specifying keel bolts, floors and structure

Discussion in 'Boat Design' started by anthem, Jan 31, 2012.

  1. michael pierzga
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    michael pierzga Senior Member

    I say use your model to test engineering. Common sense

    The amount of lead you are hanging is small. Keel bolt scantlings arnt a worry.

    I would be more concerned about the load concentration on the hull at the keel attachment.
     
  2. anthem
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    anthem Junior Member

    The internal hull structure to dissipate the keel loads into the hull is another area altogether- but one I am much more familiar with.

    I am proposing to fit substantial transverse floors corresponding to each pair of keelbolts, tied in with longitudinal stiffeners which run more or less the length of the hull- or at the very least from the forward-most ring frame supporting the forestay to the aftermost frame supporting the backstay chainplate. the transverse members will be tied in with laminated ring frames which run all the way up to deck level, and the forward-most keel floor/ring frame will tie in with the main bulkhead/ring frame supporting the main shroud chain plates. In addition the longitudinals can be heavily beefed up in the area of the keel floors and have a considerable vertical dimension, as they will also form the bunk fronts in the main cabin.

    In addition to the obvious need to make calculations to support all the timber scantlings, there is a great deal of mileage in making comparisons to similar vessels to support these calculations, which I intend to do as the project takes shape.
     
  3. michael pierzga
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    michael pierzga Senior Member

    Sounds logical.

    I dont like NUMBERS. I pefer to build a model and then break it to see how the loads are handled then use common sense scantling

    I would think the a bulbed steel blade with a broad keel flange would work. Possible to build a composite bulbed blade and flang if you are worried about dissimilar metals .

    How heavy is the proposed keel ?
     
  4. anthem
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    anthem Junior Member

    The proposed fin and bulb is to be 1250kg total.

    The only trouble is that the nature of the deadwood I will need to use to keep the hull lines fair means that the fin (fabricated steel) can't be flanged- so there is a limit to how far off the centreline the keel bolts can be-

    From reading elsewhere on the net, and in the eliasson and larsson book, the load on the keel bolt is a product of the VCG of the keel from the root, the weight of the keel and the distance between the leeward 'bearing' edge of the root in compression on the hull, and the centre of the windward keel bolt which is under tension.

    My logic is that a worst case scenario would be to calculate the loads for a design attitude of 90 degrees, with the keel in air rather than water. On this basis, after some initial rough calculations on the back of an envelope, it looks as though only ONE 3/4"UNC keel bolt would be nearly strong enough- by the time several per side are introduced, significant margins for error start getting built in. What I still don't know is how big those margins for error need to be, particularly in respect of dynamic as opposed to static loads.

    Regarding the earlier response about the shock loading properties of lead vs cast iron- the keel is to consist of a hollow fabricated steel fin (with substantial internal web structure, keel bolts carried down through and secured to several horizontal webs) and fitted with a cast lead bulb.
     
  5. michael pierzga
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    michael pierzga Senior Member

    The problem I have when even thinking of your keel is the shape of the bottom of the boat.

    You should start sketching possible keel profiles and flanges.
     
  6. sorenfdk
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    sorenfdk Yacht Designer

    You've got it all right! Use a heel angle of 90 degrees and a safety factor of 5.
    Using only one bolt is a very bad idea. If it wasn't, you would see a lot more of it! I would go for 8 bolts.
     
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  7. Paul B

    Paul B Previous Member

    This might be the worst advice given on this forum in the past year.
     
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  8. anthem
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    anthem Junior Member

    I can't see how you could arrive at a sure-fire correct solution without numbers-

    But there is validity in testing things in model scale? I have built a scale model of the boat to test my calcs on floatation, particularly with the addition of an inboard diesel engine, the weight of which wouldn't have been there previously- I hope that the model will float to its marks without any tweaks, that would indicate to me that I've got it right. I haven't yet determined the position of the tanks, batteries etc, so the positioning of these could potentially be used to my advantage to correct small errors in trim. If I've got it really wrong (although I will be extremely surprised if this is the case) I can try moving the fore/aft position of the bulb.

    Secondly, when the model is finished it can be radio controlled and sailed in order to check the ce/clr are where they should be.

    I have to say though, I wasn't really planning on breaking the model to see where it breaks... I really would rather do some sums to work that out :)
     
  9. Tim B
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    Tim B Senior Member

    Get real, numbers are great; why do you think we are where we are today. Numeracy and Literacy are the two most important skills in society.

    Anyway, building structurally correct scale models is difficult. You should be able to work out where the load paths are by inspection if you are a decent engineer (and it sounds like our boy, anthem, is at least experienced). From that, the requirement is to make sure the load paths (ie. the structural components) are big enough to take the loads, without being oversized.

    Tim B.
     
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  10. michael pierzga
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    michael pierzga Senior Member

    Paul, I'm certain that you need pages of numbers to make up for your lack of commonsense !!!
     
  11. Paul B

    Paul B Previous Member

    Lucky for you no numbes are needed to wash boats for a living.
     
  12. Tim B
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    Tim B Senior Member

    Come on guys, this isn't conducive to the discussion.

    Tim B.
     
  13. gonzo
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    gonzo Senior Member

    Without numbers you couldn't even buy the bolts. What are you going to say: send me some bolts, any will do?
     

  14. Wavewacker
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    Wavewacker Senior Member

    And what is wrong with a break away keel when you go Oooppps?
     
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