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Old 04-12-2012, 06:11   #166
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Re: Fibreglass vs Aluminium Impact Strength

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Originally Posted by atoll View Post
not if you happen to be mid-ocean when the boat bio-degrades
Being mid-ocean on a boat that hasn't been looked after properly for decades is never a good idea, regardless of the material of the boat.
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Old 04-12-2012, 06:34   #167
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Re: Fibreglass vs Aluminium Impact Strength

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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!

The Ovni 395 is 4mm plate topsides rising to 10mm underneath as the grounding plate. Thats a long way from 1/4"

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Old 04-12-2012, 07:06   #168
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Re: Fibreglass vs Aluminium Impact Strength

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North Shore boat torn open on land | Parker Marine

Lets see now, is this a FG boat?

And how about this one?
Boat on the rocks | Parker Marine
Cannot compare an open tender (probably homemade) with a properly built boat that is capable of crossing oceans. Other sailboat was cut in half to pull out of water. That is exactly as just easy with an alum boat when using a sawsall.


If the steel boat on the reef did not split a weld, get flooded from loosing the rudder/and or prop shaft or a thru hull or a rudder shaft etc it might make it IF it can be towed off -Steel is not light. And if the boat is not twisted at the frames either which will mean it's doomed. And how would all that saltwater inside do for the steel especially in all those hard to get to areas that hold the saltwater moisture.
Although the conditions here are very benign!

And speaking of welds, we met a couple that sail a very well built French boat (Garcia Passoa)which every time they were were reaching would take a good amount of water in through an underwater weld. Could not find where the water was coming from, it was only on that point of sail. They were just living with it.
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Old 04-12-2012, 08:29   #169
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Re: Fibreglass vs Aluminium Impact Strength

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The Ovni 395 is 4mm plate topsides rising to 10mm underneath as the grounding plate. Thats a long way from 1/4"

Dave
4mm is very close to 1/4". Just because it has a 10mm grounding plate means nothing. What is the thickness of the bottom? How does it progress from 4mm at the sheer to thicker bottom plating? And where does the joint between two different thickness plates occur? This is a major weakness of alloy boat construction to my mind. A fiberglass boat tapers gradually in thickness, transferring loads evenly. An alloy boat that transitions from one thickness of plate to another is setting up a weld that is stiffer on one side than the other, it will not transfer loads evenly and the weld will be stressed. A better transfer can be made if the boat is hard chined and the transfer occurs only at the chine, so that internal structure is transferring load. But as a fiberglass boat builder, I was taught never ever to transition suddenly in thickness like that, even a little bit. Alloy boat builders have no choice, unless they built the boat in strips instead of plates like a wooden hull and made each one a 1/16th thinner. But then the boat would be all welds and take a hundred years to build.
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Old 04-12-2012, 08:34   #170
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Re: Fibreglass vs Aluminium Impact Strength

minaret, We are in agreement, other the 1/4 inches is 6.35mm 2.35mm of extra thickness is substantial in metal. Agree re transition, this is why I question the wisdom of metal boats in small sizes, too many compromises. If you want custom then its a choice, otherwise GRP is a better compromise.

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Old 04-12-2012, 08:53   #171
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Re: Fibreglass vs Aluminium Impact Strength

Most of the strength of an aluminium boat comes from the very deep stringers, and frames. The plating is mainly there to keep the water out. Fibreglass is very different, most of the strength is in the "plating" with very little framing.
My own boat, for example has internal reinforcing transverse and longitudinal spaced a every 30cm (1foot). This is common.

There have been aluminium yachts constructed in a similar way to fibreglass boats. It's called the "origami" method. This still uses more internal framework than a fibreglass boat, but much less than a normal aluminium boat. Consequently the plate thickness needs to be much higher for the same strength.

In many ways an aluminium boat is constructed like a traditional wooden boat.
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Old 04-12-2012, 09:04   #172
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Re: Fibreglass vs Aluminium Impact Strength

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Cannot compare an open tender (probably homemade) with a properly built boat that is capable of crossing oceans. Other sailboat was cut in half to pull out of water.
That was not an open tender. Take a closer look. The deck and house have been torn right off the thing.
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Old 04-12-2012, 09:07   #173
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Re: Fibreglass vs Aluminium Impact Strength

Thickness of plating is related directly to the the size of the boat. So, when 12mm bottom plating is mentioned it is important to note the size, i.e a larger boat will have thicker plating than a smaller one.

Sometimes plating is increase from the nominal thickness for that size but that is usually reserved for custom designs.

So an 80' boat with 30mm bottom plating is not at as impressive as a 40' one off with 20mm plates. Just design math.
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Old 04-12-2012, 11:22   #174
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Re: Fibreglass vs Aluminium Impact Strength

I have a tremendous respect for some of you, but I feel that the conversation here is getting a bit silly.

A lot of confidence is developing around a series of flawed anecdotes, and the engineering reality of this is being hand waved at from a distance. The only person who has contributed to this discussion in a substantive and scientific way hasn't spoken up recently, probably because there's not much more to say at the level that people are willing to discuss here.

I try to avoid 'Someone on the Internet is wrong!' -- though maybe that's what Panope was thinking during his vicious axe attach on the metal plate -- but... compact, easy answers are compelling, especially when one doesn't know how much they don't know.

Maybe we should just decide that in this medium we, collectively, do not have the ability or desire to discuss this seriously in a technical way, and give up on trying to reach into engineering or yacht design from an amateur perspective, and so should tack and instead discuss 'hull materials' qualitatively from our experience and perspective as cruisers.

That's actually what I think I could learn from some of you here, at least: what it's like in the travel far and long on a metal or fiberglass boat, how you think the hull material changed your experience.
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Old 04-12-2012, 11:42   #175
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Re: Fibreglass vs Aluminium Impact Strength

I left this discussion long ago, coming back I see it still continues to be as hilarious as when I left. A bunch of people trying to convince others that one material or another is "better". it's all about the details, engineering and variables. No one material is "better".
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Old 04-12-2012, 11:56   #176
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Re: Fibreglass vs Aluminium Impact Strength

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That's actually what I think I could learn from some of you here, at least: what it's like in the travel far and long on a metal or fiberglass boat, how you think the hull material changed your experience.
I love the cruising/sailing lifestyle.
If the only option was to cruise in paper mâché boat, well sign me up

However selecting the best boat is important. This ideal, varies enormously between different people. I am constantly amazed by the diverse range of craft people successfully cruise in.
Fundamental to this decision is "what material is the hull made from". There is no ideal option, but hopefully these discussions on CF allow members to pick the material that matches their requirements best.
We may argue the best option, but a sunset on the water is just as sweet no matter what boat you have
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Old 04-12-2012, 12:15   #177
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Re: Fibreglass vs Aluminium Impact Strength

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Originally Posted by msponer View Post
I have a tremendous respect for some of you, but I feel that the conversation here is getting a bit silly.

A lot of confidence is developing around a series of flawed anecdotes, and the engineering reality of this is being hand waved at from a distance. The only person who has contributed to this discussion in a substantive and scientific way hasn't spoken up recently, probably because there's not much more to say at the level that people are willing to discuss here.

I try to avoid 'Someone on the Internet is wrong!' -- though maybe that's what Panope was thinking during his vicious axe attach on the metal plate -- but... compact, easy answers are compelling, especially when one doesn't know how much they don't know.

Maybe we should just decide that in this medium we, collectively, do not have the ability or desire to discuss this seriously in a technical way, and give up on trying to reach into engineering or yacht design from an amateur perspective, and so should tack and instead discuss 'hull materials' qualitatively from our experience and perspective as cruisers.

That's actually what I think I could learn from some of you here, at least: what it's like in the travel far and long on a metal or fiberglass boat, how you think the hull material changed your experience.
I completely agree with what you have written. Thanks for your insight.

As I brand new online forum member I find this whole business of having serious conversations with strangers rather difficult and unnatural. I suppose this is due to my undeveloped writing skills.

It has been my intention in joining this forum to try and "give back" something in exchange for the huge amount of knowledge that I have taken from it.

Serious questions: Should I have not posted my "Metal Attack" video? Should I make another Video when I find some composite to Attack? Am I adding confusion and disinformation by doing so?

I agree with others that have stated the video has more entertainment value than anything else. It was certainly satisfying for me just to unload some pent-up tension on that scrap of metal.

Again, thank you

Steve
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Old 04-12-2012, 12:17   #178
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Re: Fibreglass vs Aluminium Impact Strength

if this boat were aluminum, it would have been holed
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Old 04-12-2012, 12:38   #179
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Re: Fibreglass vs Aluminium Impact Strength

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if this boat were aluminum, it would have been holed
i love the way the blackfellah is standing on the highest part of the boat!!!
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Old 04-12-2012, 12:45   #180
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Re: Fibreglass vs Aluminium Impact Strength

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Serious questions: Should I have not posted my "Metal Attack" video? Should I make another Video when I find some composite to Attack? Am I adding confusion and disinformation by doing so?
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
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