ISO 12215 effective plating

[adventurelad]

Hi all-

I've been working through the recently released ISO 12215-5 hull scantlings (reducing to a usable spreadsheet for work) and came to section 11.6 effective plating. It seems to me that the FRP sandwich perscription (20 * (touterskin + t innerskin)) is overly conservative. Anyone else run into this? Depending on your core thickness, it produces an effective width ~1/2 of the ABS high speed craft regulation.

Difficult to compare to DNV or Timoshenko since they both only consider the beam spacing and fixity (although don't have DNV HSLC or NSC handy, anyone else??).

Any thoughts? Thanks!

Adventurelad

[Paul Kotzebue]

It seems to me the ABS approach of using a skin thickness with the same moment of inertia as the sandwich laminate to calculate effective width gives too much credit for very thin skins with a thick core.

The ISO approach of using the total skin thickness seems reasonable if you consider the core thickness when calculating the moment of inertia of the stiffener section.

[adventurelad]

Paul-

Thanks for the reply. I guess my issue with the ISO is that we've been working to the ABS for all of these years without any issues, and now we will likely need pad laminates (tapes) at the beam locations to increase the RF of the hull to a passing level.

I know, it sounds like just whining, but was really wondering if I had overlooked something in the ISO...

Adventurelad

[Paul Kotzebue]

I'm not an expert on the ISO scantling rule. But I do know that everytime there is a major revision to a small craft scantling rule, the boats get heavier (and safer).

[foxy]

I am using Hull Scant from the Wolfson Unit and have not seen that the laminates produced are much different from ABS Offshore Racing Yacht. (For ABS, I am still using ABS Construct from Vacanti) In fact, one of my clients had me run numbers on several of the models they produce to see where they stand since ISAF special regulations now require plan approval to ISO 12215 for all boats built after June 1, 2009. I ran a complete panel study of the hulls and decks and in no case did we have to increase laminate.

You might look at the laminate values you are using under each rule and make sure that you are using the same ones. In the Hull Scant program, there are three options. ISO values which add in an extra 20% Safty factor, Measured properties (meaning you know the fiber/resin ratios and laminate thickness) which do not add the safety factor, and tested values. You might look and see if your spreadsheet formula incorporated a .8 multiplier.

The one area where I do see ISO being tougher is in the keel area for both floors and what ABS calls the reinforced shell.

[adventurelad]

Thanks for the thoughts foxy, I am curious what type of boats your comments refer to (power, cruising sail, racing sail, etc.)? And were they designed to absolute minimum R.F.'s per ABS, or did you have some extra reserve? Our firm is primarily high performance racing sail boats, and the kSLS factor is a very significant increase in the hull pressures. That aside, the reduction in effective width of plating significantly impacts the beam analysis as the hull (very thin face sheets over 25 - 30 mm cores) cannot handle the tension load without more effective cross-sectional area.

As to your other comment, we have set up our program for "EL-a" (as-tested values) and Annex H analysis, so we should be getting the 'best' set of properties, so not getting the 0.8 knock-down.

Thanks for your comments!

Adventurelad

[foxy]

I deal mostly in racing sailboats although some are cruising or day sailing boats. I used to do some work with high speed power boats, but have not done any in the last 8 years or so.

We are seldom at absolute minimum ABS (offshore racing yacht) even in our high end race boats. Generally speaking for ABS, we would look at what the minimum outer skin weight needs to be and go with a core thickness that works with that. I think many designers go with inner skin weights to the formula, and ignore that more or less equal inner skins are implied although not specifically required. Leastwise most builders have told us that our inner skins were heavier than those of other firms. That may explain why I have not run across your issue.

At any rate, the new ISAF special regs require engineering with plan approval to ISO and ABS is a dead horse.

[MikeJohns]

The last ABS ORY issued is deficient in it's deck scantlings for composite construction. The pre 1997? ORY was deficient in it's keel attachment area scantlings and was modified.

'foxy' says it is now a dead horse, but there are still many yacht designers who use it even now it's discontinued.

There are aslo older boats in use clearly built to deangerous scantlings under older ORY guides and many people are quite oblivious of this. So overall the ABS OSY composite scantlings have been a bit hit and miss from an engineering viewpoint.

ISO is going to become the new standard for lightweights but it will and should be a little more robust in some areas.

[Paul Kotzebue]

Originally Posted by MikeJohns

The last ABS ORY issued is deficient in it's deck scantlings for composite construction.

I agree. I usually add a 50 psf "walking" load to the ABS design pressure when calculating composite deck scantlings.

[foxy]

Mike,

Many designers still use ABS because it was pretty much the only standard to use even though it was last updated and published in 94. ISO 12215-5 became final in 2008 and became ISAF mandated for special regs June 1 this year. ABS is no longer acceptable for boats built after June 1.

I don't think many good designers would go for minimum ABS for keels and decks, even on race boats. I also think that many newer designers really don't use ABS properly. Back when ABS actually had a plan approval process, I don't think I ever saw a set of plans go through without a red line or note somewhere. This was across a range of good, competent design offices like Farr, Nelson Marek, Bill Tripp....... whose boats I have built.

With no organization interpreting the scantlings, and reviewing plans, things just got too loose and lax all around. I'd still like ISAF to include some check of the builder to see that they are following the "approved" plans we provide. Some just do whatever they want and sign the papers anyway.

If you look at the reports of keel failures. Most conclude that the builder didn't follow the plans. Of those however, many conclude that the engineering was deficient and the failures would have occurred anyway, just at a later time.

[Ad Hoc]

The section 11.6, is not that much different from DNV's.
The formula in 11.6, or rather those 3 figures, in your case i assume 13 a), the "w", width of stiffener is added to the breadth, ie b + w. In DNV is it CL to CL of stiffener, so the same.

The inner/outer thicknesses, for sandwich panels is also similar to that of DNV in same thicknesses for minimums, inner outer, DNV table A2, pt3.ch.4 sec. 5A.

DNV, is actually not that difficult to use. Looks complex, but it is not once you have understood how the rules work.

You could also use LR's SSC rules,with their software, makes it even easier than doing by hand!