Hull Speed Formula , Pointing, and Twin Keelers

Discussion in 'Sailboats' started by Michael D, Apr 1, 2007.

  1. Michael D
    Joined: Apr 2007
    Posts: 1
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    Location: Jacksonville FL

    Michael D New Member

    Q1 Hull Speed

    The usual formula for the hull speed of a displacement hull uses a multiplier of 1.34 and occasionally one still runs into the use of 1.4. I'm trying to determine a proper multiplier for boats such as my Westerly Berwick 31 which by cursory rough measurement during slack tied seemed more like 1.44

    Research showed that the hulls on these boats are not truly displacement and the hull and keel design combined keep the aft wake flatter, extend the wave length somewhat and allow the the boat to gain a surfing effect with greater ease and maintain it longer than the usual displacement keel boats.

    One reference thought an increase of 15% to 20% over the 1.34 rule was pos)sible under optimum conditions.

    At that point I ran out of expertise, found your forum and determined to pose the question to those who are more able to provide an answer.

    Q2 My previous experience was on the 26' LOA 21' LWL Westerly Centaur the current boat is a 31' LOD 25' LWL Westerly Berwick. One of the negatives amongst many positives is in the area of pointing and light air tacking.

    Somewhat related I've combined the two. In very light airs I've found it useful to 'wear ship' meaning just go round the other way copying the square riggers. Especially since I'm at a wider angle off the wind often 45 degrees and don't have to go that far around.

    The best we seem to be able to reach in pointing is in the 30 to 35 deg. sometimes less. This falls off to a wider angle at slower speeds.

    Short of motor sailing what might I do to gain a smaller angle?

    The new rig is a furling 110% head sail and a loose foot full batten main with everything to shape the sails except mast bending.

    My thanks for any assistance you may be able to offer.

    /s/ Michael
    SV Se Langt
    Westerly Berwick 31
  2. Eric Sponberg
    Joined: Dec 2001
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    Location: On board Corroboree

    Eric Sponberg Senior Member

    On hull speed, the 1.34 multiplier comes from the physics of gravity waves. An ocean wave's speed is definitely 1.34 x sq.rt. of the length between crests. As the hull moves through the water, it creates a gravity wave starting at the bow. For any given hull, there is no rule or definite multiplier that you can apply because each boat is different and conditions vary infinitely. I once had my heavy 27' (20.33' Lwl) full-keeled sailboat scooting along at 9 knots on a broad reach in nice waves--that is a speed length ratio of 2.0. It is pointless, really, to fixate on what speed-length ratio the boat is capable of achieving--conditions can make it go all over the place. A far better indication of its performance is what do you achieve on average--what is it typically capable of? Use that for planning a voyage.

    The first thing to consider on pointing ability is the sails--how new and fresh are they? This is where your power comes from. If the sails are old, then they have stretched and don't have the best shape. Consult with a sailmaker to recut or get new sails. Modern materials and cuts will do wonders.

    At the other end of the boat, meaning the hull shape and bottom, there is not too much you can do other than keep the bottom clean and the anti-fouling up to date. The twin keel Westerly's are what they are, and known for their not really impressive performance. You would have to consider going to deeper draft (keel additions) or completely rebuilding the hull and keels in order to reduce drag and increase lift, which would improve pointing ability.

  3. PI Design
    Joined: Oct 2006
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    Location: England

    PI Design Senior Member

    Hi Michael,

    Eric is right about the 1.34 multiplier being related to the speed/length ratio of gravity waves, however a boat is not necessarily limited to this value.

    David Gerr (author of several boat design books and director of Westlawn Institute) has developed an empirical formula for estimating boat hul speed, where the 1.34 is replaced by a variable parameter (which is a function of the displacement/length ratio of the boat).

    It is discussed here and here
    . Try it for your boat and see what you get.

    There is also a discussion of a similar topic here.

    To make the boat point better, there are a thousand and one options and I'll leave it to others to offer advise. Note howeve, that VMG is more important than pointing abilty per se.


  4. water addict
    Joined: Jun 2004
    Posts: 317
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    Location: maryland

    water addict Naval Architect

    for pointing- echo the good sails, tuned rig, and clean bottom. If you have these, great!.
    If not, get them. You will be literally blown away at how much difference it makes.
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