Math puzzle for scope

[RaymondR]

Be mindful that in anchoring one is dealing with two sets of forces: the first is generally those generated by wind and current which are comparitively steady in both magnitude and direction; the second are those generated by wave action upon the vessel which tend to be cyclic, rotationaland planar with both vertical and horizontal components.

Consequent to this combination of forces two functional components are required to retain the vessels position. the horizontal restraint which can be provided by either cable of chain and other than the comparitive durabilities neither has the advantage. The cyclic wave induced motions require restraint more akin to that provided by a spring and in this instance chain tends to be superior because of the catenary it forms providing a springing action.

The advantage provided by the springing action of a chain catenary could also be provided by a long rode of nylon which in shallow water, where the vertical component of the catenary is restricted, would probably provide a superior springing action.

A combination of both with a long nylon snubber on the chain might work best, chain close to the bottom where the nasties are, energy absorbing rode up the top to provide a good long spring.

[noelex 77]

Originally Posted by Lodesman

That article seemed to be a little biased. I grabbed the equations from the other article for determining the critical force [Fc=w*(L²-H²)/2H] and ran the numbers from Smith's first scenario (30m 10mm chain, 5m depth, 15 kg kellet) using the effective weight of steel in water. Chain alone required a force of 159.25 daN (kilos) to lift the entire rode; adding a 15kg kellet bumped that required force up to 197.1 daN. Getting 38 kilos more at the bowring (24% increase in "holding power") for adding 15 kilos to the rode suggests more value than just shock absorption.

Assuming the maths is correct, your figures confirm the effect of the kellet is only slight.

If we use the ABYC force guidelines for a 10m boat the change in the "critical force" 159 daN to 197 daN is the equivalent of 19.8 knots without the kellet to 22 knots with the kellet.

Originally Posted by Lodesman

Getting 38 kilos more at the bowring (24% increase in "holding power") for adding 15 kilos to the rode suggests more value than just shock absorption.

Fortunately these figures are not the "holding power". If they were we would all be dragging at low windspeeds.

Anchors will continue to hold when the chain is off the bottom.

The ABYC guidelines overestimate the force somewhat, so in practice the windspeed where all the chain will lift off the bottom will be higher than these numbers suggest. However, the maths shows what is seen in practice ie that the chain will lift off the bottom, even with a kellet, at modest wind speeds. These sort of wind speeds should not trouble a set anchor.

Adding a kellet only slightly increases the windspeed where all the chain will start to lift, in this example a 15kg kellet only increased the windspeed where this occurred by just over 2 knots.

[Dockhead]

Originally Posted by Don C L

Thanks to you both. Noelex, that second article is the one that had what I was searching for. Looks like the only major benefit is shock absorption.

It all depends on the weight of the kellet. If it weighs a few tons, it will be pretty effective.

[noelex 77]

BTW, the reference Rode - Static Behavior makes the common mistake of assuming the shank angle matches the chain angle, or is at least significantly lifted up by the rode.

This impression contributes to the apprehension that anchor cannot possibly work when the rode is lifted from the seabed.

The reality is different. Good anchors have a remarkable ability to force their shank down and maintain the fluke at a reasonable angle to the substrate. The shank will commonly remain on the seabed despite the chain leading at quite an angle, at least until the scope is very short and/or the force is very high.

As the rode is lifted at a higher angle off the bottom the holding power is reduced, but don't imagine the shank adopting the same angle as the chain.

Here is a Rocna showing the chain leading at a significant angle (the scope was 4:1), but notice how the shank is not lifted off the seabed and the anchor is still able to maintain a reasonable attitude. The anchor will continue to bury (but not as deeply as it would with a greater scope) despite the rode angle.

Does anyone think an extra 15kg would make much practical difference to the chain angle when it is taut like this?

[Lodesman]

Originally Posted by noelex 77

Fortunately these figures are not the "holding power". If they were we would all be dragging at low windspeeds.

Anchors will continue to hold when the chain is off the bottom.

The ABYC guidelines overestimate the force somewhat, so in practice the windspeed where all the chain will lift off the bottom will be higher than these numbers suggest. However, the maths shows what is seen in practice ie that the chain will lift off the bottom, even with a kellet, at modest wind speeds. These sort of wind speeds should not trouble a set anchor.

Adding a kellet only slightly increases the windspeed where all the chain will start to lift, in this example a 15kg kellet only increased the windspeed where this occurred by just over 2 knots.

My reason for putting quotation marks around "holding power" was to differentiate the contribution of the rode from that of the anchor. Any anchor will produce maximum "hold" with the shank parallel to the seabed; as the shank is raised from the seabed, holding power of the anchor is decreased - in an unpredictable way. Obviously any anchor rode calculations should be based on the ideal of keeping the anchor shank on the seabed.

I'm not advocating the use of the kellet, but you say 'only 2 kt increase in windspeed'; I say '10% increase.' Significant? Probably not, but it is measurable. And truth be told, the calculations didn't allow for concentrating the weight of the kellet in one place along the catenary - I could only add its weight to the total weight of the rode - I'm not sure if there's a way to calculate a more realistic model of the kellet/rode relationship.

IMO, if you expect windspeed around 20 kts, then a 6:1 scope (in this case 30m of chain) would be insufficient with or without a kellet. In the example stated, putting out another 30m of chain would have brought the Fc up to 650 daN, good for 40 kts of wind.

Haven't done the calcs, but I reckon a kellet would certainly be of value in an all-rope rode. Again I believe the catenary equations in the linked article would not be truly representative of an isolated weight in the centre of a long virtually weightless rode.

[Alan Mighty]

I happen to use a kellet. Led Myne's kellet is a 14 kg (30 lb) Anchor Buddy. I don't use it all the time, only when I judge it appropriate. Led Myne's primary anchor is a 15 kg Rocna on an all-chain rode with a nylon snubber that follows Prof. John H. Knox's recommendation of being the same length as the boat and the same diameter in mm as the boat LWL in m.

For the anti-kellet fans: you've forgotten to add the ultimate scare. That's the point made by Earl Hinz that a kellet just might possibly in the very worst case be located on a rode such that it could reinforce resonance in the rode, leading to vibration that disturbs the substrate.

For those with open minds: I reckon a kellet can do two or three jobs. And the location on the rode differs, depending on which job you want the kellet to do.

For shock absorption, supplementing the work of the snubber, the kellet needs to be placed about halfway along the rode. That's when I expect the rode is subject to oscillations of tension that make the rode taut.

For constraining the swing radius, such as in a tight anchorage or a crowded anchorage, I try to locate the kellet so it is about 0.6 m (i.e. 2 of King Winklehof's feet) above the bottom. That happens to be the location that supposedly delivers a maximum supplement to the anchor's holding power. But I don't count on the kellet for holding power. That's why I changed from a CQR to a Rocna. Any (even 1%) increase in holding power is welcome in certain situations.

I don't employ the kellet at every anchorage. I've never seen my kellet reduce my holding power.

Al

[Don C L]

Yes, and after considering it all, just when things get bad you'll have the extra hassle of hauling up a heavy kellet when you have to bail out of a bad anchorage. Ok so now it seems that limiting the radius in a crowded anchorage and maybe some shock absorption remain as the only advantages?

[toddedger]

I like the idea of using my Rocna as a kellet about a boat length in front of my Fortress anchor.

[noelex 77]

A kellet (aka sentinel, angel or an anchor buddy) does have uses.

With rope or mixed rode:

1. It can be particuarly beneficial in introducing some drag on the bottom in lightish wind, which makes the boat swing much more like a boat on an all chain rode. This is nice in crowded anchorages with other boats close that are on an all chain rode.

2. In light wind it will also lower the angle of the rode so boats passing close to the bow are less likely to catch their keel on your rode.

3. Reducing keel wrap.
In very light variable wind it reduces the chance of the rode wrapping around the keel

With any kind of rode (chain, rope or mixed):

1. More reliable setting
It keeps the angle of pull of the chain lower in the initial stages of setting the anchor, when there is not much force on the anchor. Anchors will set better and more reliably. This can be especially useful when anchoring in hard substrates with an anchor that does not penetrate well.

2. Increased holding power at low windspeeds.
This only works at low, or low to medium windspeeds.
If you have a very difficult substrate. For example a thin layer of sand over smooth rock. The anchors holding may well be limited to quite modest windspeeds and a kellet will help.

3. Increased holding power at very short scopes.
At very short scopes the holding power will be low so this is much the same as 2. above.

4. Reduced sheering.
The kellet if deployed conventionally needs to be reasonably close to the bow so it will be off the ground in anything other than light wind, but the drag created in the water helps. A more effective alternative is to deploy the kellet on its own rode so it drags along the seabed, close to the bow as the boat sheers. The drag on the seabed helps.

5. Reduced swinging.
In light to moderate wind the boat will move around less, but if you have all chain rode it will make your boat move differently to your neighbours. This tends to make a collision more, not less, likely unless used with a some care. It is not normally reliable enough to prevent hitting obstructions within the swing circle.

6. Shock absorption.
This disappears in moderate wind, but it can be useful when Med-mooring with the stern close to obstructions especially with the effects of passing wakes.

7. It is something to do if bad weather is forecast.
People become apprehensive with a bad forecast and pulling out a heavy weight that has been buried in the locker is a good placebo.


This sounds like a comprehensive list of advantages, but with all chain rode there are few occasions where the kellet could not be substituted with even a very fractional increase in anchor weight. As well as simplicity this has the important advantage it will increase holding power in strong wind and is always deployed.

For some of the above examples a loop of chain either after the snubber, or joining the chain in one, or more loops forms an effective kellet. A spare anchor anchor can also be used as a kellet, and while more difficult to handle reduces the need to carry a specific weight.

[noelex 77]

Originally Posted by Alan Mighty

And the location on the rode differs, depending on which job you want the kellet to do.

+1.

If you are going to use a Kellet, it is important to know what you are trying to achieve and position the kellet accordingly.

One location rarely mentioned, even by the commercial vendors of kellets, is with the kellet on its own rode, just off the blow. This can be useful to reduce sheering. I guess it could be considered a second "anchor" rather than a Kellet, but is one of the more useful applications.

Another advantage is unlike other uses of the kellet it does not complicate retrieval of the main anchor if you start to drag.

[Bulawayo]

I find myself disagreeing with this posting. We were anchored in nice clear 5m deep water. We were laying to a 5:1 chain scope attached to a 20mm nylon bridle that is 1.5x our beam, and the breeze was around 8 knots. We decided, just for fun, to attach our scuba weight belts to the anchor chain. We ended up hooking on just over 30kg's to the chain and it appeared to make virtually immeasurable difference to the catenary. It did sag initially, as the weights were dropped, but slowly seemed to realign itself just about back to where is was. There was a visible, but very minor 'kink' in the catenary but it was so slight. On warp, I would imagine it would make a difference under light weather conditions but under those conditions there is not much to be concerned about anyway. Try suggesting anchoring with line, or even a short (<10m) chain and warp to cruisers in the Indian Ocean or Red Sea and wait a nano-second for their reaction. In the Caribbean we saw people with angels on their warps, but we also saw other boats with bouyed anchors have yet another boat try to use it as a mooring.
Why are we trying to disprove something that has been established over centuries, at least for extended cruising? All the maths in the world goes for a crock when the weather wick is turned up and is totally meaningless in the face of variable ground conditions. It is similar to some people putting down a lunch anchor. It is fine (perhaps) for known environments but for general cruising who is going to swapping their anchoring arrangments on a regular basis?