The Offshore Voyaging Reference Site

Watertight Bulkheads

One of the things that is attractive about metal construction is the ease with which structural modifications can be made. What would require substantial amounts of design, planning, physical work and cost in a GRP boat can require very little effort in metal, whether steel or aluminium.

A good example of this is the installation of watertight bulkheads. When we had Pèlerin constructed we specified an additional full depth watertight bulkhead aft of the anchor locker. As the angled back wall of that locker already formed an effective watertight bulkhead it might seem to be overkill to add a second one, but in fact the anchor locker bulkhead is too far forward and so doesn’t extend far enough below the waterline, and as there was already a designed ring frame behind it, (a) we weren’t going to lose much additional space and (b) all that had to be done was provide a plate to ‘fill’ the ring frame to make it fully watertight.  The cost was in the order of $100—negligible on our boat.

Our thinking was that in going to this extra length we were forming a double crash bulkhead that would absorb a head-on impact far more effectively. It would also be much more likely to retain full watertight integrity, which was, of course, our reason for going down this route in the first place. And with an additional watertight bulkhead aft as standard we had the best of both worlds.

Are they a must-have?

No—as John has pointed out in a very well argued earlier post, it’s important to understand the limitations of watertight bulkheads, especially when considered in light of the few sinkings caused by collisions with floating objects. But, in my view, the addition of watertight bulkheads is still worth considering if the cost and complexity equation is favourable, as it was in our case.

And whilst I’d agree that they won’t make a boat unsinkable, they will buy you time in the event of a major collision, perhaps allowing you time to effect a basic repair to reduce the ingress of water, thus preserving batteries so that you can get out an appropriate call for assistance via sat phone or VHF/DSC radio. Or, in the worst of scenarios, buying you time to abandon ship in good order. I don’t think that’s in any way a negligible advantage—quite the opposite.  And if I were to build or specify another new boat, would I do the same again? The answer is emphatically yes, and here’s why.

They could save your skin

In the event of a collision that causes significant water ingress, then your primary aim will be first to stop the water coming in and then to get it out. Consider that a collision might well occur at night and/or in a less than calm sea, and you are definitely going to be up against it, especially if you are short handed.

Let’s throw in the physical challenge of stopping the water coming in and then getting it out (even with the boat stopped), as the batteries fade and the strum boxes clog. It won’t be long before you’re faced with the unpalatable choice of getting out a mayday call before it’s impossible to do so, launching the liferaft and gathering up panic boxes and grab bags and abandoning ship—and now you’re really on your own. To me, whatever we can do to stop the water coming in, even if we’re only buying time, has to be a very good thing indeed—and that’s when a watertight bulkhead might well save your skin.

Of course, that’s a relatively easy choice to make if it’s an easy option (as it was for us). And, even in that case, there are other things worth serious consideration:

  • It’s a lot easier to design and install a simple solid bulkhead, i.e. without the need to physically pass through it. On some GRP boats where the bulkheads are structural features, the only possible bulkhead capable of being converted may be the one between the saloon and the forecabin. To do that it will therefore be necessary to add to the cost and complexity by adding a proper watertight door. Not only that, the tabbing and bonding of the bulkhead itself may well have to be beefed up to give it sufficient structural integrity to withstand the massive amount of water on the other side of the bulkhead.  And watertight doors are only of any use if you keep them closed, which may make them totally impractical for anything other than a military vessel!
  • Unless you’re planning to undergo a seriously major re-fit, installing a watertight bulkhead on an existing boat is probably be out of the question. But one simple option might be to reinforce the bulkhead abaft the anchor locker, which will at least give some protection. If it is fairly deep and extends well below the waterline, well and good.
  • Another option might be to convert the underbunk space in the forecabin to a ‘crash box’ with the inclusion of some form of structural foam to keep the water at bay even if the external shell is breached. This is one of the few modifications that can be relatively easily achieved with an existing boat, although some locker space will be lost. But I have heard a couple of reported cases where this simple modification has proved to be a life-saver.
  • Any watertight bulkhead must be just that—watertight. By which I mean with a minimum of holes. Consideration should be made at the design stage about ways to minimize the number and size of holes to allow the passage of pipework and cabling, to make provision for top quality cable glands to seal them and (at the very least) pass them through the bulkhead at the top.
  • Some people debate the value of a watertight bulkhead in the stern. Where such bulkheads certainly have a place is aboard a boat with a spade or transom hung rudder, where a massive impact upon the rudder might well cause the hull to be ruptured around the rudder tube or mountings. Obviously on such a boat the watertight bulkhead will need to be ahead of the rudder post, and the same attention paid to the passage of cables and pipework paid as above.

There’s a strong and valid argument that watertight bulkheads are not essential for an average cruising boat that spends most of its time sailing within relatively easy reach of assistance. For a yacht that will spend more time well offshore, in my view the argument starts to tilt towards acceptance. And for an ocean cruising boat, especially one that will go, say, polar, I’d argue that the case for their inclusion is hard to resist. Even if it only buys you time, those might be the most precious minutes of your life.

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