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

    Catbuilder--if i recall-- your building a sizable cat in composites?--do you think your hull will have much flex? i think you posted on my other thread on the materials for my boat the p95??..my thought was composites are quite cheap and very stiff when they are thick...especially since the joints are epoxy--wondered how this compared to say a small steel cruiser? around the 30 ft mark?
     
  2. assycat

    assycat Previous Member

    hmm good points...!
     
  3. assycat

    assycat Previous Member

    Hi daiquiri- yea--people here know there stuff...i have learned more here than with any other way of getting info on this ...
    wood was my second choice--because of steels wieght it made it impractical to use for the cat hull... but i started thinking-(overthinking?) about the flex because someone i knew just casually mentioned there is a lot of flex in a wood boat -so i got to wondering just how much...and to what exten is "good" flexing and what is bad...wood does have its appeals...its cheap too -well depending if you go for cheaper woods... the plans i have state that you can use "less expensive" woods if the west system is used and the wood coated(smeared plastered or essentially smothered in epoxy) ...
    my thinking is rot- because i am worried about ingress of water ..i think only a pinhole would let water through into the wood..George Buehler states that wood needs to breathe and not to entomb it in epoxy...so it gets really confusing..also i read in a ferro book that the ultimate goal or to quote "the holy grail" of a boat would be a totally monolithic hullform...and wood could not do this...but i dont know...to me i like composites--dont they share similar features to wood but reduce the risk of rot? and remain very solid and rigid?
     
  4. CatBuilder

    CatBuilder Previous Member

    Even with only half the bulkheads in, I can lift my hull up from the ends and there is no visible flex to it. Take that for whatever that is. I have no idea if that means anything or not. :D

    The composites I'm using are 1" (25mm), 6lb (100kg) Core Cell with 34oz (1150g) triax skins. It's pretty solid, especially once the fairing bog goes over it.

    My hull is designed to be exceptionally stiff, as the designer mentions in his literature.

    Composites aren't quite cheap at all, actually. They are probably the most expensive to build, but the maintenance later is nothing. I always prefer to pay for something up front rather than have payments. :D

    You may want to separate "wooden boats" from West System style "wooden composite boats." When we talk wooden boats, the classics often come to mind built entire of large timber and held with fasteners. These are the most expensive boats of all because the maintenance is outrageous over the years.

    West System was a pioneering company building boats from various wooden techniques which are then completely sheathed in epoxy and often covered in fiberglass on the outside to prevent damage to the wood.

    These boats will not be nearly as expensive to maintain as traditional "wooden boats" and tend to stand up to abuse better. They also happen to be the lowest cost boat to build and own, IMO. They build quickly as well. Many advantages, but you will be replacing wood that rotted from time to time. It's the nature of the beast. Just not nearly as often as a true "wooden boat." These boats also have no fasteners. The wood is bonded together with epoxy.

    An answer to your wood breathing point is that sometimes, when a boat is made of wood with an epoxy and fiberglass coating outside, the inside (bilges, etc...) will be left open with no epoxy put on. This allows the wood to breathe, according to some.

    IMO, it's not necessary to do this. Once you lock wood inside enough layers of epoxy and get those pinholes out, the wood never moves or changes in moisture content again.

    There are a lot of different materials and a lot of different techniques.

    Each has advantages and disadvantages. You will want to make a decision based on that.

    Personally, I chose composite (foam/glass) because I wanted less maintenance in the future. I am paying for it now in terms of cost and a considerable addition to the number of hours it takes to build my boat.
     
  5. assycat

    assycat Previous Member

    so for the p95-in my first thread i was discussing the use of plascore or honeycomb--but for ply epoxy- the costs of the ply was about 100.00 to 150.00 per sheet for good marine ply- which is then encapsulated with epoxy- but the plascore honeycomb is still 43.00 a sheet..which is then glassed and epoxied--same thing--so im not sure how its more expensive?
    however in a cruiser style the west system seems to be the best since i believe the strength and flex might be less--but in your case--it probably is at least as stiff as any other method--and i like lightweight--even for a cruiser i think it should be light...yea the idea of less maintenance seems like a huge bonus!!

    ...so this is why i like plascore and its fast too...the only issue is--the bottom is flat--the sides have curvature which makes for a stiff and strong section..but the bottom is flat...
     
  6. CatBuilder

    CatBuilder Previous Member

  7. assycat

    assycat Previous Member

    oh believe me--i know--actually i can get offcuts at noahs in toronto for about half price i think...check into it...

    what thickness did you say your using??
    core-cell in large sheets--i think around 300.00?? (not offcuts)
     
  8. CatBuilder

    CatBuilder Previous Member

    They're gone. They were gone a long time ago. No more off cuts. They were on a couple of tractor trailer loads that Noah's bought once. They sold it all and Gurit isn't giving them any more.

    1" (25mm)

    Depends on the exact type (density, thickness, perfs, scored, etc...) but that's a good approximate figure.
     
  9. assycat

    assycat Previous Member

    Depends on the exact type (density, thickness, perfs, scored, etc...) but that's a good approximate figure.[/QUOTE]

    wow---that must add up fast. itll pay off later. I used some 3/8ths unscored t 550 once--great stuff-man was it stiff! like ply but light and it would soak up the resin-any resin and make such a strong model(thats what i used it for)- i could break that size thickness but it took a LOT of strength and covered in glass--id never be able to do anything to it...it truly is awesome stuff...lol-so tell me- i have clothes from a past build that still have epoxy on it..it get everywhere...sometimes id get so anxious to get at the build id forget and not wear work clothes...id build in core cell too if i had the money for it...but the honeycomb is really -the best idea for me--cost effective...all around strength and weight. durable too in thicker sizes like 1 inch thick...or so...
     
  10. ancient kayaker
    Joined: Aug 2006
    Posts: 3,497
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    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    Good boats are made from composites, wood, steel, aluminum, ferro-concrete, injection-molded plastics and other materials. Since they are good boats it’s a good bet that all of these materials are good things to make boats out of. Which one is best depends on a host of requirements that have to be defined first before such a decision can be made, any attempt to obtain a decisive verdict here is doomed to failure!

    A certain amount of flex is necessary in any structure from a high-rise building to a spacecraft. The flex allows the energy of externally-applied shock and forces to be absorbed and distributed into the structure without concentrating at one point where it would cause failure or degradation.

    In a boat, flex contributes to durability and comfort. Overdone, it results in a loggy craft with poor handling and efficiency. Boat design involves compromise, like everything in life except politics during an election or before a war.
     
  11. assycat

    assycat Previous Member

    thanks for the useful info...its balancing act--my guess is steel just wouldnt flex like wood or other materials...of course for a mulithull it would be thin so maybe it would anyway--of course using something like pp honeycomb for a trawler?..does that make sense? the plascore guy was saying that people are starting to use cores for heavier displacement hulls more and more--i like the honeycomb because water cannot get into the cells...and the stuff i experimented with(see my other threads for the results)was nealry indestructable--which leaves me wondering; is there a limit to a boat being too light? i.e. if a trawler hull or a multi hull could they be too light?
    forinstance could a 40 ft heavy displacement hull built using 1 inch honeycomb and glassed heavily, be too light ? seems it would be great for reducing fuel costs in the same way that it makes for an efficient multi -in lite airs..
     
  12. ancient kayaker
    Joined: Aug 2006
    Posts: 3,497
    Likes: 147, Points: 0, Legacy Rep: 2291
    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    My own boatbuilding experience is limited to wood and a bit limited there too, but my background a an engineer helps a lot. The ideal material for amateur boat building would be strong, stiff, low density, durable, easy to obtain and handle, reasonably low cost, require simple tools and impose a minimum strain on the environment. Wood is hard to beat overall: in each category there are materials that exceed wood but fall short in other areas.

    The strength and flexing characteristics of steel, aluminum and wood aren't so far apart actually, expressed in engineering terms and related to density; steel may be more than 10x as strong but it's also more than 10x as heavy! The materials that are extreme in terms of strength and stiffness relative to density are the composites and foam materials that I do not use.

    The greater the strength and stiffness of a material the more emphasis is placed on the integrity of joints, and the boundaries with other materials become harder to manage in proportion to the difference in their physical characteristics. Ultimately each material comes with a 'system" for construction and some get paired with a suitable technology or another material: for example if you wish to get the best out of steel you will need to master welding, there's the plywood and epoxy pair - and so forth.
     
  13. CatBuilder

    CatBuilder Previous Member

    Speaking of skyscrapers flexing, I used to both live in one and work in one. On stormy days, I used to hate the feeling of the thing swaying back and forth. So very unnatural of a feeling! Kept me awake many nights feeling my bed move around.
     
  14. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Hey, are you a sailor or not?!? :mad:
    ;)
     

  15. ancient kayaker
    Joined: Aug 2006
    Posts: 3,497
    Likes: 147, Points: 0, Legacy Rep: 2291
    Location: Alliston, Ontario, Canada

    ancient kayaker aka Terry Haines

    Aw, kumon, cut him some slack! Not many sailors like being several hundred feet up the mast in a storm . . .
     
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