Hull speed

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

  1. capta

    capta

    Joined Jun 4, 2009
    3,050 posts, 1,099 likes
    Pearson 530
    na Admiralty Bay, Bequia SVG
    @ 45' wll, my theoretical hull speed is 8.99 knots. We've done 11 knots plus repeatedly and over 12 knots now and then BEATING, not surfing in any way or shape, and that's well reefed, not pushing or racing. I guess my GPS lies a lot!
    However, I have heard tales of Clipper Ships that have so far exceeded their hull speed that they have literally sailed themselves under and been lost. But of course, that's downwind.
     


  2. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    10,072 posts, 3,079 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Light weight 40 foot boat, high SA/D ratio, high righting moment, flat bottom. 17 knots in 24 true. No wake at all. This type of performance has to be baked into the boats design from day 1.

    E57AC25D-9E6C-4E9B-8231-2ABB89CB3AB0.jpeg
     


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  3. Scott T-Bird

    Scott T-Bird

    Joined Oct 26, 2008
    3,903 posts, 895 likes
    Starwind 27
    US Barnegat, NJ
    There is a COB waiting to happen as it did during the Mackinac race last summer. It is surprising to me that the manufacturer doesn't address this issue. I'd guess that the frequency is so low that it has never been considered. That CYC report didn't even mention this issue.

    A crew sliding under the lifelines could disappear from sight pretty quickly at that speed. Think of a family with a small child, not tethered, wearing an inflatable PFD (that fails). A rail 4" high off the deck would prevent a tragedy, no?
     


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  4. jon hansen

    jon hansen

    Joined May 25, 2012
    1,932 posts, 1,600 likes
    john alden caravelle 42
    us sturgeon bay, wis
    "hull speed" of any vessel is the top speed at which the stearn of the boat is still riding on the second, or stearn wave that is created by the vessel going through the water. very simple concept. faster than that means that the stearn has fallen off the stearn wave. the horsepower then needed to go faster increases exponenitially. you have to understand the dynamics of waves. as waves goes faster the crests between the waves grows father apart. simple science, nothing more.
    sure are allot of posters here that do not understand the definition of "hull speed". all boats can go faster than hull speed whether plowing through or planing over. it just takes allot more horse power.
    easy peezy :)
     


    Last edited: Mar 12, 2019
    Scott T-Bird likes this.
  5. jon hansen

    jon hansen

    Joined May 25, 2012
    1,932 posts, 1,600 likes
    john alden caravelle 42
    us sturgeon bay, wis
    read frank bethwaites book's chapter on waves. it even has pictures :)
     


  6. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    10,072 posts, 3,079 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI

    Idle speculation. In actual sailing, it happens a lot lot less than you think.

    First think about it, most boat pitching throws a crew forward or sideways. Not backward. Second if it did happen with any true degree of regularity, the manufacturers would stop the open transom design process. Third, these boats are designed to cross oceans. This particular (Hermes) boat has crossed the Atlantic 5 times. Fourth, the lifeline height is no higher than on the sidedeck The ChiMac incident this year was a COB over the side, not aft, in a huge open transom boat.

    Some boats will rig netting if kids are on board, just like you would on your siderails.
     


    Last edited: Mar 12, 2019
  7. Davidasailor26

    Davidasailor26

    Joined May 17, 2004
    1,652 posts, 401 likes
    Beneteau Oceanis 37 LE
    US Havre de Grace
    What makes you think that? It wasn't at all the impression I had when I first read the report, although re-reading it now I'm not so sure myself.

    First, the report makes the following statements: "Jon slid off the aft leeward starboard corner of the boat", "There’s nothing to hang onto in the back of a TP 52. (There is an absence of hand holds between the lifelines on a TP 52 at this location on the boat. (See photo in Appendix C)", and a picture captioned as "Open afterdeck with minimal handholds".

    Unfortunately, Appendix C and the report in general aren't especially clear on where he went over relative to the lifelines, and one of the crew did state they saw him in the water "passing by the transom", which suggests maybe he was in front of the transom when he first went in.

    (Re-reading the report I agree even more with the lack of satisfaction that @Scott T-Bird expressed in the previous thread on the subject.)
     


  8. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    10,072 posts, 3,079 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    The Commodore of the CYC told me.
     


  9. Davidasailor26

    Davidasailor26

    Joined May 17, 2004
    1,652 posts, 401 likes
    Beneteau Oceanis 37 LE
    US Havre de Grace
    Fair enough. Sure would be nice to have that kind of info in the report to provide some kind of root cause analysis though.

    Back to the hull speed topic - I get why hull speed is based on LWL, but how do changes, say, attached to the end of the boat, affect its LWL from a hull speed standpoint? If I could tow my dinghy against my transom such that the waterline were continuous, would that increase my hull speed (admittedly with losses due to added wetted surface)? I remember New Zealand trying something like this by adding a "hula" appendage to their America's Cup boat a while back, and that wasn't very successful. Was that just because of the extra drag or is the position of the stern wave limited by something more than just the aft most piece of boat touching the water?
     


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  10. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    10,072 posts, 3,079 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    No problem. The report does allude to it, with the comment that 'Jon may have hit the leeward stanchion with considerable force as he went over' Almost all MOBs go off the leeward side of the boat.
     


  11. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    10,072 posts, 3,079 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    A 'hull' is a single wave-making body. A towed dinghy is a different hull. I have seen boats that have added sugar scoops to the transom, turning a 34 footer into a 35. That probably adds 0.1 knots to the hull speed.
     


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  12. Whatfiero1

    Whatfiero1

    Joined Mar 29, 2017
    263 posts, 52 likes
    Hunter 30t
    US littlecreek
    Anyone ever thought of making a bulpus extending bow out of a 2" pvc pipe with a round capp. Cut into bow at water line and use 2 prop seals to seal and hold in place. It would start off flush then extend with rope and pulley 3' maybe.
     


  13. Daveinet

    Daveinet

    Joined Sep 20, 2014
    1,014 posts, 211 likes
    Rob Legg RL24
    US Chain O'Lakes
    It would work IF you could make the dingy support the weight of the rear of the boat. The reason the water line matters is because the water is scoped out from under the boat, causing the rear of the boat to sink. This changes the angle of the boat. When the wave length is shorter, the rear of the boat is still held level, so it doesn't angle upward. Beneteau was making boats with a wide stern, to prevent the rear of the boat from sinking. This cheats the water line some, but the boat is slighty slower in lighter winds.
     


  14. Daveinet

    Daveinet

    Joined Sep 20, 2014
    1,014 posts, 211 likes
    Rob Legg RL24
    US Chain O'Lakes
    The shape and design would need to be large enough to actually create a wave. Just a simple pipe is probably not going to initiate enough of a wave to make any meaningful difference. Its displacement is too narrow. Its all about where the bow wave is in relation to the rear of the boat and how much water is scooped out behind the bow wave.
    Hull speed is really the combination of the height of the bow wave + the hole dug out behind it. Get rid of either one and the hull speed limit goes away.
     


  15. Charlie Jones s/v Tehani

    Charlie Jones s/v Tehani

    Joined Mar 1, 2012
    1,734 posts, 680 likes
    1961 Rhodes Meridian 25
    us Texas coast
    one thing I haven't seen mentioned here is wave lenght. Waves of a certain lenght crest to crest, travel at a specific speed Always. So the waterline length of the boat and hull speed is dependent ( to a certain extent) on that wave length. Exceed it and you start trying to climb the bow wave an the stern ( or quarter wave) drops behind the ransom, and the boat squats. Physics :)
     


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  16. Scott T-Bird

    Scott T-Bird

    Joined Oct 26, 2008
    3,903 posts, 895 likes
    Starwind 27
    US Barnegat, NJ
    Hull speed is simply defined as the speed where the wave length equals the water-line length of the boat. I think there is too much made of the resistance from the quarter wave among those whom argue that a displacement boat can't exceed hull speed. Sure, there is much more resistance when a blunt-nosed, heavy boat creates a steep bow wave and quarter wave. But a boat that is lighter, has a fine entry and a wide, flat stern creates a relatively minor wave, what's the problem? The boat simply moves fast enough to create a longer wave that spreads out behind the boat. The stern of the boat simply escapes the quarter wave. The boat doesn't have to plane and climb over the bow wave, It simply escapes the relatively minor resistance of the quarter wave. You could say it is essentially "squatting" but if the wave has minor definition because of the relatively light weight and bottom design of the boat, the boat simply moves along with less significant resistance and exceeds hull speed, and visually, it isn't really "squatting".
     


  17. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    10,072 posts, 3,079 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Agree. But this is easier explained by simply saying that non-planing sailing boats can't plane because their hulls were optimized to perform in non-planing conditions.

    Take an old IOR half-tonner. 40 foot leadmine. This boat would never plane, even if you put 500hp on the transom. The designer had a downwind SA/D (30?) in mind in the design, knew what its top water speed was going to be based on that, and optimized the hull around that.

    The Pogo design team knew their target SA/D (70), and knew the boat would plane with the right hull. So they gave it that fine entry, flat bottom wide transom. Zoom!

    Some of these newer designs plane under aux power, an amazing feat.
     


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  18. Scott T-Bird

    Scott T-Bird

    Joined Oct 26, 2008
    3,903 posts, 895 likes
    Starwind 27
    US Barnegat, NJ
    Only amazing because designers are likely to limit aux power to limit weight and costs. They are increasing speed under sail, so why would they want to add HP! But then, when they are able to achieve planing with a conventional power plant (for a sailboat) that just adds to the appeal, I suppose.

    In order to understand how a sail boat exceeds hull speed, I think it is necessary to also explore how (and when) a boat achieves planing. I think there is a misconception that the only way for a boat to exceed hull speed is to become a "planing" boat. I don't agree with this. Attached is a discussion about planing that I think is interesting.

    https://www.boatdesign.net/threads/definition-of-planing.45248/

    I think that Leo and DMacPherson have reasonable definitions. I particularly relate to the graph that DMacpherson presents.

    A simple statement that brings my thoughts into focus: "There is no one binary point of planing. It is not either on or off. It is a transitional regime" I think that our displacement sailboats are capable of entering that transitional regime, where we exceed hull speed but don't really enter the regime of planing. And, of course, there are sailboats that truly do break into the regime of planing.
     


  19. Daveinet

    Daveinet

    Joined Sep 20, 2014
    1,014 posts, 211 likes
    Rob Legg RL24
    US Chain O'Lakes
    I think maybe the best definition of fully planing would be the point at which the bow wave ceases to increase with speed. Or maybe looking at it from the opposite side, the point when the water displacement decreases with speed.
    With the exception of a submarine, all boats will plane to some extent, as the forward motion will always create some lift. And a boat that is planing will also have some displacement. Even a snowmobile doing watercross has some displacement.
     


  20. Jumpstart

    Jumpstart

    Joined Jan 13, 2009
    247 posts, 54 likes
    J Boat 92
    78 US Sandusky
    I'm confused. The polars for my boat show a boat speed of 10.05knts at a true wind angle of 140 degrees in 20 knots of wind. It's a 30 ft boat with a 25.8 foot waterline. Obviously exceeding hull speed without surfing. Does that make it a planing hull?For the record boat speed in waves in 20knots is more like 12 knots due to surfing. A lot of fun in that rig is pretty unloaded as you go faster.
     


    Scott T-Bird likes this.


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