Design pressures on multihull hull-skin

Discussion in 'Multihulls' started by simon, Nov 19, 2009.

  1. simon
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    simon Senior Member

    Are there any rules on design pressures and expected pressures for multihull's hull-skins?
    I have once read an article about monohull slamming loads, where a study showed that the actual load in the forefoot can be very high and that this can lead to skin breaking, delaminating or buckling. Anything around for multihulls?

    Thanks

    Simon
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    ISO, DNV, LR, ABS...take your pick.
     
  3. jehardiman
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    jehardiman Senior Member

    Mono or multi, it makes no difference because shell impact load is a vessel speed and sea state governed calculation. As Ad Hoc says, take your pick from the rules; or calculate it directly trading off flexture against strength (energy methods). FWIW, impact loads for even moderate speeds can excede 10,000 psf.
     
  4. baeckmo
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    baeckmo Hydrodynamics

    In addition to Ad Hoc and jehardiman: Consider a "worst case", which is one hulls forefoot on one wavecrest and the other hulls aft end on another crest. In this position the total weight is carried diagonally on the approximate equivalent of one wetted hull. When dealing with cats you must always consider the cases when one hull is taking all the beating; the wet stuff is seldom flat.

    Some of the rules do not specifically treat catamaran "skin" loads, and you must be aware if the rule you use is developed for monoulls with displacement as an input factor. If so, consider the design wl to be where all load is carried by one hull, otherwise your topsides will lack strength.
     
  5. jehardiman
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    jehardiman Senior Member

    What you are talking about in your post are bending loads (i.e. primary structural loads) which is a whole seperate subject. And yes, crossbeam stress issues due to wave profile are a huge concern for large multi-hulls.

    However, what the OP was after was slamming loads (i.e. tertiary structural loads). Consider the case of any vessel (mono, multi, foil borne, SES, etc.) launching itself off the crest of one wave and slamming into the oncomming face of the following wave. I have done a failure analysis on broken structure from a wave slap (~ 17 knts in SS 4) where the load to cause failure was over 20,000 psf and could justify that load from motion data recorded onboard. Slamming is not a trivial problem for high speed open ocean vessels.
     
  6. baeckmo
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    baeckmo Hydrodynamics

    Sorry if I am unclear in my english writing, but my notes are de facto dealing with slamming. My point is that the slamming situation in a cat in oblique seas will be worse than the slamming if meeting the wave head-on, both hulls sharing the acceleration. In oblique seas, the total weight of the boat may be carried all by one bow, burying itself in an oncoming wave, or one hull slamming down sideways into next wave slope.

    Normally, the dimensioning slamming loads in monohulls occur in head seas, and the dimensioning rules are built from this situation. If each of the two cat-hulls is treated as an independent monohull when checking slamming loads in the skin structure, the pressures occurring when the catamaran is "landing" on one hull in a wave will be underestimated because of the "weight transfer".

    One of the classical papers that has been useful for our understanding of the slamming process; "On the structural design of planing craft", by Heller and Jasper, is just referring to head-on wave and using boat weight as one parameter in the case of a monohull. If we may use this reasoning for the cat case, the weight to use has to be the full boat weight applied to each hull, NOT half weight per hull, simply because sooner or later this loading will occur!

    Of course this has consequenses for the primary structural loads as well, but that was not my concern here, when commenting Simons question.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    You will find that, if you select DNV rules, for example. There is a difference between the pressures obtained when using a monohull to that of a catamaran. There is an input, for the "number of hulls"...makes a difference to the pressures obtained.

    PS..that Heller & Jasper paper is excellent. Set the "rules" in my opinion
     
  8. jehardiman
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    jehardiman Senior Member

    OK, I see what you were after now. Yes, a multi has a much higher mass moment in the XY axis (Ixy) than a mono and therefore is the limiting case. With a mono hull, because of the difference between Iyy and Ixx, only pitching down into a wave face is considered as roll is negilable. As a multi would have a Ixy larger than both Ixx or Iyy and it would be the simultaneous pitch down-roll down into a wave face that would cause the highest skin loading given constant geometry.
     
  9. simon
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    simon Senior Member

    the reason, why I am interested in this subject is that my sailing catamaran (44' and 8 tons) has a watertight horizontal bulkhead (vertically about in the middle of hull) in the bow that forms a crashbox. The boat is standing on the hard and the air inside heated up and mad the hull-skin bulge to the point where the paint and the bog started to bubble and crack at the line where it is attached to the bulkhead, as the outer layer of the hull-skin is under compression. If the air inside has heated up from 20 C to 100 C, which is a lot, it means that the air has expanded by about 27% which would result in a raise of the inside pressure of 27% of an atmosphere +/- 2700kg/m^2 = 27kN/m^2 or even less with lower temperature raise.

    My guess is that slamming can reach higher pressure than that. It is difficult to judge the stiffness of the structure, as the bulging probably affected more the sides than the bottom, because the radius that stiffens the panel.

    I have tried to use the Bureau Veritas formula for slamming loads, but it seems that there is no formula for sailing multihulls, either sailing monohulls or motor multihulls are covered.

    Simon
     
  10. catsketcher
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    catsketcher Senior Member

    This aint normal

    Gday Simon

    You have other members far better qualified than me to answer you in terms of the theory of slamming so I will give my answer from another angle.

    1- I don't think you cat will get 100 degree Celcius air in a compartment. If the boat is sandwich it would take a long time to get a hot deck to conduct heat into the compartment this way. Anyway to get 100 degrees into the air would mean more that 100 degrees on the surface. Not many composite boats will take that.
    2- Cats don't tend to bulge at bulkheads. If the boat has been built with large enough coves and strong enough tabbing then the hard spot should not develop. I had something similar to your situation on my 38ft cat. When I drilled into the compartment I got a big whoosh of air but no change in the laminate outside.

    I think you may have other issues here, maybe the boat is not well supported on its chocks. I can only hope that the boat has enough laminate - around 600 - 800gm on the inside is par for a light laminate on a 15mm core balsa cat your size.

    What puzzles me is the bubbling. If your boat is epoxy I hope you haven't painted it a dark colour lately. This would make it hotter and when epoxy gets hotter than it has ever been it goes rubbery - post curing. You don't want any load when post curing a boat. I am guessing it is epoxy as say you have bog there. If you want to know if you have any deflection fair the boat with a 1.2 metre piece of 20mm foam covered in black powder or chalk. The chalk will only be left on the high spots. If it is a deflection problem you will find out quickly. Maybe the floor is good at conducting heat or the builder has done something silly with core removal etc in this area. What is the floor and hull made from? Cedar?

    Some photos would be good
     
  11. simon
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    simon Senior Member

    there was quite a lot of pressure in the compartment and once the air was out, the hullskin was fair again. The hull is 12mm Cedar and seems pretty flexible. The outer skin is white, but the antifouling is black. I don't think that the air was 100 deg C, but just to calculate an extreme. Lower temperature makes it even worse , because that means that even a lower pressure deflects the hullskin.
    I will fit a breather hole that can be closed. I will check the tabbing on the underside and were thinking of either fitting a stringer and/or an extra frame to reinforce the panel. I will discover the internal structure, once I have cut an access hole into the bulkhead.

    Simon
     
  12. Alik
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    Alik Senior Member

    If we are talking about 'small craft' (LH<24m), then design loads on catamarans in terms of calculation methods can be subdivided into:

    a) local loads similar to monohull craft, i.e. loads on bottom sides, decks, cabins, internals;
    b) local loads on bidgedeck structure (specific for catamarans);
    c) global loads from transwerse shear, transwerse bending, londitudinal twisting.

    At present, we use ISO12215-5 to define all these loads. There are methods for panels and stiffeners for a) and b), while for c) is not that clear and one can use either FEM analysis or simplified methods.
     
  13. catsketcher
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    catsketcher Senior Member

    Mmmmmm

    Simon - as I said before I come to this from a "what works perspective" as I don't know the theory well enough like the others do. BUT I don't like the idea of the hull flexing this much unless your topsides are absolutley flat. Even then what you say is occuring is quite rare.

    I can only think that this problem along with your other problem may relate to under sizing of the scantlings. As you can't go and redo the laminates I would be thinking about going through the boat and reducing the unsupported panel sizes throughout with well tabbed frames and bulkheads. If you reduce panel span by half the deflection should reduce by 8 times (assuming it acts as a simple beam). Stress also reduces.

    If the laminate is too light inside you may want to put a bedding tape under the bulkheads to spread the loads. Two db 600 tapes one 150mm and the other 200mm would help lock the bulkhead/frames into position. I have heard of your concern in some foam race boats. The phenomenon when out sailing is called panting. I don't like the thought of a composite structure moving much because of fatigue limits and the like.

    It may be time for you to get a good reliable multihull builder to come down and look over your boat. I can guess all I want but if you got an experienced builder of cedar cats he would be able to tell you whether he has experienced what you describe and how your boat's structure compares to the norm. Be careful though - a flat panel cat will need a heavier laminate (for stiffness) than a rounded cat.

    cheers

    Phil
     
  14. simon
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    simon Senior Member

    Catsketcher, thanks a lot for your practical advice.

    I have opened the watertight compartment today. There is no backingstrip between the horizontal bulkhead and there is not tabbing on the lower side, just on the upper side. The bulkhead has been placed on a triangular shaped wood. No lamination done from the underside of the bulkhead, just glued on top. That would be the reason for the hardspot. The Hull is not supported by any frames nor stringers. in an area of about 1.5m length and 1m height on both sides. Further back in the hull, there are severall reinforcements, like the cabine sole laminated to the hull.
    I am going to get some advice by a composite builder this week.

    Wondering if I could bog over the wood to shape a radius and then laminate the tabbing over it? It is very difficult to add a backingstrip to an existing bulkhead. Is adding a bogged radius and add tabbing sufficient to spread the load?

    I am going to reinforce the hull-skin.
    What is more efficient, stringers or bulkheads(ringframes), or both? Wide stringers would distribute the load and a ring-frame could reinforce it additionally.

    Is it possible to run a stringer just from bulkhead to bulkhead? In metal-boats the stringers would end with a gusset on the bulkhead. Anything similar with composite?

    There is no comparison in the structure of my old aluminium monohull and the cat's structure. Where a central vertical keel made of 8mm aluminium holds for sure, the cat sports a joke of a plywood web. Do cats have central vertical keel and bow?

    I would be still interested in hearing, what kind of pressures there are expected in slamming conditions.

    Thanks to all for the input

    Simon
     

  15. catsketcher
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    catsketcher Senior Member

    Simon the triangular piece of wood is good building practice for a watertight compartment. You sit the floor on it as it is impossible to cove under a floor in such a compartment. It may cause a hard spot especially if it was forced in a little. It is best practice to recess every bulkhead and floor of the hull skin slightly and then push glue into the gap.

    I cannot guarantee that bigger coves and tabs would help but they should.

    As for frames and stringers I think the matter is best decided by you. Work out what you can fit and how easily it can be installed. In my cat I have a couple of shelves that the designer wanted to reduce panel size. These go fore and aft. They have a lip of cedar and are used as a bed step. If you can find a use for a shelf use one. As to ring frames I would recommend going composite. Put a bedding layer of DB 800 or so under the frame area then use thick foam (25mm) cut into strips 100mm wide or so and glue these onto the 150 mm wide tape. Chamfer the edges of the foam and then glass over them with more DB tape. If you want to get fancy put uni over the top of the foam. Of course you can always use ply but getting a new ply frame into a built boat may be trickier. Ply will also concentrate load more as it is thinner.As for ending at a frame just spread out the load with DB tape and generous coves. Spread the tape and the load will follow.

    Cats are a bit different from each other in the amount of stiffening inside. A beautifully built Chris White Atlantic 46 had very little interior stuff. It would have had to have had heavier scantlings to cope with the larger spans. When a cat in Oz split its gunwale and sank one hull/capsized, Lock Crowther mentioned that it had no interior furniture and thus the hull side was underspecced.

    As for keel webs. My cat has a 5mm one that is bulletproof. It is the shear web and my cabin sole forms the top of the beam. The hulls is incredibly stiff. Maybe the top of your web needs to be beefed up to take the compressive load.

    cheers

    Phil
     
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