Boatbuilding: Collision bulkheads as a safety factor and how they can be retrofitted
Alexander Worms
· 01.03.2025
In July 2023, a German Bavaria 38 sank off the Dutch coast within minutes. In rough weather, the improperly installed retractable bow thruster had become detached from the hull. Within a very short time, a lot of water poured in through the large opening. The crew barely managed to save themselves in the life raft, from which they were rescued. For the same reason, a Sun Odyssey 410 sank off the German Baltic coast in the spring of 2023 within a very short space of time.
In the same year, the Bavaria 34 Holiday of German sailor Martin Daldrup sinks in the South Atlantic. After more than a day in the life raft, he is rescued by a freighter. Here, too, the yacht is lost. The sailor, however, reports that everything happened very quickly, but he suspects that the water ingress came from the rudder. There was no time to tackle the leak. It was a matter of pure survival. Thanks to a satellite phone and power banks, he was able to coordinate his own rescue from the island.
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In addition, a total of four yachts have sunk in recent years following attacks by orcas off the coasts of Portugal and Spain. The animals targeted the rudder blades. If these were severely damaged, water entered them. The yachts were lost.
In all cases, the question arises as to whether a skilfully placed, watertight bulkhead would have prevented or at least delayed the sinking. In all cases, the transfer to the island or the rescue had to take place very quickly, and more time to prepare for this would have been good. But why do so few series-produced yachts have watertight compartments when they are quite common on many metal explorer yachts? What are the advantages and disadvantages? And can something like this be retrofitted to a GRP series boat if desired?
Collision bulkheads as a marketing gimmick
Firstly, it is important to distinguish between a watertight compartment and collision bulkheads. In an emergency, the former can either provide sufficient buoyancy to keep a yacht afloat in the event of major water ingress or trap water that has penetrated inside this compartment in order to maintain buoyancy.
Collision bulkheads, on the other hand, usually directly behind the anchor locker, only close off a very small part of the hull with little volume, so that only a crash in front of the bulkhead is protected. "Most yachts have this. However, this may not even be necessary, as the stern is by far the most stable part of the hull due to its shape and the enormous thickness of the laminate. It takes an enormous impact to crush it," explains yacht designer Marc Oliver von Ahlen, referring to the crash test of a Dehler 31 carried out 40 years ago by YACHT, which was unsuccessful in terms of destroying the ship.
Nevertheless, most standard yachts there have a collision bulkhead. However, this is more of a marketing ploy than genuine safety. "Ships sink for four reasons," says von Ahlen. "Firstly, keel loss. You don't stand a chance anyway. Secondly, via the shaft passage or the saildrive collar. With the former, you can still counteract this with good pumps. With the saildrive sleeve, it becomes more difficult. Thirdly, through defective hoses. We have sea valves for that. And fourthly, through collision and possible loss of rudder. And this is where watertight compartments can really help."
The experienced designer considers the risk to the hull in the event of a collision with a floating object to be rather low: "The angles are usually very acute, so it's more of a tangent. In addition, both the yacht and the drifting object are mobile, so the energy of the impact can be dissipated well." The rudder, on the other hand, is more at risk if it is free-standing. It can slip out of the hull or break off, leaving a fairly large hole through which a lot of water can penetrate.
Unable to manoeuvre, but at least not doomed to destruction
Von Ahlen agrees: "The area around the rudder is certainly the most sensible place for a watertight partition." Especially when you consider the attacks of the orcas on precisely this part of the ship. Although the rudder coker is usually pulled up to above the waterline, the height is often limited by the low cockpit floor. If the yacht dips aft into the wave, water would penetrate. "The waterline is often in the coker, and in waves even higher up. If the coker cannot be raised very high for other design reasons, a sleeve and a shaft seal on the rudder stock must be used to prevent water from entering. Both can wear out and leak," continues von Ahlen.
In the event of an accident, when the rudder is possibly even completely gone, for example after an orca attack or if the rudder stock breaks, as with the Arcona 460 "IdaLina", which sank in the Pacific, water penetrates through the open rudder coker, which is no longer sealed by the rudder stock or is damaged. Then the only thing that helps is if the rudder stock was designed as a watertight area. The ship is then unable to manoeuvre, but at least it does not immediately go to the bottom. Sealing attempts with leak sealants will usually not be successful either because of the difficult accessibility in the rudder compartment or because the hole is too large if the rudder is missing completely.
"The keel, saildrive, hoses and rudder are the main weak points of a boat."
Now, many yachts have a bulkhead installed at the foot of the aft berths anyway, which is usually structurally necessary there to provide additional torsional rigidity in the aft section. However, structural does not mean watertight. The stern area must be accessible, for example to service the steering gear. Openings are provided in the bulkhead for this purpose. There are also openings for the engine exhaust hose, the hot air hose for the heater, which is often installed aft, the hoses for the stern shower, various cables for the autopilot and the stern light. In short, the bulkhead in front of the wheelhouse is more like Swiss cheese than a thick wall.
Retrofitting collision bulkheads
This can be changed. To do this, the bulkhead should first be given a good, all-round laminate to the hull. The plywood panel from which the bulkhead is made is sufficient in terms of strength, as it is not intended to be permanent, but rather to provide a seal in the event of an accident. However, if you want to be on the safe side, you can ensure additional strength and, above all, watertightness with a few layers of glass fibre mat.
The inspection openings to the aft compartments can be replaced with watertight covers, and the lead-through for the exhaust can be sealed with sealing compound. Cables are routed upwards towards the deck as far as possible and separated watertight with deck passages, the hoses for the stern shower are fitted with bulkhead fittings.
The heating hose remains a problem. Due to its large cross-section, it is difficult to seal. Ideally, therefore, the heater should not be installed behind the bulkhead; if it is, a different location may have to be found. What remains difficult, if only because of the mobility, are the control cables. Ideally, the space in which they move is part of the watertight compartment in the stern. However, before reaching for resin and hardener, it is important to remember that the control columns must remain accessible from below for inspections. Inspection covers can also help here.
Where collision bulkheads can be installed
One possible alternative would be to seal only the area around the rudder coker with an additional laminated box. This is built as high as possible so that the coker, steering cables and autopilot can work freely. However, this effort is only worthwhile if it can actually create a relevant height above the waterline.
In the bow area, further bulkheading than that of the anchor locker is often quite easy to realise. Here, too, there will be a structural stiffener in front of the main bulkhead anyway. This is often located under the berth. However, many shipyards install storage compartments there that are accessible from aft. Again, the frame alone is sufficient for rigidity, but is of course not watertight. If you prefer not to have access to the storage compartments, for example as drawers, you can also store the items stored there from above after raising the mattresses. The structural bulkhead can then also become a watertight bulkhead. The following applies here: laminate and sealed cable ducts for the navigation lights and bow thruster, for example. Even if a water tank is stored in the bow, the pipes must be sealed accordingly.
"Anyone travelling on the North Sea and Baltic Sea is just as well served with a life raft and Epirb as with a collision bulkhead."
This makes it easy to create a large watertight compartment in the bow area, which also encloses the bow thruster, if one is installed. This is because the bow thruster - certainly on a retractable model - also represents a large opening in the hull. However, the conversion to a watertight compartment only makes sense if the bulkhead is still high enough when the area in front of it is full of water, i.e. the bow is significantly lower in the water.
Set up a watertight compartment
If you want to be on the safe side, you can design the entire area in front of the main bulkhead as a watertight compartment. The advantage of this is that the main bulkhead goes right up to the ceiling anyway and fills the opening completely. Of course, all cable penetrations also need to be sealed here. The biggest challenge is, of course, the door. If it opens forwards, i.e. the water pressure from there would press the door shut, sealing it is a lot easier. A circumferential rubber seal, sturdy hinges and a few toggle latches then help quite a lot. After all, such a bulkhead does not necessarily have to be one hundred per cent tight, it is enough if the amount of water that penetrates is less than what the pumps on board can permanently bring outboard.
Of course, a yacht can be divided into several watertight compartments, even in such a way that it is almost unsinkable, as long as not all of these compartments are flooded. However, this raises the question of whether it makes sense: "If you're travelling around the North and Baltic Seas, you're just as well served with a life raft and an Epirb and therefore don't need to disfigure the interior of your own yacht. You don't need to be prepared for the extreme," says von Ahlen.
The question of the cruising area does indeed play a major role. Most aluminium yachts have large watertight compartments that have already been designed in. This is logical, because anyone who buys such a yacht automatically thinks of areas that are, firstly, very remote and, secondly, where there may be ice in the water. The risk of collision with flotsam increases here and help is not forthcoming. The crew must be able to help themselves. The typical large series yacht will not usually sail in such areas. When designing them, comfort and utility are quite rightly at the forefront.
Isolation of certain areas not officially required
The fact that ship losses due to flooding in the bow area obviously play a subordinate role is also shown by the fact that watertight compartments are not provided for in the requirements of the Recreational Craft Directive. It does specify how to deal with openings in the hull and how high they must remain above the waterline at what angle of heel. However, sealing off certain areas is not required.
However, recent events show that a watertight area around the rudder could definitely be a safety gain. Firstly, it is the most sensitive and exposed point on a yacht, and secondly, the rudder compartment is a part of the yacht that is very rarely visited anyway. Sealing it is possible with almost no loss of comfort. So why not give it some thought?
However: "If you want a ship that is unsinkable under all circumstances, you have to buy an Etap," says von Ahlen. He designed the yachts from Belgium back in the day. Of course, this also applies to any other design in which enough foam is provided to ensure that there is always sufficient buoyancy even when flooded. However, very few yachts have this.
The desire for safety under all circumstances is of course all too understandable at sea. Whether the retrofitting of watertight compartments and the associated loss of comfort is justified is something that each owner must decide for themselves. The sailing area naturally plays a role here. However, it is important to familiarise yourself with the subject: What are the weak points of your own yacht? What can be significantly improved with a manageable amount of effort? After all, it is often enough to reduce water ingress in order to be able to pump against it for a while until help arrives. This saves valuable time for everyone involved. How could you respond to a water ingress with on-board resources if necessary? And finally, what is the plan if the ship is unstoppable? Nobody likes to think about this, but it can save lives.
Collision bulkheads at least above the waterline
The wheelhouse can be converted into a watertight compartment with little effort. It is sufficient if the bulkhead at least extends well above the waterline. The ship will then continue to drift. Please note: We are talking about the waterline when flooded. If the bulkhead has a volume of 1,000 litres, then there is an additional tonne of weight there when flooded. To determine the waterline in this condition, neighbours on the jetty can be asked to stand on the stern of the boat. In the bow area, it is ideal if a water tank is installed there. It takes up a lot of volume. If it is emptied in the event of flooding, this creates additional buoyancy.
