Swath Boat building Plans

Discussion in 'Boat Design' started by blackdaisies, Sep 1, 2008.

  1. blackdaisies
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    blackdaisies Senior Member


    This says the pros and cons of the ship. And the over all conclusion is that it is still better than ordinary boats in efficiency. The way I described to use doesn't seem to work being thinner and smaller designs under 60 feet will not do well and be wave followers, reduced seakeeping, and weight sensitivity more pronounced with even it being intolerable.

    This just means I should stick to catamarans for the design I wanted, an 8 foot wide by 33 foot boat that had ocean capabilities. Or at least the durability to sail on the ocean, so I know small boats to not do well on the ocean.
  2. srimes
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    srimes Senior Member

    have you ever designed and built a boat before? I don't want to be mean, but it doesn't sound like you have even a fundamental understanding of boat design. A 33x8 sailing swath sounds deadly.
  3. rwatson
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    rwatson Senior Member

    "a properly designed and built SWATH ship will substantially reduce motions "

    key words *properly designed *

    "The most advantageous SWATH hull form is such that its greater beam leads to large deck area "

    key words *greater beam *

    "The greater wetted surface of the submerged hulls causes greater frictional resistance and total drag at low and moderate speeds."

    Key words "greater frictional resistance and total drag at low and moderate speeds" ie. Sailing

    Its all in the fine print if one actually reads the details
  4. blackdaisies
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    blackdaisies Senior Member

    No. I have not ever designed any boat and only joined hoping someone might know more about swath boats. I'm a beginner builder at anything and want a backyard boat that will fit on a 8 foot by 33 foot boat trailer I am going to get the kit for when I get plans for a boat. I've already found a nice catamaran plan, but I wanted a little extra security on these supposedly strong and light boats for ocean traveling.

    It's a backyard project and I wanted to use ferrocrete/concrete hollow forms to build it because it would be inexpensive. It should be a fairly easy job, but I wanted to see if the swaths were any better. I have read only good things about them and their concept is just a better version of a catamaran and pontoon boat. All platform boats, and I did not mean to get in the middle of an engineers forum, if that is what this is. Some of the posters are looking at simple types of boats as well.
  5. blackdaisies
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    blackdaisies Senior Member

    I'll guess the drag would make the boat harldy movable with a sail? I'll just get the catamaran plans and not worry about it. It's a still a step up from a monohull and is still ocean worthy and a little more stable at that.
  6. rwatson
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    rwatson Senior Member

    I think some proven catamaran plans are going to make you very happy BD. All boat users, but especially builders become "engineers". You have to be to survive in what can be a hostile environment. Even the simplest Kyak is a life support system - and all designs rely on good engineering to preserve the sailor.

    There is no harm in looking at innovative ideas, and down the track, a special SWATH boat design might become available - but till then, its tens of thousands of dollars of design, let alone building.

    With cats, ferro concrete is a no go. Everyone will tell you that concrete sounds easy (because we have all built a few paths, havnt we) - but in practice its a specialist skill, and way more difficult than we would like.

    Likewise, an eight foot wide, 33 ft cat is a hard act as well.

    You may like to consider Iann farriers 33 ft Trimaran

    as a trailerable multihull solution. You could cross the Pacific in one of these. They are self build as well. You will need $80k miniumum.

    Some of the Wharram Cat designs are sort of trailerable

    Derek kelsall has some trailerable designs

    The wise thing is to go with a reputable design - just so you can at least insure the thing, let alone being sure it wont fall apart in the ocean.
  7. blackdaisies
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    blackdaisies Senior Member

    What about double keeled boats? I know it's strange, but I was hoping for stability in a narrow boat. If it were built like a cat with two keels both a few feet away, but not on the edge of the outsides of the boat, do you think that would work? How hard are long keeled boats to sail? Do you think they are slow? I know they are hard to trailer, but that sounds like it might work.

    A small boat on an ocean does't do well, but if it had one long or fin keel, it would do better, but if two were placed, how strange does that work?
  8. blackdaisies
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    blackdaisies Senior Member

    What does this sound like to anyone reading? A crab claw sail, tumblehome canoe type shape, multi chine, displacement v-hull, 1 foot? rocker equal for each end of double ender with double keels 6 feet deep at an angle, and the boat thin enough for a canal boat of 6 feet and 9" wide a total of 33 feet long?

    The Crab Claw sail is supposedly superior, cheap, and a newbie can learn very easily how to sail her, second only to a lanteen sail.

    Tumblehome canoe shapes are an inward lean of the hull with a rounded bottom and are able to keep the decks clean of water. I've also read they are stronger. Not sure if I can prove this, but I thought I read it.

    Multichine to make it easy to build and make it a better boat to maneuver than a flat sided or flat bottom boat. I think it will be as close to the round bottom of a tumblehome as needed.

    Displacement I guess is a narrow type boat with almost a tear drop shape putting the weight I'll say just before the aft or maybe not. The v-hull will be for each end equally to make it similar to a dory double ender. The v-hull tips of front and back will cut through waves.

    I am guessing what a 33 foot boat would have for a rocker, maybe 1 foot or more? About 1 and 1/2 feet for both ends equally? This helps with waves as well and will enable the boat to turn easily

    I am going to add either the double keels that are only about 10 feet or so wide, 6 feet deep, and rectangle shaped with an eliptical shaped sides for a keel for less wetted surface attatching itself to the keel, for better stability.

    Or the almost full double keel. I want to taper the ends of the long keel 5 feet before each end of the boat with the double keel being 6 feet deep at the longest point in the middle of a boat. So under the water the boat will look like two almost half circles are attatched one on each side of the bottom at an angle. The tapering should start 5 feet before the last 5 feet of the ends, making the half circle come to blend into the boat.

    It's my opinion if a sail can move a long keel hull with ballast, it can move a swath boat, but it's obvious the swath cannot be turned unless it can be literally sailed in a complete circle and not all marinas would have room for that in order to park it. It also would turn over with the sail wind with no way to self right.

    I'm sure I'll hate asking this, but how dumb does this sound? This looks to me like it would work, but what do you think? Also do all keels have ballast? Or are some just for balancing the boat? I don't want water ballast or lead and only want the keel to keep the boat stable. How unrealistic does this sound and does this self right?

    Just a thought, so if anyone knows anything at all about the keels and have an idea to tell why none of this works, I would love to hear it. I know I've had lots of good advice already, so don't think I'm ignoring anything that's been said. I'm learning as I go. I want an easy to use boat that is very, very stable and efficient. A backyard built boat that isn't too confusing is what I'm after.
  9. kengrome
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    kengrome Senior Member

    You are very wrong about this, and if you don't believe me you might try reading a story entitled:

    "The Venturesome Voyages of Captain Voss"

    This book might help to correct some of your mistaken beliefs. It is freely available on the internet and freely downloadable in .pdf format by the way.

    It sounds like you're picking and choosing features that sound great individually (based on what you can find that's been written about them) and then you're suggesting that they all be used at the same time on a single boat.

    It all sounds absolutely and completely unrealistic to me.

    Instead of trying to invent your own boat from scratch, why don't you select a handful of boats that have been used successfully in the ways you are dreaming of using yours, then figure out why none of them employ all the features you're listing?

  10. Munter
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    Munter Amateur

    Like KG says, picking features off different design types and throwing them in together may not yield the best results when combined.

    Naval architecture has been around for a long time and the designs that you see today are not just random permutations of design features but refinements over multiple iterations. Attempting to reinvent the wheel at your first attempt is probably biting off a bit too much. To be honest - I can't even picture what you've described let alone comment on it's properties!

    Keep reading though - there's stacks of info on the web, but also try to get a hold of some creditable books which contain solid design basics not just kooky ideas from backyarders on the web. Good luck! ;)
  11. blackdaisies
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    blackdaisies Senior Member


    These are some nice thin boats with big sails that do well on the ocean.






    These have canoe yawls that look like what I'm talking about.

    A canoe yacht is a larger canoe yawl, but I didn't find an url yet for them.

    This one's close to what I want, but the keels are shorter.


    The design I was talking about isn't so original or never been done before. None of the boats I'm finding work with crab claw sails or even lanteen sails. The crab claw sails turned out to be better than the lanteen. I said the opposite earlier. If they are more efficient, why doesn't anyone use them? The oriental boats also have the long thin boats, high noses in both front and back like the long tail boats and probably lanteen rigs. No pictures or plans for any of them. The only change in the keel designs is the diamond keel that keeps the bulk of the keel towards the back instead of the front like the airplane wing shaped keels. They flip it making the wider side to the back and make it diamond shaped with a sharp point cut through waves and have less resistance.

    I don't see how the design couldn't work accept if it's going to be made with ferro crete, I won't need the multichines. If I have to build with plywood, it will be because I'm not getting into steaming the boards. I thought it would all work fairly simple accept the keels. Maybe the keels would be too heavy 6 feet under, but any square shaped keel should be shaped more like a half circle maybe to they won't anchor themselves like they have a tendency to do.
  12. Qhiron
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    Qhiron Junior Member

    Large SWATH sail cats design parameters

    New considerations on this thread, or should this go to a totally new topic?

    It's pretty clear that swath design parameters focus mainly on submergence, buoyancy, drag and obviously wave amplitude considerations. What are best design considerations for max. wave amplitude? Somebody must have figured that one out. Certainly in all the literature available, there should be design parameters which define how high the main hull rides above the lowest possible draft. This, then should offer guidelines for ideal beam to draft to length parameters, right?

    So, what if we turn this design-configuring swath definition around? What is the ideal beam to draft to length oa, for an ocean-going, sail powered swath cat? Oh, if only Hobie could be here!

    Then I'd like to use this to scale relationship to size a new hull design to the max. allowable strength parameters of the construction materials proposed. Submerged, aluminum, or steel construction (ballast) with a composite carbon fiber and stressed-skin membrane hull design above water, for weight/mass reduction. Stressed membrane maximum shear strength should provide an ideal maximum dimensional size of such a vessel. Now a bunch of you are scratching your heads going: wow....

    Got any suggestions? Here's mine:

    LOA: 145-160 ft
    Beam: 48 ft (yeowsa...)
    Draft: 16 ft DW, 6 ft- littoral
    clear-to-hull air plane: 6ft-16ft
    Mast (s) Hgt. 180-220 ft. (depends upon geom. config.)
    usable deck/main hull: Loa: 110 ft., beam: 36 ft, height 16 ft

    obvious secondary propulsion systems are diesel/electric, in pontoons.
    With the large surface area of such a design, eco-PV solar panels and
    rigid wing composite sails for a quad, cross-braced mast system is proposed

    In the luxury, gigayacht designs I have yet to see anyone suggest this. I understand the probs with cats, structural torsion, cantilever, huge moment arm loads, and obviously beam width in harbors, (who'd want to come in?) but it seems that for ideal ocean going stability, platform area and largest sail/mast design configurations, this kind of swath cat design has huge advantages.

    There are two other parameters in the initial design configuration layout I'd like to propose for this kind of radical design:

    1. Adjustable draft depth, swing/hinged, or hydraulic/telescopic pontoons, or
    2. air-inflated, water ballast rigid structure pontoons.

    i.e. when shallow draft required, submerged pontoons are air buoyed, raising entire structure higher off water plane. With conditions of deep water sailing, pontoons are partially ballasted with water, the inherent structural mass increase of pontoons adding stability and inertia-resisting wave modulation under high yield wind conditions.

    As in any sailboat, less mass is better for increased performance. With the moment arm of a large beam, huge mast lengths and max sail areas can be achieved. What are the limiting factors for such a design? Manoeuverability? Turn radii, ouch?

    I assume the optimum hydro-dynamic submerged pontoon shapes, scaled, will be the Ohio class boomer sub hull shapes. Put enough money into those shapes, no?

    So, where are my major probs? I need a big job to get this started. LOL!
    And an insane builder to say: yessssss

  13. BMcF
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    BMcF Senior Member

    The inherent problem of dealing with the heeling moment created by sail propulsion as it would affect a SWATH was raised in the second post in this thread. The fundamental basis for the SWATH hull design is to use very low stability derivatives for heave, trim and list to decouple the vessel from surface wave profile. If it could withstand/react the heeling moment from sail without excessive heel angle..it wouldn't be a SWATH....

    SWATH are not particularly efficient..certainly not as compared to more conventional catamaran forms. That is the price paid for the reduced wave-forcing gained by having such a low waterplane and large underwater bouyant bodies.
  14. Qhiron
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    Qhiron Junior Member

    disagree bf
    Here's why: 1 cubic in of displacent the same
    Only diff wetted area
    Hence drag minimal, heel toe and pitch. Wrong conclusion
    With quad interactive masts
    Forces balanced as to loading
    Remember bathtub duck?
    Doesn't matter how deep duck was
    It is about stability
    Dimension needs are critical, remember
    Forces in swath are relative to overall bpuyancy and wetted surface area
    Don't go off on yangents LOL

  15. masalai
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    masalai masalai

    Lots in Australia and NZ around 40 plus ft loa as power CATAMARAN hull-forms - Most will only allow builds strictly within their design criteria in the contract to protect their name and reputation....
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