On boats, concrete was primarily used as a cheap form of ballast – however in Europe (for new builds) the use of cement-based products in steel boats HAS BEEN BANNED. There is a reason for this: 1. The inside of the hull cannot be maintained against corrosion; 2. The slightest crack allows water ingress. Corrosion occurs with moisture between the steel and the cement; and 3. The problem is compounded with cement coverings, as whenever the hull is deformed e.g. during haul out or the vessel takes the ground. The deformation of plates creates tension in the cement, which it cannot withstand, and it cracks, allowing even more moisture to become trapped between the cement and the hull (more water and oxygen = more corrosion). Cement is relatively easy to remove compared to concrete, however you have to gain access to it if it is underneath a floor and/or holding tanks and/walls etc. Then it is a major problem. What should be used is hull plating grease as it protects the steel from corrosion, penetrates any gaps between the ribs and plates and it is flexible.
The Technical Expert at a well-known insurance company was very helpful with our concerns. We telephoned him to discuss the situation, and he said: You should avoid concrete in the bilge, we recommend that loose ballast is used. •
Without the grease on the inside of the hull, there is always a problem with rusting between the ribs and hull plating. •
Grease is the best thing to use, it is much better than paint. It should be used on all boats of this type as it seals between the ribs and the plating and it is flexible whereas cement is not and it cracks when the boat is lifted or the boat is on the ground. •
With older hulls, there is the problem with rivets popping out (becoming loose). We are aware that some people use cement/ concrete and do not have problems – in our humble opinion, they are lucky.
Jackie Parry.
What also happens with the older boats, the rivets can start to weep, allow more water in, which is tolerable if the moisture can be removed. Condensation is water, causing corrosion. The problem is you cannot see it and cannot get to it and you do not know what is going on there – it is a silent killer on a hull. The main problem occurs with cement or concrete laid with less than 3 inches thick (around 70 mm), as the enormous loads of going into dry dock can crack it, separating the concrete from the steel allowing moisture to penetrate and the steel starts corroding. SisterShip 58