Hello,
I am in the process of upgrading my calypso 870 for serious offshore travelling here in Norway.
The rig has to be renewed anyway, the boat is currently on the hard and I don't think I will be able to put it in the water this year so I am using the righting moment graph (p.136) reproduced in "the rigger's apprentice" to calculate shroud load on my boat.
Here's a link with pictures of it so people can get an idea:
http://m.finn.no/boat/forsale/ad.html?finnkode=42720402
My first question is am I reasonable to base all my calculations of that graph or is there just so much variations between boats generally that I should do better to have the boat put in the water just so that I can proceed with my own inclining test (there is very little info available about the boat online, I don't even know its exact displacement)?
In the book, the text accompanying this graph says:
"note the dotted lines on either side of the solid line in the chart. These indicate likely limits to extra-solid or extra-flimsy boat RM."
In this case, what is the intended meaning of "extra-solid or extra-flimsy"?
Is it a reflection of your desired safety margin? An offshore cruiser selecting the higher foot-pounds extra-solid line while a racer would pick the lower extra-flimsy one?
Or is it a reflection of tonnage? Extra-solid = heavy displacement and extra flimsy = light displacement?
A more general question, is there some "special" safety factor used for offshore long distance cruising boats not mentioned in the book.
In the voyager's handbook, Beth Leonard writes:
"For offshore work, stays should be sized so that the working load should is no more than 25% of the breaking strengh".
The way I understand it, that would mean using a ×4 safety factor instead of the ×3 maximum recomended in the rigger's apprentice.
More intriguing, in "this old boat" Don Casey, who so seems to draw a lot of his information from the rigger's apprentice wrote regarding the transverse load formula:
"Multiply that result by 1.5 to allow for severe conditions that might heel the boat beyond 30 degrees.
(Occasionally 2.78 is used rather than 1.5, a good precaution if you are headed for high latitudes, but empirical data sugest that the higher factor is rarely essential under more normal conditions).
2.78 instead of 1.5 is quite a jump and in my calculations, nearly doubled my shroud load
so I wouldn't choose it lightly.
Any input would be appreciated.
George