Thank you all for your input.

Sadly this chapter/dream is now on hold. Sandy is now part of the ever growing South-African unemployment stats. The little funds we had spare for the project, now needs to be diverted in to keeping our land-based lives afloat.

Martin and Sandy's original post suggests that they do not have the funds at the moment to buy a finished boat.  Thus the temptation to buy some materials and start building bit by bit.  They could save up for a few years and then buy a second hand boat, if they could find something suitable.  They then face the challenge that there are not many suitable boats in South Africa, and they are expensive.  Buying overseas has other challenges, one of which is the crippling exchange rate for South African Currency.  A few years ago I inherited R150,000 from my mother's estate, which translated into $30,000 Australian.  So, although buying a boat and getting some sailing experience might sound logical, I don't think it will work for Martin and Sandy at this stage.  If possible, doing some sailing on other people's boats could be helpful.

The real issue, Martin and Sandy, is that you build a boat that is well designed and engineered.  Designing it yourself will require a huge amount of research to make up for your lack of hands-on experience.  It is hard to get that information outside of a formal course, like the Westlawn Yacht design program, assuming you do not already have an engineering background.  Crawling over similar types of boats (plywood multihulls) is useful, but getting access to them might prove impossible in SA.  You could buy a set of plans and modify them aesthetically but that can also be fraught with risk, if you compromise the structural integrity or balance.  James Wharram's "ethnic" catamaran designs are another avenue you could explore.  They are the closest thing I can think of in a set of existing plans to your concept.

Here are a couple of pictures of a Marples Seaclipper tri to inspire you. The plans are still available to purchase.  Click on the images to view a larger version.

Hello Martin, I have been following this post for a while and as an experienced sailor and cruiser of multihulls and monohulls, both offshore and coastal, and having built some boats I would certainly advise caution when hoping to design and build your own boat. Building a boat takes up a lot of money and even more time, and anything of 9 meters or more is going to take up some years of weekends and evenings. Unless you really know what you are doing this is a lot of time lost when you could be experiencing sailing and life. If you are building a boat to your own design there is a big risk that it may not work out so that will be a lot of your life lost for no good reason.

People do spend years building boats. I know someone who spent 19 years off and on building a Newick trimaran, fortunately he was then able to spend almost a decade sailing around the world enjoying the fruits of his labours. We have also had a visiting boat in New Zealand which is Wharram Tiki 38 catamaran with a double Polynesian crab claw rig. So that is a good example of someone taking an existing well proven multihull design and adding a rig that suited him. It seems to work very well.

So along with some others my advice to you would be to get out on the water in what ever you can afford and learn something about sailing and the cruising lifestyle. If you really want to build a boat invest what is really a very small amount of money in a set of plans by a reputable designer such as Bernt Kohler or as Graham has mentioned the John Marple trimarans.

Hope you are able to get out on the water soon, David

I'm not discouraging you from designing it yourself, just pointing out issues that occur to me.  It is true though that a sailing double outrigger will have significant loads and requires good engineering.  I wouldn't know where to start working out specs like that.  Looking closely at existing designs is helpful and you could go down the empirical route, though there is some risk here, compared to a properly engineered structure.  Of course, you may have some engineering expertise, unlike me! If you are looking at existing plans, you should look at John Marples' trimarans.  His Seaclipper series (not sure if the plans are still available) is built from sheet plywood with a flat-bottomed main hull.  Aesthetically, they are more of the modern western trimaran rather than the exotic Polynesian/Philippino double outrigger type, but well-proven, easy to build, and as economical as it is possible to get.

Just to keep you smiling, try this Google Images search 'junk rig trimaran image'. Somehow it pulls in monohulls and cats as well, but plenty of Tris too.