S/V Sea Witch. Polynesian outrigger Inspired Trimaran.

  • 15 Mar 2017 19:50
    Reply # 4668967 on 4602302

    Doing a quick internet search, I can only find one tube that's anywhere near to the right size:

    168.28mmx4.2mmx6000mm 6063T6  from http://www.aluminiumalloys.co.za/wp-content/

  • 15 Mar 2017 18:30
    Reply # 4668851 on 4602302

    Thanks for the input all.

    Yes buying a boat and then just converting it to a junk rig would be easier, but as stated before, and in our new video on youtube, its not a option for us.  And to buy the boat  overseas, we will have to save up months just for the tickets($1500 from South Africa to Bangkok) and accommodation($50 per night average), never mind the boat itself. Add 45% import tax to South Africa.

    We could save for the boat, at $200 a month we would have the money for a $20000 (average price here in South Africa for boats now) in about 100 months. But  by then is would not be enough due to inflation.

    So we taking the foolish path as most would say and do it ourselves, design and build. And we know the risks...living in a country where there is  53+ murders a day we know risk.

    So back to our question.

    What will be the best size tub/pipe (aluminum) to use as our mast, sail AR 2.0 reduced  by 0.9 (Chored is 4500mm) 32.63sqm?


    Martin & Sandy

  • 10 Mar 2017 10:23
    Reply # 4659060 on 4602302

    Thank you David

  • 09 Mar 2017 20:09
    Reply # 4657652 on 4602302

    Come to think of it, last I heard the proa Nixe was for sale: http://www.proa-nixe.ch/.  The owner is Swiss, the boat is in Thailand, and he can get there to sail only once a year until he retires.  He expects by then he will have ideas for an improved version.  The market for proas being a bit on the small side, you might get her cheap.  Could you sail her from Thailand to South Africa?

  • 07 Mar 2017 12:59
    Reply # 4652505 on 4602302

    The AYRS Magazines are on this page:


    Go down to the embedded Box window, and then go to page 3. It's the last entry you're looking for, Catalyst_N51_Jan_2017.pdf

    Last modified: 07 Mar 2017 13:21 | Anonymous member
  • 07 Mar 2017 10:21
    Reply # 4652386 on 4651319
    Robert Biegler wrote:

     The latest AYRS Catalyst contains an article .  I could upload a copy of the article here (once I look into how that works).  

    I think from memory (which is not what it was!) that the JRA has a corporate  membership of the AYRS.  

    Perhaps someone much more techy than me could confirm and or provide a link.

  • 06 Mar 2017 19:32
    Reply # 4651319 on 4602302

    Can you provide more information?  Primarily, what is the volume of the outer hulls?  The current rule of thumb is that the outrigger hulls should have a volume of 130% or more of the loaded displacement.  I emphasise loaded because Multihulls Magazine once listed the published and measured weights of several production multihulls.  One boat had a measured empty weight just above its designed displacement.

    I also remember seeing a calculation comparing trimarans and catamarans with and without bridgedeck and with fat or narrow hulls.  The configuration that minimised skin surface area (one component of weight and cost) was the catamaran with fat hulls.  Derek Kelsall used to have a line of designs like that.  If you want the most living space from a given amount of money, the trimaran may not be the way to go.

    Edit: I finally remembered that this series of Kelsall designs was called Tonga.  I found an example at http://www.xboat.uk/318-DEREK+KELSALL+CATAMARAN+TONGA+TINI.html.  Pictures 8 and 9 show most clearly that the hulls are fairly broad for a catamaran, and the omission of a bridge deck, the idea being that if you wanted more inside volume, it would be better to put that into larger hulls instead.  The stock designs now Kelsall's web site all feature bridge decks, though.  I suppose most customers dislike the idea of having to go outside to get from one hull to the other.

    Also consider that trimarans need to be a bit wider than catamarans of the same length.  The reason is that trimarans sail at a greater angle of heel because the lee hull submerges more than that of a catamaran, and that a beam sea will still lift the centre hull of a trimaran when it has already passed the weather hull of a catamaran of the same beam.  Therefore, if you want the trimaran to be as safe from being flipped by a wave as a catamaran of the same length, the trimaran needs to be wider.  That adds to loads and weight.

    What I would study in detail, given your design brief as I understand it, is a very wide Pacific proa with a low volume, high density windward hull, a single stayed mast and junk sails set on the stays, including the windward stay.  The latest AYRS Catalyst contains an article on why that rig theoretically should give good helm balance and pictures of a rudderless radio-controlled model that I could steer by sail trim only.  I could upload a copy of the article here (once I look into how that works).  I am even considering this for my next boat, only I have the additional constraint of a small harbour and a berth limited to 9 m x 2.8 m, not the 9 x 9 m that design should have.  I would have to fold the boat up as well, driving up cost.  And payload would be roughly that of a 6 m long catamaran.  So even if the proa were to cost less to build for a given payload and speed, it has a large footprint, which makes for high berthing costs.  If you will keep the boat on a mooring, this may not matter.  I don't have that option.

    Last modified: 10 Mar 2017 16:21 | Anonymous member
  • 06 Mar 2017 09:11
    Reply # 4650007 on 4602302

    Hello again all. We thought we would upload a picture on our design so far. So here is a link to the updated Sea Witch (again this is still a work in progress). We are slow getting use to using Freeship!.

    As for the sailplan we decided to go ahead with our Twin Sails (same size Schooner). Both sails we will be using Arne's AR 2.0 sails scaled down by 0.9(Chored is 4500mm on our sails). This will give us 32.63sqm sail each. 65.26sqm totale...if my math is correct.

    According to FreeShip! our displacement is 4.5metric tones. This give us a aspect ration of 23.94. (again depending on our maths). Should be more then we would want in a cruiser.

    If we are to use a Aluminum pole for mast what would be the safest size?

    Last modified: 06 Mar 2017 09:15 | Anonymous member
  • 26 Feb 2017 20:14
    Reply # 4636272 on 4602302

    We had a busy weekend, but we had some time today to work on the Sea Witch design.

    As Dave suggested, we are using FreeShip! to create a better 3d design.  Thanks Dave it helps a lot. Still learning a few things on Freeship! but we are getting there. This is about a 80% complete design of the main hull. Hope you enjoy

    S/V Sea Witch Freeship!

    Last modified: 26 Feb 2017 20:20 | Anonymous member
  • 23 Feb 2017 06:18
    Reply # 4627877 on 4626860
    Annie Hill wrote:Look on the bright side - it will get you used to living in the even smaller space of your boat!  Best of luck.
     Moving from the room to the Sea Witch(the boat) will be a upgrade we think. And that just talking about  the space down below, not counting couch roof, cockpit and open trampolines. :)
       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

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