A question of philosophy

[DUCRUY Jacques]

Good evening,

I frequently ask you about the method of calculation the scantling of ABS ORY and ISO.

It seems to me, now, that :

- the ABS ORY rule is OK for on offshore racing sailboat (Fastnet, Bermuda or Sydney-Hobar races) ;

- the ISO rule is OK for a sailing boat used by a "family father" (pure cruising, no racing).

In effect, if the plating thickness is nearly the same, it seems that the transverse frames & floors are stronger with ABS.

It is true or not ?

Best Regards


Jacques

[gonzo]

The method of calculation is to comply with a rule. Philosophy has nothing to do with it.

[TeddyDiver]

Originally Posted by DUCRUY Jacques

- the ABS ORY rule is OK for on offshore racing sailboat (Fastnet, Bermuda or Sydney-Hobar races) ;

- the ISO rule is OK for a sailing boat used by a "family father" (pure cruising, no racing).

Can't say anything about ABS ORY but the ISO boat standard 12215 is purely for recreational boats (though in some countries it's used for work and fishing boats too).

[Alik]

Originally Posted by TeddyDiver

Can't say anything about ABS ORY but the ISO boat standard 12215 is purely for recreational boats (though in some countries it's used for work and fishing boats too).

The standard itself does not say that it is for recreational boats, it is for small caft! We use it for some small commercial and special craft also. If we look at validation versions of 12215-5 standard, there are definitely other types of boats besides recreational.

Agree that this standard might be conservative for racing sailboat.

[Ad Hoc]

Originally Posted by DUCRUY Jacques

It is true or not ?

Neither.

Class rules etc, are used based upon their ease of use and assuming the vessel will be classed to the rules, how cheap it is to have the vessel classed. Some Class societies charge more than others.

ABS was widely used because there wasn't much, at that time, about as a guide and it was easy to use.

So, you choose one which you feel comfortable with. But ultimately these are just MINIMUM scantling requirements. The structural design still needs to be thought about carefully as well as the expected sea conditions the vessel will operate in, and hence expected laods, not covered by Class rules..

As for ISO, it states clearly that the usage is:
vessels between 2.5-24m, and has 4 main categories, ranging from A "Ocean" to D "sheltered waters".
Ergo pretty much everywhere.

[sorenfdk]

Originally Posted by Ad Hoc

ABS was widely used because there wasn't much, at that time, about as a guide and it was easy to use.

Being mandatory for racing yachts also had an influence...

[Paul Kotzebue]

Originally Posted by DUCRUY Jacques

In effect, if the plating thickness is nearly the same, it seems that the transverse frames & floors are stronger with ABS.

Are you considering any differences in effective width of plating, stiffness requirements, and allowable stress between the two scantling rules?

[DUCRUY Jacques]

Hello,

Thank you for yours answers.

If I understand correctly, I must choose a rule (Classification Society or ISO) and follow his requirements for to get a satisfying scantling (I suppose there are no "bad" rules). But I can always choose a scantling more stronger (for example the SM of the floors in case of grounding).

I admit I like the ABS ORY rules for the simplicity, but I know that this rule had few hiatus (as lack of precision about assembling of ballast bolts : minimum thickness of the hull, dimentions of backing-plates ...).

Good Evening


Jacques

[Perm Stress]

Every Class Rule has certain philosophy hidden behind the formulas and factors safety, even if it is not declared in clear words.
And, further down the philosophy line:
Designing to the Rule all you get is design ACCORDING TO THE RULE and nothing more.
Whether design will be satisfactory to YOU or YOU CLIENT depend on how much the (hidden) philosophy of the Rule is in line with that of you or your client.
The example I use to ilustrate this is following:
The Class Rules I am familiar with (Polski Rejestr Statkov, Germanisher Lloyd, ISO) specify deck loading no more as ~0.5...1.0t/m^2. While impact load to topsides, deck and couchroof in storm conditions, when a yacht is thrown sideways by breaking wave crest and hit solid water with her lee-side, could be ~2-2.5t/m2 (I have found it in "Heavy weather sailing" by A. Coles, and later in other sources, cannot remember exactly where). So, yacht designed to Class, is unlikely to survive such impact without damage. On the other side, it is (very) extremely rare event. Mostly it happen only in "Perfect storm" conditions, like Fastnet '79, more recent Sydney-Hobart debacle and the like. So all the rest of the time and all the other boats would carry this reinforcement as extra weight on deck level. Here we discover some part of philosophy....

[gonzo]

Fastnet 79 was merely a gale. The main problem was panic and poor seamanship.

[Perm Stress]

"Fastnet 79 was merely a gale. The main problem was panic and poor seamanship."

I, for one, would think more than twice to head out to such "merely a gale".
With every level of seamanship, sh*t happens.
And at that time, it suddenly became clear, that some boats have extra strength(or stability) in store, and some do not.
I remember a picture in Yachting World, taken from inside of ~40-50ft sailboat in Sydney-Hobart debacle, with some 2m^2 of deck and couchroof missing. I bet deck was designed to no more than 1t/m^2 pressure, an equivalent of 1m head of water.
In Fastnet '79 there were yachts capsized and damaged.
Some of them did not right quickly or by themselves. Where is connection with seamanship here? Poor seamanship was to abandon yacht still floating, but too small stability or damaged structures is a matter of inadequate design/build.
Open 60's some years ago did not have requirement for minimum stability at 90 degrees nor full inversion test, as required today. It turned out that some of them had angle of vanishing stability 95, 105 degrees or so.
Presumably, it was considered, that seamanship of high-profile skippers will exclude "sails in the water" events. It did not. After serial abandonment of boats, that were unable to recover their sails from the water, a much-resisted ultimate stability requirements and full inversion tests were introduced.

Please do not overrate seamanship. The sea may not agree with any level of seamanship at any moment in time.

[Alik]

Originally Posted by Perm Stress

Open 60's some years ago did not have requirement for minimum stability at 90 degrees nor full inversion test, as required today. It turned out that some of them had angle of vanishing stability 95, 105 degrees or so.

In Open60 minumum requirement was AVS=110 deg; later it was set to 127.5 deg.

So I doubt that it could have been 95-105 legally; maybe due to keel cant or some other reasons.

[Ad Hoc]

Originally Posted by DUCRUY Jacques

If I understand correctly, I must choose a rule (Classification Society or ISO) and follow his requirements for to get a satisfying scantling (I suppose there are no "bad" rules). But I can always choose a scantling more stronger (for example the SM of the floors in case of grounding).

Correct. Choose one you're happy with or understand. Stick to it, as you can't mix and match rules. Then if you feel the scantling is not not enough, from experience or from the operational envelope, then you can of course increase it.

That's it.

Also worth remembering, the scantlings are just that, series of moduli, but nothing about structural "design" in the sense of establishing load paths, or shear connections, stress concentrations, fatigue etc etc.

[Perm Stress]

Originally Posted by Alik

In Open60 minumum requirement was AVS=110 deg; later it was set to 127.5 deg.

So I doubt that it could have been 95-105 legally; maybe due to keel cant or some other reasons.

In the heyday of ketches in Whitbread Round the World race, one of them was fitted with mizzen mast at the last moment. Later in the Southern Ocean she did "sails in the water" trick and was unable to recover until mizzen mast was dumped overboard. I doubt if her stability after modification was legal, as ballast was not increased.

[Alik]

Originally Posted by Perm Stress

In the heyday of ketches Whitbread Round the World race, one of them was fitted with mizzen mast at the last moment. Later in the Southern Ocean she did "sails in the water" trick and was unable to recover until mizzen mast was dumped overboard. I doubt if her stability after modification was legal, as ballast was not increased.

As I understand You are talking about IOR Maxi class; those days stability assessment was part of IOR rating and thus it was tricky.