a better tiki 21

Discussion in 'Multihulls' started by guzzis3, May 7, 2019.

  1. Manfred.pech
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    Manfred.pech Senior Member

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  2. oldmulti
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    oldmulti Senior Member

    This is interesting. Pitching is a problem with all boats but some have less than others. Fine ends are a problem on fatter hulls but according to Hugo Myers once you have length to beam above 15 to 1 fine ends are ok. With a small simple multi you need wider waterline beam to fit EG bunks and carry the loads. As Lock Crowther found out fuller ends and wider waterline beams generally ended up with a better riding boat that could achieve the same speed for a cruising boat. As long as the boat had some asymmetry fore and aft it also helped reduce pitching. The other trick was to have a wide stern but in the form of a step or scoop stern of low buoyancy that would sink into a wave in a following sea if running downwind in large seas. It has the same effect as a canoe stern downwind. But upwind a wide low buoyancy stern hull has a lot less pitching. It also helps to centralize weight. Have a look at the Tiki 25 in Sea People magazine issue 1 november 1983 page 14. (available for free on the PCA web site). It started to head in this direction but Wharram did not pursue it.
     
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  3. Angélique
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    Angélique aka Angel (only by name)

    Thanks Old Multi for the info, here's the PDF link.
     
  4. redreuben
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    redreuben redreuben

    Those two 25’ Tikis are for me the most interesting boats in the whole Wharram catalogue.
    Except they aren’t in it.
    I read the racing column till it petered out about issue 5, such a shame it all went nowhere.
    Do you think the lightweight boat was 4mm ply ?
     
  5. oldmulti
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    oldmulti Senior Member

    Redreuben. All other wharrams around this size have 6 mm ply. There is a TIKI 21 built in the Philippines with quality 4.8 mm ply skins, but the standard is 6 mm ply. The Haiti 17 is the largest Wharram that has a 4 mm ply skins. But taking the advice of Gary, experience of Tennant Bamboo Bomber and Richard woods Strider 24 (racing version) cats, a 4 mm ply skin with some compound curves and a light glass sheathing looks very possible.
     
  6. Manfred.pech
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    Manfred.pech Senior Member

    oldsailor ?
     
  7. jamez
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    jamez Senior Member

    Heres a clip of my old Hinemoa taken just before the start of the Coastal Classic back in 2008. I had just sailed through the fleet (of 239) and was trying to get a couple of klicks upwind of the start to get a grandstand view of it. Hit a big wake (there were heaps of spectator boats and the water was fairly churned up) which you can see half way through the clip. To me this demonstrates the lack of pitch dampening that is inherent with this shape (heavily rockered, double ended hull).


    This is not limited to Wharrams though. Great Barrier Express catamarans have a fairly low prismatic, heavily rockered, well rounded V hull with a transom. In certain conditions; like a sloppy steep head sea they tend to hobby horse while similar size cats with higher prismatic coefficient, less rockered hulls pitch noticably less and sail away from them.
     
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  8. Angélique
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    Angélique aka Angel (only by name)

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  9. Manfred.pech
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    Manfred.pech Senior Member

    oldmulti = oldsailor ? Welcome back home ?
     
  10. redreuben
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    redreuben redreuben

    Oldmulti and oldsailor aren’t the same guy.
     
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  11. Manfred.pech
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    Manfred.pech Senior Member

    Thank you, redreuben !
     
  12. guzzis3
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    guzzis3 Senior Member

    Has anyone made a hull by:

    Sheathing ply or foam

    with a "flange" single skin of glass on one side

    put that panel into some station molds with the flange curving around to form the outside of a rounded shape to the waterline

    laid up solid glass inside the thin glass surface to form the hull to the waterline ?

    A bit like KSS only without the foam below the waterline....

    You would probably need to make cuts in the glass to take out any ripples where it didn't want to take a fair shape, and you might need stringers in the mold to stop bulging between the stations. If you could get it to work though it'd be fast and easy....
     
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  13. redreuben
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    redreuben redreuben

    As soon as you add fibreglass you lose the “compound-ability” that you can get with plywood.
    Either build a batten mould and sheet foam, if the hull is symmetrical you can build a half mould make two parts then reverse the frames and make the other two, or perhaps a tortured ply shape with thin ply and use that as a male or female mould, the latter gives you the option of vacuum bagging or infusion.
     
  14. oldmulti
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    oldmulti Senior Member

    Kurt Hughes "cylinder molding" technique has been used to make cheap hull molds that a glass hull can be laid up in. Cylinder molding create a "bent" sheet of ply that is then tortured into a hull shape. If you use good ply use it as a hull. Use cheap ply and use it as a mold for a hull. If the hull is solid glass and it is to do a wharram tiki type then the glass can be molded on a flat table and tortured into a wharram shape before taping in any bunks, furniture etc to provide stiffening. Richard Woods 27 and 30 wharram type designs do flat panels fiberglass or wood structures.
     

  15. guzzis3
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    guzzis3 Senior Member

    Well if you can't get any double curvature with glass how does kss work ? It should take the vertical curve easily but the bow to stern might cause some bulges and dimples. I would have thought using a single layer of say 400 gsm you could cut it transversely to relieve that, patch it with packing tape or whatever.

    Using tortured ply to make a mold is a good idea but comes with the same shape restrictions that tortured ply gives.

    I'm thinking about ways to scale this down to do a realistic experiment and see if it can be made to work. If so it presents the possibility of making a rounded hull to waterline with single curved ply or foam core sides. Solid glass to the waterline is nice, probably as nice as core in that area and it would be less fiddly than kss.

    I am probably not explaining my idea as well as I might so to try again...

    Have a look at what is kss page 4 method 2.

    http://kelsall.com/UniqueKSS/WhatIsKSS.pdf

    What DK is doing is making a flat cored panel, rolling the bilge over and making "dart cuts" to accommodate the longitudinal curve.

    What I am suggesting is omit the foam and inner skin from the bilge area, roll the bilge using 1 thin layer of glass and cut as needed to get a fair curve bow to stern, maybe run a stringer or 2 between your station molds. Patch the outside to make it resin tight. You now have a fair smooth outer skin for your hull to the waterline. I suppose in theory you could now add a core inside this skin as long as you don't bend and bash it out of shape but I'd just lay in some more glass and build up a solid hull to the waterline. Above the waterline you have a single curved cored panel, smooth and naturally fair.
     
    Last edited: Jun 27, 2019
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