How strong are hull windows?

We are often asked online and at boat shows about the strength of our hull windows. While we cannot answer for other manufacturers, we can say with complete confidence that the hull windows on Sirius Yachts are stronger than the hull surrounding them!

Safety above all else

Sirius Yachts have proper windows
When we started looking into adding hull windows, we didn’t want small portholes, we wanted proper windows!

Safety is of paramount importance on our sailboats so we thoroughly researched how we could add hull windows, not just any old way but in our Sirius way, with unrivalled safety and strength.

Many people still have reservations about hull windows and if you don’t want them, we’ll build your boat without them. But since we started building the Sirius 35 in 2009, only one owner has decided not to have the large windows in the mid-cabin. For everyone who does want them, it was important to us that we made them the best we could.

When we started looking into adding hull windows, we didn’t want small portholes, we wanted proper windows! At the time this was unheard of in sailing yachts of the size we build, but we wanted windows that you can look out of and see passing swans, the sunrise or the sea flowing past. We didn’t want them to just let in a little light, we wanted the cabin to feel like an extension of the saloon: a place to relax and still feel connected with the world outside. A light and airy space where you can have some alone-time without feeling tucked away. This is especially important if you choose to configure the mid-cabin as an office (or a multi-purpose cabin that converts into an office). Being able to see out helps your senses to process the boat’s motion, reducing the likelihood of seasickness and just enjoy the beautiful nature surrounding you.

Stronger than the hull

Reinforced area around the windows in a Sirius Yacht
Before we could add large windows, we had to ensure the area around them was reinforced

Large windows are frowned upon by traditionally minded sailors wary of the yachts that had large windows fitted times when the right techniques and materials simply weren´t available. So if we were going to add windows in the hull, we weren’t just going to make them as strong as the hull, we were going to make them significantly stronger to give total peace of mind. Every part of our yachts is built to last a lifetime: our hulls are solid GRP laminate with a thickness of 50-70mm in places and our cabin doors are 30mm thick marine plywood, cut from the bulkheads. Our hull windows are no different.

Before we could add large windows, we had to ensure the area around them was reinforced. It’s actually the strongest part of the entire hull, thanks to the extra structure added around the window frames. The windows don’t take any load from the hull, the frame just holds them in place. If you could find another way of keeping the water out, you wouldn’t need glazing at all!

The right material for our hull windows

It was important to choose the right material for the windows. We looked into using toughened glass, but it is not bulletproof, or rather the thickness of glass required for it to be bulletproof would have added a lot of weight to the yacht’s topsides. Instead, we chose ballistics-grade polycarbonate, the same material used on military vehicles, armoured vehicles, police cars, helicopters and airplanes – it is also the material used to make shutter blinds to protect portholes in extreme expedition yachts where they are typically 6mm thick. But in true Sirius style we use 12mm thick polycarbonate, which is much thicker than you need them to be for impact and sea pressure protection.

To make sure we had the right material, we had a 140kg (300lb) body-builder, attack the windows with a new 3kg (6lb) axe – and they remained intact, for much longer we all thought possible.

We now had confidence that the hull and the windows were more than strong enough, but the way the windows are fitted into the hull was just as important. For this, we looked to the aeronautics industry, and we use the same bonding techniques used on aeroplanes and helicopters. On aircraft, the windows may occasionally need to be removed and replaced when they have been damaged. We can do the same with our windows too without affecting the surrounding structure or frame.

Making them last

Sirius Yacht windows are built to last
We’ve yet to hear about any crazing, cracks or damage to the windows of a Sirius Yacht that have compromised their integrity or led to any needing to be replaced

Unlike the Perspex hatches that are used on deck, hull windows are rarely exposed to the full intensity of sunlight. Even so, longevity was also a concern for us, so we have our polycarbonate specially coated to increase its protection from UV and scratches. Our first Sirius 35DS with these windows was launched in 2009; she still has the same windows today (2021) and they are still in excellent condition. We’ve yet to hear about any crazing, cracks or damage to the windows of a Sirius Yacht that have compromised their integrity or led to any needing to be replaced. It’s true, nothing lasts forever, we estimate the lifespan of a Sirius Yacht to be about 100 to 120 years, so like the windows on your home, there will be a time when the windows and/or the adhesive on your Sirius need replacing.

When we say our windows are stronger than the hull around them, we are serious. But strength isn’t the main thing that makes our hull windows special. It’s the view you get when you’re relaxing in the cabins.

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.tf1635248110rew-s1635248110uiris1635248110@ofni1635248110

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