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Old 13-01-2014, 10:27   #1
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Alum vs fg

Over the last two decades my Dad and I built steel motorsailors in the form of gulets. That trade sorta drifted off into never land. So, we built a couple of mahogany/glassed over sailing cats - the GR 50 and GR 42. The 50 I sold after my Dad's passing and I kept the 42 which I live on, when I'm not fooling around designing or delivering a boat.

Neither of us really liked fiberglass hulls - too thick and heavy. And didn't want to spend $300k on a stiff hull. So, I thought why not aluminum - 5083/5182/5086 specifically - it can be welded easy and will take a beating. It's lighter than glass and easier to maintain. 0.25 on hull sides and 0.50 on the keels.

Any thoughts on aluminum vs glass hulls?
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Old 13-01-2014, 11:33   #2
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Re: Alum vs fg

You might want to post that on Boatdesign.
Metal Boat Building - Boat Design Forums
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Old 13-01-2014, 13:01   #3
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Re: Alum vs fg

You can build lighter in fibreglass. But not the old way - ie. inch thick solid chopped strand layups. Using foam or end-grain balsa cored construction, epoxy resins and multi-axial uni layups you get very stiff, strong and light panels.

Also with glass construction it's possible to build in such a way that the interior only needs to be painted. With metal construction you also have to build an internal liner, which adds more weight.

Metal also conducts heat and noise better, so some form of insulation is usually added. More weight.

Unless you're building a really big boat, it can usually be lighter in cored glass than alloy.
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Old 13-01-2014, 14:06   #4
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Re: Alum vs fg

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Originally Posted by 44'cruisingcat View Post
You can build lighter in fibreglass. But not the old way - ie. inch thick solid chopped strand layups. Using foam or end-grain balsa cored construction, epoxy resins and multi-axial uni layups you get very stiff, strong and light panels.

Also with glass construction it's possible to build in such a way that the interior only needs to be painted. With metal construction you also have to build an internal liner, which adds more weight.

Metal also conducts heat and noise better, so some form of insulation is usually added. More weight.

Unless you're building a really big boat, it can usually be lighter in cored glass than alloy.
Actually the two boats I finished were built with Nida-Core vaccumed bagged. They were 0.75 inch thick on the keel and 0.375 inch on the deck and hulls. Far lighter than previous attempts - but they still very heavy and stiff.

This is going to be a big boat for me - at least 60-65ft. I like plate welded aluminum - I can TIG or MIG without any problems. Sprayed insulation is planned. Also 10kw LP generators to run electric motors
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Old 13-01-2014, 14:39   #5
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Re: Alum vs fg

I've read that you must build a Cat of at least 50 feet, if you are to make it out of aluminum and not be overly heavy.

Kurt Hughes does have one design in aluminum, it's a 65 foot charter cat.

Kurt Hughes Multihull Design - Catamarans and Trimarans for Cruising and Charter - 65' Day Charter Catamaran

A 36 foot Kurt hughes was built of aluminum, to a very high standard. She came out at 5.2 tonnes, if I recall, correctly, what the owner told me. He used Kurt's CM method with aluminum sheet, and said it worked quite well. Judging by the final results, I'd say he's right.

http://www.multihulldesigns.com/desi...k/36bdcat.html



It must be said that the builder/owner is a very skilled fabricator and boat builder. This is not his first try at bat.

Cheers.
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Old 13-01-2014, 14:59   #6
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Re: Alum vs fg

I've seen a couple of Hughes designs - one in Belize and the other in BC - both tho are glass and very expensive.

I'm looking to do a green boat hence the electric motors. Three cabins: master king and two queens ensuite. Mine will be heavier with plate aluminum - hopefully built to ice standards, tho I need to check on that one.
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Old 13-01-2014, 15:21   #7
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Re: Alum vs fg

Around here aluminium is extremely popular for runabouts and crayboats due to its bashability on reef.
Many people however refer to them as aspirins.
As mentioned over 50ft aluminium is a viable material, I wouldn't, I hate the noise of the stuff. But given you are familiar with the material and have misgivings about glass, go for it.
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Old 13-01-2014, 15:41   #8
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Re: Alum vs fg

Um, you read my post right?

I guess I'm missing something. You want to build in aluminum, a cat somewhere around 65 feet.

I posted a link to plans for a 65 foot aluminum cat. Whether you put electric drive or not is up to you.

Good luck.
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Old 13-01-2014, 15:51   #9
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Re: Alum vs fg

Can't build AL boat from a female mold, unless you have a zillion dollar stamping machine.
I would never say aluminum is 'easy' to weld, and calling a foam core FG as 'very heavy',
pretty much sums up you have an agenda here.
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Old 13-01-2014, 16:29   #10
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Re: Alum vs fg

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Originally Posted by svquintana View Post
Um, you read my post right?

I guess I'm missing something. You want to build in aluminum, a cat somewhere around 65 feet.

I posted a link to plans for a 65 foot aluminum cat. Whether you put electric drive or not is up to you.

Good luck.
Paul.
Yeah, I read it and not particularly in favour of Hughes designs. Thanks anyway.
Quote:
Nimblemotors: I would never say aluminum is 'easy' to weld, and calling a foam core FG as 'very heavy',pretty much sums up you have an agenda here.
I am a welder by trade - and aluminum is easier to work with than steel - even Corten. Nida Core that we used is not heavy compared to hand-laid up. Nida is vaccum bagged with epoxy. No I don't have an agenda - I just asked for opinions.
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Old 13-01-2014, 16:36   #11
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Re: Alum vs fg

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Around here aluminium is extremely popular for runabouts and crayboats due to its bashability on reef.
Many people however refer to them as aspirins.
As mentioned over 50ft aluminium is a viable material, I wouldn't, I hate the noise of the stuff. But given you are familiar with the material and have misgivings about glass, go for it.
We refer to them as tinnies - none that I've seen are plate welded
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Old 13-01-2014, 16:40   #12
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Re: Alum vs fg

Here is a cat designer and builder based in Australia. Most of his boats are Aluminium.
Lizard Yachts Home
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Old 13-01-2014, 16:42   #13
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Re: Alum vs fg

Thanks will look it over this week.
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Old 13-01-2014, 18:51   #14
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Re: Alum vs fg

How about some pics of those cats you built.
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Old 26-01-2014, 07:04   #15
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Re: Alum vs fg

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How about some pics of those cats you built.
Sure, I'd love to find that photo album. It was last seen my Dad's place in Panama. I did send you some photos of boats we built together.
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