Will this hull work for a 12ft beach cat?

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

  1. CT249
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    CT249 Senior Member

    The difference between flat bottomed hulls and round bilge hulls appears to be pretty minimal in reality. I'm not sure how many flat-bottomed cats and chine cats you have sailing in the USA. One of the most popular cats in NZ and Australia is a V bottom chine cat, while Australia also has a popular flat-bottom cat. They are among the fastest cats of their length, indicating that chines cannot be too slow.
     
  2. CT249
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    CT249 Senior Member

    Having been hit (while on starboard tack) by a world A Class champion, I'd have to mention that wave piercing bows have downsides for a club racing or mess-around boat. They can be both fragile and damaging to other craft.

    It's a pity the kid's cat class didn't take off. There were some solemn undertakings from cat associations to get junior cat racing going when they were campaigning to get back into the Olympics, but it seems that they decided it was all too hard. As someone who sails all sort of craft (including fast cats) I'd be quite concerned if we had a fleet of juniors running around with sharp stems like those kids' cats because of the damage they could cause.
     
  3. upchurchmr
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    upchurchmr Senior Member

    Its really difficult to justify one feature out of at least 10 as being the thing that makes a cat fast.
    Perhaps we should consider sail area also?
    Perhaps the level of competition in the class?
    Total weight compared to Sail area?

    I haven't seen any flat bottomed A-class cats.
    That is the extreme edge I admit.
     
  4. Manfred.pech
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    Manfred.pech Senior Member

  5. ahen
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    ahen Junior Member

    I think I have a good base to work from now, but I don't know what I need to do to polish up the design. I hope I'm not being too much of a bother but I don't really know what I'm looking at in the calculations/unsure of the best way to change the design to get satisfactory numbers. In summary I want help making sense of the calculation tools.
     

    Attached Files:

  6. sailhand
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    sailhand Junior Member

    ahen I opened your last project ok but this file is denying me access
     
  7. ahen
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    ahen Junior Member

    How about now?
     

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  8. sailhand
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    sailhand Junior Member

    ahen no go it still wont open
     
  9. ahen
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    ahen Junior Member

    That's odd, I was able to open the file on a different computer. Ill attach pictures instead. I also included a screenshot of the calculations
    profiloeView.PNG

    bodyplanView.PNG

    planView.PNG

    perspectiveView.PNG

    hydrostatics.PNG
     
  10. ahen
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    ahen Junior Member

    | Draft -| Trim -|- Lwl ---| Bwl ---| Vol ---| Displ --| LCB -| VCB --|- CB ----| AM --| CM ----|-- AW -- | CW --- |-- CP ---|--- S --| KMtrv | KMlong |
    | 0.620 | 0.000 | 11.769 | 1.194 | 4.994 | 0.143 | 5.805 | 0.358 | 0.5731 | 0.667 | 0.9010 | 10.983 | 0.7814 | 0.6360 | 20.847 | 0.558 | 19.188 |

    I pushed the rocker down a little bit and recalculated with a draft that would make the bottom of the transom just touch the surface of the water, the previous ones had the default of .5 for a draft
     
  11. sailhand
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    sailhand Junior Member

    ahen I tried again to open the file and freeship boots up and then says file access denied. ok now you are getting familiar with the basics of hull design lets sit back and establish some parameters for your boat. the most important factor in a small hull like this is what weight the boat will be when you are sailing. this weight will be the combined weight of everything and it is very important to know. that means rigging, sails, crew, boat, provisions, equipment etc, I mean everything. an accurate calculation of this weight needs to be established so its time to do a bit of research. the best approach is to do a spreadsheet with everything you can think of and try to find a dinghy with a similar size mast and sails to that you envisage using. for a rough idea of weight for your rig you can look at various designs on the net and compare hull weight to fully rigged weight of the boat and you will get a rough idea about rig weights for your boat. find boats that are rigged the same as you wish to rig your boat ie cat rigged, sloop rigged etc. now when you started all this you said a "cruising boat" 12 foot long. That is a big ask for a 12 foot pair of hulls are you going with a solid deck which can be dry or a mesh trampoline and wet weather gear. are you thinking cabin or pitch a tent on the deck all these things need to be established. I have a dinghy with a full bridgedeck and very high bows and you can duplicate this in your build and have a dry area to sail and camp on. my dinghy is around 45kg and easy to build and works well as a camp cruiser with the deck configuration. you dont want to build my dinghy but you may like the deck layout for a cruiser as it is very dry. if you are using the boat in a warm climate and storing your sleeping gear clothes etc in dry bags then getting wet when sailing is half the fun I think. remember whatever you build chances are you are going to capsize the boat at some stage and get everything wet. if you have a really conservative rig that can be easily reefed/dropped/feathered to enable you to sail in all sorts of conditions safely I would look closely at the hobie bravo for your rig set up. free standing rigs are great in a small boat like the one you are building. easy to rig and take down even on the water and if you get caught out in bad weather you can reef easily and even throw off the sheet and let the sail depower completely and you have no problems at all. the downside to all of this is the boat will not be as fast as an all out speed machine but still go very well if done right. 12 foot was your original idea I know but the biggest improvement you could possibly make to this boat is to make it longer. that will give you more displacement and a better beam length ratio of your hulls, but it will also be a bigger heavier boat etc. just on waterline length I would maximize whatever length you have by plumbing the bows. the hull shape will vary a great deal depending on the weight you want to carry and the configuration you have,ie full solid deck, half solid deck, net etc and also the overall beam of the boat. you can do a full solid deck in foam for not a lot more weight if done correctly. you use a cheaper resin with foam than the epoxy that you have to use for ply so the cost is not wildly different and you can do a foam deck with no structural support underneath as the foam core deck can span a greater distance unsupported. to duplicate this in ply would be complex or way heavy. also the rocker in the hull will to a large degree effect the load carrying capacity of the hulls so this weight build method needs to be established so we can give you more help with the hull design. check out

    http://ikarus342000.com/DUO480Cpage.htm

    for a small cruiser design it might give you some ideas. are you thinking a smaller version of that sort of thing. for some good info and build ideas for a smallish cruiser check out this blog

    http://duo480.blogspot.com.au/searc...-max=2016-01-01T00:00:00-08:00&max-results=25

    maybe a smaller version of this with just a flat cockpit and maybe the cabin can just be a tent type arrangement. check out my dinghy on this forum by searching my posts and you will see the full flat deck and the high bows. I would keep the high bows, maybe a bit lower, to keep you dry and cut away the topsides aft of the mast to allow you to hang off the side of the boat to sail. of course all this assumes you want a solid deck cruiser in this size range. that would be a very interesting little boat I think. and I would go with a pop up tent like

    http://www.kmart.com.au/product/2-p...3X0zp9iV30cJrnZN0GnMQoE-eyuZy4JvicaAoHV8P8HAQ

    these dont need any pegs and take two seconds to set up. you can literally just throw them in the air and they pop open, the nasty part is folding them back up the first few times. be warned you cannot put weight forward in a multihull this small. you need to centralize your weight around the middle of the boat so maybe a small raised area midships on each hull that opens into the bilge and you can store your gear in there and that would also help to keep the weight down lower in the boat and help with stability. plenty to think about here for you and I look forward to your reply so we can push forward with the hull shape.
     
  12. ahen
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    ahen Junior Member

    I looked a little into weights of similar cats, and I expect to around/less than 200lbs. I don't mind getting wet, in fact I almost think it is desirable. I don't want it to have drag and spray, but I don't mind having the deck close to the water. I want a simple deck, I will probably end up having some ply across the supports to sit on for each side, but leaving the middle uncovered. I don't need storage, I plan on taking very short trips to islands in a bay (1 mile or less), places I have canoed and kayaked too easily in the past. If I do end up carrying cargo it will be small things that can be strapped to the deck. As for rigging I have been looking every now and then for used sails, but I might end up buying a new one. If that is the case free-standing sounds the easiest.
     
  13. sailhand
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    sailhand Junior Member

    ahen does that weight include you or is it just the boat. we need the weight of everything total including you as you set off from shore on your trip. this is the total displacement. where your hydrostatics say displ that is displacement and the value is in long tons which = 1000kg. so the displacement on just one hull at your designed waterline (DWL) or draft is 102kg/225lbs or .102 long tons for just one hull so you obviously double it for two hulls. so the boat will sit on its lines (DWL) at a total weight of 450lbs, this includes the boat you and everything else aboard. to have a slightly faster boat you could lean the sides in a bit and make them a bit more vertical, not too much. this will make it a slightly wetter boat but will give you a better beam length ratio at the waterline but will slightly reduce your displacement. 12 to 1 at the waterline, obviously a waterline beam of 1 foot, would give you a good compromise in this length and the plumb bow will maximise this also. if you can reduce the displacement slightly as you will not need to carry too much weight I would lower the transom a bit to reduce the rocker. and this will reduce your draft a bit. to make the boat float on its lines, ie not sinking at the back with the bows out of the water or vice versa you need to keep the centre of gravity of the boat and all its contents including you and your crew directly above the LCB or longitudinal centre of buoyancy. in freeship if you go into layers and assign a weight to your layers then it will calculate the centre of gravity of the hull for you. allow .6 of a pound per square foot or around 2.8kg per square metre and you will be close enough to the mark to give you some idea of weight of the finished hull and its centre of gravity. recalculate your hydrostatics after you have done this. also you can use the other hydrostatics calculation and set your increments at one inch and this will give you a table that tells you how far down the boat will go as you add more weight, this is very handy if you want to know how it will perform with one person and what will happen when you put extra people on board. this will help you to decide how much freeboard, height of boat/deck above water, you want with the extra weight aboard.
     
  14. ahen
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    ahen Junior Member

    200 lbs was just the boat. For the total displacement I expect about 350. I'll start making adjustments to this number.
     

  15. upchurchmr
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    upchurchmr Senior Member

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