Watertight Bulkheads

Anchor locker bulkhead

Anchor locker bulkhead looking aft.

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.

Watertight bulkhead detail

Watertight bulkhead detail

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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Colin Speedie

Colin, European Correspondent here at AAC, is a deeply experienced offshore sailor who holds a Yachtmaster licence, and a gifted photographer and talented writer who has added a whole new dimension to Attainable Adventure Cruising. In addition, since Colin and Louise are from England and had their OVNI 435, Pèlerin built in France, they bring a European perspective to our site. You can read more about Colin and Louise and their business at their website.

24 comments… add one
  • Marc Dacey Dec 13, 2013, 5:51 pm

    Clearly, we are of one mind on this. If I hadn’t bought a steel boat with a collision bulkhead, I would have considered installing one. As you said, the pluses outweigh the minuses, the biggest one of which is having to go on deck to get into the forepeak workshop space. I am debating the wisdom of a watertight hatch just under the deck from the salon to the workshop, allowing bad weather access (the forepeak hatch is steel, too, and is heavy), communication with the crew, and the ability to do stuff like work on a boom end inside the boat.

    I don’t wish to defeat my own purpose, however. On the other hand, I am not convinced that I have so much reserve buoyancy in the bow that smashing a below-WL hole in the bow would actually put water more than one foot above the current WL. Some math may be in order, but it’s from a happy place.

    • Colin Speedie Dec 14, 2013, 5:06 pm

      Hi Marc

      We also have to go through a deck hatch to gain access, and it’s a pain, but at the same time, it means we don’t have to a water tight door, which would have been far more weight and cost, and is potentially less reliable.

      Still a good idea in my view.

      Best wishes


  • Nuno Dec 13, 2013, 6:55 pm

    I couldn’t agree more.
    One forgoten chalenge is to make it safer under the tanks that normally are placed along the hull. When you get a damage under one of that tanks it is almost impossible to be able to reach the damage for stopping the water flow as the tanks are in the way.
    I do own a GRP boat that it isn’t very easy to build water tight bulkheads. But there is diferent ways to help to at least slow down a possible water intake due to a collision.
    The idea with a flexible watertight coat of foam is great and I am already considering it in some places. Thanks.
    One thing to take in consideration is also not forget that it is important to easily reach the all parts of the interior hull for a quick reparation.
    Looking forward to read the next post.
    Best regards from Sweden

    • Colin Speedie Dec 14, 2013, 5:09 pm

      Hi Nuno

      many good points, and one in particular that stands out, that with many GRP boats it can be very difficult indeed to get to underwater damage, due to internal tanks, or more likely internal mouldings. On our last boat that was the case, and it isn’t ideal, by any means.

      But as you say, there are things you can do, and that’s not negligible.

      Best wishes


  • Travis C Dec 14, 2013, 2:07 am

    I once had the misfortune of just such a rearward rudder crash on my Cal 28. The shaft ran through a fiberglass tube, tabbed to the hull and cockpit floor. The momentum crushed the hull tabbing, allowing a difficult to slow inflow of water. In hindsight, a small crash box could have been built in the space and not interfered with day-to-day operations.

    • Colin Speedie Dec 14, 2013, 5:10 pm

      Hi Travis

      Sounds like you were lucky to get away with it – and with a relatively minor modification, you can mitigate against it.

      Best wishes


  • Dick Stevenson Dec 14, 2013, 4:11 am

    Dear Colin,
    Well argued and very reasonable. One of the implications is that all your chain/rode (maybe for 2 anchors) is in the bow at all times. On a 40 foot boat, this can be quite a load to have at the tip of the boat. I know there would be a piece of me aware (and annoyed) at the sailing/pitching compromise induced by this weight in everyday life even on Alchemy, a beefy 40 footer. On Alchemy, where a watertight bulkhead is not likely for all the reasons you specify for a GRP boat, we pull the ½ + of our chain that we rarely use to amidships. When you can’t have something, it is good to look at the bright side of these compromises.
    I appreciate the brain teaser as well. It took me a while to figure that the picture was not: 1. The bilge pump in its recess with hoses out the side, 2. The windlass motor IN the anchor well, but 3. An extremely nice design having the windlass motor in the more benign (and accessible) cabin environment while the working windlass is in the anchor well.
    Thanks for the article, Dick Stevenson, s/v Alchemy

    • Colin Speedie Dec 14, 2013, 5:16 pm

      Hi Dick

      Our set-up is not quite that bad, as we have our second anchor and cable in the sail locker which is behind the watertight bulkhead, so not right forward.

      And due to the fact that our hull bottom is filled with lead or tanks, we can’t store much in the bilges, although I totally agree it’s a really good idea.

      And yes, the windlass is well sited – a neat job, well executed by the builder.

      Best wishes


  • Matt Dec 14, 2013, 9:59 am

    Nicely put, Colin. Thanks.

    For what it’s worth, the trimaran I’m currently building has three WT bulkheads in each outrigger, in addition to two full ones and three partial (below the self-draining sole) ones in the main hull. If they’re strategically placed from the start, when the boat’s still on paper, it’s possible without much of a compromise in accommodations or storage.
    And, in our case, the incremental cost is negative; a full flat bulkhead is, for the section shapes in this boat, easier and simpler to install than a ring frame despite using a bit more material.

  • RDE Dec 14, 2013, 12:31 pm

    Hi Matt,
    The best way to build a watertight bulkhead is to take off the ballast keel. LOL

    On a catamaran or trimaran with a thick core and judicious placement of foam flotation under the cabin soles and in non-usable areas like the narrow bows it is entirely possible to design a vessel that can still sail slowly and motor with both hulls punctured and flooded.

    • Colin Speedie Dec 14, 2013, 5:18 pm

      Hi Matt, Richard

      another plus for new build, and multihulls, and foam in the right places could make for a really safe boat.

      Best wishes


  • Eric Klem Dec 14, 2013, 3:47 pm

    Hi Colin,

    I agree with what you have stated in your post. Being a fiberglass boat owner, I do not have a proper collision bulkhead but I would definitely feel better with one.

    Your point on making the watertight bulkhead truly watertight should not be missed by people making this modification. One boat that I worked on had an incident at the dock where a below waterline hose blew in the midships compartment. By the time the high water alarm had gone off, the water level in the engine room was increasing rapidly and half the engine was underwater. Luckily, the engine started and the large hydraulic trash pump lowered the level while the problem was traced down (this is a big boat so it takes a while to find it). Not only was the boat still in jeopardy of sinking, other than the high water alarm, the crew would not have known where in the boat to even start looking as there was water in all compartments. The reason that these bulkheads were not totally waterproof is that it was a wooden boat and it is incredibly difficult to make traditional bulkheads that are watertight. Even in non wooden boats, I have seen many owners get careless and start putting holes in the bulkheads that are not properly sealed (silicone sealant doesn’t count, you actually need a bulkhead fitting).

    It is also worth noting that watertight doors have limitations. The obvious one is that they need to be closed to work (and it would be really hard to close one once there is water) but in most installations, a good crew will have no problem with this. The other limitation is that they are really sensitive to being adjusted right and not being bent. All doors should be chaulked regularly to ensure that they actually seal, as they get used a lot, they tend to stop sealing well. Also, I have some concerns about how some door designs would do if the bulkhead they were in was damaged. Steel and aluminum bulkheads will often bend a lot without being breached but these doors may not seal well at all. It is still much better than nothing and I have absolutely no problems with a well executed installation and disciplined crew, it just needs to be thought through.


  • Colin Speedie Dec 14, 2013, 5:22 pm

    Hi Eric

    All good points, and as you say, watertight bulkheads in wooden boats are hard to achieve. I used to skipper an old wooden boat that (supposedly) had watertight bulkheads, and they were anything but. And she finally sank when alongside a dock….

    I agree that there is always the danger of leaks unless the doors and their seals are maintained, and also that an impact might distort or bend a bulkhead, one of the reasons I went for the compromise we have is that although it’s further forward than I’d like, there’s no need for a door.

    Best wishes


  • pat synge Dec 14, 2013, 8:28 pm

    I’ll mention emergency flotation bags for those of us with boats that don’t have watertight bulkheads.

    Back in the ’80s I was involved in the development of the Float Pac system (now Turtlepac). One big dive bottle and 2 bags give almost 2000kg of flotation . They take up little space (often simply laid out under mattresses) and can be instantly deployed. Enough of them will keep you afloat and as Colin says “even if it only buys you time, those might be the most precious minutes of your life.” Better than jumping into a liferaft.

    While ultimately not as tough as water-tight bulkheads they have another use that can be invaluable. They can be used externally (lashed under the hull/keel) to reduce draft when grounded and more than one boat has been floated off using them. Having a few dive bottles aboard may be useful as well.

    • Colin Speedie Dec 15, 2013, 3:57 pm

      Hi Pat

      good point, and something I’d completely forgotten about, so thanks for bringing it up. I suppose the only thing is a question of the scale necessary to keep a monohull afloat – I seem to recall they were being heavily promoted for multihulls at one time?

      Best wishes


  • Bryce Winter Dec 14, 2013, 7:34 pm

    Hi Colin,

    Our boat is a 40 foot aluminium sailboat without a watertight bulkhead but a fairly large forepeak. The chain locker is unsealed plywood (came like that!), and is getting soft and stinky. I’ll need to do something but am unsure just what to do yet. Thanks for this overview, it’s given me some ideas!

    I did have a couple of questions – did you need to replicate your bilge pumping arrangements for each water-tight section, or do you have a self-draining system? We do have a self-draining locker forward of the windlass that I’m trying to figure out a good use for – perhaps storage for long warps. Also, I’ve yet to see a satisfactory way of dealing with water ingress through the hawse-pipe – any ideas would be interesting in your upcoming post on keeping the water out!



    • Colin Speedie Dec 15, 2013, 3:54 pm

      Hi Bryce

      Our compartments don’t have individual pumping arrangements – they are there to keep the water out of the main compartment of the boat, and rate not too massive in terms of capacity.

      When you say self-draining, I’m assuming you’re talking about draining back to the sea? Like an anchor locker? There area number of ideas about such
      areas in Part II. Ditto – hawse pipes, which can indeed be a pain, but there are things that can be done to help.

      Best wishes


  • pat synge Dec 15, 2013, 7:56 pm

    You’re right about scale, Colin, and yes, they were promoted as extra flotation for multihulls as well as a means of righting a capsized one though I haven’t heard of this actually being successful. Laszslo Torok of Turtlepac would have that information. I do know that they have prevented a number of ditched helicopters from sinking.

    There are still quite a few smaller boats out cruising the oceans and inflatable emergency flotation for boats under about 35′ is really worth considering. Especially for timber or foam sandwich boats where the hull material already contributes to the flotation and/or if the ballast ratio is modest – as in some more traditional designs. A 1000 kg lift capacity bag weighs 6kg. An 80 cu ft dive tank is needed to inflate 2 of these bags and this weighs about 15kg. So, to provide 6 tonnes of lift you are looking at 6 bags @ 6kg each + 3 bottles @ 15Kg each. A total weight of 81kg ( ~180lbs).
    Quite a substantial weight but when you look at the ‘stuff’ aboard many cruising boats carry this would seem reasonable: especially if you then decide you don’t need a liferaft and enjoy diving.

    I would much prefer to stay with my boat even if it was full of water: as long as it was floating. Far better to have all your provisions, tools and so on than to be drifting about in a liferaft. Exactly like watertight bulkheads – often repairs can be undertaken if one has the time. Another advantage over the liferaft option is that they can easily be maintained and checked (not that they need much maintenance).

    I am fortunate in having a strongly built aluminium yacht and have the luxury of watertight bulkheads and a double skinned bottom (8 integral water tanks) but, as others have commented, this is not an easy option for many.

  • pat synge Dec 15, 2013, 8:11 pm

    I’m straying slightly from the original topic here but as it is relevant will mention that Laszlo Torok of Turtlepac has also recently developed an interesting emergency hull bandage. A bit like the old technique of passing a sail under the hull and using water pressure to hold it in place to attempt to stop a leak this is a tough reinforced polyester reinforced PVC inflatable ‘blanket’. Once lashed in place it is inflated and so uses both bouyancy and water pressure to hold it securely in place and ensure a seal.

    (I should mention that I have no financial interest in this business whatsoever having only ever worked with the company for a short time in the ’80s as a consultant.)

    • Colin Speedie Dec 16, 2013, 3:09 pm

      Hi Pat

      Interesting idea, and I’ll definitely take a look at it.

      I do have some concerns about the practicality of such ‘bandages’ however, which I’ll be touching on in a later post in this series.

      Best wishes


  • Roger N Dec 16, 2013, 5:56 am

    My 43′ GRP boat has great potential for long distance cruising, but the lack of a waterproof bulkhead is surely one limitation of the design. Very interested in the idea of an internal foam layer to help reduce the “openness” of a hull breach. Wondering if there are any downsides to this approach other than the loss of some storage space…..

    • Colin Speedie Dec 16, 2013, 3:13 pm

      Hi Roger

      The best example I’ve seen had reinforced the under bunk bulkhead in the forward cabin, and then glassed in a lid to cover it. An aperture had been left and structural foam blown in, which was then sealed off once the foam had cured. Looked neat, but as you identify, some space was lost.

      That aside, it looked like a good, low cost solution, and as I mentioned in my post, I’ve heard of a couple of very supportive stories concerning boats that had been involved in collisions with floating objects.

      Best wishes


  • pat synge Dec 16, 2013, 8:11 pm

    For what it’s worth, Colin, I just found this video of an inflatable ‘collision mat’ being deployed.


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