Fibreglass vs Aluminium Impact Strength

[noelex 77]

Originally Posted by K_V_B

But most boats never end up on reefs.

True, but most boats stay in the marina. If you look at long distance cruising boats its unfortuantuantly not unusual.

Just look at Troutbridge and Mr B as couple of recent examples that spring to mind.

We all take risks sailing and this is just one extra. A metal boat does not guarantee survivability.

Because these are often the results of mistakes many beginers underestimate the risk. When you have been cruising for a while you relise even great sailors make mistakes. Dragging anchor a navigational error or falling asleep etc are not unusual and I suspect at as common a cause of boat loss as heavy weather on passage.

[carstenb]

I'll spend mine on booze, broads and boats

[K_V_B]

Originally Posted by atoll

personally i would rather spend my money on a good boat,good anchors,good tender,and a semi auto rifle

And then something happens that you're not prepared for, and you lose your boat anyway...
One of the things I'm currently doing is getting rid of a lot of stuff. I'm realising that material things really don't matter that much, and that about everything except your own good health can be replaced.
So I tend not to dwell to much about things that might happen that may cause me to lose my home or my boat (when I get around to owning one, need to sell the house first).
As long as I survive it's not necessary for the boat to survive.

[Mr B]

Lets take that demonstration apart,

If they had used 5mm aluminium sheet, it would not have had a hole in it, the dent would be a lot less. Possibly non existant,

6 mm sheet, it would have a minor scratch. End of story,

3 mm sheet, The weight would have gone straight through it. with out doubt, 3 inch hole,

The bloke doing that advertisement builds fibreglass speed boats, Not sailing Yachts,
He's very pissed off that the huge majority of speed boats in Australia are now being built from Aluminium,

Virtually indestructible compared to fibreglass ones,

And we are talking 25 to 30 foot speed boats with 2 or 300 HP outboards on the back,

Police, Harbour Patrol, Navy, are all using Aluminium Boats. The big ferries are being made from Aluminium,

England to France and Ireland, car and truck carrying ferries, big ones, Catamarans as well, all Aluminium,

And it is not Aluminium, its Duralium, an Aluminium alloy, and its tougher than steel,

A lot of the time, its just the thickness that makes the difference in the build quality,

Very Thin steel, I can tear apart in my hands, and have done to demonstrate a point,
2 mm steel plate will bend and buckle very easily, too thin for boats,
3 mm steel plate Its very hard to bend or buckle, but stiffeners will make it unbendable,
4 mm steel plate, it stays straight, with out stiffeners, but its very heavy, Trawlers, Etc,

Demonstations can be rigged to make any point, and most people would be none the wiser,

But would buy a fibreglass boat after veiwing that video with the belief that an aluminium boat would get a hole in it if they ran up on a rocky outcrop or reef,

Cheers,
Brian,

[goboatingnow]

Virtually indestructible compared to fibreglass ones,

please show me the yachts tests showing that , or is it just your "belief"

Police, Harbour Patrol, Navy, are all using Aluminium Boats. The big ferries are being made from Aluminium,

The RNLI builds all its hulls from composite.

England to France and Ireland, car and truck carrying ferries, big ones, Catamarans as well, all Aluminium,

No large displacement ferries are built from Aluminium, Fast Cats are , purely to minimise weight for the needed speed The Cunard QM2 was built from steel due to issues with her predessor the QE2.

Of course Aluminum in theory is a good material, but in a lesiure yacht it has major drawbacks. ( Cosmetic repair, skills neeeded, paint adhesion , impressed corrosion)

Steel is actually a better material, but needs more care.

Dave

[Dockhead]

Originally Posted by atoll

personally i would rather spend my money on a good boat,good anchors,good tender,and a semi auto rifle

The last of which is forbidden where you live . Looks to me like you might be a closet Yank . . .

[Mr B]

Originally Posted by goboatingnow

please show me the yachts tests showing that , or is it just your "belief"




The RNLI builds all its hulls from composite.



No large displacement ferries are built from Aluminium, Fast Cats are , purely to minimise weight for the needed speed The Cunard QM2 was built from steel due to issues with her predessor the QE2.

Of course Aluminum in theory is a good material, but in a lesiure yacht it has major drawbacks. ( Cosmetic repair, skills neeeded, paint adhesion , impressed corrosion)

Steel is actually a better material, but needs more care.

Dave

I was comparing speedboats and fishing boats which are nearly the same thing in Australia,
Terminology is different also describing boats,
Our privately owned fishing boats are not trawler type boats, More like a ski race boat but with an open cockpit and outboards on the back, and they are very common here,
25 feet is around a good common size,

I cant see any any big ships with large displacements being built in any thing other than steel in the foreseeable future,.

Fast cats are big and are built from Aluminium,

I did exclude Yachts in my comparison to fishing and speed boats,

I can only speak about what I see here commonly in Australia,
What Europe and the states have is a totally different story, They do have different looking boats,

[neilpride]

Pen Duick VI , made in DURALINOX by Tabarly specs, one of the most beated boat by storms.....still a super strong boat ....

[K_V_B]

Originally Posted by Mr B

I can only speak about what I see here commonly in Australia,
What Europe and the states have is a totally different story, They do have different looking boats,

One of the things I noticed in Spain last summer is that many commercial fishing trawlers are built of fibreglass. On the surface it appears that builders just switched from building in wood by taking a mould from an existing boat, and using that to build more boats...

The navy also has boats build from composites.

[hpeer]

Originally Posted by neilpride

Pen Duick VI , made in DURALINOX by Tabarly specs, one of the most beated boat by storms.....still a super strong boat ....

Had to look up duralinox, similar to 6061 alloy.

Saw a steel boat this summer that was just as cosmetically perfect as any glass boat. Our boats will never look like that. Functional but blemished, a reflection of the owners.

[silverp40]

Saw a well built Canadian boat (Goderich) built from Cor Ten steel get bounced around in a hurricane and bashed against a concrete dock. Not much damage except for some dents around the toerail and a damaged rudder. We were all admiring the strength of the steel boat and the owner was proud of his well built boat that "could withstand virtually anything"

A few weeks later, one of the yard guys, an old time fisherman, noticed that when looking at the boat from straight ahead towards the bow, the stbd side seemed a little different than the port side. Upon closer inspection, it was determined that the boat was "tweaked" in such way that it was twisted about 2"-3" from the center axis, a bit like an old barber shop sign. No wonder doors and floorboards were not closing properly...


So, after an adjuster came and did a detailed survey of the boat, it was deemed a total loss - there was no way to "straighten" the boat out. It sold for $500, even though it looke fine! I wonder how the sailing, motoring, etc, etc abilities were changed!

A fiberglass boat would have probably been torn up pretty bad but repaired and put back in use. They "spring back" much more than metal boats....

[55north]

Hi all,

I feel I have to try to bring some science to the party. Here goes!

The property determining the strength of a material in an impact is not the tensile strength, but the toughness. This is a combination of the strength and the ductility, or the energy that can be absorbed per unit volume in an impact. In scientific terms, it is the area under the stress-strain graph. For the mathmatically minded, that means the integral of the stress with respect to strain, between zero and the strain on failure.

The average 45% woven roving polyester laminate has a tensile strength of around 250 MPa, whereas mild steel has a tensile strength of around 410 MPa. GRP is also significantly more brittle than steel - it will snap once it has reached the limit of its elastic (springy) deformation, whereas mild steel is ductile and has a large range of plastic (bendy) deformation. This means that steel should win on toughness hands down.

However, this isn't the whole story. The GRP is approximately 4.5 times less dense than steel, with a density of 1,740 kg/m3 as opposed to 7,850 kg/m3, so is around 2.75 times stronger than steel for a given weight (specific strength). Aluminium is less ductile than steel, but has a higher specific strength.

Does the strong but brittle GRP or the more ductile but weaker steel win? Unfortunately, I can't give a definative answer here. Impact testing is very specific to the application for which it is being done, and data comparing mild steel of the thickness and quality typically found in yachts to an average GRP laminate of the same weight just doesn't seem to be available.

However, as can be seen in pictures like this, steel can do amazing things, and with regards to the number of tall tales of steel yachts surviving huge beatings, surely there's no smoke without fire.

There is a caveat though - steel is just steel, but GRP is constantly improving - vacuum infusion, kevlar, foam coring, epoxy resins, etc. etc. A modern polyester laminate of 70% continuous rovings is EIGHT TIMES stronger than the 30% CSM polyester laminates we used to see, and epoxy laminates, rare as they are, are much stronger yet. While crude layups of old, with most of the weight consisting of brittle resins, may not have fared well when compared to metals, I reckon the performance of top quality laminates may have approached or even equalled steel or aluminium. That's what we may be seeing in the video. I emphasize that this is personal opinion! If anyone out there has access to impact testing equipment, feel free to prove me wrong.

My personal view is that unless you intend to go near ice, rountinely work around reefs in bad weather, or are just a bit paranoid about things going bump in the night, the three major disadvantages of steel (rust, rust and rust!), or the cost of aluminium, far outweigh their supposed superior impact strengths. Take a look at what these folks have achieved in a GRP yacht, and it makes you wonder what we're all worrying about!

[europaflyer]

Originally Posted by K_V_B

In the end I think it's better to spend money on a good insurance rather than on aluminium.

Great mentality. The throwaway culture.

Originally Posted by K_V_B

One of the things I'm currently doing is getting rid of a lot of stuff. I'm realising that material things really don't matter that much, and that about everything except your own good health can be replaced... As long as I survive it's not necessary for the boat to survive.

Totally agree about materialism in general... only I see my health as being dependant on my boat!

[Jim Cate]

Originally Posted by 55north

Hi all,

I feel I have to try to bring some science to the party. Here goes!.....



Does the strong but brittle GRP or the more ductile but weaker steel win? Unfortunately, I can't give a definative answer here. Impact testing is very specific to the application for which it is being done, and data comparing mild steel of the thickness and quality typically found in yachts to an average GRP laminate of the same weight just doesn't seem to be available.

g'Day 55 North,

Welcome to the forum. and thanks for your well written post. It's always good to have some objective information to salt the conjecture, and you did a good job of objectifying some of the issues that we've been babbling about.

But, as to the concern of fibreglass vs steel... the appended picture of the bow of a Beneteau Oceanus 47 which contested right of way with a steel trawler is self explanatory.

I look forward to your further contributions to CF.

Cheers,

Jim

[noelex 77]

Originally Posted by 55north

There is a caveat though - steel is just steel, but GRP is constantly improving - vacuum infusion, kevlar, foam coring, epoxy resins, etc. etc. A modern polyester laminate of 70% continuous rovings is EIGHT TIMES stronger than the 30% CSM polyester laminates we used to see, and epoxy laminates, rare as they are, are much stronger yet. While crude layups of old, with most of the weight consisting of brittle resins, may not have fared well when compared to metals, I reckon the performance of top quality laminates may have approached or even equalled steel or aluminium.

Welcome to the forum 55north. A great first post.

The great majority of fiberglass boats use simple polyester resin and CSM for the majority of the layup. Cored hull structures are being used more, but these are only marginally more impact resistant and create additional problems of delamination and / or rot in core.

The technology certainly exists to create a vastly superior fiberglass structure with epoxy resins Kevlar and e glass rovings. Unfortunately few builders use this technology, and those that do, use the better materials to create structures which are as light as possible, rather than a means to primarily improve impact resistance.
Some modern boats like Jeanneau do use a small amount of Kevlar, which must help, but when used in small quantities with polyester resin the cynical would argue its more of a marketing exercise.

Personal I believe modern fibergalss boats are less impact resistant than the "overbuilt" thick long keel boats that were built in the days when manufacturers were less concerned with resin costs.
Modern fibergalss boats do many things much better than there older cousins, but If I hit a container, or reef I would not feel safer in modern boat.