the concept of unsinkable yachts
I did NOT write this article, but (unfortunately) I cannot recall where I originally got it. FWIW:
Making a Boat Unsinkable ~ by "Unknown"
Fibreglass does not float, an early objection - but the same was said of the first iron ships. If flooded, few wooden yachts with ballast and an engine will float either. So making a boat unsinkable is a problem to be tackled at the design stage. The structural requirements would make it very difficult to achieve later. There are two fundamental principles:
1) Even the lightest buoyancy material, air, requires space: for one ton of buoyancy you need one cubic metre (roughly 35 cubic feet) of air - the equivalent volume of five big oil drums. For the average 35- to 40-foot ocean cruiser weighing ten tons, there would have to be room for the equivalent of 50 oil drums! Moreover the paper displacement figure is just for the bare boat. To this must be added the two or three tons of stores, equipment and possessions - and for any ocean cruiser we really are talking about tons, and probably underestimating. The more stores and equipment, all absolutely essential, the less room for buoyancy, and conversely the more buoyancy the less room for essentials. There has to be room to work the boat and live in reasonable comfort when in harbour where the boat will spend a lot more time than at sea. Therefore the sheer space required for using trapped air as a buoyancy material rules out anything except emergency air bags, and once those are operated, on purpose or accidentally, the boat becomes unliveable and "unsailable".
2) It is not sufficient just to keep the boat afloat, decks awash. Sheer survival requires a degree of shelter, habitability and perhaps "sailability". The widow maker is exposure and hypothermia. This means the boat must float high enough for the crew to remain reasonably dry, sheltered from wind and water, able to cook, and have electrical power to operate a radio and lights. Perhaps also to run an engine, especially if a power boat, and ideally to sail after a fashion. Morale too is very important. It is far better to stick with the boat, if possible, than take to a "liferaft". However in practical terms this amount of buoyancy is almost impossible to achieve with a ballasted, well-equipped cruising monohull, although feasible with a catamaran.
Sadlers'', for example, have built unsinkable boats by use of double skinned, foam-injected hulls, but even so still have to steal essential storage space. Both skins are fairly light construction and do not allow for the tons of extra weight essential for ocean cruising. Double skinned hulls like this and foam-filled spaces can only be done while the boat is being moulded, and are therefore design features. It is claimed that damage is limited with this type of construction. This may be so with minor damage. But the foam filling must be weak, or it becomes too heavy, and would not prevent a serious impact from damaging the inner skin too. Foamed compartments can become waterlogged and are then almost impossible to dry. Even closed-cell foam will disintegrate when wet, and then flotation is lost.
Aiming at unsinkability, ships have watertight doors and bulkheads. This is not practicable on a yacht. The interference with habitability would be unacceptable on anything but the most dedicated, large, ocean-going racer. The average yacht bulkhead is not strong enough; the sheer pressure of, say, a forecabin full of water would be formidable, even without surging. Most bulkheads, being designed for inward compression only, are secured by weak angles, sometimes none at all, and would be torn adrift. Possibly the hull or deck would burst too. The weight would also affect the stability.
Well meaning bureaucrats specify collision bulkheads. Sadly they know little about boats and their thinking is based on fast, wide-fronted cars on narrow roads where head-on collisions are the rule. With boats, free to move in any direction, head-on collisions between two pointed end shapes are very unusual, rather like two spears meeting in mid-air. Most impacts are glancing blows and if between two boats, the victim is usually hit on the forward topsides or amidships where it is weak. The victim sinks and the attacker, with its strong stem taking the impact, escapes relatively unscathed. Even a hard-sailed dinghy can sink a much larger boat. Because it interferes with the accommodation, a collision bulkhead is usually placed well forward. With the usual overhang this is above the waterline and is therefore just a token, the collision bulkhead commonly being the aft end of the chain locker. Most boats have an overhang forward and fast power boats in particular sail with a pronounced bows-high trim so that any impact with rocks or debris will be well aft of any collision bulkhead. In all my years as a surveyor I remember only one case of a dangerous leak due to impact on the waterline. This was when a Moody hit a heavy mooring buoy, and it was well aft of the mandatory collision bulkhead. I have seen a few crumpled bows from hitting dock walls or lock gates, most often just resulting in bent pulpits. But I have seen plenty of underwater damage from rocks. Because any dangerous impact will be at or below the waterline, it makes more sense to have a double bottom.
The Watertight Lockers Option
One idea seldom mentioned, and more in the way of damage control than unsinkability, is to have numerous small watertight compartments. This is copying ship practice, where each watertight section is small enough that flooding it does not affect the integrity of the ship. However, on a yacht it is not practicable to divide the boat with watertight bulkheads, and even if it was, the spaces would much too large. But almost every cruising boat has dozens of lockers which could be made watertight fairly easily. This is not the same as filling them with buoyancy, as is done with "unsinkable" boats. By making the lockers watertight and fitting watertight lids, such as large dinghy hatches, they would provide some limited buoyancy yet still remain useable, just as a ship cannot afford to lose the payload of a hold.
As well as providing buoyancy, making lockers watertight would have the even more important function of containing a serious leak. With good planning the boat could virtually have a double skin over the entire underwater area. (If a pan moulding was used it would need to be stronger and better attached than usual because, being considered just accommodation, most are lightly moulded and weakly bonded. They generally break away and split if the hull is damaged.) One or more such watertight lockers could flood without unduly affecting stability and, most important, would prevent the rest of the boat filling and sinking. Essential systems like batteries and engines could be in their own watertight compartments so that the boat remains operational.
Another important factor is that having many small, watertight compartments would prevent surging, which alone can cause damage and seriously upset the stability of a flooded boat, even if nominally unsinkable. Moreover, a lot of this work could be retrospective and done in preparation for an ocean cruise.
Buoyancy and unsinkability sound like nice safety features. But do your sums. Work out the weight of the boat - plus everything you have or intend to stuff inside it. To be on the safe side double this at least; it is invariably underestimated. On the first haul-out after an Atlantic crossing every crew finds they have to raise the waterline several inches! On my 31-foot-waterline boat the rate of waterline change is roughly two inches per ton, a fairly typical figure.