deflection

Discussion in 'Boat Design' started by Adarsh Edakkote, Jan 27, 2016.

  1. rxcomposite
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    rxcomposite Senior Member

    A dhow (wooden boat) is very limber. Note the designed arch on the keel when it is not loaded. It becomes straight or reverses arch when fully loaded. The dhow is about 50 feet long and the unloaded arch is more than 6 inches.
     

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  2. Ad Hoc
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    Ad Hoc Naval Architect

    Wood is a low modulus material, ergo, it will deflect.
     
  3. daiquiri
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    daiquiri Engineering and Design

    I agree with the above. 14mm over a length of 15m gives a deflection of 0.0003 or 1:3300 , which is more than acceptable.
    For example, primary structural members of steel ships are required to have a max. deflection of 0.0056 = 1/178, which is an order of magnitude more than your case. Wooden ships are usually evaluated on individual basis but I doubt that they would be required to bend less then steel ships by as high as an order of magnitude.
    So there must be something else that bothers the surveyor.
     
  4. rxcomposite
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    rxcomposite Senior Member

    There is no survey rule on "traditional or ancient built design". The closest you can refer to is the Lloyds Register rule on wooden ships, year 1981. There is no deflection criteria, only the allowed method on construction and material choice.

    If you consult wood and timber design books, the allowable deflection criteria is 1/360 for Douglass Fir or Pine. Other timber may have different deflection criteria but I don't have the data.

    Thus, you can either tabulate the modulus of the hull, and compute the deflection OR you can measure the total deflection of the existing ship and test if it exceeds the 1/360 rule.
     
  5. NavalSArtichoke
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    NavalSArtichoke Senior Member

    A wooden dhow 50 m long is one of the longer vessels of this type. I wonder if the class surveyor was concerned that by exhibiting a hog, this vessel was not strong enough to withstand the rigors of an ocean voyage?
     
  6. daiquiri
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    daiquiri Engineering and Design

    It is still a ridiculously small deflection for that ship length. Not even sufficient to raise eventual concerns for caulking watertightness.
    I think we are missing some info from Adarsh to get the full picture of the problem.
     

  7. rxcomposite
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    rxcomposite Senior Member

    What Class is this (Dhow)?

    I know that the biggest wooden modern ship built is the 65 meter tall ship STS Tenacious (http://jst.org.uk/our-tall-ships/tenacious/) and according to the coffee table book, it was built to Lloyds standard. Maybe you can post a picture or the Class requirement for deflection?

    What confuses me is that a dhow is an ancient vessel or replica built in the traditional method of the Arabs (Indian technology actually). A wooden ship is different to a dhow although the modern direct translation of a dhow (dow) is "wooden ship".

    If it was hogging, then it was built in the traditional way as shown in the picture. I have photos of small dhows and the keel arch is prevalent in the design.
     
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