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Hull speed

Discussion in 'Ask All Sailors' started by dnimigon, Mar 11, 2019. Add this thread to a FAQ

  1. dnimigon


    Joined Mar 9, 2012
    44 posts, 2 likes
    Catalina 30
    US PVR
    Just a question for all of you. When a manufacturer states that a boat has a certain hull speed what exactly does that mean. As in our Catalina 30 has hull speed of 6 knots. Someone said it was you have full control to that speed. ?????

  2. rgranger


    Joined Jan 19, 2010
    5,847 posts, 1,402 likes
    Hunter 26
    US Smith Mountain Lake
    There are a lot of ways of explaining this but here is mine...We are sailing displacement hulls. As the boat goes faster it produces a bow wake as well as a mid-ship wave. The faster you go, the further aft the mid-ship wave moves. Once the mid-ship wave is at the stern, you are essentially sailing in the trough of a swell. To go any faster you would have to climb your own bow wake. That is not possible with a displacement hull design powered by a sail. So now you are at the maximum speed. There is a formula based upon hull length and beam.

    agprice22 and jon hansen like this.
  3. Davidasailor26


    Joined May 17, 2004
    1,558 posts, 367 likes
    Beneteau Oceanis 37 LE
    US Havre de Grace
    Like rgranger said, hull speed is a measure of how fast the boat can go while making a bow and stern wave, and not climbing up onto the bow wave. The speed is determined by a formula based only on waterline length: Hull speed in knots = 1.34 x square root of water line length in feet.

    The effect of hull speed is that it takes more and more energy to move a displacement hull as you approach that speed, and it's theoretically impossible to exceed that speed while being a displacement hull. In reality all boats will plane given enough energy (some easier than others), so hull speed isn't an absolute limit.

    Will Gilmore and jon hansen like this.
  4. Whatfiero1


    Joined Mar 29, 2017
    246 posts, 47 likes
    Hunter 30t
    US littlecreek
    Had my Hunter 30 up to 12.9 knots in a downwind ocean run with wind blowing 30 to 35 knots. I didn't believe the GPS either so my son download a speed app on phone and it said 14 mph. Only proof is my club I race with record my time on 15 mile race at 1 hour and 41 min.

    Ken Cross and jon hansen like this.
  5. David in Sandusky

    David in Sandusky

    Joined Nov 8, 2007
    1,145 posts, 178 likes
    Hunter 27_75-84
    US Sandusky Harbor Marina, Lake Erie
    Only one thing to add to rgranger and Davidasailor:
    It is not impossible to power out of the wave that embraces your bow and stern at hull speed. It just takes more power. Riding up on the bow wave and exceeding hull speed is called planing. We have planed our h27 up to 8.5 kts. vs. our hull sped of 6.2. I planed my Sunfish to 3 times its hull speed reaching in a 40 kt wind.

  6. DArcy - Islay Mist

    DArcy - Islay Mist

    Joined Feb 11, 2017
    288 posts, 248 likes
    C&C 27 MkII
    Ca Ottawa
    Sailing a Sunfish in 40 knots? Now that is impressive:worship:

    Yup, hull speed is a purely theoretical number and, on its own, has very little to do with how fast your boat can go. A narrow boat will be less affected by hull speed and a planing hull (light, flat sections aft) will just get up and fly right through it. Ponderous cruisers require a LOT of power to exceed the theoretical hull speed. @dnimigon - your Catalina will probably easily and comfortably exceed hull speed with 15 to 20 knots of wind on the beam.

  7. Cowpokee


    Joined Jan 8, 2015
    306 posts, 162 likes
    MacGregor 26S, Goman Express 30
    US Kerr Reservoir
    LWL is something I have always wondered about. Sure it is easy to measure while your boat is sitting in the marina, but when heeled over, how could one easily measure what it increases to?

    Will Gilmore and JamesG161 like this.
  8. Jackdaw


    Joined Nov 8, 2010
    9,851 posts, 2,905 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    LOL how many sailors does it take to explain ‘Hull Speed’???

    rgranger likes this.
  9. RussC


    Joined Sep 11, 2015
    1,216 posts, 562 likes
    Macgregor 25
    US Oregon lakes
    Four. three to explain it and one to tell them they're wrong. ;)

  10. Stu Jackson

    Stu Jackson

    Joined Feb 26, 2004
    20,321 posts, 819 likes
    Catalina 34
    224 CA Maple Bay, BC, Canada

    - the fifth to explain how to do a search! :):):):):)

  11. rgranger


    Joined Jan 19, 2010
    5,847 posts, 1,402 likes
    Hunter 26
    US Smith Mountain Lake
    Or what anchor to use.

    Jackdaw likes this.
  12. kappykaplan


    Joined May 1, 2011
    929 posts, 177 likes
    Pearson 37
    US Lusby MD
  13. John Tubb

    John Tubb

    Joined Feb 14, 2017
    1,016 posts, 287 likes
    O'Day 25
    US Guntersville, AL
    "Hull Speed" : the brokers creative description of the lack or hp with the included motor on a sailboat to achieve anything but minimum speed against a 20 knot current. Dreamy eyed buyer, 'Gosh this feels slow under power" Broker, "No worries you have more then enough to achieve hull speed!"

  14. David in Sandusky

    David in Sandusky

    Joined Nov 8, 2007
    1,145 posts, 178 likes
    Hunter 27_75-84
    US Sandusky Harbor Marina, Lake Erie
    By the way, the specs on this site say the Catalina 30 hull speed is 6.7 knots.

    Will Gilmore likes this.
  15. Daveinet


    Joined Sep 20, 2014
    979 posts, 188 likes
    Rob Legg RL24
    US Chain O'Lakes
    Something of interest. We had a 120 Mercruiser that died out on the water. We towed it in with a Jetski. If I remember correctly, the Jetski was 130 HP. As we tried to go faster, the Mercruiser would not plane. Full throttle on the Jetski was only about 8 mph. The bow wave was several feet tall. The rear of the boat was just sitting in a very deep hole. If you can picture a boat vertical, and then trying to drag the boat through a vertical wall of water. The harder we pulled, the more vertical the wall of water was. Apparently if you can't get some vertical lift from the prop, the boat could not go any faster.
    Obviously a sailboat has a much flatter bottom, so it will climb up over the wall of water before it gets too tall. A good planing hull hardly makes any bow wave, but just continues to raise out of the water, the faster you go. The larger cruiser sailboats are somewhere in between.

  16. rgranger


    Joined Jan 19, 2010
    5,847 posts, 1,402 likes
    Hunter 26
    US Smith Mountain Lake
    I'm not sure if that is correct. True for some dinghy designs but not a cruising sailboat. It might be that the length of your tow line from the jet ski to the disabled boat was just the wrong length and you had your boat sitting in the wake of your jet ski.

  17. Scott B

    Scott B Moderator

    Joined Sep 20, 2006
    2,592 posts, 153 likes
    Hunter 33
    CA Georgian Bay, Ontario, Canada
    and how to brew the coffee

  18. JamesG161


    Joined Feb 14, 2014
    3,547 posts, 1,281 likes
    Hunter 430
    US Waveland, MS
    Actually you can exceed the theoretical hull speed. Plus it changes with hull design and the heel angle.

    In very high seas, one might think the Up and Down action would take your boat longer to arrive at a destination [VMG] because it appears to be traveling longer distances.
    Your hull is more "Out of the water" on the crest of a wave, it will go FASTER since there is less Hull Drag. Surfing, so to speak.

    You can exceed the theoretical Hull speed by a large percentage when Surfing your hull.
    SIX :pimp:

  19. FastOlson


    Joined Apr 8, 2010
    1,052 posts, 138 likes
    Ericson Yachts Olson 34
    US Portland OR
    Note that hull *shape* is very important in "escaping" that stern wave. While truly "planning" probably will not happen with the weight we all carry around in our dual-purpose racer-cruiser designs, there are design factors that do let you enjoy more speed in spite of the water resistance.
    Beam to length ratio is also Really important. Shape of the bottom and the stern sections is important. And total weight.
    Case in point: out hull is flatter aft, without the pinch of a more-IOR 'ish shape. We have moderate weight, at 10600#, and our midship sections are arc-shaped, altho not flat. We have a long immersed waterline at a over 31 feet. Our beam is moderate, at 10' 10".
    We have maintained a bit over 10 kts for hours at a time, with a small jib only, down wind off the WA coast in the afternoon 24 kt winds. Seas were about 4'. Surfed at times up to to 12.2. (Sister ships have gone faster with a chute up, FWIW)
    The rest of the time we motor (and sail to weather)at 7 kts.
    It does take a lot more energy input to exceed the designed-in limits of your hull! :)

    JamesG161 likes this.
  20. Daveinet


    Joined Sep 20, 2014
    979 posts, 188 likes
    Rob Legg RL24
    US Chain O'Lakes
    The power boat was a very deep V-hull for good riding in rough water. A Jetski does not have much wake, the tow rope/ski rope was quite long. I understand cruisers are more displacement hulls, but still flatter than a deep V-hull. I really wish I had a photo, as it would really illustrate how the bow wave continued to grow and the boat rotated vertical. Much higher than anything I have ever seen any boat do. We assumed it would plane, but it just would not climb over the wall of water.

    I'm only trying to illustrate how and why a hull speed exists. I know enough to realize you can exceed hull speed - my first sailboat was a Hobie14. But the hull design dictates (practically) if and how one can exceed hull speed. Typically a fast sailboat design will produce very little wake, and the transition from displacement to planing is hard to define - think of a skow design.

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