Calculating chainplates

[Hernan]

I am replacing shroud chainplates in an Alberg 30 and thought it would be a good idea to check if the current rigging is strong enough or if I should increase the size.

I would really appreciate if you could review my thought process and offer some comments.

The current uppers are 1/4" 1-19 wire with 1/2" bronze turnbuckles and 3/16 x 1-1/4 chainplates.
The current lowers are 3/16" 1-19 wire with 3/8" bronze turnbuckles and 3/16 x 1-1/4 chainplates.

Since the boat is on the hard now I can't do an incline test so I used the RM calculator at http://www.seldenmast.com/en/service...alculator.html
The RM30 from the selden calculator is 15.5 kNm which translates to 11432 ft lbs

does this RM30 value seem reasonable?

I calculated shroud dimensioning loads using 3 methods.
Riggers Apprentice (with safety factor of 3):
lowers=3822
uppers=5291
this is the only method that suggests lowers should be lighter than uppers. although in the book it notes that it is prudent to make the lowers as strong as the uppers.
David Gerr nature of boats method (this is based on boat weight and does not use RM30):
lowers = 6480
uppers = 4320
Principles of yacht design - Larsson, Eliasson:
lowers=6461
uppers=5466

So my 3/16" lower shrouds seem to be undersized (breaking load 4000) and I should use 1/4" wire (breaking load 6900) for both lowers and uppers.
1/4" wire fits in 1/2" norseman studs and turnbuckles/toggles with 1/2" pins.

Now for the chainplates:
Riggers apprentice recommends 30% to 50% stronger than the wire (9977 to 10350lbs).
Should I use the ultimate tensile strength or the yield strength for this calculation?
Calculating with the yield strength of stainless (30KSI) indicates I should use 3/8"x1-1/2" chainplates
Calculating with the ultimate strength of stainles (70KSI) indicates I could 3/16"x1-1/4" which is what has been in use the last 45 years.

So to recap:
does RM30=11432 ft lbs seem OK for an Alberg 30?
how to calculate chainplate size for 1/4 wire?
Anything else i'm missing?
Should I just replace with same size since this has worked OK for 45 years?

Thanks
Hernan

[Brion Toss]

Hi,
I spoke with one of the good people at Selden, and they are using an even higher righting moment number, assuming a displacement of 4,500kg. This results in 7mm uppers, which I frankly find absurd. My guess is that the assumptions behind their algorithm do not correspond well to this vessel.
With a waterline of less than 22ft, and with a relatively low SA/D, the Alberg will be less stiff than most contemporary boats. A direct incline test would confirm or deny this; I understand that you can't do that at the moment, but maybe someone with a sister ship could?
I have amended the note about lower shroud size for the next edition or the Apprentice, as repeated tests and measurements indicate that it is never necessary to have double lower shrouds that are the same diameter as uppers, at least on a single-spreader rig.
I can only hope that the numbers you are using from Gerr and L&I are for single lowers; the total load on the lower shrouds is typically somewhat higher than on the uppers -- as much as a 60/40 ratio -- but twin lowers, as on the Alberg, argue for reduced lower shroud diameter.
Again, a direct test will bear out the above, or not.
As for the chainplates, use ultimate strength, just as for all other components. And trust the tables in L&I, Skene's, or my book.
Fair leads,
Brion Toss