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  #16  
Old 03-24-2012, 04:39 AM
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daiquiri daiquiri is offline
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Originally Posted by DCockey View Post
What knowledge is needed to use classification society and ISO scantling rules with materials used in an unconventional mix/way ?
Class society rules and ISO standards are intended for conventional and well-defined construction techniques and materials. I believe that the class society inspector will want to see a proof of validity for every unconventional solution he notices during the construction. Another good reason to get a consultancy regarding that part from a NA or an engineer, possibly at the early design stage.
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  #17  
Old 03-24-2012, 04:48 AM
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Alik Alik is offline
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Quote:
Originally Posted by daiquiri View Post
Alik, I don't have statistics to demonstrate it, but I believe that the probability of grounding gets higher as boats gets smaller, primarily because of crew experience factor and tendency to sail closer to the coast. Large yachts are more likely to have a more experienced payed skipper and crew aboard, which (at least in theory) should diminish this probabilty.
Well, it's also true that, from time to time, even on ships around 300 meters of LOA it happens to find a Schettino in command, but that's a bit out of this topic...

So, I believe that for small boats it is more sound to stay on the "excessive" (to be defined, as Ad hoc noted) side of scantlings, unless you are designing a pure racer.

Cheers
Well, You have to talk with sailing snobs, they will call 'heavy' and 'excessive' everything that is done to rules of Classification Societies. In this terms, ISO gives lighter structure compared with Societies, for small boats.

Particularly in DNV, there is a requirement of min 2400g/m2 of glass for outer skin of bottom; that's too much.
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  #18  
Old 03-24-2012, 05:05 AM
Ad Hoc Ad Hoc is offline
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Quote:
Originally Posted by Alik View Post
Particularly in DNV, there is a requirement of min 2400g/m2 of glass for outer skin of bottom; that's too much.
That is nominally 3mm thick WR.

I personally don't have a problem with this at all..especially considering its much lower Modulus compared to ally, and as noted by D, abrasion resistance too.
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  #19  
Old 03-24-2012, 05:27 PM
MikeJohns MikeJohns is offline
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Quote:
Originally Posted by daiquiri View Post
........

I'm pretty happy with Strand. It works well and is one of the most reliable software I have. ....................
Strand/Straus is very versatile, it's also rock solid these days and never crashes, even on large complex models. But I do like being able to open Rhino generated solids directly in Cosmosworks.


I'm interested in how designers use FEA in composite design, I can see some nasty traps.
Probably the best approach is to set the material properties similar to composite and treat the model as a isotropic material, identify the stresses and then design the laminate accordingly. Remembering to apply enough FOS for material properties that change through time and use. Expected life time will be a big consideration.

Has anyone done serious elastic instability studies with FEM on composite structures?
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  #20  
Old 03-24-2012, 06:11 PM
CatBuilder CatBuilder is offline
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Small side question:

Is there a program, that you can enter bulkhead shapes at various stations into, then play with the rocker and BWL on to see what different effects happen?

Maybe generate Cp and Froude or Gerr type numbers?

I have been doing this manually, on paper. It's very time consuming to try various shapes. It would be nice to pick and drag the major axis of elliptical shapes and easily come up with "fore and aft CB" in the case of rocker.

Stuff like that... dragging ellipses around and finding volumes they displace as well as summations of their buoyancies and effects on COG/CB.

Any software out to make that more simple to play with?
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  #21  
Old 03-24-2012, 06:43 PM
Ad Hoc Ad Hoc is offline
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Originally Posted by MikeJohns View Post
Probably the best approach is to set the material properties similar to composite and treat the model as a isotropic material, identify the stresses and then design the laminate accordingly.
That's pretty much how I do it.

Once I have finished the analysis, i wait until i get the confirmed results back from the coupon tests to verify what the E, v, and tensile modulus is, and then do my final reviews. I only ever use real world, coupon tested values, never theoretical, nor, do I use individual layers, since it is impossible to verify, even experimentally.

Quote:
Originally Posted by CatBuilder View Post
a program, that you can enter bulkhead shapes at various stations into, then play with the rocker and BWL on to see what different effects happen?
Not sure I follow??

It is just hydrostatic head, ie depth, and then the scantlings are determined by the length and the breadth of the structural members on the WTB.
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  #22  
Old 03-24-2012, 07:14 PM
CatBuilder CatBuilder is offline
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Quote:
Originally Posted by Ad Hoc View Post
Not sure I follow??

It is just hydrostatic head, ie depth, and then the scantlings are determined by the length and the breadth of the structural members on the WTB.
Sorry if I'm not being clear or using the correct terminology. It's a slight thread drift to ask not about structural issues (I have a well designed hull form to start with and scantlings are the same - I'm just looking to fine tune it).

I am wondering if I take my well designed hull form and then modify it (by changing deepest point of rocker, etc...), what will the effects on static trim and dynamic speed be?

I'd like to enter all my stations in and then grab onto the rocker line (as an elliptical section) and pull the point of max draft around on the computer, as well as pull the total draft around.

Basically, a tool to speed up the trial and error process of mating a hull's rocker with the weight distribution, but making sure you don't blow the performance of the hull.

I'd also like the software to allow me to change the BWL at the various stations defining the shape at the waterplane (fineness of entry, etc...) and come back to me with the effects on static stability and dynamic hull speed once again.

I've been doing this stuff by hand by approximating shapes as conic sections or ellipses (since on a long, thin multihull, that seems to be the way to go) and trying various values by trial and error on paper. I am assuming there has to be a tool written for this iterative process to save me the time of doing it all on paper. The idea would just be to grab the line that represents, say, the BWL and drag it to change its shape, or change the widths of the stations individually to change its shape, thus altering the static stability, Cp and dynamic speed estimates.

I hope that makes sense. I obviously do not know all of the terminology. I am not a trained NA, obviously, just a guy who understands some of the basics and is exercising old, unused math and physics muscles. Looking for the software to make it a little easier now that I've been doing it on paper.
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  #23  
Old 03-24-2012, 07:45 PM
Ad Hoc Ad Hoc is offline
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Originally Posted by CatBuilder View Post
I am wondering if I take my well designed hull form and then modify it (by changing deepest point of rocker, etc...), what will the effects on static trim and dynamic speed be?
Firstly, forget all the Cp nonsense. Means diddly squat.

All you need is a simple hydrostatic stability software program. If you haven't got one, or can't afford one, then sadly, it is paper and pencil with your calculator and hours of endless calculations to produce curves of LCB, MCT, LCF etc etc at varying drafts
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  #24  
Old 03-24-2012, 08:44 PM
CatBuilder CatBuilder is offline
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Really? I thought Cp meant something on 10:1 L/B hulls. Seems everything I read tells me that.

Anyway, thanks. I'll try to find some hydrostatic stability software that runs natively on OSX (.5.8)

I do imagine I can afford the software, given I'm in the middle of building a 45 LWL x 25' Beam catamaran out of Corecell and epoxy... ha ha ha

This is a project to design and build the tender.
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  #25  
Old 03-25-2012, 06:02 PM
JRD JRD is offline
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Originally Posted by CatBuilder View Post
Really? I thought Cp meant something on 10:1 L/B hulls. Seems everything I read tells me that.

Anyway, thanks. I'll try to find some hydrostatic stability software that runs natively on OSX (.5.8)

I do imagine I can afford the software, given I'm in the middle of building a 45 LWL x 25' Beam catamaran out of Corecell and epoxy... ha ha ha

This is a project to design and build the tender.
Cat, you could use Freeship or a good few others, there is a thread full of debate on which is best in the software section. I use Hullform and I think the full version is now downloadable for free. Im not sure how it compares, but it was the first such software I learned to use, so have grown used to it (Just need to find time to finish what I have started.... ). Freeship seems to be the popular one here for small boat design.
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  #26  
Old 03-25-2012, 06:58 PM
CatBuilder CatBuilder is offline
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Quote:
Originally Posted by JRD View Post
Cat, you could use Freeship or a good few others, there is a thread full of debate on which is best in the software section. I use Hullform and I think the full version is now downloadable for free. Im not sure how it compares, but it was the first such software I learned to use, so have grown used to it (Just need to find time to finish what I have started.... ). Freeship seems to be the popular one here for small boat design.
I wasn't able to find the OSX builds of Hullform or Freeship.

What are you doing? Running the Linux version and adjusting it to run on OSX?

Failing that, I guess I'm going to have to go to a fast WiFi hotspot and download myself a windows emulator. I suppose freeship looks good and I'll try modeling the hull in that. Thank you!
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  #27  
Old 03-26-2012, 05:19 AM
JRD JRD is offline
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Quote:
Originally Posted by CatBuilder View Post
I wasn't able to find the OSX builds of Hullform or Freeship.

What are you doing? Running the Linux version and adjusting it to run on OSX?
Ooops, sorry Cat, I missed the OSX bit. My life has enough complications without stepping to the darkside to spite Mr Gates
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  #28  
Old 03-28-2012, 04:24 AM
APP APP is offline
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Quote:
Originally Posted by TealTiger View Post
Hi David,

Thanks for the questions. I'm not up on this stuff yet, so I may not be using the best terminology. I'm not sure if I need the detail that your question seems to indicate to me. I certainly don't need FEA. I want to take a fairly conventional cat (like a lighter simpler Lagoon 380) and find out if my material choices and spec's are aprox. strong enough. If so, I can pay more to get it thoroughly to get it done. So before I throw a lot of money at doing a through job of what might be a waste, I'd like to do everything I can to minimize the job of finding out if I'm on the right track. I'm interested in hearing any advice the people here might have to say. Thanks.
It would be nice if someone expert could post an example with all hand made calculations for catamaran scantlings, following up the rules of any Class Society. A good example could be a CAT between 10-18 m LOA. Maybe an aluminum Cat!

Regards
APP
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  #29  
Old 03-28-2012, 04:33 AM
Ad Hoc Ad Hoc is offline
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Originally Posted by APP View Post
It would be nice if someone expert could post an example with all hand made calculations for catamaran scantlings, following up the rules of any Class Society. A good example could be a CAT between 10-18 m LOA. Maybe an aluminum Cat!
Whilst I understand your motivation, you are over thinking this.

If you download any of the free Class rules now available, it is a simple case of reading the rule, measuring the dimensions from your GA and using a hand calculator with pencil and paper. If you can do simple arithmetic and algebra, it is not difficult, just a bit time consuming. Far better than relying on a text book not suited for such vessels too.
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  #30  
Old 03-28-2012, 06:11 AM
fcfc fcfc is offline
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Quote:
Originally Posted by Ad Hoc View Post
Whilst I understand your motivation, you are over thinking this.

If you download any of the free Class rules now available, it is a simple case of reading the rule, measuring the dimensions from your GA and using a hand calculator with pencil and paper. If you can do simple arithmetic and algebra, it is not difficult, just a bit time consuming. Far better than relying on a text book not suited for such vessels too.
I think it is still currently possible, but will soon be very limited. When you look in depth to ISO 12215-9 (sailboats keels), of course there is a way with a spreadsheet (better than hand calc + pencil), but the options you have are so limited and conservative that virtually any new production or racing boat will need some kind of structural finite elements analysis.

Of course, your spreadsheet computed boat wont loose its keel. But it will be more expensive to built and/or less performing than boats with FEA study. Rather hard to sell...
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