Fibreglass vs Aluminium Impact Strength

[atoll]

Originally Posted by europaflyer

Can't begin to imagine what events they're talking about here... To the best of my knowledge the number of fatal structural failures in Airbus aircraft is 0. Failures of aluminium brackets in the wings of the A380 have been a problem recently though. If anything composite structures in aircraft have a longer fatigue life than aluminium alloys. I've always believed that aircraft composites were virtually fatigue-free, like wood, ie. could be stressed to the yield strength repeatedly without altering the material.

Composite Materials: Airbus and Boeing Aircraft Structural Composites, ductile failure, structural composites

But here we go off topic!

since aluminium has been in use for the last 100 years in air frames,and resin composites only 20-30 the comparison may take another 70 years to mature!

however combining thin alloy or steel sheet sandwich with carbon honey- combe composite in the marine sector are being looked at with great interest in the commercial marine sector.

[noelex 77]

As an alumnium boat owner I should rightly be accused of bias, but the video was produced by a fiberglass boat builder to try and counter the practical experience of small boat owners in Australia that aluminium boats are far more puncture resistant to rocks and groundings than their fiberglass counterparts. It is hardly an objective test so a little bias on my side is permitted.

There is no comment on the composition of the fiberglass structure, or the grade of aluminium used.
There are some very weak grades of alumnium and some very strong fiberglass structures. The problem is fiberglass cruising boats are mostly made with the poorest quality fiberglass structure composed of polyester resin and a layup that is mostly CSM. Aluminium cruising boats are normally made with best grade of aluminum 5083 or 5086 and have ribs and stringers every foot or so.

The video was produced as a sales pitch to counter the practical experience of boat owners that alumnium boats are much more puncture resistant than their fiberglass alternatives.

Metal has a much greater plastic range than fiberglass. Look at this Steel boat and imagine if a fiberglass boat would have remained watertight with this deformation.

[goboatingnow]

There has never been any real comparative evidence that aluminium sailing yachts are better then GRP and I suspect they are worse. Weld quality , thin skins, corrosion, and stress cracks are all concern areas. It's a triumph of marketing over common sense.

Dave

[noelex 77]

Originally Posted by Cheechako

yeah, it isnt all about published strength numbers though. Cycle fatigue is terrible in common aluminums compared with steel, as is crack propagation etc. etc Kilo for kilo it may make sense.... steel is nearly 3 times as heavy!

"The problem with aluminum is next to each weld there is a small seam of metal that is weaker then the rest. If there are stresses in that area, it will crack. Stress areas need to be over built to compensate, like stanchions, hull/deck joints, appendage fittings and super structure joints."
Yeah, no doubt about it, I think Fiberglass is pretty forgiving as far as manufacturing goes and aluminum is noisy as heck.

Fatigue is a problem in both metal and fiberglass structures. I used to be a glider instructor and if looked at the flex of wing tips you would understand how many structures are limited by their fatigue life. Cruising yachts flex very little ( at least they should do) and fatigue life is not a factor in the hull, although it can be in the mast or rigging.

Welded steel is almost as strong as plate steel. The same is not true of aluminium. The welds are weaker. The good news is the structure is always engineered for the weakest component so aluminium strength is always considered the welded strength so most of the structure is stronger.

Most fibergalss yachts are perfectly strong enough. Aluminium yachts are not necessary stronger, but they are more puncture resistant. Leave a typical fibergalss yacht on a reef with some wave action and its hull will be punctured. Leave a similar aluminium yacht on the reef and it will be damaged, dented but it will not loose it's waterproof structure, greatly facilitating recovery and repair.

[atoll]

Originally Posted by goboatingnow

There has never been any real comparative evidence that aluminium sailing yachts are better then GRP and I suspect they are worse. Weld quality , thin skins, corrosion, and stress cracks are all concern areas. It's a triumph of marketing over common sense.

Dave

i have seen a few metal boats safely anchored inside lagoons after they have been washed over the reef,but dived on many fiberglass boats that never made it beyound the breakers................

[europaflyer]

Originally Posted by noelex 77

Look at this Steel boat and imagine if a fiberglass boat would have remained watertight with this deformation.

OK, I'm impressed! But I doubt a GRP yacht would have deformed that much in the first place.

Originally Posted by noelex 77

Aluminium yachts are not necessary stronger, but they are more puncture resistant. Leave a typical fiberglass yacht on a reef with some wave action and its hull will be punctured. Leave a similar aluminium yacht on the reef and it will be damaged, dented but it will not loose it's waterproof structure, greatly facilitating recovery and repair.

I've heard people say this a lot, but previous comments would seem to go against this...


On a wider point, what about aluminium vs steel?

[David M]

No mention is being made of aluminum's ability to absorb energy, which means converting mechanical energy into heat energy which makes a difference in a collision as far as damage is concerned.

Additionally no mention is being made of elasticity either, which also makes a difference. Aluminum can deform much greater before giving up watertight integrity.

Strength to weight ratio is not the only thing that matters.

[noelex 77]

Originally Posted by europaflyer

On a wider point, what about aluminium vs steel?

Steel has better abrasion resistance. It can be argued for the same weight an aluminium boat is stronger, but most aluminium boats are built lighter than steel boats. This means most aluminium boats are less puncture resistant than steel boats.
They do have the advantage of almost no deterioration in thickness during their life.
I cannot see my 12mm aluminium bottom puncturing too easily.

Aluminium combines the advantage of steel and fiberglass, but I am biased.

[atoll]

Originally Posted by noelex 77

Steel has better abrasion resistance. It can be argued for the same weight an aluminium boat is stronger, but most aluminium boats are built lighter than steel boats. This means most aluminium boats are less puncture resistant than steel boats.
They do have the advantage of almost no deterioration in thickness during their life.
I cannot see my 12mm aluminium bottom puncturing too easily.

Aluminium combines the advantage of steel and fiberglass, but I am biased.

the bottom of my keel is 30mm alloy,reefs i have hit came off worse after grinding for a few hours between tides,couldnt say the same about a 30mm grp layup....................

[DeepFrz]

OK, I'm impressed! But I doubt a GRP yacht would have deformed that much in the first place.

I have seen a fiberglass boat, very heavily built with glass and polyester (what a lot of people would say as overbuilt), that was in collision with a large steel boat. The boat in question had areas where the FG was completely fractured. This was a large and expensive trawler type boat and was repaired by a yard well known for their quality work. I have seen the boat listed for about 1 million US less than other boats of same size same builder. Although the boat did not sink areas of the hull and deck were just shattered.

Metal cruising boats are not hot in the summer and cold in the winter because if built by a quality builder they are very heavily insulated. They are much, much quieter than your average single skin FG boat. And an aluminum cruising boat built to proper scantlings will certainly have far thicker skin below the waterline than 4 mm. 4 mm may be used on the deck of a cruising boat...maybe.

[DeepFrz]

Oh, and another thing, most aluminum cruising boats have a double bottom as the fuel tanks are built in.

[goboatingnow]

Originally Posted by atoll

i have seen a few metal boats safely anchored inside lagoons after they have been washed over the reef,but dived on many fiberglass boats that never made it beyound the breakers................

In practice only a very small number hit reefs in the first place. After the owner has paid the repair bills on the steel boat to bring it back to full cosmetic standard , including removing half the interior to allow welding , then t he full two pack repaint. I suspect he'd prefer it went to the bottom. !!!


My yard where I keep the boat specialised in steel repair. It's far more costly to repair steel to the original cosmetic standard, yet its relatively easy and cheap to do so with GRP. In fact most average cruisers would have the skills to repair non cored GRP to a good standard. Few could do so in steel with a high quality LPU finish.


Dave

[goboatingnow]

Originally Posted by DeepFrz

Oh, and another thing, most aluminum cruising boats have a double bottom as the fuel tanks are built in.

Which is a compete disaster. If you have any problems or pinhole leaks its a nightmare. All tankage on a boat should go in and out the companionway.

The fact is few boas need double bottoms, but lots need work on integral tanks. I've seen it. The boats has to be half destroyed to fix it.

You are trading an ultimate security needed in few instances with something that gives more trouble in real world applications

[goboatingnow]

Originally Posted by DeepFrz

I have seen a fiberglass boat, very heavily built with glass and polyester (what a lot of people would say as overbuilt), that was in collision with a large steel boat. The boat in question had areas where the FG was completely fractured. This was a large and expensive trawler type boat and was repaired by a yard well known for their quality work. I have seen the boat listed for about 1 million US less than other boats of same size same builder. Although the boat did not sink areas of the hull and deck were just shattered.

Metal cruising boats are not hot in the summer and cold in the winter because if built by a quality builder they are very heavily insulated. They are much, much quieter than your average single skin FG boat. And an aluminum cruising boat built to proper scantlings will certainly have far thicker skin below the waterline than 4 mm. 4 mm may be used on the deck of a cruising boat...maybe.

Yes insulated with materials often carcinogenic or a fire hazard, like sprayed polyurethane. Then you have the constant constant battle against rust, not to mention the cosmetic issues. In aluminium you have impressed corrosion issues , material incompatibility issues. Weld quality , etc.

GRP represents the best compromise between durability , else of maintenance, cost of production and user acceptance. It's why it's been such a successful boat building material. Steel and aluminium in typical cruising boats will remain an outlier

Dave

[noelex 77]

Originally Posted by atoll

the bottom of my keel is 30mm alloy,reefs i have hit came off worse after grinding for a few hours between tides,couldnt say the same about a 30mm grp layup....................

Talking to fellow cruisers its alarming how often they have ended up hitting unmarked rocks, or submerged objects ,stranded on reefs or dragging anchors onto a lee shore.
It's only anecdotal evidence, but my guess would be 1-2 incidents like this in the equivalent of a typical RTW trip.

Is this realistic?

If so do we need consider a typical long distance cruising boats is likely to encounter these problems.
Personally I have never experienced this. I am careful, but maybe I have just been lucky. Many of the boats that experienced problems were accomplished, experienced sailors.