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Dear Fritz,

Due to late advice of Victoria meeting and already made other ABS work commitments I had advised I would be unable to attend the Victoria meeting. I am however available for several meetings in Europe before end of 2002, provided suitable notice is received.

In response to Robin's proposals. Referring Robin's E mail do not recall that WG 18 elected to develop a simplified method in place of that already agreed for craft between 10 to 24 m length. I left the meeting with the understanding that the first priority was to revise the Kar factors for sailing yachts brining them into line with ABS ORY values in order to bring the ISO standard scantlings up to ABS Guide scantlings and closer to those of actual boats that Bippe had used in his verification work.

As for Robin's proposal, it should be recalled that PB1 gives the bottom design pressure for planing craft and PB2 gives the bottom design pressure for displacement mode vessels, implicitly including bottom pitch slamming pressure.

Design Bottom Pressure Equation　It is impossible to obtain appropriate bottom design pressures for these two very different modes of operation from one design pressure equation. (which is related only to planing mode bottom design pressure) as the design pressure parameters for each mode of operation are entirely different.

　The reason given for proposing the use of only one equation is stated as being that for a certain range of these different parameters, the resulting design pressures are very similar. This is an unacceptable reason when we are developing a standard that can apply over a very wide range of parameters, as once outside the range where the different parameters give similar design pressures the single equation will give inadequate or excessive design pressures for the displacement mode.

Reference to classification society Rules for high speed craft shows they always have at least one design pressure equation for the planing mode of operation and at least one different design pressure equation for the displacement mode of operation.

　This proposal to use one design bottom pressure equation and the reason given for doing so would be indefensible in court.

2) Pressure Reduction Factor Krp　There is no justification given for the revised Krp values, particularly those for secondary members and primary members that are related by a fixed reduction factor to the plating reduction factor. This approach presumes the area of the plating panel, implicitly based on area, will always be that much less than the area of shell supported by a stiffener or girder. With sandwich shell panels this is not always the case.

Consequently for this reason this proposed approach is basically flawed and needs to be corrected

3) Simplified Design Vertical Acceleration Equation　We are replying to the appendices to Robin's E mail proposal in the attachments. to this E mail.

However we do not agree with Robin's proposed "simplified" method of determining the design vertical acceleration. The detailed reasons for this are given in the attachments. Essentially the equation proposed can be shown to be derived from the statistically developed nominal default design vertical acceleration given in DNV Rules for High Speed Craft. This nominal design vertical acceleration was developed for craft most likely from 24m to 60m, and one of the conditions is that V / (L)^0.5 need not be taken greater than 3.0, while Robin proposes it apply up to V /(L)^0.5 = 18.

However the most critical aspect of this proposal is that because the simplified nominal design vertical acceleration equation proposed by Robin was developed statistically and does not include the vertical acceleration parameters it cannot be associated with the maximum design speeds and the associated maximum significant wave heights

　

The very small amount of time saved by the use of the "simplified" design vertical acceleration equation based only on V / (L)^0.5 is nor worth the inaccuracies that will be introduced by using a design vertical acceleration that is not based on any of the recognized parameters for design vertical acceleration.

Robin states "There is a body of evidence that says n_cg varies as Vⁿ, where n is less than 2" and quotes the sources. What Robin does not say is that in order to determine accurate values of maximum design speeds and associated maximum significant wave heights for the nominal design vertical acceleration obtained through its relationship with Vⁿ, the sources he refers to, DNV and UNITAS Rules, require the maximum design speeds and associated maximum significant wave heights be obtained by equating the nominal design vertical acceleration to equations virtually identical to equation (2) presently in 12215-5, which Robin is proposing to delete from 12215-5.

The very small amount of time saved by using the "simplified" nominal design vertical acceleration equation based only on V / (L)^0.5 in place of the present, correct, parametrically complete equation (2) is not worth the resulting inaccuracies. In addition because of the nominal design vertical acceleration equation does not include the relevant design parameters it is not possible to determine the operational design limits of maximum speeds and associated maximum significant wave heights for operational guidance. These operational design criteria are given by the present equation (2)

We understood from the Baltimore meeting that Bippe's comparison with actual motor yachts showed the ISO standard for motor craft to be reasonably satisfactory in terms of actual designs, needing only minor adjustment. to bring it into line. As far as the sailboats were concerned, the use of pressure reduction factors much less than in the ABS Guide and the use of average core shear design strengths instead of minimum values resulted in the ISO standard scantlings being substantially less than the actual sailboats as indicated in Bippe's comparison and substantially less than ABS ORY scantlings. I was working on correcting this situation when I had to review Robin's latest new proposal to simplify the ISO standard. I will now continue working on the Kar factors to bring 12215-5 scantlings for sail boats up to ABS ORY level.

I regret not being able to attend the meeting but I wish you a successful meeting,

　

Best Regards,

　

Bob,

　

Anlagen:　・Comments on Robin Loscombe.doc

　

file://C:＼T-Online＼eMail4＼user＼usr Fritz.Hartz 000000＼tmp＼Fritz.Hartz

　

　

----- Original Message -----

From: RCurry@eagle.org

To: Fritz.Hartz

Cc: andrew.yates@rya.org.uk ; arikawa@jmsa.or.jp ; askhamj@DFO-MPO.GC.CA ; brue@germanlloyd.org ; christer.bergstrom@sjofartsverket.se ; didier.nicol@afnor.fr ; dolto@france-nautic.com ; eivind.amble@amble-stokke.no ; Fabdiluggo@aol.com ; gijutu@jci.go.jp ; ICNN@wanadoo.fr ; ImciUli@aol.com ; jha@hytek-marine.dk ; kensukes@fmail.plala.or.jp ; kuniyuki.hayashi@marine.yamaha-motor.co.jp ; Im.rinave_sa@jp.pt ; lorne@lcdesign.freeserve.co.uk ; lpollicardo@ucina.net ; luiscorreialopes@spnavais.pt ; Marnix@nkip.nl ; michael.vollmer@sympatico.ca ; nagami@marine.yamaha-motor.co.jp ; Paulrhandley@cs.com ; pras@abycinc.org ; PvanOossanen@csi.com ; rasna@attglobal.net ; Robin.Loscombe@solent.ac.uk ; rolf.e@reyd.com ; salvatore.sorrentino@ferretti-yacht.com ; tmarhevko@nmma.org

Sent: Tuesday, June 11, 2002 6:17 PM

Subject: Re: ISo TC 188 WG 18: Follow-up Baltimore

　

Dear Fritz,

I hope you received my E mail of 7 June with comments on Robin's proposals to simplify the equation for design vertical acceleration and the use of only one equation based on planing craft operation mode to give the maximum design pressures for both the planing mode and displacement mode of operation.

I have also reviewed the attached proposals and have the following comments

It is four years since all the class societies submitted their scantling requirements for comparison, from which. LR were concluded to be conservative while ABS and GL were less so.

Regarding Robin's rough check:

a) Design pressures of any standard cannot be directly compared unless the design pressures of the standards are the same. They may be compared if normalized to the same design pressure as Robin has done.

It is possible to compare the level of any standard by the non-dimensional strength strength quotient of design pressure / design stress. The greater this value the greater the strength provided by the standard.

The non - dimensional strength quotient for plate panels is implied by t² / s² or by FS. Pd. k / yield strength, as t² / s² FS. Pd.k / yield strength : where t = required thickness, s = frame / stiffener spacing, FS = factor of safety, Pd = design pressure. k = 0.5, fixed edged plate panel with aspect ratio > 2.0

b) We are confining our review of Robin's work to the sailboat design pressures normalized to correspond to the ISO design stress, although the only "design" pressure common to all standards is that at which the strength limit state criteria is reached - which for metal boats is the "design" pressure at which the yield strength is reached (given above as FS.Pd)

- As mentioned above LR are recognized as having conservative standards.

- Considering VT design pressures at Kar = 1.0, it should be realized that VTT Ka factor as given in NBS -VTT Extended Rule 1997, is about 30% less than that in the ISO standard for Ad / Ar less than about 0.003.Consequently values of VT design pressure for Kar = 1.00 are to be expected to be greater than the ISO standard

- For GL, as we are determining design pressures at Kar = 1.0 we have referred to GL Rules for Pleasure Craft 2002 for sailing yacht shell plating and for simplicity we have referred to steel boats.

Based on calculating the required thicknesses, we find ISO standard t² / s² to be 25% greater than GL at 10m length and 6% greater than GL at 25m length

Considering the non- dimensional strength quotient of FS design pressure / design stress (or limit state design pressure / yield strength which for ISO = 1.11 Pb2 / yield strength) we find ISO to be about 25% greater han GL at L = 10m reducing to about 6% greater than GL at 25m length

　

It would be appreciated if Robin could explain in detail how the values of GL design pressures were normalized to the ISO design stress of 0.9 x yield strength, ie what was taken as GL design stress in association with GL design pressures

　

Simplifying Kar (by R L)

We have already commented in our E mail of 7 June. Subject to revision to address our comments this Kar factor may perhaps have a future in the ISO standard

　

Plywood Bulkheads (by R L)

ABS has in-house guidance, I will provide input

　

Kar - Pre and Post Baltimore.

There is always a danger in changing long-standing criteria at short notice without prior assessment of effects. For displacement hulls it appears the absolute minimum value we suggested at the Baltimore meeting has not been included,

　

General Comment

a) Rules / standards should be written with the expectation that once in use they will at some stage be thoroughly scrutinized by legal and technical experts, because of which the should be parametrically accurate and complete. If not parametrically complete the acceptance criteria (design stresses) should be adjusted to reflect the inaccuracies in the design pressures due to the possible variations in the omitted parameters.

　

Rules / standards should be calibrated with proven vessels known to have experienced the limiting sea states on which design pressures are based using the mechanical properties that accurately reflect those of the actual as-built vessel referred to. Where possible they should also be assessed against well proven existing standards

b) We mention in a) some of the fundamentals of developing standards / Rules.

It can be clearly seen that it is most advisable to include all relevant parameters both in the design load equations and in the structural response equations. Not to do so makes the standard easy to be proven inadequate.

c) There have recently been proposals to simplify the design vertical acceleration with corresponding inaccuracies in the design pressure and to use a single design pressure equation parametrically incorrect for displacement craft. These have been proposed without any realization that because of this the present design stresses are no longer appropriate for these parametrically incomplete and inaccurate design load criteria.

d) We have always been concerned that

- material testing of the FRP laminates is not required.

- the estimated mechanical properties given in ISO 12215 - 5 for design come from LR data from LR classed yachts because of which they are not likely to be the actual minimum values that exist in the real world poorest quality construction

- in assessing the standard against actual boats the mechanical properties of the actual boat laminates were not used,

instead we have used Table 13a mechanical properties. We have previously suggested because of all of these unknowns in our development and validation processes we should consider a knock- down factor on the Table 13 a. mechanical properties, both in the standard and in their use in the validation process,

　

Best Regards,

　

Bob,

　

Best Regards,

　

Bob,

　

　

Dear colleagues,

　

please find attached several papers concerning the ongoing work and those items that were addressed during the Baltimore meeting, but raised some questions afterwards.

All those who will not participate at the Victoria meeting next month: Would you please let me have your comments prior to the meeting. The main item to be solved will be the area reduction factor kar. But any other comments are welcome.

　

Best regards,

Fritz Hartz

Convener WG 18