is not under major strain until the rode and chain are in a straight line? The snubber needs to be fairly short, so that it does not exceed its breaking strain, before the rode takes up the load from the attachment point of the snubber to the rode made off to the boat. Which is what one of the previous replies was hinting at, that his snubber is smaller diameter than the rope rode. It should never have to take the full load.