Thoughts on a rig please???

Discussion in 'Sailboats' started by john.G, Apr 24, 2009.

  1. sharpii2
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    sharpii2 Senior Member

    Not quite true.

    True if you want the same rig as a wider hull.

    But if you are willing to go with a rig whose height is in proportion to its Beam and not its length, you can end up with a boat quite stable AND quite well endowed with sail area.

    But this almost always requires multiple masts. And that subtracts from windward ability.
     
  2. MikeJohns
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    MikeJohns Senior Member

    Your'e right of course, but given the specs :

    If we set a SAD of 15 we want to set around 1200 sq ft , adding a 7ft bowsprit a 5 ft bumpkin and equal masted gaff rigged schooner would produce a max heeling moment of around 24000 ft lb with a sail centre of effort 20ft above the waterline.

    If we then look for a resonable metacentric height, a draft limit of 5ft, a beam of 12 ft and a prismatic coeff .56-.58 and I suspect we are into carbon fibre spars a high ballast ratio and a light composite or plywood hull if you want reasonale dellenbaugh angle.

    Even then it's not going to be either weatherly nor particulalry comfortable for blue water work.

    It's not a design spiral I'd like to start with steel :)
     
  3. Stumble
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    Stumble Senior Member

    Have you thought about using a dagger board recessed inside the keel trunk? I know Irwin used them on 50'+ boats with a good bit of success and while it won't give the same windward performance as a deeper keel it might allow you a lot more righting moment for the same hull design, while keeping the shallow draft you seem to want.
     
  4. TeddyDiver
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    TeddyDiver Gollywobbler

    Let's forget for a minute both length and displacement. To have another way to judge slender hull is to compare to shorter boats with similar beam and draft. The real difference is then in the increased wetted surface area and higher hull speed. So in this regard we get the same stiffness if SA/L ratio remains or we can drop somewhat SA/L to gain same speed as the hull resistance drops.
     
  5. brian eiland
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    brian eiland Senior Member

    Ketch/Cutter on Twin Keel Vessel

    Just did a quick look thru this subject thread and noted all the various options that were being considered for this cruising monohull in the 55 foot range...the twin keel consideration, and the rig options. These were the thoughts going thru my head back in 1973-4 when I drew up this idea

    There are a lot more discussions of twin keel vessels over HERE, and some new photos of a steel 48 footer I seriously thought about buying a few years ago and converting to my rig
     

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  6. schakel
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    schakel environmental project Msc

    Boy o boy o boy,

    You guys yacht designers have absolutely the best job in the world. Even to think about this stuff is day dreaming to me. Aren’t hallucinations one of the occupational hazards?

    One of the possibilities is the rigging of the Reliance:
    http://landandseacollection.com/id246.html
    or in picture: http://herreshoff.org/store/product33.html

    But I certainly do not have the nautical English to describe it: Do one of you have a recommended book on sails and their names? And I do not mean the square rigged (dwars getuigde zeilen) sails because that is not my style.
     
  7. MikeJohns
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    MikeJohns Senior Member

    Well that's one way of starting but you would be be scratching to find a scaled version that you apply this to.

    Have you ever tried it right through to the final design? I'm curious if you did how you found the weights and moments, COG GZ and roll gyradius related between the full scale and the parent model.

    Anyone who has designed boats in the mid 50 foot will know how the weight creeps up particulalry for steel, I don't think the target is realistic since D will scale linearly and something like a Colvin Gazelle would be a good starter for your proposal.

    cheers
     
  8. Milan
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    Milan Senior Member

    How about this: begin with the lines of conventional 9 m long, 3 meters wide steel sloop. Stretch the hull length for sixty percent keeping beam, draft and freeboard the same. So you get 15 meter long, 3 meters wide hull, with a length/beam=1:5, with dramatically reduced displacement / length ratio, placing it into the light/ultra light category even if built in steel. Displacement and wetted area are 60 % bigger. Rigging is schooner with two masts of the same height as a sloop. Bowsprit is added. Masts spacing allows setting asymmetric spinnaker and jibs of considerable size between the masts, so working sail area could be about 50% bigger and for reaching in the light airs it could be more then double.

    I think it would be fast cruising boat easily handled on the passages by a small crew, inexpensive to build and maintain.

    That’s the boat which I plan to design and build for myself one day, when I grow up.:)
     
  9. TeddyDiver
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    TeddyDiver Gollywobbler

    Mike, I just (tried to) pointed that when different size of hulls are compared, the bigger one wins some way or another...
    chears Teddy
     
  10. sharpii2
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    sharpii2 Senior Member

    True, Teddy.

    But it would be slower, much slower in light winds.

    That is where a higher S/D pays. In light wind conditions, hull surface area really counts because most of the resistance is from friction. A low S/D almost always means a lower Sail Area/Surface area ratio.

    In blowing conditions, this all changes. Now there is enough power to push the boat faster. Fast enough to make significant waves. Now, most of the resistance is coming from making these waves. A long narrow hull makes smaller waves than a short wide one going at the same speed.

    That's the sport of design. Making trade offs. Is is, however, good to know what you are giving up as well as what you are getting when you make a design decision.

    Boats with S/D's of significantly less than 15 have crossed oceans successfully, but not at what anyone would consider a decent speed. These same boats, in a wind rich region, such as the Southern Ocean Could make quite impressive passages. Not as fast, of course, as an Open whatever, which is really a scaled up sailing dinghy, designed to skim over the ocean, made with "don't ask what the price is." technology, but fast enough and in probably better comfort than a boat designed for more wind poor regions, that can do the same job.

    A reliable, low horsepower engine and adequate tankage can help take care of the light air stuff.

    Now the entire rig can be pulled in board with little or none of it hanging over or at the ends of the boat, where it is most dangerous to work.

    As long as we are able to accept these rules, we can design a very reasonably priced (for its displacement) cruising boat, which is easier (and probably cheaper) to build and maintain. And, most likely, easier to work as well.

    You may not like the slip rent, however.
     
  11. TeddyDiver
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    TeddyDiver Gollywobbler

    But frictional drag is however linear to speed so 10% difference in driving force should mean 10% difference in speed. Speeds where this is relevant (non wave making speeds) are so slow that I wouldn't care if I do 4.2 instead of 4.6 knots half of the time if as a trade of I get 1.5 knots on the other half.
    But as you said they are trade of's and subfects of personal preferences and differing circumstances.
    And isn't that just what's the salt of the discussions here pondering with different possibilites prosan'con's etc :)
     
  12. Pericles
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    Pericles Senior Member

    Just a couple of links to encourage close hauled sailing, doncha know. :p :p The forward placed daggerboard, for example, helps upwind performance for the sampan.

    http://www.friend.ly.net/users/dadadata/junk/sampan.html
    http://www.friend.ly.net/users/dadadata/junk/platt/platt_chinese_sail.html

    Stitch thus.

    http://www.friend.ly.net/users/dadadata/junk/tutorial.html

    I cherish the idea of a lateen rig on a catamaran, but that's just my thought today. Tomorrow, who knows? A romantic voyage on a felucca? :D
     

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  13. timothy22
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    timothy22 Junior Member

    Just a note on the original "Marco Polo" design. One example named "Talaria" was delivered south round Cape Hatteras in a winter northwester by Robert Beebe, who observed that the boat had a much snappier roll than he anticipated, and that the mizzen sail was worse than useless with the wind aft of the beam and had to be struck under those conditions.
     
  14. FAST FRED
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    FAST FRED Senior Member

    "and that the mizzen sail was worse than useless with the wind aft of the beam and had to be struck under those conditions."

    That's a surprise?

    "who observed that the boat had a much snappier roll than he anticipated"

    The Marco Polo basically had trees as masts ,

    If the "modern" boat had light weight aluminum, the roll height will be very changed.

    FF
     

  15. sharpii2
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    sharpii2 Senior Member

    That's to be expected. It also shouldn't be to useful sailing up wind either. But sailing cross wind or in a close reach it should earn its keep.

    It's always good business to put as few sails as possible on a boat. Long narrow boats with shallow drafts preclude this, so one must do as one must do to get adequate sail area.

    More sails and more masts do allow one to play with balance more by feathering or striking some, depending on what course is sailed.
     
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