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Forestay Oscillation  Bottom

  • I had something weird happen last weekend and thought I'd see what everyone thinks. For those that don't know, on our Dart 18, the forestay is connected to the jib. I attach the top of the forestay to the mast, step the mast and then tie off the bottom of the forestay to the furler.

    I was out sailing and noticed that when the wind was coming over the starboard hull, the forestay began oscillating forward and back. I was watching the furler move front to back what seems like several inches. More power meant more oscillation. I tried to diagnose the problem on the water without complete success, but did notice that my port trapeze line seemed to be tangled in my port side stay.

    Anyone have any idea what was going on?

    Dana

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    Dana, Holly, Emma & Hannah

    LJ/Stu's Dart 18
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  • Were you furled or sailing with the jib?

    sounds like you were lacking forestay tension

    it should be snug in medium air and as the saying goes "the more she blows the tighter it goes" (stay tension)

    it is kind of tough to gauge the correct tension and near impossible to have it at optimal tension per any wind condition

    and as you know, we have very variable weather here - what you rig in will probably not be what you sail in at the end of the day - so we set our tension somewhere in the middle of what we have and what we expect.

    but at no time should your entire forestay go into heavy oscillation - you are knocking the air off your sail and it's not good for your air flow on the main, plus probably educing some drag

    Pros and serious racers use a loos gauge to keep it (and diamond wires) set to known optimal settings for each wind condition


    next time you rig ask one of us regulars to check your tension

    (despite being nice, do not ask the old guy in the short shorts (ed) who's wife speaks with a heavy latvian accent (ace: aka - grandma in latvian) who helped you rig last week - he doesn't have the knowledge despite owning a few cats

    trap wires seem to be magnetically attracted to side stays for some reason and love to wrap eachother around eachothery usually, you can simply pull the trap handle aft and it should un wrap.

    unless you untie your trap wires or side stays during tear-down, there really is no way for them to actually get "tied" together (although it seems to happen from time to time)



    Edited by MN3 on Nov 14, 2017 - 09:20 AM.
  • Thanks Andrew! I suspected that my forestay may not have been tight enough. I tried to get good purchase on it, but I may need to really give a good yank to make sure I'm tight enough.

    On Ed, he actually approached us and offered to help. I didn't really need the help, but I felt funny turning him down. It was a good lesson on what happens when you get "help."

    Thanks for tip on the trap wires. They seemed to magically untangle when we came in.

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    Dana, Holly, Emma & Hannah

    LJ/Stu's Dart 18
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  • Hello Dana, On my 18 square, while on the beach, I run the mainsail up the mast and hook it, attach the main sheet system, and then pull the traveller over to one side and sheet hard. At that point the side stay should go slack and you can adjust the pin down one or two holes. Then release the main sheet and pull the traveller to the other side to repeat the process. Make sure that you use the same pin setting on both sides. On a windy day, you may have to move the boat a little so that you don't power up the sail when you sheet in. Once you have tensioned the rig, go out for a sail and make sure that you do not have too much weather helm. This method rakes the mast back a bit. You do normally want just a little weather helm, but you should not have to pull hard on the rudders to keep her going straight. If you do get too much helm, just release the rig tension, and see if you have a bit of adjustment on your jib / forestay to rake the mast forward a little before re-tensioning. The loos gauge suggestion is a good idea to see what tension your boat likes best.
    Hope this helps.

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    1991 Nacra 18 square
    1979 Prindle 18 "Tigger"
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  • keep-it-simpleHello Dana, On my 18 square...


    Alright another 18sq on here!

    As for the forestay slack if you don't have mainsheet tension your forestay will always be relatively slack.

    Look at your leeward shroud when driving the boat, it will always be loose even if the rig started pretty tight. When the sail powers up and bends the mast it's height effectively shrinks slightly and causes the leeward shroud to go a little slack. The windward shroud also stretches a little bit as well. Without any mainsheet tension that slack can allow the mast to lean forward and give you some forestay slack unless your mainsail holds the mast back and generates forestay tension.

    The tension method above with using the mainsheet stack even works if you just use the halyard, assuming you have a decent quality halyard on there. The more pretension on the rig the less the rigging will stretch but the mast bending is still going to allow slack to form anyway.



    Edited by tamumpower1 on Nov 18, 2017 - 05:20 PM.

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    Matt
    '82 NACRA 18 Square "Bangarang"
    '85 Hobie 18 "Honey Badger Don't Care"
    '86 Hobie 18 "The Rippin & The Tearin"
    Clearwater, FL
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  • Matt’s post above hit the nail on the head.

    If your forestay is moving around, it is because you aren’t sheeting in the main enough. Rig tension won’t really make a difference once the sail is loaded up - even with all the rig tension in the world, if you aren’t sheeted hard enough, the forestay is going to sag.

    I also second the notion of using the main halyad (not the mainsail) when adjusting rig tension. If you use the sail to support the rig, one wrong puff and either the rig or the whole boat is going over.

    sm
  • It is incorrect to say that the mast ‘shrinks’ or the stays ‘stretch.’

    The mast bends to leeward, true. Think about what that effect would have on the lee shroud and you’ll see why the above assertion is incorrect.

    The main thing that happens is that mast compression increases. This is counteracted (mostly) by the prebend in your x-beam (dolphin striker tension etc.) The hulls also flex. These are the factors which affect your stay tension changes under load. The mast does not shrink. Given the design of the typical beachcat, it’s virtually impossible to keep the platform from flexing to some degree as the mast presses down.

    So, if your lee shrouds seem too loose, tighten the rig overall. Google is your friend to see if someone has published the specs. But while you’re Googling, be sure that you have the correct prebend in your x-beam and fwd x-beam. That said, a little lee slack is to be expected.
  • No one said that the mast itself shrinks. Matt said that the mast height shrinks (not the exact word I would have used, but still correct). When you tension the mainsheet and downhaul, the mast bends. The mast also bends simply from wind pressure acting on the sail. Any time the mast bends, the distance between the mast hound and the mast base gets shorter which causes the stays to loosen. The mast has not shrunk, but the distance between the top of the mast and the shroud anchor points has decreased.

    sm
  • Hello Dana,
    The point about sheeting hard is a good one also. No Darts in my area, but she looks a lot like the Tornado I used to have (makes sense since they are both designed by Rodney March).
    I know that I had much better control once I learned to sheet hard in windy conditions with the Tornado and I would bet your boat will be similar. You should have a lot of fun with that one. Please do let us know if the suggestions help.

    --
    1991 Nacra 18 square
    1979 Prindle 18 "Tigger"
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  • I added "effectively" to my post so it passes the technicality filter

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    Matt
    '82 NACRA 18 Square "Bangarang"
    '85 Hobie 18 "Honey Badger Don't Care"
    '86 Hobie 18 "The Rippin & The Tearin"
    Clearwater, FL
    --

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