Perfect Cruising boat?

Discussion in 'Boat Design' started by Wynand N, Nov 14, 2004.

  1. Wynand N
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    Wynand N Retired Steelboatbuilder

    What makes a perfect cruising yacht if there ever was one :?:

    For what is worth, here is my view of the perfect cruiser. :idea:

    Design Brief:

    Hull: Raduis chined steel hull for that unexpected groundings or collisions with whales, loggs, other sailers etc. Short overhangs with long waterline. Beamy hull with beam carried well aft with powerfull stern and flat buttock lines aft for downwind sailing, the only way to go really. Boarding platform or steps for ease of boarding from dingy.

    Deck: Pilot house with modern sweeping lines with inside steering for that not so perfect days. It goes without saying that the saloon floor will be raised so that you can take in the view whilst sitting down and sipping your fire water.

    Keel/Rudder: Twin (bilge) keels for various reasons:
    1. Damped rolling motion, comfortable ride.
    2. Superior righting moment when heeled (yes, work it out yourself)
    3. Less draft
    4. Boat stand upright with low tide, no need to anchor in deep water and the bottom can easily be cleaned & painted between tides
    5. Twin rudders mounted on full skegs
    Note: Because this is a cruising boat and an extra day at sea would not kill the parrot, the pro's of this setup override the cons eg. drag etc.

    Sail Plan: Cutter rig or twin headsail sloop.

    Basic Dimensions:
    LOA 12.000m About 39.4ft
    Beam 4.000m 13.1ft
    Draft 1.600m 5.3ft
    Disp 12000kg 26880lbs loaded
    Length / disp ratio: 270
    Sail area / disp ratio: 16.5

    The boat should be easily be handled by a couple.

    In fact, this design brief really mirrors the description of my retirement boat I had designed for extended cruising in the south pacific in about 3 years time. Building starts Jan 2005

    Any other sensible contributions or suggestions?


    Wynand Nortje
     
    1 person likes this.
  2. FAST FRED
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    FAST FRED Senior Member

    A fully battened main can add lots of area up high where its needed, has a great shape , and works well under most conditions.

    A very fine addition for a cruiser, or motor sailor.

    FAST FRED
     
  3. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Compared to what? :)
    If compared to the same ballast at the end of a loinger fin, then you need to think again. If compared to the same ballast on a fin the same depth as the bilge keels (but longer, we assume) then true, but only by the buoyancy of the windward keel as it emerges from the water.

    Steve "doubting Thomas..."
     
  4. Eric Sponberg
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    Eric Sponberg Senior Member

    There is no perfect cruising boat. If you ask 100 different people, you will get 110 different answers. Perfect is only how it is related to you, and even you have to make some compromises. Good luck with your new boat.

    Eric
     
  5. Wynand N
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    Wynand N Retired Steelboatbuilder

    Hi Steve,

    The statement was made with the asumption of the same draft as a finner.

    Simply put, without going into naca foils, angles of attack, toe in, toe-out etc the following:
    When the vessel with twin keels is heeled about 15 degrees, the leeward keel with 50% ballast will be hanging (pulling) straight down under the hull, and the windward keel SG with the other 50% ballast moved outboard as per normal keel. But with the keels fitted closed to the bilges at a 15 degree outwards angle , the SG of the windward keel in a 15 degree heeled position is pushed out much further from the hull SG and centre line (the fin is actually in a "30 degree" heeled position) thereby effectively increasing the rigthing moment/lever relative to the 15 degrees heel of the boat. These righting moments of the keels combined with the form stability of the hull, is in my mind a much better deal than a single fin.
    I hope I made some sense with all this 15 degrees thing :(

    O ja, twin keels also tracks better than a finner, less leeway.

    Keep on sailing

    Wynand Nortje
     
  6. brett
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    brett New Member

    I think that there is a flaw in your thinking.

    If you consider the boat as a whole and compare the VCG (centre of gravity) of the complete vessel, splitting the ballast into two parts does nothing to move the CG either downwards or to windward thus giving no additional righting moment. But as steve says, RM will only increase a little as the windward keel in raised above the waterline.

    Unless you can change the VCG of the complete vessel or immersed volume during the heeling process, RM won't change.
     
  7. brian eiland
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    brian eiland Senior Member

    Invitation to Twin Keel discussions

    Hello Wynand and all,
    Would like to invite you fellows to join in our discussions at Twin Keel research

    Just as an aside Wynand, why don't you have a look at my single-masted ketch/cutter rig.

    I could also imagine a wylie ketch rig for such a vessel.

    How about this cockpit arrangement for those more temperate climates:
     

    Attached Files:

  8. Wynand N
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    Wynand N Retired Steelboatbuilder

    Hi Brain :)

    Thank you for your views.

    Nice picture. It actually mirrors my own design of "my perfect cruiser". My deck layout is a pilothouse, quite long in length with the last 1.5 meter open as if a doghouse/dodger. Very similar as in your picture. My thought were to relax "outside" the saloon but still be protected fom the elements. Another plus is that no rain can finds its way down the companion way.
    My pilothouse ended with a sweptback arch on which the mainsail track is situated, clearing up the cockpit. Looks like we think along the same lines.

    Your mono hull design link made my day. Your views on twin keels are inline with my reasoning, especially in a heeled position. Personally I believe for a cruising yacht, a twin keeler is superior than a fin. More pros than cons.

    Perception is mainly to blame for the fin view. We grew up sailing with the believe that there is only one way to go, cruising or racing fin. It is nice to be stuck in that groove or comfort zone. The problem with being in a groove is that you do not see very far.
    People, get out of that groove and see what posibilities the horizons offer. Can all the pommies be wrong with their love for twin keelers, or do they know something others don't.

    I would like to take this oppertunity to thank everyboby for their input thus far.

    Fair winds

    Wynand Nortje
     
  9. SailDesign
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    SailDesign Old Phart! Stay upwind..

    Wynand, what you have done is split the ballast into two equal parts (correct me if I'm wrong :)) at the ends of two fins, regardless of angle. The actual CG of the combined ballast "lumps" is still at EXACTLY the same position as if it were a large bulb on a single fin on centreline. The fact that you have twin foils just means you have more drag, and must add interference drag to that new higher total.

    Steve
     
  10. Wynand N
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    Wynand N Retired Steelboatbuilder

    Hi Steve, :)

    You are quite right. The CG is still the same. Righting lever is still the same. Only so if all things in the comparison are equal?

    Let us consider this with the following criteria;
    1. Same draft
    2. Fin and bilge keels have the same aspect ratios, design
    3. Same amount of ballast

    We all know that fin keels don't pack a lot of ballast in the tip of the keel due to the general foil shape, swept back and taper towards the tip. That is why we have keel bulbs, wings etc. to pack ballast low down where it matters most.
    Only higher up in the keel do we get some volume to pack real ballast, at the cost of a rising CG.

    Now with twin keels the ballast is split 50/50 and is it possible to pack the combined CG of the ballast lower below WL than it is possible with a single fin of same draft, aspect ratio and design. This lowers the overall CG and when heeled, will have a longer righting arm...... ;)

    As for the additional induced drag of twin keels you are right again and I never disputed that fact.
    I personally feels that a twin keeler makes a lot of sense for a cruising yachtsman. It has more pros than cons.

    Fair winds

    Wynand
     
  11. OrionsSword
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    OrionsSword i dont know

    good luck with your boat
    I have a question on your righting momentum of a twin keel though because as you heel more and more the bulb/wing/spade (as are comon on many cruising boats today) becomes further and futher from the centerline of the boat thus providing more downward and righting pull were as a twin keel has as you heal more and more some of the weight gets further from the centerline but not by nearly as much and the second keel is often causing more heel or simply pulling the boat further into the drink.
    I was wondering if someone could explain to me the righting advantage of twin keels?
    Orion
     
  12. SailDesign
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    SailDesign Old Phart! Stay upwind..

    And that is the bottom line
    I'm happy, as always, to agree to disagree about that opinion, however... :)

    Steve
     
  13. Sean Herron
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    Sean Herron Senior Member

    Yeah but...

    Hello...

    Then there is the arguement of asymetrical twin keels at say your most predicted angle of heel...

    One is thus vertical and literally pulling the hull to windward...

    The other is flapping along severing the bow wave and trying to dynamically pull the boat upright...

    Right... :)

    But yes - you still are designing to same diplacement but with added wetted surface - so it's what you want - I like the 'idea' of twin asymetrical keels - but would not want to have to build them - not to mention tweaking their toes like the cadence on your sports cars steering... :)

    YUP...

    SH.

    -awake and not feeling my toes...
     
  14. Wynand N
    Joined: Oct 2004
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    Wynand N Retired Steelboatbuilder

    Hi everybody :)

    I came across the following:


    Food for thought.

    The writer is no other than the doyen of American designers, TED BREWER and the quote verbatim from his book "Ted Brewer Explains Sailboat Design.

    And by the way, my keels are of asymetrical design with a rather "high" aspect ratio of 1.1. She also carries high aspect ratio twin rudders fitted to full skegs.

    I hereby rest my case

    Fair winds

    Wynand
     

  15. yago
    Joined: Aug 2004
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    yago __

    Wynand, if you want to see some really modern bilgekeelers, very high aspect keels and twin rudders, browse through the designs of Jean Pierre Brouns, at http://perso.wanadoo.fr/jean-pierre.brouns/bateauxcom.htm

    Examples you should look at are
    - "Copain 10.20" at http://perso.wanadoo.fr/jean-pierre.brouns/doc/copain 10-20.zip

    "BOUTEFEU" at http://perso.wanadoo.fr/jean-pierre.brouns/doc/BOUTEFEU.zip it has pictures from a finished META-built boat

    ...and several others.

    Browsing his site is a bit difficult, it's in french and he packs his pdf-files into zips for download and has no other pictures on the site, but it's well worth it, very nice designs!

    Actually when designing my own Yago 31, I was tempted for some time to have lifting lateral keels with bulbs rather than the (heavier) all-inside ballast and lateral daggerboards, but got lazy and gave up on it because of the mechanical complexities... I still think that would have been the best of all worlds for my personal cruising program.
     
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