Downflooding opening in ISO 12217-1

Regarding downflooding opening in ISO12217-1, the following is defined,

3.2 Downflooding
3.2.1 downflooding opening
opening in the hull or deck (including the edge of a recess) that might admit water into the interior or bilge of a boat, or a recess, apart from those excluded in 6.1.1.6.

Opening complying with 6.1.1.6 is considered not downflooding points, these can be recess drains on watertight recesses with combined volume less than (LHBHFM)/40, or quick-draining recesses.

My question is "why would ISO12217 suggest the recess drains overboard on a boat with recesses can be considered not downflooding points?"

There are drains to drain the recess no matter it is a "watertight recess" or a "quick-draining recess", these recesses are open to sea in any time. In traditional naval architecture, when we deal with downflooding points, we consider the points which shall lead water into the vessel, such as under deck compartment ventilators, engine room air intakes etc...

Small boat with recesses in my opinion is a boat with its main deck opened, the recess bottom on such a boat is equivalent to the double bottom of a conventional ocean going vessel. Even worse, there are drains drain the water from double bottom in this case (let's assume the double bottom is always above design waterline).

We will definitely do not accept an ocean going vessel with such an arrangement, because it allows water come on board and sink the vessel directly. But why ISO12217 can accept a small boat with opening on its edge of a recess (i.e. side shell) and does not take them into consideration when doing stability assessment.

ISO12217 suggests that the freeboard FM is measured from waterline to sheerline, no matter there are recess drains or not. Again, freeboard is considered the watertight hull portion above waterline which provides sufficient reserved buoyancy to the vessel in naval architecture, why FM can be measured up to sheerline if there are drains always open to sea between waterline and sheerline?

 

Lesser of two evils?
Does it say they aren't dangerous or does it imply they are not to be considered a danger in design.
Normally, they are a valuable asset so long as they're not too large.
A huge loss of buoyancy would be needed to render them a danger, no?

 

I am talking about the recess opening on side shell of a small boat with recess. Opening like freeing ports of bulwark of ocean going vessel serve the purpose of draining green water on board, we never count the bulwark when doing stability calculation and freeboard assigment in traditional naval architecture. How come the concept is changed dramatically in ISO12217?

 

I'm outta my wheelhouse on this one.
Disregard my last.

 

Small boats have different concepts. And it is easy to resolve if one has experience with small craft.

If you want to use bulwark, limit watertight volume by deck - in stability calcs.
OR, you assess the boat as 'open' and freeboard is actually 'sheerline of bulwark', but then no drains in 'bulwark' toward sides.

 

It is not taken into account because it does not produce a volume that can be counted as a buoyancy reserve, but the amount of water that can be shipped on deck due to bulwark must be counted. That is why it is necessary to place the drainage ports, to guarantee that this embarked water is dislodged quickly.

 

You need to consider this both philosophically and topologically. You need to get your head around the fact that there can be volumes and weights inside the shell of a vessel that are not part of the buoyant displacement, waterplane, or static weight and are not considered when determining stability unless damaged. This stems from the method of buoyancy calculation; "inside to out" or "outside to in". In the former, you are concerned with defining topology; the latter is subdividing volumes.
Lets say I have an anchor locker in the bow of a 10m yacht. It is watertight to the interior but not to the deck.
If it had no drains, it could be considered part of the buoyant hull. But any water trapped in there, by down flooding or even rain, must be added to the weight of the vessel and reduced from waterplane. This is an "outside to in" method and the most typical taught or referred to in naval architecture. The volume inside the assumed shell is subdivided into "tanks" and each "tank" is individually addressed at each stability condition. Even if the anchor locker is drained to the bilge, the flooding/draining water has be addressed as a weight and a free surface.
Now lets say I drill two drain holes in the shell on each side to drain the anchor locker. Topologically, that anchor locker volume is no longer part of the buoyant vessel. The hole in the shell allows a continuous path from the outside shell to the inside of the anchor locker skin: they are the same buoyant boundary for the "inside". This is the "inside to out" method and, while much more tedious that the "outside to in" method, is much more precise and can be used on vessels that have little to no internal volume. As long as this volume is "small" or "quick draining" (i.e. complies with 6.1.1.6) it is effectively not part of the hull and there can be no down flooding through the anchor locker deck opening or drain holes. And this topology extends below the water. Instead of a drain holes, assume we have a drain pipe that exits the shell below the water line. The inside of that drain pipe is now the buoyant boundary; so the seawater in that pipe is not part of the vessels weight and the area of that pipe is not part of the waterplane area.
Additionally, this philosophy and topology can extend to much larger things. Fish holds, peak tanks, anti-roll blisters, skegs, etc; can all be free flooding and therefore not part of the determination of static stability.
Again, this assumes that the feature is not damaged. In the case of our anchor locker, if the inner skin is ruptured, that is effectively the same as having a hole in the outer shell. Finally, this is about static stability. The hydrodynamic effect of these features and even the calculation of dynamic and hydrodynamic mass must include the entrained water of these features.

Edit: to add emphasis.

 

Thank you very much for such a detail explanation. It really opens my mind.

Let me elaborate your idea in my way, and please correct me if I am wrong.

Boat with recess which is sufficiently small (i.e. (LHBHFM)/40) or quick draining, the recess drains can be ignored when we assess stability or assign freeboard. The reason for that is the volume of recess is so small that even water comes and acculmulates there doesn't make considerable adverse effect to the boat, as compared to the relatively sufficient reserved buoyancy (i.e.LHBHFM).

Similarly, quick drain recess just doesn't acculmulate water on board for too long in time, because of its quick draining feature.

In short, both the cases guarantee only "very few amount of water accumulated in the recess with no time limit" or "large amount of water accumulated for only a short time", but what important is the boat can resume to its original condition (i.e. recess dry) in both cases eventually.

Under this circumstances, the side shell (edge of recess) can be included as a part of reserved bouyancy even there are recess drains on it.

Am I right with this interpretation?

 

Jordan, if you show us the boat in question, it would be easier to guide you through ISO...
ISO is easy to apply but if you do not work with small craft it can be difficult to understand.

 

Thank you for your advice. I agree that it may be easy to apply by following the table provided in ISO12217, but I do want to understand the concept behind, so that I can make sure that I apply the standard correctly, or even develop something new on this basis.