Definition of Planing

Discussion in 'Hydrodynamics and Aerodynamics' started by Leo Lazauskas, Nov 2, 2012.

  1. daiquiri
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    daiquiri Engineering and Design

    7 degrees is a pretty high angle of attack. What are the geometrical characteristics of the sponson you are considering?
  2. Yellowjacket
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    Yellowjacket Senior Member

    My simple point was that your reference to a 7 degree trim anlge is not correct. The best L/D or lowest drag/displacement occurs at between 3 and four degrees. You mentioned 7 degrees and said they didn't run that high a trim angle because it would result in too rough a ride. That is nonsense. There is no technical reason to run trim angles that high in any planing hull that can get to that trim angle. That is, the bucket of the planing hull trim/drag curve is between 3 and 4 degrees. If you can't get to that range, you need more planing area. If you go lower than that, you need a more aft cg, or less planing surface area.

    And most racing boats run at much lower trim angles than that. Racing runabouts and small hydroplanes run at trim angles much lower than three degrees because they need to get up on a plane, and that requires more surface area to get the boat on a plane with a high pitch prop. Once the boat reaches racing speed trim angles are very small, for small racing hydroplanes and runabouts, (ABPA stock and modified class boats), trim angles as low a half a degree are very common.

    Unlimited hydroplanes are supported almost entirely by aerodynamic forces and the trim angles of the sponsons are selected for stability and control, not drag, because it's just as important to go around the corners fast as it is to go fast in a straight line.
    1 person likes this.
  3. tunnels

    tunnels Previous Member

    time out !!

    This post has been going round and round and no one is any closer To an answer ! I even forgotten what the original question was Boring !!!!If you dont know when you your boat is plaining you are an idiot putting it mildly !and should never be allowed out of your play pen !!:mad:
  4. FMS
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    FMS Senior Member

    Click on the post number 1 <<First to refresh your memory.
    The thread is about a definition of planing. A definition should fit regular and irregular boats. For example, provided a video of a boat running, do you have a definition that you and everyone else can agree upon that makes the case for which second the boat is planing and which second it's not planing? Then what if the argument is over an irregular hull: a sea knife, trihull, IVB, overpowered canoe hull...

    When you refer to boat tests published by different sources, a specific written definition of planing insures that what each test reports as "time to plane" is a fair comparison.
  5. Joakim
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    Joakim Senior Member

    The best trim angle depends on many things and at higher speeds it is most often limited by porpoising. I just calculated using my Savitsky program the optimum trim angle for a 4 m long 250 kg 20 deg V hull racing boat at 50 knots. The optimum was about 6 degrees. At 4 degrees the total drag is about 10% and at 3 degrees about 30% higher than at 6 degrees. However 6 degrees is well above the predicted porpoising limit. The predicted porpoising limit is about 4.3 degrees.

    If the V hull in the example above is made deeper, the optimum trim angle (and the porpoising limit) increases. E.g. at 30 degrees the optimum is about 7 degrees and porpoising limit about 5.3 degrees. At 10 degrees the optimum is about 5 degrees and the porpoising limit about 2 degrees.
  6. Kestrel
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    Kestrel Junior Member

    happy to see "best trim" item...
    best trim angle is one of the most important parameters to take in account for, just since very first drawing lines in planing hull design. It strictly depends on deadrise angle and Aspect Ratio of effective planing area (AR=b^2/S), so from displacement, CG position, and speed. Higher the deadrise, higher the best trim - higher the AR, higher the efficiency, but lower the best trim. When AR becomes more then 1.1-1.3 (example: increasing further speed), best trim begins to grow again. It's always the same matter of "good mix" of many parameters, playing with planing surfaces.
    A very good paper which treats the matter is: "Stepless and Stepped Planing Hulls-Graphs for Performance Prediction and Design" - E. Clement - J.D. Pope - DTMB Rep.1490 - 1961
  7. HASYB
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    HASYB Senior Member

    I still think its a natural phenomena which occurs between certain parameters in which a fluid, a gas, speed, weight, form & surface play a dynamic and balanced roll.

    Some more interesting pre-historic study material:

    Attached Files:

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    BOATMIK Deeply flawed human being

    Planing is something we "feel" rather than something that is strictly definable.

    As a boat moves it develops lift forces which might be positive (up) or sometimes negative (down).

    weight of boat = lift forces + displacement

    As some boats go faster the lift forces increase and gradually 5% or 10% or 15% or at very high speeds close to 100% of the weight might be supported by the dynamic forces.

    But to say some specific point is "planing", "semi planing" or "forced" or "displacement" mode is meaningless because the lift phenomenon and the displacing phenomenon are present at every point.

    A proper physical description should also apply to ALL craft. To say that catamarans "displace" at high speeds and dinghies "plane" at high speeds is illogical.

    The same explanation has to fit both - this shows the label "planing" is about experience rather than something that can be defined or measured in a systematic way.

    Fast boats, multihulls and planing boats exceed hull speed significantly because they don't get entrapped by the wave train.

    There are two mechanisms for this - and both multihulls and monohulls use a mix of those two methods.

    All multihulls show some evidence of lift forces at speed with depressed wakes below static waterline at speed. So are they planing or not?

    As an argument - do we lose anything if the word planing is dropped from the technical vocabulary. Do we gain an understanding if we just forget about this word as explaining anything technical.

    Michael Storer.
  9. gonzo
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    gonzo Senior Member

    Planing is not something we "feel". In a build or design contract, the way you feel won't help you in court. This thread is an attempt to define and quantify the the speed bracket, dynamic vs static lift, hull attitude, etc. which would define a range withing which a hull is planing.
  10. tunnels

    tunnels Previous Member

    What a load of old smelly socks

    This thread has been raging on for months and no one to this point of 565 posts has given a clear indication of what defines planing and what dosent .
    So in a planing hull when the tumbling water leaves the transom in a steady stream you are planning !! thats all i want to know !! and yes you can feel it and the boat likes it as well and the motor sound changes if you take the time to listen !:)
  11. Yellowjacket
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    Yellowjacket Senior Member

    Sorry tunnels, water flow generally separates from the transom a long time before you are planing. The two are totally different phenomenon. The separation of flow off of the transom is similar to airflow separating off the back of a car once you are going fast enough. The reason the flow separates is because the momentum is too great for it to stay attached. Even displacement hulls will have this condition if they go fast enough, and that has nothing to do with planing. Planing is the result of lift generation of the hull, and there have been plenty of posts in this thread that have defined it pretty well, but some of the definitions are limited to specific hull types, and none of the definitions work for all hull types. That's not horrible, since it is not an easy item to specify.
  12. tomas
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    tomas Senior Member

    I disagree. As a newcomer, I now have both a better technical as well as a visual understanding after reading through the thread. Many answers were both intelligent and informative. Thanks to all the contributors.

    As always, it's interesting when human beings strive to accurately describe natural phenomena in detail.
  13. gonzo
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    gonzo Senior Member

    We may discuss this for years. The concept of planing or displacement is at least a century old, so what is the rush?
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    BOATMIK Deeply flawed human being

    That's exactly right ...

    People are trying to define speed brackets - which they choose to apply to some boats (with less rounded hulls) and not apply to others (more rounded hulls).

    There are some saying something like ... it is when a certain percentage of weight is carried by dynamic effects.

    The whole reason that people are looking at "possible percentages" and "possible brackets" is that it is indefinable.

    You can see the instant problem with percentages ... is it 10% or 15 percent or 35% or 50%. This is not physics. It is arbitrary.

    Physics should apply to ALL systems - if two boats of the same length are going the same speed faster than hull speed it is very likely the explanation is the same - no matter the number of the hulls.

    Around hull speed there is a trap - the reason multihulls and monos overcome it is for similar reasons - less immersion in the troughs of the wave train. Planing is another word for upward lift. Boats whether mono or multi are likely to have upward lift operating from speeds in the forced/displacement mode as well as beyond - whatever their hullform.

    Then for the people who want to "speed bracket".... there it is the same inconsistency ... it is an arbitrary decision being made. Do we apply at Froude numbers 5% above "theoretical hullspeed" - which every boat exceeds to a greater or lesser extent. Or 14%? And then the false notions of "semiplaning" and "full planing" - its arbitrary labelling .. nothing more

    The whole view is so inconsistent - which is exactly why some of the super brains at the beginning of the discussion gave up without any results.

    So as to a definition ...

    Planing is colloquial expression for the upward forces on a boat that are dynamically created. They are not present when the boat is static but start building progressively as the speed increases and increase throughout the whole speed range. It is present in all types of craft. There are no specific speeds when Planing takes place. At different speeds this phenomena is capable of reducing the apparent displacement of the boat in different proportions ... but always ...

    weight = dynamic lift + hull displacement.

    It is simple ... planing is a description of a force. It is not an accurate description of a type of movement. There will ALWAYS be a displacement factor at any speed - or the boat is just not touching the water at all.

    So ... if an efficient boat is displacing partially at all speeds through its range ... how can the word "planing" be applied to its behaviour?

    It is just illogical. But it works when used as a description of a force only.

    Michael Storer

  15. gonzo
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    gonzo Senior Member

    The word planing can be easily applied. The discussion we are having is what the definition of it is. There is no argument that a planing hull has a component of lift generated by displacement. The limit at which planing is occurring is what the discussion is about. All definitions are arbitrary. We are trying to find an arbitrary limit that applies to all cases. This we haven't been able to accomplish yet.
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