Sustainable Sailing

When planning Dyneema rigging the area we have found most confusing is deciding on the size of Dyneema we should fit. In our search we have found three sites particularly helpful. However, between the sites we have found at least four ways of deciding what size is needed. Despite that, they do all agree that Dyneema needed to be sized for Stretch rather than for Strength. That is because a Dyneema line the same strength as the Stainless Steel it replaces would be too stretchy to work.

Stretchy is slightly problematic because there are multiple forms and the terminology used isn’t consistent. From Marlow Ropes we have this:

• Initial loading will result in elastic extension. This is immediate upon loading and is immediately recoverable upon release of the load (elastic contraction)
• After the elastic extension of the initial loading, the rope will experience what is known as viscoelastic extension. This is further extension over time and is fairly limited. Unlike elastic stretch, viscoelastic stretch will only recover slowly over time once the load is released.
• Finally there is creep, which is permanent, non-recoverable and time dependent. Creep occurs at the yarn molecular level when the rope is under constant load.
• Once the load is released and elastic and viscoelastic extension recovered, the rope will ultimately have experienced an element of permanent extension. This is a factor of both creep and “bedding in”, which is when individual fibre components in the rope and / or splice settle into their preferred position when under load.

Others refer to Elastic Stretch, Constructional Stretch and Creep. Unfortunately lots of the information isn’t clear about which they are referring to in their guidance.

I’m least concerned about Constructional Stretch or bedding in. Most lines are pre-stretched. If you measure a pre-stretched line before splicing then you can stretch it after and by measuring know if you have removed the constructional stretch. If your design includes lashings (which are normally setup to have plenty of adjustment) then there is only the inconvenience of a tightening a few times initially if you didn’t get rid of all the constructional stretch.

Creep will mean your rig needs re-tensioning over time. This is mostly a problem if you only use turnbuckles due to their limited range. If you have a lashing in the design you can have shorter shrouds and a longer lashing so that you have plenty of space to keep tension as creep lengthens the shroud. It can be minimised by keeping static loads as a small % of the breaking strength, so grades of Dyneema with a higher breaking strengths will creep less under the same load.

Elastic Stretch is much the same to work with as creep except that it will show up quite quickly, so a few re-tensions in the first few months should sort it. Again, increasing the line diameter reduces the problem as does being able to get enough tension to stretch out the elasticity so that the rig doesn’t flop around.

What makes this even more complicated is that a) there are lots of variations of Dyneema available, also b) each rope manufacturer has their own ways of treating Dyneema (eg pre-stretch, heat treatments, and coatings) which makes comparisons even more difficult.

In terms of suitability for us, we have got that down to this list of basic Dyneema variations (we haven’t found a comparison between the different ways of treating the same type of Dyneema):

• DM20 (least creep, but also not as strong, most expensive)
• SK99 (Strongest, similar creep to SK78)
• SK78 (the first Dyneema with reduced creep)

Rigging Doctor describes all these (and others that we are not considering), not much has changed since that was written in 2015 apart from the gradual introduction of Bio-based Dyneema (expected to reach 60% of all Dyneema by 2030) and a reduction in the premium pricing for DM20 and SK99. Also Marlow describes them all and includes comparison charts. I found the Colligo information less helpful, it feels to me that they have stayed with the same materials despite the new developments. As Jimmy Green put it in an email to me “In terms of picking between DM20 and DynIce Dux, the choice comes down to whether you want the better performing fibre (DM20) or the better performing rope once braided and heat stretched (heat stretched SK75). Marlow recommend one thing, Colligo another, they both swear by the logic!”
[Update]
See the first comment below from John Franta, Colligo Marine where he explains the difference between heat stretching at Fibre level (SK78 and SK99) vs at the Braided level (SK75). So I am going to be adding Hampidjan DynIce Dux into my calculations and it is cheaper than the LIROS D-Pro-XTR, plus available in more sizes.
[End Update]

As we go through the sizing calculations comparisons are difficult as they don’t use the same version of Dyneema. So the sites we have used are below and for each I have sized replacements for our Mizzen mast (currently 6mm or 6.5mm Stainless Steel – can’t be sure until we can visit) and our Main mast (currently 8mm we think).

So these are the sites I’ve found most useful in working sizes for our boat.

Colligo Marine

Still the biggest name that we have found producing fittings for Dyneema rigging. Their page (from 2015) Before Ordering Your Colligo Dux Rigging… links to a PDF table for sizing.

From them we get either 7 or 9mm for the Mizzen and 11mm for the main.

Jimmy Green Marine

Our preferred rope supplier, Jimmy Green Marine, has lots of information and a range of Dyneema for standing rigging from different suppliers. They sell 100m drums and 50m hanks which is handy (they can also make custom lengths with a variety of splices etc). They have been very helpful in responding to email enquiries. They make the information from manufacturers such Marlow rather more accessible.

If we follow the table they include from Marlow for their DM20 line (Marlow M-Rig Max) then sizing is huge: 11 or 12mm for the Mizzen and 15mm for the main.

Rigging Doctor

We are Patreons of Rigging Doctor, the combination of their YouTube channel and website has more practical resources on real world cruising use of Dyneema than any other I’ve found. Our preferences are going to be to tune the rig for a bit higher performance and sail a bit harder than they do but it is still be best source of information we have found. Their sizing post is Sizing for Creep. That has two ways of calculating the size.

The first is the RM30 heeling test to calculate rig loads. “RM30 is the force that is required to heel the boat over 30 degrees.” We are ruling this one out for us. a) we need to replace the mizzen rigging before we launch b) there is no dock or anything at the boatyard so difficult to do c) I’m not sure how this would work for a mizzen mast as it is shorter and so far aft, therefore it would be very difficult to heel the boat that far with just the mizzen and not very typical of the mizzen usage.

The second is a calculation based on the current rigging size. We start by calculating the designed tension of the shrouds by assuming it is no more than 20% of the breaking strain of the stainless steel. Then we choose what percentage of the breaking strength of the Dyneema we want this to be. Herb suggests under 15% or even better under 10%. I’ve taken the stainless breaking limits from the Marlow table off Jimmy Green (see above).

For 6mm stainless steel the breaking strength is 2880kg. 20% is 576kg so if we size at the 10% we get 5760kg (we can simplify the calculation to looking for a Dyneema line that is at least twice the breaking limit of the stainless it replaces). Looking at the Jimmy Green table for all the Dyneema they sell we find that 7mm is good (except Liros don’t sell 7mm so it has to be 8mm for the Liros D Pro Xtr [SK99] or 10mm for the Liros D Pro Static [DFM20] ).

For 6.5mm stainless steel the breaking strength is 3220kg. So we are looking at approx 6500kg breaking strain Dyneema. The Dyneema sizes can be the same as for the 6mm Stainless above except that the Marlow M-Rig Max (DM20) might be better in 8mm.

For 8mm stainless steel the breaking strength is 4640kg. So we are looking at approx 9300kg breaking strain Dyneema. The Dyneema sizes can all be 10mm except the Liros D Pro Static [DFM20] which would need to go upto 12mm (no 11mm available).

Our choices

The price difference of the DM20 lines over SK99 or SK78 is still huge. Jimmy Green have 100m drums of 8mm in all 3 types of Dyneema from Marlow:

Marlow Excel D12 Max 78 (SK78) is £1,145
Marlow Excel D12 Max 99 (SK99) is £1,400
Marlow M-Rig Max (DM20) is £1,337

The Liros 8mm ropes are:

LIROS D-Pro-XTR (SK99) is £868
LIROS D-Pro Static (DM20) is £1,140

The Hampidjan (recommended by Colligo) 8mm rope is

DynIce Dux Dyneema SK75 is £800

While I would love to buy Marlow as a British company, they are a lot more expensive.

We were thinking LIROS D-Pro-XTR (SK99) as by far the cheapest option (and as SK99 is stronger than SK78 we should have less creep than the cheapest Marlow option which is SK78). Of course what we have not been able to compare fully is the performance of Marlow vs Liros in heat treatment, pre-stretch and coatings. If I were only using turnbuckles for tensioning then I might have gone for DM20 to avoid running out of tensioning due to creep.

However, DynICE Dux is now back in the running, and with the possibility of 9mm for the Mizzen for about the same price as the 8mm Liros D-Pro-XTR.

But what about the size?

One seemingly easy option is to over-size. As you size up creep and stretch will always be reduced. Plus there will be more spare strength if there is UV or Chafe damage. But the disadvantages are cost (not just the line but also the thimbles) and windage (but we have a big boxy wheelhouse so are not exactly aerodynamic).

Let’s be very conservative and assume we are looking at existing stainless 6.5mm for the mizzen and 8mm for the main (will check as soon as we are allowed to visit the boat). Let’s go up whenever there doubt. So the 3 different calculations give us (for the Liros D-Pro-XTR)

Mizzen 6.5mm Stainless:

Colligo (SK75): 7mm
Marlow from Jimmy Green (DM20): 12mm
Rigging Doctor for SK99: 8mm

Main 8mm Stainless:

Colligo (SK75): 11mm
Marlow from Jimmy Green (DM20): 15mm (but 13mm is pretty close)
Rigging Doctor for SK99: 10mm

The choices get more tricky as Liros don’t make every size (no 7, 9 or 11mm).

For the moment I’m thinking of 8mm for the Mizzen (might be a bit stretchy but at the end of the day it is only the mizzen and normally loads are low because it doesn’t have a genoa). I might have gone for 9mm if Liros offered that.

For the Main I’m thinking 12mm (larger than either the Colligo and Rigging Doctor calculations) and the largest size of Liros D-Pro-XTR available).

[Update] or 9mm DynICE Dux for the Mizzen and 12mm for the Main

100m of the 8mm should be plenty for the Mizzen with enough spare to replace several shrouds.
Possibly from our back of the envelope calculations 150m of the 12mm for the Main should also give enough for several replacements. We won’t be re-rigging the main until after the 2021 season so have plenty of time to measure properly.

In the design of the Dyneema chaimplates I mentioned sizing them up, but of course the line is doubled so I’ll use the same size for the chainplates as for the shroud/stay attached to them.

One area still to be worked out is how much length to allow for creep. I need to ensure that the lashing length is enough for me to still tension the shroud at the end of it’s life.

This post has taken an age to research and write. It is based on our specific boat and shares our thinking for our uses. We are not experts but just trying to show our thinking processes. Don’t trust us for the sizing of your own rig!