Do we have a technique to minimize the impact of high waves on the structure of our catamaran?

Do we have a technique to minimize the impact of high waves on the structure of our catamaran? This is a question we were asked recently by social media friends after they completed their first offshore passage. In considering an answer I think there are 4 main areas namely rigging, weight distribution, reefing and steering. However before discussing this I want to turn to boat maintenance.

Boat maintenance

We’ve been in many storms over a decade of full time sailing on our Lagoon 440, the worst ever being a 3 day storm with winds over 70 knots, deploying a warp to survive it. This kind of situation is really frightening and challenging at times but having the knowledge that one’s boat is in great shape can help to give us courage.

Catamarans in particular being wide platforms in rough seas are prone to stresses that can either be ignored or attended to.

One of our most important items to check regularly is the structural integrity of our boat and if we find something showing signs of stress we remedy it as soon as possible before it becomes a bigger issue. When we purchased our cat new from the factory one of the first projects on a brand new boat was to enhance the boat for world sailing as to cope better with heavy sea conditions.

On many cats the bridge deck is a lot thinner in materials than that of the hulls and for good reason because one wants to keep the weight of the craft down. On some cats one will observe ribs built between the hulls running along the bridge deck direction bow to stern and this is to compensate for the flex in this area. Because the glass is thinner here and spans a wide area, it stands to reason that it will flex as waves fight for an escape route and this leads to a bone shaker effect inside the boat.

The result of this is also not friendly to bulkheads or any other areas where there are hard joints to the bridge deck from the inside because the material flexing ahead of and behind these hard joints causes the glass to eventually crack and will result in bulkheads letting go – it’s like taking a piece of wire and bending it back and forth and eventually it will break.

For this reason we’ve developed solutions on Impi to counteract for this effect e.g. ribs internally going from the front of the main bulkhead forward and I’ve seen a few people now doing the same. There is a lot being made about Lagoons and bulkheads but actually this is a common issue across brands – some owners just haven’t looked and don’t know it has happened.
Just a few weeks ago I met another Leopard owner who was a boat builder by trade (He built Mustangs in New Zealand) who arrived across the Pacific and had bulkhead failure. He discovered this by looking at his boat’s structure on arrival.
He too concluded that catamarans in general are wide platforms moving around in heavy seas and will eventually have issues. Being a boat builder he has the tools and means to make his Leopard even stronger – he is busy with that task right now.

I’ve seen a number of builds and brands with these issues and sometimes boat owners do not want to publicize these things. A year or so ago I recall the owner of another brand telling me he had inspected his bulkheads and had no issues at all only to see his boat for sale a few weeks later. The new owner sailed into an anchorage where we were anchored and asked me to take a look at his bulkheads and indeed, they had separated.

The boat builders were smart and immediately attended to the issue so it would not be broadcast over social media, however, this is not helpful to boat owners who otherwise think these issues are non existent in their brands. We always check through our boat or any other boat we may sail on for structural integrity before any passage.

To check the structural integrity of your boat means removing the cable ties that hold electrical cables or plumbing or any other obstacles that would hinder your ability to clearly see where hard points meet the bridge deck and hulls. If you are not so technically gifted to make a judgement use a technical company such as Just Catamarans to go through your boat prior to a long passage. Surveyors tend to not pick up these issues.


I think it’s important for us to remember what’s happening to the boat as we sail in these conditions because the forces at play are massive, both longitudinal, lateral and torsional.

For this reason we don’t sail with a tightly set rig on Impi – the rig tension plays a massive role in how the boat structure will feel these forces at sea. We set the rig up tensioned enough but not heavily so and a bungee cord between the leeward shrouds takes up the slack so it has a shock absorber effect and not a ‘snapping effect’. This is because as the boat moves over waves and the apparent wind shifts, the sail can momentarily stall causing the mast to whip back and forth further hammering away at not only the rig but also at the boat itself.
Rig tension in these conditions is massively important.

A highly tensioned rig also means the structure of the boat is under more tension. This is great for light to average weather conditions but not in foul weather conditions as a boat falling off a wave and burying one bow with everything tensioned is more prone to breaking something than one with less tension throughout the structure. It’s important to note that when shrouds are tensioned they pull the outer sides of our wide platform upwards against a downward pressure at the mast. We’ve had disagreements about rigging tension with the odd ‘professional rigger’ out there, but most who have the skills realize that the aim is not to tension the wire to its rated tension but to find the point at where exceeding certain tension can and probably will destroy the craft especially in heavy weather.

For this reason before we set out on any passage we check the rigging and pay particular attention not to have an over tensioned rig.

weight distribution

Often overlooked, weight distribution plays an important role on boat structure in rough seas. When loading the boat with food, fuel, spare parts, water and other items adding to the overall weight on a boat, it is important to try and keep the weight evenly distributed, with more weight aft of midship than ahead of it.

It’s also important not to exceed the payload specifications by the manufacturer and is often an important factor sailors can get wrong on boats built with huge space to store items. On some boats like ours we need to be mindful of the fact that we are carrying 1 ton of weight in water considering the 900 liter water tanks, 80 liter hot water cylinder and water in the pipes. With 900 liters stored ahead of the mast position this is a lot of added weight toward the bows and at a point where waves passing between the hulls adds impact to the bridge.

We’ve been in some sea states where we’ve dumped water to reduce weight and stresses caused by having the bows digging into waves. Slamming is a term often used with catamarans and it usually happens on a close point of sail where waves enter between the hulls, slamming initially against the leeward hull forcing water upwards.

Over years of full time sailing and constant observation we’ve come to learn that the initial impact of water being pushed upwards happens to the underside of the trampoline. Water that cannot pass through the tiny apertures on these trampolines has to find its way out towards the stern impacting the bridge deck on its journey.

For this reason on Impi, we have removed our trampolines designed for comfort at anchorage off beautiful islands and fitted a trampoline with a wider aperture netting, and at a guess we’ve removed 70% of the slamming we used to feel. It’s important to us to make sure we don’t add too much weight to the bows to reduce the initial impact on the boat.

Bridge deck clearance plays an important role too but sometimes the inertia of water traveling upwards results in a harder impact to the bridge on higher bridge deck clearance. Whatever catamaran we may be sailing on, we pay attention to how much weight we have onboard and where it is packed in order to have good weight distribution over the boat.


Following onto this, in big seas we reef, reef and reef again. Sailing by the numbers (reefing numbers recommended by the manufacturer) is fine in average seas for these calculations, however when seas become abnormally rough we take the punch out of the sails and will sometimes engage engines to assist the boat through these conditions. There’s nothing worse than a boat moving down a wave and suddenly being stopped by another wave with the inertia of the mast moving forward – both on the rigging and the structure of the boat. It gets worse when this happens to either bow separately digging in because of the torsional forces exerted through the boat.


For this reason we also hand steer in these conditions. I’ve heard people saying an autopilot does a better job at steering than a person and whilst that may be true in most conditions, the autopilot cannot see or anticipate the odd very big wave that pops up and runs toward the boat, so we always hand steer when we feel conditions are not right and we steer off waves that are breaking bringing it back afterwards. This helps to keep the torsional forces down that will be felt on the boat. However, then there’s night time and one doesn’t see much on a dark night except hopefully the white runs of breaking waves – so all we can do is our best in any situation. Eventually the auto pilot will not be controlling a boat well enough anyhow forcing us to grab the helm by hand.

In really bad seas where the safety of the boat and ourselves is compromised to that extent, we will deploy a warp with a mooring ball on the end as a drogue of sorts and let the boat go with the weather at slow speeds. This assumes we have the sea room to do this . Usually going with the weather means we can let out some genoa via the outside horns cleat on a barber hauler system of sorts (allows the clew of the genoa to be more outbound) and be the pulling force of the boat through the water whilst the warps create enough drag so the boat does not fall off wave tops or broadside down wave faces.

So yes, in summary we do have techniques to reduce ‘hammering the boat’ but ultimately when all this is done there’s not much more to do. On Impi we like to keep the craft active in the water i.e. keep moving with the system bringing the bows up (to weather) in gaps that allow us to and bearing away from waves that would do most harm but not to forsake all to maintain our course. The course can always be corrected once the worst of the storm has passed us by. For more about this see the blog below.

Where sea room is becoming an issue we need to consider a few things.

Assuming we need to motor into the weather slowly, then rudder position on the boat plays a role because where the rudder is set aft of the prop, the prop wash over the rudder will allow rudder steering at low speeds. Sometimes one wants this in rough seas especially in seas ahead of midship.

The disadvantage is that rudders set far back like this catch air as the stern lifts through the back of a wave in following seas. In our case where the rudder is ahead of the prop, due to no prop wash we need a certain speed through the water (in rough seas that would be 4knots minimum) to maintain sufficient water flow over the rudders for steering. This means we would need to bash into weather at around 4 – 5 knots if escaping a windward shore.

The advantage we have is found in breaking seas from astern – we don’t loose the rudder to air as soon so we have better steering in those conditions delaying a need to deploy warps if it comes to that. This FORCES us to sometimes go with the weather as opposed to ‘across it’.

Interesting topic – so much more one can share but hopefully others will engage and share away too !


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