Latest News: Forums Technical converting to Centre Main

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  • #3750
    Tonym
    Member

    I have a MK1 GRP rigged as Aft Main. I would like to be able to alter this to a centre main arrangement.

    does anybody have a simple schematic or even photos as to how this is normally rigged ?

    I am not certain if I need any boom fittings.

    Thanks

    Tony

    #7039
    Bob Harland
    Participant

    Tony,
    It depends on what version your boom is. The “modern” booms (i.e. those that were supplied from about 20yrs ago) have a track on the underside that will take a standard Proctor Slide;
    http://www.sailboats.co.uk/Product~Proctor_Heavy_Duty_Boom_Slider_511-714-02.html
    If you don’t have a track then I think a centre main will be more difficult as it’s essential that the boom fittings are strong.
    You will then want;

      i) block with a becket on the aft end of the boom – or a single block and somewhere else to secure the fixed end of the mainsheet. e.g Harken 38mm mainsheet ball bearing blocks
      ii) single block at the centre of the boom
      iii) single block on a strop – on the transom
      iv) swivel jammer plus ratchet block on the boat on the aft slope of the centreboard case. eg Ronstan RF7 swivel base, plus harken hexaratchet 57mm block
      v) some means of stopping the mainsheet loop under the boom from getting caught around the helms neck. This can be another single block on a slide, or I prefer some 50mm webbing, fastened with plastic plates – e.g. RWO 2891

    You will probably be able to re-use some of your existing fittings.
    To fix the centre jammer you may need to fashion a timber base, see http://wayfarer.org.uk/forum/viewtopic.php?t=483&highlight=center+centre+main

    Hopefully these pictures will help too;




    #7044
    Swiebertje
    Participant

    What a great contribution Bob, you must have spent some time on it! I like the pictures. Though I fully support your views, please allow me to make two additional comments.

    I would not recommend the use of a block with becket. In my experience the main sheet then has a tendency to twist. This is avoided by an extra boom slider, a little further back from the sheet block. The bitter end is tied to the extra slider and because of the (small) distance between the bitter end and the block the sheet doesn’t twist anymore. This setup also allows a slightly higher bridle adjustment.

    My second remark is about a cheap alternative to guide the sheet along the boom. A small piece of rubber hose will do the job just as well. I used some left over, one inch hose that I got for free from my chandler, who would have thrown it away otherwise. It is about four to five inches long and cut in to a trapezium shape using 45 degrees angles. The long side is pop riveted to the underside of the boom (self tapping screws are just as good). It is nice and soft if I bump my head against it and it didn’t cost me a penny.

    #7057
    Tonym
    Member

    Couple of questions occur …

    My current Aft main setup has the mainsheet passing through multiple wheel blocks top and bottom (i.e. believe this gives extra purchase). I note your centre main does not have this so does that make it harder to control ?

    And secondly, why do you need to run the main sheet under the boom to the aft at all ? why not just have a block under the boom directly above the block on the aft of the centreboard casing. Hence having no lines aft of the mainsheet pulleys at all.

    any comments ?

    #7058
    John1162
    Member

    I can see what you are thinking. On my old Fireball the centre main was a series of pulleys from the jammer on the centre board case to the boom and back. A friend of mine has this arrangement and it works well. If you intend to go racing you will of course be out of class with this system.

    #7059
    Bob Harland
    Participant

    @Tonym wrote:

    Couple of questions occur …

    My current Aft main setup has the mainsheet passing through multiple wheel blocks top and bottom (i.e. believe this gives extra purchase). I note your centre main does not have this so does that make it harder to control ?

    any comments ?

    Tony, you are correct that having more blocks will give you more purchase/mechanical advantage. The common setup is as you see on our boat which is a 3:1 advantage. In a breeze that usually equates to a reasonable pull on the mainsheet when beating, hence the reason for having a ratchet block to help sheeting in, and of course the jammer when the sheet is steady. In light winds you can switch the ratchet off.
    The drawback of having extra blocks to give 4:1 is that you have to pull/release more mainsheet to get the same boom movement. More blocks also equal more friction. So with a 4:1 setup it is particularly difficult to dump the main in a gust.
    Overall I think the 3:1 setup is the best compromise, but if you are happy with using a 4:1 setup there is no reason for not continuing with it.
    Do make sure the fixed end of the mainsheet starts at the boom, otherwise you just have friction and no advantage.

    And secondly, why do you need to run the main sheet under the boom to the aft at all ? why not just have a block under the boom directly above the block on the aft of the centreboard casing. Hence having no lines aft of the mainsheet pulleys at all.

    If you do any racing then class rules do not permit a full centre mainsheet, you are only allowed to tail the mainsheet at the centre.
    There would also be more strain on the fittings on the boat – especially on gybing. The old Wayfarer Book had a photo of just such an arrangement, so it can be done.

    hope that helps

    #7061
    Swiebertje
    Participant

    @Tonym wrote:

    And secondly, why do you need to run the main sheet under the boom to the aft at all ? why not just have a block under the boom directly above the block on the aft of the centreboard casing. Hence having no lines aft of the mainsheet pulleys at all.

    Another reason to leave the purchase at the transom is the bridle rope setup most of us use. The bridle rope setup ensures sufficient sheet angle to maximize the power of the sail on any course, including close hauled. Almost no leech tension is generated by a bridle setup. (Creating leech tension is the kicker’s function). I don’t see how you can setup something similarly simple with a full center sheet setup ❓

    If you happen to be among the few that still prefer a sheet rail, the story is the same. How are you going to fit a sheet rail in the center of the boat ❓

    #7065
    matoi
    Member

    Dear Tony and all,

    I have just returned from a 9 day cruise. My father and I installed a full centre mainsheet system just before departure. When I catch some time I will post a log with some more details on why/how/consequences/opinions etc… But if you need a good reason to be convinced: I am a novice dinghy sailor, but still we performed a couple of gybes with reefed main and jib up in 35+ knot wind and open sea waves with full control. In my opinion the biggest contribution for this is exactly this full centre mainsheet system.
    Inspite of full centre system’s theoretical (and as I am told by Uncle Al less than minor) negative effect on close hauled course, mainsheet that goes to transom on my Wayfarer (World) is absolutely history.

    Best regards,

    Mato

    #7129
    Dave Bevan
    Member

    Hi, What size sheets are you using?
    I’ve got 10mm braid on the genoa and main and they’re comfortable to handle but I think they may be too heavy, and it means I have very large blocks on the boom and bridle. I’ve got an aft main at the mo, although I’m also thinking of switching to a centre-main.
    I also notice Bob’s block isn’t at the end of the boom? Mine is on a shackle on the aluminium casting on the end of the boom

    #7132
    Bob Harland
    Participant

    The normal diametre for Wayfarer sheets is 8mm – genoa and main.
    I think 10mm is too heavy, and although 10mm is kinder on the hands the sheets will not pay out so easily.

    There are a couple of reasons for having the block on the boom inboard a bit; When using the boom crutch and the tent it keeps everything neatly inside the tent.
    Gybing there is less of a tendency for the mainsheet to snag on the quarters.

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