Acat design process for a novice

Discussion in 'Software' started by Catboy, Jan 5, 2011.

  1. Catboy
    Joined: Mar 2009
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    Location: usa

    Catboy Junior Member

    I'm interested in designing an Acat. I'm wondering if it is feasible for someone with a non-existent N.A. background (but with some software experience unrelated to the NA field) forging ahead with this project?

    I am an AutoCAD user and have a MaxsurfT license also. I have spent enough time with MaxsurfT to feel confidant that I can generate a shape that looks good. So, first question is; Is this a reasonable software package to design an Acat with? I am not interested in using the wrong tools.

    Secondly, I'm not interested in designing an Acat that isn't competitive, so my intent is to generate some pleasing shapes, lay them over other designs via point clouds, refining from there and hopefully getting someone involved with more experience in analyzing the hydrodynamics, etc.

    So is this just stupid or actually doable or what? Thoughts appreciated.
  2. ACuttle
    Joined: Aug 2008
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    ACuttle Marine Design Engineer

    I'm not that sure what you are looking to build, I haven't heard of an Acat before?

    There are two answers to these questions unfortunately:
    Yes - if you've got a basic CAD or software background then picking up something like Maxsurf is pretty easy and fairly quickly you'll be able to generate something that looks like a working hull form.


    No - using the software and producing something that looks like a boat is unfortunately the smallest part of the process. The experience and knowledge that is needed covers the aspects of stability, drag, design etc. and isn't so easily picked up. There are plenty of books on the subject but be prepared for a fair bit of reading and even then you will likely not have covered it all.

    I don't mean to be discouraging but NA's exist for a reason and as you say you'll need someone who knows what they're doing to cover the hydrodynamics. Having someone experienced involved is likely something you'll want from the word go rather than just down the line. I could easily draw and model up something that looks like a plane or a car but I'd never think of designing one.

    If you can have some more information on what you are looking for then perhaps I could give some more complete advice.
  3. alidesigner
    Joined: Nov 2006
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    alidesigner Senior Member

    Maxsurf T limits you to 3 surfaces so it will be a challenge (but still possible using multiple files) to design a Cat in it. You might need to upgrade to the next level which gives you 6 surfaces.
  4. Humberto
    Joined: Mar 2008
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    Humberto Junior Member

    I assume that you mean an A class catamaran (Acat). If you doesn't, this reply will be non useful.

    When designing a competitive Acat, the less of your problems is which software you use. You might use Rhinoceros for the surfaces and maxsurf for the very basic calculations. You may better use Catia (expensive) or walk to the free side and use freeship or my own jSDN.

    The problem arises with the different "navigation modes" of Acats. Two hulls floating (upwind and downwind), one hull flying constant (upwind), one hull fying up and down (downwind - the wild thing). You have to perform calculations for each navigation mode. But not only the standard calculations, but also: longitudinal GZ, detailed moments of inertia, and quite important, CFDs of the different navigation modes (assuming you are not designing appendages ...). Here you will need to use ANSYS (CFX or Fluent) or any other CFD.

    You might need additional tools, like an specialised VPP for Acats (be it programmed or Excel coded), and visualizations tools for "actually seeing shape differences between Acat modesl". Good luck for getting hull files.

    Finally, after such hard work, one thing will be missing. The dynamic performance of the design. This is of paramount importance in this type of boats. Model testing is very expensive, so what it is usually done is 1:1 model testing, taht is, you build the boat and try it.

    I hope this helps, and good luck.
    1 person likes this.

  5. Erwan
    Joined: Oct 2005
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    Location: France

    Erwan Senior Member

    Great CFD challenge for the new year


    In NA , just like in economy or sociology, it can help to have a little picture of the history, in this case, it would be with a designer perspective.

    Your project could have been mine, so I ll try to provide you with little informations and what was the underlying philosophy of the designs.

    Going back to the late 80's you had basically 2 designs available for european customers.
    The most widly available and cheap (in fact the low cost A-Cat) was the A-Cat from Bimare in Italy.
    The first design until 89 was a Tornado shape in reduction, with just a little change for the bow rake. The hulls were 36cm wide.

    these larger hull beam enabled you to fly a hull earlier, but these shapes are much less forgiving in choppy water, and conversly, with thin hull, you might be stuck in light winds. At least it was the case 15 years ago, but the different Flyer's design have demonstrated that the light wind handicap has disapeared, the best evidence is that bimare has not won any World tittle since 94 or 91.

    A few happy mates had access to Greg Goodall design/ Jim Boyer. MK2 3 4
    The hull shape were herited from plywood buildind techniques with thin hull 29/30 cm wide, sharp keel line in the front and minimum rocker.

    In 1996 at the A-Cat world in Spain, Glenn Asbhy who had just started the A-Cat a few weeks ago, coming from the Paper-Tiger cat serie has smoked the former Italian World champion Elgidio Babbi by 8 minutes in the first race, which was 2 kilometers difference in a 20 knts wind and big waves conditions.

    The thin hull shape designed by Greg Goodall were perfect compared to the fat hull philosophy, in these very tonic conditions.
    The biggest edge of the thin hulls was paradoxally there ability to be pushed further downwind in the big breeze and remaining very forgiving.
    Their main drawback of these hull lines was that you feel to have a Truck on the tiller.

    (But Glenn would have make it with any boat I think)

    these 2 very different design philosophies remained quite similar with marginal refinement until the Flyer 1 breakthrough in 99.

    In 89 bimare released its new shape, very modern marketing look, it was basically developable shape for the first 2/3 of the hull and not developable in the aft part, because of the hudge amount of rocker. Compared to Goodall design you had a hudge manoeuvrability. Hull were around 34cm wide.

    Later in 96, bimare modified this shape, removing a large stake of the aft rocker, manoeuvrability was enough, and 14 years later with the cheapest old glass polyester technology, this shape was still performing well windward in strong wind a few months ago, versus last bimare designs. Of course plate-form rigidity is like a used chewing-gum and close to 0 compared to modern boats, the trampoline is very low and drags in the water more than half of the time downwind but it works windward as lond as the sail is ok.

    Since 1999, with the new Nils Bunderbuck's design philosophy, it is more a global approach.
    The pierce-wave design aims to provide a smoother ride in the wave in order to minimize pitching and achieve a greater plate-form stability, so you minimize the changes in wind speed at the top of the rig so its efficiency rises, and the overall boat lift/drag ratio is better.
    In other words, the modern design philosophy can lead you to a more draggy design on the paper, but the theorical wave-drag give-up is more than balanced by the extra power you extract from the rig.

    So you will have to deal with wave drag, viscous drag, but also boat behavior, which is less quantifyable than a Froude number gap or a few square feet of wetted area.

    You should also consider some side-effects which are not always intentional or may be intentional I don't know but significant:
    The pierce-wave design philosophy prefers flattish hull shape section along the keel line instead of former sharp V.
    A positive side-effect is that when you sail the wild-thing in gusty irregular wind, you just need a millimeter pull on the tiller to bear away in the gust:
    In other words, you can place the boat in its best configuration at minimum cost (little drag associate to little rudder change): you can "extract" VMG downwind more efficiently.

    Behind hull shape philosophy, you can find other reasons, for instance, in the early 90's the German were the biggest export market for bimare, so the design of the hull was ajusted for a heavy sailor market, that is why bimare had always wider hull beam than Greg Goodall shapes, even if today only frenchies and Yankees sail bimare, in the meantime all German Swiss Dutsch have all swapped for Flyer/ Scheurer/ Tools/ or similar thin hull design philosophy.

    Your starting point should be to map the drag of the global boat windward(Hydro drag + Aero drag) in 4/5 m/s true wind speed range in order to have the meat hooked on the wire but not overpowered yet. You should find something around 1.2 for the "3D implicit" lift coefficient of the the rig, in order to fly the hull full trapeze at this windspeed.

    I did that years ago, but on another computer, I didn't use any software;
    For the wave drag, I "interpolated" from the SERIE 64 of the DTMB.
    I had 2,24 square meters for wetted area, and used friction coef from ICCCT ???
    Centerboard area was 0,15 sq mt with 2.8 geometric aspect ratio.
    Other assumption regarding sail/boat angle, sail twist, I am not sure but I think I remember to have used 17° for the sail twist and 22° for ??? may be sail CE-chord / Boat angle not sure.

    I remember the following results:
    Wind assumptions were: around 4.5 m/s true wind speed and boat speed a bit less and apparent wind speed windward around 7.5/ 8 m/s.

    The lateral aerodynamic force of the rig to balance the righting moment was around 675 Newtons.

    The global drag (Hydro + Aero) was around 170 Newtons with a bit more than 50% for the aero drag, and 60% of the aero drag was induced drag , I don't remember exactly, but it should be around.

    Centerboard and rudder drags are far from marginal, should be considered thouroughly.

    So much things to add about volume placement, especially with lifting curved foil, but it would require a handbook, and much more competent persons can do that.
    The above mentionned remarks are not much more than common knowledge or cat-parking culture.

    Hope it can help a little, and my best wishes for the new year and this smart project.


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