Need scantling for 17m motor yacht in ISO SCT or Hullscant will pay for it

Discussion in 'Class Societies' started by linda.vrdoljak, Feb 2, 2015.

  1. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    I understand that these g's popped up from ISO12215-5 formula, but...

    Is the boat design suitable for this speed? The answer is NO. Too much chine beam and too low deadrise. Say, on our interceptor that is very similar size (LWL=13.4m, 52kts), the design g's are much lower (2.5-4.4g depending on calculation method), and measured g's during sea trials are lower as well.

    10g shocks at 1/100 occurrence level (commonly used for structural design) is above the limit average person can resist. In reality, one has to slow down long before that...
     
  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,033, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    5g is about the limit of bearability (is that a word ?) for most people, and you would need to be in pretty good shape physically to withstand it for long. How about "tolerance" instead of "bearability" ? :D
     
  3. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    Actually, the number of g's depend on occurrence level. Savitsky and Koelbel in their paper present the acceptable levels, and show that accelerations follow exponential distribution. For human tolerance on high speed craft usually 1/10 level is used, and for structures 1/100, the difference is about 1.7. So before giving any figures we should reference the 1/n level.

    But regardless of that, I fully agree that 5g is the limit. Stravinsky gives 4...6g @1/10 as physical injury level, and 1.5g for special personnel on short-term function without mitigation.

    FYI: Physically able male can maintain grip 0.6...1.6g by his hands, 0.3...0.8 by one hand.
     
  4. Mr Efficiency
    Joined: Oct 2010
    Posts: 10,401
    Likes: 1,033, Points: 113, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Any boat that routinely dishes out more than 3g accelerations is either being subject to excessive operation, or is unsuitable for the job, or both. It isn't pleasure boating much past 3g, even with the precaution of bent knee joints, and bracing for shocks.
     
  5. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    100% agree. And thus there is no need to design structure of pleasure boat for such g levels.
     
  6. TANSL
    Joined: Sep 2011
    Posts: 6,949
    Likes: 543, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Clearlly this requirement makes the boat enters fully into the field of standards to be met by high-speed vessels, much more stringent than what may be the ISO standards for pleasure boats.
    According to what has been said above, besides the strength of the structure, the fixation of the equipment, etc., that ship is completely uncomfortable for passengers.
     
  7. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    I am sure You would agree with me, this is why it is critically important to hire professional designer/naval architect to do the COMPLETE job. Not just build something and ask for advice on website for some part of engineeing.

    We used to perform parametric study that includes seakeeping in terms of g levels, at early stage of every design.
     
  8. TANSL
    Joined: Sep 2011
    Posts: 6,949
    Likes: 543, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    By other means of communication I am advising linda.vrdoljak that calculations presented by me can not be correct for many many reasons. Since he, as he says, already have an architect, he should entrust the project, the whole project, this architect to develop it.
    I sincerely believe that linda.vrdoljak probably misguided or without sufficient information, does not realize the size and complexity of what he is trying.
     
  9. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    This is typical situation with pleasure boat project, nobody wants to pay for proper complete design work...
     
  10. TANSL
    Joined: Sep 2011
    Posts: 6,949
    Likes: 543, Points: 123, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Indeed this is one of the problems.
     
    Last edited: Feb 21, 2015
  11. linda.vrdoljak
    Joined: Jan 2015
    Posts: 37
    Likes: 1, Points: 8, Legacy Rep: 14
    Location: Croatia

    linda.vrdoljak Junior Member

    Re

    I have to disagree with many of you, for several reasons, I have asked scantling so I can study it, and I said will pay for it. I have asked what drawing you need, no answer.

    Agreed design suggestion under iso team was;
    3g for family type (normal yacht) (30knots?)
    6g for high performance yacht (tops out at 50?)

    And Gentleman, many of American offshore high speed boats builders are laughing at us right now, we discuss 30 vs 50 knots, and they hit 5x higher speeds over "family" type.

    So in my case 20ton (will be less) @ 50 knots gives me 4.8g, I dont see problem not to design it for 6g accelerations, even it doesnt reach 5, and if it gets to 4,5 I have faster boat than Pershing. As suggested by iso-team if vessel is high performance type (6), this came from thickess idea and hull stress. At the begining I left iso graph vs rest of class rules under 3-6-10g accelerations, outstanding comparasson of hull thickness, done by iso people.
    Abs and danish hsc, is not much thicker, either. You can compare the scantlings under same accelerations.
    At the begining I asked just for scantling report to compare the numbers.


    Some had word against Safeheaven hulls, but lets be honest here, no other boat or there is few to hold such abuse.
    https://m.youtube.com/watch?v=_dVkeh7wUm4

    This is what I want and to hit 50-60knots + full interior.

    You dont need anything over 20-25 knots I my opinion, but let make things more fun.

    Its normal for speed boats to have 5-6g, here is military Rib they build here in Croatia, (even 2x the speed on navtek stepped one, I studdied this one for stepped portion) antishock seats wich are (designed to damp up to 6g) shocks on ours. And boat in this proportions gives some 5.75g empty, less if loaded, at max speed, so numbers align. This one does 55+ knots.
    https://m.youtube.com/watch?v=_APsDUNXfVo

    Alik, long thin boat will be faster but I dont wanna sit on top of the motor. It's about the same as Pershing 54, and this one hits 45knots at 26ton.
    I need some deck space, at 4.66m max beam we had problem to fit good catwalk (cant do 50, even not 40cm) and good saloon walkway, even now is tight. This is full interior and not Fountain/ Cigarette semi-interior.

    But this most important there is one big advantage over Pershing monohul, it is stepped hull, it has 3.5ton less water to push aside for each 1m of movment.
    Lets say 30konts 15m/sek 52tons resistance? Yes.

    20t@50knots is 6.2MJ of kinetic energy
    pershing has 6.0
    And here is 7 for Kinetik from Serbia
    32t@42knots.
    http://www.artofkinetik.com/pdf/hedonist-hard-top-brochure.pdf

    I cant affort this kind of propulsion and to go heavy, so I go down in weight to pass 50 speed mark.

    I know I can hit 50-60knots with better engines, surface drive and slippery stepped hull = less triming angle anytime = better speed. It has been done many times, 20m stepped hull yacht (California) hit 60+ with 1 1500hp cat engine.
    You can allways make it heavier and take big motors out but no one goes in oppsite direction, speed.

    50knots means speed, and weight reduction and better motor is proven formula, there is even better aproach if stepped hull is used, so many extra advantages if done right, viewing angle to, its flat, its perfect. Jumping and slamming is impoved vs non stepped hulls.

    But if I do 30knots 3.1g, just as any other 55-56 yacht.

    And I dont understand why would you stick 2 extra heavy rovings (1000g +) with no csm in between, csm must be used at least 300, 450 looks even better for me at yard, to fill the "gaps". You can get it close to 40% fiber ratio with rollers.
     
  12. Alik
    Joined: Jul 2003
    Posts: 3,024
    Likes: 318, Points: 93, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    I am sure You don't understand the issue. The hull should be purposely designed for the speed. Not the scantlings only but the shape as well. If You take hull originally intended for slower speeds, and just try to make it run faster by adding the horsepower and increasing laminate - this approach does not work. The hull meeds to run comfortably regardless of speed.
     
  13. vkstratis
    Joined: Feb 2015
    Posts: 69
    Likes: 5, Points: 8, Legacy Rep: 14
    Location: Athens, Greece

    vkstratis Naval Architect

    Exactly. Thickness will depend on unsupported size of panel, panel aspect ratio which is dependent of stiffener spacing. A scantlings report is always done together with construction drawings. You will also a need sections from lines plan, to determine bulkhead panel thicknesses and stiffeners as well as location of bulkheads which is usually determined from floodable length calculations if watertight division is required.
     
  14. vkstratis
    Joined: Feb 2015
    Posts: 69
    Likes: 5, Points: 8, Legacy Rep: 14
    Location: Athens, Greece

    vkstratis Naval Architect

    This is too thin. I have recently done scantlings for a 20m workboat using ABS and for much less speed (24knots max) bottom panel thickness was min. 17mm. But this is dependent on girder, stiffener spacing.
     

  15. vkstratis
    Joined: Feb 2015
    Posts: 69
    Likes: 5, Points: 8, Legacy Rep: 14
    Location: Athens, Greece

    vkstratis Naval Architect

    ISO scantlings rules are for pleasure or professional pleasure boats, not for racing ones. You have to understand that its not a mater of "fitting" extra layers of woven rovings between csm. You will have to get the minimum expected thickness AS WELL AS the minimum required mechanical properties of the laminate in order to conform. This is with any scantling rules not only with ISO standard. In other words you will have to make detailed construction drawings for hull, deck, superstructure including details such as stiffeners, bulkheads, etc. Make the calcs, revise your plans, re-do the calcs, and finally make laminate samples which will be verified at certified lab for their mechanical properties. 40% fiber ratio is nothing by itself. It could also be a laminate disaster.
     
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.