Cantilevered Davits - Engineering Problem

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

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  1. oceannavigator2

    oceannavigator2 Previous Member

    Hi,

    Could someone help me figure out the right material and scantlings to use to make a set of cantilever davits, such as these?

    The RIB will weigh 500-600 lbs with outboard and fuel tank, assuming it isn't full of water!

    Possible thoughts are 1) Buy some pre fab fiberglass square tube in vinylester and glass those on. No idea how to spec it to the supplier. 2) Use solid foam core and glass it. Plenty of uni along the length. How much glass? 3) Construct a hollow, but cored square tube in house.

    Any ideas about the easiest and best way to do these davits?

    Not really a math guy.

    [​IMG]

    [
     

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  2. Mr Efficiency
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    Mr Efficiency Senior Member

    It looks like a job for Alumin(i)um alloy. Probably "I" beam. Aesthetically may not look too flash though.
     
  3. daiquiri
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    daiquiri Engineering and Design

    On-board davits are a particular kind of animals, from both engineering and normative points of view. They can be potentially dangerous and even deadly things if not properly designed and constructed. And frankly, judging from your questions, I think that you need a professional help there.
    Several things have to be assessed before undertaking this project, like:
    1) is the deckhouse structure suitable for loads created by a davit of this type?
    2) does the boat have sufficient stability margin for heeling moments created by the davit (if the davit extends sideways)?
    3) was the vessel classed by a classification society (CS)?
    4) shall the davit be approved by a CS?
    If the answer to either n.3 or n.4 is yes, it will imply a number of mandatory requirements by CS which shall almost certainly require a professional designer to help you in this project.

    That said, can you tell a bit more about the boat which should host the davit, perhaps with a couple of pics and dimensional data?

    Cheers
     
  4. groper
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    groper Senior Member

    What if the bungs are left in and it fills up with rain water? How much does it weigh then?

    Are we going to design it so that in this worse case condition, the ship sets sail as has the dynamic load of it aswell?

    I'm not a professional, but I would approach the problem as follows;

    Assume the rib is not full of water.
    Design the beams to take a dynamic load of +-1g for ocean going vessels, less if a river boat or similar. So upto double the static weight.
    Design the beam to keep maximum deflection less than 3% span as in glass it will be deflection driven rather than strength driven.
    Apply safety factor of 3.

    This should have no problem if rib is full of water and boat is stationary.

    To design further, need to know dimensions, and as daiquiri said, deck house needs to be able to take the loads transferred from the horizontal members.

    What's the "roof" made of? This may also need to be structural to support lateral loads as the boat heels. For this reason also, I would not Consider I beams either... RHS or similar would be a better choice for the non vertical vector.
     
  5. rwatson
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    rwatson Senior Member

    Lets assume a 3 mtre lenght, 1.5 metre wide, .2 metre deep hollow - thats .9 cubic metres, or 900 kilos, nearly a Tonne, or Short Ton

    Add about 200 kilos for the boat itself, and you have a heavy beast.

    The other problem coould be a lifting system that could get a swamped RIB from the water to storage, so the lifting system needs to be substantial..

    These Stainless Steel ones, the material you will need to look for ( Aluminium aint gonna cut it ) - hold about 100 kilos each, so you are goign to need better than these
    http://www.go2marine.com/product/92879F/stainless-dinghy-davit-system.html
     

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  6. SukiSolo
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    SukiSolo Senior Member

    I think I would add a body (ie person) maybe two at least to the RIB full of water. Where is it intended to lower it? From inside the RIB? or on deck?.
    Someone might get into it and drain it or check it starts/turns over, prior to full lowering. It will also be a bit one ended load wise because the engine will be the heavy lump at the stern. It would not be unheard of to have an 'elephant trunk' fastened up or blocked before now....

    The main cross beam(s) is not per se a hard calculation, however I fully support daquiri's post and would sugest getting a professional to look at it. There are other design considerations involved especially the ergonomics of using it in a rolling sea without causing injury etc. If you need certification, surely it is a must to get the design looked at properly?

    I would not be too fussed about material, hell even timber could do it fine! 'I' beam, tube both circular and rectangular can do the job if the correct size. It is important to recognise that the load will not always be vertical with the motion of the main vessel.

    Some more pics would be helpful.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    If weight is critical, alloy is far more appropriate than SS, not to mention way cheaper, and " I " beam is quite capable of dealing with loads in more than one direction.
     
  8. oceannavigator2

    oceannavigator2 Previous Member

    A lot of thoughtful responses. Thank you. Where to begin? There were a lot of questions, so let me start by answering them.

    The boat is a private one. Catamaran, so righting moment and class society not an issue. Just like every other catamaran you see with a dinghy on davits. Certainly, the steel davits rwatson shows in his post, or similar, could be used. The roof is 1" corecell with 34oz e glass on both faces. With the help of the posts and local reinforcements, it is able to take the boom's load.

    Want to use the cantilevered beam since it is a more elegant solution than metal.


    [​IMG]

    [​IMG]
     
  9. daiquiri
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    daiquiri Engineering and Design

    The first photo shows a transom which identifies it as Bahia cat: http://sailboatdata.com/viewrecord.asp?class_id=5003
    It makes me guess that you probably do not carry more than 10-12 pax during your trips, so you are not subject to SOLAS. Which means that you have a plenty of free room (free of mandatory rules) for playing with numbers. That's a good start. :)

    The first thing that I am noting is that your tender looks like a 3.5-meter model. The worst case for a davit scantling (and for your vessel buoyancy and stability) will happen when your tender is filled with water and you are in a seaway. It can happen if a storm catches you while at the open sea, for example. You could cover it with a canvas, but does it guarantee that it won't fill with water under any circumstance? I think it doesn't.

    A 3.5 m RIB can hold some 1.0 cu.m of water, the weight of which sums up to the weight of the RIB. So we are talking about the total of around 1.3 tonnes hanging out on your transom. This will be the SWL (safe working load) of your davit. A common practice in the design of merchant-ship davits is to multiply the SWL by a coefficient which takes into account accelerations of the boat in a seaway, and which can typically have a value of around 1.5 . In absence of similar dynamic-loads data for pleasure yachts, I would stick to that practice.

    So the design load for the scantling of the davit structure shall be approximately 2.0 tonnes.

    Now the crucial part - what design stress should you use for the scantling? If it were a davit subject to SOLAS and CS rules, the maximum allowable stress in any point of the davit structure would be assumed equal to the ultimate stress divided by 4.5 . For ropes, the mandatory safety factor (FOS) is 5.0 and for loose items it is 6.0 .

    If there will be persons in the RIB during the hoisting/lifting operations, I would warmly suggest you to stick to the above FOS values.

    If the RIB will be hoisted/lowered with no persons on board, then you can relax these FOS values and save some weight. In that case only, you can use a FOS of 3.5-4.0 (on ultimate stress) for the davit structure but should stick to FOS of at least 5 for ropes.

    This leads you to a choice of a 10 mm polyester rope.

    Now let's try to calculate main loads acting on the davit and on the supporting deck structures. I will start by considering a possible preliminary arrangement of the RIB and of davit arms:
    Bahia davit 1.jpg
    All dimensions are in meters (sorry for that, if you are an "imperial" guy :p ) and are estimated from the internet pics of Bahia 46. So they have to be confirmed through actual measurements.

    With these dimensions and scantling weights, the reactions on the catamaran structure can be calculated. They are shown in this pic:
    Bahia davit 2.jpg

    All the numbers have been truncated to 1 decimal place (this is an approximate analysis anyways). The reaction loads of 3.7 t and 1.7 t are distributed between port and starboard columns, and hence each column will see half of that load. There will be lateral force components too, due to dynamic accelerations in a seaway, but if the RIB is well secured against lateral motions, these forces will be transmitted to the lateral hulls by the lashing system. So for a time being, they can be omitted from the calculations of the davit arms.

    The first thing that you should notice is that the vertical reaction force can have a pretty high value. Approximately 1.9 tonnes will push on each column of the rooftop, so the first thing to address is the structural resistance of the rooftop and whether it can bear these loads. If the structure you thought to use is similar to this one (red box):
    Bahia 46.jpg

    then the answer is straightforwardly - NO. You need a new rooftop, or an additional and much more beefy structure to serve as the basement of the davit arms. That is the first important conclusion. This is where a professional survey of the boat is recommended, in order to establish the best strong points of the boat structure to which the davit supports can be mounted.

    The next point is - what materials do you want to use for the davit? The simplest and least expensive solution would be to use standard square-section stainless steel tubes. A 120 mm square section with 5 mm wall thickness could be sufficient, from the preliminary calcs. With a careful design and good metalworking, they can be made to a very nice, shine and attractive shapes, which can even add to the looks of the boat. But that is a matter of personal taste, of course.

    Other options are GRP or carbon laminates (but that means a much higher cost and engineering complexity), or buying a custom-designed and built davit from a specialist firm, like this one: http://www.femstrutture.com/products/FEM-Davits/Gruette-Cranes/Davit-Crane.html

    Enough from me for now, I believe this will give you a plenty of stuff to think about in the next couple of days. :)

    Cheers
     
    Last edited: Mar 24, 2014
  10. oceannavigator2

    oceannavigator2 Previous Member

    Amazing reply, Daiquiri. Thank you. I looked at cantilever diagrams just like the ones you drew up with the real loads on them. I was not sure how to get the numbers to work.

    I did have a good laugh at the last picture with the red box. :) Not even in my imagination could I picture that structure working. Definitely -NO.

    The roof is rather just the normal deckhouse roof, continued aft to the transom. It is the same roof we see in the last picture you posted that is above and touching the windows along the port side. The real cabin top roof is simply extended all the way back with full laminate of 34oz glass, 1" corecell, 34oz glass.

    I will work on going over the engineering post today. And thank you. This is a great start.

    PS: One of my best friends in the world is from Treviglio in Bergamo. This is not so far from you?
     
  11. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    By fixing the davit arms more forward (towards the cabin rooftop, as by your indications) and more backwards (see the attached drawing), the reaction forces decrease considerably. See the attached pdf.

    Treviglio is 3 hrs drive from where I am now (near Trieste), but I know that part of Italy very well. I have lived some 50 km from there till a year ago. :)

    Cheers
     

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  12. oceannavigator2

    oceannavigator2 Previous Member

    That is a big reduction and a layout that seems more realistic. I will get real dimensions.

    I think one point that should be considered is this-

    Which is designed to fail first? The davit arm or the entire roof of the deck house?

    I am inclined to say the davit arm.
     
  13. daiquiri
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    daiquiri Engineering and Design

    If the rooftop fails, davit inevitably fails too. So you lose both.
    So let the davit fail first, at least you'll still have a shelter. ;)
     
  14. alan white
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    alan white Senior Member

    Steel might be a good choice. Well epoxied against rusting of course. Here's why: Cost will be very low compared to other materials. Constructions can be open, meaning windage will be low using pipe or tube. Any good welder can do a masterful job so no specialized technology is going to make for problems. Steel has wonderful properties when it comes to failure, meaning it maintains a lot of strength when bent or damaged (witness how steel hulls manage reefs, etc.).
    Finally, aesthetically, steel can be pretty nice to look at due to the way curves can be built into the design and more elegant and lighter looking designs can be done. Ditto for aluminum, though cost will be much higher and welds not as strong without proper treatment.
     

  15. rwatson
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    rwatson Senior Member

    As I alluded before - steel is the optimum method for this type of construction.

    But - there is no reason it cant be covered in foam/glass for windage, safety, aesthetics and rust protection, and will look like the preferred solution.

    If the internal hidden structure is galvanised, you can save a heap of money over stainless, and stainless welding.
     
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