Dynamic Behavior (2) - Heterogeneous Rode

1. Introduction

The results of our study of the dynamic behavior of homogeneous rodes have confirmed those of the static study, i.e.:

• An all-textile rode cannot get tangent to the bottom, because it lacks weight in its lower part.
• The high weight of an all-chain rode does not prevent from severe overtensions under gusts, because (1) the chain lacks elasticity, (2) the weight of its upper part is useless.

So, we will revisit the technologies we studied in the static behavior chapter:

• Adding a heavy weight (sometimes named "kellet", "sentinel" or "angel") down a nylon line.
• Using some length of chain in the lower part, with a line up to the bow (mixed rode).

From a practical point of view, the latter divides into 2 categories:

• A fixed but moderate length of chain, permanently spliced to a long nylon line: whatever the water height, one pays out all the chain plus the necessary length of nylon.
• A long chain rode, which is payed out according to the water height, as usual, but which is then linked to the fixed point of the boat via a moderate length of nylon rope.

Note: In the examples below, we keep the same water depth as in the previous page (5 m - 16 ft) and the same long scope (11:1).

2. Long Line + Kellet

In the static study, we found that the effectiveness of the kellet is maximum if it is placed close to the anchor. In this case, the tension, velocity and position variations are the same than without a kellet (the elasticity prevails), but the angulation is significantly reduced: with a 25 kg (55 lb) kellet down 55 m (180 ft) of nylon (18 mm - 3/4 in. size), it peaks at less than 3 degrees (fig. 2.2.1):

Figure 2.2.1 - Line + Kellet rode (step-gust)

To maintain a zero angulation all the way, a 73 kg kellet would be necessary!

3. Short Chain + Long Line

In a sense, such a rode can be considered as a derivation of the Line + Kellet version, in which we replace the kellet with some length of chain. For a given chain weight, a short and big chain is better than a long and thin one, because it puts the weight closer to the anchor. So, let's try 20 m (66 ft) of 10 mm (3/8 in.) chain, spliced to 35 m (115 ft) of 18 mm (3/4 in.) nylon (fig. 2.2.2 - same scales as 2.1.6):

Figure 2.2.2 - Short chain + long line (step-gust)

Except for the shorter oscillation period, the tension and velocity variations are about the same than with the Line + Kellet rode. On the other hand, the position and angulation variations are significantly lower. In brief, for a small increase in the total on board weight (18 kg - 40 lb), a chain section is more efficient than a kellet, and much easier to handle. In addition, the chain section , which lies on the bottom most of the time, is much less prone to chafe than a nylon one in aggressive environments e.g. coral heads.

4. Long Chain + Short Line

The problem with the above rodes is veering them in when the wind is strong – although some windlass manufacturers claim their models are designed to "swallow" rope as well as chain. On the other hand, a long chain with a matched windlass is ideal, but, as we saw in the previous page, it suffers from severe overtensions for lack of elasticity.

So, assuming the parameters are the same as above, let us first veer out 45 m (147 ft) of chain. Then, by any appropriate means, e.g. a chain hook or a standard snap hook, we take up a link of chain with 10 m (33 ft) of nylon. The other end of the nylon rope is tied to the bollard – this takes the load off the windlass, and it cancels unpleasant rattle at the roller level, too! Thus, we still have a 55 m (180 ft) long rode, but which includes a short elastic section (fig. 2.2.3 - same scales as above):

Figure 2.2.3 - Long chain + short line (step-gust)

The oscillations are twice faster, the overtensions and angulation are barely increased, but the velocity and the swinging radius are significantly reduced. The price to pay for a better comfort and safety is a moderate increase of the on board weight (16 kg - 36 lb more than in the Short Chain + Long Line version).

In addition, this solution is well adapted to higher water depths, provided the available chain is permanently spliced to a nylon extension. For example, if you bought 55 m (180 ft) of chain, you can splice an equal length of nylon to it, so you get a king-size rode! When there is no need for veering the whole chain, you use the hook with the additional short line to take the chain up. If the whole chain is necessary, you simply pay out more rode (up to 110 m - 360 ft).

5. Simulate Your Own Rode

To go into the above subjects in greater depth, to try other cases, to compare various situations etc., you can use the spreadsheet dyn_mix.xls (requires Microsoft® Excel 2011 or +).

Hint: To compare the performances of 2 rodes, you can duplicate the spreadsheet file under a different name (e.g. dyn_mix2.xls), so you can open both files and display their windows side by side.

6. Conclusions

Let's check all 3 solutions against the criteria defined at the top of the dynamic behavior chapter (Req. 4, 5 and 6), plus various criteria:

Obviously, the Long Chain + Short Nylon Line is the winner, except for small boats that have on-board weight problems. Actually, there is no boundary between the mixed-rode versions: one can choose any Chain/Nylon mix inside a wide range, say, from 40/60 to 80/20, with no significant performance differences (test it yourself with our spreadsheet!). Practically, this involves having 2 rode elements at one's disposal before attempting to anchor:

• A long mixed-rode, typically with a 50/50 length ratio. The nylon section should be spliced into the last links of the chain, so that it does not jam through the windlass (fig. 2.2.4).

Figure 2.2.4 - Chain – Nylon junction

• An auxiliary 10 to 15 m (30 to 50 ft) nylon rope, ended with a chain hook (fig. 2.2.5). This device is used to take the chain up when the water height and/or the wind conditions do not need to veer all the chain section of the rode.

Figure 2.2.5 - Chain hooks

In every case, there must be at least 10 m of nylon in the active rode - more precisely, between the bollard and the chain section (fig. 2.2.6). If you're rich ;-) or you don't want the hook in the water, you can shorten the auxiliary rope, under the condition it is fitted with a spring or rubber snubber like those used on dock lines (fig. 2.2.7). When choosing such a device, however, check that its elasticity range matches the tension range it will have to cope with!

Figure 2.2.6 - Separate windlass and bollard tyings		Figure 2.2.7 - Line snubbers

Please remember that, even with a heavy rode, strong gusts always require a very high scope to maintain the anchor (almost) flat on the bottom.

Waves also require higher scopes: since they lift the bow up, they increase the effective height to consider when deciding the rode length to veer out. For example, if the waves cause 1 m (3 ft) pitching at bow and the scope for a calm sea is 6:1, adding 6 m (18 ft) more rode (preferably nylon, cf. fig. 2.2.8) would be safe.

Figure 2.2.8 - "I definitely prefer nylon rodes." – "Me too!"

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