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Aluminum cabin & steel hull?
Posted: Mon Jul 23, 2007 2:08 pm
I have been practicing with a mig welder and an aluminum spool gun. I am retired. After I build a couple of small aluminum boats I want to tackle a trawler type ocean vessel for the inside passage. I already developed an epoxy sensitivity so I do not want to do a plywood hull. Is it possible to build a steel hull and attach an aluminum cabin (weight factor) without having an electrolisis problem?
Posted: Mon Jul 23, 2007 4:51 pm
Yeah, its been done. I'll look it up. I think it was in David Gerr's book.
Posted: Mon Jul 23, 2007 5:58 pm
There are special metal bonding strips used in the transition from steel to aluminum. Sorry, can't remember what they're called. One of the ships I worked on had a steel hull and aluminum superstructure. It was joined with overlapping sheets that were bolted through a rubber like gasket. I think the bolts were some sort of "unobtanium" metal. Sorry couldn't have been of more help.
Posted: Mon Jul 23, 2007 6:28 pm
I found that reference. He lista 3 ways to do it.
Triclad. Triclad is a strip of aluminum and steel cladded to a inner layer, just weld steel on one side and aluminum on the other.
Another way is to weld a steel flat bar in place and bolt the aluminum to that with a insulating gasket.
then he mentioned welding a stainless flat bar to the mild steel, and simply bolt the aluminum to the ss flat bar.
Posted: Wed Aug 08, 2007 12:12 am
The traditional method of having aluminum superstructure w/ steel hull & deck is to use a rubber gasket with stainless bolts. An alternative is to use transitional material - aluminum/steel strips(triclad, etc.) which are explosion welded together. The transitional material is extremely expensive & brings up some cathodic corrosion issues. I have kicked around the idea of using transitional material when building my offshore sailboat - for the structural integrity of the hull/cabinside joint & would probably go with magnesium anodes. If you are planning on cruising more protected waters, the rubber gasket joint would work & be cheaper. If, however, by "inside passage" you mean the arctic, I'd have some concerns over the rubber cold-cracking. Then again, I'd have similar concerns with the aluminum.
Posted: Thu Aug 09, 2007 6:23 am
What would be the advantage to building a boat using the two different metals? My only experience has been on the Gulf of Mexico so obvioulsy I don't deal with weather extremes, is that the reason for the steel hull? In my area it appears aluminum will last as long if not longer than steel hulls,(if properly protected) so what is the advantage of the steel hull?
Posted: Fri Aug 10, 2007 4:10 am
In weighing the advantages/disadvantages of building from steel or aluminum, some of the considerations would be as follows:
1)Steel plate is more ductile than aluminum, that is, more willing to stretch & bend, than fracture(tear).
2)It is far easier to repair a damaged steel hull or deck, especially in a remote location, or while at sea.
3)Building with aluminum requires more than twice the dollar investment in material, than with steel.
4)An aluminum hulled boat is limited in where it may anchor/moor in a marina as the aluminum is prone to act as an anode for any steel in the immediate area(e.g., other steel boats/structures in close proximity, whether at surface or sunken).
5)A steel hull does not require isolation from installations - engine, generator, etc. - other than where electrical flow is present.
6)However, Aluminum is much more corrosion resistant and lighter/faster.
Without trying to sound like some "metal guru", allow me to explain from what perspective I draw my opinions. I am a welder & shipfitter/fabricator, most of my employment being with Vancouver Drydock(Washington Marine Group), though I do emergency marine repairs for other companies, as well. I "grew up" in the steel industry; my great-grandfather was a shipwright, my grandfather a boilermaker in the shipyards, my father a former boilermaker in the shipyards and, later, a metallurgical engineer, in whose firm I was employed for 5 years as a welding inspector.
There is a Yahoo group on the internet - "origamiboats" - which you may find of interest. Though many of the members are too inclined to "parrot" the views of a single builder/designer(Brent Swain), and Brent has very strong opinions of what is & is not correct practice in design, construction, and material choice, the group has a vast amount of archived information on the topic of steel vs. aluminum construction. A word of caution, however; the most outspoken of members, while having credible offshore sailing experience, are only as knowledgeable in metal construction as one can be by referencing articles on the internet. The concept of Brent's design/building of boats is K.I.S.S., to the point of being unaccepting of many traditional design & construction methods. I was a long-standing member of the group, until recently, when I felt that I had gathered much of what the group had to offer. Still, it is an interesting resource.
In my daily work, I repair/refit/build ships, barges, etc. . As has been my experience, a properly protected & maintained steel hull will keep it's structural integrity for generations. Personally, I would only build a hull in steel for offshore purposes, as it can be - in most cases - repaired at sea or on a remote shoreline, unlike the alternative hull materials. One can carry a small inverter welder or have an alternator welder installed on his/her engine or genset. In fact, most freighters, factory fish ships, etc., have one or more welders on them.
I have , recently, chosen to order plans/patterns for the Glen-L Lodestar55, after years of deliberating on most of the available designs. I have owned several "plastic" boats, and have modified the structures of two; a Contessa 32, and; a Cal 3-30. Great boats, but my arms itch just thinking about them(damn fibreglass!).lol
Posted: Fri Aug 10, 2007 6:14 am
Thanks for the reply. I teach welding at our area Votech. We have several shipyards in the area and I do a lot of training for them. Other than barges I would say the rest is split about 60/40 of steel over alum in the boats they construct. When I have asked why one would be chosen over the other the most common comment is "it is what the customer wants." I have noticed that most of the work type boats are generally steel and passenger or voyage type vessels are usually aluminum.
I had not thought of ease of repair as being one of the reasons for choosing steel, though it does make a lot of sense.
Posted: Wed Sep 26, 2007 7:39 am
I worked on a steel crabber out of Kodiak for 20 years, The boat was built by my Great Uncle in 1967. Steel hull, alum house. It was a pain in the but. We replaced the rubber gasket between the steel and the aluminum house maybe twice and all of the stainlees bolts (even these would corrode from electrolisis). It is a lot of work to maintain. But the boat had the origanal house on her when she sunk in 1997, she was 30 years old. She went down due to the stupidity of a guy my Father had running her at the time.
steel to aluminum joint
Posted: Sat Jan 19, 2008 1:00 pm
The USN class DD-963 ships all had steel hulls with aluminum superstructures. The connection joint was steel bonded to aluminum. I was told the material is produced using an explosive technique. I do not know how well the joint held up over time. All that class was decomissioned very early for USN ships 1983-1995 so they could have developed hull problems. The older DLG-16 class ships used a rubber gasket and bronze rivits to hold the steel hull to the aluminum superstructure. Those ships were in service from 1960 to the middle 1990's.
I spent several years on both of those type ships and the aluminum parts, exposed to the weather were constantly a problem.
The most durable ships were all steel. The USS New Jersey built in 1943 is still in good shape as a museum piece someplace on the Atlantic Coast of the US.
Aluminum House on Steel Hull
Posted: Thu Jan 24, 2008 1:58 pm
I put an aluminum house on a crabber like oleman describes and it lasted the life of the boat.
We welded a steel angle to the steel deck after the original house was cut off, and on that heavily painted steel we laid a fiberglass angle strip. The fiberglass was covered with an aluminum angle and the whole thing was drilled for SS fasteners.
Once the joint was made the lower foot of the aluminum cabin was coated with a very heavy paint. I believe the paint was called coal tar epoxy - not sure of the brand. That coating was more like peanut butter than paint, but the joint survived.
These boats' deck are often awash and that contributes to electrolysis if there are exposed metals of different types. But by coating and passivating the SS parts you should be able to get a good serviceable joint from steel deck to aluminum cabin structure.