The ISO 12217 Stability Series is currently under revision

The following is my contribution to revising ISO 12217 as communicated to ABYC. ABYC is the American National Standards Institute (ANSI) accredited TAG representative for U.S. positions on activities and ballots related to ISO TC 188. I'm posting my comments here so that other's may weigh in. I'm not at liberty to share the entire regulation but encourage others to contact their nation's TAG representative if you do not have access to a copy. My prior related threads include:

• https://www.boatdesign.net/threads/electric-launch-design-optimized-for-semi-displacement-speeds.68648
• https://www.boatdesign.net/threads/how-wide-should-it-be-a-method-proposed.12440

Note that ISO does not distinguish between crew and passengers, so "crew limit" (CL) means all persons aboard. ISO uses the variable m(LDC) for full load displacement in kilograms.

Introduction:

I’m submitting this because there’s scant information offered by Section 5.1 of ISO 12217-1:2022(E) concerning how one might develop a target CL for preliminary design from basic vessel parameters prior to finalizing a righting moments curve. The proposed language and supporting derivation below are suggested so that Section 5.1 might offer a starting point to users prior to any test being undertaken. The addition could either appear as 5.1.2 or be incorporated into Section 6.2 as 6.2.4: Estimating CL for a Candidate New Design (and be referenced from Section 5.1). The same math can be utilized to solve for a target Righting Moment and to write expressions suggesting other vessel parameters in the preliminary design process once a target CL and a target VCGs are determined.

Concerning existing vessels or designs that are sufficiently final for a righting moments curve to be determined, it would be helpful were Section 5.1 to reference the Offset Load Test Calculation Worksheet of Appendix J.

Determining a target Crew Limit CL relative to Hull Length (LH):

LH is as defined by these diagrams from ISO 8666:


A preliminary relationship between minimum righting moment (RMreq’d) and Hull Length (LH), for the purpose of anticipating a maximum Crew Limit (CL), is:
[from B.3.2.4]

RMmin@o0(R) = 9.806m(LDC) * sin[11.5+[(24-LH)^3 / 520]] * GM´

derivation:
RM = 9.806m(LDC) * sinϕ0(R) * GM´
RM = 9.806m(LDC) * sin[11.5+[(24-LH)^3/520]] * GM´
​

CL = GM´ * 9.806m(LDC) * tan[11.5+[(24 - LH)^3 / 520]] / 480BC

derivation:
HM = RM [@ equilibrium]
HM = 480CL * BC * cosϕ [from B.3.2.4]
480CL * BC * cosϕ = 9.806 * sin[11.5+[(24-LH)^3/520]] * GM´ * m(LDC)
CL = 9.806m(LDC) * sin[11.5+[(24-LH)^3/520]] * GM´ / (480BC * cosϕ0(R))
CL = GM´ * 9.806m(LDC) * tan[11.5+[(24-LH)^3/520]] / 480BC
​

GM´= 480BC * CL / [9.806m(LDC) * tan[11.5 + [(24 - LH)^3 / 520]]]

derivation:
CL = GM´ * 9.806m(LDC) * tan[11.5+[(24-LH)^3/520]] / 480BC
CL/GM´ = 9.806m(LDC) * tan[11.5+[(24-LH)^3/520]] / 480BC
GM´= 480BC * CL / [9.806m(LDC) * tan[11.5+[(24-LH)^3/520]]]​

Deriving an approximate preliminary waterline beam

From this point on I will employ approximations & assumptions as follows:

• GM(initial) may or may not equal GM´, depending on hull shape. Nonetheless, posit that GM(initial) = GM´
• Posit that waterline beam (BWL) = BC

[I'm having difficulty with format problems as I attempt to paste from MSWord. Please see the attached PDF file for my full remarks, equations, and an example as transmitted to ABYC - including everything from here onward.]

 

You're missing the point of Design.

The CL..is a payload. The client defines what they want to carry...the payload.
A vessel is then designed to carry the payload....

One does not develop a CL..a payload...it is defined exactly, by the client.

Just as one does not use the rules to establish, v, what speed the vessel will do. The speed, like the payload, is a function of the Design, holistically.

 

I agree entirely, As Hoc. My solution allows one to solve the problem iteratively. Is a better, more direct approach possible? It may well be.

What my formulae will allow, when written into a spreadsheet or computer program, is rapid cycling through alternative design parameters.

If you look at the example in the PDF file attached to the first post, you'll see that one can supply a CL and solve for GMreq'd by the ISO offset load test using:

GM´= 480BC * CL / [9.806m(LDC) * tan[11.5+[(24 - LH)^3 / 520]]]

If one assumes twice the arm of the crew weight being on one side, Bc, and the waterline beam are probably close, one then knows quite a bit about the relationships that define the boat's capacity from this expression.

Will you be submitting any suggestions for improving ISO regulations, AdHoc?

 

What does this mean....rapid cycling?

 

An iterative process will always be limited by the time it takes to perform each iteration.

Where I'm going to claim the virtue of experience in "wholistic" design is that, within a constrained type, designing around a target full load GM of about 0.7 meters, then applying some math to work out parameters that align with the SOR, works out well. GM is nice in that it can remain nearly fixed for vessels of various sizes.

 

A certain initial GM value does not guarantee that stability and buoyancy are sufficient, that is, it does not guarantee compliance with ISO 12217. Boat design cannot be simplified with magic formulas or shortcuts along the way.

 

I agree, but a GM required can be derived from the offset load test, which can then be utilized in designing a boat that has the intact stability to comply with that test, then be checked against the other portions of the regulation PRIOR TO BEING BUILT. It's not intended to be a shortcut to guarantee compliance - it's intended as a first check on the adequacy of a preliminary design's basic parameters to POTENTIALLY comply. In the world of ships there are many instances of vessels having to add expensive sponsons because adding ballast was not a sufficient strategy to bring a vessel into compliance. This is a way to get off on the right foot early to avoid costly mistakes, in my view.

 

Gosh,. you really do not understand the process of Design at all.
One does not build a boat and then launch it and see, oh wow, I wonder if it is stable or will pass rules/regs.

What you are poorly attempting to describe, is the normal MO design process.
Intact stability is checked before the Design is even sent to the builder....or to Class/Flag for approval.

I would suggest you go and work in a design office for a few years, then you may start to grasp your own failings on what you keep pronouncing as a Eureka moment, owing to ignorance on the subject.

 

I have worked in design offices, and am attempting to share things I've learned. Some of that work has been bringing boats into intact stability compliance. In some instances, things would have worked out better if the boat had another 25cm of beam and another 25cm of freeboard, but not having that I was faced with a vicious cycle of diminishing returns. Yes, the VCG would lower if I added ballast, but so would the metacenter, and the freeboard would decrease.

One of our new designs was conceived as having a scaled down hull from her big sister, but in that case, also, we would have had to add considerable ballast - more than the larger boat carried, for full compliance with the passenger vessel standard we employed. We got it sorted out with appropriate beam, but it was experiences like that which led me to be concerned with the way stability scales and how to understand it at the outset of preliminary design.

My concern is: boats get built without fully accounting for everything. Too often a picture of a boat is the basis for a contract - then the NA feels constrained to design the boat that was sold to the client, be it right or be it wrong. In my view there's benefit to being able to do some quick, but mathematically sound "ballparking" before a preliminary design is drawn and contracted.

To raise a second, related ISO 12217 point briefly, the references to several weights assumed per person at B3.2 and B3.3 are confusing. The 85kg weight accords best with the U.S. standard of 185 lbs documented at https://www.dco.uscg.mil/aawpp/. In my view, the weight of an individual should be changed at 3.5.3 from 75 to 85kg, for consistency. The 98kg weight used in places should be explained as including supplies worn, carried, or free to shift, consistent with 3.4.4 note 2.