I have only just started researching
Then I recommend looking into traditional boats. Millenia of informal R&D have gone into creating economical craft with materials that have only modest mechanical properties. Rigs like those of modern windsurfers get their shape and adaptation to wind strength from the mechanical properties of expensive materials and sophisticated production methods, which translates into not economical by the standards you probably must apply. Also, if you think of the whole life cycle, fibre-reinforced plastics are a pain in the backside when it comes to recycling. Untreated wood can decay or be burned, aluminium can be recycled, so I would focus on those. Ropes and sail fabric will likely be some variety of plastic. That is difficult to avoid. But if you want easy fabrication with local methods and materials, you need to look at rigs with modest mechanical stresses. Traditional rigs had to limit those stresses, because the available materials could not take much. Therefore those would be a good starting point. And there have been studies of such rigs. One secondary report is in Practical Boat Owner #264, December 1988, pages 52 - 55. I know there was another report, possibly UN-funded, on giving sail-powered artisanal fishing boats more stability. The best method they found was a double Bruce foiler. I can't find the reference for that now, I might have seen a secondary report in Practical Boat Owner. The boat resembled this one, only still lower tech, with the foils just being flat plywood sheets.
See also
Ngalawa - Wikipedia https://en.wikipedia.org/wiki/Ngalawa for a very low tech version from East Africa.
Your design is amazing but i think it may be a little too complex
It may well be too complex. Then again, if you put a conventional rig on a tinnie hull, you will need a rudder, because the outboard leg will not do the job. The paravane can steer the boat. An easily lowered and started electric outboard could power and steer a rudderless paravane-stabilised boat through a tack. And there may be a way to gybe without a rudder. If I get on the water before the sailing season is over, I will try. So the rig could have exactly the same complexity as the auxiliary you were proposing, the paravanes would replace the rudder and possibly a keel, and the load on the hull structure might be less. So maybe the complexity of the paravane-stabilised boat is more visible, but not necessarily greater.
The kite is an option I hadn't considered but it may be the simple design I could be looking for.
Hobbyists have made their own ram-air traction kites on sewing machines. Not having tried myself, I don't know how much knowledge and skill is needed. Certainly more than making a crab claw sail, which can get a good shape from a flat sheet of fabric, as you can see here:
If a kite with an inflatable spar is needed, then the airtight bladder/inner tube may add more complexity to manufacturing, and I have no idea how much. How important is it to your project that things can be made locally? How long do the bladders last, especially when exposed to possibly tropical sun?
if I was looking into installing a centreboard on the tinnie would I have to worry about the extra lift making the boat prone to tipping and therefore have to make some sort of ballast?
You need lateral resistance for courses other than straight downwind. I guess that a tinnie hull with a rudder and a crab claw rig in the forward half of the hull should be good enough for about 30 degrees either side of downwind. If you place the rig well aft, so that most of the lateral load is on the rudder, and you make the rudder large enough and give it good balance, then you should be able to sail a lot closer to the wind. See the interview with Matt Layden in Small Craft Advisor #57, pages 17-22.
Most arrangements of foils in air and water create a heeling moment, which you need to oppose with a righting moment. Whether you need ballast for that depends partly on the size of the boat and the skill of the crew. Alternative ways of getting righting moment would be the Bruce foiler shown in the first picture, the paravanes in my last post, or not generating a heeling moment in the first place, as is possible with kites. I forgot the most interesting configuration, the Windfly Rig. It's inventor linked to a now defunct web site on this short thread on
The WindFly Rig - Fast and stable kite boats - no heel, no leeway - Amateur Yacht Research Society https://www.ayrs.org/phpbb/viewtopic.php?t=2284, but the pdf to which he linked is still accessible. The part most relevant to you is that the whole rig clamps to the transom in place of an outboard, so there would be absolutely no modification to the hull of the tinnie. The drawbacks are that the rig seems to needs a traction kite with an inflatable spar so that water launch is possible, and probably it needs low friction blocks. And the web site being defunct may indicate that the inventor failed to find investors, so the rig would not be readily available. Is the availability of specific parts, rather than generic types of parts, a criterion in your project?
I looked up tinnies, and found one with a plate thickness of 1.6mm, about the same as my aluminium canoe. That is about half the thickness that I have seen described as the minimum for welding, though a video of manufacturing an aluminium canoe does show some welding. So you will need to check under what circumstances and with which techniques it is possible to weld aluminium as thin as that in a tinnie. Riveting may be safer. But if you can't weld, then a centreboard or daggerboard is out, you will need to fit leeboards. Look up
Open Canoe Sailing Group - for canoe sailors across the UK https://www.ocsg.org.uk/ for examples.
Or if the centreboard itself was made of solid steel could it perform both jobs?
Yes,
if you can provide a structure that holds it in place,
if that fixed keel doesn't interfere too much with pulling the tinnies out of the water at the end of the working day or if leaving them in the water is a viable alternative (needs more infrastructure), and
if you can find a way to weld steel to aluminium, and
if you can avoid galvanic corrosion between steel and aluminium.
