standing rigging diameter?

[county]

My boat a Mao Ta 36' had 5/16 cable on the very heavy wooden mast with antiquated external Forespar furling main unit.

The new to me aluminum mast came with almost new 9/32 cable, its seems to me that a reduction of 0.032 in diameter would be acceptable considering the substantial loss of weight aloft.

or maybe I'm being foolish?

The boat #s are

L.O.A. 36'2"
L.W.L. 32'6"
Beam 11'2"
Displacement 28,000#
E=15'7"
P=39'
I=43'5"
J=18'8"

The boat is a full keeled cutter rigged Mao-Ta 36 if you are not familiar with the model it is nearly identical to the Union 36 or Hans Christian 36

Any thoughts?
Thanks.

[county]

Am I missing something obvious?

When running the numbers with Brion Toss's formulas I get a R.M. somewhere in the neighborhood of 12,097# but I feel like some of the necessary data is still not available thru The Rigger's Apprentice. How do I know what % of the load is divided between the shrouds. Even using Toss's example he shows 45% for the uppers and 32.5% apiece for double lowers, but then what % do the intermediate backs take up and how does he come up with those % in the first place? Also doesn't having double lowers and intermediate backs reduce the 45% load that the uppers carry? Its also confusing to me how displacement doesn't effect R.M.

Any insight on this?

Maybe I need Skene's Elements of Yacht Design but honestly the last time I picked that up I got dizzy.

[Brion Toss]

Hi again,
Good for you, seriously, for questioning those numbers. I don't like it when people accept things I say blindly. Let's start with the displacement question. That is a factor, but only when coupled with the length of the righting arm, and this in turn has to do with the distance between the center of gravity and the center of buoyancy, at a given angle of heel. Thus a light boat with a lot of distance between those points could be more stable -- and have higher rigging loads -- than a heavier one with shorter distance.
The load on the uppers is approximate, but not grossly so, and you can prove it, either on paper, or directly, with load cells. Same with the lowers.
As for the "intermediate backs," if you mean aft-led intermediate shrouds, they are typically at such a sharp angle that they provide negligible lateral support, and can be safely ignored. They also don't provide significant aft support, which is why runners make more sense for your boat.
Finally, it is just not possible to have an R.M. of only 12,000 or so, except at very low angles of heel. How did you derive that number?
Fair leads,
Brion Toss

[county]

Hi Brion,
Thanks for the reply!

Sorry, the shroud load 12,000# was mislabeled as RM.

I am using the graph in The Riggers Apprentice from Skene's Elements of Yacht Design (Foot-Pounds/D.W.L in FT.)

33' L.W.L RM=45,000
beam=11.17’

(RM*1.5)/(1/2*beam)
=(45,000*1.5)/(1/2*11.17’)
=67,500/5.58
=12,097#

Am I doing this calculation correctly?

How do I calculate what % of the load is divided among the shrouds?

How do I find a reliable source to determine load-bearing of 1/19 (the websites I looked at have varying loads)?

What I refer to as "intermediate backs" are shrouds led from the mast hound where the inner forestay connects to the same chainplate as the aft lower shrouds, connecting aft of the lower shrouds. I will refer to them as aft intermediate shrouds. My boat was designed with aft intermediate shrouds. I would like to avoid the added complexity of running backstays. Why do you think runners make more sense for my boat? I believe many cutter-rigged boats have gone below the Great Capes without them.

Thanks for your input. It is really appreciated,
Karl

[Brion Toss]

Hello again,
First, the data I have for your boat shows a waterline of 32ft. Even at 33ft, you would probably be well shy of 45,000lbs rm at 30 degrees. A direct incline test could confirm this, and I recommend one. Meanwhile, 5/16" uppers and 9/32" lowers look right. For more details, see the "Rigger's Apprentice," or be in touch with the shop to set up a consultation.
As far as runners go, as I said before, those aft-led intermediates are not making a useful contribution, and runners will. It would not be adding needless complexity, but restoring needed complexity. And there are ways to make them very simple to use.
True, many cutter-rigged boats have sailed below the Capes without them, but hey, some of those boats didn't have standing backstays either. Your intermediates were a misguided attempt, long-since abandoned by people like Bob Perry, to get rid of runners. The attempt resulted in poor staying, compromised pointing in storms, chafed mains'ls, and a higher incidence of unintentional jibes.
As for wire strength, there are international standards for a given diameter, construction, and alloy. Whose numbers are you looking at?
Fair leads,
Brion Toss