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Old 03-06-2007, 02:03   #1
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How Are Aluminum Hulls ?

I'm looking for my first sailing cat in the 40-50 foot range. I've seen some aluminum hulls advertised. Any negatives or advantages?
Aircraft do really well in aluminum.
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Dave
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Old 03-06-2007, 11:53   #2
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For example is an aluminum hull noisier? Lighter? How about general longevity compared to composites?


I was interested in this one.....

The Catamaran Company

Any comments ?
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Old 03-06-2007, 16:36   #3
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Silkline Catamarans

Peter Kerr does a alloy cat as well, can't find a website.

Seaeagle 1

They can be noisy and need lining.

They can have bad electolysis and dissolve in saltwater if not set up properly.

Dave
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Old 03-06-2007, 20:12   #4
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I looked at alloy as a building material too. Like everything it has it's pro's and con's. What seemed to be the consensus was that around 45 feet was the lower practical limit for an aluminium cat. Because aluminium tends to deform when it's welded, you need to use reasonably thick plate to get a decent appearance, which would make a boat less than 40 feet or so heavy.

Other difficulties include getting paint to stay on permanently - some people choose not to paint because of this, and as Dave says, you need to insulate, and be extremely careful about electrolysis. You'll be limited in your choice of antifouls too, and some of the ones suitable for aluminium don't work very well.

In it's favour, it's fairly tough, and installing additional deck fittings is relatively simple. Just drill and tap, more or less.
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Old 03-06-2007, 21:44   #5
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Quote:
How are aluminum hulls?
They're good. Expensive and sometimes no so easy to repair, but good.
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Old 03-06-2007, 22:26   #6
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Quote:
Originally Posted by Sailormann
They're good. Expensive and sometimes no so easy to repair, but good.
Not realy true.

I did my apprenticeship building 100 ft plus alloy gin palaces and have done alloy from 17 ft centre consoles up.

For me alloy is cheap and very easy to work with.

Interestingly my last 2 build's for myself have been composite,and I think you'll find composite structures will be much more expensive to repair than an alloy one.

Dave
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Old 04-06-2007, 02:41   #7
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Quote:
Originally Posted by 44'cruisingcat
In it's favour, it's fairly tough, and installing additional deck fittings is relatively simple. Just drill and tap, more or less.
I would agree with this to a degree. It will depend on the fitting and the thickness of the aluminium in this area. The other problem to remember is that the fixing to go into the tap must be chosen carefully or electrolysis will occur. I would drill through and put an insulator between the fixing and the hole and put a reinforcing plate behind personally.

Like "44'cruisingcat" I also looked into alloy as a building material. It does have many advantages. The boat must be built by someone experienced in welding Alloy materials tho. If not done properly the area around the weld will be significantly weaker than the rest of the material.

The other thing to look out for is age hardening. Aluminium suffers badly from this so if the boat flexes alot the alumium will fail around this point eventually. This is the main reason I decided not to go with alloy in the end. If the boat is designed to be stiff you'll be ok.

Alloy is very easy to repair if a competent welder can be found. The area to be repaired will normally be fairly local, unlike GRP where the hole could be fairly small but the damage to the GRP can spread for another 3-5 foot.

If you choose not to paint the hull ensure you check for stray currents when in marina's. Your boats electrical system maybe set up correctly but that doesn't guarentee that everyone elses is.

Check out the Metal Boat Society Forum for info. I was a member of this and found it to be a very useful resource whilst I was looking into monohulls and at the start when i was looking into Alloy Cats.
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Old 04-06-2007, 04:05   #8
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Even for those experienced in welding steels, welding aluminum alloys can present quite a challenge.
In most cases, a weld in an aluminum alloy is weaker, often to a significant degree (ie: 50%), than the alloy being welded.


Common Mistakes Made in the Design of Aluminum Weldments
By Frank G. Armao, Senior Application Engineer, The Lincoln Electric Company
Common Mistakes Made in the Design of Aluminum Weldments | Lincoln Electric

A Guide to Aluminum Welding ~ from Welding Design and Fabrication magazine
A Guide to Aluminum Welding | Lincoln Electric

Aluminum: Experience in Application - What you should know about welding aluminum
Aluminum: Experience in Application | Lincoln Electric
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Old 04-06-2007, 05:30   #9
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There are basically two types of aluminium construction - standard skin on frame, or "strongall". Strongall is frameless, using thicker alloy instead for strength and is considered to be very tough. Pro-meta in France build strongall cats, and FairMetal boats in Michigan can build a strongall cat if you supply the design. As has been already stated the downsides are electrolysis, 50% loss of material strength where it's welded, and it's prone to metal-fatigue. Alloy would be my #1 choice if building a monohull, but for the reason of metal-fatigue alone, I would have reservations about alloy cats. You'd definately want a good surveyor that knows the material.
Peter Kerr's site was Lizard Yachts, but it's gone - I think he's involved with Silkline. Dave forgot to mention Easton cats.

Kevin
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Old 04-06-2007, 10:57   #10
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I would agree with Lodesman about mono vs multi.

From what I've seen out there alum has a tendancy to crack next to welds and in high stress areas (stanchion bases, cabin corners and cleats).

On a Cat the twisting effect would bother me! Although, the designers/builders should have calculated in the necessary preventives.

The other problem, once alum has become oxidized, welding is near impossible. The material has to be cut/ground back to clean metal to be welded. An older boat with lots of bad corrosion could be a nightmare.

And screwing in a SS screw in an area of much seawater exposeure, one would have serious problems tring to remove that screw. It's best the drill thru and back up with a nut..................._/)
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Old 04-06-2007, 12:08   #11
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Speaking of aircraft, the newest Boeing jet will be all composite construction, except for engines and landing gear of course.
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Old 04-06-2007, 16:02   #12
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[quote=delmarrey].

The other problem, once alum has become oxidized, welding is near impossible. The material has to be cut/ground back to clean metal to be welded. An older boat with lots of bad corrosion could be a nightmare.

You alway's should clean and wire buff alloy before welding, even on new plate, agreed, lot's of bad corrosion in hard to acces areas is a PITA

And screwing in a SS screw in an area of much seawater exposeure, one would have serious problems tring to remove that screw. It's best the drill thru and back up with a nut..................._/)

This is what you need to use on dissimilar metal's. Never had a SS bolt grow into alloy if enough Duralac was used Whitworths: Duralac Jointing Compound

Dave
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Old 06-06-2007, 06:57   #13
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Quote:
Originally Posted by Lodesman
Peter Kerr's site was Lizard Yachts, but it's gone - I think he's involved with Silkline. Dave forgot to mention Easton cats.

Kevin
Peter is most certainly NOT involved with Silkline. He is happily building aluminium cats at Tin Can Bay north of Brisbane Australia. His web site isnt currently functional - but I will ring him tomorrow and see if he has a new one - or what the story is. Meanwhile he is at the end of the phone.
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Old 06-06-2007, 07:01   #14
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Oh and as for the suitability of aluminium - In monos possibly the most travelled guy around, Jimmy Cornell runs an aluminium boat and in multis some of peters boats are going on 20 years old, and there are like bucket loads of medium and large multihulled commercial cowthers and the like.
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Old 06-06-2007, 08:21   #15
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Quote:
Originally Posted by Factor
Peter is most certainly NOT involved with Silkline.
Sorry - didn't mean for you to get yer knickers in a twist. It was merely an assumption on my part - Kerr's site disappeared about the same time Silkline came into being, and I'm fairly confident his boat "Lizard of Oz" is in charter with the Silkline prototypes: Faraway Expeditions - Phuket sailing and diving charters, Thailand Honest mistake - really. Anyway, if you do talk to hime, I'd really like to know if he's still selling plans?

Your point about commercial alloy multi's being around for awhile is valid, but I think it's also fair to say a significant number have had stress-fracture remediation. I believe the early wave-piercers were particularly prone. Not necessarily a design flaw - like aircraft, it sometimes takes a lot of miles for cracks to happen. That's why I said a good surveyor, who knows how to look for cracks is vital. In a mono, stress fractures are both less likely to occur, and less likely to be catastrophic.

Kevin
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