Cantilevered Davits - Engineering Problem

Discussion in 'Boat Design' started by oceannavigator2, Mar 22, 2014.

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  1. CDK
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    CDK retired engineer

    The davit that carries the bow of the RIB should incorporate a safety device to prevent damage from overload. The simplest solution is a shear pin in the winch, a more elegant one is a spring loaded clutch.

    I designed my davits and had them cut and folded from stainless steel plate in a machine shop where they make equipment for the diary industry. No mark-up for marine purposes, just the costs of the material (by weight) and 2 working hours. Drilling, welding and polishing took me a weekend.
     
  2. daiquiri
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    daiquiri Engineering and Design

    Smart. :) I guess that you have chosen the bow davit arm for the shear pin because you don't want the other side, which carries the outboard motor, to fall in water?
     
  3. CDK
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    CDK retired engineer

    Indeed! I also would attach a lanyard to the bow to limit the downward angle.
     
  4. oceannavigator2

    oceannavigator2 Previous Member

    That's a great idea for planned failure. Good thinking, CDK.

    I have to discuss the steel idea for a bit. Wouldn't this be the heaviest way to do it? Or are the size of the hollow steel tubes so small compared to the foam and glass method that they are lighter or the same weight? Galvanized steel would be painted black if that is an option. Is it? Then, also, there has been no talk of actual material size. What size galvanized, steel, square tube would I need to use?
     
  5. daiquiri
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    daiquiri Engineering and Design

    Steel tubes are definitely the least expensive and technically least demanding solution. They are standardized structural components with well-known mechanical properties, and hence the relative engineering calculations are simple and reliable. If I were you, that would be my way to go.

    That said, by considering following 5 mm-thick square hollow sections:

    a) 130x130 tube:
    - linear weight: 18.8 kg/m

    b) 140x140 tube:
    - linear weight: 20.4 kg/m

    c) 150x150 tube:
    - linear weight: 21.9 kg/m

    d) 160x160 tube:
    - linear weight: 23.5 kg/m

    And the loads and scantling criteria indicated in the post #9, in the attached pdf you can see the results of beam stress calculations.


    So, if we consider the following 4 steel grades and assuming a FOS of 4 (no persons on board the RIB during davit operations, ok?):

    1) S235 mild steel:
    - yield stress: 235 MPa
    - ultimate stress: 360 MPa
    - admissible stress: 360/4 = 90 MPa

    2) S275 mild steel:
    - yield stress: 275 MPa
    - ultimate stress: 430 MPa
    - admissible stress: 430/4 = 108 MPa

    3) S355 mild steel:
    - yield stress: 355 MPa
    - ultimate stress: 510 MPa
    - admissible stress: 510/4 = 128 MPa

    4) AISI 316L stainless steel (approximate values):
    - yield stress: 200 MPa
    - ultimate stress: 500 MPa
    - admissible stress: 500/4 = 125 MPa

    in which the bold letters indicate the maximum stress allowed by each material, we can deduce the minimum tube sections to be used with each material (with some reasonable allowance):

    1) S235 mild steel:
    - minimum square hollow section (SHS): 150x150x5 mm
    - approximate weight of each davit arm: 94 kg

    2) S275 mild steel:
    - minimum SHS: 140x140x5 mm
    - approximate weight of each davit arm: 88 kg

    3) S355 mild steel:
    - minimum SHS: 130x130x5 mm
    - approximate weight of each davit arm: 81 kg

    4) AISI 316L stainless steel:
    - minimum SHS: 160x160x5 mm (due to wild variability of published mechanical properties)
    - approximate weight of each davit arm: 101 kg

    It's your choice now. :)

    Cheers
     

    Attached Files:

  6. Ad Hoc
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    Ad Hoc Naval Architect

    D

    SOLAS only states UTS for life rafts etc simply because these are used, hopefully, only once! Which is also why the FoS is low.

    For davits that are used regularly one should never use UTS, always yield.

    You need to take into account workmanship, environment, attachments, load paths, fatigue, corrosion, poor maintenance, possible poor repairs later in-service etc etc, hence always use yield, never UTS.
     
  7. daiquiri
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    daiquiri Engineering and Design

    Thanks for that remark AH. It's a good food for thought, for sure. :)
    However, please also note three things relative to this particular case:
    1. by using a FOS of 4 relative to UTS of a mild steel, one gets a FOS of at least 2.6- 2.8 relative to its yield stress, to be multiplied by the dynamic accelerations factor of 1.5 (see the post #9). At the end you get an overall FOS (relative to yield) of around 4, which is pretty much what is commonly used in the yachting industry. And in some cases it is even much higher than the common practice. Riggers, for example, use a FOS of 1.5-2.0 for calculating ropes, sheaves and fittings on competition sailboats, and 2.5 on cruising yachts. The rest of the structure often follows that philosophy (I realize that this might start a fierce discussion now). See this old post of mine on that regard: http://www.boatdesign.net/forums/sailboats/forces-fittings-48753.html#post660686.
    2. A FOS of 4 to UTS (2.6-2.8 to yield) imo places the structure inside a pretty much safe range of stresses with regard to fatigue (35-40% of yield stress).
    3. The position of the RIB appears to be pretty much out of reach of violent hits by waves. And plus, I have placed a warning back in the post #9 against having a man on board the RIB during the hoisting operations. I believe that the davit layout and the transom configuration of this catamaran allows a lowering and hoisting without a sailor on the RIB. Should a person be on board the RIB during hoisting/lowering, the applied FOS should certainly be higher.
    All imo of course, but I am always open to suggestions - especially when they come from an experienced NA like you, my friend. I would be interested to learn what values of FOS would you use in this case.

    Cheers
     
  8. rasorinc
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    rasorinc Senior Member

    Alternative to galvanizing MIGHT be to powder coat the steel parts. I have no knowledge though of the durability of the coating in salt water.
     
  9. daiquiri
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    daiquiri Engineering and Design

    By the way, Ad Hoc, I always have this problem with AISI 316L - if you take a look at spec sheets from 10 different manufacturers, you will find 10 different values for both yield and ultimate stress. Different by a big percentage. I have seen values of declared yield stress ranging from 170 to 260 MPa and UTS anything between 460 and 590 MPa. That's crazy, considering that the material's composition is defined to a pretty narrow range.

    Just a couple of examples:
    http://www.efunda.com/materials/all..._Type_316L&prop=all&Page_Title=AISI Type 316L.
    http://www.stal.com.cn/pdffile/316316l317317l.pdf
    http://www.globalmetals.com.au/_pdf/Stainless_Steel/Stainless_Steel_316.pdf
    http://www.aksteel.com/pdf/markets_products/stainless/austenitic/316_316l_data_bulletin.pdf

    In the latest calculations report for a lifeboat davit I had submitted to the CS, I even had to omit the value of the yield stress for components made of 316L because it was too indeterminate. I had just stated that the UTS is 500 MPa, and they were happy with that.

    How do you explain this huge scatter of certified data relative to AISI 316L, and what value do you take in your calculations, before builder has bought the material?

    Cheers
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    In essence it is quality control, most figures are "sales" figures.

    Joe Bloggs down the road if he wants to buy SS he can get some...but what garde is it. Don't care, just want SS. Which is rather different from, I want SS..what grade...316L...ok we have some of that, here you go. Only to find when it arrives and fabricated, it doesn't behave as you thought. Which is again different from, I want SS ok..what grade..316L..and I want a certificate. Ok it arrives comes with a cert. Hmmm..you notice the cert is either out of date, or a compliance with a "body" you're not familiar with or simply stating that it was "witnessed" only; but by whom?. Ok..you want SS 316L, but with current certification and the certification has the plant the process the source of the raw materials the materials being identified witnessed and then the same raw materials being alloyed and then the process and equipment used all witnessed and verified and then the final alloy is independently tested to ensure that the final alloy has XX properties and is consistent etc etc. and is tested regularly. Aaahh..that's the 316L I want.!!!

    As you can appreciate there is a significant difference in availability and of course cost from the first example of SS 316L to the last. Which is where the mismatch occurs and what is actually being "sold".

    I always, always use Class values when designing, never ever sales values. Reason, not just because some 90% of my work is all Class certified, but because of consistency. You need to ensure that the values you are using are real and not an elevated "sales" value. And I also stress to my clients to buy proper Class certified alloy, never off the back of a lorry type stuff which is cheaper and has "some" paper work!!

    Typical LR & DNV Class values are noted here:

    LR Stailess Steels.jpg DNV Stainless Steels.jpg

    You can see even they have a slight difference; 195MPa & 170MPa. But it would be owing to the mills source and verification of raw materials and processes etc. The "fudge factors" used in each Class rule when calculating the design allowable stress limits usually equalise in that the final scantling ends up about the same.
     
  11. daiquiri
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    daiquiri Engineering and Design

    I understand, and it makes perfect sense.
    But do you keep using the CS minimum values even if you are given almost double those values by the actual supplier of the materials, backed by an inspection certificate?

    See this example: http://www.yokogawa.com/eu/an/analytical/eu-certificates/Heatnr253465.pdf . Found it in internet, but I have similar originals in the office, relative to a Duplex plates. In this case, it is a inspection certificate relative to a 316L round bars.
    The results show following measured values: Rp0.2 = 472 MPa, Rm (UTS) = 662 MPa.
    That is 177% higher value of yield stress and 36% higher UTS than what is indicated by the LR, for example.

    What do you do in this case? Do you take advantage of these data and make a somewhat lighter structure, or do you ignore it and go for CS minimum values?

    This is a doubt I am ultimately facing pretty often. Till now I had always opted for the CS values, except for one case in which the piece didn't pass the FEM calculation by a tight margin and the shape couldn't be changed. In that occasion, we have used the certificated values (much higher than the CS minimums) given by the steel manufacturer, and managed to get the approval.

    Cheers
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    Yes.

    You need to look at the cert. Does the cert state it complies with the Class rules you are using??...99% of the time it will be no!

    Thus always always err on the side of caution when in doubt. No one will thank you especially the lawyer interrogating you in a courtroom after a structural failure that cost someone their life when asked why did you use twice the value...er...the cert said so...ok Mr.D..wbut WHERE does it say it complies with LR or DNV rules??

    There's your acid test!

    QA comes in many forms. One of them is yourself. Can you prove to yourself that the material is high quality and does what it says on the tin? If the cert seems fine, you could elect to use it. But how do you know? If you want to be sure, get a coupon and mechanically test it yourself independently and then have the test witnessed by the Class surveyor, or other as your "proof". That's why a Class approved alloy with its stamped approved and witnessed Class cert is more expensive and often takes longer. The QA side has been done for you. Thus which to choose??...pay your money and take your choice! Class certs also come in different categories depending upon the amount of witnessing and testing that has been done.
     
    Last edited: Mar 24, 2014
  13. daiquiri
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    daiquiri Engineering and Design

    Yes, very clear. And in fact, the only time we had to use mechanical resistance data supplied by the manufacturer (the FEM failure case in the previous post) a coupon testing was performed and CS inspector's stamp was put on it. The design was approved, so I guess that's sufficient when it comes to lawyers. :)

    Thanks AH. Chatting with you is always very instructive. :)

    Cheers
     
  14. Barry
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    Barry Senior Member

    While the I beam can be sized easily for a vertical load its load carrying capacity deteriorates quickly when loads are applied laterally.

    Using 5 inch standard steel I beam, 10 pounds per foot, 5 inch web height, 3 inch flange width and .210 web thickness as an example

    If a vertical load is applied on the beam which causes a 10,000 inchpound moment when the I beam web is vertical, the resultant stress in the outer edge is 2,032 psi
    When the web is horizontal, the same moment will result in a stress of 12,295 psi

    So as the load is changed from vertical "y" to a horizontal "x" axis the load carrying capacity of the beam drops by a factor of 6 times



    With the square tube the area moment of inertia is the same in both vertical and horizontal loadings. Certainly as the load is applied at angles off vertical the neutral axis changes with a change in the direction of the loading with a corresponding change in the stresses but it is not as significant as the load carrying capacity as an I beam as loads go from vertical to horizontal as shown by the stresses in a purely vertically loaded beam and a purely horizontally loaded beam

    In the early 70's, an instructor had told us that a 3 degree in loading of an I beam ( but might have been a channel) can reduce its load carrying capacity by 10%
     

  15. oceannavigator2

    oceannavigator2 Previous Member

    The 160-200kg weight is too much.

    I have a bad habit of instinctually putting in the FOS, which is contributing to this excess design weight. The actual measurements are:

    Davit beam length from pillar/post: 1m
    Davit beam length across roof to pick up bulkhead: 2.5m
    Beam of dinghy: 2m
    Weight of dinghy: 150kg including motor and fuel (no passengers ride up)

    Maybe to save weight on the davits, also it is a good idea not to have a FOS of 4x the weight of the dinghy loaded with water. Maybe just 1.5 or 2x maximum FOS for this very rare occasion? Or maybe I prefer it just break off in this case of 2x the weight fillee with water? The davits seem excessively heavy and strong compared to any I see on the market that are available and work fine in these applications.

    I will go back and play with post #9 some more.
     
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