weight aluminium vs GRP yacht

[innomare]

Your opinions please...

What would roughly be the difference in weight of a hull built in aluminium and the same one built in epoxy composite? Hull length aprox. 40 m - 2 decks.

Cheers,

Bruno

[RANCHI OTTO]

Very roughly estimation for :

Hull length > 40.0 m
Breadth > 4.0 m
Depth > 3.5 m

Alu > 35'000 kg
GRP > 42'000 kg

For small speed this figures can be reduced.

RANCHI Otto

[Eric Sponberg]

Bruno,

You cannot get that specific without designing both the aluminum hull and the composite hull and comparing the weight results for both. The structures are going to be quite a bit different, and it is difficult to make direct comparisons until you know the totals for both.

I have done some average comparisons in that last few years on basic boat dimensions and selling price. I have been interested in the prices of boats built of different materials, and also to see if there are any detectable trends based on material, length, and displacement.

The following applies only to cruising sailboats. Data has come from Ocean Navigator magazine and Blue Water Sailing because these magazines review boat designs on a regular basis and usually publish the list price. Here is a summary of what I have found over the last few years, covering 121 yachts so far:

Aluminum Boats:
No. of Boats: 21
Max LOA: 159 ft.
Min LOA: 43.92 ft.
Max Displ: 842,240 lbs.
Min Displ: 11,400 lbs.
Ave. Price: US$778,000
Ave. Density (Displ/LBD): 8.94 (L=LOA, B=Beam, D=Draft)
Cost/Ft: US$14,414
Cost/lb: US$20.85

Steel Boats:
No. of Boats: 6
Max LOA: 53.00 ft.
Min LOA: 35.00 ft.
Max Displ: 47,000 lbs.
Min Displ: 15,000 lbs.
Ave. Price: US$616,000
Ave. Density (Displ/LBD): 8.60
Cost/Ft: US$12,636
Cost/lb: US$17.72

Composite Boats:
No. of Boats: 84
Max LOA: 74.58 ft.
Min LOA: 21.00 ft.
Max Displ: 244,458 lbs.
Min Displ: 7,055 lbs.
Ave. Price: US$673,757
Ave. Density (Displ/LBD): 7.56
Cost/Ft: US$13,481
Cost/lb: US$19.51

Wood Boats:
No. of Boats: 10
Max LOA: 124.00 ft.
Min LOA: 30.50 ft.
Max Displ: 350,000 lbs.
Min Displ: 10,000 lbs.
Ave. Price: US$960,556
Ave. Density (Displ/LBD): 8.88
Cost/Ft: US$16,407
Cost/lb: US$21.56

Here is what I find in this data, and what I conclude:

1. The price of the boat is retail, fully equipped, so I do not have enough data to separate out the effects of interior fitment such as plush interior vs. plain interior, or the costs of different electronics packages, which will be more elaborate on larger yachts. However, the average costs are interesting.

2. The weights include the weight of the ballast, which may be 35% to 50% of the boat weight. This is a significant factor comparing boats on a cost per lb. basis. Just keep that in mind.

3. On the average, assuming different types of boats average out reasonably, and based on density, Aluminum boats are the heaviest, followed closely by wood, steel, and then composite. This would probably answer your question: A composite hull, on average, would be lighter than an aluminum hull, if you assume that the boats are equipped the same and have the same amount of ballast. Individual cases, of course, could vary from this average.

4. On a cost/Ft basis, Wood boats are the most expensive, followed in order by Aluminum, Composite, and steel, being the cheapest.

5. On a cost/lb basis, the same order applies, Wood boats are the most expensive, followed in order by Aluminum, Composite, and steel again being the cheapest.

6. If you plot a scatter graph and eyeball a trend line through the results, the trends in costs are clearest when plotted cost vs. length and cost vs. Disp. The cost vs. length appears nearly a cubic relationship, as one one expect. Cost vs. Displ. is almost linear, but not quite; it is perhaps somewhere between linear and quadratic (vary by the square of length). Cost plotted against other parameters like density or displ/length ratio are not nearly as clear--too scattered.

I offer this information for whatever you can make of it yourselves. I continue to collect data as the magazines come in. I have not done the same yet with motoryachts.

Eric

[innomare]

Thanks guys!

Again this forum amazes me.

The yacht in question - sorry I didn't mention it before - is to be a motoryacht with topspeed at about 30 knots.

In Eric's figures composites turn out lighter than aluminium which is what I would expect, but with Otto it's the other way round.

What surprises me also is that steel boats would be lighter per displacement than aluminium. Steel seems to be the clear overall winner in Eric's figures. However almost all sailing yachts I see are either composite or aluminium built? I suppose this is size-related as the steel boats in the database are on average as small as the smallest aluminium boat. Or maybe it has to do with the ballast?

I would rank them like this:
Weight + wood steel aluminium composite -
Cost (for custom build) + wood composite aluminium steel -

With series construction composite will evidently become cheaper.

Bruno

[mackid068]

Steel seems to be the best for strength, or so I've heard.

[Eric Sponberg]

It is very difficult to judge on strength alone. Stiffness also plays a factor. One should also consider ductility--steel and aluminum are ductile, composite and wood are not. Of the materials shown, and we'll assume composites means only fiberglass and not carbon and Kevlar. Here are comparable strengths and stiffnesses of the materials only:

A36 Steel:
Ultimate tensile strength: 58,000 psi (400 MPa)
Yield strength: 36,000 psi (248 MPa)
Modulus of elasticity (stiffness): 29,000,000 psi (200 GPa)

5083-H321 Aluminum (as for plating)
Ultimate tensile strength: 44,000 psi (303 MPa)
Yield strength: 31,000 psi (214 MPa)
Modulus of elasticity: 10,400,000 psi (71.7 GPa)

Composites: (varies all over the map)
Ultimate tensile strength (knitted bidirectional laminates): 30,000 to 60,000 psi (206 to 414 MPa)
Yield strength: there is none, laminates are brittle
Modulus of elasticity: 1,900,000 to 3,000,000 psi (13 to 207 GPa)

Wood: (also varies considerably, considering different species, methods of construction, etc., and is much harder to define)
Ultimate tensile strength: can be defined only by additional engineering of the type of construction
Yield strength: very little definition of yield. Wood composites are very brittle.
Modulus of elasticity: can be defined only by additional engineering of the type of construction

Steel and aluminum are very strong, steel is 3 times stiffer than aluminum, and both are many times stiffer than composites and wood.

Composites and wood make up for their relatively weak stiffness by gaining extra thickness--composites with cores, and wood by it's mere light weight and thickness.

So one can't necessarily say which material is stronger or stiffer. Again, for any given design, you have to judge a totally engineered and designed structure for each material in order to judge strength, stiffness, weight, cost of materials, and cost of labor to build. It is a complex problem with no easy answers.

Eric

[marshmat]

I'd agree with Eric. There's too many factors involved to make a general statement on the weight of your hull.
It's very hard to get weights until you have a structure, which will be hard to get until you have the hull shape, which you can't finalize until you know weights.

So here's a very rough, quick rundown of what I typically see (by no means is this definitive):

Composite: Used mostly for complex curved shapes in mass-produced hulls, popular becuase it's easier to make the exact same hull every time in mass production. Used where cost is a big factor, and hull will not take a lot of beating and damage.

Aluminum: Popular for lightweight hulls, usually in low production volumes. Usually stiffer, stronger and lighter than fibreglass hull of the same size. Also good for hulls that might take a beating as it bends where fibreglass shatters. Not easy to mass-produce so not as popular as composites for production hulls. Used where strength, weight and ability to take damage/neglect are important, but cost is not.

Steel: Popular for heavy, working vessels that take a beating. Like aluminum, not easy to mass-produce like fibreglass is. Used when strength and ability to sustain damage and neglect are more important than weight and speed.

Wood: Used today mainly for its looks. Can be used to build a very solid vessel, and can take a lot more abuse than fibreglass or even some metal boats can. Harder to work with though, and expensive. Requires a lot of maintenance.

Again, this is by no means definitive, just an informal survey of what's generally seen out on the lakes.

[Karsten]

If you want a motoryacht that is 40m and does 30kn the weight will become a major issue. The more weight the bigger the engine the more fuel the bigger the loads the heavyer the structure.

Also I don't agree that fibreglass hulls don't take beating. I kow of some impact tests where a sandwich laminate only showed some scratches whereas the same weight steel and aluminium panels were completely destroyed. It all depends on the materials and exact design. In your case I recon you should go for the fibreglass option and also think about some unidirectional carbon for example in the beam cappings. The material is more expansive but if you take the savings in labour, resin, weight and fuel into account it might end up to be cheaper.

[Eric Sponberg]

On commercial craft, where the hull and deck do take extreme impact loads, aluminum and steel are the only way to go. Fiberglass hulls just won't stand up to the abuse. In a yacht, where the crew takes extreme care in maneuvering, fiberglass is acceptable. However, you will not gain any appreciable weight or cost savings merely by capping the internal beams and girders with carbon fiber. You won't gain any strength in the hull and deck skins, which comprise roughly 2/3rds of the structural weight. Again, if you really want to fine tune the structural differences between aluminum and fiberglass, you have to compare detailed structures of each to the other. This doubles the process of design and construction for any particularly design.

If you want to build only one of these boats, maybe fiberglass will be the better way to go, but you have to invest in the mold to do so. If you are building only one, then aluminum is the way to go. Even then, if you build a few yachts to the same design, aluminum, with CAD/CAM cutting, can be quite economical.

Eric

[Eric Sponberg]

Correction, if you want to build MORE THAN ONE of the boats, fiberglass is the way to go. Sorry about that. See, you should proof-read your posts before you send them!

Eric

[mackid068]

I concur. If you want to mass produce, fiberglass is best, but if you want strength (and, I assume, ease of repair which comes with metal, I'm pretty sure), then steel or aluminum are much better.

[Bob Mott]

Hello Bruno,

Have a look at Sealium made by Alcan/Pechinery Assoire France. Stronger than 5083 - more corrosion resistance - better strength in the welds.

We will be building a number of 15.4 metre sailing cats in Thailand out of the new version alloy purchased direct from France.

Alcan have weight comparisons steel to alloy mainly and one graph alloy to GRP.

In both cases alloys are much lighter.

I can believe steel can be lighter than alloy??

The founder of ATL composites is living here in Thailand - we are looking into a alloy hull up to the waterline with composite panels from there up. The adhesives are available. The issue is expansion as alloy expands different to composite. Having it to only just above the waterline will reduce expansion problems.

I favour alloy and have had a number of alloy cats - have a 14.4 metre sailing charter cat now and more to come.

If you were not aware there is a transition plate made by Alcan alloying alloy to be welded on one side and steel on the other.

They have an excellant brochure out about their products along with the history of alloy.

The Amerca's cup yacht Defender that won the cup in 1895 was a steel hulled yacht with alloy riveted hull.

over and out

Bob

[Bob Mott]

Forgot to add Tot Zeins as I see you are in Holland

Bob