Block size for a davit

Discussion in 'Sailboats' started by boony, Nov 9, 2015.

  1. boony
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    boony Junior Member

    What sort of block size should be used for a davit? It's to lift an inflatable with a 5hp on the back. All up about 80kg. The blocks themselves will be triples for purchase but wondering if 50mm or 60mm sheaves would be suitable?

    Cheers
     
  2. hoytedow
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    hoytedow Wood Butcher

    I am no expert so may need correcting. My rule learned from others is to choose rope capable of lifting twice any expected load, then size the blocks to match the rope diameter and lifting power.
     
  3. daiquiri
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    daiquiri Engineering and Design

    The bigger the diameter, the less pulling force will be dispersed into friction.
    The smaller the diameter, the more pulling force shall be required for a given hoisted load.
    It happens because the mechanical efficiency of a pulley decreases with sheave diameter, for a given rope diameter.

    Generally, it is recommended that:
    1) The rope is chosen with a safety factor at least equal to 6. It gives a rope which will be well-suited for long duration (UV resistance is to be considered) and high number of hoists with occasional shock loads.
    2) The sheave diameter is not less than 10 times the rope diameter, in order to have less friction during hoisting.

    In your case:
    1) The load is 80 kg, and since it is a light outboard-motor dinghy, assume that most of the weight will be carried by the aft davit arm.
    2) Multiply by a factor of at least 2.0 for vertical accelerations during navigation in wavy seas. It gives 2x80=160 kg per davit arm during navigation.
    3) Multiply by a factor of 1.25 for vertical accelerations during hoisting in partially protected seas. It gives 1.25x80=100 kg per davit arm during hoisting.

    The above are very small loads. For a 3-fold purchase, a static tension in a single line is 160/6 = 26.7 kg, which is so low that even a 6mm braided nylon rope can easily handle it. With a safety factor of 6, the required minimum tensile strength of the rope is 26.7x6 = 160 kg. A 6 mm nylon rope easily carries 800 kg, so it is more than sufficient.
    A sheave for a 6 mm rope should have a minimum diameter of 10x6 = 60 mm.

    So, the answer to your question about sheaves is - yes, 60 mm will suffice.
    However, bear in mind that the aft rope will require a pull of approxim. 25 kg (if good-quality, 90%-efficient sheaves are used), so a small manual winch will be required for hoisting.

    Hope it helps.

    Cheers
     
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  4. Stumble
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    Stumble Senior Member

    Are you using one set on the bow and one on the stern?

    The reality is that modern ropes are so strong that you don't size for strength anymore you size for handeling size. For heavy pulling I try to stay above 1/4" line to make it easy. Then size the blocks to that.

    As for strength... there isn't a block sized for 1/4" line that can't handle 20 times the expected load from this.
     
  5. boony
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    boony Junior Member

    Thank you all for your very informative replies....learnt a few things. There will be one set of blocks (with cam cleat) at the bow and the other at the stern and usually a person on each side pulling.

    Yes even small diametre ropes now have very high working and breaking loads. The boat owner wants to use between 8 and 12mm line. 8mm will largely suffice for the load but as mentioned there is the handling (comfort) to consider so 10mm might be better for comfort.
     
  6. Stumble
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    Stumble Senior Member

    1.75mm would work for load, terrible for handeling. For this I would probably go with3/8 Tenex. Plenty strong, easy to splice (single braid), cheap, and has a very nice hand. It can also be tapered into 3/16 dyneema if you want to minimize the working line size.
     
  7. daiquiri
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    daiquiri Engineering and Design

    Friction losses will be high for 10 mm rope. They are pretty high on any rope for such small blocks, but a sheave/rope diameter ratio of 60/10=6 is rather unfavorable.

    So, beware of the pulling force which will be required on the free end of the rope. For your load, it might be as high as 20-25 kg, which in turn can be too high for some hands (and backbones too), especially if a wave hits the boat during hoisting/lowering operation.

    So what you might need is a small manual winch like this one: http://www.haacon.de/media/produkte/seilwinde421.pdf, to be mounted on the base of the davit arm.
    It costs $200 and we have used it for some commercial applications. Very nice little contraption. Be sure it is the stainless version.
    Then again, if you decide to use a manual winch, there is no need for a triple-purchase block. A gun tackle or even a simple pulley will do the job. Simpler is better.

    Another thing to take care of - the tensioning of the unloaded block. A double or triple-purchase block with such low sheave/rope diameter ratio will be pretty slack when unloaded, and will probably require an additional weight (like 3-4 kg) on the suspended block in order to make it move downwards.

    Cheers
     
  8. boony
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    boony Junior Member

    I have to admit I don't understand how friction would vary when using a different diametre line for the same sized block. I can understand though that the larger the diametre of the sheave the less friction.

    What strikes me however following this chat is that most sailboats have sheaves completely undersized for the required task in terms of friction.

    Re the manual winch, that looks like something I might be able to use for my lift keel in a larger version.
     
  9. daiquiri
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    daiquiri Engineering and Design

    The main culprit is the internal friction between the rope strands. It is not so hard to understand if you imagine a rope bent around a circular sheave. The fibers in contact with the sheave have a bend radius equal to the sheave radius R (= half sheave diameter = D/2). The external fibers have a bend radius R+d = D/2 + d, where d is the rope diameter. So the external fibers are more stretched than the internal ones.

    During the hoisting, this difference in length translates into a movement of the fibers relative to each other, which creates dissipation of power into heat. The higher the relative sliding speed between the fibers, the higher the power dissipated into heat - with the consequent rope heating and wear.

    So you can imagine that the higher the ratio (R+d)/R, the higher the friction. This ratio can be written as:
    (R+d)/R = (D/2+d)/(D/2) = 1 + 2 d/D.
    So the higher the ratio d/D (or the lower its inverse D/d), the higher the power loss due to friction.

    Check this thread for more info on this, plus some measurements: http://www.boatdesign.net/forums/sa...friction-measurements-calculations-47062.html

    That is pretty much true. Sheave friction and efficiency are too often completely overlooked when block purchase is calculated, which can lead to some nasty surprises later on - like required rope tension much higher than expected and the increased rope wear and failure rate.
     
  10. DCockey
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    DCockey Senior Member

    If there is any possibility of the dinghy filling with water while on the davits I would include the weight of the water in the load calculations. Also if any possibility of a person being in the dinghy while hoisted the persons weight should be included.
     
  11. daiquiri
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    daiquiri Engineering and Design

    Yes, water has to be included if present. Though small dinghies are usually stowed in inclined or vertical position for the navigation, both to save space and to avoid this swimming-pool effect.

    Human operators should not be allowed in the dinghy during hoisting, because too many things can go wrong. If their presence is required, than much more stringent safety factors and sheave/rope diameter ratio would have to be adopted, leading to both costly and heavy davit design. But we did have discussions in the past in this forum, in which boat owners argued that they don't give a damn about hefty safety factors, because its their life anyways...
     
  12. DCockey
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    DCockey Senior Member

    Besides a person being in a dinghy during hoisting, another possibility is a person climbing into or onto a stowed dinghy, perhaps to repair rigging, work on the dinghies engine, etc. Perhaps a worst case situation would be a stowed dinghy in some situations is a person climbing into a stowed dinghy filled with water to bail it.

    Also, even if no one is "allowed" into a dinghy while it is hoisted there still might be the special occasion when it is hoisted with someone aboard.
     
  13. daiquiri
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    daiquiri Engineering and Design

    That is true. But there is a safety factor of 2 on the load, and (on top of it) the safety factor on 6 on the rope and lose items. Add to this a safety factor of 4.5 on davit material (on top of the SF 2 of the load) and you have covered all the casual overloads.
    Needless to say, every davit needs to have a well-visible plate with the maximum safe working load (SWL). It is the operator's care to not overload it.

    Some would also call it - natural selection. ;)
     
  14. Stumble
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    Stumble Senior Member

    You guys are seriously overthinking this. It's an 80kg weight, split in half since each side is lifted seperatly. That leaves 40kg load per side. A block w/ a fiddle on one end and a simple double block on the other leaves 13kg of pull needed.

    Friction, non-static loads, water weight, whatever is all seriously overthinking this. I won't argue that they play a part, but in this application there is no need to redesign from first priniple. The guy has a real actively light weight dinghy he needs to get out of the water using a simple block and tackle.
     

  15. daiquiri
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    daiquiri Engineering and Design

    I prefer describing the general process of choosing the correct block and rope so that the next time someone asks a similar question I can simply give a link to this reply, instead of re-writing it all over again. And anyways, one can never learn too much of something - just too little.

    Besides that, the calculation of the pulling force on the block and tackle takes a bit more than a simple weight/mechanical advantage division, for the reasons explained before. Each turn of the rope requires an extra force due to friction, the multiplication factor being 1/efficiency.
    Again, check this thread for some measured data: http://www.boatdesign.net/forums/sa...friction-measurements-calculations-47062.html

    So, for 40 kg load, the single luff tackle you have described, and averagely efficient sheaves (80-90%), a pull force between 17 kg and 20 kg can be expected. Which is up to 50% more than what you have calculated, and might be too much for some hands. ;)
     
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