Boom Loads on 24 ft Cat

Discussion in 'Multihulls' started by YoungGrumpy, Mar 26, 2015.

  1. YoungGrumpy
    Joined: May 2012
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    Location: New Jersey

    YoungGrumpy Junior Member

    Boom Loads on 24 ft Cat
    Since the snow is almost melted, spring commissioning time!
    As a result of a time/budget restrictions, I’ve sailed last summer with a repurposed boom (from something like O’day 30). The boat is Seawind 24, with the Main’s size Luff 30 ft, leech 31'5" and Foot 11'6".
    That boom is helluva strong and soo heavy, so I would like to lighten it up a little bit.
    How light can I go, and what mods would that require?
    Like, instead of using SS bracket, attach the mainsheet tackle to a soft belt (what is the proper term?) below the boom, and the sail’s clew above?
    All the comments and advice are greatly appreciated.
     
  2. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Gday Grumpy

    I am not really sure why multi have strong booms - almost none have a vang. My 23ft cat is getting a carbon sailboard two piece mast as a boom, complete with rubber universal as a gooseneck.

    If you attach the mainsheet directly to the clew loop the boom is only in compression. Of course in reality the boom will be bent when someone leans on it or it hits a stay. I would go light and keep the loads and hardware off the boom if you can.

    As for reefing try leading the lines forward to the mast.
     
  3. Jim Caldwell
    Joined: Aug 2013
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    Jim Caldwell Senior Member

    Put a load sensor on your stay and you will find out!
     
  4. YoungGrumpy
    Joined: May 2012
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    YoungGrumpy Junior Member

    The last answer shows a depth of an education added to the width of knowledge. Or, the other way around?
    The load sensor will show the load on that stay, and how much good will that do for me? :confused::rolleyes::!:
     
  5. YoungGrumpy
    Joined: May 2012
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    YoungGrumpy Junior Member

    Catsketcher,

    How big is your main? What happens when you reef?
    Thanks
     
  6. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Many multis go the boomless route - some Farriers have done it but it can be a pain downwind. I think that you need to start with the basic engineering premise of the part - for a 24ft cat with no boom that is pretty low compression loading.

    For reefing you need to get a little innovative. I have not built my rig yet but the boom will be a two piece carbon mast from a sailboard. The sheet will attched directly to the clew of the main. On reflection I think there is about 50cm between the sheet and clew attachment on the Seawind 24 so you may need beefing up.

    If you put the winch on the boom you need a bigger section to handle torsion loads. On my wishbone cat I reef at the mast. On the 7.3m cat I will need to take the lines forward. Single line reefing is what I am after and the lines will lead to the back of the cockpit. So again the boom is only under compression.

    I like composites so the above approach works for me. On top of this I need a small light boom that comes apart for trailering so the two piece is my choice. Could be different for you.

    Main luff - approx 8 m Foot 3m

    cheers

    Phil
     
  7. teamvmg
    Joined: May 2008
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    Location: christchurch,uk

    teamvmg Senior Member

    if your mainsheet takes off the boom from the same position as the clew of the sail, the boom does very little and a prodder boom will do

    Some boats have long boom and short sail so the loads on the boom are a lot more

    Yes, reefing can change the whole geometry of the set up and, althought the sail is smaller, the loads on the boom can get higher
     
  8. Petros
    Joined: Oct 2007
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    Petros Senior Member

    you can not design for all possible loads that might occur on a boom. one unplanned for misshap would damage it. boom strenght has evolved based on a long history of similar designs, find a boat with a similar size sail and see what they use.

    Or you can go light, and if it breaks, build a stronger one. repeat until it does not break. that is how the current sizing rules were developed.
     

  9. ThomD
    Joined: Mar 2009
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    ThomD Senior Member

    Presumably something like the boom on a Hobie. If that seems too heavy, how much lighter would you want it to be. Once you figure that target, and you know the kinds of materials you can work with and afford, all you need to do is think about how to arrange them to get the most out of them, and then think on whether you have built a superior, or too light structure. Short of a lot of engineering talent, the easiest is to manage problems. Identify the weight savings you need, think of the best way of getting them and either re-design or build.
     
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