Will this hull work for a 12ft beach cat?

Discussion in 'Multihulls' started by ahen, Mar 1, 2017.

  1. guzzis3
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    guzzis3 Senior Member

    I don't think that's the point.

    I'm putting dyneema shrouds on mine despite several people advising against it. Might be a disaster, but I might learn something if it is.

    This project is a cheap simple boat. He's not trying to design his own 12 meter bridgedeck cat. If you reread his posts he realises it probably won't be a great boat. He wants to try something really manageable and learn from it. He doesn't want to copy someone else's boat. So it goes pear shaped, so what ? He's wasted a few hundred dollars and maybe 150 hours. He has specifically said, after so many people told him to build a proven design, that he doesn't want to do that.

    If no one ever tried anything new we'd all be stuck sailing oak timbered blunt nosed barques and schooners.

    He has asked specifically for input on his ideas so he doesn't make a complete hash of it. I think we would be most helpful giving that instead of telling him to do something else...
     
  2. guzzis3
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    guzzis3 Senior Member

    Hi,

    Looks like you've decided on chined ply. That's one decision made :)

    Here for what it's worth is my 2c.

    As others have said a v hull is not ideal for a few reasons. Your better off designing a dory hull. This sort of shape \_/

    More fullness forward is imperative. If you leave it that fine you're more likely to pitchpole (capsize). Cats need buoyancy forward or the bow digs in and the boat falls over diagonally.

    I think the numbers you presented above have confused some people. Also some of the information. Can you clarify :

    Draft: 18''
    Buoyant force: ~3000 N if fully submerged

    Is 18" the draft to the decks or the design waterline ? In other words how much draft with a parson on board ?

    The prismatic coefficient is about .72, but that includes all the freeboard.

    That's a good start but as I said more fullness forward, toward the bow. These are very fine hulls anyway, it won't go faster with very pointy bows and it won't go much slower with fairly blunt ones, unless you build it like a tug boat :)

    Once I finish the hulls I plan on connecting them with 6' wooden beams that will be the main structure of the craft, from which the center board, rudders, and mast will be attached.

    My little cat is 11'6" and 5'6" wide. 6' is enough, you can go wider if you wish, or not. Most modern cats are about 60% length to beam, so 6' 9", but it varies a lot on beach cats because the crew weight has such a huge influence....most 14' cats are 8' or a bit under, say 7'6".

    What I am more concerned about is your statement above. Read literally it sounds like your mounting the rudder/s and board/s on the connecting beams ? Is that what you are planning ? If so your making potential problems for yourself.

    By the way I happened to weigh my boat the other day. Less rig it's 44 kg or about 95lb. With rig it will be less than 60 kg or 130 lb. Unless you build your boat VERY heavy it should come in well under 90kg or 200lb. There are plenty of 14' cats that weigh 90kg. I know your building a heavy boat that's fine, but a 12' cat is a fair percentage smaller than a 14'.

    Anyway good luck...
     
  3. ahen
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    ahen Junior Member

    Thanks for the info, I realize I used the wrong word. The 18" is the distance from the deck to the keel, not from the waterline to the keel. When I redesign should both the front and back halves of the hull have similar volumes? Should I keep the back half at least a little more buoyant than the front to counteract my weight or make them equal?
    EDIT: A better way to ask this question in terms of the advice sailhand gave me: Where should I make the keel panel widest? Right in the middle of the boat or should it be shifted towards the stern?

    Thanks for shying me away from veed hulls, the flat bottom will probably make the construction easier, which is what I want anyways.

    I plan to attach the rudders to the hulls, but maybe having a centerboard attached to the connecting beams.
     
  4. sailhand
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    sailhand Junior Member

    ahen now you are getting close to the right idea. there are several forces at play when a cat is sailing. the weight of the crew is forcing the hulls down at the centre of gravity of the crew. the sail is generating lift/force towards the low pressure area to leeward of the sail. the centre of this effort is close enough to the centre of the sail area, use both mainsail and headsail to calculate this. the centreboard/daggerboard is generating lift to windward to stop you just going sideways, and the centre of effort of that is the centre of the submerged section of the board. both of these are like the wing of an aeroplane and as water is much denser than air the board can be much smaller and achieve similar countering force to the sail. there is also the lateral resistance of the hull shape itself under the water. the dory type hull has more lateral resistance than a rounded hull shape and will prove beneficial in resisting leeway movement when under sail. the reason people spend years at university is to learn how to balance all these forces to get a nice balanced sailing boat that steers straight. imagine if you put the centre of effort of the sail area well forward and the dagger board well aft, every time you try to sail the sails would push the bow away from the wind and the dagger boards would hold the transom to windward and prevent them from slipping sideways. net result would be a boat that steered away from the wind as soon as you try to sail and you will be fighting with the helm to try and keep it straight. if you swap this around and put the sails aft and the daggerboard too far forward the boat will steer into the wind when you try to sail and again you will be fighting the helm. Imagine you are riding a pushbike that always wants to turn left. if you take one hand off the handlebars to scratch your nose the bike will turn hard left, this would be unusable, you need to balance your boat right or it will not sail very well at all. if you get the balance close enough you can adjust the centre of effort of sail fore and aft a bit by raking the mast backwards or forwards but this has limits. so you must be close. search this forum for "balancing a cat" for more information. you must consider the lateral resistance of your submerged hull in the equation as well as the rudders to a degree. nowadays you can calculate the centre of lateral resistance of all these forces easily with a computer, a simple way to double check after you have built the hulls is to place them in the water with loads similar to that you intend to place on them in the right position fore and aft and find a the point on the hull that you can push on and the boat will move sideways with the bows and the transoms moving together. if you push on the side of the hull and the transoms move sideways faster than the bows you need to move the point you push on further forward and vice versa if the opposite occurs. that will give you an approximation of the centre of lateral resistance but you can do this much more easily on a computer. you must then match the centre of effort of the sailplan to this point to get a somewhere near balanced boat. just like your pushbike, you can let the handlebars go and it will go straight ahead not veer one way or the other. this will allow all the forces to interact with each other to provide forward propulsion which is what you want. if you hold a bar of soap in your hand and squeeze it till it pops out of your hand you are doing the same thing. applying opposing lateral forces to provide motion in a different direction. of course all this changes when you are sailing downwind as you are simply being blown along to a large extent and all the forces are naturally aligned. looking at existing designs gives you a good starting point as the rocker and hull volume effects the submerged shape of the hull. the submerged shape effects the centre of lateral resistance and on it goes. like the environment everything interacts with each other to form a complex system, change one component and it changes everything else. to say what would be the widest point on the keel panel would be dependent on many factors so you cannott simply make an arbitrary decision like that without examining all the other factors involved. rocker will also effect the centre of lateral resistance, it also effects how the boat will pitch/hobbyhorse in the waves and also how quickly it will tack so you need to examine all of these things together to form a system that works in harmony. We are trying to help you but you must understand that you are creating a system that works in harmony and therefore there is no definitive answer for us to give. if I asked you how many whales should we have in the ocean you cannot tell me because there are so many factors to be considered just like in a boat design you are searching for a magic formula and each different paramater you define effects the answer. that is the reason you should examine existing designs to see how they came up with the answer. If you look at a design that sails well with a certain weight in a certain area and you want to carry more weight in the same place or move that weight forward then you need to change the hull volumes and rocker etc to suit, that is easier than starting from scratch designing a very complex system and your results will be more reliable and less prone to failure. you must make some arbitrary decisions and base your design on that and you can move forward from there, but these need to be ealistic and to do that you need to have some knowledge of what you want to acheive. as I said in my last post a beam length ratio of 12:1 is where I would start sor a more cruise orientated boat. this is a boat that will carry a little more weight for expedetion/raid type boating than simply just racing with the lightest weight. You are also building from ply and your boat will possibly be a little heavy as you are a first time builder. a draft of six inches may be a better starting point, maybe centre of bouyancy 3 or 4 % aft of the midpoint of the hull and a shallower run aft to keep your transoms out of the water as much as possible. a wider hull will be shallower and a skinny hull will be deeper, keep this in mind as you go from the bows to the transom. look at pictures of the arrow cat and the arafuru cat, both flat bottom ply hulls in small cats, and you will notice deeper skinnier towards the bow and flatter fatter towards the transom I will pm you a drawing of a generic type hull I am drawing at the moment just for you to get ideas, you will need to change parameters in this hull to suit your needs it is not suitable but is not too far away from what you could use. I just scribbled it up to give you an idea a bit closer to the mark.
    Goodluck
     
  5. upchurchmr
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    upchurchmr Senior Member

    Wow.

    Flat bottom is not so good, a round bottom will have less drag.
    Lets see what that sets off!
     
  6. sailhand
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    sailhand Junior Member

    here is the drawing and all the properties need to be changed like where the maximum beam is, the rocker, the height of the hulls,etc etc but it gives you some idea of where to start. it is not wildly wrong it is close to what you need but you have to work out the parameters you want to suit what you are trying to acheive.
     

    Attached Files:

  7. guzzis3
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    guzzis3 Senior Member

    Yeah again we already covered this in the previous thread. It probably would have been better to keep the one thread rather than start another.

    Huge post there sailhand :D

    I was thinking about a 5 panel dory so 3 below the waterline and a pair of vertical to get more freeboard but a 3 panel is easier I suppose.

    With a 12' boat you really have to manage displacement carefully. Mine is incredibly sensitive to where I am on the boat and is a submarine in any sort of chop. The nacra 350 has MUCH higher hulls so can carry an adult and child. I describe the experience as "intimate", you really have to be as one with the boat, far more so than on a 14' cat.
     
  8. upchurchmr
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    upchurchmr Senior Member

    Well the issue of flat vs round has been discussed about a thousand times.

    Interesting that even designers are inconsistent on the subject.
    I've been looking at the W-17 trimaran and talked about my desire for a round bilged boat (instead of the flat bottomed design) with the designer. He made all kind of "excuses"/ rationale and then drew a round bilged boat for his W-22 and said how wonderful it is.

    I guess if I remembered the other thread I wouldn't have felt the need.
     
  9. guzzis3
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    guzzis3 Senior Member

    It wasn't meant as an attack on you. It's just a pity we have started the same conversation over again.

    Ahen started this thread:

    http://www.boatdesign.net/forums/mu...sing-catamaran-design-construction-57203.html

    And then went on to explain himself. He wants to DESIGN his own boat and build it. Several people suggested tortured ply, established designes etc etc.

    As I say above if he was being sensible he wouldn't be building at all, you can pick up an old beach cat for loose change, but that isn't the point. Nor is it to design, first up straight out of the box, the greatest sailing craft ever. He' having a go, trying something new and in a sensible manageable package. Yep round hulls make sense, so does solid glass or foam, so does getting a set of afura cadet plans and building that, so does buying an old prindle and modifying it to suit. But IMO the best course is to do whatever makes him happy :)

    He's comfortable with chined ply for various reasons, so we'll go with that. V hull probably has some problems, maybe the dory will be better. He's talking about a board in the bridgedeck, that might get tricky but we'll see, or he'll change his mind. No matter what happens all he's risking is materials to build a 12' cat and the associated time. No big deal either way. And THAT I think is very sensible...

    Unlike me who's doing the same exercise with a 7 meter trailerable bridgedeck cat :D I must be out of my cotton picking mind...
     
  10. upchurchmr
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    upchurchmr Senior Member

    I'm perfectly fine with him wanting to do his own design.
    IMHO most beginners think that chined/ flat bottomed shapes are necessary because its "too hard" to build round.

    I blame Bolger for this attitude. Bad looking/ slow is fine because it is easy.

    Personally I just wanted him to realize he had a choice.
     
  11. ahen
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    ahen Junior Member

    I spent some time messing around with freeship. I attached what I have so far. I made it look somewhat like the pics sailhand sent for now, Ill mess around more with calculations etc. when I have more time.
     

    Attached Files:

  12. upchurchmr
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    upchurchmr Senior Member

    Couldn't open the file
     
  13. ahen
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    ahen Junior Member

    Thats odd. I was able to open it on a different computer after I downloaded it from the thread. I added some faces so Ill upload it again
     

    Attached Files:

  14. sailhand
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    sailhand Junior Member

    ahen hullshape looks better but it is 12 metres (40ft) long and displacement is 3.8ton a bit bigger than you started with. try file>new and set your dimensions for 12ft and 1ft 6in beam and draft 6 inches 7 points longtitudinal and 3 vertical and start again. you will have the right dimensions for what you are trying to do and accurate figures when you do the hydrostatics calculations which are in the calculations menu you will also have 6 points along the hull, that is every two feet to keep it simple. if you want a control point every foot you could go 13 but i find it easier to let the computer do the fairing (smoothing of the lines of the hull by making "fair" curves)
     

  15. Manfred.pech
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    Manfred.pech Senior Member

    In the history of small catamarans the designers have changed the lines of their designs. Very successful have been the plans for small catamarans of the board of Ron Given. Especially from his PAPER TIGER plans hundreds have been sold and boats built. Also his TIGER CUB design has been very successful.

    [​IMG]

    But now wave piercer hulls and squarehead sails are preferred and some like foils/hydrodynamic wings. Newer lines you will find from Chris Tuckers DS12 (DarkSide12) here: http://www.ctmd.com.au/ds12.php . Or: http://www.ctmd.com.au/DS12.pdf

    These lines have been modified by amateur cat builders and boats like KIWI 12 or VUDU 12 have been built from moulds and sailed: http://www.catsailor.com/forums/ubbthreads.php?ubb=showflat&Number=243944&page=1 .

    [​IMG]

    [​IMG]

    [​IMG]

    Hope this will help a bit. You do not need to start from scratch. https://www.youtube.com/watch?v=buZxq2oeNnQ
     
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