16ft beach cat design advice

Discussion in 'Multihulls' started by ch3oh, Mar 9, 2015.

  1. ch3oh
    Joined: Mar 2015
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    ch3oh Junior Member

    Hi! I'm designing and building a 16ft beach in 5mm ply stitch&glue finished with epoxy and 200g glass. Doing it on the cheap, so bought a bowless hobie 16 to salvage mast, tramp framing, rudders for starters, forestay and trapezes from.
    LOA is 4,95m, max. beam 2.50 (hobie pylons are to be offset and frame is sunk to 50mm over rounded deck level. Hulls are f16-ish with decks a tad higher and a bit more volume because of cheap and safe target weight of 90kg.
    So in with some questions!

    Q1: Should I opt for daggerboards (offset plumbing allows this) or fixed keels? Opting for speed allthough I'm underpowered and overweight. Simplicity and forgiveness of design might account for 5% perf. loss.

    Q2: If daggers, how far back should i place them? Surface vise I assume I'll need a tad less than lower aspect f16's. Don't want to go over 250mm wide so I might have to squeeze some carbon over ply core into the budget. I haven't learned the coe etc. enough yet, but I'm eager to do the math if someone points me towards some nice reading.

    Q3: I have a revised sailplan and more beam (and will trap on the hull edges) compared to a H16 so my righting moment increases. Is there any point of adding diamonds to the rig? Tube is thankfully one piece alu. Fathead trim might benefit from some sideways stiffness and ability to pre-bend some 50mm.

    I'll hopefully find the time to do a full set of publishable free drawings as the build goes. Here's the current (early, simple) sailplan, rake could be increased a tad. Using qcad to draw, I have done only some minor cad work so don't mind it.
     

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  2. teamvmg
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    teamvmg Senior Member

    I would have thought that hulls for daggerboards were much easier to build than asymmetric hulls or skegs

    Ditch the heavy pylons/side rails and build the front and back beams into the hulls

    The rig of the 16 works quite well, the standard sailplan is as big as the classic tornado was - so should push you along fine
     
  3. Corley
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    Corley epoxy coated

    Place the daggerboards in the hulls at the centroid of the sail area. The low tech method is to make a scale model of the sail area in paper and balance it on a pin. With a rear biased rig like the one shown they will be well back.
     
  4. ch3oh
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    ch3oh Junior Member

    Thanks for your input! I have been fiddling with ditching the pylons, rails and corner castings. It might save me a lot of weight and easen the decking. I'd still prefer the beams to come off for easy transport so I'm mostly worried about high stresses involved in the joints. I quess laminating hollow ply blocks to go like 150mm inside beams and placing bulkheads in line could work out nicely? Skegs would have to be binded to the inner structure whilst a glass daggerboard trunk would be easier to do but boards would have to be built. Standard sails are gone, and planned ones are on the works. I'm using 50mm round alu for boom&pole.
     
  5. ch3oh
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    ch3oh Junior Member

    Corley: I:ll def give the pin method a try if I can't get it out of my cad! Does placing the surface areas in line produce a neutral weather helm? Should I think about rudder generated lift?
     
  6. Corley
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    Corley epoxy coated

    It should be close to neutral possibly with just a little weather helm. Vertical daggers rather than raked seem to be the best compromise overall on cats but I'm happy to be corrected if that is discredited by new data. I wouldn't consider using an assymetric hull shape as daggerboards are so far superior. You get to choose your optimal hull shape for target displacement and minimum wetted surface area without having to compromise. On rudders I'm not sure I doubt lift would be an issue but I note that more modern beach cats are going for higher aspect rudders I'm not sure how that works as I've not sailed on one. Older beach cats favoured somewhat thicker sections and were generally lower aspect.
     
  7. rcnesneg
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    rcnesneg Senior Member

    This is the direction cats seem to be heading. Move the crew farther and farther aft, sails farther aft, daggerboards somewhere between front and rear cross-bars. Here is a good example.
    [​IMG]
    Note: That was an A-class, so mainsail only- meaning it is balanced for even more sail farther back than yours, so you could probably place daggerboards a bit farther forward, more like this Tornado (below, blue spinnaker)

    [​IMG]
     
  8. Erwan
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    Erwan Senior Member

    Have a look at the VIPER from AHPC, GREG Goodall is by far one of the best designer for beach cat.
    Hull shape is great and seaworthy for any conditions and his boats hads always a lot of smart tricks that make sailing easy and safe.

    Regards

    EK
     
  9. ch3oh
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    ch3oh Junior Member

    Viper is super nice, simple yet effective. Makes me realize how unrealistic it is to hit 2 digit weights though (She's 129 Kg) Board aspect ratio seems to be on the high side of feasible home build. I'll take some coarse board measurements from it's sailplan to have a reference! I allready copied lots of details from the well documented tuning guide, some applied to my boom.
     
  10. CT 249
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    CT 249 Senior Member

    The Viper is designed to be heavier than it could be; rumour is that it's because the SCHRS likes heavy boats. For a guy from Greg's background it's a heavy boat; I once had a great conversation with Jim Boyer (Greg's old partner, now retired) about the way that he physically found it hard to make a boat heavy enough for European rules.

    Mind you, the European/US concept of cheaper, heavier boats works pretty well; we recently moved from light cats to a much heavier US/Euro style cat and we're loving its durability and ease of handling, rather to our surprise. It's like a big fast V8 compared to a highly stressed turbo 4 cylinder.

    The Taipan 4.9, the earlier boat Greg co-designed, is just over 100kg fully rigged and it can be built at home in ply with success. The 'glass hulls last for ages and I think a well-maintained timber hull lasts just as well.

    It may pay to look at the home-built Taipans and the home-built Mosquitoes, which are even lighter (under 80kg fully rigged) for the same length, albeit with a much more modest rig. They will give you an indication of things like the aspect ratio you can get with home-built boards.

    Whether an A Class shape would work with a Hobie 16 mast is another issue. A lot of hull shape development is driven by rig and construction technology. The modern A masts are featherweight and therefore exert dramatically less pitching moment than a Hobie 16 mast, therefore the hull shape could be very different as well.

    The aspect ratio of foils is also related to quality, of course, but in this case sailor quality as well. The top A Class guys are brilliant, they don't stuff up tacks and therefore they can use high aspect foils which don't work as well in puffy winds or in inexpert hands. That pic is of Landy at Lake Garda, so it's a former world champ and Olympic medallist sailmaker sailing at one of the best sailing spots in the world. Mere mortals face different challenges and arguably do better with different designs!
     
  11. ch3oh
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    ch3oh Junior Member

    Hulls are to be made of 3-ply finnish birch that glues really well and doesn't absorb water or insane amounts of epoxy. It's about 35% denser than okume and won't bend quite as well so decks can't have tight enough radius to line up with sides. Sheets are 1500x3000 so one joint lenghtwise is needed. Upsides are 50% cost, aviability and ever lasting construction. In fact I'll pick up the sheets this week, so I'll have to do some finishing touches for those hull lines, show them for your input and make a new 1:10 scale model to check out the shape development. I made up my mind on ditching the exess tubing as I estimated it to save up to 10 kg in alu weight. Laminated inserts to go inside those beam ends can be made way overkill in the same weight range (like 2 full bulkheads per joint). As per the foils&chord I figure I run into flutter issues way before I should worry about stalling them :) A-cats are some aerospace grade beasts.
     
  12. waynemarlow
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    waynemarlow Senior Member

    have a look at http://www.catsailor.com/forums/ubbthreads.php?ubb=showflat&Number=252629&page=1 theres lots in that thread which is very relavent to your design as it was based on a A class hull design.

    Lessons I learnt was that at 16ft an A Class design has too little bouyancy and most of the later F16 designs are now quite fat ( such as the Nacra F16 ) and short and yet not that slow, to allow for weight carrying.
    Rig weight has a real impact on the way the boat handles
    You should be able to get to under 100kgs without spinny kit on
    Do put a spinny kit on, its way worth the effort.
    Look closely at how the F16's have developed for ideas, although they may not look it, they are right up there with speeds.
     
  13. ch3oh
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    ch3oh Junior Member

    Here's the side plank layout. Not showing any cutouts is not to piss you off, I just want to be 100% sure it'll develop, so some sorry guy going for it doesn't end up with a pile of firewood :D Heck, they might even be altered 5 minutes before glue-in if it feels like a torture..
    It'll hopefully still give you an idea where and how the volyme is located. A bit fatter and slightly less aft. than most f16:s.
    I think I'll call her The Flyin' something, after all I've studied thousands of plans, building in plywood, never sailed a multi, might be tranny rail meat involved.. The ordered 1500x3000 plywood had them skin plys facing the wrong way, so we have 3 joints and loosing some material. If I end up making full plans, I'll draw a placement scetch according to standard sheet sizes. Meanwhile I'll try to minimize my current losses.
     

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  14. teamvmg
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    teamvmg Senior Member

    Am I reading that right? - the max waterline beam of a hull is 146mm?

    You wont need any daggerboards or centerboards, come to think of it, forget the rudders too - cos it will only do straight lines!
     

  15. ch3oh
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    ch3oh Junior Member

    Oh god no ;) those are the planks that bend to form the round plywood skin quite like on a blade catamaran. Joining angle will vary from 0 deg. in stern to 50 deg. in bow. max. beam will be 430mm at 58% length.
     
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