Open source 12-15m high performance/semi-cruising catamaran design

Discussion in 'Multihulls' started by groper, May 10, 2017.

  1. Doug Lord
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    Doug Lord Flight Ready

    Kurt Hughes foil assist trimaran design-note size and depth of curved ama foil:


    Trimaran Kurt Hughes 2012 w foils.jpg
     
    Last edited: May 10, 2017
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  2. Doug Lord
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    Doug Lord Flight Ready

    Cruiser racer cat with surface piecing foils and rudder T-foils:
    Groper-foil+ T foil.jpg
     
    Last edited: May 10, 2017
  3. UpOnStands
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    UpOnStands Senior Member

    Question: How transferable is the trimaran system of semi-lift foils to cats? The foil assist works well in tris as the amas are so far outboard that their lever arm is significant.
    Here we have a 14m cat with 7m beam so lever arm is 3.0-3.5 m. The cats hulls are designed for accommodation and load carrying whereas the amas are designed to generate righting moment and counteract sail loads.
    To my mind there is a design disconnect.
    Are we looking for 30%, 50%, 70% lift from the cat foils?
     
  4. Doug Lord
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    Doug Lord Flight Ready

    Curved foils on ocean going trimarans (for foil assist) are at a deeper angle than you show for your foils-see previous post. In my opinion, a mostly horizontal foil will work poorly at the position you show it. I think with all the considerations you have mentioned a "Z" foil comes closer to doing everything you want , particularly in combination with rudder T-foils. I think interior room with the hull shape you show would be similar with the Z foil and your curved foil. By using two foils to do the job a single foil can do you're unnecessarily complicating the design, building and sailing of the boat.
    Z foil cruising.png
     
  5. Doug Lord
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    Doug Lord Flight Ready

    Catana cruiser (racer?) with curved foil assist foils:

    catana59.jpg catana59- curved foil.jpg Catana-59-performance-confort-croisiere-2.jpg
     
  6. groper
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    groper Senior Member

    Look, the foil angles, dimensions and what not can and most likely will be changed, this is an initial concept rendering... the renders are only shown to help visualize the concept thats all - dont read into it too far...

    Heres a farrier system;
    [​IMG]

    By the time the boat heels and is pressed, the curved foils are getting pretty close to horizontal. Once that foil is moving through the water at 20kts - can you imagine how much vertical resistance to heave its going to produce?
    do the math - F=Cl.0.5.rho.v^2.A
    assume an area of 1m^2 and an angle of attack of just 3 degrees @ 20kts = ~1.7 tonnes!
    Sharp pitching motions will likely cause more than 3degrees AoA so the damping effect could be alot more.
    The pitch and heave stabilization effect is huge and its very evident in the videos of the ocean racing tris and just how steady their pitch and heave is whilst running at speed - thats why im interested in the idea more than anything else - seakeeping.
     
  7. Gwion
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    Gwion Junior Member

    Ok back to general design.

    I often ponder where modern multihull design is taking the cruisers amongst us.
    By that I mean we see design trends come and go.We see rounded side decks, reverse bows and buoyancy pushed into the ends .
    Whilst some of these features look great, and perform great in certain situations, I often wonder what effect some of these features actually have on the big picture.

    Who can deny the Bill Gibs Wahoo looks spectacular.

    After reading Michaels Blog from Surf Sail Roam on windward performance I couldn't help but wonder wether we have designed ourselves into a corner so to speak.
    Upwind Catamaran Sailing onboard ROAM http://sailsurfroam.com/blog-michael/2016/9/24/vbx0ubebpk3r917mbp03893ie1vv6v
    Michael talks of windward performance
    QUOTE"As soon as the TWS approaches 20 knots, anything over 7 knots SOG is very unpleasant due to the sea state."UNQUOTE

    20 knot tradewinds is not strong. Infact sometimes one wishes the trades would drop to 20 knots.
    Have we got to the stage where the designers are more interested in potential top end speed than designing a boat that can actually be sailed hard to windward without
    destroying the crew?

    Sharply "v"'d forward sections without massive reserve buoyancy certainly smooth out the lumps a little. Yes , the tendency for these hulls to pitch does increase but then again so does the addition of massively tall rigs.

    Has the pendulum swung too far?.

    Would boats like Surf Sail Roam actually be better allround boats, and hence faster if some of the extremes were dialled back?.

    Certainly not better on the race track, but better and quicker on the cruising track. Is Surf Sail Roam a race boat? I wouldn't have said so.
    One thing is for sure its a lovely boat that anyone would be proud to own.

    A 12 to 15 meter boat is still a small boat. 20 knots sustained for any length of time with a small crew is going to be tough.
     
    Last edited: May 11, 2017
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  8. groper
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    groper Senior Member

    Its funny you brought up ROAM... i designed the system and supplied the components for his watermaker :) So i know of Mick and his boat and travels quite well.
    ROAM is still very much a cruising boat IMHO. Micks comments about sailing upwind are very valid and i agree with him 100% - anything above 7kts upwind in a decent blow gets pretty uncomfortable for long passages and its pretty hard on the boat too. However;

    A good performance boat with low windage will get a better pointing angle upwind. You can still keep the boat at 7kts but do it at a higher pointing angle provided you have good boards and less windage. Thus youll still get better milage upwind at the same comfortable speeds for long upwind passages for this reason. The condomorans on the market these days will hardly go upwind at all - they always end up motor sailing as they simply have too much windage and leeway and can barely make 80degrees to the wind.
    A good performance boat will cover alot more miles in the same winds when the wave action is not an issue and you can really stretch its legs. 20kts sustained is not a problem in a following sea state...

    None of the current boats on the market have attempted to use a horizontal foil to this extent beyond a few high performance boats such as the Beiker b53 "Fujin"with its curved foils- and which has proven faster then all the gunboats its raced against and most other performance cruising multihulls out there- save the full on race boats.
    The new Multi 50 trimaran one design class is also using these foils this year.
    Just about all modern high speed catamaran passenger ferries are using forward foils and stern interceptors usually with active control - again the benefits to seakeeping are enormous and the million dollar systems they are buying can be justified because it works so well.
    As far as i can see - its simply that the performance cruising designs are a bit late to catch on! and the fact that most cruising designs are biased towards comfort than performance - market forces at play here... but the important thing to note is that the forward foils will improve their comfort in a seaway!
     
  9. groper
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    groper Senior Member

    How do you raise the board once the bend in the board reaches the hull?
    Can you remove the board entirely from above or do they have to be inserted from below? If so - its a non starter for a cruising boat.
    I can see problems with marine growth, deeper draft, not to mention being able to completely eliminate the drag of the angled part of the board in light conditions - its always in the water. This is not a round the bouys racer or a model doug - theres alot of real world problems boats have when they live in the water full time and go cruising the world. Theres no way your looking at hauling the boat onto the hard ($$$$$) simply so you can do a repair on the boards after you bump a reef or something - the boards have to be removable from above or your asking for a nightmare in terms on maintenance and repairs, cleaning etc...
     
  10. catsketcher
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    catsketcher Senior Member

    My sailing upwind tends to be similar in vein to Roam - the boat is more capable of dealing with stress than me. We slow down when the boat moves too fast up or down. It can handle the speed but we can't.
    It is easy to over estimate the need for speed. One thing the Australian designers seem to have done is over estimate the desire for speed in owners. There are more older owners who want slow and solid condomarans rather than detuned cruiser/racers - I don't but I am outnumbered by this larger cohort.
    I don't think anyone will be able to get around the maximum acceleration limits that humans can handle on small boats. Going fast involves large accelerations unless you increase mass or length. Even the use of foils will largely be counter to comfort. A horizontal foil that resists heave will mean that the boat will be held down when waves rise, making it much wetter. A foil that holds the boat up for a while might then ventilate as it comes over a wave (going to windward) and the whole structure falls down.
    Watching Moths in waves is really fun. They don't do well in large seas. I would want my boat to heave easily in seas so that she is as dry as possible. Moths handle chop beautifully but have problems when the waves get to a longer wavelength.
    Rather than resisting heave I would like to have better pitch damping and maybe some active small foils to smooth out smaller motions like horizontal sideways (sway) and yaw.
    Like Ad Hoc I can't see how a project like this will work unless someone does work on beams, someone writes a database for hulls, someone writes more generic stuff for rigs and then people can pick and choose parameters rather than produce a specific design.

    cheers

    Phil
     
  11. groper
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    groper Senior Member

    Yep i fully agree - which is why im attempting to design a boat rather than buying a condomaran... much less drama to go out and buy one, i dont particularly want to build again but if i have to to get what i want then so be it...
    i have to disagree here. There is a multitude of study papers which all evidence a very significant reduction in pitch and heave motion when foils are introduced. Sometimes as much as 50% reduction in accelerations and amplitude. Remeber this is no a full foiling design like a moth!
    This doesnt seem to happen if you watch the videos of the big racing trimarans so other than a hunch, what do you have to base that upon besides a moth which isnt really comparable to this idea i dont think?
    yep agree once again, Adhoc is probably correct.... i was hoping someone would jump in here and say its been done before or had examples of similar etc ... i posted the idea on SA and again noone seems to really have anything to say about the idea or why it wouldnt work well or at all. Looks like ill have to go it alone again... might build a model first tho :)
     
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  12. Ad Hoc
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    Ad Hoc Naval Architect

    I think there in lies your problem.

    Without the DNA (SOR) defined even armchair critics are finding it hard to comment on something is yet to be 'something'.!

    As it is your boat, you need to identify what YOU want the boat to achieve - what is the objective, overall. Once you've done that and sketched it out after running some basic numbers...it is open season. :eek:
     
  13. Doug Lord
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    Doug Lord Flight Ready

    ------------------------
    The Z boards are being used on a number of boats and are designed to be fully retractable. The trunk has to be a bit larger to accommodate the board.
    Induced drag from the angled part can be nearly 100% eliminated by adjusting the AOI to zero. Since this is not a full flying foiler draft should not be a problem because the boards will be smaller and, again they are fully retractable from above. You will have the same problems with marine growth on the foil you sketched except twice as much!
     
  14. groper
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    groper Senior Member

    Damn Doug - i wish you would think a bit more sometimes... no growth on the boards when you lift them clear of the waterline when not in use!
    How much cruising or real world sailing have you done Doug, any? Z foils are completely impractical for this in many ways, i dont have time to explain that beyond what i already have- ill leave you to think about it...
     

  15. Doug Lord
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    Doug Lord Flight Ready

    Thats just ridiculous! Z boards are no more affected by marine growth than any other board.
    I would really appreciate it if you could explain the difference in marine growth between the foils you drew and a Z foil . Surely it would be enlightening to others as well.
    But, of course, only when you have time.
     
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