Offshore capable rig and correctly interpreting the rigger's apprentice

[Ole1291]

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

[Brion Toss]

Hi,
Good questions all.First, flimsy: some monohulls are stiffer than others; the solid line in the chart is a reliable average resistance to heel, and in my experience is good for most boats. You might try running the formula for the higher an lower values, and see if you get much if any difference in wire size.
The variance is definitely not about safety factor; that is added later. It is about the vessel's inherent stiffness. An incline test is always a good idea, if only to check the data from that formula.
When Beth talks about "working load," it could mean whatever you want, but the load at 30 degrees of heel is generally used. Ultimate load is usually about 1.5 times that (more on this number later), and you add a safety factor onto that. Do the math, and you will see that a safety factor of about 2.7 gives you a break strength of 4 times the working load. This is an important bit of math.
Finally, that 2.78 number has nothing at all to do with high latitudes; it is a component of mast design, wherein the jibstay, forestay, and backstay add compression to that imposed by the shrouds. So righting moment at 30 degrees, times 1.5 is total shroud load, and the same moment, times 2.78 is total mast compression.
Fair leads,
Brion Toss

[Ole1291]

Originally Posted by Brion Toss

Hi,
Good questions all.First, flimsy: some monohulls are stiffer than others; the solid line in the chart is a reliable average resistance to heel, and in my experience is good for most boats. You might try running the formula for the higher an lower values, and see if you get much if any difference in wire size.
The variance is definitely not about safety factor; that is added later. It is about the vessel's inherent stiffness. An incline test is always a good idea, if only to check the data from that formula.
When Beth talks about "working load," it could mean whatever you want, but the load at 30 degrees of heel is generally used. Ultimate load is usually about 1.5 times that (more on this number later), and you add a safety factor onto that. Do the math, and you will see that a safety factor of about 2.7 gives you a break strength of 4 times the working load. This is an important bit of math.
Finally, that 2.78 number has nothing at all to do with high latitudes; it is a component of mast design, wherein the jibstay, forestay, and backstay add compression to that imposed by the shrouds. So righting moment at 30 degrees, times 1.5 is total shroud load, and the same moment, times 2.78 is total mast compression.
Fair leads,
Brion Toss

Now it's all much clearer to me.
Yes, I forgot the 1.5 factor was for ultimate load , my mistake. With your explanation it makes perfect sense.
It looked weird that Casey used the precise same number (2.78) that was in your book for the mast compression calculation, so I half suspected a misunderstanding.

Thanks for having taken the time to answer my questions, it actually helped me a lot.

George