sail track questions

Discussion in 'Boat Design' started by Pammie, Feb 16, 2020.

  1. Pammie
    Joined: Dec 2015
    Posts: 109
    Likes: 16, Points: 18, Legacy Rep: 10
    Location: Eindhoven, Netherlands

    Pammie Senior Member

    On my F9XR trimaran I have a aluminium mast with a Tides Marine sailtrack made of UHMW. With the previous owner it has broken at the connection from the inside to ouside part. This is at the upper sections of the mast. It has been repaired with bolts screwed into the mast. Past summer another slightly lower section tore loose, and I repaired it likewise. I think the masttrack is quite old, might be even original (17 years old). There is also some play between mast and sailtrack (not measured that yet).
    As it is not an ideal solution (slight release of a bolt might block the mast track and prevent the mainsail coming down. I thought of replacing the mast track. Tides Marine specifies max sail area as 300-600 sqft. I suppose depending on the actual luff groove width. My main is 41 m2 so 450 ft2. It has quite some squared top.
    My own calculation: max diagonal force in the sail (boomend to masttop) at 26 knots 12000 N. (calculation from small trimaan design). With an angle of 30 degrees (prop. less) the force radial to the mast is 6000 N. The topcar touches the masttrack over 150 mm. UHMW has a low modulus. The lowest luff groove width is 0,35 " (not measured the actual size) = 9 mm. So load on the luffgroove connection is 6000/1500 = 4 MPa. Max load for UHMW is 23 MPa. design load: 11,5 MPa? Leaves a factor 3 for dynamic loads?
    My questions: does my calculation make sense? What about aging of UHMW? What about play?

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  2. Pammie
    Joined: Dec 2015
    Posts: 109
    Likes: 16, Points: 18, Legacy Rep: 10
    Location: Eindhoven, Netherlands

    Pammie Senior Member

    Nobody?? Did some research myself.. Maybe should done that before asking ;). It seems degrading in thermoplastics as PE is quite some issue as there is a lot of research about special mixes to minimize this effect. Degrading (of mechanical properties) is mostly because of UV, combined with temperature and moist. This report shows degrading very clear. Fig 7 shows sigma max reduction of more than 50 % with (accelerated) aging for HDPE.
     
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  3. Ilan Voyager
    Joined: May 2004
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    Location: Cancun Mexico

    Ilan Voyager Senior Member

    You're right. Mainly UV, none plastic resists to UV. And their life span is rather short compared to metals. But they do slide better than alu and are easier to fabricate.
    UHMW lacks of rigidity but it's pretty strong. No worry for that, change your track as counseled by Tides Marine. Ask Tides for the life span, that will be funny.
    And change it before it breaks, the real life span is rather short. These plastics are generally rated in industry in exterior marine environment and high stress for 5 years.
    On practical advises about multihulls go to a specialized forum of multi sailors.
     
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