Aluminum Catamaran Build

[noelex 77]

Originally Posted by ozolli

It's also worth noting that 6082 loses 40% of its mechanical properties when welded, so basically gets down to 5 series mechanical properties.

Yes, good point.

All the aluminium alloys lose strength in the weld area, this is why designers use the “welded strength” of the material when performing structural calculations. In practice the weld area only involves a small area of the total aluminium used so this approach is conservative.

I would not suggest any particular six series aluminium to replace the 6061. This is really a job for professionals, but many older designs were drawn up before the newer grade of six series aluminums became available. So while 6061 may be specified in the design it worth exploring the possibility of replacing this with one of the newer six series. This is especially true in a highly stressed lightweight design such as a catamaran. Likewise the latest cutting methods mean it is now practical to substitute one of the more corrosion resistant five series aluminums in some places where the six series was traditionally required.

I would emphasise again that any substitutions or changes should only be undertaken with professional advice, preferably with the input from the original designer.

[elsener44]

Thank you for the thorough feedback.
The design is a Mumby 48.
I will be using a pretty high end pulse and double pulse capable mig welder with push pull feeder. I haven't welded alu yet and I-m aware I'll need to do a few practice runs. From talking to welders with alu experience it seems setting the machine up properly is half the battle. Plus it helps I'm no dummy using tools and machining.
I do not intend to do any fairing compound or paint above the waterline except the top sides and below the waterline. So yes I'll try my best to get it as straight as possible. I don't intend to enter any beauty contest with it though
The bulkheads are going to be 5383. I will definitely look into getting as much of it in 5383 but I do know the design does call for some T-shaped stringers. I'm ok with some 6061 or if I can find 6063 or 6082 as they won't be minimal if any exposure.
Do you have any suggestion on what grade filler you would recommend?
Thanks again for the detailed feedback!

[elsener44]

Sounds like the best 6 series substitute for the 6061 would be the 6063 if available?

[elsener44]

I do plan on visiting some alu boat builders in the PNW and get some input as well.
Thanks Captain Mark!

[noelex 77]

Originally Posted by elsener44

I do not intend to do any fairing compound or paint above the waterline except the top sides and below the waterline. So yes I'll try my best to get it as straight as possible. I don't intend to enter any beauty contest with it though

This is the best approach in my view.

However, it is tough even for professionals who have welded aluminium all their careers to achieve an acceptably smooth result with 3mm. Aluminium this thin buckles and distorts over the weld area very easily.

Make sure you see some examples of this thinner material in its unfaired state before proceeding.

[noelex 77]

Originally Posted by elsener44

I will be using a pretty high end pulse and double pulse capable mig welder with push pull feeder. I haven't welded alu yet and I-m aware I'll need to do a few practice runs.

A word of caution. This is a huge undertaking.

An approach worth considering is having the shell professionally welded. They will do a better job, particularly if aiming for an unpainted finish. There is still an enormous amount of work fitting out the interior and installing systems that will keep you busy for many years.

[elsener44]

I appreciate your word of caution. I actually spent a few hours with a weld instructor welding alu today and I have to say after getting the machine tuned right and a few hours of running different welds ... it is not difficult at all. Actually easier than some other materials. I can only speak for myself but in my opinion it is easy to weld alu with the right equipment and setup. We tested the welds after and they where 100% legit and looked the part.
Also I'm getting a welding schedule from the builder to minimize any distortion and from what I understand no welds are more than3 inches continues for that reason.
I will be working on the boat full time once the build has started as well as hiring some people along the way. I don't plan on dragging it on for years as a side project.
I will document the process and total time spent.
Thanks again for the feedback. Cheers

[hpeer]

Another resource is The Metal Boat Society. Very active in PNW and oriented towards new builds.

[elsener44]

Very cool resource! Thank you . I will definitely get in touch with them!
Cheers

[Dave_S]

Does Mumby give you an estimated number of hours to reach each milestone. Schionning say x,x00 hours build time for each model which is only a rough guide. I'd be interested to know how many hours they estimate and how many hours it takes you. I think it would be possible to slash the times if you were confident in what your doing and don't spend too much time having beers and pointing at stuff.

Looking forward to seeing how you go with it.

Have you decided on power/propulsion yet? I guess it affects the design.

[elsener44]

Yes the build time for the shell before fit out is estimated at 1600 hours ... not including beers and pointing at stuff I am planning on working on it full time pretty much everyday on it once I got everything in place. That's why I am getting as much as possible squared away now.
Yes propulsion I will go with a parallel/serial hybrid system. One diesel engine in one hull and the electric motor in the other. It will have either a high output alternator or integrel alternator on the engine. Two hybrid controllers (one in each hull) and controller to be able to regen on the electric engine since it has folding props it needs a smart controller to keep the blades open for regen. Yes it will effect design a little because of the weight difference. I plan to put a hydronic heater system where the electric engine goes to balance weight.

[Harmen]

I would love to visit and follow your progress... I am presently at my other home in Anaheim, not too far from you. Planning to return home to Subic Bay soon, and travel from there to visit Tim Mumby's yard in Carmen, Cebu. Hopefully to get in line for one of his next builds.
I would recommend staying with the 5000 series alloys as much as possible, including the filler material. Stay away from 4000 series fillers to reduce corrosion problems. 5000 series are non-heat-treatable, and have full strength as-welded, whereas most other series suffer considerable loss of properties in the HAZ.
I need to dig through my supplier files for a list of the many raw material suppliers in the Socal area. I retired 10 years ago....
7000 series have the highest strength, but are corrosion prone plus are totally no-weldable.
I have written hundreds of weld procedures for most weldable alloys during my years in the aerospace and energy research industries.
Let's meet up for coffee. 7149148686

[Manateeman]

Please take time to contact Miller welding and to view their technical pages.
We also forgot ...Welding Tips and Tricks...with Jody. He has a lot of great videos and we strongly suggest that they are worth the time.
If someone were to ask me how much experience is necessary to do full strength aluminum welds...at least a year...we would want quite a bit of under the hood time before any hull or structural work. Please consider a professional for the hull.
If you spend time in a shop building vessels in aluminum, ask them how they qualify welders for structural work. Invest in professional help for this critical phase...its money well spent. Aluminum welding looks easy but I can assure you it is a skill which is deceptively attractive. Thin aluminum heat control is difficult.
We have seen failures from many minor errors. The more you learn and weld, the more you will appreciate just how much skill and attention to cleanliness matter.
I don’t want to discourage you but please consider letting a professional weld the hull, deck, tanks. It will be money well spent.
Happy trails to you.
Captain Mark and the “ Miller Blue” manatee crew.

[elsener44]

Yes I am planning on working with Miller welding. I have seen some of the Jody youtube videos. I am not a newby to any metal work including alu. However the welding part will be a work in progress. I do intend to hire a pro for at least some of the hull build ... specially crucial components till I'm up to snuff. I talked to Tim and consulted with him as well. I will definitely stay away from the 4000 series.
I plan on x-raying the welds as well and I am a stickler for quality ... I am swiss after all
I appreciate your input about quality and I will definitely get the appropriate professional help when needed. I do however trust my quality of workmanship and I'm sure I'll be up to par at some point during the build with the appropriate pro help.
I do plan on having "plastic tanks" for water instead of the alu as per plans. Onlly the fuel tanks will be alu.
Harmen, I will take you on that offer, thanks!
Cheers guys and thank you for all your valuable input!

[NevilleCat]

Presume you would have checked out the Sailing Life on Jupiter series?
As a total numpty as far as building in aluminium goes, I was pretty shocked at all the tabbing work necessary to create fair welds in such thin material. But I guess it is all just part of the build process time.
Seen here at 8:13 and 8:38

https://www.youtube.com/watch?v=SRf08QXT3fw