Forestay Sag

Hi Steve,

First, I want to say thanks for the great website and books you and Linda have done. They’ve been very helpful to me.

I’m designing the rig for my boat now, a 42′ steel cutter. She has a genuine loaded for cruising displacement of 34,000 lbs.

I am doing a large roach main (80%)with swept spreaders and no standing backstay. Doing the rig stress calculations has made it clear that the large majority of the stress on the rig is from the shroud tension required to counter the jibstay and cutter stay tension. A reduction in tension on the forestays causes a great reduction in shroud and mast loads, but at the cost of increased forestay sag.

I’d greatly appreciate your thoughts on an appropriate sag percentage to use for the 30 degrees of heel stress calculations.

I am going to fit masthead runners to help tighten the jibstay and take some load off the rig, but am designing the rig to not require them.

Best Regards,

Hi Paul:

I’d check on the forestay sag tradoffs with your sailmaker as this is a sail shape issue. Less is a lot better, but with swept spreaders, as you have noted, it does load the caps.

Our approach has been to start with a given size headstay, and then work the cap shrouds and mast compression from the breaking strength on the headstay.

Posted by Steve Dashew  (May 16, 2010)

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