Intermediates on a single spreader mast?

[Newsong]

I recently bought "The Complete Rigger's Apprentice" (second edition) in preparation for replacing the rigging on my Cape Dory 330 cutter.

My mast has single spreaders and aft-led intermediates. I like the idea of moving the intermediates out onto the spreaders and adding running backs.

In the book figure 5-25 gives the percentage of maximum shroud load allotted to each shroud. I did not see what percentage to use for intermediates on a single spreader mast.

What is a safe percentage to use to size these shrouds?

I'm sure the intermediates take some load off of the uppers, is there a good way to calculate how much? (My calculations have my uppers right at the top end of 1/4 wire. I am trying to decide if I should move up to 9/32.)

[Brion Toss]

Hi,
I think it is safe to say that those intermediates are doing little or nothing to stabilize the mast or make use of loads. Try sailing with them tuned, then slack, and see if you can notice any difference. I would just replace then with runners, and sail away,
Fair leads,
Brion Toss

[Newsong]

Brion, Thanks for getting back to me.

A couple more questions.

Am I understanding you correctly, the mast does not need any lateral staying at the forestay?

How do I determine how far back to lead the running backs?

[Brion Toss]

Hi again,
A small portion of the stay load is expressed laterally when the loads are highest (on the wind in a breeze), and the mast itself provides the needed lateral resistance. Your current intermediates are at such a steep angle that they provide very little lateral support, but impose a great deal of compression load on the spar. Runners will do about as well laterally, but will be far more effective fore-and-aft, given their wider angle.
As for placement, the more angle the better, but it is best if the end of the boom is aft of the runners, so you don't risk ripping the mains'l leach out in a jibe. Deck traffic is another concern, as you don't want the runners impeding that. House sides and other obstructions are also a layout consideration. And access for fasteners is another factor. All in all, this calls for careful design work.
Fair leads,
Brion Toss

[Newsong]

Great! That makes things a little easier than I was thinking.

Thanks again for the help