Wave-Piercing, Pitching damping or marketing?

Discussion in 'Sailboats' started by Erwan, Jun 24, 2007.

  1. Erwan
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    Erwan Senior Member

    Hi Everybody,

    I try to find out the actual advantage of pierce-wave design for catamaran.
    As these designs are usually combined with larger beam section, it is not clear wether the supposed advantage comes from less hull drag as "pierce-wave" suggests, or from less pitching and therefore better rig efficiency ?

    Thank you in advance.
     
  2. frosh
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    frosh Senior Member

    My understanding of the wave piercing design which has been done much more in power cats than in sailing craft is mainly to allow higher average speeds through rough sea conditions. In sailing catamarans there is the very considerable risk of capsize by pitchpole which is not a factor in power cats. A bouyant bow section will lead to a very rapid slow down of the leeward hull if it suddenly is pressed hard by the rig forces, especially if a large wave is also encountered simultaneously by the leeward bow. The rig has considerable momentum and wants to continue it's forward motion at the same speed it was travelling at just prior to the burying of the leeward bow. A capsize over the nose may result, and at the least, a shuddering de-acceleration with huge stresses being created especially on the mast and the staying system, and also the forward hull sections.
    The wave piercing bows will in theory de-accelerate much less, and all the bad events described above will be minimized. Also in theory the hulls should continue sailing even when the bow(s) are fully under the water surface. It doesn't always work and the notable maxi cat Team Phillips did break up in trials even though it was a definite wave piercer.
    See story in this link: http://www.solarnavigator.net/team_phillips.htm
     
  3. MalSmith
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    MalSmith Boat designing looney

    While it is true that wave piercing bows have been used on power cats to achieve higher average speeds in rough conditions, the reason has nothing to do with reducing hull resistance. A limiting factor for high speed ferry operation is seasickness. Wave piercing bows are used in an attempt to reduce the passenger seasickness incidence by reducing the pitching motions of the vessel. I am not aware that there is any significant reduction in resistance due to wave piercing. In fact the reverse may be true, but it would depend on the details of the design. I was involved in some early development work for wave piercing catamarans, and I can state that for a wave piercing bow to be effective, a very significant reduction in reserve bouyancy above the bow is required. Most sailing multihulls I have seen which are touted as being of wave piercing design, do not even come close to being true wave piercers.

    I do not know for sure why wave pieercing bow on sailing multihulls as are fashionable at the moment, but the two potential benefits which come to mind are reduction in crew fatigue and, as mentioned, reduced rig motion. Both of these benefits will be realised only on upwind legs.

    On downwind legs I would think that wave piercing bows would tend to be a liability rather than a benefit, as pitchpole resistance is considerably reduced due to the necessary reduction in reserve bouyancy. I doubt that any reduction in decelaration when stabbing into the back of a wave would offset the loss of longitudinal stability and hence sail carrying ability.

    Mal.
     
  4. Frosty

    Frosty Previous Member

    So-- what is the definition of a wave piercing hull?

    Semi displacement bulbous bow?
     
  5. MalSmith
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    MalSmith Boat designing looney

    I guess a wave piercing bow is one such that the shape provides the kind of wave piercing behaviour that you desire!! Personally I think that an effective wave piercing bow could be defined in very broad terms as a bow shape for which the enclosed volume above the waterline is less than the volume below the waterline, for at least 20% of the waterline length aft of the stem. The effectiveness of any wave piercing bow is effected by the sea state. i.e the wave height and encounter frequency, and the dynamic properties of the hull, i.e the radius of gyration and the damping effects of the hull shape itself. Any given wave peircing hull will be most effective in one particlar sea state, and less effective to varying degrees in most other sea states. The trick is firstly to pick the right design sea state, and then be able to tune the hull shape for that condition.

    Mal.
     
  6. Erwan
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    Erwan Senior Member

    Thank you very much , your references and comments are very usefull. The definition you provide for wave-piercing bow is an interesting benchmark to keep in mind. I primarily look at A-Cat hull shapes which seem to associate wave-piercing features with larger and flatter hull (U shape) with maximum beam a bit forward (55% vs 61%) and in addition to dynamic lift these hull shapes seem to be designed in order to dampen pitching motion. But on the other hand in choppy water it seems that the boat rebound on waves like a tennis ball, it has been noticed by competitors at the last F18 World in Australia. But F18 Cats are twice heavier than A-Cats for the same lenght: 18 feet. I think I have too investigate further on relationship between pitching, center & area of flotation and so on.
     
  7. frosh
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    frosh Senior Member

    Real world example of wave pierce bows.

    I found this interesting link written by highly respected yacht designer and multi-talented, Meade Gougeon. It looks at the extremely competitive A class catamaran, a devolopment class with only a few restrictions. There is also a photo of the Nacra A2 showing very obvious reverse bows, where bouyancy below the waterline is significantly greater than bouyancy towards the deck in the bow sections. This trend has been the class standard for a few years now, and is still a feature of the world's top A class designs. The article by Meade is the second link.
    http://www.a-cat.org/id59.htm
    http://www.sailingworld.com/article.jsp?ID=34924&typeID=395&catID=635
     
  8. MalSmith
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    MalSmith Boat designing looney

    Another practical, but more obscure example of wave piercing hull forms can be found in the amas of traditional Polynesian proas. These are usually made from solid logs with a specific gravity of about 0.5, which gives a 50/50 volume distribution above and below the waterline. It has been speculated that one advantage of this, apart from simplicity of the solid log construction, is that the resulting wave piercing nature of the ama reduces torsional loads on the aka structure by minimising movement due to independent pitching of the ama, which would wear out the structure more quickly.

    It makes sense that for good wave piercing properties the volume distribution should be about 50/50 above and below the waterline, since the upward buoyant force of the fully immersed hull section will then be equal to the downward weight force when the section is fully airborne. Our findings from tank testing that we got better performance with less than 50% volume above the waterline in the bow region were probably due to the practical restriction that we had to have significantly more that 50% volume above the waterline from about 20% LWL aft of the stem.

    One interesting observation from the tank tests was that for a conventional bow, the average running trim in waves was a few degrees bow up, whereas with the wave piercing bow, the average running trim was closer to zero. From this one could speculate that the improved average running trim may result in lower average hull resistance.

    Mal.
     
  9. Otter 33
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    Otter 33 Junior Member

    thoughts on the subject

    I am definitely not the “resident expert” on this subject – But I’m currently building a small “wave piercing” power cat that I have designed… and hopefully these comments can be of use to someone. Please correct any misassumptions I may have about this subject.

    To me, wave piercing effectively means:
    Length to beam ratio of the hulls are near 20:1 and as high as 40:1.
    Very fine forward entries of the bows.
    Very low volume bows
    Forward bow sections (roughly first 1/3 of hull shape) elliptical/symmetrical or have a low drag section. To create a low drag section for the bow you would consider the conditions that will exist on the bow when they travel into a large wave. Imagine slicing the bows off, and then using them as a keel, with the “deck” surface now acting as the leading edge of the keel. This is a good thought experiment to create an efficient bow section. A low drag shape in this condition will reduce the tendency to nose dive. A high drag section in this condition (like the bows of a Hobie 16) will increase the tendency of the boat to trip.

    In regards to the radical looking hull shapes on newer A-cats: To me it seems A-cat’s have been wave piercing for some time now. The newer radical looking bow shapes may increase the speed of the boat through wholesale reduction of windage as opposed to changing the wave piercing affects of the hulls. For a boat like the A-cat- you may be seeing performance gains from a design decision to remove any parasitic, above water, unnecessary hull surface.

    Also, the trend in A-cats of moving the underwater volume of the hull forward may also be of benefit only to the A-cat and not larger catamarans. It is there to counteract the sailing forces on the boat as opposed to creating a more efficient hull shape. A more efficient wave piercing hull shape would bring the underwater volume further aft- that’s if you did not have to consider the rig. Note that A-cats have very high aspect ratio rigs with the center of effort a lot higher than any other sailboat I can think of, while weighing proportionally less than any other sailboat I can think of. This creates a huge lever arm which pushes the bows down… while the boats have very little comparative weight aft to counteract this (even with the one crew trappesed near the windward rudder). Also note that the tendency in A-cats is to move the rig further and further aft. For these reasons, I would have to conclude that the tendency to move the underwater hull volume forward is to counteract a very high aspect rig on a very lightweight platform, as opposed to having any “wave piercing” or hull drag benefits.
     
  10. frosh
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    frosh Senior Member

    Hi Otter, your reasoning seems quite sound re the A cats and the need to reduce the tendency to depress the leeward bow, and reduce parasitic aerodynamic drag. This might be as, or even a more important reason as wave piercing for the unusual reverse bows seen almost universally on this class of cat.
     
  11. Erwan
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    Erwan Senior Member

    Thank you everybody

    Thank you everybody for your relevant and very interesting comments. As for many boat, the issue is the optimum trade-off with many criterias to be considered, and difficult to modelize, that is why Yacth design is still and art.

    Regards

    EK
     
  12. Retired Geek
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    Retired Geek Junior Member

    taking wave piercing to the next level

    not quite ready for the water (one or two weeks away yet) but hope to have the LR2 competing shortly...btw, design waterline is where the unskinned hulls were joined
     

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  13. Erwan
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    Erwan Senior Member

    Thank You Retired Greek for the Photos

    Great boat, obviously, according to the carbon beam and high aspect ratio daggerboard it seems to be a A-Cat. Hulls seem to be made with carbon or dark foam ? limited area for the hulls must lead to light weight
    Is it your own design ?
    The place you live seems to be near the water, happy man !!!

    Thank you again

    Cheers

    EK
    erwankerauzen@free.fr
     
  14. Erwan
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    Erwan Senior Member

    Sorry / Thank you Retired Geek

    Sorry for the misspelling in the former msg
     

  15. Retired Geek
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    Retired Geek Junior Member

    Yes, its an A-Class, core is corecell with carbon inner and outer skins, weighs about 34 kg as shown before paint, boards are very high AR (just 140mm chord and draw about 1.15m), rudders are just shorter versions of the boards.
     
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