[Public-List] Dynamic Loads while sailing

[Roger L. Kingsland]

Jim,

On the subject of mast compression loads I called Mike Kaufman, the navel
architect/surveyor in Annapolis who surveyed my boat to ask how he might
quickly get a rough number.  He thinks taking 1.2 times the breaking
strength of all stays/shrouds under load would work.  It seems the greatest
cross sectional area of wire would be under load on a broad reach or close
reach where the windward shroud, main stay and one (windward) baby stay
would be under load (one could argue that both baby stays would be under
load as the wind moves toward the beam but there would then be a reduction
in the load on the shroud which has greater strength).  One 3/16" baby stay
at 4,000 pounds and one 1/4" stay plus one shroud at 6,900 pounds each total
17,800 pounds, 21,360 after the 1.2 mark up.  

On the unlikely chance that all shrouds/stays under load would reach
breaking strength at the same time, I would be inclined to discount the
safety multiplier and use more like 18,000 to 20,000 or roughly double the
weight of the boat; all things considered, fairly close to the 150%
mentioned below.  I guess the point is, next time you haul an A30, just lift
it by the mast (just kidding, of course).
 
Roger 148


-----Original Message-----
From: public-list-bounces at lists.alberg30.org
[mailto:public-list-bounces at lists.alberg30.org] On Behalf Of Jim Davis
Sent: Friday, January 15, 2010 3:24 PM
To: public-list at lists.alberg30.org
Subject: Re: [Public-List] Dynamic Loads while sailing

>From L. Francis Herreshoff's "The Common Sense of Yacht Design, Vol 1"
page 114.
This indicates there could be a loading in excess of five tons on the deck
of an A30.
Just something more for the thought process.

I apologies for the poor OCR.

Jim Davis
S/V Isa Lei
CT 35

But our sail is not flat, so that more likely the strains are about as shown
in Figure 151,  and in this diagram the total length of the lines added
together would equal the pull downward of the mainsheet so that we see the
strain are mostly near the masthead and after end of boom. If the strains of
the mainsail are rather concentrated aloft, the strains of the  jib or
headsail are all concentrated in one spot, and if the forestay pulls forward
as much as the mainsail pulls aft then we have an enormous  strain in one
spot; and as a matter of fact, when the backstays are carried  unnecessarily
tight the strain at this point is more than most people realize .  So the
fore and aft strains on the mast are the pull of the mainsheet and backstay
which has to be counteracted by the pull of the forestay. These strains may
be about as follows (in strong breezes) as compared to the weight or
displacement of the yacht;  mainsheet 12 per cent, backstay 13 per cent,
forestay 25 per cent, depending on the angle of pull. While the side strains
on the mast are not as 

great as the fore and aft strains, still as the shrouds do not have as
favorable  leads the strain on the shrouds is great, particularly on narrow
yachts or where the shrouds' are carried inboard to clear a lappiiig jib. At
times this shroud strain is as much as 100 percent of the yacht's weight, so
that with the halyards and headsail sheets pulling downward the mast must at
times  take a compression strain of over 150 per cent of the yacht's weight.



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