Fibreglass vs Aluminium Impact Strength

[minaret]

Originally Posted by noelex 77

Hi Steve the Internet is changing. To post a video especially produced for CF to answer a thread is great.
If a picture means a thousand words then a video is an encyclopaedia

The breathless commentary as you pounded the aluminium was fantastic.

For those that posted it not a scientific test I agree completely. There was no science, no controlled environment, no control pieces. We don't even know he grade of aluminium, but we saw a man pounding the s..t out of of a boatbuilding material. I think most people can relate to, and be impressed by the forces involved.
Go bash the s..t out of fibreglass and video the results

Couldn't agree more, please do. But also please be careful, fiberglass is a lot more "bouncy" and won't bend like that, you could hurt yourself easily with the rebound. Having sledgehammered glass before I can tell you it's usually not a good idea, it can really hurt your wrist. I mentioned G-10 fiberglass plate in a joking fashion because if you strike a thick piece of it full force with a sledgehammer, you will definitely hurt yourself and not the plate. It was an extreme example of what a modern fiberglass laminate can be like. I bet there are destructive testing videos of it on YouTube, but my boat is in the yard so I am restricted to the iPad right now and can't link.

[Jim Cate]

Originally Posted by minaret

Definitely not, 1/4" plate is quite common on production boats in the mid size range (say 40'). In the same size range a solid glass lam will usually be as thin as 3/4" at the top of the sheer, most of the hull will be 1" and the keel much more. I just mentioned 3/4" because we happen to have lots kicking around right now that we hand laminated ourselves, much like a production boat. Try your test on a piece of G-10 1" plate for a real eye opener. Using this math to match your 3/16 th plate you would have to go up to an 1 1/2" laminate. Good luck!

G'Day all,

I'm curious about your laminate thickness numbers, Minaret. Looking back at the damaged Bene 47 whose picture I attached in post 89, the layup in the bow sections where the failure occurred was much less than the 3/4 inch you mention as typical at the sheer line. I wasn't able to measure it, and the ragged fracture may have been deceiving, but it appeared to be on the order of 1/4 inch in this highly stressed area... certainly less than 1/2 inch.

Would it be unreasonable to expect that this was common practice in the manufacture of modern production boats of the French design school?

Cheers,

Jim

[foolishsailor]

Originally Posted by minaret

Impressive. But more in the realm of the custom expedition yacht than anything production. Expense wise we could certainly compare it to a carbon Kevlar hybrid pre-preg epoxy vacuum layup. Neither can be afforded by 99% of boaters. Hardly a typical example.

Im sailing an 80s French Trisbal centerboarder and the hull is completely fair except for the welds which were left un grinded. Only painted deck and waterline. My bilge keel plate is 20mm, plate to turn of bilge is 12mm, plate to gunnel is 8mm, deck plate is 4-5ish, Bow, or stem, is formed of 200mm tube with 10mm wall. She has welded tbar frames at 30cm center, and an epoxy sealed weighted bilge.

I have banged her off docks, plastic boats, steel trawlers, floating steel fuel drums, and a rock or two.

She is 42' and only weighs 9 tons fully laden and sails incredibly well and for me meets the illusory definition of a bluewater cruiser.

I paid under 100k for her and fully refit her to bristol for about 50k more.

Just to add this to the data points for discussion sake as what i am reading doesnt match up entirely to my experience in either plastic , wood or alloy boat ownership or sailing experience.

Seems there is alot of focus on the extremes of each materials benefits amd failures and not alot of diacussion on the realities of why people choose a given material.

I tend to bang on about statistical realities when discussing cruising so lets do that here.

What are the most common issues that are minimized by material choice?

1. Cosmetic abrasion: this can be major even if not life threatening. Think that guy who let his sugar scoop get under the dock during bad weather

2. Hull flexing: even well built boats flex

3. Rigging attachment points

4. Superstructure attachment points for modifications

5. Mounting strength: think drag device. What do you have to do to your cleats or bow roller area to meet the bullet proof requirements

6. Hull maintenance: keep the material at full strength, optimal weight and minimal cost

On and on...

Edit: forgot this bit

Its not the impact with a container or the perfect storm that should be informing a decision for hull material any more than those extreme examples should inform outfitting for health and safety.

I bought my boat specifically because i knew that old alloy boats ate easy to accurately survey, if the thickness of the plate is sound the boat is sounds. Their upkeep is nonexistent if you sort the electrical, they are easy to modify and if you accept the above the pain that comes from the wallet when painti g the bottom is a minor inconvienence.

Each material has its pros and cons. I have owned cruising boats in plastic, wood and alloy and havent seen evidence in this thread to change my opinion on the superiority of alloy for my needs and cruising style.

That said it wouldnt even occur to me to challenge another boat owners decision...

[msponer]

I love the metal attack video, it was a great contribution.

Here's some of the reasons why our first boat was fiberglass and our second aluminum:

• I wanted a different pain, some new horrible thing that is not grinding fiberglass. I want new nightmares that are not 'rotten core' in a satanically hot tropical boat yard, covered in fine itchy dust that won't wash off, sleeping in it and eventually growing allergic to it... What are the nightmares with aluminum? Ripping out the interior furniture to replace a panel? Scrapping her when stray electrical currents turns her into aluminum foil? Anything else?
• We are feeling 'done with leaks', and I have this wishful thought that our metal boat can be made to not leak, at all. Dusty bilges, and storage that I never have to worry about getting wet, would make me so, so happy. I was never, actually, able to stop my fiberglass boat from leaking when we were actively making passages. I'd rebed, it wouldn't leak, and then on the next multi-week passage, it'd start leaking again. This might be my problem, I'd love to know if other folks, who've regularly travel far with a fiberglass boat, are able to have dusty bilges.
• I feel a bit more confident in the ability to thoroughly survey a used metal boat than a fiberglass boat, to be more confident that there are no hidden monsters. Clean aluminum plate just looks... easily and knowably right. This is probably fiction, but when I look at used fiberglass boats, I feel there's a bigger risk in hidden problems (ex: Bumfuzzle's cat). Is this true? I don't know.

Foolish and Nolex -- I'm beginning to feel like a lightweight. My 48' boat is 9mm thick near the keel, 6mm on the rest of the hull, and 4.2mm on the deck. I've never measured the spacing between the stringers -- maybe ~20cm. The frames are about 40cm apart. Before we found this boat, we were going to commission a Van De Stadt with 5mm plate and frames 40cm apart.

[foolishsailor]

Jeez i didnt even think of the pleasures of a dry boat!!! Thanks for reminding me. A well insulated alloy boat is a Msponer said, dry as dust. Not just bilges either, as there is no residual water in bilges or surfaces the boat feels drier inside, even the surface mold issue in cabinets and surfaces is almost nonexistent.

Our boat is fully insulated and a pleasure to live aboard, but i must admit the noise and environment of a good woodie is still my favorite, but that will have to wait till the wife and kids no longer want to sail with their old man.

[limejucer]

I liked the previous aluminium video so i did a search and you americans came up trumps with this video of fiber glass vs pistol and assault rifle. it looked to be less than half inch Grp though .
infusion fiberglass ballistic test - YouTube

[Kettlewell]

Let me preface this to say that I think we're comparing apples, to oranges, to bananas. You can't just say fiberglass is stronger than aluminum and have it mean anything. I always think of those photos of a stalk of straw driven into a telephone poll during a tornado--see, straw is stronger than wood!

How about wood? A 45-foot or so fiberglass cat dragged into my 32-foot wooden (ply and glass) cat in the Bahamas during a severe thunderstorm. I was hit so hard I was knocked off my feet in the cockpit. Thought for sure I must have been holed by that force, but only scratches on the wood while my boat knocked a rather large hole in his stern where he hit us. Another time I hit a 100-lb propane tank offshore, no damage.

My point being that there is more to construction strength than looking at the raw material in isolation. It is what the designer and boatbuilder does with it that matters.

[silverp40]

My point being that there is more to construction strength than looking at the raw material in isolation. It is what the designer and boatbuilder does with it that matters.[/QUOTE]

[neilpride]

This Ovni hit the rocks at Canary islands ,, no puncturing or hole in the hull, i guess the repair is big $$$ ,....

[Mr B]

Originally Posted by silverp40

My point being that there is more to construction strength than looking at the raw material in isolation. It is what the designer and boatbuilder does with it that matters.

[/QUOTE]

+10

[silverp40]

Originally Posted by neilpride

This Ovni hit the rocks at Canary islands ,, no puncturing or hole in the hull, i guess the repair is big $$$ ,....

Interesting how the weld on the shaft tube split apart...and the skeg also. Must have run in the rocks at full speed....

[noelex 77]

Originally Posted by msponer

Foolish and Nolex -- I'm beginning to feel like a lightweight. My 48' boat is 9mm thick near the keel, 6mm on the rest of the hull, and 4.2mm on the deck. I've never measured the spacing between the stringers -- maybe ~20cm. The frames are about 40cm apart. Before we found this boat, we were going to commission a Van De Stadt with 5mm plate and frames 40cm apart.

Those sort of thickness of your boat are fine.
A lot of aluminium cruising boats are built for ice (or light ice) conditions. All the retractable keel models are designed to sit on the bottom on a regular basis.
It's easy to add some more thickness and it does not tend to add much weight.

On the other hand some aluminium boats have been built as light as possible for racing. Before the advent of carbon / epoxy laminates aluminium produced the lightest boats. Because most of the strength is in the framing very thin plating can be used satisfactorily, but these boats which are now mostly retired from the race circuit, are not the ideal for cruising, but they are rare.

It is difficult to weld aluminium,in a fair way, less than 4mm so generally this is the thinnest you will see, but occasional slightly thinner is used. (Note small dinghies and tenders are constructed differently they are usually pressed into shape, not welded, and much thinner aluminium can be used)

As always the devil is in the details. The strength of different 5xxx series aluminiums and often 6xxx series extrusions varies by 30-40%. and there are the occasional horror stories of boats built from non marine grade aluminium. The framing thickness, depth, spacing and section (are the stringers flat bar or T section for example) all make an enormous difference. Round bilge is slightly stronger than hard chine, if other things are equal.

Generally people that want an aluminium boat, want something that is not available on the fibreglass market and a strong puncture resistant hull is usually on their list of priorities.

[europaflyer]

That Ovni might not have holes in it, but from a repair perspective the damage looks utterly catastrophic. I find it hard to believe that a well built GRP hull would have fared much worse, but the damage would be a lot easier to repair - weeks rather than months if doing the work yourself.

And that's the main message I've taken from this thread. Steel or aluminium might be a good idea when things get really, really serious due to the large range of plastic deformation (although we have heard arguments against this). However, in any other situation they are somewhat impractical, from my perspective. Clearly a lot of people swear by metal hulls though, and that's just fine. I totally appreciate the desire for a tough hull, and I see the appeal of metal, but for me the supposed benefits are outweighed by the disadvantages.

More than anything, I've yet to be convinced that a panel of GRP is any less tough than a panel of steel or aluminium of the same weight, especially if it's a high quality modern laminate. I loved Steve's mad axe attack video, unscientific as it was, but I would also enjoy seeing the same treatment to a GRP panel (but it's springy stuff, don't hurt yourself!). Also, if anyone has the drive to do some more scientific impact testing, you would be doing everyone a huge favour. Comparative impact strength testing of different yacht construction materials seems to be a pretty much unresearched area, or at least an unpublished one. To illustrate my point, this thread is now the first result on Google for 'aluminium impact strength'!

[neilpride]

That Ovni probably is a nightmare to repair, probably a GRP boat just sank in the same circunstances , who know??

[noelex 77]

It's sad to see a boat like this. It's an old photo I think this occurred about 2005, does anyone know where the boat is now?

The important thing is that its not holed.
Once a boat becomes holed the chance of recovery is much lower and if it is recovered the interior and electrics are likely to need extensive repair and replacement.

It is obviously a major repair job but its likely to be easier than fibreglass. In both materials the damage needs to be cut out, but with aluminium there is no need to scarf the joint when repairing just weld in in a new piece into place. On an Ovni all the panels are flat sheets so there is nothing that is complicated.
It needs a very good welder and therefore it unlikely to be repairable by the owner, but how many fibreglass boat owners would undertake, themselves such an extensive repair?
New rudders would be required to be fabricated just like fibreglass boat.
The keel is a swing keel so there is likely to be little damage and if damaged its only a flat sheet. Although there is no keel to repair it does mean the hull takes all the damage, without some protection for the keel.