How much snubber / bridle is too much?

[MathiasW]

Originally Posted by Westcliffe01

We were anchored in 8ft of water when we started out.

Fortunately, I haven't been exposed to anything close to such a violent storm yet. So I am not in a position to tell from experience. But chain does not do well energy-absorbing-wise in shallow water, and so you were relying entirely on the rope for absorbing the shock loads.

[rslifkin]

Originally Posted by MathiasW

Thinking about this a bit more... One needs to create some means that this device can sink very quickly with little water resistance, but when pulling it up, it has a lot more water resistance. Otherwise, after the gust / swell, it will not return to the low energy state quickly enough. And on the way down there is no need to dissipate energy, anyway...

So something like one of those flopper stopper plates with the flaps or valves in them? I wonder what hanging one of those from the rode part way up would do. Or just hanging one off the bow to damp pitching motion (and the extra water drag up forward might reduce yawing as well).

[Just Another Sa]

Originally Posted by MathiasW

And on the way down there is no need to dissipate energy, anyway...

Dissipating energy is just as useful when surging forward than during the backwards stroke. Why do you think it's not?

If you could somehow dissipate so effectively that boat speed forward is never much above zero, the rode would effectively have a constant tension having the same value as in the static maximum load case without effect of gusts and waves. That would be a major improvement over no dissipation, as well as relative to a case representing a reality with rode arrangements in common use.

[CassidyNZ]

Originally Posted by raahell

You're looking for new ideas so here's one - possibly from left field.

Kellets seem like a good idea from the point of view of keeping the anchor imbedded, but as noted - in terms of energy absorption - lifting a weight of 10-20 kg a meter or two doesn't absorb very much energy. So why not utilize the density of water by designing a kellet weight that includes a frictional element?

Imagine a kellet with a structure that opens up like an upside down umbrella. When a gust forces the sailboat backwards and the chain is raised by the increased tension, the umbrella shape resists the upward thrust, absorbing much more more energy than the weight itself.

I know, tons of issues to work out (!) but that's what the design process is all about. I imagine that the "umbrella" component could be made completely with line and fabric, (patterned after sea anchors and JSDs, lots to learn there). Prototyping, testing and customization for boat displacement, geometry etc, therefore, is very do-able, assuming one knows or has access to someone that knows how to sew and has a good machine

I would suspect that when conditions are quiet and the wind shifts or tide changes and the boat repositions itself on the other side of the anchor, any folding umbrella-type device that is dragged across the sea floor would probably self-destruct in short order.

Whenever I hear of new possible solutions to problems my first thought is “How would I design/construct it?” On this concept, I have no idea.

[MathiasW]

Originally Posted by Just Another Sa

Dissipating energy is just as useful when surging forward than during the backwards stroke. Why do you think it's not?

If you could somehow dissipate so effectively that boat speed forward is never much above zero, the rode would effectively have a constant tension having the same value as in the static maximum load case without effect of gusts and waves. That would be a major improvement over no dissipation, as well as relative to a case representing a reality with rode arrangements in common use.

I was not thinking about forward and backward, but rather up and down. After a gust or swell, we want the chain to dissipate its energy as quickly as possible - which means it should get lower as quickly as possible. Anything impeding that motion I would consider not useful.

[Rothblum]

Originally Posted by MathiasW

Yes, indeed I am!

That looks like a very different approach. Need to do some basis assessment about the energy one could lose per second this way...

I worked on a project very similar to this idea for the US Navy. We were trying to measure very low frequency sound signals with moored hydrophones. To add a lot of mass to the hydrophone suspension system, I used large hollow spheres made of fiberglass. For our purposes, the spheres had to be relatively rigid so they wouldn't have low frequency structural resonances. But for absorbing the energy of a rising and falling chain catenary, they could be fairly flexible. In fact, instead of spheres, for purposes of dissipating energy, they could be discs, or umbrellas, as proposed. The problem would be to design the umbrellas so that the umbrella "handles" stayed mostly parallel to the motion. I know that I have been harping on "added mass", but the added or apparent mass of a disc moving through water is significant. It is about equal to the mass of a sphere of water with the same diameter as the disc. An "umbrella" with a diameter of 1.24 m has an added mass of approximately 1 metric ton. (1000 kg). If this seems unbelievable, consider that, in principle, as the disc moves through the water, it imparts a velocity to the surrounding water that extends to infinity. That is for the frictionless or zero viscosity case. In the real world case, damping is huge, so it is a win-win. I'll bet that one could approximate the effect of a disc with a balloon that could be stored in the anchor locker deflated and filled with water and clipped to the rode if and when necessary. Best place to attach would be where the motion of the rode is greatest, so, about in the middle of the catenary.

On reviewing this post, I just realized that Mathias wrote that impeding the chain from going back to it's deepest position would not be productive. True, the umbrella or disc or balloon would impede the return of the rode to the relaxed catenary. Adding virtual mass to the rode would certainly have the effect of placing a huge mass and large damping between the tugging of the boat on the rode and the anchor. Seems like this would at a minimum reduce the unsteady force on the anchor and the boat, as well as dissipating energy. Maybe the place for such a device would be the midpoint of the flexible bridle. Just add a few discs or balloons to the bridle.

[wingssail]

Originally Posted by MathiasW

I was not thinking about forward and backward, but rather up and down. After a gust or swell, we want the chain to dissipate its energy as quickly as possible - which means it should get lower as quickly as possible. Anything impeding that motion I would consider not useful.

I'm sorry guys, you are spending a lot of mental energy on a problem which does not exist.

You do not need to think about heavy kellets or flopper stoppers or other sollutions needed to absorb the energy of gusting or rough anchorages.

1. If you know a blow is coming, MOVE!!!
2. If you can't move, put on your heaviest anchor.
3. Put out more chain, heavy chain. It is rarely going to go totally straight so you don't need a rube goldberg approach.
4. You are not going to rig the apparatus every time you anchor, and if you know a blow is coming, you have some time to better prepare.

You are better advised to prepare yourselves for sailing to a safe anchorage and avoiding this nonesense.

[raahell]

Re: the idea of a kellet with "water friction-adding" element:


Rothblum has the closest engineering experience (so far) and confirms that the forces will have a positive impact.


Wingssail has a valid point. There's no substitute for a long length of heavy chain and a oversized anchor. My other takeaway from his advice is that any extra device that is going to be added to an anchoring system has to be simple and robust in construction and easy to deploy. Otherwise it will not be widely accepted.


CassidyNZ also has a great point. Using soft materials wouldn't work if the device would possibly end up on the bottom and subject to chafe. Still as a method of protoyping/testing, it's a way to sort out effects of various shapes and sizes. The actual device would have to be pretty bomb-proof.


I'm just thinking of those small dinghy anchors with flukes that dig into the ground when deployed but when stored away the flukes fold up into a compact form. Imagine a variation on that with the flukes designed to present more surface area to upward movement through water, as opposed to digging into the sea bottom. With the correct relative locations of Centre of Gravity and Centre of Area of each fluke in relation to the fluke's pivot point, the flukes would open in upward motion (giving maximum resistance to movement) and close in downward motion (giving less resistance to movement). If such anchors exist and resist damage on the sea bottom, then this device could probably also.

[rslifkin]

In my mind, adding something to damp movement would be intended more to solve an issue with the motion of the specific boat (yawing, violent pitching, etc.) to make it more comfortable to be aboard in bad conditions when there's no better option. Any resulting increase in holding power would be a bonus. But I'd already want a big anchor and chain / rode / snubber combo that can handle the forces and keep the boat where I left it.