Latest News: Forums Technical MKIV setup – mast pre bend

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  • #3923
    Dave Bevan
    Member

    We’ve had our new boat (MKIV racer) for a few weeks, and just checked our mast rake/prebend measurements.
    We had to move shroud-plates down one to get correct mast rake (it was OK, but at the minimum), and also move heel pin forward to get correct pivot-pin position, but we’re still lacking mast pre-bend (only getting about 10mm).
    I note that the spreader-blades have fore and aft slots for the shrouds and ours is in the foremost slots. Can we move them into the rearmost slot to give us more pre-bend, or is it necessary to change the spreader deflection further aft? – yes, I know we’ll need to check the other measurements too!

    Hartley recommended settings
    Rake 23’9″ to 23’11”
    Tension 350lbs
    Prebend 40mm

    #8134
    Colin Parkstone
    Participant

    Dave,Hello.

    If when you look up the shroud line from base to hounds the line is deflecting forward at the spreaders with all the tensions on then the mast is being held straighter.

    Your find that if you let the spreaders aft it will then let the mast prebend more until you get to the 40mm prebend point.

    Having seen some Hyde sails and the masive luff round they have the 40mm is probably what is needed to make them work!!!

    If your going to use any other make your have to go back to the prebend that sail needs and not what is used for the Hydes

    Also, when you set the prebend,have the chocks out. Then when you have the prebend put them back in to hold the bend!

    Hope that makes some sense ?

    Good Luck,

    CP 🙂

    #8135
    Dave Bevan
    Member

    Thanks Colin, All makes sense.
    I wanted to check if I can use the other (aft) slot in the spreader tip to achieve the required pre-bend – I assume this will have the same effect as changing the spreader angle aft, but wanted to ensure there isn’t a good reason for not doing it this way?
    Just for confusion (mainly mine), we have McNamara race sails, and Hyde cruising sails. The settings on my first post are for Hyde sails.

    #8145
    Colin Parkstone
    Participant

    Dave, I have not yet seen the second slot in the arms but if it is open at the aft side of the arm it may pull out if your arms are pulling the shrouds forward ???
    We have a new boat at the club ,so I will try and have a look for you!

    C P 🙂

    #8151
    Dave Bevan
    Member

    I found this on the Selden website. http://www.seldenmast.co.uk/technical_support/spreader_end_cap.asp
    Looks like it is OK as long as you use the correct-sized side of the end-cap to clamp it in.

    We increased the spreader length (they were about 1/2″ short) and moved the shrouds to the aft slots to achieve the correct pre-bend for our McNamara sails.

    #8153
    Colin Parkstone
    Participant

    Dave and all Mk4 owners a heads up!!
    Please talk to Mark or Richard but the local Hyde sails owner has been told that the prebend should be 4mm, not the 40mm quoted.

    C P

    #8160
    Dave Bevan
    Member

    Our setup guide (for Hyde sails) gives a prebend of 40mm, but spoke with Richard today, and he recommended around 20mm (certainly not 4mm!). 25mm is recommended for our MacNamara sails. Be interested to hear other MK4 sailors’ experiences, and hopefully I’ll pickup some tips at the inlands next weekend.

    #8162
    Swiebertje
    Participant

    @Dave Bevan wrote:

    I wanted to check if I can use the other (aft) slot in the spreader tip to achieve the required pre-bend – I assume this will have the same effect as changing the spreader angle aft, but wanted to ensure there isn’t a good reason for not doing it this way?

    The answer is Yes, or rather it does not matter which slot you use since you are measuring from the shroud to the mast (or from shroud to shroud).

    Perhaps Collin can answer me this:
    In all these years it has been unclear to me what point on the mast we use for this measurement. It makes a significant difference whether we measure to the front, middle or back of the spreader bracket. And don’t say “use the pivot pin” because the latest spreader bracket allows the pivot pin to be moved around as well.

    #8163
    bigal
    Member

    Ian Porter held a teach-in at Shoreham last week-end and said you measured to the wall of the mast at the point which represented the tangential point measuring from the shroud !

    I think this is the same as saying take the shortest distance -queries to Ian please !!

    #8164
    Colin Parkstone
    Participant

    Swiebertje,

    Can you help me in return?

    When you say (what point on the mast we use for THIS measurement)

    What is This?

    When I was talking about mast pre bend, it was the measurement at spreader brecket height taken from the main halyard to the rear face of the mast. ie 40 mm /4mm or any other?

    If your talking about the spreader length, i take it from the shoud wire to the nearest side of the mast, ie shortest distance.

    Any Help ?

    CP 😕

    #8167
    Swiebertje
    Participant

    I was referring to the spreader length measurement. Until now I was guessing it was the shortest distance but I have never seen this mentioned in any of the set-up documents. I have accidentally done it right all the time 😉

    #8171
    bigal
    Member

    No accident – they call it experience at our age !!!!

    #8172
    Fantasia
    Member

    I have noticed that the rig geometry of the Mk IV is very similar to my Mk 1 woody across the beam. These are the settings that I use:-

    My boat is a Mk 1 Porters woody and I use Pinnell & Bax sails, measurements taken with the rig under tension of 350 pounds, standard Selden mast etc.

    940mm across shrouds at spreaders. Spreader length 515mm (shortest distance). Spreader deflection 205mm. giving a prebend of 42mm at the spreaders. I sail with the mast raked at 23′ 5″ when we expect to be overpowered and at 23′ 7″ for light winds.

    These settings are fast upwind for medium and strong winds. Very light winds are another story as I and/or my settings are slow.

    The fore and aft geometry or the measurement point for rake is obviouslt different for the Mk IV

    #8174
    bigal
    Member

    ian P adds 3″ for the rake and also appears to have added an inch to previous suggestions – the new ones are thus 23’9″ to 23’11”

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