Planing Trimarans

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

  1. Capn Mud
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    Capn Mud Junior Member

    Very interesting point of view Cav.


    Gary, I think that is exactly what the planing trimaran guys are saying, that the vaka is carrying a significant amount of the load in partial dynamic lift rather than displacement of the vaka and/or ama.

    And here is the entire crux of the disagreement. How significant is the load carried by the main hull in partial dynamic lift compared to the overall load and how much does it increase performance.

    I have to say that from my observations, on the F27 and even the Weta most of the time, and rough force diagrams I tend to agree with Gary. To me the effect of this is likely to be so small as to make little odds in a tri designed with a rig to provide the power advantage available by virtue of having the leeward ama available to stablisie the boat and carry more sail.

    Cheers,
    Andrew
     
  2. Chris Ostlind

    Chris Ostlind Previous Member

    Well, isn't that interesting, is right.

    Since you are using Fischer's comments within the scope of this argument, you might as well fully absorb his comments at the blog called Cat Sailing News.
    http://catsailingnews.blogspot.com/2010/05/cs-interview-martin-fischer.html


    CS - Testing models in pools is still used? 1:5 Protoypes are not necessary any more?

    MF:"Tank testing is still used, but I reckon that nowadays numerical simulations are more accurate than tank testing, at least for sail boats. The reason is the following: Let’s assume we have a model of a boat at a scale 1:4 (this is already a big and expensive model). In order to get the wave pattern in the tank right, we must reduce the speed of the model by a factor of square_root(4), hence by a factor of 2.

    This ensure that the so called Froude number of the model and the real boat are the same. So far so good.

    In order to get the viscous drag right, we must ensure that the Reynolds numbers of the model and the real boat are the same, and to do so we must increase the speed of the model by a factor of 4! And that’s the problem.
    It is impossible to get the Froude number and the Reynolds number right at the same time. There are techniques to compensate for that, which result in correction factors, but these correction factors also exhibit errors.

    To my understanding the errors of the correction factors are nowadays more important than the errors of numerical simulations, which is the reason why I have a strong preference for the simulations. Another advantage of a numerical simulation is that it provides detailed information. It is possible to study in detail why one shape is more dragy than another one."[/I]

    The guy who is sitting right at the top of the heap for hydrodynamics in multihull design, as well as a current monohull for Cammas at Groupama, says that models don't get it anymore. He gets much better and more trustworthy results with computer simulations.

    So, for those who place an inordinate amount of trust in the so-called, relevant, examples provided by even smaller models... are very clearly moving in the exact opposite direction for reliable data.

    Yes, I'd say that is very interesting and then some when one considers the emphasis being placed on the wrong direction testing examples of the Trapwing.

    Care to comment on that bit of thorniness?
     
  3. Capn Mud
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    Capn Mud Junior Member

    Off Topic?

    Doug,

    Martin's credentials as a designer notwithsatanding I am not sure how this advances the discussion - we aren't talking about flying main hulls are we?

    And if we are I think it actually advances the non-planing argument really. Once the main hull is flying (assuming non-foiling boat) it can't be taking ANY of the load so it must all be being taken by the ama in displacement mode. That is not to say that that is not a way to get good sailing perfomance but what it definitely isnt is planing. Unless you wish to say the long narrow leeward ama is planing - but surely not.

    As per my previous post the issue really comes down to whether at the point between the main hull being in displacement and flying whether there is significant load being taken in dynamic lift by the main hull. The fact that the boat may get up and fly says to me that there is probably very little and that matches my observations while sailing.

    Cheers,
    Andrew
     
  4. cavalier mk2
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    cavalier mk2 Senior Member

    I liked the Ostracizers earlier comments on how no one in the cat fraternity would support the notion of planing and yet doesn't acknowledge the fact that the good doctor designs partially planing cats to use his own terms. Martin also says in the article that the testing he refers to is financially out of reach for him unless he is hired or sponsored. The idea of doing the best with the resources available to you is lost on some who would rather talk than do. If the main hull is planing on a tri the displacement is reduced and less load is placed through the aka onto the amas depressing them less. Less means more here. I think Mr. Ostlind actually understands some of these things but doesn't want to admit it to the posse, he has 4 years invested in obscuring the issue. Ultimately the whole question shows there is no single way to design a boat that is best. Choose the features you like, planing/high length beam etc... and enjoy the ride, the sea is the best teacher for finding what works for you or doesn't. We all would love to test our ideas on a great program, for a cruising boat they should also give you an idea of how habitable the ride is going to be. Maybe a simulator where you can hop in and see if you're going to come out shaken or stirred. If there are programmers on the design site who are interested in working on a testing program we would all benefit, I know I'm trying to interest my computer friends.
     
  5. cavalier mk2
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    cavalier mk2 Senior Member

    I suppose load lines put on a ama or slender cat hull could be filmed at speed in flat water to measure the actual displacement it is carrying while the other hulls fly. You would need protected water with good wind.
     
  6. Gary Baigent
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    Gary Baigent Senior Member

    Thankyou Capt. Mud - that is possibly clear enough for the mischief making malcontents here to comprehend. It is similar to what I wrote earlier about BMW-O stooging around before the start of race 1 bullshitting Ernesto and Brad by keeping the boat flat, while the cat pranced about like a prima donna, then BMW-O winding on the sheets and partially lifting the central main hull so that it skimmed the water surface, "They're planing," shouts Doug histerically then the trimmer winds on more power and totally flies the vaka - what fo you call that Doug and Cav? - going from "planing" to hull flying in a few seconds .... maybe you could call that planing in the AIR.
     
  7. cavalier mk2
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    cavalier mk2 Senior Member

    Like most cruising trimarans of its era the Nicol isn't designed to fly the main hull so planing does help. For a does planing reduce displacement test and flyying a main hull test I would suggest something like the weta with taped on load lines on the main hull and amas. You could do the same with a F boat but it is more expensive. Great job Gary ! In the old days that was called sandbagging after a boat class where fooling the opposition was an important part of the race prep.
     
  8. Chris Ostlind

    Chris Ostlind Previous Member

    Partially planing cats, huh?

    Isn't that something like our friendly Nicol driver being just a little bit pregnant? Nicely enough, Cav has wrangled a very sad bit of error by omission when he says, partially planing cats. Due to simple reasoning, a partially planing cat is also partially something else, as well; that being partially not planing, in its simplest form. Once again, we have the claim that is full of promise, yet delivers nothing but sorta, kinda vapor for the trouble.

    That same kind of trouble lives with the gang of bubbas on this thread who think that planing is a thing they can just wish on the boat of choice... and then sit back and make crinkled-up faces when substantive issues are presented to the contrary.

    There is a partial remedy for the perpetual frown in the visage of Botox, but I'm afraid that no magic elixir is going to save the clown squad from the faces they have created due to loss of logic.

    Now, you see why this thread has gone nowhere in all this time. It's an uncooked kettle of fish that can't be delivered in anything but an inedible state. Claims are made, but nothing is proved. Nobody has the sand to actually perform the suggested acid test, so the claims fly forth as if they were blowing out of a county fair cotton candy machine... all frothy and full of hot air.

    Good on ya, guys. Ya started with nothing, ya argued for days now and ya still got Nothin'.

    Great effort, gents. A truly memorable effort and lots of laughs.
     
  9. Capn Mud
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    Capn Mud Junior Member

    I have both irony and sarcasm and know how to use them

    I guess that would be that little thing they call "sarcasm"? Are you licensed for use of irony and sarcasm Chris? ;) :p
     
  10. Capn Mud
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    Capn Mud Junior Member

    Holy flying trimarans batman

    No Gary - I think by Doug's logic the trimaran would be FLYING.

    The main hul planes = the trimaran planes;
    Therefore if the main hull flies = the trimaran flies.
     
  11. Gary Baigent
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    Gary Baigent Senior Member

    I do believe we're actually getting somewhere and have actually shot down the fantasist wackos from Planet Mogog - but you know, extremist believers in the tooth fairy, they still keep believing. It will be a real disappointment to leave this turkey of a thread ... ... but maybe this is finally the end, the end end ,,,,.
     
  12. cavalier mk2
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    cavalier mk2 Senior Member

    You kids need to read more books. At least constructive test suggestions were made. The definition of planing is partial dynamic lift but you chaps are what we call ballast.....
     
  13. Gary Baigent
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    Gary Baigent Senior Member

    Give it up Cav - you're embarrassing everyone and yourself.
     
  14. CT 249
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    CT 249 Senior Member

    With respect, is that in fact a definition of planing? Many (most??) craft create partial dynamic lift when they are not planing; as I understand it, any time the motion of the craft forces water molecules to be diverted downwards, it's creating partial dynamic lift.

    One time this happens is when sailing a "sinker" windsurfer (ie a board that lacks the volume to support the sailor and rig by virtue of buoyancy). Sometimes, you'll be up to your hips in water when you get a little bit more breeze. The board will accelerate through "partial dynamic lift" and therefore lift itself up so you are (say) thigh-deep and moving at about 2 knots. According to the "planing = partial dynamic lift", a sinker sailboard is therefore planing when it's completely underwater and moving at 2 knots or less.

    Surely that wouldn't be planing as we know it?

    Another guide some use is when the water coming off the transom runs "clean", but that can happen in craft that are clearly not planing.

    I've read plenty of books and NA journals and dealt with quite a few experts in the field, and it seems that there is not a universally-acknowledged definition of planing, therefore we may all be talking at cross purposes. A classic example of the confusion is that Uffa Fox, often said to be the inventor of the planing dinghy, later said to Westell that he didn't think that dinghies really planed at all!
     

  15. Doug Lord
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    Doug Lord Flight Ready

    The nature of the planing main hull is that the dynamic lift reduces main hull wetted surface over and above the reduction in main hull wetted surface achieved as a result of heel(and partial ama loading).
     
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