should a boat flex? or be rigid in rough water?

Discussion in 'Wooden Boat Building and Restoration' started by assycat, Mar 8, 2012.

  1. assycat

    assycat Previous Member

    Im curious as to the properties of wood compared to metal when it comes to flex in the hull. here is what i percieve on this so far- im no engineer.
    so i dont have the slightest when it comes to this:


    it seems to me metal boats (or even ferro)win hands down on the issue of flex in a hull. I have heard that wood boats flex more than other materials.

    If so -i wouldnt like wood(ply) as a boat material because I see wood as being held with fasteners etc. and the properties of wood make for a boat that flexes(bending modulus???) too much. I know in ships, steel also flexes. in fact metal can fatigue - when you bend a metal coat hanger back and forth- it heats up and breaks apart. this simulates hogging and sagging in a hull.
    so is steel truly stronger than wood and more rigid?- i know that flex can be important too such as when a mast needs to give some to absorb the stresses of the forces of the wind pushing the boat and yet not snap...now, is this the same idea with a boat?? i have also seen demonstrations on youtuibe regarding how wood ply will weaken very rapidly when submersed.

    see this great vid-http://www.youtube.com/watch?v=_ymEoivrrK0&feature=plcp&context=C4d44e47VDvjVQa1PpcFNORBmmlLNcD7VvPM0fmKzuhXOP9unickA%3D note how it weakened!!

    My thinking is that metal is superior as is ferro in this regard because it does not flex as much in a seaway to my knowledge anyway. with fastenings --again the coat hanger effect comes in doesnt it? yes i know wood is glued too but wont that flex also in time weaken a wood hull faster or pull away from the members if overloaded?? also the wood would have more joints in it meaning it has less monolithic properties to absorb stress and pressure of the water against the hull???

    can a wood boat be made to have the same stiffness factors? would that be a good thing? can epoxy/ply composites be made to be as stiff as steel? id like to build in wood, but this is my concern...i like the idea of cored hulls.
    sorry this has so much info and questions but its important to me to learn...thanks!!
     
  2. TeddyDiver
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    TeddyDiver Gollywobbler

    No it doesn't, no, yes but not the whole thruth, yes but it does effect to all materials not only fastenings, no, no, yes, no, yes... no worries and my pleasure .)
    BR Teddy
     
  3. assycat

    assycat Previous Member

    haha--thanks Teddy! much obliged!
     
  4. kvsgkvng
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    kvsgkvng Senior Member

    flexure is bad for a structure unless it is accounter for in design

    Since am an engineer dealing with this topic once in a while, here is my take on the subject.

    Ideally any structure (and vessels are structures on a spring mat to that matter) should not move or flex. But this is an asymptote and any structure will flex under a load. It is up to a client (you) to think where the limit is.

    The difference in structures flexing or not would be expressed in its useful life. The more it flexes the more design accommodations should be made to prolong a structure's life. Here comes in view the fatigue you are mentioning.

    I believe it all depends what boat you want. If it is a pleasure craft, then go ahead flex a little, but overall -- it is that springy feeling when your small craft goes over a wave.

    The hull should not "flex" locally, like when you press it with something hard.

    When you see a wooden boat held by fasteners, remember that that flexing eventually is being transferred by those fasteners and it destroys this connection.

    The definition of structure is that it is not movable or deformable under normal conditions; a mechanism on the contrary, as a shape, can be changed.

    So, the final word – none of the boat and/or boat components should “flex.” It is a bad thing for a structure(s). Can we avoid it – no, but we can account for its effect and try to extend the life of a structure. The more flex, the sooner any structure would die unless it is specifically designed to take that punishment (inflatable, rubberized joints, thin shells etc.)

    Those were just my thoughts. You asked a general question and I tried to provide a general answer.
     
  5. assycat

    assycat Previous Member

    NO this is great!..i find this very helpful--thanks!

    so do you think steel has an advantage over wood or a core for a hull? with repsect to flex?
     
  6. TeddyDiver
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    TeddyDiver Gollywobbler

    Never do such consumtions. Steel flexes as any other material you can think. It also has limits for the flexing and for the number of cycles. So there's no simple yes/no answer to your question...
     
  7. assycat

    assycat Previous Member

    Teddy- actually it DOES answer it--quite nicely in fact(i think)--so in other words- the materials if well built really become just a preference? and there may not be an advantage - if one looks just at only flex and not other factors such as intended use of the vessel or type of vessel etc.. based solely on flex --it would be safe to say - if given equally well built vessels, steel has no advantage over wood plastics or other and vice versa??
     
  8. TeddyDiver
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    TeddyDiver Gollywobbler

    To make the point simplified (over simplified and not accurate) steel, aluminium and carbon laminate are stiff materials vs wood and glass as "soft" ones. Steel deforms under stress. Aluminium breaks under cycled loads, carbon is fragile. Wood gets rot. Glassfiber blisters.. Get the picture?
    BR Teddy
     
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  9. assycat

    assycat Previous Member

    yes very well put--thanks i do get it...i have learned much today...
    awesome--cheers!
     
  10. TeddyDiver
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    TeddyDiver Gollywobbler

    Depens of the vessel...:)
     
  11. Petros
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    Petros Senior Member

    it depends on the size you are talking about. Too much flex makes the hull difficult to control in rough seas, like a car with soft suspension on a rough road. But a small amount of flex can actually improve comfort.

    I know this from sea kayaks, I have paddled many miles both in hard shell factory boats, and flexible skin-on-frame wood kayaks. A little bit of flex, to match the sea conditions, actually improves comfort and control. Too stiff makes for a rough ride, too soft makes it hard to control. Just like on a car suspension, sports cars are stiffer for high performance driving, and luxury cars are soft, and will not handle rough roads or performance driving very well.

    I would think this would be true will all hull sizes, but very large hulls likely do not benefit much from flex. Of course you would not be building large hulls from wood any more.

    So it would depend on what conditions you are designing the hull for, and the size and type of boat you are building.
     
  12. kvsgkvng
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    kvsgkvng Senior Member

    it is all up to how much you want to spend :)

    In theory there is no preference. All materials, when correctly designed should not "flex" to the point when it becomes perceptible. It just happens that incorrectly overbuilding a steel/concrete hull, compared to thin wooden hull, it appears that steel hull feels stiffer.

    Properly designed wooden hull would have very little of controllable and predictable "flex" (deflections). There is a catch though...

    Because wood is a softer material, it would require a lot more of wood thickness in the hull to be comparable with stiffer materials such as steel or aluminum.

    It should all boil down to cost of manufacturing, expected service life and of course the funds available.

    If you have got the money, I would go first for steel then aluminum. If you have not so much money then I would go for composites with "S"-type glass fiber. After that comes "E"-type glass fiber -- it is cheaper and less stiff. Remember, composites require more labor intensive expenses (ha, another corporate lingo!). If you have a little money go for wood.

    Again, it depends what boat size you have in mind and what its purpose - pleasure or business.

    Personnaly I would want to have a properly designed steel boat with positive flotation and cathodic protection in places all over this vessel compared to wood or composites.
     
  13. CatBuilder

    CatBuilder Previous Member

    It may depend on the size, but a stiff boat is the best boat you can have coming from a professional mariner's point of view.

    You do not want a flexing boat for all the reasons mentioned above, especially the fatigue factor. Flex any material over and over and it'll eventually fall apart. The less flex there is, the long the boat will last.

    Also, don't forget the bulkheads and/or stringers. Local stiffeners like that are part of all boats and go a long way toward making a hull that might otherwise deflect, stay true and stiff.

    I would tend to disagree slightly with the previous post in that the idea boat is one make from composites. Well, for multihulls anyway. The metal boats are too heavy, save aluminum. I guess that's a very specific niche though. :)

    However, a composite boat with no wood will last an entire lifetime with little to no maintenance. I'd say if you are rich, build in wood. If you are not, build in composite. That way, you don't have to spend money and time year after year replacing rotten wood.
     
  14. michael pierzga
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    michael pierzga Senior Member

    You could argue the point forever !!

    With any engineering , Flex is good because it relieves stress..... and Flex is bad because It point loads pieces of the structure that dont flex.

    In the perfect world structures that must be stiff don't flex and structures that must be malleable, flex.

    " Flex " is in the responsibility of the designer.



    Think of a Mast...if it doesnt flex you cant change the shape of the sails.

    Does a vessels hull " Flex" YES !!!!!!!!!!!!!! How much is good ? Speak with the designer.
     

  15. daiquiri
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    daiquiri Engineering and Design

    Assycat, you have got very good replies so far.

    Steel boat can flex in a same manner as a wood or grp boat does. The amount of flexing will be determined by scantlings. One can design a wooden boat which is as stiff as a steel one, if he does his calculations for the same deflection in all three cases. As a simple example, you can try to analyse a simple case of a cantilever beam with a rectangular cross-section. you will discover that a beam made of wood, designed to carry the same weight and flex the same amount as a steel beam, can weigh up to 70% less.

    For small boats, wood is competitive and in several respects better than metals:
    • It can be designed to flex the same way as steel does, at much lower structural weight.
    • It's fatigue resistance is similar to that of aluminum. If designed for 30% of yield stress it will resist 1 million cycles.
    • If properly waterproofed, wood is a good thermal and current insulator, and very little additional insulation is necessary to reach the same level of a well-insulated steel hull.
    • Wood is "alive" and, warm, and gives a very pleasant sensation when touched with a hand or walked upon with bare feet.
    And so on.

    As the size of the boat grows, wood will loose it's advantage over metallic materials or grp because the structural sizing becomes too big and difficult to work with. But on a smaller size, I just love to see wooden boats.

    As you can see, wood as a boatbuilding material has many qualities. It all depends on what boat you need and how much time and money are you willing to spend, both for building of your boat and on it's future maintenance.

    Cheers
     
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