Forestay design

[MichaelRoberts]

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

 

[patzefran]

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 !

 

[gonzo]

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.

 

[TANSL]

Wow

 

[rberrey]

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

 

[MichaelRoberts]

Thanks for your comments.
Today I'm seeking a bit more advice on the forestay for our catamaran
Because the headsail furler becomes very expensive as wire size goes up we are now looking at 14 mm 1x19 stainless wire
The indications are it has a breaking strength of about 20 tonnes
Allowing a safety factor of 3, that means max tension of about 6 tonnes
The boat will take it, not so sure if the mast will be too happy with 6 tonnes
To sum up, no use going for 16 mm wire
Does 14 mm, or even 12 mm seem ok?
Maybe someone with a big Lagoon or Fontaine Pajot or Outremer could tell me what their forestay diameter is
Many many thanks
Michael