Dyneema forestays and backstays

All the posts I have been writing about Dyneema rigging and chainplates have been mostly focused on Shrouds (the standing rigging that holds masts up from the sides). Much of it also applies to Stays (the standing rigging that holds masts up from the bow and stern of the boat). However, there are some differences, for us especially because we have a ketch rig (two masts).

So I’ve been checking out how to apply the work I’ve done for Shrouds to Stays. It is quite different for our Mizzen and Main mast so I’ll write about them separately.

Main Mast

As I have mentioned in other posts (eg Why Dyneema standing rigging?) we are not going to be replacing our forestay with a Dyneema synthetic rope. The roller furling for our genoa would chafe through a Dyneema forestay very quickly as it puts the forsetay inside a metal tube that is rotated to roll up the sail around it.

However, we are planning a removable inner forestay (see Progress on Sails for our first mention of this) and this will be Dyneema. With all that we have learnt we will probably fit a DIY Cheeky Tang (see Dyneema Termination and Chainplate update) for this. Earlier we would probably have used a Bluewave Forged T Eye (as mentioned in Termination of Dyneema Shrouds. The most contentious issue?), however, if this is to be capable of acting as an emergency forestay, holding up the whole mast, then we will want a thicker line than 8mm.

Our current plan for the inner forestay is to have it as far aft from the bow as we can manage. The limits are set by the space required for the dinghy on the deck and where we can reinforce the underside of the deck enough. This will allow us the option of setting a staysail so we have a cutter rig (two smaller jibs instead of a genoa). Depending on how high we fit it to the mast we might need to add running backstays (which our mizzen already has so see below).

To hoist a sail at the inner forestay we will need to add a sheeve to the mast just below where the inner forestay attaches for the halyard (and at the bottom for the halyard to come back out of the mast).

Our backstay is currently slightly complicated and the tension can’t be adjusted (something you often want to do when sailing to control the tension of the forestay which changes the shape of the genoa).

It starts with a single wire at the top of the mast.

Part way down the single backstay is split so that one can go each side of the mizzen mast.

I’m assuming this is to save weight, although it might also help avoid the back of the sail (the leech) from rubbing on the backstay when sailing upwind (the sail won’t be pulled in as far as the centreline where the single backstay section is, but it might be pulled in far enough to rub against one of the double lines if they go all the way to the top of the mast (because the sail “sticks” out from a straight line from the top of the mast to the end of the boom – this is called “roach” and it is supported by sail battens). Our current sail doesn’t have roach but the original design adds 21 square feet of roach (and it is in a very efficient place near the top of the sail).

The problem with the single to double backstay is that instead of having the safety feature of two independent backstays you have multiple single points of failure.

So looking at where the single backstay attaches to the top of the mast.

I’m thinking we can replace that pin with a longer bolt and two DIY Cheeky Tangs so that we have 2 independent backstays right from the top of the mast. With Dyneema lines being so light we would save a lot of weight and add redundancy.

If we find that we need the backstay central at the top to miss the sail then we can add a couple of low friction rings to pull the two lines together at an appropriate point. If one backstay fails then the other will be a bit slack but will still be there.

A similar technique is commonly used to tension the backstays. A line with a low friction ring is used to connect the two backstays. This line is then pulled down to pull the backstays together (and tension them) or slides up to allow the backstays to separate (and follow a more direct line) thus slackening them. This technique automatically compensates for any differences in the tension of the two backstays (the slacker one always moves more inwards to balance the tension).

Mizzen Mast

Our mizzen mast doesn’t have a forestay or a backstay. A forestay would stop the main boom from being able to swing from side to side. A backstay would require a much shorter mizzen boom (and so smaller sail) or something sticking out the back of the boat to fasten it to.

So instead we have 2 shrouds that come forward from the top of the mizzen mast to the sides of the boat. The main boom just misses them. These stop the mast falling backwards.

Then we have two shrouds (side stays) that start just below the spreader and are angled slightly aft. These stop the mast falling forward but as they do not go to the top of the mast they are not enough to hold the mast up if the sail is pushing forward. For this we have running backstays, one each side. These provide the extra support for the mast, but to let the sail out fully they have to be released. So when you tack or gybe you loosen one and tighten the other so that the boom can move across the boat.

The net effect is that our mizzen basically has 4 shrouds per side. One per side is a running backstay and so you need a means to tension and release it as needed. As far as replacing them with Dyneema there is no need for any difference in the shrouds themselves.

Remaining issues

  • We need to finally confirm the current sizes of the stainless steel wire (need to be allowed back to Wales).
  • Then we can finally calculate the size of Dyneema to replace our stainless steel wire.
  • We need to decide where to protect with a Chafe/UV sleeve.

Other than that we are close to getting on with building and fitting it all 🙂

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