Reverse-Engineering of Modern Cat hull shape

Discussion in 'Hydrodynamics and Aerodynamics' started by Erwan, Jul 30, 2012.

  1. Erwan
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    Erwan Senior Member

    Hi Everybody,

    First of all I d like to post a link as a support for discussion:

    http://www.catsailingnews.com/2012/07/etnz-ac72-trickle-up.html#more

    You will find pictures and comments regarding some modern cat hull shape, especially the radical hull shape observed on the F18 Cirrus R.

    The bow design is at stake, and searching for "spoon bow" or "Davidson bow" as mentionned in the comments did not bring new elements.

    Instead in old academic papers:

    "Procedures for hydrodynamics evaluation of planning hull in smooth and rough water" Daniel Savitsky & P Ward Brown.

    It is mentionned, as far as I understood (although for different kind of hull shape): Either in the pre-planing range (1<Volume Froude<2) or full planing Fvol>2 the total drag is always lower if the bow is not immersed.

    The Flyer hull shape and most of A-Cat hull shape, advertized as pierce-wave designs, seem to sail with the bow out of the water all the time, just like if optimized as a semi-planning hull.

    Lower lenght/ displacement ratio makes similar design more challenging for a F18, unlless you put a lot of rocker in the front of the hull, just like the CirrusR.

    These observations lead to an implicit question:
    In Cat design is optimization for planing or semi-displacement hull, the direction to go in priority?

    Of course, planing and pierce-wave features are not exclusive, keeping in mind that pierce-wave is more related to volume above flotation than underwater volume.

    Regards Everybody

    EK
     

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  2. TJ Cameron
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    TJ Cameron Junior Member

    Erwan,

    This topic of optimized beach cat hull shape I find very interesting. I hope we get some posting.

    Here are some of my thoughts:

    I am a bit skeptical of the flat bottoms of the DNA A-cat and the Cirrus R. Mostly for the reason that to provide lift the flat bottom must be inclined at a positive angle of attack. That is generally lacking in the DNA for most of the hull length (except at the bow) unless the bow rises substantially. The bottom of the Cirrus R appears to have a small positive angle for only about the last 300 mm. Lifting force at the absolute stern is usually not helpful going downwind at high speed.

    On the other hand, both of these boats are quite successful. I don't see how their success can be explained by any current theory. The DNA may in part be successful because the wide flat stern adds pitch stability which helps to optimize the curved lifting foil performance.

    Does the Cirrus R sail substantially bow up downwind so that the whole keel is at a positive angle of attack?

    I have used the Godzilla program quite a bit to try to understand optimum beach catamaran hull shapes at low Froude numbers (sub-planing). For this speed range the program usually shows immersed bows to be equally low drag as non-immersed bows. At higher speeds, the non-immersed bows are lower drag at least for non-planing /non-foiling. The program also shows that wide transom sterns have less drag than tapered sterns at speeds above 3 knots.

    TJ
     
  3. groper
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    groper Senior Member

    TJ, you need to understand that the program developer of Michlet and Godzilla, Leo, puts a caveat on the accuracy of predictions involving transom sterns. Nobody to date, has successfully developed an algorithm that accurately predicts transom drag at all speeds for all variations of any given transom. That said however, the program does seem to give reasonably accurate results for resistance despite this.

    One thing ive noticed when modelling different slender hulls of the same displacement, is that only very minor changes in resistance are evident once you already have a good shape. These racing cats, have naturally evolved into pretty ideal shapes already. The americas cup AC45`s have about the minimum drag hull shape for their mean speed regime (very high froude number). These small racing cats also operate mostly at very high froude number and are considered high speed displacement hulls as no substantial dynamic lift is produced by the hull itself and are also very similar in shape to each other and the AC45`s. The reason i use the AC45 as an example, is that many very intelligent engineers have worked to produce an ideal shape using the latest CFD technology with virtually limitless budgets to the very best of our modern ability, ie, its the best we can do.

    So, when people change the shapes as they experiment with new designs, they really are perusing the last 0.5% of hull drag - even tho most privateers do not realize how little of a difference they could be chasing. They go for it anyway, because its human nature to live in hope they might discover the next big thing.

    IMHO, id say bigger gains are to be had from improving other elements of the boat and its rig. The success of this radical hull shaped cat, could well be and most likely IS, from other design or materials improvements employed - which people do not notice because they are too busy looking at the radical shaped hulls ;)
     
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  4. Doug Lord
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    Doug Lord Flight Ready

    ================
    Excellent point! For all multihulls, it's my opinion that the area of the biggest gain for the part in the water is lifting foils-now being used to the greatest extent in history.
     
  5. k2mav
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    k2mav Junior Member

    Doug, in the F18 at least, you CAN´T play with lifting foils, so nothing to do in that area.

    The Cirrus R is confirmed in racing (gps data in regatta) and by the competition (other builders) being the fastest boat downwind. I assign part to its flat/fat stern/transom. All this taking out the rigs out of the equation.

    In the other hand I have reports of being somew how "slow" in calm weather (too much drag?)

    You guys need to sail an F18 and feel the planning of the modern hulls above 16-18 knots of wind , even upwind in ocassions.

    Whichever the case right now you have at least 4-5 designs in the F18 that compete at equal level: Wildcat, Infusion, C2, Cirrus R, even the old Tiger in good hands.
     
  6. idkfa
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    idkfa Senior Member

  7. Doug Lord
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    Doug Lord Flight Ready

  8. k2mav
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    k2mav Junior Member

    Phantom.. but here Erwan is trying to discuss hull shape, there are 10 thousand threads on foils already, so here try to keep focused mainly on modern cat hull shape.
     
  9. Doug Lord
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    Doug Lord Flight Ready

    ---------------
    Except(for the most part) in the classes where lifting foils are illegal, they are critical to cat(and tri) performance AND to hull design. Lifting foils have changed the way hulls are designed-the hull is designed to go with the foil system.
     
  10. groper
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    groper Senior Member

    The only thing left to discuss on hull shape, is for what speed do you optimize for?

    The logical response would be to carry a GPS and log average speeds over as many racing seasons as possible, find the mean or perhaps median speed regime, then optimize the hull for that - this is probably what they already do so..>?

    When you change the hull shape, you can have less drag at higher speeds and more at lower speeds, or you do the opposite and have more drag at higher speed and less at lower speed. From the already very advanced designs we have arrived at today, you cant make a new hull with less drag at all speeds.

    There is no magic bullet on hull design here, so what are we discussing? This discussion doesnt have far to go from this point other than tiny optimization changes. This would be similar to discussing the suspension settings or other car setup features of a race car for a given track. The car is essentially the same, just different settings for different tracks or weather conditions etc.
     
  11. k2mav
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    k2mav Junior Member

    Following your thought of line tell me why the ETNZ even bother to have that hull shape then.

    The same for all the little insignificant details that made the cat hull design evolve to what we have now.

    And finally explain why two boats builded in the same exact rule/weight, mast, and sails , the one with the insignificant rare detail (Cirrus R shape hull) over performs downwind the now standard and more conservative wp hull shape .

    I agree these little details have minimal impact overall on hulls of the same generation, But they have real effect in some conditions, for ie downwind in the F18 class, and I think the discussion here that Erwan established is understanding this evolution and how this new shapes works.

    Again, Why if not worth even analyzing it,
    ETNZ goes that way. As opposed, say, the more standard (now) AC45 hull shape (as we didn´t see any other 72 yet)
     
  12. groper
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    groper Senior Member

    You answered your own question... if it does indeed go better downwind, and there is no other difference on the boat besides the hull, then one might argue this shape has less drag whilst trimmed down attitude and high speed (downwind = higher speeds, trim down), therefore it has been optimized for this speed regime or race leg. If this were an accurate interpretation of the facts, then it would not surprise anyone if the hulls had a little more drag going upwind or in lighter air where the trim and speed are no longer favorable.

    Do you see the point i am making? IMHO, you can only 'tune' things from where we are today in terms of hull shape, and there are bigger gains to be had by spending your efforts elsewhere in the boat design, once you know what speed regime to optimize for.
     
  13. Erwan
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    Erwan Senior Member

    Thank you all for taking time to post on this subject, especially because my first post was both a bit candid and confused.

    I should have use "Guessing the design philosophy" instead of "Reverse engineering....." it would have been more appropriate. Also, below I will make an attempt to reformulate in a clearer way my first post:

    1-The starting point is when sailing my cheap A-Cat (a 2008 design), I noticed the bow "knuckle" is not in the water when sailing windward.
    I had the same remark from people sailing Flyer 1 a few years ago, and Ben Moon who sailed both NIKITA and DNA made similar comments about the DNA windward. (Don't know for NIKITA but guess it's pretty similar than Flyer1).

    2-When looking at speed/displacement ratio, an A-Cat windward @ 7.5 knts is already in the pre-planning mode(speed/lenght ratio> 1.7) as defined by Marchaj.(but for monhull, so I am not sure it is relevant, as wave drag of slender hulls is very different from classic dinghies hull).

    3-I guess that F18 larger displacement, precludes the boat to sail the bow knuckle out of the water windward as long as the hull shape (keel line) is pretty "standard". (to be confirmed by Wildcat, C2 sailors) So in order to take advantage of the "not immersed bow advantage" , it is mandatory to have an unorthodox rocker in the front (That is why the Cirrus-R)

    4-That is why theses observations combined with the Savistky workpaper, lead me to wonder if it makes sense to say that modern A-Cat hull & Cirrus R' shapes are optimized for pre-planning or semi-displacement mode?

    But I guess that is not the whole story. Modern hull shape style, advertized as pierce-wave design, actually demonstrated an ability to minimize pitch, compare to former design philosophy. That is why one must not rule out the possibility that modern hull shapes could have a slightly higher drag than old shapes, but ability to minimize pitch makes the rig more efficient in such a way that it more than offsets the increment in hull drag.(I will call that "a new global trade-off approach")

    In other words, even if I agree with Groper that digging hard for low drag cannot lead to a significant breakthrough, one can find another "trade-off" which lead to an overall significant better package.

    Thank you TJ Cameron for sharing your Godzilla research. Regarding the lift, I am not confortable making calculation because I just remenber an old workpaper addressing efficiency of hydroplane amas according to their lenght/beam ratio. The vertical (Y) axis of the graph was the lift coef (Cl) and the horizontal axis was the length/beam ratio. Then each curve was addressing an angle of incidence.

    I cannot find it anymore, just remenber that the global shape of the curves were quite similar to a function like y= 1/(x^n), downward sloping curve, going horizontally asymptotic with the increasing lenght/beam ratio along the x-axis.

    The maximum lenght/beam ratio investigated was about 8 and the asymptotic lower limit for lift coefficient was about Cl=0.02, for low angle of incidence consistent with an A-Cat hull moving in the water.

    Using conservative assumptions for the planning area which provides lift (the 2.5 meters hull lenght just before the centerboard and 20cm usefull wide, 15 knts boat speed); The basic lifting formula gives around 280 Newtons for lift force with Cl=0.02. Of course it is a very rought proxy, but it provides an idea.

    When sailing my cheap A-Cat windward in 10/12 knts true wind speed, I can testify, you feel the hull skimming and accelerating in the gust, a feeling you did not have with the older V shapes, so if no significant dynamic lift, at least there is something happening with flat bottom hulls?

    Thanks K2MAV for your remark, you mentionned the 16/18 speed range for a boat twice heavier than an A-Cat. So it is very consistent with what you can feel with an A-Cat in the 10/12 speed range.

    The curved foil for A-Cat brings another parameter, providing a significant "synthetic buoyancy" which increases with speed^2. When combined with the appropriate hull shape , we have again what I call a new trade-off approach.

    In conclusion, I would say that a thorought understanding of existing packages is a necessary but not sufficient condition to achieve the next leap-frog in increasing our small cat performance, that is the main reason of this topic. Another reason is I am not able to understand all that myself,

    Thanks for reading and even more for writing.
     
  14. k2mav
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    k2mav Junior Member


    But of course, all these comments on fine tuning are valid only, as you are rightly mentioning, covering current designs.

    I'm actually involved in the F18 Open Project, and the last of my worries is to have a breaktrough hull shape design as right now I'm not seeing any great chance of an overall performance gain.

    But this does not mean that I'm not going to take all developments into account, I'm taking them as reference, to have an overall bullet proof performer and learn from others worst/best features.

    As I remarked, the F18 class itself is the best exponent of you remarking how little gain can be achieved today.

    My question/interest or Erwan point it is how these shapes modify performance on targeted design goal. For ie the planing/sub planing features of modern hulls, being the extreme point the Cirrus R in the F18.

    The other feature of the Cirrus R is the marked bow rocker.... little gain in current scenario, but worth to ETNZ team design to give it a try, and there lies my interest at least.

    These little tuning features are responsible for what we have today, in the F18 you went from the Hobie Tiger to the Cirrus R.

    That's the point, analyzing this evo, and not gaining in a glimpse nowadays a revolutionary design feature.
    Martin-
     

  15. idkfa
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    idkfa Senior Member

    Let's for a moment accept that there is some lift created by these narrow hulls, what's the effect?

    let's say the hulls rise 2-3cm, what's the result? wetted drag decreases a few percent. Wave drag will be practically the same, ie little change in water-plane shape, plus some depth.

    Since wave drag dominates at higher speeds, there is little change in overall speed. Put it this way, you need foils to take you air-borne, before speed jumps.

    The speed increase downwind can be explained by the larger transom, it may look like planning, it may feel like planning, you may want it to be planning, but is it still only light-displacement.

    Finally, squared hulls have a narrower beam for the same disp versus rounded or V-ed ones, (though more wetted surface) hence they can have lower wave-drag, and thus a higher top-end speed, like accel in a gust when already at speed.
     
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