Design of 16' foot beach catamaran

Discussion in 'Multihulls' started by hashtag_laeuft, Aug 15, 2017.

  1. hashtag_laeuft
    Joined: Aug 2017
    Posts: 14
    Likes: 0, Points: 1
    Location: Hamburg

    hashtag_laeuft Junior Member

    Hello at all ... I'm new to this forum and highly interested in boat design.

    I have already built an catamaran out of playwood ... more precisely a Novacat 17.
    Now I want to design my own one and I hope that I can get some support from you guys here.

    My idea is to create a double chined hull that can be built in stitch-and-glue method. So I want to keep it simple but it should have a modern hull shape. My firs question is regarding the Cp. I read that one can use a negative rocker - like on the new AC'72 catamarans - to decrease the Cp and thereby also the resistance. Many new designs in the F16 and F18 class picked this up. Today I read something else - that fast semi-planing or planing hulls should have a higher Cp about 0.66 or 0.68 for a speed/length ratio of 1.5 to 1.6. So why do they use this negative rocker? Is it used to reach faster the required velocity to foil and because the hulls are not planing anymore?

    My current design has a Cp of 0.561. Can you suggest me some literature for me as reference? Maybe from an F16 catamaran? I saw something about an Open Source F16 project here in the forum - maybe someone can send me some hydrostatics of these hulls. Would be nice ;)

    I already have the following books: -> Principles of Yacht Design - Larsson, Eliasson, Orchy
    -> Aero-Hydrodynamics and the Performance of Sailing Yachts - Fabio Fossati


    That's it for the beginning ...
    I'm looking forward to get plenty of response and have some interesting conversations here.
    Sorry for my bad english ... i'll practise on it ;)

    Best greatings from Hamburg
    Nico
     
  2. Skyak
    Joined: Jul 2012
    Posts: 1,194
    Likes: 25, Points: 38, Legacy Rep: 152
    Location: United States

    Skyak Senior Member

    This is just an opinion, possibly lacking education, but I think you are wasting your time considering something so esoteric and advanced on a chine hull. Why do AC boats do it? It may be just to get a few more inches of foil span while fitting the rules to the fraction of a mm. It sure as hell isn't to optimize Cp for displacement mode it will never see if it is sailed right.

    Cp tweeks are for displacement mode. If you are designing a fast 16' chine cat, draw the boat with the windward hull just out of the water and design the lee hull to plane efficiently. That would look like the bottom surface of a surfboard at the appropriate angle -with positive rocker.

    I won't even start on how negative rocker would impair tacking...
     
  3. upchurchmr
    Joined: Feb 2011
    Posts: 2,783
    Likes: 78, Points: 48, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    Perhaps I'm behind on current design.
    Can you list the "planning" catamarans you know of?
     
  4. Skyak
    Joined: Jul 2012
    Posts: 1,194
    Likes: 25, Points: 38, Legacy Rep: 152
    Location: United States

    Skyak Senior Member

    No I can't but it would be about all of them 16' or less designed since the late 80s. The ones with transoms close to full width that are wet at rest, dry at speed.

    Maybe the term is wrong -I mean Fn 2.5 +

    The point is that to make a chine hull out of conic surfaces he is better off designing for dynamic lift to support a taller more powerful rig.

    The best example of a fast planning hull that I have seen is on Randy Smyth's "scizzor" (the 2017 amas)

    [​IMG]

    Looking back, I see the OP never said "fast", he said "modern" and talked about AC cats -I presumed speed was the objective.
     
  5. upchurchmr
    Joined: Feb 2011
    Posts: 2,783
    Likes: 78, Points: 48, Legacy Rep: 579
    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    I don't think there is a single "production" cat up to 20' that is an actual planning hull.
    Why do you think planing is desirable?
    When you are going slow (lite wind) you are going to want the transom out of the water. Planing doesn't exist.
    Going fast you want it in the water, because that is what gives you more bow reserve bouyancy - to prevent a capsize over the bow.
    You will get cleanly separated flow at the transom due to your speed. But again planing is not the point.
    Actually if you had a planing hull (at the back) it would be harder to sink the stern and avoid capsize over the bow.

    To get anything out of planing you would need the majority of the hull planing, but that would be slow, slow, slow in lite weather.

    Unless you want a world record beating top end sailboat you need performance at lite and heavy.

    So where should the planing surface be to "support a taller more powerful rig"?
    Preventing capsize over the bow or directly to leeward?
     
  6. Skyak
    Joined: Jul 2012
    Posts: 1,194
    Likes: 25, Points: 38, Legacy Rep: 152
    Location: United States

    Skyak Senior Member

    You seem to be stuck on the term planning -as if it specifies a hull design. The term is used to describe hulls optimized for speeds Fn 2.5+
    At that speed dynamic lift can be used to lower drag. A hull that is designed for displacement (Fn 0 to 1.3) will have far higher drag at higher speeds.

    I think most modern production cats are planning hulls.

    Lite wind performance is important for class race boats -but to be clear they are by no means designed or optimized for low speed. They only care that they are faster than the other boats, not that they are as fast as possible in the conditions. The OP is not designing for class racing (or he failed to mention it). Class racers don't have the option to increase rig height -the OP can and I recommend that he do so to address light winds.

    The challenge to designing fast small cats is supporting the leeward bow against the drive of a very tall rig. This pushes the hull design to be wider farther forward than you would want for a slender displacement hull. Width IS what you want for dynamic lift -combined with an appropriate attack angle. Dynamic lift is also the reason that this wide forward section is flat on the bottom (If a production hull was designed right for displacement the bottom would be round). This is what I see in modern non-foiling fast cats. Side comment -now with lifting foils being added to the the latest cats I see the hulls giving up their dynamic lift features -lift at speed is the foils job now.

    If you are designing a displacement hull trying to support a tall rig on a 16' cat no rocker or the negative rocker like the OP asked would put the most buoyancy forward per hull width. That is not what I would recommend and not what I see on any fast modern (non foiling) cats.
     
  7. patzefran
    Joined: Feb 2011
    Posts: 175
    Likes: 10, Points: 18, Legacy Rep: 10
    Location: france

    patzefran patzefran

    "You seem to be stuck on the term planning -as if it specifies a hull design. The term is used to describe hulls optimized for speeds Fn 2.5+
    At that speed dynamic lift can be used to lower drag. A hull that is designed for displacement (Fn 0 to 1.3) will have far higher drag at higher speeds."
    If I analyze your sentence you say you are free to name this design as planning or whatever you want. But the only physical fact is that athougth these hulls reach high froude number they never plane
    in real world !
    About the negative curvature in the aft run, they were originally created for semi displacement motor boat (cf David Tailor series 64) to counter act the tendency to get gow high at speed.
    When applied to the sailing catamarans, they were used first together with lifting C boards (no flying boats !) which gives the same tendency to lift the bow. Some dynamic lift aft help to control bow up pitching moment and reduce the draq. A fast racing cat catamaran without C board don't need negative aft curvature, only high length to beam ratio, high prismatic coefficient (very low longitudinal curvature), together with low displacement length ratio.
     
  8. patzefran
    Joined: Feb 2011
    Posts: 175
    Likes: 10, Points: 18, Legacy Rep: 10
    Location: france

    patzefran patzefran

    sorry for the bugs, I meant drag instead of drac and bow instead of goow !
     
  9. hashtag_laeuft
    Joined: Aug 2017
    Posts: 14
    Likes: 0, Points: 1
    Location: Hamburg

    hashtag_laeuft Junior Member

    Hey guys ... thanks's for your answers.

    my focus was not realy the "planing" - right - rather than to know more about why negativ rocker is used for plenty new designs.
    Please check the hull lines of for example the Duma F16, Cirrus Q, Viper F16, Raptor F16, Nacra .... etc. all these have a negativ rocker, but why?
    I could imagine that it's used to minimise wave risistance dotu pushing down the stern-wave licke the ductails on big ships. So please don't hang on that "planing" term - that was my fault.

    But the picture of the amas of that sizzor are locking quite interesting - thank you Skyak.
    Again - it is not my goal to design a "planing" hull but an as fast as possible and modern looking double chined hull. And no - this is no waste of time. I just want to learn more about cat design.

    greatings
    nico
     
  10. patzefran
    Joined: Feb 2011
    Posts: 175
    Likes: 10, Points: 18, Legacy Rep: 10
    Location: france

    patzefran patzefran

    You should look at the 14 ft Paper Tiger designed in the sixties in New Zeland (3500 built !). Chined hulls , not "Modern" but very nice and effective lines, weight less than 70 kg.
    Very active class in New Zeland and Australia.
     
  11. Manfred.pech
    Joined: Apr 2010
    Posts: 567
    Likes: 70, Points: 28, Legacy Rep: 319
    Location: EU

    Manfred.pech Senior Member

    Hi h_l, negative rocker - or hollow (concave) lines - are a rare thing in hull design and most people do not understand them saying hollow lines will cause suction because they do not understand the laws of Bernoulli - but they do exist:

    Athol Burns [​IMG]

    Bernd Kohler of Icarus
    [​IMG]


    0r new design [​IMG]

    Convex lines or positive rocker produce suction and hollow lines will give you lift.
    [​IMG]

    But ---- it is not so easy to draw the lines for a catamaran hull with negative rocker. It depends all on the way you combine areas of lift and suction. Therefore you need a lot of tank testing and a navel architect/designer with experience.

    If you will read about it: German "YACHT", 1, 1988, S. 30 ff.
     
    Last edited: Aug 16, 2017
  12. patzefran
    Joined: Feb 2011
    Posts: 175
    Likes: 10, Points: 18, Legacy Rep: 10
    Location: france

    patzefran patzefran

    Manfred,
    On your cat assuming a single 80 kg crew and 200 kg total displacement, you need two hulls suporting half load each to get planning at about 15 kt, right ?
    I assume it is the reason you need 4 m beam : If you fly a hull = no planning (It could explain also why A cats need 4 foils to fly with 40 cm foils beam !) .
    If you assume a planing angle around alfa =4°, you get optimal total drag to weight ratio D/W= tan(alfa)=0.07, and you must add the friction drag,
    The total drag is same order as a high L/B displacement catamaran flying a hull at same velocity. The draw back is the performance loss at lower velocity.
    I assume it is the reason this kind of design did not reached expectations, like for example Yves Parlier Hydraplaneur.
     
  13. Manfred.pech
    Joined: Apr 2010
    Posts: 567
    Likes: 70, Points: 28, Legacy Rep: 319
    Location: EU

    Manfred.pech Senior Member

    Patzefran, as I understood "hashtag-laeuft" from Hamburg - he is not not interested in planing:
    "my focus was not realy the "planing" - right - rather than to know more about why negativ rocker is used for plenty new designs.
    Please check the hull lines of for example the Duma F16, Cirrus Q, Viper F16, Raptor F16, Nacra .... etc. all these have a negativ rocker, but why?
    I could imagine that it's used to minimise wave risistance dotu pushing down the stern-wave licke the ductails on big ships. So please don't hang on that "planing" term - that was my fault."

    In my opinion negative rocker can optimize the flow around the hull within certain circumstances - may be - fn 0.5 - 1.2. But to find the right position for the negative rocker and the depth it needs a lot of tests.
     
  14. patzefran
    Joined: Feb 2011
    Posts: 175
    Likes: 10, Points: 18, Legacy Rep: 10
    Location: france

    patzefran patzefran

    Manfred,
    I agree about aft negative rocker. I think most design were inspired by Serie 64 David Taylor bassin shapes, without any tests !
    I my opinion Roucayrolles /Akena comes with this solution for his outriggers to limit bow up from C boards , I don't know if he ran any tests .
    Have you performance returns from your P5000 E design ?
     

  15. hashtag_laeuft
    Joined: Aug 2017
    Posts: 14
    Likes: 0, Points: 1
    Location: Hamburg

    hashtag_laeuft Junior Member

    Hi guys,
    thanks for your contribution, Manfred !! As you already did - I repeat for all that I'm not interested in planing - this was a fault by myself. So please don't use this as an discussion thread if a catamaran is able to plane or not. Even if it was quite interesting till now.

    I'm trying to design a 16' catamaran which looks like one of the F16 class and getting some experience in boatdesign. There I was interested in how rocker (negative rocker) in correlation with Cp is influencing the performance of a single hull in it's upright position. Manfred mentioned already Bernoulli's Law (which I'm familiar with) and the characteristics of convex and hollow lines.

    I am a kind of naval architect but with no experience in boatdesign. I just finished my studies this year on ship technology and marine engineering. I did a lot of CFD calculations also resistance predictions of big and fast ships.

    As patzefran mentioned - I think in many cases negative rocker is used because it's trendy nowadays. But to perform some numerical test would a doublebody configuration be okay to just have a look at the pressure distribution around the hull to check which design is better or would you suggest to use free surface simulation including dynamic trim and sinkage? When I know about the pressure around the hull - how can I continue to optimise my design to find the right position and depth of the rocker. Should I just test some different variations to see which is best or can I get some keys from the results to know about in which direction I have to go?

    As always - many thanks for your answers and please stay factual and forget about that planing stuff ;)

    Best greetings
    Nico
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.