Improving stability of a fractionally rigged mast.

Discussion in 'Sailboats' started by Midday Gun, Apr 7, 2019.

  1. Midday Gun
    Joined: Mar 2019
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    Location: UK

    Midday Gun Junior Member

    As per the title.

    Pretty standard setup, single spreader 7/8ths swept fractional rig, shrouds to the deck, tie bar to the hull.
    The problem I have is that the mid section of the mast has a tendancy to pant forward at the hounds in heavy weather & strong winds.

    Normally this would be because the swept back lowers aren't tight enough, but the rig is already setup with the caps @ 25% breaking strain & the lowers also very tight to pull the mast back into column. With the backstay on hard the situation improves a little, but there is still a worrying amount of movement.

    The boat hits a wave, the midsection pants forward, increasing the bend of the mast, this means the hounds move downwards, which then lets the forestay sag to leeward, not good for speed when racing, and not good for my peace of mind when punching into a sea even when cruising.
    This video shows the movement I can achieve even at the dockside, this is the rig fully setup tight with the backstay on hard.

    VID_20180505_113601 - Streamable

    We're getting some deflection of the hull where the chainplates terminate, which can't be helping matters, and there are plans afoot to beef up these attachments, but before we dive in we're looking at making some changes to the rig geometry to try and improve the mast stability.

    Options we're looking at are:
    - Leaving it as it is & beefing up the chainplates
    - Adding rake to the spreaders & moving the chainplates further aft
    - Keeping the same rake & move the spreaders outboard
    - Add more rake & move the spreaders outboard.

    Overlapping headsails are not a concern, we only sail with a 110% which sheets inside the shrouds.
    Runners & Checkstays are off the cards, big rating penalty & I'd rather have a rig that's stable without them.

    Reading material on rig modifications seems pretty sparse online, could anyone comment on how those options would affect the rig stability, or give some ideas or suggestions then it would be greatly appreciated.

    IMG_20190307_152307.jpg IMG_20190308_114804.jpg
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    You could use running lower backstays during rough weather.
  3. Midday Gun
    Joined: Mar 2019
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    Midday Gun Junior Member

    I could, but I'd rather not or at least its a last resort.

    Just having them is a rating hit, and it feels like something of a band aid. I'm just interesting in exploring potential modifications to the rig.

    I know that on the south coast, some of the refitted quarter / half tonners have moved to external chainplates and got rid of their runners, and they just use small genoas that can sheet inboard of the shrouds.
    Unfortunately nowhere local to me as done anything similar so I've not much to go on.
  4. philSweet
    Joined: May 2008
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    philSweet Senior Member

    This is why double lowers are used on so many boats. But switching up a size on all the shrouds would probably help a lot. The video shows a vortex train driven oscillation. At the dock, this can sometimes be disrupted by winding a line around the mast 4 or 5 turns in a spiral (At the least, it cuts down on the halyard slap). What boat? What wind speed causes the vortex panting and what are the X and Y dimensions of your mast? What are the shroud diameters?

    Light bendy racing boats can't be fixed. The whole boat flexes in a seaway and the thin stretchy shrouds actually lower stress concentrations. But on a stiff boat, you should be able to size the shrouds to stop that. Using the breaking strength of the shouds, they should have a total strength of about 10 times the boat weight on each side of the boat, and be pretensioned to about the boat's weight. This doesn't work for large boats, but should be a good ballpark for yours. I hate panting boats.
  5. Midday Gun
    Joined: Mar 2019
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    Location: UK

    Midday Gun Junior Member

    Actually the video shows me shaking the base of the mast to demonstrate the movement I'm getting, most other masts are either solid or at least a lot more damped.

    Shrouds are 6mm dyform, orginally 6mm 1x19.

    Boat is a Contessa 27, Rob humphries design, mast is a proctor section, maybe a little on the light side, but I'd still not expect movement like I get. I think the minimal spreader sweep doesn't help. - CONTESSA 27 Sailboat
  6. CT 249
    Joined: Dec 2004
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    CT 249 Senior Member

    That really does appear to be very little spreader sweep. I can't recall the sweep on my 28'ers spreaders, but I'd guess my boat has 5 degrees more rake. The chainplates would be about where the set of stanchions aft of the mast lie on your boat. Even with a flexy old 2000kg wooden boat I don't think I get that much movement when I rattle the mast in a similar fashion, despite the fact that I use very low rig tensions due to the boat's age and the fact that I'm running a very light Etchells/J-24 mast section that is deck stepped.

    I found Matthew Sheahan's book on rigging to be interesting; it's cheap second-hand. I can't recall exactly what I used it for, but it's worthwhile looking at.

    One thing you could look at doing is experimenting with a mini-stay running from the chainplates to the gooseneck. It's not a perfect idea but I had them for a while when I was using an in-line rig and had seen them on the old Paul Whiting or Murray Ross designs. By restricting bend right low down they seemed to lock up the mid section of the stick. They are dead easy to jury rig for an experiment, and would probably only cost one pip under IRC. I did extend my stays with ss plates at one stage during my rig experimentation, so if you do decide to change spreaders and stay base you can extend the existing stays for very little financial cost, although there is some cost in appearance.

  7. Midday Gun
    Joined: Mar 2019
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    Location: UK

    Midday Gun Junior Member

    Hey thanks, I'll try and get a copy of that book to read through.

    Don't mind paying for new shrouds if it makes a difference, I'll try your idea to the goose-neck, though I'm not sure if it will work, I can't put a huge amount of tension on it because the chainplates aren't designed to take that direction of load.
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