single spreader mast design

Discussion in 'Sailboats' started by Antc, Dec 31, 2014.

  1. Antc
    Joined: Jan 2011
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    Antc Junior Member

    Hello all

    i have an eboat that is having some mods done to it and we want to, make the rig better, the mast is 8.10m high with a single spreader set up and we are looking at widening the shroud base, bringing the shroud base back and widening the spreaders.
    as i understand it the spreaders want to be at the same rake and the same width as the shroud base. with the shroud entry and exit angle to teh spreader being equal.

    what i am struggling what to do with the shroud base.

    It is currently 10deg back from the mast and 800mm wide, the mast pants like a fat dog even with around 25% strain rig tension so we want to change. would widening just it to 1400 be wise or would we also want to rake back more?

    is there an optimum angle for these things ?

  2. gonzo
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    gonzo Senior Member

    Do you have single or double lowers?
  3. Richard Woods
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    Richard Woods Woods Designs

  4. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    Would it still be categorised as an E boat if you made the change?Or is it not a matter that concerns you?
  5. sharpii2
    Joined: May 2004
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    sharpii2 Senior Member

    Which part of the mast is panting? Above the spreaders? Below the spreaders? Both above and below?

    If it's below the spreaders, widening the shroud base on deck may be all you need to do.

    Widening the spreaders will also cut down on compression loads on the mast, but will require re designing them, giving them a larger section, so they don't buckle, due to their longer length.

    Doing this will create more weight and windage, and make the jenny harder to set (it has to curve around the spreader).
  6. gggGuest
    Joined: Feb 2005
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    gggGuest ...

    The E boat is a pretty numerous and well established boat. They've done a lot of racing miles. It would be suprising to run into a fundamental rig design issue that isn't pretty well understood if this rig is standard. Have you attempted to contact other owners to see how their rigs behave?
  7. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    Concur with ggg. It sounds a bit like the mast section itself is too whippy, see what other owners are using. The masthead genoa/jib does not help, compared to fractional, but you have a backstay to aid adjustment. On smaller boats a small increase in mast section usually cures any panting, another alternative being to sleeve the lower part. Simpler in some ways than adding lowers although there is a weight penalty.

    You also need to check what the spreaders are actually doing. Quite easy to get an S bend in the mast upwind with overlong spreaders and even this may cause a bit of panting. Best thing is to get a set of photos from dead astern with the boat going upwind right on the point of needing to reduce sail. Examine the stills and see what shape the mast is deflecting to sideways. If the spreaders are too short everthing opens up too early, too long and an S bend with closed (and closing) slot which is exactly what you don't want especially with a masthead rig.

    Copied from E-Boat rules on the mast.

    7.2 MAST

    7.2.1 The mast shall be made of aluminium alloy extrusion (not less than 90% aluminium content) with a continuous luff groove which shall be an integral part of the extrusion.

    7.2.2 Subject to Rule 7.2.3 the cross section of the mast in the horizontal plane shall be constants throughout its length.

    7.2.3 Within a distance of 1.68 metres of its top end the cross section of the mast may be tapered.

    7.2.4 The untapered fore and aft cross section of the mast shall be not less than 0.105 metre.

    7.2.5 Any external cross section of the mast in the horizontal plane shall be not more than 0.121 metre and not less than 70 millimetres.

    7.2.6 The external radius of the leading edge of the mast shall be not less than 19 millimetres.

    7.2.7 The external section of the mast shall nowhere be concave with the exception of the groove for the mainsail bolt rope.

    7.2.8 The wall thickness of the mast shall not be less than 2.38 millimetres.

    7.2.9 The weight of the plain extrusion shall not be less than 1.10 kilograms/ metre.

    7.2.10 Distinctive colour measurement bands of not less than 20 millimetres in width shall be painted on the mast to define the limits of P as defined in Rule 6.1.

    7.2.11 The lower edge of the top band shall not be above the upper limit of I as defined in Rule 6.2.

    7.2.12 Permanently bent masts and rotating masts are not permitted.

    7.2.13 The mast shall be stepped on deck such that the maximum dimension for J as defined in Rule 6.3 is not exceeded. The mast heel shall not be moved whilst racing. The bearing point of tube or sheave that supports the spinnaker halyard shall not be more than 61 millimetres above the upper limit of I and shall not project forward of the forward face of the mast by more than 92 millimetres. The spinnaker pole attachment point on the mast shall be no more than 1.96 metres above the deck. A band may be required to define this limit.

    The angle outward from the mast is more important than the angle back as it provides the side support. If that is also 10 deg + it should be more than adequate, I believe somewhere around 8 deg is the minimum to actually do anything useful witout adding too much compression. You should have a system that allows different rake aft either by pins or dynamic anyway to tune for different wind strengths.
  8. Martin B.
    Joined: Aug 2013
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    Martin B. Junior Member

    If you are not bothered about E Boat class rules then consider going to a double spreader rig. Had a similar problem with a narrow beam light displacement John Spencer Scimitar 35' with a slender section mast and the mast pumped fore and aft and bulged/buckled sideways above and below the spreaders.
    With a bit of basic geometry, worked out that with the same shroud to mast angle at the head but shorter and higher spreaders there was little or no change in shroud and mast loads. Added a second set of (now lower) spreaders with same intermediate shroud to mast angle and nominally same length as the (now) shortened upper spreaders. Shortened the original lower shrouds to suit and it all worked out exceptionally well. Genoas no longer fouled on the spreaders, mast sideways bend completely eliminated/controlled and pumping easily controlled ( via an occasionally used only) light weight pair of running "backstays " originating from the location of the original mid mast spreaders ( no new holes there) down to aft end of cabin sides.
    The main aim was to reduce the slenderness ratio of the mast section - forgotten the exact engineering something like "panel" length of mast over it's section modulus; the 'new' panel length was now only 1/3 of mast length not 1/2 so L/Z ratio much better.
    Sure the current engineers will correct my engineering but been retired some 20+ years now!

  9. Antc
    Joined: Jan 2011
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    Location: Rutland

    Antc Junior Member

    thank-you for all your replys
    Firstly on the E-boat class rules thing I do not care, there is no OD racing any more and so Race IRC anyway. i do not use the number 1 so the number 2 would sheet inside a wider shroud base,

    the last 'Eboat' i raced against was called witchcraft in the solent and she had changed her rig to a double spreader set up, with a bigger main and a smaller genoa prompting this debate.

    mast panting is an established problem with the Eboat, (people use to lock the spinnaker pole to the bow from the mast upwind )

    when i have looked up the mast in 18-20kts the mast pants with the waves but there is minimal if any deflection sideways
    the whole of the mast pants and we run quite high tension on the shrouds and the lowers to minimise this.

    the reason i would want single wider spreaders is because i think this would be the simplest solution to the problem, the benateau first 21 has the same section as the e boat with bigger sails and much wider spreaders and shrouds, and the mast behaves a lot better (admittedly it is a 7/8 rig)

    i have just measured the mast and it looks as though the angle at the hounds is about 11.4 degrees

    the spreders are 740 mm long (from the outer edge of the mast) with a 100 mm deflection so i think about 7.57 degrees

    is there an established set of 'rules' to shroud base / angle that is out there as i think wider is the way to go

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