New to cats

Discussion in 'Multihulls' started by davekf, Feb 17, 2009.

  1. davekf
    Joined: Feb 2009
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    davekf New Member

    I'm new to catamarans, so forgive dumb questions. I want to build a small cruising cat, about 20 -23 feet. I like the Woods Wizard because it fits my building area and it would be easy to trailer.

    My question:
    It seems that most cats have a lot of unused space in front of the bridge deck between the bows. Could I extend the cockpit forward and the cabin to take advantage of this space:?: I'm thinking something similar to the Jarcat but I like the better stability of the Wizard.

    I would be sailing mostly on lakes, with maybe some trips to do some coastal cruising. I don't have the need for speed, just room for my family of 5 and safety for the same.

    Thanks for any info / education you can provide.

    -Dave:)
     
  2. robherc
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    robherc Designer/Hobbyist

    You can put a "trampoline" deck up there, but I wouldn't recommend pushing your cabin more than a few inches further forward as you'd be moving the CG (center of Gravity) of the boat forewords of the CB (Center of Buoyancy) and shoving the bows down into the water. Doing this would cause loss of efficiency, loss of stability, and a large tendency to "pitch-pole" (boat digs the nose in & flips...throwing everyone & everything in/on it into the water).


    ...just my $0.02, take it or leave it.
     
  3. Chris Ostlind

    Chris Ostlind Previous Member

    Interesting you should ask, Dave.

    I live in Salt Lake City, I happen to have a design for a cruising cat at 21' in length and would be happy to answer any questions you may have with regards to your search for a small cruising cat that will fit your needs.

    There is an article about the Gato at my website if you would like to read about the specifics of the boat. http://www.lunadadesign.com/gato-especial-21.html#more-18

    You can write me directly at Chris@Wedgesail.com

    The boat shown below is the Gato Especial for stitch and glue marine ply construction. The first boat is under construction right now in Athens, Greece. Yeah, I know... wouldn't we all like to have the Aegean and the Greek Isles as our cruising grounds?
     

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  4. robherc
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    robherc Designer/Hobbyist

    Dave:

    Pay careful attention to Chris...he's a designer with experience, and has re-designed the hulls on this boat so that they have a further-forward CB...this makes HIS design safe, but doesn't change what I said earlier for if you decide to move the cabin forward on a design that wasn't meant for it.


    ....just my $0.02, take it or leave it.
     
  5. catsketcher
    Joined: Mar 2006
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    catsketcher Senior Member

    Just don't

    That is tempting but like many temptations should be resisted - especially on the size of boat you are thinking of. As Rob says you will change the centre of gravity but also you will make the boat much more prone to nosediving and capsize. Having loose mesh up front lets waves that dump over the bow flow away without helping trip the boat up.

    Some large low performance sheltered water cats do this, but you will be out on a limb if you do too.

    cheers

    Phil Thompson
     
  6. catsketcher
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    catsketcher Senior Member

    Darn

    In between me reading the post and writing it alonmg comes Chris to show a good looking design that does this. I will go back to my hole

    Phil
     
  7. davekf
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    davekf New Member

    So, the key is to keep the center of buoyancy and the center of gravity the same. That makes sense now.

    In order to move the CB forward with the CG what needs to happen? Do the hulls need to be wider at the bow, or have more flare, or what?

    Chris... I have looked at the Gato. It looks similar in size to the Jarcat but I think the drawings are much better looking. Is there room for 3 single berths in the hulls? Even though speed is not real important to me, how fast do you think it will go having a rig that is small enough to stay safe and not have much risk of capsizing?

    I like the Wizard for the fact that it seems like it would have more stability with the wider beam. I would like it better if it had a bigger cockpit. I know it would be more work to launch and retreive, but to me it would be worth the extra 15 or 20 minutes.

    Thanks for all of your replies.

    -Dave:)
     
  8. robherc
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    robherc Designer/Hobbyist

    EXACTLY

    Actually, there are a few things that can be done (usually in some combination) to move your CB fore. You can:
    Narrow the stern & transoms (or even make it a canoe stern or double-ender);
    Reduce the draft in the after portion of the hulls;
    Increase the draft in the fore portion of the hulls;
    Increase the beam in the fore hull portions;
    Change the shape of the bilges to give more buoyancy fore (or less aft).

    These are the methods that readily come to mind for me. There may be others & I'm sure Chris could tell you how to combine them to good effect in countless different ways. The important part is that you have a professional work with you to ensure that the CB and CG are in the right places before you build anything, otherwise the boat's actions will most likely NOT be to your liking.
     
  9. Chris Ostlind

    Chris Ostlind Previous Member

    Dave,

    The Gato is spec'd with a rig from a Hobie 16 at 214 sq. ft. This is slightly less than the form stability of the Wizard, with its 277 sq. ft of sail area and wider beam of just under 13'. The Wizard will be slightly faster under sail when compared to the Gato at 8.5' of trailer legal beam. If the Gato is built at a ten-foot beam, then things change quite a bit, but trailering long distances takes on a bit more hassle.

    Like any design strategy, there are always going to be trade-offs. There's no such thing as the perfect boat, as the moment you want to do something outside of your design criteria.... there goes the perfection idea.

    You really do need to look at all the functional elements of how you'd like your sailing experience to unfold. Richard Woods is a designer of the highest order and I thoroughly enjoy his design work and comprehensive thinking for boats in this category. The Wizard is a very nicely conceived boat and will serve you very well if that is the style of boating you wish to pursue.

    There is room for three single berths in the Gato interior. It all has to do with how you designate the sleeping areas within the allotted space.



    When it comes to the extra time involved, I would say that the Gato will be launchable in a decent amount of time less than the Wizard. You simply have to raise the mast and launch, whereas the Wizard involves some shuffling around of major parts to be made ready for the mast raising. You are the only guy who can make that decision, as it will likely be you who is directly involved with the process.

    Sit down and make a list of all the things that are important to you. Time to build, cost of build, cost of trailer, maintenance considerations, storage potential, towing needs, personal preferences for aesthetics, comfort aboard and sailing pleasure are but a few of the issues you should consider.

    I'm sure that if you look at it as an opportunity for enhancing your sense of adventure, the correct answers will come to you and you'll make the best decision for your needs.

    Whatever you end up doing... make it about the fun of fufilling your dreams out on the water.


    Addendum... Phil's comments above about having an open weave tramp surface forward are to be regarded. The design of the Gato, as it is shown, should be considered more for a skipper who is ready to reef early, taking the speed issues out of the equation of pitchpoling. There is a decent sized volume of the cabin slope at the bow that could easily be converted to a trampoline surface if the boat is going to be used regularly in conditions where an easily rising bow would be advantageous.
     
  10. yipster
    Joined: Oct 2002
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    yipster designer

    perfect reply's but question keeps playing in my head
    f.e. how a smal sharp flying center hull would behave..
    [​IMG]
    http://www.revolutiondesign.com.au/projects.html
     
  11. rayaldridge
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    rayaldridge Senior Member

    It should probably be pointed out that when it comes to stability, the important measurement, so far as beam is concerned, is the distance between the centerlines of the hulls. This is a difficult issue for designers if they want to draw a boat that can be trailed in one piece, and launched without extending the sailing beam. And cats with some accomodations are particularly hard hit by this fact, because the wider the individual hulls are, the smaller the functional beam, if limited to highway legal overall beam.

    As an example, my little open cat Slider has an overall beam of 8.5 feet, because I wanted the boat to be as easy as a bassboat to launch and retrieve. Because the hulls are about 2.5 feet wide, in order to get the functional beam for purposes of calculating form stability, I have to subtract 2.5 from 8.5-- yielding a functional beam of 6 feet. The unfortunate fact is that this is less than the functional beam of a Hobie 16, because the Hobie's hulls are much narrower, resulting in a wider functional beam. Weight plays into this calculation, but it's important to realize that a cat with any accomodations, and a highway legal sailing beam has no more transverse stability than a Hobie 16. If the boat has hulls with an individual beam of 4 feet, then functional beam of an 8.5 BoA cat is only 4.5 feet-- much less than a Hobie 16.

    One way to address the problem is asymmetric hulls. In Thomas Firth Jones' Weekender, an 18' X 8.5' cat, Jones drew the boat with vertical topsides on the outside of his hulls, and strong flare on the inside-- which resulted in a wider stance, in functional terms. Even so, he only put 180 sq. ft. of sail on the boat-- and when the first builder took the boat out in heavy air, he came back vowing to sandbag the bilges. With Slider's symmetric hulls, I only dared to put 140 sq. ft. of sail on her, because the design goal was comfortable safe cruising. I didn't want to have to worry about capsizing the boat in a strong breeze, because unlike a Hobie, Slider could not be righted by her crew.

    But the best solution (if speed is a serious concern) is to extend the beam. Taking the Woods Wizard as an example, if the hulls are 4 feet wide, then from a sailing beam of 13 feet, the functional beam is 9 feet, more than twice the functional beam of a highway legal boat with hulls of similar beam. This is an enormous difference in stability. The Wizard will be much faster than a narrow boat like Slider, and not just because it has twice the sail area. It will be able to hold onto that sail area when the narrower boat will have to reef down for safety.

    One reason Slider is only 16 feet long is that at that length, an overall beam of 8.5 feet is not terrible, though less than optimal. But the tradeoff is that Slider is slow compared to a beach cat, or even a little tri like the Windriders.

    If you go the other way, you end up with something like the Gougeon 32, which was a big cat in every respect but beam, and though it was fast in light air, it was (in my opinion, at least) unacceptably prone to capsize.
     
  12. Chris Ostlind

    Chris Ostlind Previous Member

    Here's a way to look at part of the issue as described by Ray.

    In the images below, you see two cats bow-on. The teal boat is the current version of my Gato Especial in Euro beam trailer trim. The orange boat is the Gato Esp. in US trailer beam with the V1 hull design.

    Aside from the obvious difference in overall beam, you can see that the boat has very different stances when it comes to the placement of the, in the water, hull components.

    The V1 design of the Gato E initially showed a set of wet surfaces planted squarely in the middle of the overall hull structure. This reduced the effective stability beam of the boat by a small, but important amount. All trailer beam boats struggle with this issue... how to get the max stability without monkeying too much with the aesthetics, or the original conceptual design function.

    The later iteration of the Euro boat shows that I have moved the water immersed components out to very nearly the limits of the stability potential, while still being able to retain a spray chine on the outer hull surface. I like what this chine does for the boat for several reasons. It adds strength fore and aft to the hull, it deflects water away from the upper reaches of the boat, especially aft and it lends a nice, clean line to the longitudinal form of the boat down low.

    So, the Euro design morphed into a different take on the placement of the waterborne hull components without sacrificing any of the internal volume of either hull.

    This feature is now on all the Gato Especial designs which includes the illustrated Euro trailer version, the North American trailer legal boat and the oversized, but still trailer legal beam version at 10’. All of the boats now enjoy stability numbers just a small slice away from the maximum capable for each design, while still maintaining the aesthetic/pragmatic components I originally sought with the exterior hull spray chine.
     

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  13. davekf
    Joined: Feb 2009
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    davekf New Member

    stability

    I think I've learned now what the difference is in stability between the monohulls and the cats. Someone please correct me if I'm out in left field somewhere.

    A mono begins heeling with little force. The more it heels, the more force that is required to heel it. That continues until the maximum righting moment.

    The cats are already at their maximum righting angle just sitting flat in the water. If a hull starts to fly, it will take progressively less force to push it over.

    So, in a mono you can tell when you need to reef the sail by using the heeling angle as a guide. How do you know when to reef in a cat?

    -Dave:)
     
  14. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    "..how a smal sharp flying center hull would behave.."

    Behave how, structurally, hydroelastically, hydrodynamically, hydrostatically etc etc?
     

  15. Chris Ostlind

    Chris Ostlind Previous Member


    Cruising cat? When you start to lift the windward hull... or sooner, if you see weather building.

    Racing cat? When you leave pucker marks on the windward deck ;-)

    No, really, it depends on the cat and the circumstances. Beach cats... no reefing. If it gets too windy, you shut it down for the day.

    Bigger faster ocean racing cats?... better to be a bit less fast than scared to death of losing it far from shore. Most of time when you reef, you'll be faster anyway, as the boat gets back into its optimal trim with control.
     
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