Cat Ketch 30

Discussion in 'Boat Design' started by INW, Dec 2, 2014.

  1. INW
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    INW Junior Member

    I am looking for some help in developing plans for a cat boat. I want to stretch an existing design to a more/longer DWL. Then build an archetechtural model using a modern free standing cat ketch rig. I have some lofted half block lines on a disc to get started. I don't have the computer skills or software.
    Can anybody help?
     
  2. PAR
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    Which design are you going to stretch? How much of a stretch? Is it a proportional enlargement or a stretch?
     
  3. INW
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    INW Junior Member

    Cat ketch

    I would like to stretch a 22' cat boat to 30'. It has about a 10' beam which is what I would want. I'm new at these thread sites. I think I can include the lines I have.
    well that didnt work but I can email it to you.

    Ian
     
  4. PAR
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    PAR Yacht Designer/Builder

    An enlargement or a stretch to 30' from 22' is just too much to expect any real success, even if it is a fat 22'er. The hull form will become too distorted, the weights and centers too far off reasonable expectations, to make this a viable option. Simply put, you need a hull much closer to the size you desire, where a simple stretch might be sufficient. The end result will not be as you imagine, for example if you take a fat 22' boat, which is a 1.5 - 2 ton boat and enlarge to 30', it would become a 6 - 8 ton boat, which is way more than it should be. Hell, I have a 37' carvel (1.5" cedar over oak frames) powerboat that's 8 tons, just to give you an idea of how far off it would be. Find a 27'er and stretch it the few feet you need.
     
  5. philSweet
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    philSweet Senior Member

    What PAR said, plus the fact that a ketch rig actually wants a different hull than a catboat. The way sail is reduced for reefing changes things. Differing aspect ratio of the sail plan and a different downwind configuration changes things. You want a hull designed for a two stick rig. The location of the cockpit and the weight distribution of the crew and the relative weight of the crew vs the hull all change a huge amount going from 22 to 30. Draft and freeboard would usually change in a nonproportional way. Take a look at Roger Martins boat for some ideas.

    http://www.rodgermartindesign.com
     
  6. INW
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    INW Junior Member

    I would like you too, to see these lines. I believe what your saying. Things like the center board and rudder would have to different. Is what your saying the boat probably would not sit on it's lines right? That's why I want to build the model. Here are some of the reasons I would like to use a cat boat like design for this type of ketch.
    There are plenty of Cat Ketch sharpies which have similar parameters. But sharpies are narrow and have a tendency to sail on their ear going to weather. I like the beamyness, full bilges with lots of reserve boyancy. There is plenty of room for spartan accomadations and even a diesel engine. I love the lines and old world classic looks to them. They would look even better if they had a longer length to beam ratio. It looks to me like it would balance and foot better on all points of sail than a big heavy clunky over canvased gaff rig.
    Yes, I would probably have to redesign the whole boat, but what the hell, I can't lose my *** on a model can I? When I vision stretching it out in my mind, it has a similar prismatic coefficient as some of those REAL beamy ocean racers so broad they have to have twin rudders.
    So...
    Can you help me?

    Ian Walter Bastrop Tx. 2vbxr77@gmail.com
     
  7. philSweet
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    philSweet Senior Member

    Sharpies sail rather flat. Never more than 10 degrees, which is similar to a catboat. The sharpie hull just doesn't work very well above 10 degrees of heel. If you go to a heavier boat then you can get some ballast down lower and the sailing heel angle will tend to increase. But these changes all involve costs and complexity which far exceed the tiny gain in performance potential. If you want to use a ketch, it is important to get the sails as far away from each other as possible. This is why you see long skinny ketches. In the size you are talking about, you can often slam the mizzen right down to the deck level, and that helps lower the heeling moment.

    Catboats can really exploit the benefits from a gaff rig in that the rig weight is much less a factor than it would be in other boats. I think a ketch rig on a catboat would be a complete dog to sail. You have twice the complication, but derive utterly no benefit from it. If you like catboats, stick to a single pole and one of the conventional rigs such as bald gaff, gaff sloop, gunter rig, or lug.

    The boat will be much easier to sail than if you try to fiddle a ketch rig onto it. If it worked, there would already be thousands of them out there, and there isn't

    look at the Nonsuch boats.

    I can't imagine much worse than this - http://www.cmdboats.com/images/cb21_photo.jpg
    but apparently, there are plans available.
     
  8. PAR
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    PAR Yacht Designer/Builder

    The cat boat was derived from a purpose specific sailor, that was repeatedly modified, eventually into a nearly unsailable freak in many cases. If the typical and modestly designed cat, was stretched (not proportionately enlarged), the lines could be made into something fairly reasonable. It would take some serious skill, to know what a reasonable set of distorted cat boat lines would look like, after a stretch and no stretch ratios or formulas will help in this regard. You either know, or you're just guessing, which usually doesn't work out very well.

    Again, which design are we talking about? Is it a stretch or an enlargement? Do you really know or are you simply interested in a cat ketch about 30' long? If simply just liking on cat ketch, but wish it to be 30% larger, then look for a 30' cat ketch or discuss you proposal with the designer or another, to produce a new one off. A 30% stretch or enlargement will alter everything so significantly, that everything needs to be redone, so it needs to be re-engineered.
     
  9. INW
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    INW Junior Member

    Cat Ketch

    Well it is more than a guess. I worked in the marine service and repair business
    for quite a few years, over 20 in fact. I did alot of sailing, Ive helped build a few boats. Ive been on a first name basis with several yacht designers. Ive studied quite a bit on the subject my self. You might say it is an educated hunch.
    Thanks but none of the responders here have been much help. Seems to me you guys are the ones who are guessing.

    INW
     
  10. Eric Sponberg
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    Eric Sponberg Senior Member

    INW, basically what the responders so far are saying is that stretching a 22' design to 30' is basically starting with a whole new design. Everything will change--lines, displacement, construction weight, rig design--everything. So whatever is established in the way of design and construction parameters for the 22' will be quite different for the 30' boat. Yes, you can create new hull lines at 30' based on the shape and style of the 22'er, but it will have to be a new design. Therefore, you need to talk to a naval architect who is familiar with cat boats and cat ketch rigs to see what a new design will cost. There really isn't any other way around this--it will be a brand new design.

    Phil Sweet mentioned looking at the Nonsuch boats--that's probably good advice because Nonsuch's came in various models form 22' to 36', all with cat rigs, and they are pretty fat, shallow boats. I know of one cat-ketch rigged Nonsuch 36, owned by a friend of mine here in St. Augustine (the only one built, he tells me), which is close to what you are talking about, just a bit bigger. So a cat-ketch rig on a boat similar to the Nonsuch 30 is likely what you are talking about. The designer of the Nonsuch's is Mark Ellis, in Canada.

    Creating new hull lines and new construction details is what is required to achieve what you want. I expect you will not find a stock design close to what you are looking for. You may find an old cat boat design, but it would still have to be modeled and engineered for modern design and construction techniques--you're still looking at a custom design, at a custom price.

    I hope that helps.

    Eric
     
  11. philSweet
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    philSweet Senior Member

    We are guessing - guessing what you want in your boat. The questions that we have asked you have not answered.

    My guesses so far are -

    The lines or what ever it is you have are not going to be of any use to anybody who is going to draw up the boat you want. There is a process for setting out the requirements so that a boat can be designed. It doesn't begin with some pieces of drafting from a boat that frankly can't be relevant structurally, hydrodynamically, stability wise, or GA wise.

    But there is no reason at all that the dozens of designers here can't help you. What you seem to want is a specific look to the boat, one that evokes memories of a particular place or time. Nothing wrong with that. The topsides can be grafted onto a hull and the GA can be fiddled with to suit the whole.

    Take a look at a thread here called the design spiral. Post the info needed to get once around the loop and see what happens. And post some of your ideas regarding aesthetics or what you want the craft to evoke.

    <cross posted with Eric>
     
  12. philSweet
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    philSweet Senior Member

    INW, here are some pics of small ketches and maybe a couple yawls. They aren't all freestanding, but that doesn't matter. They are each, IMO, a plausible way to use two sticks on a small boat. Would any of these hull forms suit your wishes?

    It is much easier to use a yawl rig on a boat this size. With modern materials, the loss in efficiency is just too apparent. When you start bumping into bridges that don't raise and lower, that's when you usually start looking at ketches.
     

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  13. INW
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    INW Junior Member

    Cat Ketch

    I could build myself an "Egret" if thats what I wanted. I thought about it. But there is not enough room in them. Especially with that barn door center board in the middle of the cabin.

    INW
     

  14. PAR
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    PAR Yacht Designer/Builder

    This is a typical problem with sharpie and other shoal or canoe body hull forms. There's just no hull volume, particularly depth, for headroom.

    Try this, draw a 6' 2" square to scale on a sheet of paper, while sitting at the kitchen table. Now draw a horizontal line, to the same scale, about 30' long. See how much draft and freeboard you can live with and roughly draw a sheer line, keel profile, etc. You'll quickly find that you need some draft to prevent the boat from looking like it's got a Winnebago parked on it's deck. This is particularly true with sailboats, where keeping windage down is a major concern.

    You're going to have to get a design penned up for you, have a semi or full custom worked up, possibly from one of your half models. At this point you're best advised to work up a solid SOR of your needs and desires, prioritizing them as best as you can. This is the starting point.
     
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