Rosie,
Your question regarding stainless steels - SS is a mild steel alloyed to a minimum of 13% chromium, and often has a % of carbon content as well. This makes it a real pain to weld, especially in thick hull-plating sizes, and therefore it can be difficult to find shipyards with welders who are qualified to repair this material. Unless SS is welded properly, the weld heat can produce extensive cracking, which then leads to early failure of the welds. It is also more expensive than mild steel or toughened steel plate.
Super yachts frequently have SS anchor boxes so less damage is incurred when dropping/raising the anchors, but superyacht owners tend to have more money than shipping companies! We used to have a lot of trouble here in Auckland trying to find qualified people who could weld THICK SS plating correctly.
Also, strangely, some SS's do not like environments that have NO oxygen - it tends to fail faster in these conditions than does mild steel.
Around 2004 a new superyacht (which, fortunately, I was no longer involved with) was launched here in Auckland, and the yard crew decided to pressurise the 316L Stainless Steel fire mains to test the pumps and manifolds. The hoses, etc were all run out and tested, but, the crew DID NOT purge the SS pipework systems with fresh water after the tests. Many months later, they pressurised the system again and a number of guest suites (all that beautiful joinery, and full of electronics!) and other spaces got wet when the pipework welds began to fail because of crevice cracking. Unfortunately, because the firemains had been installed first, all the remaining systems pipework and cable trays/wiring were now covering the pipework - it was a major.
Regarding propeller cavitation, this is generally caused by imploding air bubbles on the thrust face of the propeller blades, and is dependent on shaft speed, blade tip clearance to the hull, prop blade pitch, diameter (the bigger the diameter, the faster the circumferential speed), propeller immersion, propeller material, water flow into the blades, etc.
One ship I was on with a controllable pitch propeller was found to have a bad cavitation condition when the blade pitch was between Dead Slow Ahead-Zero Pitch-Dead Slow Astern positions, but also had a considerable problem near Full Ahead. The blade design was not good, and after a major failure at sea, then continual blade cracking & ongoing replacement, it was decided to retrofit a totally new propeller and shafting system, with the blade design being of the skewed (ie, the blade profile is 'swept back') type. All the cavitation & vibration disappeared, you could safely leave a cup of coffee on your table without it vibrating over the edge, and life was a lot more pleasant. Although the blade areas of both props were identical, the ship also had a small speed increase resulting from the more efficient design, which had more of the blade area located toward the outside of the propeller circumference and thus developed more thrust for the same rpm. Fortunately, the ship also had sufficient installed horsepower to deliver this thrust. Refer to Before & After photo attached.
Many different solutions have been tried in order to reduce the cavitation effects - these days, large propellers sometimes do not exceed 75 rpm at full speed, this giving a more efficient propeller, with cavitation effectively eliminated.
Peter,
With reference to your mention of double hulls, all TANKERS are required to have double hulls around the cargo tanks from this year, but this requirement does not apply to vessels that are not tankers...extract from IMO Rules...
A speeded up programme for the gradual phasing-out of single-hull oil tankers - notwithstanding a number of limited exemptions, single-hull oil tankers will not be allowed to continue operation beyond 2010. The exemptions foreseen by the IMO concern a limited number of single-hull tankers which will be subject to new and more severe regular technical inspections.
Cheers
Skilly
