Malo37
Well-Known Member
Probably not practical on a used boat unless it is totally stripped out to a bare hull. Cheaper to build a new one I would guess
(just noticed earlier posts on 'flame spray zinc'.)
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Steel boats - hull maintenance and longevity
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(just noticed earlier posts on 'flame spray zinc'.)
GTom
Well-Known Member
There are quite steep differences between used boat prices and building costs (factor of 2x?). <4-500$/foot (about the cost of a full osmosis treatment for a FG boat) might be justified (unless you can buy a 40' zinced hull for 10k$). Question is where the real numbers stand, I'd expect it to be cheaper.
coenvanwyk
Active Member
Interesting thread! I built my hull with 3mm steel, and will launch soon. My plan was to go with flame sprayed zinc, but I could not find anyone to do it. Tom Colvin in his book on steel boat building feels modern epoxy coatings are so good that it is not worth going for sprayed zink. There are zinc rich coatings, and aluminium/zinc coatings you can spray on which sound fantastic, but the price is also quite something, and application requires some expertise. I eventually took the advice of industrial experts and went for an epoxy undercoat intended for the inside of tanks, which is so hard you can cut thread into it.
I would think the secret is proper preparation, and then internal construction so that you can remove panels and inspect the inside of the hull often. Mine has removable panels, but it is hard not to build furniture over it. We will see once she is in the water how it holds up.
I met a sailor in the Pacific who built a steel hull in France, and scrimped on the paint. Five years later he was desperately looking for somewhere to do sandblasting and repainting outside. Inside was his biggest worry and he was facing the prospect of stripping out everything and blasting, then re-painting.
I would think the secret is proper preparation, and then internal construction so that you can remove panels and inspect the inside of the hull often. Mine has removable panels, but it is hard not to build furniture over it. We will see once she is in the water how it holds up.
I met a sailor in the Pacific who built a steel hull in France, and scrimped on the paint. Five years later he was desperately looking for somewhere to do sandblasting and repainting outside. Inside was his biggest worry and he was facing the prospect of stripping out everything and blasting, then re-painting.
Malo37
Well-Known Member
3mm sounds awfully light to me. New build barges are typically 10mm below the waterline, 8mm sides and 4mm topsides or something like that.
Malo37
Well-Known Member
There is quite a lot of information on the topic of zinc coating in various barge and inland waterway forums. They reckon that electric arc plasma spray does a better job than flame spray and that it's fairly easy as a do it yourself job once the hull has been blasted. The cost is apparently no more than a good epoxy job and can be expected to last 30 years or more.
coenvanwyk
Active Member
I know, people told me there is little room for corrosion. They also told me a steel boat under 35 feet would be too heavy. Life is a voyage of discovery.
Bajansailor
Well-Known Member
GTom
Well-Known Member
So you leave little room for corrosion, yet you skip the zinc? Sounds like a risky plan to me... Are you using the stronger, less corroding corten?
coenvanwyk
Active Member
There are questions about Corten and crevice corrosion. Bad excuse for "was not able to source at the time." So we went with mild steel and 3mm epoxy inside and out. Call ot SRP (Steel Reinforced Plastic) as opposed to GRP (;>
Time will tell.
Time will tell.
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coenvanwyk
Active Member
Danny Greene did a great job with Brazen, I wish there was more info available abvout her, especially about the interior layout. I wrote to him to ask, but did not get much of a reply. So I tried my own arrangements, to the dismay of a number of carpenters and assistants. Time will tell if my plan will work. There are two points where I would like to disagree with Mr Greene: It is possible to weld steel of less than ¼ inch with success: even an atrful bodger like me can do a fair job with 2mm steel, especially if it goes on round bar stringers. Some experts with the right equipment can weld razor blades, although I am not sure why one would want to... And it is possible to build steel hulls under 34 feet: Tom Thumb was build to demonstrate that it is possible to build a seaworthy steel boat to 24 feet, and smaller ones had been built. I went a little bigger, and time will reveal the magnitude of my folly.
GTom
Well-Known Member
Corten is not just about corrosion - which it undoubtedly does better in the more problematic INSIDE - but it is 40% stronger. Meaning a 3mm corten hull is as strong as 4mm mild steel.
I'd be worried about different heat expansion coeffs of steel and the thick epoxy layer, but indeed, time will tell.
coenvanwyk
Active Member
Corten is also quite a bit more expensive, but the cost of steel is a very small percentage of the total cost. Some people on the boardesign.net warned that it is hard to weld, you need specialised equipment. YOu get arguments in all directions. I considered doing the deck in 2CR12, which I could get, but thenthe welding of dissimilar steels become problematic. In the end a friend and mentor, ex merchant marine, said: "Oil tankers are built of mild steel. Corrosion is a fact of life. Deal with it."
So far, Dreamtime having been exposed to extremely high temperatures, (42C for a while) in direct, unfiltered sun, we have not seen signs of problems with differential expansion, but that would probably come when my kids have inherited the boat...
So far, Dreamtime having been exposed to extremely high temperatures, (42C for a while) in direct, unfiltered sun, we have not seen signs of problems with differential expansion, but that would probably come when my kids have inherited the boat...
GTom
Well-Known Member
I wish them and the boat all the best
As for my project, I am undecided, exactly halfway between steel and GRP, I am having my "GRP day" today, my wife would prefer the "safer" steel
Pity that aluminum is so expensive
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rogerthebodger
Well-Known Member
Corten does not do well in salt ladened environments as it seems the "stable rust protective surface" of corten continues to corrode in a marine environment.
I have a steel boat built of 6 mm and 8 mm thick hull and 3 mm deck. The main problems I have is the deck which is painted with epoxy after grit blasting. The epoxy cracks at any change of section thus allowing water to get in and start rusting the steel under the epoxy.
Below the waterline outside is painted with epoxy tar and shows no sign of rust to date
The main issue inside is where water is lying in side so I have fitted sumps of 316 stainless to catch any water that gets into the bilge.
If I were to build again I would have a aluminium deck and superstructure bolted to the steel hull insulating the two to prevent the steel and aluminium interaction.
coenvanwyk I see you are in Pretoria where is your boat at the moment. My boat is in Durban at the marina.
You can see pics of my build by following the link in my sig below
GTom
Well-Known Member
You don't have much salt inside, and in the countless inaccessible/difficult to a access corners corten could come handy... AFAIK the corrosion pattern of corten is also more hull-friendly, it doesn't blister like mild steel...
rogerthebodger
Well-Known Member
Well I have had sea water in my bilge and when the water evaporates it can leave pure salt or very highly concentrated salty water.
inaccessible /difficult to access corners should be eliminated by proper design and in most cases its the turn of the bilge where the rust starts and this should be east to access.
Never known any form of steel to blister. The paint can blister but with proper preparation and thick coats of a low water permeability paint to reduce the reverse osmosis through to the underlying steel.
Good anode protection will also help.
Brent Swain
N/A
For more info, check out
http://groups.yahoo.com/group/origamiboats
My 31 ft steel sloop is 33 years old and requires less maintenance than my last one did after 10 years.The trick is lots of epoxy on clean steel ,inside and out. Mine has 30 gallons of epoxy tar, on a 31 footer ,as good, after 33 years, as the day I put it on.
The last boat I pulled together, the owner had given away his last boat, a Roberts , because it had rusted thru from the inside, due to NO paint inside. He said on this boat he planned on at least 11 coats of epoxy tar, before foaming.
In cold latitudes, a metal boat is almost unliveable without spray foam insulation , so limber holes where the foam is, becomes irrelevant .
Yes, plastic lasts longer, if tied to a dock most of its life. If used full time, it gets the same treatment as a work boat so work boat priorities are best, and steel become superior. Welded down cleats and fittings don't work loose, and there is no better bedding compound than weld.
Last week ,I met a guy who was sailing his plastic boat in Mexico, and hit a whale .The boat sank quickly, and he ended up in open ocean in his dinghy. Luckily it was calm and he had a VHF , so only had to row a couple of hours before being picked up. He said, had it been rougher, he would have died. He agreed when we said he should have been in a steel boat. On will never know how many of the missing at sea people would have survived, had they been in a steel boat. I suspect, many .
I have hit a lot of logs, etc, at night , at hull speed, with zero damage ,which would have sunk me , had I been in in a plastic boat. Friends, who had crossed the Pacific several times in a plastic boat, said their steel boat gave them huge peace of mind sailing hull speed on a dark, moonless night.
Steel boats in the 30 to 36 ft range make roughly the same passage times as most moderate displacement plastic boats.
http://groups.yahoo.com/group/origamiboats
My 31 ft steel sloop is 33 years old and requires less maintenance than my last one did after 10 years.The trick is lots of epoxy on clean steel ,inside and out. Mine has 30 gallons of epoxy tar, on a 31 footer ,as good, after 33 years, as the day I put it on.
The last boat I pulled together, the owner had given away his last boat, a Roberts , because it had rusted thru from the inside, due to NO paint inside. He said on this boat he planned on at least 11 coats of epoxy tar, before foaming.
In cold latitudes, a metal boat is almost unliveable without spray foam insulation , so limber holes where the foam is, becomes irrelevant .
Yes, plastic lasts longer, if tied to a dock most of its life. If used full time, it gets the same treatment as a work boat so work boat priorities are best, and steel become superior. Welded down cleats and fittings don't work loose, and there is no better bedding compound than weld.
Last week ,I met a guy who was sailing his plastic boat in Mexico, and hit a whale .The boat sank quickly, and he ended up in open ocean in his dinghy. Luckily it was calm and he had a VHF , so only had to row a couple of hours before being picked up. He said, had it been rougher, he would have died. He agreed when we said he should have been in a steel boat. On will never know how many of the missing at sea people would have survived, had they been in a steel boat. I suspect, many .
I have hit a lot of logs, etc, at night , at hull speed, with zero damage ,which would have sunk me , had I been in in a plastic boat. Friends, who had crossed the Pacific several times in a plastic boat, said their steel boat gave them huge peace of mind sailing hull speed on a dark, moonless night.
Steel boats in the 30 to 36 ft range make roughly the same passage times as most moderate displacement plastic boats.
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Brent Swain
N/A
In Auckland, I was moored off A&G Price boatyard in the 70s. The foreman said they had tried Corten and were disappointed with it .It only came in plate, no angles, no round or flat bar, and those in mild steel reacted with the corten, as did the welds. It had no advantages, and was much more expensive, and more prone to distortion. They went back to mild steel, happily.
Brent Swain
N/A
Sometimes the equipment to flame spray is cheaper than hiring someone to do it. Friends have found that out , and did it.
GTom
Well-Known Member
I had the impression that the Dutch yards build in corten. Surprisingly no rust on 99% of the Dutch built steel boats I've seen so far - true, they can be mild steel properly zincked.