We're looking for the trash fence...

Boat Math

Posted on Tue 21 October 2008 in articles

NOTE: I have updated this entry with accurate chainplate beam measurements. 

Here’s some hardcore math as an outcome of my time at the Brion Toss Rigging workshop this past weekend. The point of this exercise is to be able to determine the loads and thus the wire sizes needed for the standing portion of the rigging on my Pearson 424.

Feel free to leave a comment if I messed anything up. I’m sure I did, see the footnotes for caveats and sources.

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](#_ftn1_2455 "_ftnref1_2455")
    • Single Spreader Single Lower Mizzen Shrouds[[2](#_ftn2_2455 "_ftnref2_2455")
      • 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](#_ftn3_2455 "_ftnref3_2455")
  • Beam at the Main Chainplate = 9.54'
  • Beam at the Mizzen Chainplate = 10.83'
  • Safety Factor 2.75[[5](#_ftn5_2455 "_ftnref5_2455")

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

[[1](#_ftnref1_2455 "_ftn1_2455") This formula is not in the Rigger’s Apprentice. Brion Toss provided at his workshop 18 October 2008

[[2](#_ftnref2_2455 "_ftn2_2455") 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](#_ftnref3_2455 "_ftn3_2455") I derived this number from the RM30 table Figure 5-24 pg. 136 in the Rigger’s Apprentice.

[[5](#_ftnref5_2455 "_ftn5_2455") Brion supplied this as a fairly standard constant used in rigging at his workshop 18 October 2008