How do I center & align mast bracket (pics)

Discussion in 'Sailboats' started by Amateur_Expert, Jun 28, 2012.

  1. Amateur_Expert
    Joined: Feb 2012
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    Location: Wichita KS

    Amateur_Expert Junior Member

    Hello,

    I am replacing the mast step/bracket in a '74 Laguna Windrose. Someone had replaced the mast and the original bracket was too big (and rigged up with some washers and the holes were drilled crooked), cumbersome and a little bent up.

    Quick Background:
    I am replacing the mast step/bracket/cradle with a 'dwyer tabernacle'. I
    think it will work out nice. I will be able to use the factory holes in the mast
    (the other holes where drilled out crooked for the old bracket - this
    mast is a replacement and is not original to the step).

    I grinded out most of the fiberglass where the step used to mount, it was
    cracked, had been drilled out a couple of times and discolored from 40 yrs of water damage.

    I am slowly rebuilding the glass from the inside upside=down one thin
    layer at a time, it's getting beefy, got about 20 layers up so far. Also
    plan to add 2 addition crossmembers and glass it all together. I will
    then glass a larger portion of the deck where the step mounts. I have
    not strated grinding top-side yet. The original back hole is still there
    but there's more than the original (someone had this bracket off
    before and drilled another hole).

    My question is:
    How do I center and align this new bracket??! Someone suggested I use a
    plumb bob and try to go off the center of the keel (which is not right
    under the mast). I want to get the bracket centered and also aligned so
    it's not cocked a little to one side of the other. Does the rudder come
    into play in the equasion? Also... is it ok to move the bracket forward
    slightly if the rigging allows (I want to center it over the crossmembers
    and wood inside the glass in the deck)?


    Please help. This is my first sailboat project. I am thinking that you
    would want this straight and centered. I have not been able to find anything on actually drilling new holes for a mast bracket.

    Thanks!

    Shawn T
    Wichita KS
     

    Attached Files:

  2. sean9c
    Joined: Jan 2011
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    sean9c Senior Member

    I'd center the mast athwartship between the chainplates. That will keep the rig geometry the same on both sides. Put a stick across the boat then drop a plumb down to the chainplate. If the spreaders are inline with the mast (not swept back) I'd do the same thing for the fore/aft location.
    You'll drive yourself crazy trying the align the rig to the keel,rudder, bow, stern, boats are always all out of whack.
     
  3. sean9c
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    sean9c Senior Member

    I should have also mentioned that you should set the mast base on the deck so it's level. That will allow the bottom of the mast to set flat on the base.
    From the pics it didn't look like there was much structure in the boat to take the mast compression load. I guess what's there must work.
     
  4. masalai
    Joined: Oct 2007
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    Location: cruising, Australia

    masalai masalai

    Hi AE,
    Mostly, and as sean says, boats do not align perfectly, locating by eye is best, select the fore/aft position, visually align to midships (port/stbd positioning) confirming with a 'string line' running from bow to stern and also by half the beam at the mast position... (Notice a difference? - step back and view as one would walking around looking at boats in a marina - does this confirm that the position is 'right'?) - - Make adjustments so the 'dockside experts/inspectors' can affirm everything appears to be "ship-shape and Bristol fashion"...

    I do not know your boat, but compressive loads can be humungus...

    My cat can accept a 6500kg compressive load at the mast-step, (the radar mast is sitting there at present), and the loading is far less than a traditional 'bermuda' or other format of fore and aft rig with boomed mainsail and genoa/jib configuration, as I will have, when fitted, a hich-hiker rig which entails 2 flat cut genoa like sails, one from each bow of my 40 x 21 ft catamaran with an empty weight of 4000 to 4500kg and fully loaded ready to cruise, up to 6500kg...

    The images seem to suggest you are on the right tack... an engineer with aircraft design skills may be able to assist you with a design showing how to spread and carry the mast loads... I use box girders throughout my boat, giving exceptional stiffness and load spreading...
     
  5. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    The setup that you took off may have been done that way on purpose so that the mast will be straight fore and aft, but will come down a bit off to one side to leave the cockpit open and not interfere with the helm. Or it could have just been a bad job.

    Is there a fitting on the transom for a mast crutch? Aim the mast base at that and drill the holes so the mast will be straight when it is up.

    The usual procedure is- Before removing the old one...... sorry, I couldn't resist.:D

    It's going to be a matter of splitting the differences on on old boat like that.
    I see some one beat me to the submit button. Yes, I'd work off the chainplate
    positions as well on that one.
     
  6. Amateur_Expert
    Joined: Feb 2012
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    Amateur_Expert Junior Member

    sean9 thanks,
    since Im a novice I had to look up some of these terms but better now than later!

    So you say run a stick across the top and drop a line to the side chainplates and use the center of this, correct?

    I don't know what you mean by "spreaders swept back" but I can measure from the fore and aft stays, is this what you mean?

    As for compression, I share your concern. Most sailboats I have seen have a pillar or something inside the cabin for support of the mast. This boat has a wooden crossmember encased in glass and also has a board in the center (both were broken). I am rebuilding them, reglassing them as well as adding two more longer ones. I pasted a couple of photos of how it looked before.
     

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  7. Amateur_Expert
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    Amateur_Expert Junior Member

    Thanks masalai,
    I had not intended to seek out an engineer on this even though that would be a great idea but this is an $800 boat so I am hopefully not doing the wrong thing.

    I am looking at it this way as far as support goes. I am "beefing it up good" so to speak. Since there only a crack as opposed to a more severe break in the crossmember I decided to cut out he bad parts and splice in new pieces. I will then add one more crossmember on each side and finish it off with the stainless reinforcement that was in place. I have about 20 layers of glass so far and I went about 12" out in all directions. ALSO, I plan to further reinforce the deck with a few layers of roving. In the end it will be stronger and thicker so hopefully it will support the mast.

    The numbers you gave are very significant.. does the mast indeed exert 6500kg of pressure??!? Thats like 14,330 lbs right? Surely that can't be right!
     
  8. Amateur_Expert
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    Amateur_Expert Junior Member

    Thanks Phil,
    I do have a mast crutch (that also holds on the rudder). You say I should center between that and what else?

    Yeah you are right, but I figured I had to lose the holes because I had to grind out all the rot. I figured I'd just re-measure, the grinding had to be done. I did however try to save them by grinding inside the cabin but ai ended up burning through.

    The original rear hole is there so I could get close but someone had drilled a couple holes before.

    Thanks all for the replies.
     
  9. sean9c
    Joined: Jan 2011
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    sean9c Senior Member

    Yes, you're just trying to put the center of the mast halfway between the chainplates. So put a stick across the boat, drop a plumb down to the chainplate, (pick the center of one of the holes the clevis pin goes in), do the same on the other side, measure between the two and take half that number. That's the athwartship centerline for your mast
    For the fore/aft position if your spreader stick out 90 degrees to the centerline of the mast (they are not swept back) measure from the leading edge of the mast to the center of the spreader, remember that number, now put a straight line between the two chainplates. At your mast step measure forward the amount you got from your spreader measurement and that's where the leading edge of the mast will go.
    As far as engineering goes remember that whatever was there before worked for a lot of years if you can get back to what was there in the beginning you should be good.
     
  10. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    A plumb bob only works if the boat is dead nuts level. If you don't know how the rig is setup, you'll have hell to pay, to get things trued up.

    You have one chain plate hole with the same dimensions on each side. I'd use this as reference. The other reference is the point of the bow. I'd establish the point of the bow and put a drywall screw in it, to serve as a marker, which is easily fixed later. Hang a tape measure on the point of the bow and measure back to the chain plate references. If they're they same then you can split the difference and establish an accurate boat centerline. If they're not the same (don't be surprised if they aren't) just select an arbitrary number, longer then the step is aft of the bow and swing this arc, so it hit both rails. Mark half the distance between them, then snap a chalk line from the established bow point to this mark. That's the boat's centerline and equal distant from the chain plates hopefully.

    You basically know where the step is suppose to live, so if it's off an inch of so, don't worry about it. With the tape measure on the bow point again swing another arc, so it hits both rails at the mast step. Again stand up blocking on the rails, the same on both sides and place a straight edge over them. This will be close enough for you to build a level pad to place the step on.

    [​IMG]

    I don't have a lot of information about these, but the rig is visible above. They were better then most production boats of their day, but needed some wind to get going well and seriously wants a reef at SCA. It would be helpful to know what her length is. I'm assuming it's the 18, but there's also a 22, 24, 25, 26 and likely more variants as the company what through ownership changes.

    If it's the 18, the chain plates are in volume with the mast, so you want to be fairly precise about this.

    [​IMG]

    This is what's supposed to be under the mast step on the 18 (above).

    [​IMG]

    This is the relationship the chain plates have to the step (above).
     
  11. sean9c
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    sean9c Senior Member

    I agree that the boat should be roughly level and I should have mentioned that, but as your plumb line is only as long as the height difference from the cabin top to the chainplates I would think that getting the boat eyeball level would be sufficient
    Not that it matters but I can't figure out how measuring back from the bow to the chainplates and swinging arcs helps find the centerline, but I'm sure that it's just me.
     
  12. Amateur_Expert
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    Amateur_Expert Junior Member

    Thanks alot for taking the time PAR but Im lost a little.

    Chainplate holes:
    I pasted a pic below. As seen, I have the chainplates out. There is a
    slot there. I had planned to remove the stainless cover and clean it up
    since it looks like some dirt under there (the slot plate is held on by 2
    little screws). Do I want to slide the plates back in first and bolt
    them in (they bolt onto that big brown center board)? That seems the way
    to go since those plates connect to the shrouds... I gather you would
    want them in tight first before measuring, does this sound right or
    measure off the slot?

    Point of the bow:
    Sorry but what did you mean by this? Are you talking about the tip??
    Put in a drywall screw, I got that, just wondering how you establish
    where to put the screw.

    Measure back to chainplate references:
    Are you saying measure from the point of the bow to the side plates?
    Someone may also have mentioned before about the aft - that aft stay is
    offset of center. And you are thinking don't worry about keel in
    conjunction with where mast sits?

    Ok I may have got lost here... you said "Hang a tape measure on the point
    of the bow and measure back to the chain plate references. If they're
    they same then you can split the difference and establish an accurate
    boat centerline. If they're not the same (don't be surprised if they
    aren't) just select an arbitrary number, longer then the step is aft of
    the bow and swing this arc, so it hit both rails."

    Did you mean swing the arc form the side plates (or to one plate and one
    mark if they are off?? What rails did you mean (if you mean the teak
    rails I have removed them)?

    You said "Mark half the distance between them, then snap a chalk line
    from the established bow point to this mark. That's the boat's centerline
    and equal distant from the chain plates hopefully."

    You could of course just measure between the plates but I think I
    understand the bow point coming into play. Should you run the line to
    the stern?

    You said "You basically know where the step is suppose to live, so if
    it's off an inch of so, don't worry about it. With the tape measure on
    the bow point again swing another arc, so it hits both rails at the mast
    step. Again stand up blocking on the rails, the same on both sides and
    place a straight edge over them. This will be close enough for you to
    build a level pad to place the step on."

    Are you meaning to arch it in the different direction? Also still don't
    know about what you mean by rails or "blocking on the rails". I am also
    unclear as to where to place the strait edge.

    This is an 18', sorry I forgot to mention that. Is the one in the pic
    yours?

    Also you mentioned about chain plates in volume with the mast... did you
    mean the mast should be centered in between them? Also does it matter if
    the mast is back or forward a little bit or does it need to be ball on
    dead accurate in the middle?? I guess where you mark from would make a
    difference.

    Sorry so many questions. I have never owned a sailboat nor am I real
    technical as far as math goes.. I am more a hands-on guy!

    Also you had included a photo of the big brown center board in the cabin.
    I pasted mine below, its hard to see but its there. The piece missing at the top was broken (which is one reason i am beefing it up). I am also going a
    step further and glassing the whole think in all the way around as well
    as adding a couple additional crossmembers. Should have about 50 thin
    layers inside total (and then will beef up the deck).
     

    Attached Files:

  13. Paul B

    Paul B Previous Member

    I would do this.


    You have a headstay fitting on the boat. You can use that instead of putting another hole in your boat. That is is you want to swing arcs (I wouldn't).



    This is a very important issue. Do NOT allow the CL of the mast at the deck to be behind the chainplates. If anything you want the CL of the mast to be a bit forward of the chainplates. If it is forward, the rig, when loaded, will be trying to bend the mast back. This is good. If the chainplates are in front of the mast CL the rigging will be trying to bend the mast forward. Not good.

    So you might want to simply pull a string tight across the boat from one chainplate to the other, or use a laser level if you have one. Mark that as your chainplate location line.

    Your boat has a molded in centerline between the nonskid. Mark the middle of that at the CP location line and measure to each chainplate. If it is pretty close you are in good shape to use that as you mast's centerline.

    Now position the mast step so the CL of the mast is just forward of the chainplate location line. An inch might be a good number. Measure from the front of the mast to the headstay fitting. Does that match the published "J" dimension?

    How does that line up with the structure below? Is the mast pretty well centered over it?

    If so, you are probably as close as you need to be.
     
  14. Amateur_Expert
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    Amateur_Expert Junior Member

    Thanks Paul,

    What do you mean by "CL"??

    Also can you tell me whre the "molded centerline" is supposed to be?

    Thanks. Shawn
     

  15. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    The textured ares on the deck cap have smooth separations between them (waterways) and one of these roughly define the centerline of the boat. They aren't especially accurate for several reasons, but will get you close.

    Paul is correct in that just placing the new mast step on the patch where the old one lived will likely be just fine. If you need to move it forward a bit to land on your reinforced area and center over the ring frame below, than do so. In the picture, the step looks to be slightly overlapping the textured area a bit. Grind down the texture in this area so you get a flat surface and it would look "right", if you ground enough away to make a new, extended waterway around the front of the new step location.

    If I was doing this, I would slide a yard stick or flat bar stock up through the chain plate slots on the deck, high enough to clear the cabintop. Both sides being the exact same height off the deck and braced to be perpendicular to it. You can stretch a string or place a straight edge (preferred), on these chain plate extensions and from this, you can measure to the stemhead fitting and the mast step location. In an ideal world, the mast step will be equal distant from each chain plate and the chain plates will be equal distant, from the stemhead fitting. You can bet they will not be, but if they're close, do worry about it, as the rig can be adjusted for the slight length differences.

    So, the key would be to get the mast centered between the chain plate slots and looking at the sail plan, the slots look to be just slight aft of the mast centerline, maybe an inch. The chain plate locations are what they are and their relationship with the stemhead fitting too, so move the step around, until you're mast centerline is abreast or slight forward of the plates and the mast is centered between the plates, as well as over the ring frame below.
     
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