Pros and Cons to different types of keels

Discussion in 'Boat Design' started by Emma23, Oct 19, 2010.

  1. Emma23
    Joined: Oct 2010
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    Emma23 New Member

    Hello,

    I am looking at buying my first sail cruiser, something smallish,
    I was wondering could anyone recommend some reading material on the pros and cons of different design features, or would have any reccomendations themselves,
    At the moment I am looking at a boat with a shoal keel, 1.1mtr.
    I intend to do a lot of Coastal sailing in the West of Ireland. (if that helps for suggestions...)

    Thanks,
    Emma
     
  2. PAR
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    PAR Yacht Designer/Builder

    Most folks have keel requirements based on their local sailing area with it's general depths and sailing needs. If your area is shoal, then you don't have much choice and some sort of retractable set of appendages are necessary. This would include drop keels, centerboards, daggerboards, leeboards, etc. On the other hand one shoal draft option and common in northern latitudes, is twin fixed bilge keels. These appendages don't retract, but are short enough to permit getting into shallow places and taking to ground bolt upright if necessary.

    The other types of keels are fixed in depth. These range from full length, square faced monoliths attached to the centerline of the boat, to very skinny fins with big streamlined bulbs attached at their tip. Each have benefits and draw backs. A bulb/fin keel can offer great preformance potential, but also can be delicate it you tend to smack into things. A full length keel can protect your rudder, prop and other gear if you like to go bump in the night, but usually has a decided preformance disadvantage to more modern types.

    Ultimately, it's your call, based on experience, desires and local requirements.
     
  3. gonzo
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    gonzo Senior Member

    If you are in these parts of the world, twin keels or centerboards are the way to go. With the huge tides, they are the only boats to sit straight.
     
  4. Petros
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    Petros Senior Member

    the deeper and more high aspect ratio the keel, the better the performance. It has less drag and points better. But the trade off is it is not good for shallow anchorages, and does not lend itself to trailering, so the retractable keel is your best bet if you need to go into shallow waters.

    Low aspect ratio keel or blended hull keels (shoal draft) perform less as well, have more leeway the higher you point, but are less sensitive to beginner mistakes.

    The old fashion full length keels are simply nostalgic and does not perform well, they do have a bit more comfort in rough seas because of the high pitch inertia, but few designers would consider that a worth wile trade off today.

    good luck.
     
  5. RThompson
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    RThompson Senior Member

    Something else to put in the pot and consider is the actual sailing performance of the yacht. Even though you may not be racing the keel will have great impact on how well the yacht sails -and who wants to sail slow if its not necessary? For the same weight and cost there is a big difference in sailing performance for different types of keel.

    Assuming you are talking about a smallish keelboat:
    The keel performs two functions for sailing performance:

    1. Put ballast as low as possible
    The first point is self explanatory - the deeper the keel the better its ballast performance.

    2. Provide lift to counter the force of the sails.
    The second point leads to how well the keel works as a vertical underwater wing. The fundamental dimension here is the aspect ratio of the keel. ie how deep it is (d) compared to its surface area (a). Thus aspect ratio = d/a.
    Within reason the higher the aspect ratio the better it will perform, which inturn implies the deeper the keel the better its sailing performance will be. -(these two reasons explain why race yachts have long skinny keels with a heavy bulb at the bottom)

    So for sailling performance one generally looks for the deepest keel possible.
    However that must be balanced against the practicalities of actually sailing -eg shoal draft, putting the boat on the hardstand or drying out on a mooring -Here the practicalities usually mean the shallowest keel possible.

    Twin keels (or bilge keels) is a good example of the compromise mentioned above. Two short fat keels is about as poor (sailing - edit) performance as one could get from a sailing yacht keel. However the payoff is significant in that the yacht will be quite stable and easier to climb up to when dried out on the hard, and the yacht will be able to sail in shallow water where deeper keels cannot venture.

    If you are inclined to learn about how and why yachts are designed the way they are you could try reading: Principles of Yacht Design by Larrs Larsson and Rolf Eliasson - modern classic and easy reading.
    Good luck

    ah-ha, Petros, already said this.. :)
     
    Last edited: Oct 21, 2010
  6. FAST FRED
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    FAST FRED Senior Member

    Two short fat keels is about as poor performance as one could get from a sailing yacht keel.

    If the Daffynition of "good performance" is the ability to race to windward pointing higher than the next boat to get to a mark a 1/2 second earlier, this might be true.

    Cruising boats seldom DESIRE to point as high as can be done , "only fools and Yachtsmen bash to windward".

    In most offshore work the boats will need to sail less high as it takes HP to climb waves.

    10Ft waves take LOADS more power than smooth water.

    The cruisers ability to take the ground , handle well under AP or self steering and have the HP to climb the usual combers is PERFORMANCE , to me.

    The full keel does most of this , with 300+ years of refinement.

    Other advantages, like the ability to hide a really big 2 blade prop behind deadwood also add to a cruisers "performance".

    A Porsche is a poor way to get 20 sheets of ply , and a weeks groceries home.

    A pick up truck is not great at Le Mans,

    chose your "performance" needs.



    FF
     
  7. gonzo
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    gonzo Senior Member

    There are 25 foot tides in this area. Performance has to take that into account. It doesn't make any difference that you can point, if the tides will prevent you form getting there anyway.
     
  8. dskira

    dskira Previous Member

    I am one of these one who think you have no "trade off" for what it means, and my customer do not find it nostalgic, but very efficient and they ask me why they don't design it more often on cruising boat.
    It is perhaps because new designer spend to much time to read glossy trendy magazine, and not enough time to sail? Or just they just don't know the trade properly?
    It is always dangerous to make general statement about designers and design features without a proper backing.
    As for the "today" quite an amazing word that come back very often :p
    Daniel
     
  9. PAR
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    PAR Yacht Designer/Builder

    Attached is my most popular design, currently being built in several different countries around the world. A throw back to the Buzzard Bay boats, with slack bilges and a full length keel. The latest version of this design is a glued lapstrake schooner, being built about 150 miles south of the arctic circle in Finland. I don't know what I must have been thinking . . .
     

    Attached Files:

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  10. Ad Hoc
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    Ad Hoc Naval Architect

    And she's a lil' beauty too :)
     
  11. PAR
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    PAR Yacht Designer/Builder

    Well, thank you John. I tried hard to get the qualities just right. She sails just as you'd expect, initially tender, but then about 8 to 10 degrees into it, she firms up and rides like a '53 Buick. A very comfortable motion for a small boat.
     
  12. RThompson
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    RThompson Senior Member

    The drag on a keel (and hence part of the horsepower required to drive it) is basically dependent on two things: its aspect ratio and its surface area. Lift induced drag is a function of aspect ratio and viscous drag is a function of surface area. So irrespective of its pointing ability generally bilge keels will consume more horsepower for the same speed due to increased wet surface area (compared to a fin/bulb arrangement).
    However, as I implied earlier a yachts keel is a compromise between two extremes: outright sailing performance and practicality. Hence the measurement of overall performance of the yacht will include aspects of the keels performance, whether it be the ability to sail faster, the ability to provide protection for the propeller, or seakeeping - or any combination of many different things.
    It is the operational profile of the yacht that should decide priorities for the keel. This discussion is about cruising yachts so obviously one would look for a cruising keel. - well that opens the flood gates, some people go cruising in race boats, and some go racing in cruising boats. So the definition of cruising is different for different people.
     
  13. dskira

    dskira Previous Member

    You forget the strength of a long keel design. The fin is a rupture point on the hull, well documented on Bavaria and other boat. The long keel with return garboard (built down) gave a natural strength to the whole hull.
    The sea is the sea, you don't have different definition of cruising, just uninformed or ignorant peoples who make the wrong decision following stupid advise.
    Daniel
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    I'm curious: in actual practice, how many cruising sailboats get sailed upwind for a significant length of time? Or do they just motor along or wait for a change of wind?

    Not talking about the round-the-world or cross-the ocean types here, just the along-the-coast types.
     

  15. PAR
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    PAR Yacht Designer/Builder

    98% of the time a cruiser is anchored, moored or berthed. Up wind work is a necessity, but it doesn't have to be a smash and bash to windward, when all you have to do is ease the sheets and make a few more tacks.
     
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