specifying keel bolts, floors and structure

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

  1. anthem
    Joined: Sep 2011
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    Location: essex uk

    anthem Junior Member

    Hi all,

    I have a 1930 built Abeking and Rasmussen 30 square metre which I am beginning to rebuild.

    As part of the rebuild project I am intending to fit a more modern keel and rudder layout, much like that on 'Rumbleseat', the famous 30 square metre from the USA. Pic here- https://picasaweb.google.com/110515486074762033828/AssortedBoats#5644894215062243394

    In the case of 'Rumbleseat' the deadwood was sawn off below the bilge sump, and the new fin is bolted onto the resulting flat surface, with keel bolts into relatively conventional hardwood floors inside the hull. The top of the fin is simply a flat surface with no flange, the width of the fin made to match the width of the remaining deadwood so the new fin is fair with the old hull.

    I have the questionable luxury of rebuilding the entire deadwood from scratch, so I am not constrained by the width of the old ballast or the depth of the bilge sump, therefore I am free to design and build the most hydrodynamically efficient arrangement which will work structurally. I have built a 1/8 scale model of the hull which has shown that with a deadwood as little as 4" wide the hull lines can be made perfectly fair. Obviously from a performance point of view the narrower the deadwood, the better- but the flip side is that a narrower surface onto which to bolt the fabricated steel fin is more difficult structurally, imposing far more load on the hull/keel joint.

    So, I was wondering if anyone can offer any pointers? Working from a completely blank canvas, but knowing the weight of the fin and bulb required, the desired chord of the new fin, and the overall draft-

    Where do I start with specifying the diameter and number of keelbolts and their layout on the top of the fin?

    Knowing that a wider fin for a given chord than would be desirable hydrodynamically would be better structurally in this application, where does the best compromise lie between the two?

    Is there anywhere I can find info on the load required to cause permanent elongation in bolts of a given diameter? In Lbs or Kgs, which I can easily understand and interpret?

    When working out the required load capacity of keel bolts, how much is it normal to allow for shock loadings rather than theoretical maximum static load, which is much easier to calculate?

    Lots of questions, but I think that will do for now... Hopefully somebody can help?

    Thanks!
     
  2. Stumble
    Joined: Oct 2008
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    Stumble Senior Member

    Anthem,

    You really, really need to hire a NA to help you here. You raised some serious questions about keel design and attachment, and that will take some serious work to get right.
     
  3. philSweet
    Joined: May 2008
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    philSweet Senior Member

    What do you plan to do with the boat when you are done? What are your ambitions?
     
  4. anthem
    Joined: Sep 2011
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    anthem Junior Member

    Thanks for the comments- I appreciate that this is an area a professional would often be required to deal with, but if it's a case of either paying for the services of a naval architect or learning to be one, I'd prefer the latter:)

    As regards what I want to do with the boat- A mixture of round the cans and longer passage racing under IRC, spirit of tradition and classics regattas, and cruising over reasonable distances in the UK and Europe.

    I should probably add, though, that it really is a case of thinking of one thing at a time- and although sailing in the above capacities is obviously the long term goal, my current ambition is to make good inroads with the project. I enjoy working on boats as much as I enjoy sailing them and love getting my teeth into a problem.
     
  5. sorenfdk
    Joined: Feb 2002
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    sorenfdk Yacht Designer

    Then you should start studying! Although four years of studying will probably cost you more than hiring a professional...
    Or, to put it another way: You can't learn Naval Architecture or Yacht Design from forums like these! You can pose your questions, and you might receive useful answers, but that's it! And remember: the answers come without any guarantee at all! You will have to judge yourself whether the answers are useful or dangerous - lives are at stake here!
    If you still don't want to pay for professional help, you can try and read Larsson & Eliasson: Principles of Yacht Design and/or Gerr: Boat Strength. These two will take you a long way towards your goal!
    BTW: We need more information to be able to answer your questions! How deep is the keel? How heavy? How far down is the CoG situated?
     
  6. anthem
    Joined: Sep 2011
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    anthem Junior Member

    The overall draft is 2000mm

    The draft of the hull to the bottom of the proposed deadwood is 850mm

    The depth of the proposed new fin is 1150mm from the root where it meets the deadwood to the lowest point of the bulb.

    The chord of the fin at the root is 800mm, tapering to 680mm where it meets the bulb. The aft 150mm of this will be a trim tab, so is not part of the structure.

    The VCG of the entire assembly (fin and bulb combined) is 1000mm from the root of the fin/bottom of the deadwood.

    The combined weight of the fin and bulb is 1250kg. The VCG above of 1000mm from the root is based on the entire 1250kg being in the bulb- in practice it will be slightly higher, but this offers a worst case scenario.
     
  7. Tim B
    Joined: Jan 2003
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    Tim B Senior Member

    Although the mathematics is not difficult in this case (it's just a moment calculation, a loading estimate, and a safety factor); there are various materials questions to answer, and build specs required. This is where a practicing NA (by which I mean someone who's producing designs, not just someone who's been on the course) will be able to advise. There are plenty of Naval Architects around the south coast (perhaps not so much Essex itself). It would be worth your while talking to them. E-mail me off forum if you want me to point you in the right direction.

    Sorry to re-iterate everyone else's advice, but it's good advice;

    Tim B.
     
  8. gonzo
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    gonzo Senior Member

    You can't think of just one thing at a time in boat design. You need to think of it as a whole. Everything is interconnected and affects every other element of the design. It will be an expensive failure if you start working to "make inroads in the project" without a defined goal and steps to achieve it.
     
  9. anthem
    Joined: Sep 2011
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    anthem Junior Member

    Gonzo- I appreciate your advice, very sound words- I'd like to clarify a bit though as it isn't really an accurate judgement of my situation-

    I'm accutely aware of the need to have the entire project planned in advance, and all elements of the design and construction details worked out before proceeding. I have a good knowledge of the principles of yacht design, and have been able to make all other calculations which affect the centre of gravity, flotation and sailing qualities of the boat with the proposed new foils without any problems or the need to ask any questions.

    I'm merely trying to gain information on one small area of this particular project where I feel my knowledge isn't sufficient, hence posting on this forum.

    I'm a professional rigger, not a professional designer- in terms of my interest in design and construction on my own projects as opposed to my customers' yachts, I'm just a guy who likes to get involved in things and push the boundaries of what I know and can achieve.

    My first big project was to a 40' IOR one tonner which I owned at the age of 19. Her foam and kevlar hull had more or less entirely delaminated. I removed her engine, took off her keel, and turned her upside down with two cranes in the yard where I worked at the time. I then built a complete new epoxy/glass/foam hull on her existing frames and inner hull skin, having first removed all the old hull outer skin and core. All the wisdom in the boatyard said this wouldn't be possible. At least two local surveyors weren't very optimistic either, and before I started I knew nothing about what I was doing- but I read a lot and researched from whatever sources I could find, and got the job done. Also during this project I moved the engine ten feet aft, extended the coachroof and shortened the cockpit, fitted a full cruising interior, moved the chainplates, modified the rig with slightly swept spreaders to do away with the running backstays, etc.

    I enjoyed sailing her for seven years, and eventually sold her for good money and with a clean bill of health from a surveyor.

    Between then and now I also built a 28' trimaran from scratch, but that's another story altogether.

    I think when I used the term 'making inroads' it might have appeared that I wanted to make progress with something for the sake of progress, which I agree wouldn't be the way to go. What I'm actually trying to do is make appropriate progress with the correct bits of the project, in the correct order and with nothing left to guesswork.

    I'm not getting on my high horse and have no grievance with anything which has been said, I really appreciate everyone's responses and help- but it is frustrating for me that after the ease with which I have been able to achieve what I want to achieve in the past, I'm now trying very simply to specify some conventional keel bolts on a totally non-exotic wooden boat, and struggling to find the information I need!
     
  10. gonzo
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    gonzo Senior Member

    Looks like you have experience with boats, so there is hope ;) Shock loading depends a lot on the material of the keel. Lead is soft and tends to absorb shocks, while cast iron hits hard.
     
  11. michael pierzga
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    michael pierzga Senior Member

    You have a challenging hull shape. When thinking, keep in mind Gonzos advice about " The WHole Structure" . Somehow you need to transmitt loads from your new narrow base fin into the structure. Your scale model should help...bolt a scale keel to it ...then contemplate.
     
  12. Paul B

    Paul B Previous Member

    As Soren said, Larsson & Eliasson: Principles of Yacht Design can give you some ideas. In my copy (2nd Edition) some very relevant information can be found on pps 239 - 243, and pp 289.

    In your country I think David Hollum may have some experience in doing something similar to what you are considering. You might want to contact him, if for nothing else to look over what you finally calculate to get his (paid) opinion before doing the construction.
     
  13. fastwave
    Joined: Jan 2007
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    Location: europe

    fastwave Junior Member

    Dear Paul,
    I believe you have a beautiful boat and would like to add a few points. There is need to mention that by doing the modifications teh boat will no longer measure as a 30sqm. This is a shame since the class is still very active. Check the number of boats at the european championships and you will be surprised.
    Since you have the $$ and oppurtunity to make significant changes, I would perhaps consider keeping the updates within the class rules. I have deisgned a new 30sqm which due to the recession is on stanby with a couple of owners, but I can assure you the performance increase is a nice step (nowehere near what a fin bulb would be though). A word of warning- this would be a bigger task than the one you suggest. Just drawing a legal boat is a nightmare and quite a few boats out there are illegal in the 1st place.

    One last thing, do not to forget is the righting moment change and its effect on old wooden masts -rigging etc.

    Good luck
     
  14. anthem
    Joined: Sep 2011
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    anthem Junior Member

    The point about the fin and bulb with seperate rudder making the boat no longer compliant with the class rules is a valid one, and something I considered for all of about ten seconds before putting to one side as not a significant consideration. Class racing is not what I want the boat for, in any case there are very few if any other sq metre class boats actively racing in the uk.

    I'm simply trying to recreate a proven concept, very similar appendage modifications have been carried out already to a very similar boat, and she proved to be excellent under modern handicapping systems and fast and practical for passage making. That is the goal, not to be class compliant.
     

  15. anthem
    Joined: Sep 2011
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    anthem Junior Member

    The rig isn't an issue either, the boat will be fitted with a new rig engineered according to the righting moment of the hull with the new keel, not the original.

    Paul, your point is very valid and indeed something I forgot to mention in my original post- whatever the outcome I will certainly get my calculations checked by a professional before I commit to building anything- but if I simply walk into a naval architect's office and say 'please can you calculate this for me', then I have learnt nothing and have no sense of satisfaction having solved a problem.

    I would far prefer to work everything out myself, then ask a professional to run through the sums just to make sure there isn't anything glaringly obvious I have miscalculated or missed altogether.
     
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