Aluminium vs fibreglass, which is the best hull material for your yacht?

What’s the best hull material for an ocean cruising sailboat, fibreglass (GRP) or aluminium? It’s an important decision for any boat buyer to make and both materials have their pros and cons. In essence, the right choice depends on two fundamental factors: the yacht’s size and where you plan to cruise. But there are other practical considerations too, some of which might surprise you.

If you’re in the market for a new sailboat and confused by all the contradictory advice and opinions online, here’s what you need to know.

Compare like with like

First, it’s important to compare hulls of the same quality. There’s a massive difference in strength, durability and toughness between a mass-produced GRP hull and a premium GRP hull like our Sirius yachts. An aluminium yacht might be a lot stronger than a typical GRP production yacht but unless it’s heavily overbuilt it won’t be significantly stronger than a Sirius. If GRP laminate is built to last, not just to the minimum specification, you get an almost indestructible boat.

Weight vs strength

Now let’s compare the inherent properties of the two materials. On aluminium hulls, thicker panels are used low down and thinner ones higher up, giving a hull skin that is strong and reasonably light. If aluminium is used to create the skin of the hull, it can’t be much less than 5mm thick without losing too much strength, on a 50ft/15m boat this is fine. However, on smaller hulls, the topsides (the part of the hull above the waterline) can’t go below the panel thickness of around 5mm either, so a smaller aluminium hull has a higher centre of gravity than a larger aluminium hull or a GRP hull (all other things being equal). The skin thickness of the topsides will either be thick, strong and heavy or light, thin and weak. Neither is an ideal solution.

Pound for pound, GRP is lighter and stronger than aluminium, so about half the weight of GRP can be used to achieve the same strength and stiffness

By contrast, GRP does not have an abrupt cut-off point in terms of skin thickness and can be precisely optimised for any part within the hull. Aluminium is isotropic, meaning it behaves the same way in all directions. GRP, like wood, is anisotropic so it behaves differently depending on the direction of force applied. While this might be considered a ‘win’ for aluminium, being able to reinforce or tailor the GRP structure to be much stronger along its load paths without having increase internal reinforcement is a definite benefit for GRP. Lastly, pound for pound, GRP is lighter and stronger than aluminium, so about half the weight of GRP can be used to achieve the same strength and stiffness.

Interior space

Aluminium yachts have less space inside. This is because aluminium sheet doesn’t have the required stiffness without an internal supporting structure. (Actually, it does if you make it thick enough, but we’re talking about sailing boats, not barges.) To keep the weight down, an aluminium yacht hull needs to have a thin skin with reinforcing frames and stringers inside it every half metre, or couple of feet, along the entire length of the hull and all the way around it. This reduces the amount of usable space inside the hull and the space reduction is more significant in smaller yachts than in larger ones.

Structure of an aluminium hull
The structure needed for an aluminium hull can cheat sub-50ft yachts of interior of space. This is one reason why we build our yachts out of fibreglass and not aluminium

For a typical aluminium 50-footer (15m) with a beam of around 15ft (4.5m), the supporting frames will be 4-6in thick (10-15cm). To make a comparison, let’s assume that it’s 5in (12.5cm) including the thickness of the hull skin. The frames are on both sides, so the usable interior beam is reduced by 10in (25cm). Thus the yacht loses around 5% of its beam to the internal supporting structure.

If we scale the hull size down to 35ft (10.7m), the frame thickness still has to be the same and now it accounts for 8% of the yacht’s beam.

The way we build our boats and the service we deliver is as unique as our yachts

By contrast, a GRP hull – even one with a Divinycell foam core, like our Sirius 35DS – is no more than 2in (5cm) thick and it doesn’t need any framework inside for support. This is just 2% of the yacht’s beam, so its interior is 6% wider than one built in aluminium. That translates to 6in (15cm) of extra space, which in practical terms is the difference between being able to comfortably get past someone in the galley and having to squeeze past them.

More space inside also gives you more scope for interior customisation and there’s another practical consideration. The lattice of frames and stringers inside an aluminium hull makes it more awkward to clean and maintain, especially down in the bilges.

This extra space enables us to build the interior on two levels so not only are you getting more width, you are getting extra floorspace thanks to our clever use of space.

The interior and foam insulation on an aluminium yacht
The interior and foam insulation on an aluminium yacht would have to be removed before welding could start

Insulation

Aluminium is an excellent thermal conductor, so aluminium boats need a lot of extra insulation to stay warm in winter and cool in summer. This can be expanded foam, or foam sheets the thickness of the frames. The insulation material adds a bit of weight; more importantly it can also trap moisture and make it difficult to inspect the hull surface from inside.

If expanding foam is used, it tends to make welding repairs more difficult. The foam will need to be cut out from a wide area all around the damage to avoid the risk of the welding equipment starting a fire.

All this added insulation does have the benefit of making aluminium yachts quiet, but we can achieve the same acoustic levels in our boats because of the construction methods, materials and build quality we employ.

Osmosis vs corrosion

Aluminium hulls are immune from osmosis, but so are our premium-quality GRP hulls. We solve the problem by using modern resins and coating the underwater surfaces with epoxy. And be aware that aluminium immersed in seawater has a constant battle with galvanic corrosion, which doesn’t affect GRP.

You will never see a GRP yacht suffer from galvanic corrosion
“You’ll never see an aluminium yacht with osmosis.”… but you’ll never see a GRP suffer from galvanic corrosion

With expert installation and mounting of equipment to minimise the stray electrical currents that cause galvanic corrosion, an aluminium hull can last for a very long time. Unlike on a fibreglass yacht, though, you cannot install electronic devices or mount deckware without knowing exactly what you are doing. You also have to be careful how you store metal items in the bilges as galvanic and electric corrosion can work frighteningly fast. Throughout its life, too, the underwater surfaces must be well cared for, with plenty of sacrificial anodes and regular, careful inspections.

This is fine if you’re the first and only owner of the yacht, but the ever-present potential for galvanic and electrical corrosion can cause doubt in the minds of buyers. You might know you have been meticulous, but they don’t, and this can affect the value of aluminium yachts on the second-hand market.

Scratching or abrading the underwater antifoul and gelcoat can give shudders to a GRP boat owner. But scratching the underwater surface coating is a real worry for owners of aluminium hulls.

Good GRP hulls can also last a lifetime – and without any special treatment. Many fibreglass yachts built 60 years ago are still happily cruising the world’s oceans today and the materials we use in our Sirius hulls are much stronger and longer-lasting than the basic GRP that those boats were built with.

It’s not just below the waterline where aluminium is more vulnerable than GRP. On deck too, aluminium can be susceptible to crevice corrosion and pitting.

Cost considerations

For small and medium-size yachts, there’s a substantial cost saving with GRP because the material is inherently more suitable than aluminium for efficient series production. It might make sense, cost-wise, to build a 55ft-plus (17m) full custom one-off yacht in aluminium. Any smaller than that, though, and it’s more economical to build the hull and deck in GRP – even with the cost of tooling and mould construction included. That’s why many full custom builders still choose to build in fibreglass, using short-run moulds. For a semi-custom build, like all of our yachts and most other top-quality ocean cruisers, it makes sense to choose GRP and invest the cost saved in upgrading the yacht’s gear and equipment, or in a higher-quality interior.

Ease of repair

For long-distance cruising, ease of repair is an important consideration. The worst-case scenario could involve you needing to repair the boat yourself, in a remote part of the world, which is much easier for you to do if the hull is GRP. You can carry fibreglass mat, resin and catalyst on board, and a repair can often be carried out in less than an hour. It’s also worth bearing in mind that almost any boatyard anywhere in the world can repair a GRP hull and get you sailing again. To repair an aluminium hull you first need to remove the surrounding interior and insulation to minimise the risk of fire, source the right grade of aluminium (unless you take spare panels with you), then find someone with TIG welding equipment, the power to run the equipment close to the yacht, and the knowledge and skills to use it.

Ease of repair is an important consideration for long-distance cruisers
Ease of repair is an important consideration for long-distance cruisers. This is one reason we use GRP and not aluminium for our yachts

Wear and tear

Another consideration is the appearance of the yacht. The battleship grey of bare aluminium can look purposeful but it can’t be described as attractive. Painting it is difficult, expensive and most aluminium yacht builders don’t recommend it. Gelcoat, on the other hand, comes in all shades of colours and any cracks or scratches can easily be repaired for a relatively low cost.

When aluminium yachts get dented or scratched, they tend to wear their scars with pride. Dents can be hammered out if you can access the inside of the hull at the right place, but if you can’t, there they stay.

It’s not just the boat, that takes knocks, you do too. A GRP cockpit can have all the seating and edges nicely rounded; GRP is more forgiving on the body than metal is. This makes the cockpits of GRP boats more comfortable and easier to live with.

You don't need an aluminium yacht for a high-latitude adventure
We don’t think you need an aluminium yacht for a high-latitude adventure

How are you really intending to use your boat?

Aluminium does have a higher puncture resistance and a much higher abrasion resistance than GRP. That’s why it’s favoured by expedition yachts that are designed and built specifically for long-term polar exploration, navigating in pack ice and getting ice-bound in harbours on a regular basis. If those are your plans, or if you want a large, full-custom yacht, you can get the hull built by a specialist aluminium fabricator and then fit it out accordingly.

If you’re looking to buy an aluminium yacht with the thought that one day, maybe, you perhaps might go to the Arctic, are you prepared to live with the downsides of aluminium until that time? Will that time ever come? We see SUVs and 4×4 vehicles on the road all the time, bought with the thought that they can go off road, or for the really bad weather that happens once every few years. Until that time, the owners have the downsides (poor fuel economy, mediocre handling, high luggage area, a step up or down every time you go somewhere) to put up with every single day.

There’s a strong argument for choosing a GRP hull
If you still want a “go-everywhere cruiser” but your plans involve less extreme sailing for most of the time with occasional high-latitude cruising, there’s a strong argument for choosing GRP

If you still want a “go-everywhere cruiser” but your plans involve less extreme sailing for most of the time with occasional high-latitude cruising, there’s a strong argument for choosing GRP. Our extremely strong hulls can be reinforced even further with layers of Kevlar to safeguard against ice punctures. We already have crash boxes in the bow as standard, and we can also fit stainless steel bow protectors or any other solution you want.

Plenty of GRP boats have sailed through the North-West Passage and some have even transited the North East Passage in recent years. These include standard mass-produced yachts that are nowhere near as strong as a Sirius. And throughout the other 98% of the world’s cruising grounds, for all the reasons above, GRP is clearly the best material for a sub-50ft ocean-going yacht. That’s why the vast majority of yachts, including long-distance cruisers, are made of GRP.

Sirius Yachts logo white

General Manager – Torsten Schmidt
SIRIUS-WERFT GmbH
Ascheberger Straße 68
24306 Plön/Holstein

Tel: 0049 – 4522 – 744 61-0

Fax: 0049 – 4522 – 744 61-29
Email: ed.tf1635260282rew-s1635260282uiris1635260282@ofni1635260282

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