Some thoughts on unsinkability

Some leisure boat owners are questioning whether the money spent on liferafts couldn’t more usefully be employed in making the main boat unsinkable. After all, conventional boating wisdom has it that we should stay on board as long as possible in any emergency situation, with the possible exception of a fire. Abandoning to a liferaft is considered a retrograde step, and a last resort.

There is some logic in this thinking. Our boat has on and within it everything we need for survival in the long term. If we can protect the vital, life-sustaining elements from damage or loss, we will have a much better chance of living to fight (or float) another day!

Thus, many leisure boat enthusiasts have explored the concept of unsinkability. Some modern designs, yachts and powerboats, embrace unsinkability and incorporate buoyancy mechanisms, usually foams, within the structure. But for the most part, boat owners have to find a way of adding sufficient buoyancy as a retro-fit in order to make their boats unsinkable.

Now, we need to look at some terms and agree some definitions. ‘Unsinkable’ might mean that if the boat is holed or swamped, such that it is totally flooded, it will remain at or above the surface and will not sink. But would that really be of any use? Surely what we really need is for the boat to remain sufficiently high out of the water that we may be able to effect some repairs and bail out the flood. So we might need more than just unsinkability in its’ strict sense; we may need some form of positive buoyancy that will ensure that the boat is habitable (and hopefully repairable) if it is totally flooded.

OK, but how do we achieve buoyancy? Well, in order to keep an object buoyant, the overall density must be less than the density of water, which is 1000Kg/m3. Therefore, to make an item float that would normally sink, we need to increase its’ relative volume so that it becomes less dense than water. Air is ideal, if we can find a way to trap it in the places where we need buoyancy.

Think of it this way. Your boat is basically a saucer. While it is empty, the saucer will float because the inside is filled with air, and its’ total density is less than that of water.. If it is flooded, the air is displaced by water and the saucer will sink. If we could ensure that the inside of the saucer was filled with something much lighter than water, which would take the place of any flooding, the saucer may not sink. In your boat are many spaces filled with air, which help the boat to remain buoyant. However, if that air is replaced by water, that buoyancy is lost and the boat will sink.

The question is, how can we trap air inside the boat in such a way that it cannot be replaced by water in the event of flooding? And, how much air do we need to trap?

To answer the second question first, we need only calculate the total mass of the boat fully laden. That is, with everything that is likely to be on board at any time, including people, stores, equipment, engine etc. For example, let’s say our boat weighs 1.5 tonnes empty. Then we add 4 people at 85Kg each, an engine at 40Kg, stores at 30Kg, food and water at 30Kg, odds and ends at 20Kg. The total mass is 1.96 tonnes, say 2 tonnes. In order to calculate the amount of buoyancy we need to add, we need to know the density of the trapped air. Let’s assume it is trapped in low-density closed-cell foam, which has a density of 15Kg/m3, the calculation is as follows:

Buoyancy per cubic metre of foam = Water density – Foam density
= 1000 – 15
= 985Kg

Foam required to support boat = Boat mass / Buoyancy per m3 of foam
= 2000 / 985
= 2.03m3

Therefore, around 2m3 of foam with a density of 15Kg/m3 will provide sufficient buoyancy to stop our 2 tonnes of boat sinking. The more foam we are able to add, the higher the boat will sit when flooded. The calculations for this are more complex, and require us to work out the volume of the boat to determine its’ inherent density, but it can be done.

Foam is not the only way to add buoyancy. Air bladders are available which can be inflated by compressed air when required, but this tends to be a very expensive option, and usually requires the addition of some foam.

Foam itself is available in preformed slabs, for cutting and fitting, or as a liquid for foam-in-place application. The latter is the cheapest method by far, but once the foam is in place it is extremely difficult to remove.

Another consideration is that whichever buoyancy method is used, it has to be balanced throughout the boat, for obvious reasons. Therefore, a thorough understanding of the space available and its’ location is essential, to ensure that buoyancy is evenly distributed throughout the boat.

For further information on liquid foam-in-place, visit

For preformed slabs, visit Sherlock Foams (Plastazote) at

For air bladders, visit

Ralft. 06/09

Reproduced from the Internet Archive copy of the previous site.

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