Inflatable sailing catamaran

Discussion in 'Boat Design' started by va262, Aug 31, 2011.

  1. va262
    Joined: Aug 2011
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    Location: Poland, Europe

    va262 New Member

    I project infatable sailing catamaran. I am projecting not exactly building. The floats will be made by professional persons. The rest will be done by other people (also proffesionals). I only will make holes in pipes in frames and fold the catamaran. Simply, I don't have possibility to make it by myself because I live in a flat not a house.

    The reasons for choosing such construction:
    - speed (because it is a catamaran),
    - a small place to store it,
    - a (small) car to go with it;

    floats (cylindric in shape): diameter 0,45m, length 4,2m. Perhaps I will choose bigger version: diameter 0,5m, length 5m.

    The weight of the crew will be about 260kg which is 2 persons with (big) bagage or 3-4 persons.

    Sail: balanced lugsail. What area of sail? I suppose 6-7m^2 should be ok?

    The frame will be folded and made from alu pipes of length 1,5-1,8m.

    I have also problem with joining the mast with the rest of the frame. I don't know what forces are here on balanced lugsail? But I have read that lugsail has small requirements for strength of a mast and other spars. Is that a truth? I'm a bit confused because it doesn't matter a sail is bermudian or lug, there is still some force.

    I also want to add centerboard. Is this a good idea with lugsail? I mean: does it have any sens? I know that catamarans sometimes don't have centerboards but only if they have v-shaped hulls.

    What do you think what will be the speed of this cat? Is possible to (easily) reach 11 knots in middle strength wind? I know, too small amount of info but an aproximation?
  2. keysdisease
    Joined: Mar 2006
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    Location: South Florida USA

    keysdisease Senior Member

  3. va262
    Joined: Aug 2011
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    Location: Poland, Europe

    va262 New Member

    Thanks, but I saw these links before. I don't want to buy. One of the reason is the price - I can make it much cheaper. The other reason is: I will make more flexible construction. For example I will have the possibility to easily change something. The next reason: I am engineer (IT however :) ) but it is interesting to construct something.
  4. freeman1
    Joined: Apr 2012
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    Location: china

    freeman1 Junior Member

    Hi va262 I am starting to do the same thing as you and has same questions:) Do you have answer for your question? If so, can you share?
  5. 176inches
    Joined: Apr 2012
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    Location: NW Ohio

    176inches Junior Member

    I have built (but not yet sailed) a cat ketch with lugsails. Lugsails have a lower center of effort and gravity, and need lower masts, so the forces on the masts and the strength of attachments should be lower. A balanced lugsail also needs no boomvang and is very gentle in a gybe.

    Now for centerboards: I have seen inflatable catamarans sail and the leeway can be pretty bad (due to smooth, rounded, high-riding profile). Without a centerboard I am not confident it would be able to tack at all, so do plan for one, and a pretty good-sized one too. Leeboards are perhaps a simpler alternative. And when you design it make sure you calulate the centers of effort and lateral resistance. You want them to be close together, and preferably the CLR very slightly forward of the COE: a slight weather helm is good in emergencies.
    Last edited: Apr 25, 2012
  6. va262
    Joined: Aug 2011
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    Location: Poland, Europe

    va262 New Member

    Finally, I haven't done the cat yet. Perhaps, it was easy to predict :) But I will share some information about my unsuccesfull project. I hope it will be usefull.

    1. Idea
    The idea was to construct folded luger cat for doing trips up to 500km for 20 days. So no more than 25-50km per day.
    A folded cat can be transported even in small car or on plane or train. So the whole world is open for sailing. Other approach was to make such folded cat with pipes of full length. But such construction will be only trailerable or transportable on roof of car. But it will be simplier in construction.

    Other advanteges: after some time it is relatively easy to change the construction. For example change the size of the sail, height of the mast, size of the platform.

    There were several problems to solve.

    2. Joining the pipes
    I had some ideas. It was the most important problem. Because the whole construction has to be rigid but strong. The most simple solution was to connect two pipes with the small third one and fasten them with screw. There are special nuts for securing against the unscrew. Probably the whole idea should be extended by some other elements. For example bolts and nuts can lean on small elements with shape of saddle (for adjusting to shape of pipe). I am not sure did I explained it clearly.

    3. Problem of sail
    Some people think the simple sheet of cloth should be enough as a sail. But some say, the sail need special shape to be most effective. I planned to find some information on internet and in library. There should be some books about yacht modelling for example.
    Choosing a luger as a sail was an obvious. There are other kinds of sails but personally I like the look of the luger. Note also with cat and its speed advantages no one needs a sail which will be such effective as bermudan (triangular).

    4. Floats
    The cost of floats was about from $260 up to $450 per one. It depends from material (0,9mm or 1,2mm) and number of chambers. The length 4,2m and diameter 0,45m. Pressure up to 0,25bar. The floats had to be done by proffesionals who builts inflantable boats.

    There were 2 projects of floats: one and two chamber. The second one had to be done along the float. So in case of problems (broken one chamber) there still be a long float.

    5. Other problems
    Testing is the most serious problem. Whatever you buy to use, it is usually a thing the producer gives some guarantee and it is usually tested. Lack of serious testing means that a folded cat can split its construction few miles from shore when the wheather is going to be worse...
    Other problem is the weight of such cat. Above some weight its folding capabilities are discusable.

    6. Why the project was unsuccesful
    I realized there are simplier solution for short trips. The project itself is not simple. It requires a lot of knowledge and time. The most important and most difficult problem is to create solution for joining all the pipes. It determines the strength of the construction and safety. Without serious (and probably tested too) solution it has no sense to create such folded cat. Perhaps it is good idea make a model of such joinment and test it (try to break).

    In the near future I want to do such cat. But I am not sure will it be folded or trailered one. Currently for short trips I buy inflantable boat and outboard engine. Yes, it is not the same as folded cat with luger sail but at least it is simplier solution for short time.
  7. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

    Comments in CAPS below from my 30+ years experience with tiny inflatable boats. Have experience with transporting boats by air as checked baggage which is the ultimate challenge, since any other transport would be much easier...

    Hope it helps.



  8. DimitriW
    Joined: Jun 2013
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    Location: belgium

    DimitriW Junior Member

    Check out my project/thread: inflatable sailing catamaran with tubeclamps
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