Foil Assist for current kayak trimaran then building dedicated trimaran

Discussion in 'Multihulls' started by kaymaran, Dec 15, 2015.

  1. kaymaran

    kaymaran Previous Member

    Hi everyone,

    I’m a new member here (although I've been reading the site for about a year now) so please try to understand if I ask something dim.

    Also apologies up front as there is quite a lot of info here so I understand if you get bored part way through.

    I have some small sailing experience on Hobies/Darts/Lasers and Argos recently and some building experience having built a CLC Sport Tandem sea kayak and then converted it to a hybrid trimaran with the CLC Mk3 sailrig. (Also well into old cars so have a bit of mechanical ability.)

    This has been great but the recommended mast was far too bendy so I have put on some stays and added an RS200 jib and it flies along. (one of the videos below shows the mast all over the place but was far from the worst it looked...

    It is however still very slow to tack as it is very light, 22ft long, the leeboard is too short and the mast too far forward - all being remedied over the winter I hope.

    I have then recently added some test wooden trampoline frames so my wife can hop out of the front cockpit to help keep it flat when breezy but don’t have any decent video of that yet.

    Here’s a couple of links showing some of the development and some videos:
    Some early build photos and test with various windsurf rigs:

    A few photos of main rig, jib and trampolines

    A video of the trampolines being tested unfortunately in almost non-existent wind

    and since then it has been raining / no daylight / blowing a gale so nothing useful to report.

    As a kayak by the way these Sport Tandems are amazingly fast (and tippy) – it simply flies along and can take a load stuff in the for and aft water tight hatches.

    Once the updates have been implemented the next steps are building a new dedicated trimaran about 15ft long and I have just bought an ISO dinghy with loads of spare sails and a spare mast etc. I bought have some Zest plans from Richard Woods but am also developing my own design that I am finishing off shortly as well.

    Main aims of the project are:
    • made from ply again with fibreglass covering
    • 4.75m long
    • aiming for something that is sort of like a Weta so one or 2 handed, fast and light to manouevre and easy to store on land
    • width to be finalised but quite square for stability - this is key as my mobility is not great following several broken neck and back motorbike incidents so we want as much speed as possible with some degree of safety so the wife is ok with me doing this. More extended stays in hospital are frowned upon...
    • keep it as light as possible. Based on the kayak experience very low weights with great strength are achievable. I can put the kayak on the roof of the car by myself if needed – the main hull is around 30kg I think – including loads of reinforcing for the mast and multiple mast steps as I am experimenting with multiple mast configuration s. Aiming for it to be the same weight as the Weta so approx 100kg all up if possible. Me and the ever forgiving wife come in at 130kg max.
    • outriggers to be as long as the main hull with high volume forward to reduce pitch poling but kept quite narrow
    • main hull bow volume reasonable as well for same reason but probably open throughout as only for thrashing around and not looking to overnight on it. Might put camping gear on it to go round the Isle of Wight or something but that would be it
    • mesh trampolines with raised outer rail on outside for sitting on / trapezing if required. This means we won’t be standing on the outrigger directly so it can be kept light
    • open transom for easy draining
    • crossbeams and outriggers easily removable for easy storage in dinghy space on land and can be either aluminium or laminated wood as per kayak (but beefed up obviously) and possibly with hollowed centre to reduce weight. The strength of the laminated douglas fir crossbeams I have for the kaymaran is quite amazing.

    Many more things as part of the design but I think those are key.

    The final one though is where I really need some help.

    I am extremely interested in the concept of foiling and although I’m not looking for a fully foiling vehicle - certainly not in the first iteration anyway – I am looking for foiling assist especially on the outriggers at the front beam again to reduce the risk of pitch poling and reduce wetted area of the outrigger. Further on after testing then more options become possible.

    I am thinking about a hydrofoil that could be simply slotted into the outer part of crossbeams when wanted that would be at roughly 45 degrees but I could vary this (although not on the fly and then once right angle found I could fix in place) or a simple inverted T to start with but this is obviously less flexible.

    I guess my main question is how do I calculate the lifting forces involved on the hydrofoil?

    I have found a spreadsheet from the web that looks at length/width/depth/area etc from here:

    are there any other similar resources available?

    Looking at this if I use for example:
    foil area 1.29 sq ft using 60cm x 20cm
    Speed 5 m/s so approx 10 knots
    Assuming the figure already given of 0.05 for thickness so thickness of this foil would be 1cm – seems incredibly flat?
    What is the foil depth divided by chord? The depth in the water? If so I’m assuming this is 1 ie about 20cm below the water
    Aspect ratio of 3 (60/20)

    This gives a lifting force of 1000 pounds.....

    If instead the thickness is 2cm it brings it down to 975 pounds. I am obviously missing something very obvious here.

    What foil profile is it assuming? It appears to be assuming a flat bottom but what sort of shape?

    I will be experimenting shortly with various profiles and intending to use the profiles as a basis and can hopefully get some decent strength by using extra internal supports and weld it up then a decent sheath of epoxy and fibreglass.

    Does anyone have any thoughts around a profile to try to emulate and where I can get dimensions from?

    I can knock up some tests and apply to a test bed and rag behind a friends boat to see what lift can be generated.

    Instead of the plug in option I could build essentially a dagger board case in the outriggers that an inverted T foil could be mounted in it and height adjustment and angle can be manually adjusted for testing. There is also the option of the wave piercing foil being mounted to the inside the outrigger and angled inwards so reducing some width issues.

    Has anyone any thoughts around this and strengthening / mounting techniques for the foils as I want to ensure I take account of as much as possible before getting into the full build.

    Finally for a laugh I will be attaching a sizeable kite surfing kite to my current kaymaran and possibly the new project just because it seems likely to offer some amusement...

    Any and all thoughts welcome.

    As long as they are nice....
  2. Doug Lord
    Joined: May 2009
    Posts: 16,679
    Likes: 351, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    Foil Assist

    I recommend that you order this book right now-it will be a big help: "Hydrofoils Design Build Fly" by Ray Vellinga. Available from Amazon:

    A foil section to consider is the H105 by Tom Speer: Without a doubt the best foil type for a full flying tri ama is the UptiP foil pioneered in AC 34 by Team New Zealand but it will take a lot of research to get it right. Angled straight foils can work for foil assist. You have to decide how much of the total weight you want the foil to lift-for high performance foil assist that will be between 60 and 80%(flying the main hull). That leaves the ama to provide 20 to 40% of load carrying. The boat would benefit from a T-foil rudder. This assumes that the engineering has been done to allow the main hull to fly. You can do foil assist with lower capacity foils but your gain will be significantly reduced if you don't want to fly the main hull. Again, read Rays book to learn about hydrofoils and how to calculate lift. And spend time researching boats similar to the boat you want to wind up with.
    The "Small Trimarans Under 20'" thread here may have something that interests you. Good Luck!

    PS- don't shortchange yourself by not doing the research you need to do to fully understand the concepts involved and don't hesitate to contact a yacht designer/engineer for some help.
  3. kaymaran

    kaymaran Previous Member

    Thanks Doug, that book is currently sitting under my christmas tree waiting for the 25th courtesy of my ever understanding wife.

    The initial plan is not lifting the main hull as that is far too ambitious but gaining experience of getting some lift more for extra support and safety initially is what I am after.

    Has anyone on here used the profiles and had any success using for lift instead of rudder/dagger boards?

    I am also in contact with Gareth Roberts from windknife and has a trimaran plan in the pipeline apparently so that could be useful as well.

    I'm super busy with work for the next 6 weeks then going to get cracking into the initial build of the outriggers as they can be lashed onto the kaymaran or the ISO while I then get on with the main hull.
  4. Doug Lord
    Joined: May 2009
    Posts: 16,679
    Likes: 351, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    The windknife concept looks real good unless you need an asymmetrical foil.
  5. kaymaran

    kaymaran Previous Member

  6. Doug Lord
    Joined: May 2009
    Posts: 16,679
    Likes: 351, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready


  7. kaymaran

    kaymaran Previous Member

    I met up with Gareth Roberts from and have picked up a section of his aluminium extrusion and will get on with getting a first trial hydrofoil prepared over the next few weeks but work will be flat out until end of January.

    Will pop up some info as things develop and I will use the kaymaran as a test bed while I carry on preparing the new project.
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