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I just want to talk to one narrow point in the discussion above....You often see people say that "Ferrocement only gets stronger over time." That is a bit misleading. The portland cement used in ferrocement is a material that increases in compressive strength over time. Depending on the specific mixture, placement technique and curing method the cement typically reaches 75% to 85% of its ultimate compressive strength within a week and 90% to 99% of its ultimate compressive strength in 28 days. From there on out it will somewhat forever increase in compressive strength infinitely but at a very decreased rate of increase. That part is absolutely true......
What is misleading is to say that ferrocement continually gets stronger over time. Over time as portland cement increases in compressive strength it decreases in ductility. Impacts that might have achieved flexure without damage will over time cause failure. In engineering terms failure does mean catastrophic failure, but it does mean the formation of small fissures, sometimes microscopic and sometimes larger hairline cracking. On a macro level this cracking concentrates loads and cumulative allows greater flexure on a structure that is less able to withstand flexure over time due to lost ductility.
Similarly the steel reinforcing in the matrix also loses ductility over time as the numerous cycles of small amounts of flexure work hardens and fatigues the steel. The loss of ductility and tensile strength results in a material that is less resistant to impact and bending than it would have been when new. This long term weakening in one form or another is typical of almost all boat building materials (Fatigue resistance is often cited as one of the advantages of cold molded wooden construction.) and is certainly not unique to ferrocement and should not be viewed as a 'deal breaker' in and of itself.
The issue of hairline cracking is a tough one to really address. Depending on the building methods of the individual boat and the maintenance that has been performed, it can be either a negligable problem or it can mean widespread and seriously weakening corrosion of the steel and destruction of the bond between the portland cement and the steel. Depending on who you believe, while properly constructed ferrocement has very good resistance to the migration of water when new, the issue of macro cracking over time means that this impremiability is greatly reduced, which is why you sometimes see recommendations to barrier coat ferrocement in much the same way as you would a FRP hull.
Respectfully,
Jeff
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