Forstay sizing without prior model

Discussion in 'Sailboats' started by DavidJ24, Aug 5, 2015.

  1. DavidJ24
    Joined: Aug 2015
    Posts: 1
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Minnesota

    DavidJ24 New Member

    Hi Fellow Sailors. I am looking for actionable advise on how to size a forestay for my West Indies 38 (Same boat as later Out Island 38) by Morgan.

    I have done a bit of reading in forums and here are the facts I think are relevant:
    I am replacing something called a dynafurl and/or twinstay which is a furling system with no wire forstay It is a 5/8" aluminum rod with two groves that takes the whole load (its broken now). So I have no original wire size to go from.
    There is a working jib stay about 3 feet back.
    The Forestay length is 51'
    The boat displacment is 17000 lbs
    Its a mast head sloop.
    I was also told that with this info a rigger will make me a stay. So what method is he using?

    I know that a sailboat rig is complicated and that statics and dynamics is a hard class in college. My Bro is a ME and he wont touch the problem. Many rig questions cannot be answered without real calculation but I think this one can just on the basis of how common the proven examples are.

    Since there are only a few reasonable choices of wire diameters and millions of examples of masthead sloops I am looking at this as a multiple choice answer:

    Should I buy wire in: A. 5/16 B. 3/8 C. some number of 32nds in between. I would love to hear from anyone that has a similar displacement boat and forestay length who is happy with performance etc.

    Does anyone have possible sources of info on what the original spec was for this boat. Or a rule of thumb calculation. Or set of parameters.

    Thanks for any help and insight!
  2. philSweet
    Joined: May 2008
    Posts: 2,630
    Likes: 390, Points: 83, Legacy Rep: 1082
    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    If your backstay is 5/16, use 3/8. My Catalina 38 is a bit more high strung, and that's what I run on it. The C-38 racers that use smaller stuff change it yearly. You size the forestay bigger because it has more tension than the backstay due to the angles, and if you are having a bad day, you want the mast to fall forwards, not backwards into the cockpit.
  3. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 497, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Without running the numbers, your only choice is an educated guess, so Phil's suggestion seems reasonable.
  4. Eric Sponberg
    Joined: Dec 2001
    Posts: 2,019
    Likes: 232, Points: 73, Legacy Rep: 2917
    Location: On board Corroboree

    Eric Sponberg Senior Member

    David, another guideline is that the lower shrouds are some of the heaviest loaded in the rig, and so the headstay should be at least the diameter (and of the same material--i.e. stainless steel wire) as the lower shrouds. Also, whatever diameter you use for the headstay, use the same for the backstay since the angles these stays make to the mast are actually not that different from each other, at least not loaded so differently that you would need to make them different sizes. I have used this guideline for over 30 years, and it works well.

    I hope that helps.


  5. markvannote
    Joined: Aug 2015
    Posts: 1
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Wanchese, NC

    markvannote New Member


    The naval architect and spar designer have already done the work for you. The size of the wire will be spelled out for you in the pin hole size in the chainplate on your deck and the fitting on the rig. If the holes are either 1/2" or 9/16" you are looking at 5/16" wire. If the holes are 5/8" you are looking at 3/8" wire.

    Hope this helps.

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.