Forestay design

Hello again catamaran designers and cruisers
This time I humbly seek your advice about a suitable forestay for the good ship Archimedes
(she/he/it is a 60 foot catamaran that I have been building for many years - and hope to launch this year - the forestay is 18 m long)
The questions are
1. Tension and sag - the formula I use is T=qL^2/8h (where T=tension, q=lateral wind load per m, L=length, h=sag). The curve is neither a catenary, nor a parabola because of non uniform loading so I just use a parabola as first approximation. Is this an appropriate formula?
2. Thus if wind load in the middle of the 40 sq. m blade headsail is about 300N (20 kN - 10 kPa on 3 sq. m) the required tension will be 40,500 N to limit the sag to 300 mm. That's 4 tonnes!
3. Is a sag of 300mm OK?
4. How should I terminate the stay at the bow end - it needs to have lateral and longitudinal flexibility.
5, I am considering 16 mm 1x19 wire, breaking load about 15 tonnes - is this overly conservative?

I hope for some comments from experienced cruisers and perhaps a rig designer
Thank you
Michael

 

Hi,
IMHO, looks your approach is reasonable.
You should ask to a sailmaker the appropriate sag for a headsail with 18 m long luff. Sag is a design parameter for headsails.
If you are able to provide the luff tension for 300 mm sag, the sailmaker will design the sail for it !

 

Seems like you are assuming the forestay is attached to a rigid structure. The deformation (strain) of the rest of the boat has to be taken into consideration too. Also, take into consideration the stretch of the cable.

 

Wow

 

You have a good looking boat , few people can say they built a 60 footer .