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