plating thickness?

Discussion in 'Boat Design' started by rlackey, Jun 24, 2005.

  1. sorenfdk
    Joined: Feb 2002
    Posts: 511
    Likes: 27, Points: 28, Legacy Rep: 394
    Location: Denmark

    sorenfdk Yacht Designer

    Exactly. Except that people who don't know enough about the problem, also don't know that they should pick a greater safety factor.
     
  2. rlackey
    Joined: Jun 2005
    Posts: 49
    Likes: 1, Points: 0, Legacy Rep: 19
    Location: Cape Town, South Africa

    rlackey Junior Member

    Okay! now were getting somewhere, this is all great input!

    I'll post some renderings of the hull tomorrow.

    Rich
     
  3. rlackey
    Joined: Jun 2005
    Posts: 49
    Likes: 1, Points: 0, Legacy Rep: 19
    Location: Cape Town, South Africa

    rlackey Junior Member

    Here's the hull, I posted these before in a different thread. Tad, I would appreciate some opinions.

    From what you have said so far I've got two problems (challenges sounds a bit better ;) ) with this design, firstly the lack of a single deck running the whole length of the hull as it has a raised deck forward and a lower deck aft. Secondly is the small freeboard at the stern, which may get pretty close to the water at a large degree of heel.

    It is inspired by a 1930's style power yacht, and I loved the raised deck style but the above concerns may be good reasons why I have never seen a sailboat like this.

    It has two full decks below the sheer line, and a third superstructure deck above plus the pilothouse.

    Stability has also been an issue, but I may get rid of the pilothouse on the uppermost deck as this will bring the C of G a bit lower and increase stability.

    This is not a self righting sailboat though! If it capsizes, it's staying that way!

    However, I'm not about to give it up without a good fight. I think it could be a beautiful design with heaps of space inside and out.

    Rich

    Oops, I wanted to add that as you can see I haven't designed a keel yet, when I said between 5 - 6m draft, that was board up also, you are very right, it would need to be more like 8m with the board down. So much for taking her into shallow waters. I am looking into a few more exotic keel designs that may allow a shallower draft.

    Before I can really calculate loading from the rig accurately I need to design it first, along with the keel, so it seems I still have a ways to go yet.
     

    Attached Files:

  4. Tad
    Joined: Mar 2002
    Posts: 2,307
    Likes: 191, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    Rich,

    Okay, cool boat.

    You are the designer so you get to decide where and how to compromise. A big question is what is this boat? Is she a charter yacht, (with MCA compliance implied?) and is she a true sailing vessel or a sail assisted power boat? Why would someone want this boat, what does she do that no other currently available yacht does?

    Where are the edges of "modern interpretation"? The double-ended form is lovely at low speed when it will leave no wake. But modern interpretation (for me) implies better performance and thus higher speeds under both sail and power. This form is less than ideal in that light, and I would really like to see more bearing (width) aft to damp out pitching.

    Where does the mast go, behind what I assume is the pilothouse? What about sheeting and staying of the mast and sails? Is there a bowsprit? Have you taken a look at Dellenbaugh angle and deck edge immersion? What about a ketch or schooner rig?

    My guess would be that the aft deck is too low. Does it have full bulwarks? I would solve the structural problems by extending the upper (boat?) deck full length. On the other hand if the existing main deck level extends full length then you only need a shear wall (diagonal tie) amidships between the upper and main deck.

    Big dinghies and toy space are required on modern boats, I get those positions and any possible holes in the topsides worked out before getting too deeply into construction.

    If she is to be a "real" sailing vessel I would be inclined to drop the upper house 1/2 level. This would reduce windage, lower the rig and sail centers, and perhaps give you the opportunity to have a flying bridge aft of the PH.


    Tad
     
  5. rlackey
    Joined: Jun 2005
    Posts: 49
    Likes: 1, Points: 0, Legacy Rep: 19
    Location: Cape Town, South Africa

    rlackey Junior Member

    Thanks for the input Tad,

    Okay, she's a personal design that I would one day like to build just for my own use and pleasure should I ever have the financial resources to undertake construction of such a large boat. I would like to think that when refined and finished, she would be of interest to others largely due to her styling more than anything else.

    I have not been designing her necessarily for any commercial purpose, or with any thought to selling the design to a third party but I intend to comply with MCA regulations regardless.

    I first saw the basic design on a motor yacht, and I loved the lines, the space and the layout, but I wanted a sailing yacht not a motor yacht. The idea is that she will be a full sailing yacht, not a sail assisted motor yacht, and in fact, as with the Tiara, her mast height would also be around 60m.

    Yes the mast would be directly behind the pilot house, but the more I'm looking at it, the more I think your idea is a good one in terms of dropping the upper house half level and having a flying bridge aft of the PH, I am thinking of just getting rid of the PH altogether. Dropping the upper house half level will sacrifice using that whole internal space as another full height deck, there is plenty of space though, so maybe this is a small compromise.

    I suppose there is not much modern interpretation in there yet. I have played around with different designs at the stern, including a much more modern full transom that had much more width, perhaps I should revisit it. Again I like the aesthetics of the narrower stern, but I see what you are saying about how it limits performance. I'll work on it.

    Yes, there will be a bowsprit, and the main deck does extend the whole length, so that does indeed help with structural rigidity.

    I have also considered other rigs, I don't have my heart set on a sloop, or even on any single aspect of the design as it currently stands, it's a work in progress and you have been a great help so far!

    If I am honest, if I were to build her, I would probably be retired and I would live on her and I am not sure how much open ocean cruising she would ever see, but I don't want a glorified house boat that looks like a sail boat, I want a sail boat that is fully capable of open ocean cruising at speed, and to sail well, so she needs to be designed as such.

    I'll be working on the hull form some more now.

    Thanks! :D :D

    Rich
     
  6. rlackey
    Joined: Jun 2005
    Posts: 49
    Likes: 1, Points: 0, Legacy Rep: 19
    Location: Cape Town, South Africa

    rlackey Junior Member

    Looking back through the 34 revisions to the hull form, the submerged portion hasn't changed dramatically at the stern even with the more modern transom, it wasn't in fact any wider than it is now, and as far as I can tell from what I have been learning in 'principles of yacht design' and looking at more modern hulls ,the below waterline form at the stern is not all that different to the modern forms, it just has a different form above the waterline, but surely this shouldn't impact anything more than aesthetics? Of course it isn't a submerged transom, but I thought this was only found on motor yachts. Perhaps the renderings don't show this well. I will post some more specific to this area though.

    I will also check the hull at up to 40 deg of heel and see just how close that lower aft deck is to the water. I know I have done this before, and as far as I remember, though it is low, it wasn't submerged. If I were to add width to the stern, it would however probably become a problem.

    Rich
     
  7. Tad
    Joined: Mar 2002
    Posts: 2,307
    Likes: 191, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    Rich,

    I would expect the prismatic (canoe body) to be somewhat low (.52-.53) with the double ended form. Pulling more volume into the ends will raise this to maybe .55-.56, but it means the waterlines are rather full aft, with a sharp zig towards the centerline after station 9. This is really hard to manage with the double-ended form above water. What about a clipper style fantail? It would still be a problem with the waterlines but will provide more width and volume above the water.

    MCA compliance requires watertight bulkheads and a floodable length study, with minimum freeboard requirements when various compartments are flooded. Absorbing work if you can get it!

    All the best, Tad
     
  8. rlackey
    Joined: Jun 2005
    Posts: 49
    Likes: 1, Points: 0, Legacy Rep: 19
    Location: Cape Town, South Africa

    rlackey Junior Member

    Hi Tad!

    Well, I know for certain that the prismatic coefficient of the canoe body is 0.56 already, this was the figure I wanted and spent a long time trying to reach it.

    I really appreciate your thoughts. Thanks again!

    Rich
     
  9. Tad
    Joined: Mar 2002
    Posts: 2,307
    Likes: 191, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    Rich,

    Well, that shows you what I know! I would be interested in seeing some hull lines, it's very dificult to tell much about the shape from what you posted above.

    Tad
     
  10. rlackey
    Joined: Jun 2005
    Posts: 49
    Likes: 1, Points: 0, Legacy Rep: 19
    Location: Cape Town, South Africa

    rlackey Junior Member

    Hi Tad,

    I hope I haven't caused offense. I meant in no way to tell you that you don't know what you are talking about. If that came across, I apologize, I really do appreciate your help.

    Of course it is almost impossible to determine such specifications from the pictures I posted, and of course my calculations could be wrong.

    I'll post some lines drawings, and if you don't mind taking a look, maybe you can tell more from those.

    I have dropped the upper house half level and things already look better.

    Thanks,

    Rich
     
  11. Tad
    Joined: Mar 2002
    Posts: 2,307
    Likes: 191, Points: 73, Legacy Rep: 2281
    Location: Flattop Islands

    Tad Boat Designer

    Rich,

    No, no offence at all. I try to take this fairly lightly, but forget to add the little grinning faces :D :D

    The calculation of CP will require a first guess at your overall displacement, and that is a huge question, how realistic is it?

    There are three rules for designing sucessful large boats; Watch your weights, watch you weights, and watch your weights. Things like carbon spars, composite or alloy deckhouses, and the electrical system in this size vessel can gain or lose tens of tons. It's what keeps designer's up at night. And why we can demand huge salaries :D

    Take care, Tad
     
  12. JimCooper
    Joined: Feb 2005
    Posts: 81
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Varies, Aberdeen

    JimCooper Junior Member

    Rlackey

    I hope this is just tilting at windfarms

    Have you calced the windage on all that superstructure? Theres a lot of similar sized deep sea fishing vessels that wouldn't throw up such a lot of frontal area simply for the extra fuel you have to burn..

    Your anchor load in a blow is going to be horrific. You have designed a small pretty looking ship. Take some advice Forget the sailing keel. Your're only going to sail that broad off anyway. To windward the superstructure is just one large sail constantly aback .
    You'd better throw in a couple of good big donks, she'd never claw to windward in any conditions no matter what keel you gave her.

    If you want a sailing vessel you need a lot of changes, any designers here are remiss if they don't tell you this.

    My humble opinion
    Jim
     
  13. kmorin
    Joined: Apr 2005
    Posts: 185
    Likes: 18, Points: 0, Legacy Rep: 231
    Location: Alaska

    kmorin Senior Member

    beginning point

    rlackey, I suggest that you get Dave Gerr's book The Elements of Boat Stength (ISBN 071365287X) which uses a 'scantling number' derived from the displacment, length, breath etc; and that number is used throughout the book to calculate plate thickness, frame depth, flanges and spacing as well as most other nominal frame element's. It is very detailed and exhaustive of metal framing and design conventions are illustrated to confirm interconnections.

    While the book isn't THE final solution to the application of all rules society's when applied to a specific boat, it will give you a "beginning" and that is what you'd mentioned was your current focus.

    You can use the formula's in a spread sheet to work out a very reliable first draft that can be checked against the various international rules' for given members.

    This method (using Dave's book) starts you right out with a conservative practicing designer's guide to sizing structural components without interperting "rules". Also you could add multipliers in whatever increment you decide to 'beef-up' any member that may require conformance to a higher standard of construction if you found that necessary.

    (Full Disclosure:) I've found the results of Dave Gerr's calculations to be very satisfactory in small planing boats, but I have no experience with anything HALF the size you're contemplating.

    Cheers,
    kmorin
     
  14. kmorin
    Joined: Apr 2005
    Posts: 185
    Likes: 18, Points: 0, Legacy Rep: 231
    Location: Alaska

    kmorin Senior Member

    rlackey, since posting I've looked at the Gerr book and used your posted dim.s for a guide, although I don't have the Depth of your Hull, just draft so I estimated about 5 meters.

    For example, the bottom worked out to 14 mm plate, topside 10 to 11 mm and transom 15 to 16 mm. 9mm decks and 7 to 8mm cabin sides. 650 mm spacing for longs, which are 125 mm deep and 15 mm thick. (assuming a flat bar type for example) The bottom frames would be 436 mm deep, of 14 mm plate with a flange the same width as the ht of the frame's bottom member and a thickness of 18mm for the flange. These frames would be about 175 cm spaced using the formula in this book.

    Just a quick look at the ease with which you could approximate the construction with adequate scantlings as you're likely to go around the design cycle a few more times this approximation will help in hull mass, LCG of the hull sans equipment, and other values for use in the preliminary cycles' estimations and approximations.

    Cheers,
    kmorin
     

  15. rlackey
    Joined: Jun 2005
    Posts: 49
    Likes: 1, Points: 0, Legacy Rep: 19
    Location: Cape Town, South Africa

    rlackey Junior Member

    Thanks guys, I've ordered the book, and the upper superstructure is going through a major redesign. I really appreciate the help. I'll post some new renderings soon.

    Rich
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.