Originally Posted by Hartley18
I notice no-one's mentioned concrete. As a kid seeing a world-traveller's properly-built concrete hull come to grief on a reef in north queensland wasn't a pleasant experience either.
Actually that was one of the main reasons my parents had an Adams-designed steel-hulled 52-footer built not long after..
I want to start with an apologize to you and say that I am not ignoring you. I know that I owe you more information on the work that I am doing on the trade-off between sail area and sail power. Its hard to explain in few words and I have not been comfortable with my first draft explanation.
Regarding ferrocement, the panel weight for a typical under 40 foot long Ferrocement boat is 5 to 7.5 lbs (depending on percentage of reinforcing used) roughly the same weight as a steel boat plated in 3/16" to 1/4" plate. Depending on the actual design of the ferrocement materials involved, for an equal weight that typically results in the F.C.having roughly 1.25 times the bending strength, 3 times the stiffness but only between .8 and .9 times the impact resistance of steel.
I also want to touch on Outbound's point about steel having non-directional strength distribution and also his point on ductility. This is an important point when used on boats with a large panel dimensions such as the Origami designs or a boat with minimal internal framing.
Non-directional materials offer little or no advantage on conventionally constructed boats, where normal load paths are well understood and even highly focused point loads such as an impact or a prolonged grounding on a sharp edge, distribute across the shortest dimension of the panel and therefore have an orientation.
Ductility is a similar issue. Its true that low strength steels will deform more than almost any other material without breeching. This is less true for the higher strength steels (often used in boat building) which trade lower ductility for higher strength.
But to me, ductility has almost no benefit. To me what is critical is at what load the material will rupture in an impact situation and when compared on a pound by pound basis, both glass and wood have higher impact resistances.
But it is also important to reitterate that not all ferrocement or fiberglass boats are created equally. For example, the fiberglass panel I used in my example above was specifically designed for impact and bending strength. The resin used is the same vinylester popularized in crash and military helmets. The core was specifically chosen for its energy absorbing ability. The laminate used almost no non-directional materials. In the Naval Academy studies, it was also the least expensive of the panels tested. That laminate is acceptable when comparing custom built boats designed for low cost and impact resistance as prime objectives. But it needs to be said that there are almost no production boats that actually employ that resin and laminate schedule.
I would also like to repeat a comment that I made in another discussion on this topic. Anyone who has ever held a piece of 1/4” steel plate in their hand could not help but be impressed with how seemingly indestructible this material would appear to be. And anyone who has ever worked on a poorly built fiberglass boat, will also come away with an intuitive sense of the potential vulnerability of fiberglass.
But my post is intended to clarify the reality of situation, which is that when compared on a pound for pound basis, using accepted engineering data, steel just does not do all that well against properly designed and properly build composite or cold molded boats. While I gladly admit that I strongly prefer lighter weight boats and better sailing boats for my own use than can typically produced in steel, that still does not change the realities of the materials in question.