Watertight Bulkheads

[rob2]

It doesn't need to be deckhead height. If you are flooded that high, it's time to leave.

If you lose the keel and invert then that line of thought will be proven wrong. A crash bulkhead to limit flooding if the bows are damaged may not be taken to the bulkhead if the pumps are adequate - but then that's exactly what they believed with the Titanic! In addition, unless the watertight properties are compromised by a lesser fastening then, in the event of a sudden inrush of ocean, the whole apparatus is useless - there is never as much time as you would hope during an emergency. It's probably best to consider watertightness of some volume as simply a flotation device and better that it should have no frequently used openings. For example, sealing the lockers under the bunks might be sufficient to keep the holed hull afloat, but on a cruising boat it would make access to storage very slow, so the hatches may not regularly be screwed down.

Rob.

P.S. had to post to celebrate regaining access by changing the browser.

 

[pmagowan]

If you lose the keel and invert then that line of thought will be proven wrong. A crash bulkhead to limit flooding if the bows are damaged may not be taken to the bulkhead if the pumps are adequate - but then that's exactly what they believed with the Titanic! In addition, unless the watertight properties are compromised by a lesser fastening then, in the event of a sudden inrush of ocean, the whole apparatus is useless - there is never as much time as you would hope during an emergency. It's probably best to consider watertightness of some volume as simply a flotation device and better that it should have no frequently used openings. For example, sealing the lockers under the bunks might be sufficient to keep the holed hull afloat, but on a cruising boat it would make access to storage very slow, so the hatches may not regularly be screwed down.

Rob.

P.S. had to post to celebrate regaining access by changing the browser.

It is one of these things that you never expect to need but when you do it is priceless. I am designing a boat that can be used for open water cruising in areas where help would not be quick to respond, which is why these concepts interest me. Like the threads on risk of fire I think we need to accept that there is always a level of risk and our actions will never eliminate it but we should look to minimise risks with reasonable precautions. I thought about the V-berth as a potential flotation device and also a separate crash bulkhead even though it does not go very high. It would be possible to put in hatches that were easy enough to remove for storage of things that are not in everyday use. I suspect that V-berth storage is generally not used daily anyway (in a 40'+ boat) as it is a pain to lift the mattress. Every bit of the hull that has a sealed 'tank' inside it has potential breach protection as well as flotation benefits, so water, fuel and waste could also be used.

Looking at the detail I think it would be easy enough to create a watertight bulkhead with a door in it for the forward cabin. The door is the biggest problem but it is more to do with minimising the weight and 'clunkiness' so that it is not an everyday problem to use. I think that the plan would be that in general use the door would be used as normal and left either open or closed but in conditions where there are increased risks the door could be closed and the main seal engaged with a central handle controlling multiple 'dogs'. I am not sure whether is is necessary for the door to be watertight each way or only from the forward side as it is unlikely that the breach would be aft and if it was there would be limited benefit to a sealed forward cabin. I would envisage access to the forward cabin, when flooded, would be through a deck hatch.

Any breach in the forward hull would essentially not sink the boat but flood only the forward cabin. This would allow a more calm response and plenty of time to find the source of the problem and fix it.

 

[WilliamUK]

Titanic had partial-height bulkheads. Their partialness was a major factor in her sinking as the flooding from the hull damage spilled over the bulkheads one at a time, filling successive sections and pulling her into the famous nose-first dive to the bottom.
Something to consider.

 

[Stemar]

I'm no expert, but it seems to me that the multi-point systems on grey funnel jobs are designed to seal from either side, even with a degree of distortion in the bulkhead. To stay afloat in the event of a collision with a container or similar, a door that opens into the forepeak and closes against a neoprene seal wouldn't need much in the way of locking shut. As long as the ordinary lock holds it against the seal, when you need a bit more, water pressure would force the door against the seal.

Something else to consider would be to seal the under bunk area. That strikes me as the most likely place to get a hole and restricting the 'oggin to that would be better that giving up the whole forepeak. Jissel has a single area with three openings for access. Were I to do such a thing, I'd divide it into three as well, thereby restricting ingress to a few hundred litres. In an ideal world, one would seal the lockers and the bulkhead, but doing the bulkhead wouldn't be realistic for me, and probably not in a fair few other smaller boats.

 

[AntarcticPilot]

One worry I'd have is that a bulkhead might well distort or even fracture under the loads imposed by an impact sufficient to breach the hull. After all, by definition, you're imposing forces greater than the fracture limit of the material of the hull, and it is unlikely that a bulkhead would be stronger than the hull. So, your door would have to be engineered to work even if the bulkhead had distorted - a tall order.

Might not a better solution be to have boards pre-cut to overlap the door aperture, with strong-backs to bolt them to? Then in the event of an uncontrollable leak, you place the board on the side nearest the leak, and screw the strong backs on the side away from the leak? The board could be coated on one side with a soft rubber, to ensure a seal. The boards would be of a shape and size to store under the mattress of a berth.

 

[pmagowan]

One worry I'd have is that a bulkhead might well distort or even fracture under the loads imposed by an impact sufficient to breach the hull. After all, by definition, you're imposing forces greater than the fracture limit of the material of the hull, and it is unlikely that a bulkhead would be stronger than the hull. So, your door would have to be engineered to work even if the bulkhead had distorted - a tall order.

Might not a better solution be to have boards pre-cut to overlap the door aperture, with strong-backs to bolt them to? Then in the event of an uncontrollable leak, you place the board on the side nearest the leak, and screw the strong backs on the side away from the leak? The board could be coated on one side with a soft rubber, to ensure a seal. The boards would be of a shape and size to store under the mattress of a berth.

That could work but I suspect in a situation of that severity you are not going to be able to spare the time for such a solution. Worst case scenario if, as you suggest, the bulkhead buckles then I think the watertight door would simply slow things down so that you could abandon ship as required. However, I think that with the average position of the forward cabin bulkhead the liklihood of the impact causing it to fail would be low. Once you get beyond the forward quarter of the bow then you are dealing with a glancing blow where the hull should be able to take it, especially when reinforced by the bulkhead. I suspect, but need to do more research, that the most likely place to get holed on a collision is very much further forward and probably just to one side or the other of the hull/keel join.

I am looking for a solution that would be least disruptive and also easiest to use in the event of a collision resulting in a hull breach. I suspect that anything too involved is likely to be problematic in such circumstances. I also suspect that a composite bulkhead with a core material, ply skins and fibreglass coating would be quite resistant to deforming shape such that it failed. Remember we are likely dealing with an impact at around 6 knots with a floating object.

 

[AntarcticPilot]

That could work but I suspect in a situation of that severity you are not going to be able to spare the time for such a solution. Worst case scenario if, as you suggest, the bulkhead buckles then I think the watertight door would simply slow things down so that you could abandon ship as required. However, I think that with the average position of the forward cabin bulkhead the liklihood of the impact causing it to fail would be low. Once you get beyond the forward quarter of the bow then you are dealing with a glancing blow where the hull should be able to take it, especially when reinforced by the bulkhead. I suspect, but need to do more research, that the most likely place to get holed on a collision is very much further forward and probably just to one side or the other of the hull/keel join.

I am looking for a solution that would be least disruptive and also easiest to use in the event of a collision resulting in a hull breach. I suspect that anything too involved is likely to be problematic in such circumstances. I also suspect that a composite bulkhead with a core material, ply skins and fibreglass coating would be quite resistant to deforming shape such that it failed. Remember we are likely dealing with an impact at around 6 knots with a floating object.

Good points.

The experience of the crash-test boat suggests that even quite a large breach in a hull doesn't flood the boat that rapidly; the team had ample time to test various systems to stem the flow of water. In no case did the water from a hole about a foot long get much above the cabin sole before they had an effective means of stemming the flow in place. YMMV in the case of a boat at sea in a storm! Point is, though, that a breach large enough to flood the boat rapidly is also likely to extend a significant part of the length of the boat, and might well span a bulkhead.

Another point from the same crash-test video is that it took an enormous amount of force to breach the hull of a GRP boat; they had to resort to an angle grinder; a sledge hammer wasn't enough! I suspect the same would go for the construction you're proposing, which is effectively wood reinforced epoxy.

Of course there are cases where boats have flooded rapidly, but even they have given the crew time for an orderly evacuation - both the Baileys and the Robinsons suffered rapid sinkings, but both were able to abandon ship with enough equipment to allow their survival. The recent sad case of Cheeki Rafiki was an inversion, and the boat didn't actually sink for many days.

Another point is the method of attachment of the bulkhead. Again, I can only speak for GRP construction, and your wood construction may well be stronger. but in a GRP vessel, the wooden bulkheads are attached to the hull relatively weakly, and could easily break from the hull in a collision sufficient to breach the hull.

One other thought - if you seal a water-tight door or whatever, you're effectively saying that whatever is the other side of that bulkhead is abandoned. So you're giving up the possibility of stemming the leak. Again, the crash test boat suggests that there are lots of things that could be done to at least slow the inflow of water, using materials that we all have on board. But several of the techniques tested required access from the inside.

Many years ago, I happened to be aboard a plywood boat that was holed in the River Humber. I think that a sheet of ply was effectively stove in; we must have hit a log or other floating debris. The boat was small - perhaps 18' - and I was a child, so my memory is distant and perhaps not accurate! But even then, the water was kept under control by a mixture of using berth cushions and baling with a bucket. We made it to a safe harbour - fortunately we were making passage with my father's boat, who towed us in, so if she had sunk, we'd have been Ok!

 

[rogerthebodger]

" Remember we are likely dealing with an impact at around 6 knots with a floating object. "

If its a container or the like it is more likely to take your keel off in which a watertight bulkhead would not help or even a high power bilge pump.

Some years age we had a GRP boat loose it keel and it instantly inverted found some time later still afloat but inverted (all lost).

I considered fitting watertight bulkheads to my boat when I was building here but didn't in the end as I considered being steel the likelihood of a bow breach as low and if it did a watertight bulkhead any further aft than the forward cabin aft end would result in the bow being very low in the water and make the boat liable to sink due to ingress of water through the deck hatches aft of the forward cabin.

The Titanic would stay afloat with 4 sections breaches but it was 5 so its bow was too low in the water which allowed the water to top the following bulkheads.

If you do go for watertight bulkheads you need to be positive that the bow will not sink so the boat becomes unrecoverable. The Titanic had a lot more weight aft of the damage to keep it stable than a shorter yacht.

I think it was said that it all of Titanic life jackets was places in the forward damaged section it would have been kept afloat, don't know if that is true but it a nice dea.

 

[pmagowan]

Good points.

I think we must assume that any collision large enough to breach through two compartments is likely to end in a sinking and we can't really plan for it any more than we would with a conventional boat so we are really planning for a breach of the forward hull.

I am pretty confident that I can make a boat that is strong enough to survive everything but the most extreme situation. The composite structure does provide amazing strength in a monocoque type approach although I would have frames and would use a frame as the support for a watertight bulkhead. This can all be designed into the build so that frames and bulkheads coincide with each other to provide additional strength. It can also be done with berths and seating as well as shelving, essentially acting as structural elements. I am also considering the use of more exotic fibres in the hull section as I want the boat to be useful in areas where ice may be encountered.

I think, at the design stage, there is a lot of oportunity for tweaking things so that stuff you need can also meet an alternative need. An anchor locker being a crash bulkhead, V berths being sealed storage and flotation, water, waste and fuel tanks providing a 'double hull' etc.

Access to the forward cabin, in the even of inundation would have to be through a deck hatch but I suppose with the door opening forward that you would not necessarily have to seal the door until the water was over the lip and starting to flood the main cabin, and even then the bilge pumps may be able to buy you time.

At the end of the day I think the greatest risk of significant water coming into the boat is through holes that are man made, through-hulls. If access to all of these are good and there are bungs available then the main problem is determining which one has failed and quickly.

 

[pmagowan]

" Remember we are likely dealing with an impact at around 6 knots with a floating object. "

If its a container or the like it is more likely to take your keel off in which a watertight bulkhead would not help or even a high power bilge pump.

Some years age we had a GRP boat loose it keel and it instantly inverted found some time later still afloat but inverted (all lost).

I considered fitting watertight bulkheads to my boat when I was building here but didn't in the end as I considered being steel the likelihood of a bow breach as low and if it did a watertight bulkhead any further aft than the forward cabin aft end would result in the bow being very low in the water and make the boat liable to sink due to ingress of water through the deck hatches aft of the forward cabin.

The Titanic would stay afloat with 4 sections breaches but it was 5 so its bow was too low in the water which allowed the water to top the following bulkheads.

If you do go for watertight bulkheads you need to be positive that the bow will not sink so the boat becomes unrecoverable. The Titanic had a lot more weight aft of the damage to keep it stable than a shorter yacht.

I think it was said that it all of Titanic life jackets was places in the forward damaged section it would have been kept afloat, don't know if that is true but it a nice dea.

As far as the keel goes I will be going for a long fin keel to reduce the likelihood of it being ripped off in such a collision. With the shape of the hull and keel you would expect the force to be dissipated by forcing the boat to ride up, onto the object you collided with. I think it is possible to make such a keel that it would be impossible to 'knock' it off with a collision at full speed.

 

[Twister_Ken]

One thought, if you have a sealed forecabin flooded to height x, is free surface effect, which might reduce stability so much that the boat becomes untenable. Don't know if that's the case, but with 1 cu mt of water weighing in at a ton (and you'd probably have more than 1 cu mt in a flooded forecabin) rushing from side to side in response to wave action it could become a problem

 

[rogerthebodger]

That's my theory on the design of my keel plus the structure is integral to the whole hull structure.

 

[pmagowan]

One thought, if you have a sealed forecabin flooded to height x, is free surface effect, which might reduce stability so much that the boat becomes untenable. Don't know if that's the case, but with 1 cu mt of water weighing in at a ton (and you'd probably have more than 1 cu mt in a flooded forecabin) rushing from side to side in response to wave action it could become a problem

This can reasonably easily be calculated into the stability calculations. I suspect it would not be a problem as, if you think about the whole hull then the non-flooded portion is going to be supporting the flooded portion and actually reducing the level of flooding. It will essentially cause a change in the waterline related to the loss of buoyancy of this forward section. There will be no weight forced above the waterline that would cause the stability to be such that the boat would want to turn over, for example. The forward section is small in comparrison to the remaining floating section and is still likely to retain significant buoyancy from V-berths etc.

 

[Tim Good]

To the OP... I my 43ft seastream the forward cabin is designed to be watertight or at least sufficient enough for you to fix the problem and pump it back out.

The bunk itself is above the waterline and each locker beneath is sealed with bolt down lids. Then the whole cabin is also sealed with a bulk head to deck level and the door / frame is about 10mm thicker than the others in the boat with up rated hinges, a watertight rubber seal around the frame and then large levers that lock down the door from outside. Once locked down the only outlet is via pipe to a bilge pump further aft which can be switch from electric to auxiliary. It was designed by Ian Anderson so I guess plenty of thought would have gone into it. Hope I never have to rely on it but on a long passage to higher latitudes I'll leave it locked off as a refrigerator room.

If you want me to send you some photos just shout.

 

[john_morris_uk]

The ideas mentioned on here are venturing into the realms of commercial ship/boat design, where full height water tight bulkheads are the norm, with the appropriate bilge pumping and cable/pipe runs. The only small boats I can think of with that sort of protection are lifeboats and pilot cutters. If you get the chance to have a close look inside a proper pilot cutter you will realise how flimsy the average leisure craft are!

I would like to emphasise the various points made about not underestimating the forces involved. Proper watertight integrity is very difficult to achieve and if it was as straightforward as some people suggest, more yacht builders would have included it as a selling point to the anxious. Watertight doors and hatches that remain watertight as bullheads distort are large and heavy. The steel built challenge boats have water tight compartments and the doors in them are substantial.

 

[pmagowan]

I would like to emphasise the various points made about not underestimating the forces involved. Proper watertight integrity is very difficult to achieve and if it was as straightforward as some people suggest, more yacht builders would have included it as a selling point to the anxious. Watertight doors and hatches that remain watertight as bullheads distort are large and heavy. The steel built challenge boats have water tight compartments and the doors in them are substantial.

I am not sure why it would be a problem. Water pressure is easy to calculate as are the scantings for bulkheads. If you think about it the transom is just a watertight bulkhead at the end of the boat. Using plyboard and glass fibre it should be relatively straight forward. The main problems I envisage is the changes to wiring layout that would be required. The doors and frames would need to be a bit bulkier as per Northcave but this is not designed to be holding back water permanently. I would have thought a forward opening door with a rubber seal would seal itself quite well from water pressure alone.

My current plan is to have all the berths and saloon seats as watertight compartments with simple watertight hatches for access to storage. I recon I can get at least 8 tonnes of bouyancy from these alone. Then a watertight crash bulkhead fore and aft and the forward cabin being seperated by a watertight bulkhead. With built in water and fuel tanks this scenario would give me a second hull in most areas, massive bouyancy and great defense against all but the most severe breach of the hull. It would also do so with minimal disruption to the 'flow' of the boat layout. It would require additional flood sensors and additional bilge pumping complexity, but nothing overly complicated. It would also make storage in these spaces a minor inconvenience but then it already is.

I think since most boats do not get used for blue water cruising most people do not consider these things high on their priorities. There will be extra work and complexity in the design but for a home build I think that is justified. Obviously commercial operations have different priorities.

 

[pmagowan]

To the OP... I my 43ft seastream the forward cabin is designed to be watertight or at least sufficient enough for you to fix the problem and pump it back out.

The bunk itself is above the waterline and each locker beneath is sealed with bolt down lids. Then the whole cabin is also sealed with a bulk head to deck level and the door / frame is about 10mm thicker than the others in the boat with up rated hinges, a watertight rubber seal around the frame and then large levers that lock down the door from outside. Once locked down the only outlet is via pipe to a bilge pump further aft which can be switch from electric to auxiliary. It was designed by Ian Anderson so I guess plenty of thought would have gone into it. Hope I never have to rely on it but on a long passage to higher latitudes I'll leave it locked off as a refrigerator room.

If you want me to send you some photos just shout.

If you could post any pictures I would be very interested. Particularly in the structure of the door. There is no point in reinventing the wheel if someone has already done the work. Thanks

 

[Tim Good]

If you could post any pictures I would be very interested. Particularly in the structure of the door. There is no point in reinventing the wheel if someone has already done the work. Thanks

Probably not down to the boat for a week but will take some photos then and post.

 

[pmagowan]

Probably not down to the boat for a week but will take some photos then and post.

Cool, thanks.

 

[prv]

The main problems I envisage is the changes to wiring layout that would be required.

Shouldn't make too much difference - the average forepeak doesn't have tons of electrical gear in it to run wiring to. A well fitted out boat would have all the wiring high up anyway - either above the headlining or in the corner between deck and topsides - so even if you made no attempt to seal them you're fine until full submersion. But a couple of glands for the windlass cables plus one of those drill-your-own-holes multi-glands for other wiring should cover even that.

Pete