Cats Planing

Discussion in 'Multihulls' started by Coen, Aug 28, 2004.


Does a cat always plane?

  1. Yes it does

    0 vote(s)
  2. No, your laminar/turbulent is correct

  3. No, other

  1. Coen
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    Coen Junior Member

    Okay, I read several threats on your forum 'bout this, but I want to hear your opinions. This because I last had a little confrontation with one of my professors (Dutch Peter >> Mr. V/d Boogert, you might know him).

    He said: any boat has a maximum hull speed. You know the formula; something like 2.43 x sqrt(Lwl). When a boat gets over its max hull speed, it starts planing.

    My reply was that a normal catamaran doesn't plane. This because it doesn't have enough surface to plane on, certainly when on just one hull (most of the times). But a catamaran easily goes over its maximum hull speed.

    I think it has something to do with the hull going laminar or turbulent through the water. I have a vision that a sailing yacht goes laminar through the water untill its max. hull speed, and then goes over its own created bow wave. A catamaran is so designed that at low speeds the hull will not go laminar ways around the hull, but turbulent and therefore, with brute power (large sail area) can reach much higher speeds.

    What do you think??
  2. yipster
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    yipster designer

    most boats will plane going downhill on a bigger wave... even normal beach cats but what is a normal cat... i know the cats slender beam is getting the higher hullspeed and thats oposite to your theory that laminar flow gives less drag than turbulent i belive... but i'm not a prof :)
    i wait with my vote...
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    FAST FRED Senior Member

    The argument has gone on for a long time , as the orginal formula was for boats that has a L/B ratio of 3-1 or 4-1 .

    When you getb over about 6-1 the historical formula doesnt work.

    They knew this before WW1 , which is why you find do many 10-1 or more boats from that NO POWER era.

    The AYRS has been working (mostly on sail ) for 4 decades on speed , and has a hull resistance calc site ,

    This is for Cat hulls , not conventional Lead Sleds, and is very user friendly.

  4. tspeer
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    tspeer Senior Member

    Planing has nothing to do with whether the boundary layer is laminar or turbulent.

    Planing is not directly related to the length/beam ratio of the hull.

    Planing means the boat is supported, in part, by dynamic lift on the hull instead of being 100% supported by buoyancy.

    If you drive a fat hull at high speed without any dynamic lift, the wave drag is tremendous. This is what the "hull speed" formula is all about. It doesn't mean there's an absolute limit for that hull. But the power required to go faster is more than is typically available from the powerplants installed in these types of boats. If you want to see what it's like when you do have lots of power to drive a displacement-type hull, watch a tugboat race.

    If the boat can start planing, then the drag from the dynamic lift - which actually decreases with speed - can be less than the wave drag of the displacement hull. So it's commonly assumed that if a boat is traveling faster than its hull speed, it must be because the boat is planing. Otherwise it wouldn't have the power to go that fast.

    But if you have a displacement hull that does not exhibit a dramatic drag rise, then you can drive it fast without having to resort to dynamic lift. So just because the boat is going at high speed doesn't mean that it is planing. The way you minimize the drag rise is by making the hull slender.

    The drag from dynamic lift is inversely proportional to the square of the width of the planing surface and inversely proportional to the square of the speed. So a wide hull producing the same amount of dynamic lift will have less drag from that source than will a slender hull carrying the same amount of lift. This is one reason planing catamarans have not been all that successful as sailboats. Even if they do plane, the drag due to lift may not result in a net reduction in drag unless they are going very fast. If they produce dynamic lift at too low a speed, the drag can be greater than if they'd supported all their weight with buoyancy instead. It's also why boats that spend their whole lives planing tend to be comparatively short and wide.

    I see this in my trimaran whenever the wind comes up. The main hull is somewhat flattened and will plane. The amas have very narrow V sections in the stern and definitely do not plane. Since we sail offwind with the apparent wind at 90 degrees, the leeward hull carries a significant portion of the weight of the boat, if not the majority of the weight. There's a definite increase in speed when the displacement of the main hull drops and it starts to plane. But both the narrow lee hull operating in displacement mode and the planing main hull are definitely moving at the same speed. Or else it would quicky cease to be a trimaran!

    Laminar vs turbulent flow has to do with what's happening very close to the hull surface. This boundary layer region is where skin friction originates. The skin friction is proportional to the wetted area and proportional to speed squared. I don't think you'll find much laminar flow on a full-sized boat moving at anything like planing speeds. Small disturbances in the boundary layer get amplified and start forming tiny eddies near the surface (turbulent flow) before the water flows very far from the bow.

    A planing boat reduces this source of drag simply because the dynamic lit raises the boat out of the water and reduces the amount of wetted area. The flow is still turbulent over the vast majority of the wetted surface.
  5. redcoopers
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    redcoopers Member

    Tom is exactly correct. One thing to watch out for is terminology between sailors and powerboaters.

    When any boat starts moving, the center of gravity will begin to sink. As it sinks, dynamic lift will begin and the rate of sinkage will decrease. The rate of sinkage will become zero, and afterwhich, the boat will lift out of its lowest position. Lift will continue to increase, and at a certain speed, the CG will rise above that of the standing position. Technically, this is the planing speed - when the CG moves above where it would be when the boat is at a standstill.

    Planing speed on a powerboat (i.e., non-displacement hull) is around Fn = 0.7.

    We sailboaters say that we start to "plane" when the boat no longer sinks into the waves, but begins to move up (around Fn = 0.4, i.e., hull speed). It's not really good terminology, but, whatever...

    Also, there is no dependence on Reynolds number - only Froude number. You can plane in laminar flow, the transition zone, or while fully turbulent.

  6. Doug Lord

    Doug Lord Guest

    Planing Cats and so forth

    Yves Parlier's boat has been written about extensively in Seahorse and unfortunately,so far, has not lived up to expectations from a racing standpoint.
    It uses stepped planing hulls with rudder t-foils(necessary because of the pitch instability of the stepped hulls.)
    and theoretically over 20k hull speed has as little as 1/5th the drag of a displacement hulled cat or tri.
    There is a small beach cat being produced with what are claimed to be planing hulls featuring
    a sort of step but I can't remember the name...
  7. Ilan Voyager
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    Ilan Voyager Senior Member

    Mr Speer your explanation is cristal clear and magistral.

    The K factor is the famous 1.4 of the formula of speed lenght hull which seems to be the "here you won't go more fast" of a lot of people. When Froude invented it as a simple help, the K factor was 1.2, and on heavy 17th century battleships it's 1 to 1.1...

    Mr Gerr made a formula based on the displacement, pretty precise with boat with a ratio L/Width of 3.5 to 5 or 6. From my former job (I'm retired now) I have empiric formulae (not public alas) made for warships (ratio until 10) using the length, width, displacement, depth and form coefficient.

    I've taken the freedom to take this table from the article "Comparing cruising multihull design features" by Charles E. Kanter Copyright © 1998, 1999 Southwinds Media. It's only for educative purposes so I do not infringe the copyright.

    Bill Roberts does not need to be presented as you know all he is the architect of very fast sailing multis.

    Bill Roberts Presentation -- The "K" Factor
    LOA / BMAX / Disp Ratio/FinenessSpeed/ MAX speed / D/L3 / K Factor
    27 16" 650 20 27 kts 30 5.2
    27 23" 1300 14.3 20 59 3.8
    27 28" 1950 11.7 17 89 3.3
    27 32" 2600 10.2 15 119 2.8
    27 72" 7500 4.0 7.3 - 1.4

    I follow with great interest the boat of Yves Parlier: only the results will confirm or infirm the validity of the theory of a planing cat. However I'm afraid that the cat is heavily handicapped in light wind and upwind. The true good race is for Mediatis is donwwind with a strong breeze. It's the same problem with hydrofoils as the Hydroptere (a part cavitation problems above 40 knots).

    The problem with these boats is they are too "specialised", needing special conditions for taking advantage of their technical features. That kills the possibility of a all around speed potential.

    The 60 footer tris are very, very fast: able to make 9.1 miles at 32.15 knots of average speed as did Gitana XI a few days ago.

    I join a pic of Gitana X flying on a hull

    About the "Fred" hull resistance calculator: the results have to be taken with high prudence; the calcs are made for a hull of a very particular shape (not a lot of boats have hulls of such shape) but it's friendly and very educative. It shows clearly the influence of displacement and width. The apparition of the hump of planing boats is clearly representated. It's worth to play with it even if the results are aberrant and do not find the results for slims hulls as accurate as I've found with Michlet of Mr Lazauskas.

    Michlet is painful to use but the results are pretty accurate. The quality of the works of Mr Lazauskas is widely known.

    Attached Files:

  8. Ilan Voyager
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    Ilan Voyager Senior Member

    I forgot the objet of the thread:

    Big Multihulls with "elliptic" hulls, as design Gilles Ollier and others, do not plane. The architects prefer to get better caracteristics of deadening the lateral and vertical mouvements. The rigs are very powerful and put a lot of pressure on the hulls.

    The small cats with flat sterns and U sections do plane but it do not signifies that they are faster in regatta because of the (small) hump before getting the plane and the difficulty to stay planing. The penalty in drag is high as I have experimented that on my own small cats.

    Fast windsurf have the same problem: planing works only with a lot of wind.
  9. Phosphor
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    Phosphor Junior Member

    A cat can't plane unless you get a good hull shape, such as a flatter transom. Those things have a small enough prismatic coefficient so that they don't have to plane. They don't have the wave making resistance nearly as much as mono-hulls do, and get past that speed hump well enough so they don't plane (I'm talking about small beach things like the Trac 16 or a small Hobie cat). I don't know enough to comment on large cats. I don't see why not if they have the right shape and power...
  10. Coen
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    Coen Junior Member

    Hey guys.

    Haven't been here for a week. During weekdays I live in Hilversum, a place near Amsterdam in Holland. In Hilversum I have no internet and therefor can not check your replies. At this moment I am little short of time so i will have to look at all your replies next weekend. See ya then!

  11. Dutch Peter
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    Dutch Peter Senior Member

    Don't know him. He didn't teach at Rotterdam before 1999!
  12. gggGuest
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    gggGuest ...

    A simple thought (or practical if you like) experiement deals with the " if it exceeds 1.7rootL or whatever it must be planing" argument.

    Take a flat sheet of metal. Drag it vertically, edge first through the water. Is it exceeding "hull speed"? Yes. Is it planing? No.

    In more detail Tom is definitive.
  13. marshmat
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    marshmat Senior Member

    A lot more variables involved than just # of hulls, which is all we get from "does a cat plane?". The shape of those hulls matters; so does their size, the speed, the weight, the applied forces. Any hull will plane if you put enough power behind it- strap the first stage of a Saturn V rocket to a tugboat, and it'll plane. Planing efficiently tends to require relatively flat bottom sections, light weight, high speed and a clean breakaway at the transom. Kind of hard to generalize any more than that- some wavepiercing hulls can run speed/length ratios of 2.0 in displacement mode, while some ski/wakeboard boats will be planing at 1.5.
  14. GregOrca
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    GregOrca New Member

    Video of Planing sailing cat from 1980

    People interested in seeing if planing catamarans work might like to watch this footage from 1980 of a production planing cat.
    you will need the latest version of quicktime to view it and it's about 40mb in size. Well worth it though.
    Unfortunately the version sold in the USA was quite different, with dagger boards almost half the depth and a broad wide barge-like transom.

    This footage shows the original Australian version performing nicely despite a terribly cut sail.

  15. Retired Geek
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    Retired Geek Junior Member

    Over time Ive found that the most accurate means of calculating the onset of planning is the relationship between instanteous displacement or volume (speed in fps * frontal area)/lift of the hull. In general I've found that the transom is clean at about 2 and real drag reduction starts at around 1. Most of this calc depends on your ability to derive a value for the lift ... wide flat surfaces tend to have dCL/dA's starting around 0.06 and long narrow shapes get as low as 0.0235 ..there are a ton of papers out there that describe how to calc this.
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