Ok everyone, it's that time of the year again: time to talk about concrete

Yeah, I've seen the old threads where you talk down to people bringing it up. But there's a reason it keeps getting brought up: you don't actually give a good answer. I've looked at your boats: they're heavy, so clearly the weight issue is a red herring. People buy heavy boats all the time. Way heavier than I'm talking about. If you want to prove me wrong, stop pulling stuff our of your *** and prove it. Let's compare modulus of elongation. Let's compare weight. But the fact is that you haven't done so because you don't know and your giving answers drawn from your understanding of convention. And the simple fact is that you don't know how light a modern concrete boat would be because no one has made one (other than the canoes).

The fact is that concrete boats were built in the 70s. They weren't fast, but that's not why they fell out of use. They got less popular because they got a reputation for being unsafe (mostly undeserved), and a fiberglass got cheaper. Now it's way lighter, way more ductile, and way easier to work with, and the cost of labor has meanwhile gone way up, and you're just spending your time offering destructive criticism WITH NOTHING TO BACK IT UP.

Your analysis lacks rigour, and can safely be disregarded. I feel sorry for anyone who was discouraged by your self-proclaimed expertise.

EDIT: plans linked on your site for a 20 ft, 350 lb dory. Ironic, that.
Spira International Inc - Puget Challenge Offshore Rowing Dory http://www.spirainternational.com/hp_puge.php

 

Uh... no, my engineering chops are just fine thank you. When's the last time you sat an engineering materials class?

 

It's been some decades, though both of my degrees (structural at Maryland and chemical at Delaware) have been well exercised since.

Ferro didn't fall out of favor because of GRP, which was very well established for over a decade previous. It fell out of favor for two primary reasons; it produced children only a mother could love with 80% plus left (after the plastering) remaining to complete the build and weight. A CCA or trawler style of yacht could be fattened up enough to accommodate the extra mass, though a pig usually resulted, but mostly it was the marketing strategies of (Jack) Simpson Marine trying to sell plans. He did more to damage the reputation of ferro than anything else.

During the 70's there were dozens of ferro builds in my area, littering lawns and backyards, with big for sale signs spray painted on their flanks. Houses couldn't be sold, until the monolith in the backyard was removed (free yacht with sale of home). 95% of these were knocked down with a loader and dragged to the landfill. Some were professionally built and these looked as good as production craft, though they still were overly heavy, lacked resale value and were (still are) extremely difficult to survey accurately without x-rays. In fact, if you'd like to get a real idea, call your marine insurance agent and tell him you've bought a ferro cement yacht and see what he says. In the USA they're all but uninsurable, unless you go with a declared value, which is the highest premium approuch to insurance.

Lastly, ferro requires a lot of labor, plus careful engineering on the armature. I'd recommend Benford designs as a well done and thought out completely. They're not very comfortable to be in, which you should have a go at. The noise is an eye opener, repair can be nightmaris, as is the amount of insulation necessary. The real question here is how many ferro yacht have you been on? I've been on several.

 

I will throw out a couple of products that have popped up over the last few years that might would allow for a better product concrete wise . Helix micro bar , or other mix of SS micro bar and Carbon composite for reinforcement , and a product such as Delvo Master set to put the concrete to sleep allowing the microbes to bond for a much stronger PSI

 

This subject is obviously highly technical as regards the materials used, but it strikes me as problematic to translate it accurately and economically into a good hull shell at a weight that would be acceptable, regardless of the material mix..

 

Material mix will dictate both concrete thickness , and weight . With a lite weight mix of 70 pounds per cubic yard , at 2 inches thick a hull would be about 12 lbs a sq ft . With the Delvo added to a 5000 psi mix we turned it into a 11,000 mix , better than a 50% gain . I have little doubt that a lite weight concrete mix could be designed today where it could compete with other hull materials if the money was right , that being said , I bought foam and glass .

 

If there was a breakthrough, or series of gradual improvements that made concrete-based boat building more feasible, we'd have heard about it. Seemingly has yet to happen.

 

Touché. Although in this case I think I still hold that there have been developments in this particular material that are pertinent to this discussion.

I think that's fair, and I think it's also fair to say that concrete is a much more difficult material to work with than it would seem at first. Barring any fancy non-metallic reinforcement or admixtures, the strength and general performance of concrete is highly variable based on water content, aggregate selection and curing conditions (among other things). With that said, I still think that the introduction of newer designs has made building ridiculously strong concrete much more idiot proof, and that someone reasonably skilled with concrete could make a really neat boat for (relatively) cheap.

Your point about the hull only being a small part of the build is also not something I intended to minimize, except insofar as to isolate one part of the build that is expensive and time consuming. Also, while particular attention has been paid to weight in the past 40 years or so, the hull shapes of your average cruiser are surprisingly varied, and (unfortunately to my mind), often driven by whatever the latest racing rules are, rather than what would actually work best.

So, I think it's probably counterproductive to refer to what I'm suggesting as ferrocement, despite the historical nomenclature, as a) I'm not proposing anything containing steel/iron and b) I think there are enough relevant dis-similarities that the term lends itself to making assumptions that may or may not be true. Corrosion, impact resistance, strength for a given weight : all of those things are substantially improved, and I worry that calling it "ferro" makes it difficult to tack one's thinking.

I bring this up because of the armature. I suppose when we say "careful" engineering, I'm not sure we would agree on the degree of engineering that qualifies (although, strictly speaking, I would probably recommend a third party trust your advice over mine in that area. I'm much more comfortable with "cobbled together" than the average person). What I mean by that is, I think, taken as a whole, I'd be comfortable sketching a hull in CAD, putting in the parameters of the material (tensile strength, etc.), and doing whatever testing can be done easily (with free software), making adjustments for an appropriate factor of safety and then maybe have a naval architect look it over. I'm not sure, but I would be afraid that would strike you as starry-eyed. But I think as I alluded to above, the various modern-ish reinforcing materials and admixtures make it a lot easier to build a lot of margin into your concrete that simply wasn't possible in the 1970's.

As far as riding in a concrete boat: if I have it would have been on a little Vietnamese river boat/barge and I have no recollection of its properties other than I remember being shocked that someone would try and build a boat out of concrete (we saw several). I have been on large steel boats, and smaller (34' Grand Banks, being the biggest I can recall) glass boats. I recall steel being very noisy, probably not helped by the fact that they have all been large diesel boats not exactly intended for creature comfort. I can also recall being amazed at being able to hear the water flow over the boat through the hull on fiberglass (this was on small sailboats). But I wonder how the sound insulation properties of GFRC compared to steel reinforced concrete? And I wonder if adding pozzolons accentuates or deadens the sound conduction properties? (it seems like it would accentuate them, if anything). In either case though, it seems like this would be something that one would have to determine experimentally.

Finally, labor. This I can't comment on, as I haven't built any boats concrete or otherwise, so any comparison I would make would be someone else's. Projects always take longer than expected when you're doing them for the first time, and much more so when one is deviating from what's been done before and/or clearly documented. With that said, there are two things that make me think this is worth pursuing (from a labor standpoint). Well, actually three if you count the fact that I find it interesting for its own sake because no one seems to be doing it, and thus it is something that I can learn for myself (and maybe others) that isn't available to be learned in a book, or a class, or online. But also that I think that concrete has quite a few "shortcuts" or perhaps more appropriately, labor saving measures that could be implemented if someone put the time in to figure out how. The shotcrete approach being one example, but I suspect I could think of more if I took the time. But secondly, and perhaps most importantly, is the foamie revolution (and to a lesser extent the quadcopter revolution) in R/C flying. I would argue that the ability to make any crazy design you can think of for ridiculously cheap has pushed R/C model design in a way that was simply impossible when one had to spend hours and hours gluing balsa together. Currently, hull design is expensive from a testing perspective. No one wants to spend big money building a hull that sucks balls. So hull design is iterative and unrewarding of imagination. Radically reducing the cost and build time of a hull would lower the barrier to experimentation, and push hull design forward in a way that is similar to how R/C design has been pushed forward. That's my hope any way. And if I build a few GFRC hulls and it's not worth it, I'll be out the time, but not too much money.

 

I agree with much of what you've just mentioned, though there are some misconceptions. On an average sailing cruiser (30'- 35') the hull shell isn't a big portion of the build time (labor) in the more common material choices. One of the problems with a concrete shell is the "plastering", which has to be done all in one shot and by skilled plasters. Guncrete pool builders are a good source for these skilled laborers, though not a particularly large base to select from, nor all that familiar with shaping boats to precise dimensions, if needing a hull shell produced in a timely fashion. Yes, you can save some application time, shooting the mix into a mold, but now you have to build a mold too, possibly both a male and female if thicknesses are to be accurately controlled. One of the issues these "gunned" applications have had is consistency and thickness. So much so, in highly stressed structures, like tunnels and large retaining walls, yeah, it's shot in, but also placed under pressure, to insure a void free, consistent density and thickness. This doesn't sound cheap to me, nor was it in the "big build" in Boston.

I agree in the recent advances in formulations and reinforcements, though testing on dynamic structures isn't exactly cheap. Modeling is an option, though not something an insurance company is going to get on board with, without real tests. This would be true of any object or contraption that places humans in an "unnatural environment". This is insurance-speak for failure modes which pretty much guarantee people will die. New high rise elevator designs, aircraft, being in a vessel farther from shore than you can swim back to, etc. are examples of these "modes". It's considerably more than a safety margin.

Hull design now has reached a point where a pretty fine line can be drawn, where performance and expectations can be accurately predicted. This wasn't always the case. Not but a few decades ago, it took quite a while (5 - 10 years) for a class to develop a design and move onto the next big breakthrough. Now, this is months of development, assuming the funds exist. No one does build a design that sucks out of the box. This isn't the old gun and run brigade stuff, where the old school master NA dreams up the newest design, pens it up and hopes for the best result for his client. This just isn't done any more, way too costly and problematic. Designs can be done, that actually float dead bang on the LWL the builder was told to paint on. This wasn't always the case. In fact, most old school designs required some "trimming ballast" which was typically about 10% of the actual ballast, just to get the boat to settle on her lines plumb and square. My last design floated dead bang where I said it would and this isn't unusual. This is because of accurate weight and volumetric studies.

Will this eventually work out to a new hull shell material option, well maybe, but given the current misconceptions associated with ferro, don't hold your breath, at least not in the USA. Weight is still going to be an issue, even with lightweight formulations, especially given some of the other choices. If you want to bet on something, picture the world's largest 3D printer, popping out hulls in various materials.

 

Yaaay ... another "concrete boat" thread. (I'd post a link to THE "Concrete Submarine", but the moderator would just remove it.)

Call me cynical, but appears that of all the "concrete revelation" threads that pop up over the years, the proposers spend a lot of time saying how great the idea, how easy, how cheap and argue the point with experienced boatbuilders - about 2 years before they even start the project. Then nothing happens. Oh, yes - and usually they don't have any money (there is one of those active at the moment).

Don't get me wrong - I really WANT this project to SUCCEED, with lots of photos and detailed explanations. Even just a test panel - is that too much to ask ?

But just don't lead me on again, my poor heart just couldn't stand it.

Sob.

 

The test panel(s) would be a very good start. If that doesn't stack up, no need to worry the head about other prospective hurdles.

 

Ever had test panels tested? Tool and machine time on these pieces of equipment aren't cheap.

 

I'm sure a materials engineer could suggest some tests that would not break the bank, and give a good indication of comparative performance, without having to take over a university lab.

 

Lol. I might be able to swing a test panel. I have some concrete, some superplasticizer and can get grout and sand for cheap enough. And "no money" is relative, I suppose. I don't have "**** you" money to just drop on random projects, but that's in part because my kids are young, and every penny I spend on a project is a penny that doesn't go towards their college. Also, like I said, I'm in the middle of finishing a basement.

 

I think having cylinders compression tested isn't that expensive? Not sure about flexural tests though.