Doesn't the factor of safety calculation depend on what stress you assume to be the ultimate stress for which you are designing?
Bob Perry's calculation of FS=4 could result in the exact same specification as RichH's calculation of FS=6 if they are considering different ultimate stress criteria...?
I think this quote from Eric W. Sponberg sums it up better then I could. Just to put this person into perspective, Mr. Sponberg is a life member of the Society of Naval Architects and Marine Engineers as well as a licensed Professional Engineer in the state of Connecticut. Mr. Sponberg is also a corporate member and licensed Chartered Engineer of the Royal Institution of Naval Architects in the United Kingdom. He has a naval architectural degree from the University of Michigan. He is a contributor to both technical and popular magazines and a former technical editor for Cruising World. His web site is www.sponbergyachtdesign.com
. The snippet by the way, is from a thread discussing mast design originally appearing 7-20-06 on www.boatdesign.net
“There is not really any "rule of thumb" because generally the loads are pretty easy to identify, and they all come from the righting moment of the boat. Typically what I and many designers do is assume that the wind loading on the sails is equal and opposite to the righting moment of the boat. This moment is assumed to come from a distributed load along the mast multiplied by the center of its area above the waterline or above the CLR, depending on how you like to design. "Skene's Elements of Yacht Design" by Francis Kinney (therefore what used to be standard practice by Sparkman and Stephens) assumed that this distributed load along the mast was uniform, i.e. a constant value up the mast. Other designers such as myself assume that the load is larger at the top of the mast, smaller at the bottom. In my case, the top half of the mast is uniformly loaded, and the bottom half of the mast has an increasing distribution from zero at deck level to the same value at mid-height as the upper half of the mast. To summarize, then, the lower half of the mast is a triangular load distribution, and the upper half is a uniform distribution. The total moment of these two distributions (center of its distribution for each area to the waterline) is equal and opposite to the righting moment of the boat.
I hope that's clear. This is a live load. Therefore, you need some factor of safety on top of that somewhere in the analysis, either an allowable stress under live load, or multiply your live load by some FoS and calculate back to the yield stress, ultimate stress, or buckling stress, whichever is appropriate. Typical factors of safety may be anywhere from 1.1 to 4.0. My personal belief is that if you have to use a FoS over 5.0, you don't know enough about the problem to engineer it properly. FoS over 5 and you are overbuilding.
So to answer your question, there is a fairly well defined procedure for identifying the loads, therefore we don't need to rely on a "rule of thumb".”
All the best,