Calculating Shroud Loads

[Patrick_Seattle]

I posted this in my blog, but I thought I'd repost here and see if anyone cares to offer a critic of my math or formulas. The basics of this I picked up at the workshop this passed weekend which was well worth the time and money. These calculations are for a Pearson 424 Ketch.


My Formulas:

• Main Shroud Load = (RM30 * 1.5) / .5 (Beam at the Main Chainplates)
  • Single Spreader Double Lower Main Shrouds
    • Upper Shroud Load – 45% Main Shroud Load
    • Lower Shroud Load – 32.5% Main Shroud Load
• Mizzen Shroud Load = (RM30 * 0.5) / .5 (Beam at the Mizzen Chainplates)[1]
  • Single Spreader Single Lower Mizzen Shrouds[2]
    • Upper Shroud Load – 45% Main Shroud Load
    • Lower Shroud Load – 50% Main Shroud Load
• Wire Rating Requirements = The Specific Shroud Load * SafetyFactor

My Inputs:

• DWL = 33’ 8”
• RM30 = 47000 lbs[3]
• Beam at the Main Chainplate = 9.54'
• Beam at the Mizzen Chainplate = 10.83'
• Safety Factor 2.75[5]

My Findings:

• Main Shroud Load = 14,465 Lbs
  • Upper Main Shroud Load = 17,900 Lbs
  • Lower Main Shrouds Load = 12,928 Lbs
• Mizzen Shroud Load = 4,246 Lbs
  • Upper Mizzen Shroud Load = 5,254 Lbs
  • Lower Mizzen Shroud Load = 6,423 Lbs

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[1] This formula is not in the Rigger’s Apprentice. Brion Toss provided at his workshop 18 October 2008
[2] I have assumed that the load per wire ratios for mizzen mast shrouds are the same as those for main mast shrouds with the same numbers of shrouds and spreaders as in figure 5-25 pg. 138 in the Rigger’s Apprentice.
[3] I derived this number from the RM30 table Figure 5-24 pg. 136 in the Rigger’s Apprentice.
[5] Brion supplied this as a fairly standard constant used in rigging at his workshop 18 October 2008

[Brian Duff]

yeah, i didn't work through your math but 3/8" uppers, 5/16" lowers on the main seem sensible

1/4" and 7/32" on the mizzen fit right too...

one thought however is does the boat have 5/8" holes in the upper/forestay/backstay chainplates ?

[Patrick_Seattle]

I am pretty sue at this point I'll be doing some tang work too.

As I read in the parts list from 1981 (not measured) the size of the wires installed are as follows:

• Main uppers, lowers (fore and aft) are 5/16"
• Mizzen uppers are 3/16" and 7/32"
• Jibstay is 3/8"
• The fresh water stay (Brion called it something else) is 3/16"
• I don't have the specs on the split backstay

Anyway, when the stick is pulled I'll be having the masts refinished so there will be an opportunity to size things appropriately as well.