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  • #4114
    Algol
    Member

    I’m thinking of converting my Mk II to centre main.

    Does anyone know of a diagram online anywhere showing what I need to make the conversion?

    I think I need:

    A block on the bridle above the transom; a block above that at the end of the boom (sheet becketted here?); a loop to stop the mainsheet dropping onto the helm; a block above the swivel jammer; block on swivel jammer.

    Or . . .

    A block on the bridle above the transom (sheet becketted here?); block at end of boom; a 2nd block just in front of that one; loop; block above the swivel jammer; block on swivel jammer.

    The 2nd version gives more mech advantage, right?

    What dia mainsheet would I use? 8mm?

    And I’m wondering about the bridle. What control do I need over it? Up and down, allowing the transome block to be raised and lowered? Side to side, to allow off centreline sheeting? (<<<< these will effectively do the same thing, won't they? IE, allow the boom to be sheeted with a slack leech?) And does the bridle need to be adjustable from both sides of the boat by the helm?

    #9008
    Swiebertje
    Participant

    @Algol wrote:

    Does anyone know of a diagram online anywhere showing what I need to make the conversion?

    Yes, here: http://www.wayfarer-international.org/WIT/race.related/RiggingTips/UncleAl/Mainsheet/mainsheet.html
    @Algol wrote:

    I think I need:
    A block on the bridle above the transom; a block above that at the end of the boom (sheet becketted here?); a loop to stop the mainsheet dropping onto the helm; a block above the swivel jammer; block on swivel jammer.

    Correct, but no becket is needed. If you tie the bitter end to an eye on the boom you can pull in the boom in an inch or two further. The bridle block will then touch the boom rather then the other block. Another nice feature is that you can untie the sheet from the boom end and have a single purchase for light weather sailing. Obviously you need a figure eight knot in the end to prevent the sheet from sliding through the bridle block.
    @Algol wrote:

    Or . . .
    A block on the bridle above the transom (sheet becketted here?); block at end of boom; a 2nd block just in front of that one; loop; block above the swivel jammer; block on swivel jammer.
    The 2nd version gives more mech advantage, right?

    Yes, but it’s far too much purchase. You don’t need it and think of all that rope you need to pull in.
    @Algol wrote:

    What dia mainsheet would I use? 8mm?

    Yes, though smaller would be better but 8mm is nice on the hands. I use a 10mm sheet, just because it is nicer to handle. An expensive alternative is a 8 or 10mm tapered sheet, thin and light where it needs to be and thick and soft in the hands.
    @Algol wrote:

    And I’m wondering about the bridle. What control do I need over it? Up and down, allowing the transome block to be raised and lowered? Side to side, to allow off centreline sheeting? (<<<< these will effectively do the same thing, won't they? IE, allow the boom to be sheeted with a slack leech?)

    No control needed. Once the bridle length is adjusted there is no need to change it. Even those that have an adjustable bridle hardly ever use it. More bridle tech here: http://www.wayfarer-international.org/WIT/race.related/RiggingTips/UncleAl/Bridle/98Bridle.htm

    #9017
    Algol
    Member

    OK, so I bought a load of lovely Harken gear to convert to centre main.

    Now I need to plan to make a block to sit on the sloping rear of the c/b case to screw the swivel jammer onto.

    Question: What height should the swivel jammer be at?

    As my plans are at present the swivel jammer’s base will be an inch or so below the level of the thwart.

    #9018
    Swiebertje
    Participant

    @Algol wrote:

    OK, so I bought a load of lovely Harken gear to convert to centre main.

    Note that the middle block on the boom can be replaced by some triangular cut rubber hose or some sail cloth to save some money. Study the picture of other boats on WIT for details. I use Ronstan which costs less then Harken. Only the block that really matters is by Harken, the auto ratchet on the swivel. No other brand has such a nice block.
    @Algol wrote:

    Now I need to plan to make a block to sit on the sloping rear of the c/b case to screw the swivel jammer onto.

    Something like this perhaps: http://www.wt.dk/Images/Diverse/cenklods16.jpg
    @Algol wrote:

    Question: What height should the swivel jammer be at?

    What ever you feel comfortable with. There is only one thing to consider; Most crews like to slide with their bum over the thwart, and in light weather they often sit in the middle of the thwart. Make sure the main block is positioned some distance aft of the thwart to allow the crew to sit or tack in comfort. For the same reason most center main boats have the first block on the boom not plum above the swivel-jammer but a few inch further aft. This creates a bit more head room (back room?) for the crew between the kicker and the main halyard.
    @Algol wrote:

    As my plans are at present the swivel jammer’s base will be an inch or so below the level of the thwart.

    Sounds good to me.

    #9020
    Algol
    Member

    Regarding fixing the wooden swivel jammer mounting onto the c/b case . . . Is screwing it down from the underside of the lip of the c/b case sufficiently strong? (I’ll reinforce the screw head position with a hardwood strip either side or use penny washers if they’ll fit)

    #9021
    Swiebertje
    Participant

    Four good sized self threading screws should be sufficient to fix the block. Then the swivel is also fixed with four self threaders. Particularly in hardwood it is important to drill holes with diameters equal to the core diameter of the self threaders. I would use marine grade ply rather then wood for ply cannot split. You are after all, putting eight fair sized screws in a tiny block. Before drilling, make sure the swivel screws will not be obstructed by the bottom screws. (Don’t ask 😳 ).

    Using penny washers will help to spread the load over a larger area and are a good idea indeed.

    Some dip the screws in varnish or epoxy before final fitting. It locks the screws in place so they won’t wobble out on a country road. It also seals the screw holes and toughens the (wooden) threads in the holes.

    #9022
    SeaHolly
    Member

    What is the best combination of jammer, swivel mount and block to use for the centre main , without worrying about remortgaging the house?

    Are there alternatives that will do the job?

    Regards

    #9023
    Algol
    Member

    @SeaHolly wrote:

    What is the best combination of jammer, swivel mount and block to use for the centre main , without worrying about remortgaging the house?

    Are there alternatives that will do the job?

    Well, I bought one of these:

    http://www.tridentuk.com/sailing/product-Harken_Little_Swivel_Base___150_Cam_Matic-HA205.html

    and one of these:

    http://www.tridentuk.com/sailing/product-57mm_Carbo_Ratchamatic-HA2625.html

    Swivel jammers seem to be roughtly the same price no matter what make you get.

    You could save on the ratchet block by getting something like this, but I think most people think the ratch block is the most important component

    http://www.tridentuk.com/sailing/product-Auto_Ratchet_Block_52mm-JHHA4621_1.html

    #9026
    Swiebertje
    Participant

    So far I have not come across anything that beats the big Harken auto ratchet block and the Harken swivel base. I tried the Ronstan swivel for a while and though it is a very nice piece of equipment, with more adjustment options then my trusted old Harken swivel, I did switch back to Harken. Maybe it is something personal? Perhaps the contents of your wallet and your personal preferences are a better guide here.

    N.B. An auto ratchet block is a block that automatically switches to ratchet mode above a certain load. Below that load set point it is a free running block. The Harken block does it at the right load each and every time where others may have some variance in the load set point. But who am i to judge that, I don’t have a lot of experience with auto ratchet blocks from other brands. Note that most of the fleet seems to prefer the Harken auto ratchet block. There must be a reason…..

    There are cheap alternatives like this one for example: http://www.sailboats.co.uk/Catalogue~Rwo_Main_Jammer_Standard_Base~p_R1950~c4621.html Since it has a sheave it would save you a block. It is minimal and I have seen them break too but it is good enough when you are on a tight budget. Though I consider a good swivel base and a good ratchet block are worth the money, there are other ways to save money besides the ‘el cheapo’ RWO jammer. Try in Google: “swivel cleat uk”. This Google search should set you on course for Harken swivel base alternatives. And do you really need that ratchet block now? Perhaps you could make due with a simple block for the time being? And finally: visit boat jumbles that many clubs have as a season opener.

    #9050
    Algol
    Member

    So I got the job done today. V pleased with it, but the proof will be in the sailing tomorrow.

    Thanks for your help Swiebertje

    #9051
    bigal
    Member

    Hi Algol

    I see no signs of a pump action spi uphaul – were you the poster thinking about trying it !!

    #9052
    Algol
    Member

    @bigal wrote:

    Hi Algol

    I see no signs of a pump action spi uphaul – were you the poster thinking about trying it !!

    Haha, it was me. I was put off by the wise owls here. I did re-route the existing system tho as it had been dragging badly

    #9054
    Swiebertje
    Participant

    @Algol wrote:

    So I got the job done today. V pleased with it, but the proof will be in the sailing tomorrow.

    That setup looks really nice and would look good on a boat in the top of the fleet. I see you went for the Harken gear all the way. You won’t regret it. And the cherry on top is that nice shiny varnish.

    Two final remarks:
    1. To hold up the sheet in the middle an expensive block can be avoided by adding a piece of sail cloth from side to side or by a 4″ long piece of 1″ garden hose, tapered at the ends similar to your spi-pole drain pipes.

    2. I would fix the bridle much further out, near the end of the track. A simple way to do this is by drilling a horizontal hole through the track and then putting a pin of a round bar D-style shackle through it. These shackles are nice and smooth and won’t “bite” your bridle rope.
    And if you insist on an adjustable bridle, take the rope through the D-shackle forward to a clam cleat on the inside of the gunwale, just in front of the aft bulkhead, where you can easily get at it. Then mark the bridle’s ends with some whipping twine to ease symmetrical cleating (port and starboard). There are more complex solutions that adjust both sides together, but that involves using expensive sheaves or blocks.

    #9055
    bigal
    Member

    ……. but surely the whole point of having a boat is to put expensive gear on it – and in the UK the expenditure serves to reduce the inheritance tax liability in due course!! Think ahead !!

    #9056
    tempest51
    Member

    Can anyone tell from the images if the jammer is the high or the low base Harken that is available?

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