Planing Trimarans

Discussion in 'Multihulls' started by Doug Lord, Sep 30, 2006.

  1. Chris Ostlind

    Chris Ostlind Previous Member

    Trestles report:

    Loooong, 4-6' faces with tasty peeling crests provided amazing nose rides all day long. Afternoon breezes demolished some of the shape in the classic sense, but the water was still reasonably warm and my old dude, long boarders buzz was raging. Air temps in the high 70's and pretty much ideal conditions with all the young guys gone off to school on Monday.

    Pardon the off-topic shenanigans, gents
  2. Doug Lord

    Doug Lord Guest

    1) "Planing ratio's don't address planing beam"
    Absoutely wrong: the Philips -Birt formula DOES address beam in the factor waterplane area times a given waterline length.
    2) Taipan 4.9- In either one person or two person configurations the Taipan ranges from 2.05 to 2.09 clearly above the figure required for planing in the Philips-Birt formula.
    3) Crowther Twiggy- Crowther was known for narrow main hulls but I can't find any info on the beam of the Twiggy main hull-I doubt the area you gave because that means the mainhull was 10/1-considerably wider than I've been led to believe one would find on a Crowther.
    CT249 updated the beam of the mainhull to 2'on a 28' waterline; that produces a result according to of 3.13--way outside the planing range.
    ---4) Marstrom M20- According to the Marstrom site the boat "semi-planes"-that is not planing. According to the Philips-Birt formula it would not plane.( 2.05)
    ---5) In looking at the Formula board I come up with 1.5 for the Formula Board -higher than that for the speed board. As happens with many "rules of thumb" if you push them way out to the edges they may not be accurate -as in this case.

    Clearly, in the cases you presented, where verifiable information is available, the Philips -Birt formula predicts the actual facts of the boats performance reasonably closely for a rule of thumb.( Except in the cases of the boards) As it did in combination with the other ratio's used in describing both the F24 and F22 where both were shown to have the Sail Area and righting moment required to plane.Not only that but the ratio's used along with Bethwaites formula and Philips-Birt's formula
    agree 100% with the observed facts on the F24.
    SA/D,D/l, beam to length ratio and others are used on multihulls and monohulls. On the Farriers, we're using the Bethwaite formula to look at the characteristics of the power applied to the main hull at a particular lb.per sq.ft. sailing pressure and weight. Philips-Birt looks at the area the main hull has vs. it's weight to come up with a rule of thumb about planing-it says nothing about power but the other ratio's do.
    Last edited by a moderator: Oct 25, 2006
  3. CT 249
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    CT 249 Senior Member

    Sorry, I admit I have a problem reading some of your posts clearly. It's the dogmatism that gets me. However, I shouldn't have said that the PB formula ignores waterline beam as it clearly doesn't.

    Nor, perhaps, should you simply ignore Tennant's reference to the fact that cats of similar dimensions can be either planing or non-planing, or the fact that Fox etc said that Avenger was a planing boat when her contemporaries of identical dimensions were not. Savitsky also breaks planing, semi-dosplacement and displacement hulls into three categories according to hull shape. Savitsky, we can assume, would know. Yet these factors that he speaks of as dividing displacement from planing hulls are not covered in your numbers.

    Surely you must agree that hull shape plays a part in whether a boat planes or not. Therefore, the numbers presented simply don't tell all the story. Isn't something like the Marstrom evidence that planing is not a simple yes/no question and that people can reasonably hold different views? In the same way, some of the top Mothies think one way when it comes to planing, some top Mothies think the other. Andy Dovell has plenty of info on Moths - most of the recent world champions were designed on his computers - yet he doesn't pretend to know whether they plane. If simple numbers proved the case, there wouldn't be a debate with reasonable people on each side - but they do not, and there IS such a debate.

    Can I ask what definition of "planing" you are following in this thread? The one Savitsky says hydrodynamacists use? The other one he speaks of? The one guys like Andy Dovell follow? The transom running dry definition? The "normal" sailor's definition (bow up, transom dry)? The displacement entirely supported by dynamic lift? The displaement mainly supported by dynamic lift?

    Re formulas. I may have my numbers wrong here. Presented for inspection and comment.

    Taipan; 16' LOA, 2 hulls each about 1' max beam waterline, PC of say .6 = 19.2sq ft waterplane. 19.2 x 16 (length) = 307

    95kg rigged+75kg sailor = 374lb. 374 divided by 307= 1.2. Well into the PB formula. Yet even when both hulls are in the water (running in mild thing mode) the T 4.9 doesn't plane.

    Re Twiggy beam; I think I was thinking of 1ft ama beam, 2 ft main hull beam waterline.

    M20 - the site of a project headed by a renowned multi builder/designer and two hydrodynamacists says the boat does not plane. The Marstrom site says it's a semi planer. This is merely another example of a case that shows that the question is not as simple as some think.

    Interestingly, a Formula board doesn't plane according to D PB (40lb board, 180lb sailor; 7' LOA; 3' wide; PC say .8. 7x3=21x.8=17szqft waterplane. 17 x 7= 119.
    220/119= 1.8. Yet Rohan says FW boards are quicker than a foiler Moth downwind in a breeze.

    A speed board doesn't plane according to D PB - 220lb, 7 ft x 1ft, PC of say .6 = waterplane of 4.2. 220 / (4.2 x 7=29.4)= 7.4. So Erik Beale's board isn't a planing hull. The inclined rig can be a factor at times, but you can also get a speed board to plane (not as quickly) holding the rig upright, so the inclined rig isn't the entire excuse.

    Maybe my numbers are wrong; I'm cruddy at numbers. However, when the world's fastest bit of kit (anyone want to do YPE's numbers?) can't plane according to the formula, it may not be conclusive proof.

    Simply, and finally, people like Savitsky and Tennant and Fox seem to say that hull shape is a vital ingredient in whether a boat planes or not - ergo just getting the numbers "right" isn't proof a boat planes. Therefore the people who don't think a Farrier planes may have a right to their way of looking at it.

  4. Willallison
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    Willallison Senior Member

    In many cases, the inflatable tubes run clear of the water when on the plane. In 2 of ours (one an Avon RIB, the other an older inflatable with 'soft' bottom) the tubes remain partially submerged.
    In my opinion - and it is only that - the tubes are definitely not operating in the planing mode.
    Again - in my opinion - the boat is.....
  5. ActionPotential
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    ActionPotential Junior Member

    If a video of an F-9R center hull with the bow right out of the water and a power boat like wake/rooster tail out the back is not enough for skeptics to believe it is (skimming across the top of the water) then even a video from the side would not convince either.
    Apparently, that this (skimming across the top of the water) may be caused by the wind, which not unnaturally causes a sailboat boat to heel, which causes the main hull of my boats to lift and (skim across the top of the water), is somehow cheating, or 'marketing hype'.
    The 'marketing hype' is in my opinion, calling all skimming across the top of the water 'planing'.
    Surely everybody know these tri's are really fast and often get their centre hulls skimming across the top of the water and are going faster than the monohull sports boats that are planing in the same conditions.
    However look at some of the other trimaran designs which have their accomodation above the waterline and go faster then these in the 'planing conditions' but can't match them in lighter conditions because of their greater wetted surface when the main hull is sailing at its DWL.
  6. Doug Lord

    Doug Lord Guest

    planing tri

    CT 249 , the method I used on the Taipan and Marstrom was to estimate the beam to length ratio, multiply that by 16(Taipan; Marstrom 20) then estimate the area of one hull's waterplane by multiplying the result by .65. I tried two beam to length ratio's on the Taipan and two weights and the range of results was from 2.05 to 3.57.
    It's my opinion that you have to analyze just one hull to determine it's "planeability " according to Philips-Birt-otherwise you skew the results inaccurately.
    On the Marstrom ,according to one site the "numbers are similar to a Tornado" so I assumed a 20/1 beam to length ratio which gives a 1' wide hull 20 feet long with a rectangular area of 20 sq.ft.. I multiplied that by a factor of .68 to give a waterplane area of 13.6sq. ft. Following Phillips-Birt I multiplied 13.6 X 20 =272. All up boat weight is an amazingly light 240 lb + 320 crew for an all up of 560. 560 divided by 272=2.05 .Outside of the planing range....
    CT, I introduced the Philips-Birt formula because it mathematically "creates" a "planing shape" with the numbers -if one is there. It won't be accurate for every conceivable shape , I don't imagine since it is a rule of thumb. But it has been accurate for the shapes you brought up as well as for the F24 and F22. In the very first post it is mentioned that the F22 has a "planing shape" I believe. At least Ian has said that. And Philips Birt confirms the "planeability" of those two main hulls.
  7. yipster
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    yipster designer

    normally not even semi planning fast cat, or would you make an argument its not a tri
  8. PI Design
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    PI Design Senior Member

    Some points, so that we can calm down, reflect, take stock and hopefully reach an amicable conclusion. Yeah right...
    The thread is becoming quite difficult to read, so i aplogise if I repeat earlier statements or wrongly accuse people.

    1. There are many incorrect 'definitions' of planing. Whilst I am not sure, off the top of my head, what the precise definiton is, in principle planing is defined by the creation of a stagnation point and spray with a forward velocity component. Any other definition is layman's bunkem. Therefore, additional support from ama's is not part of the criteria that excludes or includes planing.
    2. I agree that there are many claims made in adverts that do not meet reality. Beyond that, I think Chris Ostlind exaggerates most the potential problems and issues.
    3.In general, I believe a trimaran CAN plane. Whether the F-22,or whatever, actually does, I do not know and I have not seen hard and fast evidence one way or another presented here. Doug has presented some rules of thumb that suggest they might, but clearly this is not definitive proof, but then I wouldn't expect that.
    4. Demanding professional video footage for evidence is childish and ridiculous. Why should anyone go to this expense to justify such a claim? If you don't believe that the F-22 planes, you hire the camera man etc. Even then, is planing something that can visually proven? Do you believe everything you see on TV? Do you only believe things you see on TV?
    5. In response to an earlier post, the Savitsky method for planing calcs does include the beam of the craft, both in a beamwise Froude number and in a length/beam ratio. It also includes the deadrise angle, so hull form is accounted for in the method, if somewhat crudely.
  9. PI Design
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    PI Design Senior Member

    One more thing to note: most of the pioneering research into planing hulls was conducted in the 1930's by the Germans, trying to develop sea planes. Clearly these has two hulls and wings! - are you saying that that these flying boats do not plane before 'take off'? If so, most experimental planing data (used to derive Savitsky's empirical, and widely used, formulae) was obtained from craft that didn't actually plane! Now that would present a problem!
  10. Chris Ostlind

    Chris Ostlind Previous Member

    Interesting take, PI, though I'd never agree with you on the assessment. Perhaps a rather lengthy list of applications for video analysis (and film before that) regarding technical studies would be suitable for you? In fact, you can pretty much name the industry of your choice and I will be willing to bet that video has been used to analyze the product, especially through non-destructive testing regimens. As a result of this overwhelming evidence to the contrary, I can only assume that your comment is coming from a position of being totally ignorant of the realities of the application. Video analysis and broadcast TV are two entirely different disciplines.

    So, my questions to you would be... Do you always believe everything you see with your eyes? Are you sure that what you saw was actually what was going on?

    There are whole websites out there, dedicated to the perception of vision phenomenon where you can learn about how much you missed with your simple, one-time viewing pursuit. Do take the time to visit them.

    The above is not a trick set of questions. I'm suggesting that you face reality about human perception and our ability to filter-out our pre-conceived ideas as to what may or may not be happening. Perhaps you feel the same way about tank testing or wind tunnel analysis for boats and rigging? Both of those can rely heavily on video analysis so that technical observations can be reviewed later for accuracy and nuance.

    Most tapes of this sort are viewed un-edited by the technicians so that a full sense of what was going on can be put into perspective without the "enhancements" that can be added through the editing process. Later, after all analysis is complete, an edited version can be produced that hits the high points and carefully documents each shot with inserted text to give reference to the specific data as being shown.

    Just exactly what you may have problems with in regards to that pursuit, is a mystery to me. Is more, documented information a serious stumbling block for you? Are you the type that wants less information from which to make a decision?

    And, no, I'm not going to fund the video claim study for this marketing scheme. The proof is incumbent upon the designer/marketer.

    A couple of years ago, we had a rather large Space Shuttle come apart during re-entry with a serious loss of life. Without the video systems in place to track the vehicle, analyze its behavior/condition and allow repeat viewings as cause and effect hypotheses were being put forward, we would have only been able to guess as to the lessons that needed to be learned.

    Or, are you suggesting that you caught the falling debris the first time around as it damaged the leading edge of the wing surface and that you could have told all the engineers about the problem without the aid of the video tape for slow-mo analysis after the fact? Now, that is what I would call a childish statement.

    In case you missed it in your rush to judgement... the argument is not about the fat main hull and its singular ability to plane. The argument is about just what constitutes a planing trimaran?

    Myself, along with others in the thread, are of the opinion that if the main hull uses the ama to support a significant portion of the overall boat weight in order to allow the main hull to plane, the boat, as a complete unit, is not planing.

    Others feel that the ama can be doing whatever it wants to do by design and the boat can still be called a planing trimaran. They want to extend the concept of planing for an entire vehicle to allow for a multi-tasking, planing/displacement hybrid definition. They have spent a lot of time arguing that the hull, itself, can actually plane and have essentially ignored the fact that the vessel is in a serious, definition limbo due to the same, multi-tasking realities.

    As an aside: the realities of the hull of the F22 being able to plane (either with, or without, ama assistance) are purely theoretical at the moment as none of the boats are anywhere near completion at present. The unfinished concept awaits sailing trials and then, I'm sure, the argument will enter an entirely new set of arguments based on what it can or can not do.

    Since Ian admits that he's never claimed the main hull would plane on its own without the ama support, I feel the claim is specious and amounts to a marketing position and nothing more.

    If you want to argue that point, then set sail, PI. We'd all love to hear your well formulated arguments. If they are anything like that one about "what you see on TV", then I'm sure we are in for a really interesting ride.

    Just a note in closing. I don't need to hire a qualified cameraman... I am one by profession. Going on 37 years now. And I don't want or need the job. Ian's an engineer, he should already know the power of proper video analysis in this application. If he doesn't, then there's very little I can do about convincing him. He'll have to come to that reality all by himself.
  11. yipster
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    yipster designer

    "empirical" is the word i've been thinking of mentioning here for weeks and agree the whole argument must be seen a bit wider, well said and i'm out of here :p
  12. Doug Lord

    Doug Lord Guest

    Planing / Eric Sponberg

    Because I find this one of the best descriptions of planing written by anybody anywhere I'm posting it here. Originally from the "Calculating Planing Velocity" thread under "Boat Design" on this forum:
  13. CT 249
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    CT 249 Senior Member

    It's an interesting piece, but from this layman's point of view, it's still interesting to see that other sources say that hull shape IS a vital factor in whether a boat can plane. One, already mentioned, is Tennant, who is a good designer and who specifically compares two boats of similar dimensions, where one is a displacement boat and the other a planing hull.

    Another would be Daniel Savitsky. In his Athens address ("On the subject of high speed monohulls") he seems to spend a lot of time talking about the influence of hull shape.

    In speaking of the displacement hull form, he talks about the way the displacement hull form's "hull resistance begins to increase dramatically
    and thereby becomes a practical barrier to further increases in speed." He speaks of the way the "upper limit of application of a displacement hull form is at a SLR which is approximately equal to 1.3."

    "In order to operate satisfactorily at higher SLR (Savitsky says) the displacement ship hull form must be replaced by a configuration that is compatible with the hydrodynamic phenomenon associated with higher speeds."

    He goes on to describe the shape - not the displacement or length, but the shape - of the planing, semi-planing in contrast to the displacement hull. He notes that bare hull resistance "of any hull is dependent upon its precise hull shape, loading, and speed."

    He mentions that "the speed/length ratio was shown to be the dominant factor in selecting a suitable hull shape for a given application and each monhull type was identified to have a maximum value of speed/length ratio above which it will experience sudden large increases in resistance/displacement ratio."

    He compares power requirements for "for the three 70 ft. mono-hull types, each having a displacement of 30 LT". So he's speaking of three separate types (displacement, semi displacement, planing) that have the same LOA and same displacement - yet they operate very differently because of the hull shape, it seems.

    So Savitsky's talk seems to be based on the belief that shape IS important, and that hull shape can be the difference between a boat that has an "upper limit of application" of S/L 1.3, and a hard-chine boat of the same tonnage and length which is "the recommended hull form when operating at SLR > 3.0."

    I'm a layman, I know, but it seems there that one of the great authorities on the planing hull does think that shape has a lot to do with the ability to plane efficiently, or indeed to plane at all.

    Off topic; one very interesting piece of the Savitsky address deals with the semi-displacement hull at speed; "For further increases in
    speed, the trim is further reduced, the spray is intensified and the total wetted area becomes significantly larger that the static wetted area." (my italics).

    Very interesting. Savitsky's semi-displacement shape sounds (on a quick look) to seem a lot like an early planing dinghy. I thought one of the reasons planing was effective in such boats was because it reduced wsa?
  14. PI Design
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    PI Design Senior Member

    Yipster - thanks.

    Chris O - Its obvious we are going to have to agree to disagree, so I'll say my piece and move on. I do not have a problem with video analysis, per se. Never said I did. But your repeated demands for professional video footage are unrealistic. Video footage is not the only, nor necessarily the best, way of proving whether planing occurs. You appear to demand that this professional footage is taken to satisfy the curiosity of acedemics. It is not the designers job to do this. At any rate, I have never seen a video (professional or otherwise) of a ferry being capsized to demonstrate its stability. I have never seen footage of how leopards got their spots, but they've got them. I don't need to see a grocer with four apples giving two away to prove that 4-2=2. You get my point. On the other hand, if you do insist on video evidence, and you are a professional cameraman, it seems a bit mean not to offer your services!

    I wonder, have you ever taken the time to hire a professional cameraman with specific instructions as to what to look for and what angles on the boat (or chase boat) may actually create the desired, end result? If you want it to be really well done, then hire a multi-camera crew, have them roll tape on all scenarios in unison with matched time codes so the tapes can be referenced correctly and provide a factual, non-intoned narrative to support the prepared documentation… if you can.

    I wouldn't expect the videos to show anything even close to the claims (or the inability to meet the claims) without a person who was trained and experientially qualified to render the proper imagery. The simple process of getting an engineer out on the tramp (or wherever) to shoot a cheesy, amateur camera in the general direction of the vaka hull is not the answer.

    Instead, what we see is non-attributed video with absolutely no reference to the statistical evidence of the particular shooting session for reference.

    If this aspect of your design work is so important that you repeatedly make the claims, why then do you not spend the money to properly document the phenomenon so that interested and educated persons may analyze it for content? Without this proper, scientific documentation, you are simply claiming that which is not in existence. It then reverts to what is known as marketing hype.

    Doug - with the greatest respect to Eric S, I don't really like that description of planing. As Eric admits there are exceptions to the rule, so it is not hugely useful. Planing can occur at almost any speed-length ratio, dependant upon the design. If the hull is light enough, flat enough, etc it may plane at a ratio of 1, if it is a Hobie 16 it may not plane until a ratio of 20. A ratio of 2.5 may fit with a certain style of design, but it is not a way to define planing. Planing and hull speed are not related in any physical sense, beyond the fact that a humpless transition will occur if planing begins before hull speed is reached.

    CT - You are right, hull shape definitely does affect planing ability, which is why, for example, most planing craft have hard chines. The difficulty is in accounting for the effects in the mathematical models. Simple rule of thumb approaches are a useful stab at determining whether a hull will plane, but more detailed analysis (and, yes, ultimately trials) is required for accurate results. Savitsky's method, for example, accounts for the aspect ratio of the wetted surface area. CFD is getting better, but requires a very skilled practioner to give accurate results rather than pretty pictures. Finally, it is worth noting that whilst too much spray is an undesirable thing, as it increases drag, sprayless planing is an impossibility. It is the production of spray that allows planing to occur.

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    FAST FRED Senior Member

    For boat speeds from 1.34 to about 2.5, the boat is in the semi-planing (or semi-displacement) regime. That is, not fully planing, and not fully in displacement, just somewhere between the two. At speeds above 2.5 or so, the boat is, or should be, fully planing.

    This is the orthodox religion for FAT boats L\B 3-1 or 4-1.

    Once you get to the land of skinny boats 6-1 to 20-1 its a whole different set of rules.

    Most of the historic commuters were lean and FAST with heavy , not very powerfull engines. 250hp 3000 lbs.Gasoline.

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