Life Expectancy ???

[jet_morgan2000]

What is the 'life' of a ferro cement boat ? Any ideas on that one ?
Is there a 'norm' if the boat has been constructed well ?
Cheers !

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[ccscott49]

Theres are structures built by the romans from cement/concrete, that are sound and still standing, so how long is that piece of string you're holding?

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[jet_morgan2000]

Bridges & other stuff ! Yes !
But I don't think their Triremes were Ferro Cement or any of the ships they used to invade Britain with or else we would see the hulls stuck on the beaches of Southern England.
Take your point though !
Thanks for the reply.

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[ccscott49]

Apparently it takes 20 years, for cement to reach it's full strength! I've only ever seen ferro boats fall to bits, due to abuseor stranding, they seem to stand the test of time, some of the professional built hartleys, have a superb finish aswell. I'm not a ferro fan, but can see the point of them.

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[Tradewinds]

Some info here

http://www.geocities.com/bill_dietrich/BoatHull.html

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[Jacket]

The romans structures have lasted so long because they had no steel in them.

The concrete you don't have to worry about (normally), its the steel mesh that causes problems. Concrete is slightly porous, and so over time oxygen, chlorides and the like diffuse into the concrete. Once these reach the steel, it will start rusting. As rust has a larger volume than steel the reinforcement expands, which causes the concrete to flake off. You can see this on many buildings and bridges from the '60's and'70's.

The mix used on ferocement boats is a much higher quality (hopefully) than that used for buildings, so the oxygen will diffuse more slowly. However, the concrete cover over the steel will be much less, so the oxygen will have less distance to travel.

So how long a ferocement boat will last is a bit of a lottery. However, there are things that can be done to prevent the steel mesh from rusting- fitting a cathode, for example. a good paint job will also help, as it will effectively seal the surface.

Deterioration of reinforced concrete becaomes apparent well before there is any significant reduction in strength, and is easily repaired, so provided you examine the boat regularly, there shouldn't be a problem.

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[global_odyssey]

Stainless and ferro?

Am I missing something - or has anyone ever used stainless steel materials for building ferrocement boats?
OK it's going to cost more and would be a little harder to work, but the rust problems would be eliminated.
I dare say other hi-teck mesh/wire materials could be added in if someone cared to experiment. It might make ferrocement more widely acceptable.

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[jet_morgan2000]

Thanks for the reply ... The next question is ''How would you spot any potential trouble areas ? '' ... Is it as easy as looking for signs of rust on the outerskin, or are there electrical conductivity tests that can be carried out by the average bloke ?
I never knew messing about with boats could be so much fun !!!
Cheers !

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[ccscott49]

Re: Stainless and ferro?

Stainless steel corrodes just as fast as normal steel, when deprived of oxygen, so this wouldn't work. Any more exotic, materials, such as monel, would make the hull very expensive and cheapness was the main reason for ferro cement boats in the first place.

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[Jacket]

Re: Stainless and ferro?

Stainless steel has been used in reinforced concrete for some high profile buildings (if the correct grades are used it doesn't corrode in the RC concrete, as its not deprived of oxygen- the whole point being that oxygen is diffusing into the concrete. Corrosion at cracks can occur, but that's a whole different problem). However the cost is many times greater, rather than just slightly greater. There is also the major problem that stainless doesn't like being bent without tearing, so the reinforcement cage is much harder to fabricate.

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[Jacket]

Look for rust stains on the concrete, hairline cracks running parallel with the reinforcement and extreeme cases, signs of the outer layer of concrete beginning to flake off. I'd recomend painting the boat (especially below the waterline) a light colour, as fine cracks will be much easier to spot- in the lab we always paint concrete structures white before testing them, as it makes otherwise invisible cracks easy to spot.

Yes, there are electrical conductivity tests that can be carried out- its regularly done on bridges. However, the results are variable to say the least, and are done more to keep politicians happy, rather than to provide data for the assessing engineers (IMHO).

If you're worried about the condition of the reinforcement, remove some of the concrete and have a look at it- its easy enough to replace the concrete afterwards.

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[MainlySteam]

<<<How would you spot any potential trouble areas ?>>>

With difficulty. I recently had an assignment recovering a company which had approx 40 surveyors surveying commercial vessels from ships to the smallest. Firstly, small conventional commercial vessels are rarely made of ferro-cement and probably for good reason. Secondly, because of the difficulties in identifying problems (and despite urban legend, the difficulties in specifying long lived repairs) and the liabilites associated with unsuccessful surveys we decided to refuse to survey ferrocement vessels or be involved with them in any way at all - they were nicknamed by our Marine Manager as "Oh, my God boats". These were vessels that were originally built under survey.

Concrete is a low tensile strength material and one is dependant on the armature for tensile strength. The many cases of old concrete structures surviving are mainly structures relying on the compressive strength of concrete.

The cost of a hull of a yacht is a relatively small proportion of its construction cost so unless the builder put a cheap or homemade fitout on/in it, its completed cost will not be greatly lower than a vessel of higher cost/labour hull material. In my experience, this generally results in most concrete pleasure vessels being poorly fitted out and that is where the greatest part of the construction cost saving is.

As others have pointed out oxygen and water permeate the concrete to the steel (and again despite urban legend, water also permeates epoxy coatings). The alkaline environment serves to protect the steel but, again as others have pointed out, problems in steel reinforced structures where the covering of concrete over the steel is far greater, give plenty of examples of failure.

You get what you pay for and that is why they are cheap on the second hand market. You may get a good one - but there again I was looking at a reputedly good one yesterday and the tell tails of insolvable problems with the armature were apparant.

I am not knocking ferro-cement, all materials have their problems and advantages. I think the only advantage of ferro-cement is its cheapness on the second hand market or, for a new build, one can be built cheaply as long as you put cheap fitout materials and hardware on it.

John

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[AndrewJ]

A friend of mine had a ferro boat and his problem was the thru hulls, the metal ones
all of these appear to have set up a battery and he would go thru zincs very rapidly. You could look down over the stern and
see the bubbles from the zinc's deteriorating, at least in port. (it would seem the steel mesh was touching some of his outside
metal and reacting as a battery in some manner).

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[ccscott49]

Simple answer, don't use metal through hulls on ferro boats, I thought he would have known that, use glass re-inforced plastic ones.

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