basic multihull knoweledge

Discussion in 'Multihulls' started by neptunkryssare, Oct 23, 2012.

  1. tspeer
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    tspeer Senior Member

    Dave Keiper's Hydrofoil Voyager is back in print. This is his account of Williwaw, the only flying hydrofoil sailboat that's crossed an ocean. It had foils that could be retracted for hullborne operation or deployed for flying.

    In the book, he describes how having the foils down during rough conditions improved the damping and helped to make the boat more seaworthy. Which was probably a good thing, because when the conditions got too rough to fly, you wouldn't want to be climbing out on the foils to unbolt them, the way Keiper had to do to retract them!
     
  2. warwick
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    warwick Senior Member

    From what understand, Gary Baigent likes to use a T foil on the rudder, for longitudinal stability, I would think that could be one way of doing it. I am not sure of Doug Lords approach.

    I was following Gary, when He was writing for the NZ sea spray magazine. However before his foiling experiments.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    As already noted by tspeer above, the foil, in this case could only provide damping.

    Since long.t stability is governed by the waterplane inertia and the volume, to obtain the BM for the hull.

    If the hull is totally foil borne, that is a dynamic stability issue and is subtly different, and then it could come into play depending upon the amount of vertical force it can generate in over coming pitching moments.
     
  4. neptunkryssare
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    neptunkryssare Peter

    Thank you very much for the tip. I ordered the book. It will be the first book I read as a papier-book since many years.
     
  5. luckystrike
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    luckystrike Power Kraut

    Have you seen this piece of crap in reality? Of course this is a poor photo to decide, but if the mast, boom, sail, shrouds, beams, rudder, daggerboards, trampolin and mainsheet have the right size fot your future project, are factory made and in really good condition, this is a bargain!!!! These parts are the biggest cost factors in building a small boat.

    I think with a little chaffer you could get it for 4500 SEK or even less.

    Best Regards, Michel
     
  6. neptunkryssare
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    neptunkryssare Peter

    I am petty sure I can get it for less than so. And I agree that there is some good donor-parts in this cat.

    But to me living on a budget. To be out with my son coming summer, when he gonna sail his own built boat, is more important to me/us, than have something that I can make som use of later on.

    I posteed in another thread a pic about the boat I hopfully is going to buy, a Flipperjolle, also called Fabola flipper scow - which quite much will match the boat he have built -, and for much less money than a cat that sucks™.
    Here is a boat of that type I am going to see and maybe buy tomorow.. [​IMG]

    75 Kg boat, 10 sqm sail plus genua. 2000 sek. I think not much can beat that.

    To reduce drag I have some plans about make the bottom slightly slanted up a bit from the back.
    Maybe it also can be a good candidate to try foils on if I can get bigger sails.

    Later, I will use that rig and sail as parts in the first 'real' little cat or tri I will start build coming fall, it's a part of the plan.
    Also - I think its easier to me to build a rudder and one or two daggerboards than to sew a sail (ok I will make a sail to another very low budget project which I have in mind - hopefully soon - In that case, more about it in another thread).

    And regarding the cat, I felt a little (very) pissed because the seller didn't want me to try it on water.
     
  7. luckystrike
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    luckystrike Power Kraut

    Ok, back to multihull basics.

    Forget to use that rig on a cat or tri, as long as you want to go fast ... and I understood you want exactly that.

    See the "normal" Dinghies (you posted photos from) as a lawn mower and the cat/tri you want to build as a Ferrari. If you do what you have written you will have a Ferrari witha lawn mower engine.

    Cats and Tris (the faster ones) have special rigs to provide the power they need to go fast. That means high aspect ratio mainsails with full length battens, a rotating mast with a teardrop shape and special shaped sail profiles. This is the reason why the" piece of crab cat" had bargain potential if they were in good shape. They were multihull parts.

    The normal rig of a dinghy is far to low and the profile of the sails are all wrong to make a cat go fast.

    On the other hand you are absolutly right, beeing out sailing is a absolute priority, especially if you sail your own dinghy together with your son in his own. Better than staring on a pile of wood inside your workshop that will become a cat or tri someday.

    Foiling: Don't expect to get good results from foils on an old dinghy. It needs an experieced designer and engeneer to design and calculate a workable platform. No way to fit some foils (without carbon) on an old dinghy and expect it to go airborne. To much wheight, inefficient sails, the hull will not go fast enough to reach lift off speed.

    Best luck to you, Michel
     
  8. warwick
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    warwick Senior Member

    Another approach you could consider later on, you might be able to use one of your catamaran hulls as main hull and build some medium size floats, then add foils later on.
     
  9. neptunkryssare
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    neptunkryssare Peter

    Lucky
    -------
    Ok, I read what you say.
    About that catamaran. I don't know about that rig - what it was designed for from the beginning as the cat itself is homebuilt and the rig probably is picked from somewhere.

    Is there any way to modify a donor rig so it suits a multi.?

    I payed my attention to the hulls, and I am petty sure that with good plans I could make better hull, daggerboard and rudder. I still think so. As I belive you are right regarding the rig, I maybe will rethink and make a bid for the cat.

    But not about 4000, I simple can't afford that right now. So I keep play cool and give him an offer at the end of the period he have for his ad.

    And until next autumn I am gethering knoweledge, and in theese cases that I am wrong . which it seems that I am in this case, then I am glad you o anyone else tell me so.

    As my son is within an organisation where they build boats and manufacture their own carboon masts (simplest method : vacumm infusion - male mold), I am pretty sure I cold get their help build a mast. I doubt they take very much for their time.

    What I don't know. is the price of carbon, he have not made his mast yet so we have not bought what's needed for that.

    But - is carboon that pricy so its better to buy a alu rig for 4500 sek / 670 usd.?

    About the foils, yes I should probably forget about them - even though I maybe can manufacture foils at my sons organsiation. But sure, for now I don't have the knoweledge. And even If I could gain that knoweledge until then, it's better spending my time learning other things and play with foils at a later point.

    Thankyou for your time
     
  10. neptunkryssare
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    neptunkryssare Peter

    Is that really vise.?
    It's a v-hull and the seller have told that theese two hulls can barely wear two adult men, and not sail with them.
     
  11. warwick
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    warwick Senior Member

    You are correct Peter, I was not aware of the hull shape.

    Would it be possible to build up the hull volume with foam.

    Gary was able to build a foiler from the remains of his catamaran Supple jack, It is mentioned else where on the forum. ( I inquired into Supple Jacks where abouts)
     
  12. luckystrike
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    luckystrike Power Kraut

    Hi Peter,

    just short comments this morning.

    Is there any way to modify a donor rig so it suits a multi.?

    No way!

    I payed my attention to the hulls, and I am petty sure that with good plans I could make better hull, daggerboard and rudder. I still think so. As I belive you are right regarding the rig, I maybe will rethink and make a bid for the cat.

    But not about 4000, I simple can't afford that right now. So I keep play cool and give him an offer at the end of the period he have for his ad.

    If the hulls are so bad, make a very very low offer and expect to win! When I started designing and building my tri I wanted to build a 26 footer, but bought a mast and mainsail for a 30 foot for a very low price. So, now Iam building a 30' Trimaran now.

    What I don't know. is the price of carbon, he have not made his mast yet so we have not bought what's needed for that.

    Carbon is very expensive. Here in Geermany the prices start at 15-20€/m² for very light fabric, three or four times the price of similar strength glass.

    But - is carboon that pricy so its better to buy a alu rig for 4500 sek / 670 usd.?

    Expect to buy a mainsail in good used condition for around 800€ (Tornado or similar race class). Sails you can get on ebay for 300€ are worn out and not worth the money. And you need the complete rig, so there is the mast profile, boom, shrouds, sails, halyards, sheets, trimlines, fittings, turning blocks, traveller and so on. Don't expect to pay less than 50€ for every single item on this list if bought new. A used mainshhet system with traveller is not cheaper than 150€ used cond.

    Best regards, Michel
     
  13. neptunkryssare
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    neptunkryssare Peter

    Ok, as this thread is named 'basic multihull knoweledge' , (I should for sure have given it a better name if I was natural english), I would really like to know where the big difference is between the rig on a monohull vs on a multihull.

    I should be really glad get some understanding about the principles regarding this.

    Which litterature should I read to better understand mast and rigging on multihulls.?
     
  14. neptunkryssare
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    neptunkryssare Peter

    What do you think about this one.?
    http://213.132.112.100/images_full/31/3121465721.jpg
    Could I convert it to a mainhull for a tri.?
    It's 4 meters in length, and 80 cm where it's widest.

    I suppose that I have to completely cut away the bottom so I can change the boats profile.
    Then I hopefully can change the shape of the bottom (?) so it's more like the bottom on a tri, and glue it back with some fiberglass .
    Also - because the sides is wider at it's upper part , I have to reshape that as well.

    The boat seems a little wobbly, but that makes it more easy to reshape.

    And the price then - the seller told me 'as the oars is not origin, I can sell the boat to you for 400 sek without oars' - It's about 38 euro. He even offer to transfer the boat to me.

    The reason I am asking - I see a piece of glassfiber which I hopefully can do something with. And I think it could be a good exercise for me before I try to do something real serious (I am now looking around for rigs now).

    If it goes well, then I can make a pair of outriggeers for it, if not - then I have not lost to much money - but probably lerned a lot.

    I don't want to take this project to seriously, I will use sheap materials for the most parts, so it would be more like a 'full scale model' of a small trimmaran.
     

  15. rattus
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    rattus Señor Member

    It would probably be best to source a rig from another multihull of similar dimensions - think in terms of strength to accommodate righting moment.

    I'm making these numbers up, but proportionally they illustrate the point. Assume a monohull with a righting moment of 10000 nm, and a similarly size catamaran with an RM of 50000 nm. The monohull has the ability or luxury of heeling with increasing wind, thus "softening" the load on the rig. The catamaran will resist the heeling force by a factor of 5x, and thus the rig must be designed to withstand 5x the stress - BOTH for the mast and rigging as well as the sails.

    If you were to consider a monohull rig, think in terms of similar righting moment, not boat length.

    Mike
     
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