Multiplait thickness

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ChrisE

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I've temporarily mislaid my copy of yacht data and am struggling to work out what size of multiplait is needed for a 38' 10 ton yacht. The mp is to be used in conjunction with 10mm chain as the main rode. I've looked at Jimmy Green who recommends 16mm and Marlow braids who say 20mm. The boat will be going to some pretty exposed places and whilst I don't want to stint on my safety margins I also don't want to overspecify (or should that be overspend!)
Thanks in advance.
Chris
 
I've just been through these calculations for my boat and if you haven't already considered it, don't forget 'the weakest link' and all that. Before you settle on the correct MP diameter, divide the quoted safe working load by two (to allow for chafe, knots, splices, etc.) and check the max safe load of all the other components. Anchor, anchor connector or shackle, chain, and last, but by no means least, an estimate of the strength of the strong point you intend to attach it to. Nylon will stretch up to 25% to its safe working load so if you are working on 50% of that value, the total stretch at max intended load will be 12.5% of the total length of nylon you have out. You will get more stretch with a smaller size so you should be more comfortable and have less shock loading on the deck fittings.
 
For a 12 tonne (10 tonne dry) independantly ranging yacht we use 20 mm 3 strand laid (not multiplait) nylon but that as a back up rode. Main cable is 10 mm chain.

It is worth considering having it strong enough to serve as a tow rope should that ever prove to be required in which case the loads in seas are likely to be heavier than heavy weather anchoring. Nylon also has strength difficulties under heavy cyclic load, especially such as when towing, and loses some strength (10% is often quoted) when wet.

Hope that is of some help.
 
[ QUOTE ]
You will get more stretch with a smaller size

[/ QUOTE ]

this would be true if the elasticity of rope was constant, which is not the case, especially for polyamide. Total elongation will depend on the load vs breaking strain you apply

The graph of elongation to load is S shaped: roughly, almost linear from 0 to 20% of breaking strain, where elongation is +-13%, but after that to get to 23% elongation the load should increase to around 80% of BS.

If you want to use the rope as a shock absorber, it is more efficient to have a greater diameter rope (greater BS) which will work in the first part of the curve, where elongation will be greater.
 
[ QUOTE ]
If you want to use the rope as a shock absorber, it is more efficient to have a greater diameter rope (greater BS) which will work in the first part of the curve, where elongation will be greater.

[/ QUOTE ]I don't think that's what you meant to say, surely? It cannot possibly be that you get a greater elongation for a given load with a thicker rope. Strain, i.e extension per unit length, is an inverse function of the diameter of the rope. If you want to use the rope as shock absorber then using a very large diameter rope isn't the best choice.

Going one stage further, if the rope is operated in the linear area (i.e. where Hooke's Law applies) then for a given material, i.e. a given Young's Modulus, then Stress/Strain is constant. Stress = Force per unit area and Strain = extension per unit length. Provided one operates in the linear region it only needs a bit of algebraic substitution to see what will change as the cross sectional area of the rope changes.
 
I agree with you of course if you operate in the linear region of elongation, but as the load increases the elongation curve is much steeper (less elongation).

Let's take a typical approximation: with a load from 0 to A (A=20%BS) elongation is 13%; with a load from 0 to B (B=80%BS) elongation is 23%.

Suppose a cable working from 0 load to Ts (Ts=static load, chain weight for ex, or basic constant forces on the rode), then from Ts to Td (Td= dynamic load, the max load you want it to have, ex load when the boat veering on one side is pulled back by the anchor-cable).

The shock absorbing properties of cable are practically useless between 0 and Td (a steel rope would be fine, just to hold the chain, shock absorbing is mainly done by catenary), but they must be maximized between Ts and Td (ex. when the catenary is almost taut and you need the elongation of nylon to absorb the final shock).

Suppose I want 10m elongation to absorb a shock.
This elongation must be obtained between Ts and Td (the elongation existing between 0 and Ts is lost, it does not contribute to shock absorbing).

If one keeps Td (max load) = to 20% of rope BS, the cable will work inside the part of the curve where it has greater elasticity.

Suppose Td=3 times Ts, just to make it simple.

If the cable BS is dimensioned to 5x max load, then Td=20% of BS; Ts=6.6% of BS.
Elongation at Ts will be 3.5% (useless elongation), elongation at Td will be 13%. Useful elongation is 10%.
To get my 10m elongation, I will need a 100m cable: from 0 to Ts it will stretch to 103m (3m of useless elongation), from Ts to Td it will stretch to 113m, giving the needed 10m.

With a smaller cable, ex. dimensioned to 3x max load: Td=11%BS; Ts=33%BS. Elongation will be roughly 7% at Ts; 15% at Td. Useful elongation is 8%.
To get my 10m elongation, I will need a 125m cable. From 0 to Ts it will have stretched 8.75m to 133.75, at Td it will have stretched 18.75m to 143.75m (between these two you find the needed 10m).

This longer, thinner cable would have of course stretched more than the thicker one, but a big part of its stretch will have been useless (the first 8.75m to get to Ts), whereas the shorter, thicker cable will work with best efficiency.
 
[ QUOTE ]
I agree with you of course if you operate in the linear region of elongation, but as the load increases the elongation curve is much steeper (less elongation).

Let's take a typical approximation: with a load from 0 to A (A=20%BS) elongation is 13%; with a load from 0 to B (B=80%BS) elongation is 23%.......

[/ QUOTE ]May I firstly refer to a publication by Marlow Ropes which may be found at http://www.marlowropes.com/offshore/physical.htm
.......................Quote................................

Extension and Elasticity
Rope extension and elasticity are important rope characteristics because they will determine rope behaviour in terms of peak loads and mooring excursions. Synthetic fibre ropes differ from steel because their load-extension characteristics are non-linear and time dependent.

The overall extension of a rope is made up of several different components:

Elastic Extension
Elastic extension is the extension that is immediately recoverable upon the release of the load. In a continuously working environment elastic extension will dominate the rope behaviour.

Visco-elastic Extension
Visco-elastic extension is only recoverable with time after the release of the load. The behaviour of ropes subject to occasional high loads will be significantly influenced by this visco-elastic component.

Permanent Extension
Permanent extension is non-recoverable. It will occur when a new rope is first used or when a rope is subject to an unusually high load. It occurs as a result of the individual fibre components of the rope 'bedding in' to their preferred positions. Continuous loading of some ropes can also lead to further permanent extension due to creep at the molecular level.

........................End of Quote..................................

Clearly Marlow Ropes expect their ropes to be normally used in the elastic (i.e. linear, or Hooke's) strain region. The non-elastic extension is indeed an issue in extremis but is not the issue under normal circumstances. What we are looking for is a rope that will give a comfortable ride while providing adequate safety margin under extreme conditions. While I follow your argument regarding the rode under extreme loads, and do not take issue with you there, provided that the ultimate strength is great enough the ride will be softer with a smaller diameter rope.

What you have highlighted is the need to consider the extra non-linear extension when attempting to calculate peak loading on the deck fittings in severe conditions; clearly the nylon rode can go a long way towards reducing these loads to more reasonable figures.

Like many others, I have always used a chain hook with a nylon snubber using fairly light nylon on the basis that in the event of failure of the nylon the chain would be the backup. This is arranged so the the slack chain is slightly less than 25% of the length of the nylon so the chain is starting to share the load before the nylon has failed. I have not yet had to anchor in conditions that would test this but many others here have, and report good results.
 
Thanks everybody for your help and the extremely informative answers.

BTW I've answered my own question regarding BS of 3-strand vs mp. According to Marlow their 16mm 3 strand breaks at 5520lbs and the mp at 6640lbs. The two results concur with one source (American Yachting and Boating Council) I've read which states that the loads on a 40 ft boat in 42knot winds are upto 2800lbs and in 60 knot winds upto 4800lbs.

So it looks as though it will be 16mm multiplait, as I've absolutely no intention of anchoring anywhere where the winds exceed those speeds!
 
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