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post #28 of Old 01-23-2012
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I dynamically tune my boats ... but I have a healthy dose of material science, metallurgy, engineering, amateur sailmaking, etc. background; so my reasons may be quite different from anyone else.

I dynamically tune for the following reasons:
1. first and foremost - sail shape ... if the forestay tension (improper sag when windloaded) doesnt precisely match the leading edge shape of the sail as cut/designed, then I readjust (while underway) to get it 'close'.
2. Most rigging is stainless steel and that presents a special problem all by the very nature of 300 series stainless --- vulnerability to fatigue failure. Fatigue is vastly accelerated any time 300 series stainless is cyclically loaded beyond ~30,000 psi (even that the material has a ~90000 psi 'ultimate tensile strength' ... but that applies only to 'ductile failure' & non repetitive load conditions). The all important material characteristic for 'boat rigging' and plates, etc, I adhere to, is that I expect that the material/rigging WILL catastrophically fail when there are more than (estimated) 1 million 'cycles' (about 1 circumnavigation) where the component goes beyond 30K psi. Keep that loading UNDER 30K psi (30% of rig tension, etc.) and the part 'can' last virtually 'forever' (theoretically from a materials science or metallurgical point of view).

So, I do use a tension gage the rigging while getting the boat over 'towards' a 45 heel angle ... as after the 45 heel angle the loads diminish due to 'trigonometry'. If I cant get the boat to 45 I simply measure what I have at the max. angle and then calculate what the loads would be @ 45 over, etc. ... and then assuming that my rig has an appropriate inbuilt design safety factor (probably at least 3X+ for a "perry-boat") I arrive at the max. tension and then reduce the 'static' tensions as needed and still to get the needed mast 'pre-bends', forestay 'shape', etc.
When Im sailing 'hard', I simply dont want any part of the rig tension to (much) 'go over' 30% UTS ... and will apply 'helpers' (runners, etc.) to get what I want ... (hopefully) less than 30% UTS at 'max'. conditions.
When 'sailing' I monitor the backstay tension (gage attached) and if that backstay is going much over 30%, I then reduce/reef of change a sail, head off, etc. etc.
(FWIW I sail a Taiwanese made Ty37 and the Formosans are long noted for their lack of metallurgy expertise ... and I claim so too many 'riggers' also fit in this category but 'graciously understand' and 'accept' this .... as I dont want my cost of maintenance exceeding the cost of the national debt of the USA.).

I didnt do this when I was actively racing my 'sport boat' (dynamic on-the-fly mast raking and 'very' bendable mast, independent forestay tension, etc. and over-design Safety Factor of 1.5X) and did lose a few masts, rigs, etc. overboard - all due to 'catastrophic' fatigue failure.

Thats my 'impression' of dynamic tuning ... for sail 'shaping' and for keeping the rig up without undue fatigue failure 'surprises'.

Ultimately one has to consider that since there ARE noted rig failures, increasingly insurance underwriters demanding (panic, seemingly) a total rigging changeout every 10 years, etc. ... that the 'inherent design and selection' of the these materials ... is quite 'faulty' ... and 'someone' is erroneously designing rigging etc. in accordance to 'ductile' values and not 'fatigue endurance' values. Cant be otherwise.

Last edited by RichH; 01-23-2012 at 01:20 PM.
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