Brian and GSHols, we had similar questions about aluminium when we first started looking at what material to use for the hull and superstructure of the new boat. Our former boat was all steel and I'm a big fan of metal boats in general so it was down to steel vs AL for our choices. I won't debate the pros and cons of steel vs AL here as these are well known topics and one will likely appeal to each of you more than the other.
Before I get into it, I think it is helpful to agree, or at least tell you that I believe there is no "best" material when it comes to boat hulls until we specify the context of the boat itself, size, use cases, owner preferences, etc. Great boats can be built out of any steel, FRP, composite, wood or AL and each one has its pros and cons which will suite different applications and different captains differently.
Secondly and echoing some of Sunchasers comments above I don't think there can be any doubt that AL is one of the very viable materials to choose from for building boats alongside steel, FRP, composites and wood. I also don’t think that it is any coincident that in my travels around the world and my research it seems that the greatest growth of new boat construction is in AL. I kept running into more and more situations where new shipyards starting up as well as companies combing back to life after various recessions or other factors knocked their business down, are chosing to change over to or go more exclusively with alluminium. I don’t have statistics on this, just my observation over he past several years.
So the choice is about finding what material best suits a given boat, use case and owner. If you'd like to read more about our use case and ourselves that led to our choosing AL for our new boat I have recently posted an article outlining ourselves, our use case and the key attributes our design needed to follow. You can read the posting here:
http://mobius.world/project-goldilocks-mission-impossible-or-just-right/
So I want to be clear that I am NOT any expert in AL and have not owned an AL boat other than small ones. I have worked extensively with aluminium for a long time in non marine applications building things with AL and have done a fair amount of casting, welding and machining AL in my previous life as an Industrial Ed/Shop teacher and with my many hobby activities. What I can tell you is what I've learned from all this and from my extensive research into aluminium as a boat building material over the past few years.
Someone who does have a LOT of experience with aluminium boats and has recently written some excellent articles on the topic is John Harries who runs the Attainable Adventures site at
https://www.morganscloud.com/2017/11/11/22-aluminum-boat-care-tips-part-1/ The articles are behind a pay wall so you’ll need to be a member to read them but if you are considering AL for a future boat or own one already it would be worth the minimal subscription price for just these articles alone IMHO.
In our case we know steel well and so we researched aluminium extensively and had been doing so for years anyway as our dinghy was an AL bottom and we had lots of aluminium gear on the steel boat which provided good experiential learning. The result was that we believe AL is the best choice for us and our boat and I'll skip across the high points here in my attempt to provide some degree of answers to your questions and also why we settled on AL. Please just keep in mind that I am NOT building a case for AL being the best material for boat hulls nor that you should go with AL, I’m simply reporting on why we decided it was the best choice for US and OUR new boat.
We like the combination of very high strength and abrasion resistance for contact with harder bits above and below the WL, along with its elasticity to absorb and bend with blunt forces but not puncture. All up to a point of course and we in no way delude ourselves into thinking that AL or any other material is truly bullet proof but we believe that AL offers a great combination of these physical properties which match up well with our use cases. We do long passages and spend more of our time in very remote places and have lots of “hard bits’ around us from coral in the tropic to ice in the high latitudes so we WILL hit all of the above at some point and want the hull material to give us the best chance to “survive and thrive” as I call it, meaning not need to change our destination or journey to head to a yard ASAP when go aground, push through icy bits, etc.
Secondly we think AL offers us the least amount of overall maintenance time and costs. Our hull will stay unpainted above the WL and we will have no SS or wood on the exterior. Not needing to paint the hull saves money and time both to build and to maintain over the years. For us, we like the gray military look of unpainted aluminum as we want the whole exterior of the boat to give off a strong “don’t mess with me” vibe to any viewing us from ashore or other boats with ill intent. We fully appreciate that many of you will find this to be “ugly” and we’re good with that. Eye of the beholder and we have our reasons for liking raw aluminium that make it the best choice for us.
The “no paint” rule applies equally to the inside AL surfaces of the boat as well simply because of the potential for poor adherence of the paint to the AL which creates an ideal environment for water, fresh or salt, to get in there and keep the area wet and cycling lots of oxygen out of the air. This is a feeding frenzy and breeding ground for oxidisation of steel or aluminium and needs to be avoided at all costs. This is particularly important for the bilge areas or other places that have the potential of being wet off and on. Great thing in this case is that the best solution is NO PAINT or other any other sprayed on materials which can create corrosion inviting environments. As we boat owners know all to painfully, this is a very rare example where the best “at all costs” solution is also the cheapest.
Aluminium is also my favorite material to work with vs steel or FRP. I’ve worked with all of these materials to a reasonable extent over the years, owned, renovated and maintained an all steel boat for over 12 years but given my choice I’d chose aluminium to work with. I can use pretty much the same carbide tools I use and have aboard for wood working. It is very easy to machine with mills, lathes and CNC routers all of which I have onboard. And it is a joy to weld which I also have onboard.
Corrosion and especially that due to electrolysis is one of the biggest concerns so no surprise that you guys are in that boat. (sorry couldn’t resist) While the concern is very real these are all very solvable problems as is corrosion and electrolysis potential in other materials and in all boats really as we all have various bits such as props, shafts, through hulls and fittings which are made from various metals subject to the detrimental effects of electrolysis and needing proper prevention and monitoring The question of “hot” marinas for examples is not something that affects just aluminum boats and components, but all metals. Therefore ways of protecting against this are quite well known and proven, we just need to be sure to do it all with great discipline.
Some things we have learned to do and will be doing with our new boat in include:
• Isolation Transformer. I would regard this as a must have piece of equipment for ANY metal boat for sure and really for all boats. It guarantees that you keep the hull and the whole boat physically isolated from any stray current or faulty aspects of shore power supplies. We spend almost no time in marinas so insist on hving an isolation transformer for other reasons as well not the least of which is that for world cruising an isolation transformer also gives you the ability to have any AC voltage from 100-240v coming IN through your shore power connection and equally chose any AC voltage out. You still need to deal with the difference of 50 vs 60 Hz but the number of electrical consumers which are fussy about 50 vs 60 Hz is very small and getting smaller all the time.
• Isolated Ground: Also good practice for any boat regardless of material it is made from but particularly important for AL and steel. In short the hull is for keeping the boat afloat NOT for conducting current anywhere for anything. All grounds need to be isolated via their wiring and the engine in particular needs to be isolated from the boat itself via its motor mounts and then ideally have all the electrical items on the engine to have their own ground wires and not be using the metal of the engine itself to be a ground. Make sure to isolate the prop shaft as well.
• While more expensive it is ideal to have dedicated ground wires for all your DC circuits by using double pole breakers.
• Have good meters to tell you that there is no current flowing through the hull, especially very low currents. Takes a bit of work and hunting down good meters but critical to do so and a great part of SWAN for boat owners. (Sleep Well At Night)
• IF you own an AL hulled boat you want to have a good low current testers on board and check the boats hull for any current at regular intervals. This can be done with a good sensitive multimeter though it is a bit more time consuming. There are also several meters you can install more permanently which make checking this very quick and you can also use low power LED’s to build your own quick check system. However you do it, the point is to KNOW that there is no current flowing through the hull. No current = No corrosion + SWAN
• The other highly valuable, I’d argue mandatory, tester to have onboard any metal boat, especially AL, is a silver chloride reference electrode. You can look these up to learn more if they are new to you but in use you put them into the water and connect the other end to a multimeter which enables you to the voltage difference between the water the boat is immersed in and the hull.
• Also as with any boat, we will have a well sized set of anodes attached, most likely zinc.
As for concern about the protecting the overall hull material from corrosion AL is similar to steel in that your greatest concern is for rusting/oxidizing from the inside out, not the other way around. Below the water line all boats need to have some coating to deter growth and therefore assuming the bottom paint is kept in good shape, the hull is never in contact with the water it is floating in. Above the WL in our case as I’ve noted we will not paint the AL at all and this turns out to be a good thing in that AL is quite naturally looking after itself in that the surface layer converts itself to a thin hard coating of Aluminium Oxide which serves as a great barrier. It is when we paint this surface and have places where the paint does not adhere well or otherwise allows water to sit there cycling oxygen through the dampness that corrosion sets in and often takes off. Not much different than we learned with steel though aluminium does oxidise a bit differently.
Finally for today, the concern about the proverbial “penny in the bilge” is really just another important example of the need for isolation, in this case isolation of dissimilar metals. As with metal components on all boats you want to try to eliminate or reduce as much as possible, ANY contact between dissimilar metals. Copper and AL don’t get along well and there is some truth to the copper penny in the bilge, however it has also been blown out of all proportion and AL boat owners will tell you about finding stray bits of copper wire for example in a bilge crevice that was sitting there for months or years in some cases and had only penetrated a microscopic amount of the aluminium. And again as with all boats, one of the keys to preventing this is to design, build and maintain super CLEAN bilges. We end up with very few bilges at all on the new boat as we have almost all tanks below floors and the WL, and in the engine room and workshop areas where there are purposely low spots they are designed to be very open and easily viewed and have either no flooring overtop or where needed we will make flooring grates from expanded AL welded into AL flat bar frames. Very good to walk on and yet very easy to see through all the way underneath.
I hope this all helps to provide some insights and answers to your original question GSHols about “the long-term upkeep of an aluminum hull?” In our opinion when you take the big picture view of overall boat ownership costs and time, an aluminium hull will require the least time and money to maintain over the life of the boat. Perhaps even more importantly, based on our time aboard other aluminium boats and talking with their owners, that lifespan will likely be much longer as well.
What’s not to like??
Wayne
