Forestay Forces

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wofff

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I have inherited my grand Father's 18' day sailor from the early 1900's. It is gaff rigged, and has a small jib wrapped around a pole for furling. The wire forestay runs through the pole from the head of the mast to a mounting on the bow but has chaffed over the years and needs replacing. My question is, is there a rule of thumb with regard to the forces the stay needs to withstand ? weight of boat x sail area ? Any ideas would be appreciated as the stay needs to be small enough to be fixed in a mount at the bottom and strong enough that the lot doesn't come crashing down.
 
If you have the original forestay replace with the same size, or the next size up if the furling pole will take it.
My rule of thumb is for the forestay to be one size up from the standing rigging as the forestay has to take variable side loads from the headsails. However, I am not a rigger, but have re-rigged three of my own boats, which included upgrading the forestays on my present boat.
 
srm said:
If you have the original forestay replace with the same size, or the next size up if the furling pole will take it.
My rule of thumb is for the forestay to be one size up from the standing rigging as the forestay has to take variable side loads from the headsails. However, I am not a rigger, but have re-rigged three of my own boats, which included upgrading the forestays on my present boat.
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You're right in that the head stay should match the strongest shroud at least. The reason for this is prevent the mast falling aft into the cockpit in case of failure. Side (bending) forces should be accommodated with bi-axial toggles. Compared to the shrouds, the actual loads on the forestay are fairly modest and are generally calculated with 1lbs per sqft of foresail area.
 
The original had thinner wire making up the cable than I can get in stainless with the same diameter, and it has to do a figure of eight in the mounting at the bow with two grub screws securing it. So. I'm wondering if I can get away with a smaller diameter. I'm struggling with the same diameter as original - can't get it back on itself like the original, and since it looks massively over engineered for an 18 foot boat - hence the question. You've nearly answered my question 1lbs per sqft of foresail area, - is there a calculation ??
 
Flexibilty of the wire isn't only a function of the wire's diameter, but also the wire construction. 1x19 is very stiff, but 7x7 and 7x19 are very much more flexible, especially in smaller diameters.

It's not ureasonable to simply size it as the same as the rest of the rigging wires on the boat. Traditionally the forestay was one size larger because the jib hanks used to wear the galvanising off and the forestay was always more rusty than the other rigging.
 
wofff said:
The original had thinner wire making up the cable than I can get in stainless with the same diameter, and it has to do a figure of eight in the mounting at the bow with two grub screws securing it. So. I'm wondering if I can get away with a smaller diameter. I'm struggling with the same diameter as original - can't get it back on itself like the original, and since it looks massively over engineered for an 18 foot boat - hence the question. You've nearly answered my question 1lbs per sqft of foresail area, - is there a calculation ??
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As Motor Sailor points out there are different types of wire construction. For standing rigging 1x7 SS is the most common these days and uses pressed or Norseman style terminals. 1x7 wire is not really suitable for bending an eye into it using Nicopress sleeves or such.
Do I understand that your current headstay is galvanized? The Terminal you are describing seems rather unusual and if it is the type I'm thinking of, I would consider it unsuitable for the purpose, especially with stainless cable which is much more "slippery".
From you description I suspect that your current headstay might be 7x7 galva. Your best option would be to use 7x7 SS and get an eye made with a Nicopress sleeve to replicate your current set-up as it appears to be functional and has lasted the test of time.
 
Thanks for your replies. I have done quite a bit of further research, and it basically is a "Black Magic". But for anyone interested, the Rule of Thumb I was looking for, the SWL of the stay should be equal to the displacement of the boat. As previously mentioned, this is a rule of thumb, but it makes sense in my case - where the boat weighs less than one tonne and 12mm wire has a SWL of 1.2 tonnes and a min break load of 7 tonnes, I'm going with that !
Replicating the original set up is what I want to do. She's a historic boat and I want to save her "unique" features, but without the risk of crashing down around me ears.
 
wofff said:
Thanks for your replies. I have done quite a bit of further research, and it basically is a "Black Magic". But for anyone interested, the Rule of Thumb I was looking for, the SWL of the stay should be equal to the displacement of the boat. As previously mentioned, this is a rule of thumb, but it makes sense in my case - where the boat weighs less than one tonne and 12mm wire has a SWL of 1.2 tonnes and a min break load of 7 tonnes, I'm going with that !
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I'm sorry, but that is gobbledygook: 12mm wire? I ton displacement?

The forestay is sized equal or slightly more than the strongest shroud. For classic boats it's good to use old books. Skenes says the (RM x 1.5) ÷ 1/2 beam at chainplates. You have to be a bit conservative of Righting Moment in small boats as the crew weight on the rail can be a huge factor. But guessing about your boat we have (5000 x 1.5) ÷ 3 = 2500lbs. as the Breaking Strain of a suitable forestay. That's 5/32 or 4mm.
 
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wofff said:
The original had thinner wire making up the cable than I can get in stainless with the same diameter, and it has to do a figure of eight in the mounting at the bow with two grub screws securing it. So. I'm wondering if I can get away with a smaller diameter. I'm struggling with the same diameter as original - can't get it back on itself like the original, and since it looks massively over engineered for an 18 foot boat - hence the question. You've nearly answered my question 1lbs per sqft of foresail area, - is there a calculation ??
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Q9
 
wofff said:
where the boat weighs less than one tonne and 12mm wire has a SWL of 1.2 tonnes and a min break load of 7 tonnes,
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You must be joking!

My boat displaces about 8 tons, cutter rig, and is set up for ocean passages. When I bought her the forestays were 7mm ss 1x19, as was all the other standing rigging. I regarded this as a bit light for forestays so re-rigged with new roller reefing on 8mm ss 1x19. The bobstay on a short platform bowsprit is only 10 mm ss.

Flexible constructions are not as strong as 1 x 19 but even so a 12 mm forestay on 1 ton displacement seems excessive. Perhaps using wire with a breaking strain of the boat's displacement would be more appropriate.

I do remember coming across a 'rule of thumb' when re-rigging a 42 ft sloop that the total strength of the shrouds should be equal to the boat's displacement.
 
Thank you for your replies - the original measured slightly less than 1/2" and I believe it would have been original from circa 1905 and although it has survived the test of time well had frayed at the top of the mast. So my dilemma was whether to reduce to 10mm, hence the original enquiry.
" I'm sorry, but that is gobbledygook", may sound funny, but this is a serious enquiry, and as you all know, there are never any silly questions just silly answers.
It is an interesting (to me) rig. Takes two men to mount the stumpy mast, and the boom and yard are fabricated from the best part of a tree each and together require some strength to hoist. It has "Top lifts", which when lifting or lowering, are a constant reminder as to how much weight is in the air. I just don't want the lot to come crashing down.
Thanks again.
 
wofff said:
" I'm sorry, but that is gobbledygook", may sound funny, but this is a serious enquiry,
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And that was a serious reply. The first bit of gobbledygook is to use SWL in any rule of thumb, or even calculation for sizing the rig. All wire for rigging is quoted as Breaking Strain.
Then there's no 18 foot gaffer that ever needed a 12mm forestay. One that size is unseen on anything less than a deep draft, oceangoing 40footer or larger. Even 10mm is for a boat in the middle to upper 30foot range.

The weight of the mast has no bearing on stay size. It's purely the forces at play when the boat is heeled to 30 degrees. It's a combination of the weight of the boat plus the effect of the beam, ballast and crew keeping it upright against the force of the wind in the sails . The 'stiffer' the boat, the stronger the rig needs to be.

6 or 7mm would be the size of stays on a deep keel, 10ft beam, 5 ton yacht.
4mm is the size of shrouds on a 2 ton, 22 ft yacht.
 
Motor_Sailor said:
I'm sorry, but that is gobbledygook: 12mm wire? I ton displacement?

The forestay is sized equal or slightly more than the strongest shroud. For classic boats it's good to use old books. Skenes says the (RM x 1.5) ÷ 1/2 beam at chainplates. You have to be a bit conservative of Righting Moment in small boats as the crew weight on the rail can be a huge factor. But guessing about your boat we have (5000 x 1.5) ÷ 3 = 2500lbs. as the Breaking Strain of a suitable forestay. That's 5/32 or 4mm.
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Hi Motorsailor, I agree on using Skenes, but how did you arrive at a RM (righting moment) of 5000 lbs on a boat displacing less than one tonne (as OP said) even with a crew hiking out?
 
I said I guessed, based on

- a generous displacement of a long ton, with a crew of 700 lbs crew, together equal to 3000lbs.

I took a GM to be 3' 4", assuming it to be a wide and flattish hull with a high form stability and movable ballast.

RM30 = Displ x GM x sin 30°
 
Motor_Sailor said:
I said I guessed, based on

- a generous displacement of a long ton, with a crew of 700 lbs crew, together equal to 3000lbs.

I took a GM to be 3' 4", assuming it to be a wide and flattish hull with a high form stability and movable ballast.

RM30 = Displ x GM x sin 30°
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Hmm, yes, perhaps. Very well fed crew + dog and all hiking out, including the dog. If the OP's rig is as heavy and substantial as he says, I think a GM of 3 and a bit ambitious. None the less, 12mm for a head stay on a 18 footer is somewhat, ahem, "overengineered".
 
Well as we have no details of the boat apart from its length, what is you more expert guess?

I was sinply using all figures on the high side to get the biggest conceivable stay size to compare with the 12mm suggested.
If you've got a more accurate method, over to you.
 
Motor_Sailor said:
Well as we have no details of the boat apart from its length, what is you more expert guess?

I was sinply using all figures on the high side to get the biggest conceivable stay size to compare with the 12mm suggested.
If you've got a more accurate method, over to you.
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With a RM of 5000 ft/lbs he could, assuming a SA of 150sqft and CE height of 9', carry this sail up to the top of a F7. (???) But, as you said, all is conjecture.
 
Sorry but none of you are making sense if you ask me. The righting moment applies to the shrouds, the forestay doesn't really get involved with that. The main loading on a forestay is coming from the mainsheet especially as you sheet in on a blowy day but being a gaffer the loadings are not as high as with a Bermudian . 12mm for an 18ft boat does seem excessive, but how are you measuring the wire. Circumference or Dia. Nearly 1/2 circumference would be much more likely IMHO.
 
Aquaboy said:
Sorry but none of you are making sense if you ask me. The righting moment applies to the shrouds, the forestay doesn't really get involved with that. The main loading on a forestay is coming from the mainsheet especially as you sheet in on a blowy day but being a gaffer the loadings are not as high as with a Bermudian . 12mm for an 18ft boat does seem excessive, but how are you measuring the wire. Circumference or Dia. Nearly 1/2 circumference would be much more likely IMHO.
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The rule (of thumb) to make the forestay the size of the strongest wire has nothing to do with righting moment directly, but with the need to add an extra margin of safety in preventing the mast from falling aft into the cockpit and swatting the crew. The actual loads on the forestay are, relative to the righting moment, much less.
 
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