Why can't most catamarans get over the hump ?

Discussion in 'Multihulls' started by tommymonza, May 4, 2014.

  1. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed.....length displacement ratio shows that shape plays no part, or very little, in such matters. Its all about length-displacement ratio (L/D). Shape is a major misnomer.
     
  2. ALL AT SEA
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    ALL AT SEA Junior Member

    http://www.gumtree.com.au/s-ad/pialba/sail-boats/catamaran-10m/1039395379

    does this come close?
     
  3. rwatson
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    rwatson Senior Member

    So a curved hull cross section doesnt reduce planing capability at all ??
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    If shape was irrelevant they should go backwards as good as forwards, that I doubt. And that boat ALL AT SEA posted, the hulls on that look super slim. Not so sure about the wisdom of that.
     
  5. waikikin
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    waikikin Senior Member

    AAHH... if they have no shape they don't exist....
    without displacement they like a tumble weed....
    without a purpose they have no use.....................................?
    with too much they go too slow............
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Hmmm...seems no matter how many times this old chestnut comes up the lack of understanding of simple hydrodynamics goes over most people's head; why..no idea!?. Ok, here it is again (despite the fact that I and others have noted this on BD forum on various threads previously), see this for starters:

    L-D ratio-1.jpg

    As you can see, with increasing L/D ratio the effect on resistance is minimal at best. Does a convex compared to a concave hull not make a difference, of course, but the effect is minor when you actually look at the main parameter that affects resistance, the length-displacement ratio...i.e. shape has little influence overall.

    Well, simple "block shapes" have been tested by others too, noted here:

    various resistance.jpg

    The various shapes are all from pure semi-circular (minimal WSA) to pure hard chine to canoe (double enders) and so forth. And yup, even a simple block of wood with a pointed end has been tested and shock horror, not the difference everyone "thinks". Note how close all the various changes in actual hull shape (shape that "everyone thinks makes a difference) are in terms of overall resistance.The difference is very minor and when actually looking at very crude block of wood with a pointy bow compared to the classical minimal WSA of a semi-circular hull or a hard chine, not much either...but its a crude block of wood i hear many cry, surely it MUST be different!!

    Intuitively everyone "thinks" shapes plays a major part. Only those that have little practical experience of actual hydrodynamics and their real effects state as such.

    With increasing L/D ratio...shape plays minimal, if any, part in the total resistance. This is nothing new. It is only new to those that are not naval architects. Just look at PNA..even they state it, along with many other excellent references, inc Molland..but is always overlooked by those with fancy 3D software and peddling a "claim" of sorts as being "new" or "revolutionary". The science does not lie and is not buried either, only those with something to peddle claim otherwise... :eek:

    L-D ratio-2.jpg Molland LD ratio 1995.jpg Molland LD varying BT ratio 1995.jpg

    So you can bleat all you like, sadly the science does not support your assumption of shape being a major influence.
     
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  7. pogo
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    pogo ingenious dilletante

  8. tommymonza
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    tommymonza Junior Member

    To go with one idea and stick with it for a minute.

    So lets say I decide to more or less duplicate the bottom design of a Tornado . The one change is I would add significant freeboard as I choose to go through and over a wave top and not through and under a wave top as a low freeboard cat will do' I can live with the extra windage.

    The design of the hull would be very similar to a Stilletto 28 but a little longer and slimmer hull for better performance and a Plumb bow for my aesthetics.

    The Tornado hull is already a cylindrical tail dragger from the start yet everyone says what a superior design it is especially for its age.

    So the tail dragging wake producing is there

    Why not take the cylindrical portion and flatten it out with a soft radius to the hull sides?

    Or what I thought would be a little more over the edge since we are shooting for a planning hull.

    Why not have a flat planning surface on the bottom with an extension of the planning surface jutting out 4 to 6 inches on both sides of the hull kind of like a lets say 6 inch by 1 inch thick edge that would run half the length of both sides of the hulls .

    Would all that added extra planning surface create horrible drag in light conditions that the boat would be a total dog?

    My other concern with the planning wings added would be they would dig in when trying to come about but I think that could be over come with a large rudder surface and having the daggerboards further aft than they would be placed traditionally.
     
  9. ALL AT SEA
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    ALL AT SEA Junior Member

    No, i wasn't suggesting that this'd olane, but it is a similar size pod cat to the description, and not too dissimilar to an off the beacg cat such as a Prindal... i quite like it:)
     
  10. rwatson
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    rwatson Senior Member

    Well, I don't recall saying (let alone bleating ) that shape was *the* major influence, but it is an important one according to at least one acknowledged expert.

    For example, Savitsky discusses the negative pressure created by curved hulls in this paper

    http://www.staatsgeheim.com/wp-content/uploads/Savitskyreport.pdf


    Negative Pressures on Convex Surfaces
    One other major hydrodynamic phenomenon associated with high speed itself is the generation of negative pressures along convex surfaces of the hull i.e. convex longitudinal buttock lines or convex curvatures in the transverse plane—particularly in the bilge area.


    He also lists the typical successful planing hull features :-

    "Hard-Chine Planing Hulls
    : The hard-chine planning hull is configured to develop positive dynamic bottom pressures at high speed. These positive pressures actually lift the hull and thereby reduce the buoyant component of hull support. As a consequence the wetted bottom area, when planning, is substantially smaller than the static wetted area....

    Complete avoidance of convex surfaces (except for the bow area which is out of the water at planing speeds) to avoid the development of bottom suction pressures.

    Sharp edge chines at the intersection of the bottom and sides to insure complete separation of the transverse flow component from the bottom.

    A deeply submerged wide transom with a sharp trailing edge to insure complete separation of the longitudinal flow from the bottom- thus insuring that the entire transom is ventilated to the atmosphere.

    Straight horizontal buttock lines at the aft end.

    Vee-bottom transverse sections with the deadrise increasing towards the bow. The deadrise is required to reduce the wave impact loads in a seaway and to provide lateral wetted surface required for course- keeping stability and maneuvering. "



    Are you saying that all these design considerations are not important in designing a successful planing hull ?
     
    Last edited: May 4, 2014
  11. rwatson
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    rwatson Senior Member

    yes. In the paper i quoted in my previous post

    "The hard-chine planning hull is configured to develop positive dynamic bottom pressures at high speed. These positive pressures actually lift the hull and thereby reduce the buoyant component of hull support. As a consequence the wetted bottom area, when planning, is substantially maller than the static wetted area. Unfortunately, the addition of induced drag associated with the development of dynamic lift results in a total resistance- weight ratio that is substantially greater than those for a displacement or semi-displacement vessel at their design speeds. "
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    I am well aware you have repeated this numerous times, but why is the block of wood "pointy" if shape is irrelevant ? :D If the block of wood is a 4x2, are you saying the resistance is the same if it is laying flat on the water (4" wide) or standing on edge (2" wide) ? In this case the length and displacement is the same, but beam doubled when laying flat. Or is what you are saying assuming the same length to beam ratio, just varying displacement ?
     
  13. rwatson
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    rwatson Senior Member

    The point Ad Hoc is making is valid, but its not the whole story. I would like to see a planing design in commercial operation that didn't use hard chines and a sharp transom.

    The optimization of a planing hull requires these features. I can get a stone to 'plane' - but the power requirements are way too high
     
    Last edited: May 4, 2014
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    There have been a few small planing power cats that employed rounded hulls sans chines, one drawback seems to be a propensity to porpoise, more so than hard chine types, alarmingly so is some cases. Even a hard chine planing cat will porpoise somewhat without substantial flats at the chine.
     

  15. Ad Hoc
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    Ad Hoc Naval Architect

    All you are demonstrating by cutting and pasting text from another source is that you can cut and paste. Since the comprehension has been overlooked.

    Look at the figure above again. What do you see?...a range of L/D ratio hull forms with an ever increasing L/D ratio, the resistance reduces and the prismatic hump also reduces.

    If you take one hull, whatever its shape, and establish the L/D ratio and plot it onto that graph, you'll see that is falls pretty much on the line that closely matches your L/D ratio. If you then alter its shape, all you are doing is moving that "line" up or down a very small amount. But the change is not earth shattering as everyone keeps thinking it 'must' be, it is minor. It wont start moving the line well above or well below its baseline compared to the other L/D ratios. (look at those from Molland again too).

    But the problem is compounded when you start making larger changes to the designs SOR. Since in doing so you're introducing other aspects into the mix which are not related but it seems constantly assumed to be the same. For example that of LCG. Why not have the LCG at the transom, for super high speed and another at midships..what about hull shape there then?....its very very different. Sure is...one will have extreme trim when stationary the other will float near level trim! Is that a result of the hull shape or just poor design, since a hull form with LCG at midships is going to perform very differently from that with one at or near the transom.

    The LCG is not an input into the L/D ratio. You design a hull suitable for your LCG. Then changing the deadrise, or convexity, for example, will alter the resistance characteristics of THAT hull compared to its parent unmodified hull. But to suggest that either the parent or the modified hull can be used for the hull form with the LCG in a totally different location, say amidships, simply misses the whole point and just exposes a general lack of understanding of what is hydrodynamics and "hull shape". Since the hull forms for either would be totally different simply because the arrangement requires a totally different hull to work in the first place.

    Since what you are suggesting is like buying a Ferrari and then going off road and not understanding why the wheels fall off, the car gets smashed up and ends up a total right-off and all the same time saying..hey, its a car...it's a car it must be the same! Each car each hull has its range of applicability. Thus you need to compare like for like.

    So if you buy a Ferrari, will it be different from that of say an Aston Martin...of course, but is the difference between either the Ferrari or the Aston the same as either of them against a Land Rover? All are still cars, all have 4 wheels etc etc....

    Understanding the objective, the SOR, helps one to understand the importance of the L/D ratio...and that hull shapes plays a very minor roll. Whether one hull produces more negative pressure than another...so what?...these are absolutes. Only a computer program deals with absolutes, it then becomes a "measure" of difference. But taking a step back and looking at the whole scheme of things, it plays a minor role when comparing one with another for an exact purpose...not an unrelated one.

    I can list endless hull changes/shapes myself from all the boats we have done over the past several decades all of which have an effect in one way or another. But this is more about operational performance, for example is there spray on the wheelhouse windows one shape there is another there is not, but the total resistance per se is not altered. I could list many more...but are classic non-sequiturs.

    If you design and if you design on a daily basis all these things become obvious. Those that don't, your only frame of reference is computer programs that show absolutes and text that describe the effects that changes have, when comparing one against another, but little else.

    See above.

    But ask yourself this question. When you would like a 3D shape that is to float and move with a forward speed through the water would you select a bow shape that has flat vertical face, like a block of wood, or would you select a bow shape that allows the water to pass either side with relative ease?

    If you select the former, then nothing will move through the water with any appreciable speed at all. Which begs the obvious question, what is the operational profile, the SOR of the boat.....too make a lot of water splash up and scare bystanders on the shore line???. Then the objective has been achieved. But if the later, you would like to move at "some speed" with relative ease you suddenly have an SOR, thus, you're going to have to have a bow shape that allows water to flow past. Once you make the mental leap that the bow, for any hull, requires a shape to allow water to flow past, the rest is....see above.

    Again see above, as the simplicity appears to be going over your head what shape means in terms of an SOR.
     
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