Whale strike on a cruising cat: which compartment has to hold, and how do you test a bulkhead?

[gonzo]

My question is about free surface effect, not in particular about your boat. You previously said the compartment would completely fill, so how can above the waterline tanks fill without fully submerging?

 

[fallguy]

Semantics now.

The compartment will completely fill … up to the level of the water… if the water level is higher than a breached compartment; assuming the breach is not above the wl, it would completely fill… doesn’t mean the boat would sink …

As far as free surface effect, we certainly agree a catastrophic event such as a whale strike would offer uncertain results as fluid entering hull spaces would add uncertainty. The best hope would be not sinking or breaking up in high seas. I think considering the movement of fluid inside a hull or part of a hull is just a compounding issue.

More important than all of these. Is the vessel pos buoyant? I’ve done some calcs on my own boat and the boat is, but the engine weight on the back is rather heavy. I’d say the vessel buoyancy is way more important. And for a sailing cat, can a badly submerged hull still keep the mast vertical?

That is, all the static matters first. If you can get to worrying about water sloshing about; that would be best.

 

[gonzo]

Semantics now.

The compartment will completely fill … up to the level of the water… if the water level is higher than a breached compartment; assuming the breach is not above the wl, it would completely fill… doesn’t mean the boat would sink …

It is not semantics. It is an engineering requirement that a partially flooded compartment must have bulkheads that withstand the stresses caused by the water free surface effect.

 

[fallguy]

It is not semantics. It is an engineering requirement that a partially flooded compartment must have bulkheads that withstand the stresses caused by the water free surface effect.

The semantics is about the flooding of the watertights and associated water levels.

But I don’t believe the ISO standards even evaluate or post requirements about less than intact structures. So, the issue of sloshing water inside is largely mute in design, with the obvious exception of closed bulkheads on ships, which is not much in the scope of most small multihulls. I don’t profess wide knowledge of the ISO standards, but pretty sure they don’t design for unknown catastrophe, within reason.

The vessel’s buoyancy when holed would be paramount, as I suggested earlier.

You, as a wise sailor which I am sure of, understand well the order of things. The hull is breached, does it stay afloat or sink, that is #1. If it stays afloat, can it move or will it break apart, that is #2. A theoretical whale strike in most multihulls of say 20M or less are difficult to design for. The larger 20M cat has living spaces below. Things like the bed can be made so a watertight is below, but there is little certainty on the extent of the calamity. Some fair estimates can be made and probably the breach would be less than the bed base, but really nothing is certain.

My boat bottom is foam core. A whale would do horrible things to it.

 

[montero]

In my world of catamarans, losing an one entire hull doesn't mean a loss of buoyancy—I still have the other one.Much better than mono.

 

[fallguy]

In my world of catamarans, losing an one entire hull doesn't mean a loss of buoyancy—I still have the other one.Much better than mono.

Your comment assumes a positive buoyant vessel. To suggest catamarans have some special feature without this remark is invalid. A holed cat that is not pos buoyant can sink one one side, then the bdeck can sink and there is little to stop a sideways hull from taking on water.

We are at a point of hair splitting on this thread. This happens on threads without definitive answers and this one is a perfect candidate.

We get into a LOT of discussions on semantics about where water lines are and how water sloshes and if a vessel is a cat it won’t sink, etc. It can start to seem like a personal attack or ad hominem when the real issue is there is no clear answer.

What happens when a whale smashes your boat? It depends on a lot of conditionals. Will it float, will the sea smash it to bits, is it a pos buoyant vessel, will the mast take it over (and go).

The best answer is you better have offshore distress calling.

I’m out. Good day fellas.

 

[gonzo]

The semantics is about the flooding of the watertights and associated water levels.

But I don’t believe the ISO standards even evaluate or post requirements about less than intact structures. So, the issue of sloshing water inside is largely mute in design

It is a structural concern; not semantics. Also, it is not a reference to ISO or any other standards. A compartment that is suppose to provide reserve buoyancy in case of damage to the hull, has to be engineered for the expected worst case scenario stresses. The free surface effect can be considerable. If the bulkhead fails, it will cease to make the compartment reserve buoyancy.

We get into a LOT of discussions on semantics about where water lines are and how water sloshes and if a vessel is a cat it won’t sink, etc. It can start to seem like a personal attack or ad hominem when the real issue is there is no clear answer.

This is not personal. There is a clear answer. Calculating the stresses of the water's free surface effect is something not so difficult in engineering. In fact, that is a basic calculation for applications like tankers.

 

[montero]

Your comment assumes a positive buoyant vessel. To suggest catamarans have some special feature without this remark is invalid. A holed cat that is not pos buoyant can sink one one side, then the bdeck can sink and there is little to stop a sideways hull from taking on water.

We are at a point of hair splitting on this thread. This happens on threads without definitive answers and this one is a perfect candidate.

We get into a LOT of discussions on semantics about where water lines are and how water sloshes and if a vessel is a cat it won’t sink, etc. It can start to seem like a personal attack or ad hominem when the real issue is there is no clear answer.

What happens when a whale smashes your boat? It depends on a lot of conditionals. Will it float, will the sea smash it to bits, is it a pos buoyant vessel, will the mast take it over (and go).

The best answer is you better have offshore distress calling.

I’m out. Good day fellas.

I wrote "in my world of catamarans."

Can you imagine a situation where no one shows up within a reasonable amount of time?