Cox's Bay Skimmer

Discussion in 'Sailboats' started by Doug Lord, Jan 11, 2014.

  1. petereng
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    petereng Senior Member

    Hi Gary,
    How are the masts made? Are they timber or composite? Weve just got our "new" tri on the water and I'm thinking about a big wing for it. Regards Peter S
     

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  2. Gary Baigent
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    Gary Baigent Senior Member

    Here is an earlier reply for Flash Harry's wing mast. The Skimmer's masts are constructed in a similar way to the Sid beam interior photograph, except instead of a box section central beam, is a simple single scarfed together I ply beam. Otherwise the same. Skimmer's masts are very light, have hung in there well.
    "For what it is worth Hydro, here is the plan of Flash Harry's 30 foot wing mast. It is a simple and inexpensive construction of tensioned 3mm ply over frames attached to a 4mm ply I beam and is 22 years old now - and has suffered a hard life with absolutely no problems. The mast with halyards and rigging weighs 20 kgs. You could build an extended version for your Shuttleworth and it would do the job. Carbon uni-directionals are used in the high load and bending areas, that is the hounds beak fitting, wrapped halfway round the wing and in the thickest of the chord section running up and down the halfway area between hounds and base. If you want a straight luff, just reverse the leading and trailing edges. The alloy sail track is again a simple but effective extrusion used for curtains (you have to bench saw off the too wide flanges) from Alcan here in NZ. You could find the equivalent in your country."
     

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  3. Gary Baigent
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    Gary Baigent Senior Member

    Just put 2 reefs in the Skimmer at Point Chevalier on the Waitemata; very sheltered and South Pacific in close, different outside where we struggled with full rig. Skimmer looks a bit odd with reefed fore main but sails better/safer with it, then you just have to play the main. Rosie Bogle photograph: they live right on the water edge.
     

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  4. Tom.151
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    Tom.151 Senior Member

    Am interested in learning what the section shape is for Skimmer's masts - a "standard" section of some kind? Are both masts the same section - chord? - thickness?

    If you were to build a 'next generation Skimmer' would you have any reason to alter the mast sections? What's your thinking about the chord of the masts as a percent of the total chord (mast + sail) for a boat similar to Skimmer (same LOA and sail area, but more weight ;) ) - keep them the same, smaller/larger as % of total?

    Thanks,

    PS love the later pic of the reefed fore main - wicked cool
     
  5. Gary Baigent
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    Gary Baigent Senior Member

    hi Tom, I can't find the plans/drawings of the Skimmer's mast section (maybe the original client has them) but here is Sid's wing mast, which is similar proportion but elongated a little to 500 mm chord x 155mm thickness - in your language close to 20 x 6 inches. The Skimmer's are 300 x 100mm or so, 12 x 4 ins - so you can see the ratios I like, which are not scientific because I'm more an arty farty than scientist - but which seem to work well. Both masts are the same, the sails ditto, and the foot measurement is around 2 metres (have forgotten exact figures but will measure them later today).
    If I was building a Skimmer bis of 6.5 metres, think I'd keep the same ratios (because once the whole boat and heavy cradle got blown over on the hard in hard wind) but if you wanted to be greedy for speed, sure, increase the mast chord/thickness and height... and then lower them if moored for a length of time. On Skimmer bis, would lift the cockpit floor a few more inches and have DSS foil going across the boat just forward of the daggerboard case - and that extra righting moment and power would handle your extra mast height and increased sail area.
    The Skimmer, as is, carries 21 or so square metres of sail and masts and is quite powerful enough for two crew; solo, you have to reef early. With two you still have to reef the fore main at 14-15 knots TWS, in 20TWS, two reefs forward.
    The second plan is Groucho Marx's mast and the other two images are Skimmer bis 6.5m.
     

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  6. Doug Lord
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    Doug Lord Flight Ready

    bis

    Gary, again you have taught me: I had no idea what "bis" meant so I looked it up -"second version of a thing". Never saw that usage in mine entire life and I thought I read a lot.....
     
  7. Tom.151
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    Tom.151 Senior Member

    Thanks very much Gary, very kind and generous of you to provide the info and your thoughts. Look forward to the actual dimensions.

    How would you describe the cut of the sails... approx how much camber? and how far (%) back from the front of the mast?

    Just so I'm not missing something here... are the current mast profiles the same shape/size as the previous rotating masts? I'm wanting to have a freestanding rig (as the original Skimmer was). Yes, i'd probably try to rig an asym with the halyard taken back to the windward rail to provide a bit of runner support for the mast.

    If you ever come across pics of the rotating bits of the mast and base they would be a great help.

    Sincere thanks,
    Tom
     
  8. Gary Baigent
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    Gary Baigent Senior Member

    Sorry for late reply; the sails are cut dead flat, no camber ... because with rotating mast(s) you induce camber by the amount of rotation you set the mast spanner(s) to. In heavy airs you can flatten spanner and decrease sail camber - but it is better to reef down and keep normal to flat camber - because there is some drag set up in the luff area by the centred mast, which doesn't help when you want to de-power, just imo. In downwind sailing you over-rotate the mast; in fact I like the mast to be always over rotated to get a sweet flow of to leeward air onto the sail - and downwind in lighter airs you'll find, to get this flow correct, the mast will be set at near right angles to the centreline of your hull.
    First shot is Skimmer with unstayed masts; slightly difficult to see rotation controls but second shows the Groucho setup: very simple, double purchase going to cleats.
     

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  9. petereng
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    petereng Senior Member

    Hi Gary, so if the sails are dead flat you use very light battens with no shaping? So you could cut the sail from a single piece of fabic if available? Also you take the boom to the mast foot, have you tried the boom to the deck to remove the torque? or is the torque from the boom desirable? thanks Peter
     

  10. Gary Baigent
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    Gary Baigent Senior Member

    Yes, you could have the sail made from flat material.
    The top three battens I use are stiffened with carbon fibre because the originals were too soft, but the lower ones are standard glass versions.
    You could take the boom to a separate, fixed base position - but you get some distortion (because the connection will probably be near the bearing pivot point) between sail and boom near the tack area when the mast is fully rotated; the greater the mast chord, the more the distortion.
    But with boom connection on the mast base tack point, mainsheet tension pushes the mast round to the rotated position, which keeps the mast steady, doesn't flap. A flapping mast kills sail flow and performance.
     
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