Going up a couple of mm on forestay thickness.

[B27]

A heavier forestay will be tuned to a much lower musical note if it's the same tension.
That is as much help as most of the posts on this thread.
Andsarkit hints at some of the possible issues. But I don't understand the way he is presenting it.
A heavier wire has stretched less for the same tension.
So when the wind loads the rig, that wire goes slack and out of control, when the lighter wire would still have tension in it.
Whether that matters depends on the design of the rig.
I think it's more of an issue with shrouds than forestays, which often go slack downwind anyway?

I'd suggest reading Ivar Dedekam (sp???)'s book and others before making significant changes.
6 to 8mm is a 78% change, not subtle!

Maybe use the right size wire and sleeve it with ptfe tube?

 

[Buck Turgidson]

The purpose of the pre-tension in the rigging is to prevent it going slack on the lee side and to hold the mast in position. A thicker wire can achieve this with less tension as the stretch will be less for a given force. A larger wire can be tensioned to less than15% of breaking strain to achieve the same stretch under load as 15% of the smaller wire.
I have a deck stepped mast and similar boats have suffered mast step failure. I have used Dyform rigging so that I can achieve similar performance with less tension and less chance if a depressed mast step.

Yes but you have the same rigging on both sides. Fore/aft is different as the load is shared between the lowers and the forestay. If you increase the diameter of the forestay and tension the backstay to its normal amount then that load will result in the forestay being at a lower % of its max load and therefore stretching less. now when you want to tighten the forestay or bend the mast (masthead/fractional rig) then you tension the backstay. That load is transferred to the lower fwd shrouds and the forestay but the forestay is now stronger so stretches less, now you need to add even more tension to the backstay keep doing that until it snaps and you will see why if you change the diameter for your forestay you need to consider the rest of the rigging too!

You wouldn't increase the diameter of your starboard shrouds without doing the same on the port!

 

[Metalicmike]

Main thing is not to over stress your rig, if you are not racing it is better to keep tension to just enough to stop the leeward shrouds flapping when heeled. I don't like strain gauges because if you get it wrong it gets really expensive. We are addicted to gadgets, I follow Parley Revival on YouTube and they have just installed Wi-Fi stress gauges on their shrouds. I can understand their concerns as they had issues with failed bulkheads on their Cat, now they can set alarms if anything gets over stressed

 

[srm]

I only advocate going up one size on forestays. On my last boat this was from 7 to 8 mm. Going from 6mm to 8mm sounds a bit extreme.
My change came up in conversation with a pro rigger when I asked if there was a market for the old reefing gear. He agreed with my increasing the forestay size. My boats are masthead rigged with twin backstays used for cruising. I want a safe reliable rig. Bendy rigs and racing may well be a different matter.
Talk of shroud tensions is irrelevant to the OP.
Once the internet came along I found an excellent guide to setting up rigs on the Kemp Masts website. Selden now has that info and more. One point I remember from the Kemp article is that you are unlikely to over tension a rig if using hand tools with no more that 12 inch levers.
Problems with mast supports sounds like a serious design/build failure unless someone is playing with long levers or hydraulic tensioners.

 

[Refueler]

Funny because all the literature I have read uses 15-20% of breaking load as a guide.

So you extrapolate to think that applies regardless ???

Wrong.

It applies when you keep to the designed wire gauge and material for the boat.

You start tensioning that 8mm on a boat designed for 6mm to same % of BL - you can start to think about deck compression etc as the resulting mast downforce is now significantly greater.

The matter here is that OP is suggesting he will significantly increase his forestay wire gauge ... IMHO - not a good idea ....

I might consider 7mm if I was going offshore etc. - but better I feel is as another says - buy another same forestay as now as spare if really worried.

Second - I have only seen stays part due to external force / incident and then I can only remember 3 I observed ... in over 60yrs of on boats.

 

[Refueler]

Yes but you have the same rigging on both sides. Fore/aft is different as the load is shared between the lowers and the forestay. If you increase the diameter of the forestay and tension the backstay to its normal amount then that load will result in the forestay being at a lower % of its max load and therefore stretching less. now when you want to tighten the forestay or bend the mast (masthead/fractional rig) then you tension the backstay. That load is transferred to the lower fwd shrouds and the forestay but the forestay is now stronger so stretches less, now you need to add even more tension to the backstay keep doing that until it snaps and you will see why if you change the diameter for your forestay you need to consider the rest of the rigging too!

You wouldn't increase the diameter of your starboard shrouds without doing the same on the port!

NOW you are talking sense ... NOW you realise the increased gauge of wire ...

If deckstepped - first thing to observe is deck compression as the coachroof cries out in pain!

 

[Metalicmike]

Correct me if I am wrong, by adjusting the backstay you are reducing or increasing the bow in the forestay thereby adjusting the head or genoa sail shape. The load on the forestay will be the same no matter the forestay thickness

 

[zoidberg]

Correct me if I am wrong

Someone, somewhere, please show this chap a Diagram of Forces....

....and the costly consequences of over-compression of a thin-walled tube.

 

[Metalicmike]

Someone, somewhere, please show this chap a Diagram of Forces....

....and the costly consequences of over-compression of a thin-walled tube.

I totally agree that with a thicker forstay then you could easily over compress the mast but that is not what i was saying.

 

[andsarkit]

I have thought about it again and I was wrong about shroud tension. The numbers are just to show the principal and not real life.

It doesn't matter what size of wire , for a given tension the lee shroud will go slack at the same loading. However the tension will be a smaller percentage of the breaking strain of he larger wire and the smaller wire will allow more movement of the masthead.
A slack forestay is a result of stretch of the forestay and backstay. If the forestay is larger it will stretch less for the same load and will sag less but unless you change the backstay as well it will probably have little beneficial effect.

I know of one case of a Co26 with a collapsed mast step probably due to water ingress from holes drilled for mast wires and rotting the crossbeam in a 50 year old boat. High Potential, that did rather well in the RTI has a stainless mast support tube which blocks access to the forepeak but allows them to increase the rigging tensions when racing.

 

[B27]

...

It doesn't matter what size of wire , for a given tension the lee shroud will go slack at the same loading. ....

Only if the mast doesn't bend, or the boat doesn't change shape at all.
Suppose in your diagram, the mast curves to shorten the chord by 10mm.
Your light wire then keeps the lee shroud tight for more wind load.

 

[Metalicmike]

I think we can all agree that the rig should be light enough to maintain stability, strong enough to support the mast under full load and set up to get the best out of our sail plan. Few of us have the perfect setup and many of us have our own Ideas. The great thing about this forum is that we can learn from a vast pool of experience and also accept and explore new ideas that challenge or preconceptions.

 

[neil_s]

I had my new forestay sleeved with a piece of pneumatic air line plastic tube. I bought the tube and asked the rigger to slide it on before he crimped up the end terminals. Works fine in the Rotostay!

 

[fredrussell]

OP here. Thanks for all replies- looks like I’ve opened a can of worms here! I’ll summarise all the advice to “7mm ok, 8mm too much” and go with that for now.

 

[Refueler]

TBH - I would still be staying with original 6mm and find way to solve the 'tubing.

7mm if really needed - but certainly not 8mm.

 

[fredrussell]

Indeed. Some Parker 31’s came with 7mm stay as standard, so I’m sure that’ll be fine. There’s a good chance my Rotostay furler will be going strong in ten years TBH, it’s a wonderfully simple design.

 

[Refueler]

Small world ! I met a guy other guy day - CA member - in Ventspils who is cruising Baltic and making way back to UK. He has a one-off based on the Parker 31 .... as I understood it was built for the designer own use ...
I'm still in contact with him ... let me ask him ...

This is what I asked :

... your boat is based on Parker 31 ? Can you tell me what furling gear you have and what size stays .. 6 or 7mm ?? Guy asking about increasing his Parker 31 forestay from 6mm to 8mm and I advised him not to go more than 7mm. Seems he's worried about the Rotostay furler because it has a 'sleeved' forestay ?

OK _ this is his reply ....

Yes, based on the 31. Will have to check. I would say 6. I use a furlex. Recomend your friend joins the Parker and Seal Association. Only £12 per year. But this would put him in direct contact with the other 31 owners. He needs to speak to them.

His boat is 32ft based on the 31 and carries more sail / mast ... this says to me 6mm - 7mm is more than good enough.

Dunno if it helps ...