Question about determining chain plate alignment (alberg30)

Discussion in 'Sailboats' started by wesley Sherman, Oct 26, 2020.

  1. wesley Sherman
    Joined: Jan 2020
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    Location: New York

    wesley Sherman Junior Member

    In the process of rebuilding my Alberg, I made a pattern of the angle of the chain plates before removing them. Currently and sadly I have lost the patterns I made due to a move and someone thought they were not important.
    So now how does one know the proper alignment. I know how to align to the mast but not the angle of the chain plate to the rigging. I am new to this but I am believing the plates need to be in close alignment to the rigging there attached to.
    Is there some formula used? Mast height/shroud length.

    Any help for this rookie would be appreciated.
     
  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Hi Wesley - you could do a rough calculation by scaling some dimensions off the general arrangement drawing here -
    https://sailboatdata.com/sailboat/alberg-30

    I did a rough calculation for the angles of the forward and aft lower side shroud chainplates, and I got 78 degrees to horizontal for the forward chainplate, and 76 degrees for the aft chainplate.
    I calculated this using a simple triangle with a 90 degree angle - the height of the triangle is the height from the deck to the crosstrees, and the width of the base of the triangle is the half breadth locally in way of that chainplate.
    I measured the height as approx 5300 mm, and the half breadth in way of the forward chainplate as approx 1120 mm, and in way of the aft chainplate as approx 1270 mm.
    The tangent of the angle is the height / half breadth - for the aft chainplate this is 5300 / 1270 = 4.17.
    And the inverse tangent of 4.17 is 76 degrees.
    This is assuming that the GA drawing is reasonably accurate!

    For the cap shroud chainplates, I think that they most probably come down fairly vertically?

    You could apply the same calculation method to calculate the angles of the forestay and backstay chainplates, relative to a horizontal datum.
     
  3. wesley Sherman
    Joined: Jan 2020
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    Location: New York

    wesley Sherman Junior Member

    OmG I am so impressed with people that can roll up math equations like I lift my belly to put on my belt. Lol... I haven't heard the word inverse tangent, In let say 4+ decades. I so love math smart people.. even though you gave me a memory headache with all your math and terminology.

    I thank you so much for your input Bajansailor, I do remember the caps were pretty much vertical or slightly off.. My son 9 years old math solution was looking at a drawing of the Alberg 30, He took the drawing I had printed and went to the glass slider taped it vertical, whipped out his cell phone opened an app called Dioptra ( don't ask why he has that on there lol) Laid his cell phone along the lines of the rigging lines and came out with dad the front one is 83.9 degrees the one in the back is 85.9 degrees, looks up at me and says that solves it. I am regretting getting him interested in science to be sure.. I wish life were that easy. His reasoning is that the man that made this boat design is sure he made it to scale. I can't belittle his reasoning but I said to him I think we need to be sure.. so we don't make a mistake. Apps don't lie he states.

    Thank you so much for your information. This helps a lot to get me moving forward.
     
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  4. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Your son's Dioptra app does sound quite amazing.

    However - I presume that he was measuring the lower side shrouds, same as I was?
    If this is the case, then the forward shroud will be at a slightly steeper angle to the horizontal than the aft lower shroud, as the half breadth is a bit less.
    But he says the aft one is a bit steeper?
    Or more likely he is referring to the forestay and the backstay in the side profile view?
    I calculate an angle of 76 degrees to horizontal for the backstay.

    If your son's measurements are for the fore and aft stays, do you have any drawings that show end elevations with the rig, or a midship section?
    If you do, then maybe your son could try his app on the angles for the side and cap shrouds as well?

    And another method - use a traditional school protractor on a printed copy of a drawing, and this will tell you the angle of any rigging wire fairly accurately.
     
    Last edited: Oct 26, 2020
  5. wesley Sherman
    Joined: Jan 2020
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    Location: New York

    wesley Sherman Junior Member

    My son is a piece of work for sure however smart he is.. he understand so much but i was relaying a story of amusement. Have to say he keeps my life alive. lol He was actually trying to measure the aft lower (85.9) an forward lowers (83.9). That is where h he got his dimensions from a side elevation drawing. His angle for back stay is (66.7). Trust me these are not scientific mathematical measurements by any means.. just sharing some joy form my sons interest to help me in his way. Hopefully this Alberg will last long enough for him as well. As me he loves this slow boat. I gave him a old protractor and a clear copy of Alberg side view.. So he is learning how to use a protector off Google today. I have hope for his future.

    ***Another question however the aft lower and forward lower should the chain plate knees also be slightly angled to the rigging when replacing rather than parallels to the hull as they were. As thy were seemed as they would be pulling at a slight angle the way the knees were installed.
     

  6. M&M Ovenden
    Joined: Jan 2006
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    Location: Ottawa

    M&M Ovenden Senior Member

    I'm sure there must be Alberg 30 online groups (Facebook?) and you could ask for photos and measurements ? Would be a good double check.

    Cheers,
    Mark
     
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