Designing for ferrocement vs wood or steel

What are the differences when designing for wood or steel construction vs designing a similar hull form for ferrocement? I've noticed most of the plans out there say they can be built in either wood or steel, but only the Hartley's also give a FC option.

Since I know someone is going to ask "Why ferrocement", the reasons are cost, maintainability and the ability to build it at home over time.

 

Wood is universally the preferred building material for home built craft. It's easy to work, readily available, tools are easy to find, etc. In craft large enough to consider ferro cement, steel is well ahead in preference, durability, toughness, resale value, alteration and repair, etc. Again the material is fairly easy to work, tools are commonly available as are plans. Then of course we can't forget 'glass in it's many variations. All of the methods and materials I've mentioned hands down and by a huge margin are preferred over ferro cement.

The differences in designing for the materials is a complicated question covering many different subjects. Weight is a major consideration in ferro which is why you don't see many 14' ferro designs. Though there are 14' ferro designs and I actually have a full set of plans for a 14' ferro cement boat here about 6' from where I'm typing. Very clever engineering by an old friend, went into that boat and understandably considering it's building material. His own admission suggests a cold molded or strip planked version of the same hull would preform considerably better and be much easier to build.

I guess what I'm getting at is ferro isn't well thought of in the USA. There are several reasons for this, but suffice it to say, you can't give them away. I know of a 45' ferro hull that's upside down in a guy's side yard not very far from here. It's been there for years with a spray painted "4 sale" on it's flanks. I've talked to the guy and the hull is free, just get a crane and flatbed and you can have it. In other countries this isn't the case and a market exists, so designs are available.

From a technical stand point, ferro should be a good building material, if properly designed and built to the plan. Unfortunately, you can't tell if the hull was built to the plan after it has been built, which makes inspection quite difficult. This coupled with other "issues" makes it a less desirable hull material then others more commonly used. Personally, I wouldn't own one, though I've been on a few well built ones. Repair and alteration issues, athwart stiffness, not to mention resale and insurance issues just keep me away from the stuff.

Lastly, the choice of hull material has a very small foot print in routine maintenance and the hull shell itself makes up a fairly small percentage of the total new build project outlay, in materials, labor and cost. Given these facts, it's usually best to select a different material for the hull shell, particularly in light of the particularities of ferro construction.

 

Well said PAR. Additional point, there are dozens of these discussions here on these.

 

Another point: Surveyors hate them as they are very difficult to survey and Insurance companies are very reluctant to write a policy as a result.

 

They can be surveyed, but it's costly to examine the wire armature. The net result is they don't get surveyed often enough, so their value declines and insurance companies have no choice but to place them lower and lower on the desirable attributes lists.

 

They can be inspected for metal structure, with concrete X-ray equipment. I have had to do this a few times on buildings with questionable foundations, it is not as costly as one would think, and there are firms that specialize in doing it. But that is not a reason you should choose ferro-cement construction.

As a structural engineer I would say there is no advantage at all with ferro-cement hulls at lengths less than about 50 feet. Smaller hulls would be much lighter made with other materials, and less costly to move around on land. There are weight and cost advantages on very large vessels.

On very large barges and similar "work boats" they do have many advantages. The biggest negative issue I think is they are difficult to repair if badly damaged, especially with compound curved hulls. Also I suspect that once corrosive sea water penetrates to the metal framing, and it starts corroding away, there is no easy or inexpensive way to repair this kind of damage.

They have fallen out of favor for pleasure boats, but I think are still used in 3rd world countries to make barges, ferries, bridge floats and similar hulls. I have recently assisted in the redesign of some concrete decks on some very large floating docks used to do on-water maintenence on large navy vessels, they needed a good wearing, all-weather surface that service vehicles could dive on to supply maintenance crews.

They are good for large floating structures because of the low cost materials and relatively low skilled labor. In more prosperous countries the hull cost are only a small part of the total construction and equipment outfitting cost of a finished pleasure yacht, so there is no point.

 

I think this is where most people make the mistake. The hull cannot be built over time, it has about the same "pot-life" limitations as a 'glass hull. There was a company called Fibersteel Corp. (West Sacramento, CA) that produced a 55' ketch as well as barges, grain silos, and other things of laminated concrete. When building the hull (including deck and bulkheads), it was less than a week from start to finish in order to prevent corrosion of the reinforcing and allow chemical bounding of the concrete. These hull were incredably strong, but difficult to finish the interior due smoothness, hardness, and thinness of the structure.

http://ferrocement.net/ferro/files/lfcdefinition.pdf

 

Thanks for the feedback everyone.

Based on the comments, I think I'm going to scrap the FC idea in favor of foam sandwich construction, which has headaches of it's own, but at least it seems more forgiving in terms of fixing one's mistakes.

I also got a reminder today of why I don't want to do traditional wood construction as I was watching a crew replacing large sections of timber on the side of a fishing boat. I'm still looking at the possibility of cold molded wood with a GRP covering.

 

Dont be afraid to build a wood boat. If you do it with the right plans and use the right fasteners, it'l outlast you without any serious problems.

 

I'm not so much afraid of building a wood boat as I am of the maintenance requirements. Wood is actually the medium I'm most comfortable in, but it looks like it is more labor intensive to keep up than any other material. If you coat the hull with a couple of layers of fiberglass after construction do you get the advantages of a FG hull? What are the tradeoffs?

 

For a home builder, I can think of no better hull material than cold molded wood. This is because of the relative small amount of materal and effort lost to the moulds, the time insensitivity between major steps, and the ease with which the material is worked. While cold molded wood is not forever, with modern epoxies, surface coatings, and proper fastening techniques, its lifespan is much longer than it's builders and its maintence is no more than fiberglass (i.e. both have to be protected from UV and water entry at the fasteners).

 

I'll jump on the wooden band wagon too. Molded hulls are less "user friendly" then strip planked, for the home builder, but wooden "core" and "composite" hulls are easy to build, use inexpensive materials, don't require a huge skill level, need only common tools and are as durable as 'glass hulls. Hell, strip planked hulls can be built by trained monkeys. Insurance companies also see the advantages and differences between a wooden hull and a wood core or composite (as they usually call it) hull.

 

I'd not coat the wood with Fiberglass. I think that's a foolish thing to do.

Just primer and paint.
Seal the cracks etc to keep as much water from soaking into the wood as possible. Keep it dry inside, a fan going when the thing is shut up, stuff like that is all you need to do.
Use the right fasteners. Bronze below the water line, Stainless above.

Even my Biggest Boat, a 33' Chris Craft, 1953, which I came across in 1993, would have lasted many more years if the guy living in it would have known a bit about Wood.

It never had been glassed anywhere. They put Canvas up on top and painted that down.
The water couldn't run off those upper decks. Make them rounded so the Water cant stand anywhere.

Another error which might have been unintended was the weight and balance.
Originally that had two 8 Cylinder Chrysler Crowns.
They replaced the two huge heavy engines with two Izusu four cylinder diesels.
That lightend up the aft end, so the forfoot sat down and all the bilge ran to the forfoot.

There wasn't a pump up there, and he kept the thing locked up tight.
All rot needs is a Warm, Windless, moist place and he provided it. So I lost a nice boat.

You do it in wood. You wont be dissapointed and wont be working yourself to death either.

 

Wood lasts generations with minimal care. I would coat the outside with a single layer of glass say 7 pound and over that on the keel a layer of FG tape that is scrape resistant such as dynel.

 

Contrary to popular thinking ferrocement construction is not significantly cheaper then more widely accepted methods. Back in the early 70's my father and I did a lot of work in developing a system to build good FC boats. By the time you use create a quality steel armature and cement mixture your costs approach the cost of other methods. As many others have stated the cost of the hull is a small part of the cost of building and equipping a viable boat.

To my thinking the best back yard building methods are glassed strip planking stitch and glue ply and steel for the bigger boats.