Rigging Screw Sizes

[smithy]

My lower shrouds have 10mm rigging screws on them and the caps have 12mm. Is this unusual they look a bit light considering the wire is 7mm. Is the loading lighter on the lowers?
Have been looking at the breaking strains of rigging screws and they seem to vary wildly that's even if the are willing to give them.

 

[john_morris_uk]

My lower shrouds have 10mm rigging screws on them and the caps have 12mm. Is this unusual they look a bit light considering the wire is 7mm. Is the loading lighter on the lowers?
Have been looking at the breaking strains of rigging screws and they seem to vary wildly that's even if the are willing to give them.

A rigger would be the best person to check with.

If you are going to replace them, make sure you get bronze bodies and s/s threads and not all s/s. All stainless gan seize up through 'galling'.

 

[reeac]

Sounds about right. My quick search reveals that s/s 1x19 7 mm. wire has a breaking load of about 4000Kg., for M10 bronze rigging screws it's 3500 Kg. and for M12 it's 5100Kg. Cap stays are set tighter than lowers as the latter have less to do - they simply fix the mid-point of the mast to prevent buckling under compression.

 

[TimBennet]

Cap stays are set tighter than lowers as the latter have less to do - they simply fix the mid-point of the mast to prevent buckling under compression.

Only if you never put your mainsail up. If you do, I hope they oppose the bending moment from forces transmitted by the sail luff as well.

Traditionally rigging screws are sized at twice the size of the wire. So 1/4 inch wire had rigging screws with 1/2 inch pins, with 9/32 inch wire staying with 1/2 inch.

So, the old rule of thumb (for an offshore boat) would have 7mm (9/32) matched to 12.7mm (1/2") rigging screws.

As your 10mm ones are less than this 'conservative' approach, you need to have a chat to a rigger, so you're happy with any solution. Perhaps the wire on the lowers is unnecessarily oversized, or the chainplates are such that you can't, or shouldn't bother, fitting a larger size.

 

[30boat]

Only if you never put your mainsail up. If you do, I hope they oppose the bending moment from forces transmitted by the sail luff as well.

Traditionally rigging screws are sized at twice the size of the wire. So 1/4 inch wire had rigging screws with 1/2 inch pins, with 9/32 inch wire staying with 1/2 inch.

So, the old rule of thumb (for an offshore boat) would have 7mm (9/32) matched to 12.7mm (1/2") rigging screws.

As your 10mm ones are less than this 'conservative' approach, you need to have a chat to a rigger, so you're happy with any solution. Perhaps the wire on the lowers is unnecessarily oversized, or the chainplates are such that you can't, or shouldn't bother, fitting a larger size.

All of the above makes sense.I would check the chainplates and if suitable fit 12mm rigging screws.My lowers are 7mm wire with 12mm screws.

 

[smithy]

Looks like the way to go is up them to 12mm, unless i can get some reassurance from a rigger.
Ive had a good search about, can only find one set of M12 screws that are short enough to fit in and these only have a breaking strain of 2000kg.
If I fit the normal length of M12 (250 closed) it means shortening shrouds and fitting Satlok connectors, a bit of an awkward job afloat.

Anyone know where I can source M12 screws (200 closed pin to pin) with a decent breaking strain? Have spent a few hours looking already
Not really worrying about galling a dod of copper grease seems to prevent it.

 

[30boat]

Looks like the way to go is up them to 12mm, unless i can get some reassurance from a rigger.
Ive had a good search about, can only find one set of M12 screws that are short enough to fit in and these only have a breaking strain of 2000kg.
If I fit the normal length of M12 (250 closed) it means shortening shrouds and fitting Satlok connectors, a bit of an awkward job afloat.

Anyone know where I can source M12 screws (200 closed pin to pin) with a decent breaking strain? Have spent a few hours looking already
Not really worrying about galling a dod of copper grease seems to prevent it.

Don't go for closed bodied screws.They do fail because they trap moisture and dirt.Open bodied rigging screws allow you to check how much thread you have left and the locking system is better than with closed bodied ones.Locknuts put unfair stresses on the threads and can come undone.
I've replaced all my terminals with Staloks once and the boat was afloat.Since they must be done one at a time it's usually safe.