(This is from a previous discussion of Ferrocement)
My take on ferro-cement is that it is, in fact, pound for pound the weakest of all of the commonly used boat building materials. Ferro-cement operates by the same principle as fiberglass, in other words, a high tensile strength reinforcing held by a high compressive strength, low tensile strength cement. The cement in ferro-cement ideally is a high strength Portland cement. The cement in fiberglass is polyester, vinylester or epoxy resin. The tensile reinforcing material in ferro-cement is steel (sometimes with glass fiber, and in fiberglass its glass in a variety of forms, kevlar, carbon and all kinds of new variations on these materials.
Ferro-cement''s weight comes from a number of sources. First of all, no matter how small the boat, there is a practical limit to how thin ferro-cement can be. Ferro-cement needs to have a minimum thickness in order to have sufficient depth of material to protect the reinforcement from moisture. Because of this boats below 40 to 45 feet are generally considered too small to use ferro-cement efficiently. (i.e. they weigh more than they would in some other material.)
The implication of the weight issue is not readily obvious. At the risk of sounding like a broken record, Weight in and of itself does nothing good for a boat. It does not make it stronger, or more comfortable or more stabile. Weight does increase the stress on the various parts of a boat. It increases the size of a sail plan required to achieve a particular speed. It increases drag and typically means that for a given draft a boat will have a less efficient keel (i.e trading off greater drag for the same amount of leeway.)
In order to carry more sail area the boat needs greater form stability, which comes at the price or a choppier motion and greater drag, or greater ballast or deeper ballast which adds more weight and drag and perhaps depth.
To keep the weight down, many ferro-cement cement boats have reduced ballast ratios when compared to other construction techniques. This means that they need more sail area because of their weight but they can''t carry more sail area because of reduced ballast ratios without using lower aspect rigs which are by their very nature less efficient.
This is further complicated by the fact a higher proportion of the weight in a ferro-cement boat is carried in the in the topsides (and sometimes decks). This means a high center of gravity which has a variety of implications; reduced stability, wider roll angles, smaller angles of ultimate stability, and more prone to excitation rolling (which may be slightly offset by the greater inertial moments due to weight).
Then there is maintenance costs. In a study performed some years back looking at the life costs of various materials, ferro-cement-cement came out as the highest maintenance cost material (if I remember worst to best was ferro-cement, steel, conventional wood, aluminum, fiberglass, cold molded wood) Of course as with any generalized study there will be case by case exceptions and given the comparatively small sampling of non-FRP boats it can be easily skewed by a few bad apples.
Other problems with ferro-cement are the difficulty of connecting things to it, and prevention of rot in wood in contact with ferro-cement. The difficulty in bolting to ferro-cement is that ferro-cement hates localized loadings. It’s hard to glue things to ferro-cement. secondary bonds are greatly weaker than primary bonds.
Then there is the market value thing. ferro-cement does have a reputation in the States that does not match the comparatively high regard that it is held in other countries. Some of this is just plain unfair prejudice but some of this comes from real shortcomings in the materials as noted above. A well-built ferro-cement boat can be a good cruising boat. But the image of the crudely finished ‘hippie’ built cement and rust buckets still clouds the perception of ferro-cement for many North Americans.
The other problem is telling whether the boat that you are looking at is a good boat. It is very hard to do non- destructive survey techniques to tell whether the original work was done well and is in good condition. While sounding will reveal any major separations in the cement to reinforcing bond, it does little to determine the affects of fatigue, poor curing practices or cold joints. With Ferro-cement it is particularly important to maintain the ferro-cement parts in good condtion. That can be very significant. People who buy boats because they are priced well below the market, often are overly frugal or just plain do not have the money that it takes to properly maintain a boat. Anotherwise good Ferro-cement boat left to poor maintenance and miss-handling can quickly become a poster child for why North American’s don’t trust Ferro-cement
To me the real cost of owning a boat is the difference between what you paid for the boat, the cost of upgrades and maintenance and the price that you can get when you sell the boat. The problem with a lot of low value boats is that the sales price is always limited no matter how much you put into the boat. This too works against ferro cement boats.
I guess my conclusion is if you are strictly looking for an initial up front cost boat and don''t mind putting some sweat equity in, and you can look past the sailing shortcomings, and you actually find one that was well built and well maintained, a ferro-cement boat might work out fine for you. For most of us, they do not.