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  • #3875

    Having followed Paul Wrens set up guide to tuning the mast I have reached the stage of mast pre-bend.
    The pre-bend is approx. half an inch. Clearly it needs to increase.
    Paul’s guide says “adjust the spreaders backward a small amount.”
    P51 of Wayfarer book says “To some extent this distance can be adjusted by means of mast chocks. Alternatively you will have to use a different combination of spreader angle and chocks.”
    What might an experienced tuner advise? Could an explanation be given regarding the use of chocks at the front of the mast?
    John.

    #7869
    Anonymous
    Inactive

    How much pre-bend you have depends on a number of things, such as the cut of the mainsail, the stiffness of the mast, the weight of the crew, the amount of kicker / cunningham applied, and of course the weather conditions.

    Winding the spreaders so that they deflect the shrouds aft will encourage the mast to pre-bend, with the degree of bend increasing proportionally with rig tension.

    In heavy airs when you are tending to use a lot of kicker then having no chocks in front of the mast will allow it to bend a lot to flatten and de-power the sail.

    In medium winds where the boat is underpowered you typically want to resist the tendency of the mast to bend in order to keep the sail full. Thus you put chocks in front of the mast so that applying kicker powers the leach up but doesn’t flatten the sail.

    The other extreme is very light conditions, when you want the sail flat but without using kicker so as not to hook the leach. This you can achieve with spreader deflection and by putting chocks behind the mast gate if there is something solid to chock against – or with a mast ram or lever that can pull as well as pushing.

    #7870
    Colin Parkstone
    Participant

    John,

    Have you got the 1/2 in prebend with or without any chocks in ?

    I would try to set the mast up without the chocks in at first,get the prebend you want, then put the chocks in to hold that position.

    Make sure the pin is loose in the tabernacle at all times!!

    C P 🙂

    #7873

    Thanks very much for the help. At the sailing club last night I was given the very general advice regarding controls that it was very important to have a powerful kicker and cunningham/outhaul to depower the rig in a blow. As a new racer in a mixed fleet the advice was to enable me to depower the rig. Today still on my learning curve without adjusting the pre-bend I managed to bend the mast a significant amount and the cunningham moved the luff significantly. As you can see I am a beginner re set up matters.
    John.

    #7876

    Surely the only purpose of pre-bend is to match the mast to the luff of the sail?

    After that, once the sail is hoisted, you are no longer in “pre-bend” territory, but into “mast-bend”.

    Longer spreaders, with enough rig tension to make them work, will push the mast forward in the middle = pre-bend.
    Spreaders swept back more, with enough rig tension to make them work, will do the same thing.

    Once you have “pre-bend” in the mast without any chocks at deck level, then people tend to put in the chocks to REMOVE the bend, so they start sailing with a straight mast.
    Then you hoist the main, and this pulls the top of the mast back = bending. This also moves the effort in the sail backwards: hence the cunningham to pull it forwards again.

    Set the spreader length and angle to the “class norm” (tuning guides from the sail makers, normally).
    Pull up the jib to give the required tension (you need a tension guide to know when 350 to 400 lbs has been applied). Check the mast rake, if necessary adjust shrouds and job halyard.
    Then chocks to give a straight mast.
    Then hoist the main, and use enough kicker to give a good shape in medium winds. Less in light winds and much more when it blows to bend the mast and flatten the sail to de-power.

    #7877
    Colin Parkstone
    Participant

    A couple of points,
    The mast gate on the Wayfarer is longer than the movement the mast will move because of the mast pin.

    This must not have any weight on it as it will damage the mast and tabernacle.

    So the first chocks are needed to stop the mast going any further forward at deck and pin level when, you have the desired pre bend

    Then when you add more chocks in front of the mast it will push the mast aft and straighter till the pin then binds again.

    C P

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