How big a sea can I handle in my trailer boat?

But things tend to be a little bit different for trailer boats. Wind can have a huge effect on boats, and it is as much about the water as it is about the amount of metal or fibreglass that separates you from it.

As part of National Safe Boating Week, we will look at several topics that focus on ensuring you have a great day – which in some instances will be a decision to not head out.

In this article, we look at how the length of your boat can have a big influence on your decision as to whether to head out at all on a windy day.

The biggest influence on the water is waves. Sometimes they can be a nuisance, particularly if you are prone to seasickness, but in the wrong mix of conditions, they can represent a big threat of either capsizing or swamping a boat.

Waves are created when wind moves across the water, transferring its energy. The more wind, the bigger the waves, which means you can usually get an idea of how high the waves are likely to be on any particular day.

However, waves are also affected by other factors such as gravity, the movement of tides and currents, and how long the wind has been blowing on the water (known as fetch). 

You can estimate the speed of wind across water by looking at the formation of whitecaps – an indication that the wind has transferred enough energy to make the wave’s crest unstable. If the whitecaps are on the lee side of the wind, the windspeed is around 12 to 15 knots. If you see whitecaps on the windward side of the wave, the wind speed is pushing at least 22-25 knots.

Waves are measured by their height, and are usually tightly packed together

While waves are the result of wind moving across water, swell is the remnants of weather events that have happened some distance away, such as a storm.

Swell takes a long time to run out of energy, so can travel long distances. Sometimes the swell is formed in the wake of a storm, but it can also warn of an approaching storm as the energy of the weather event radiates out from the source.

The depth of the water can also influence swell, meaning it can get taller and closer together as the depth decreases.

Swell is measured by its height, but it is also measured by the distance between its peaks. The swell is considered to be short when it measures less than 100 metres, average when it measures between 100 and 200 metres, and long when it measures more than 200 metres between peaks.

Likewise, the swell is classed as low when it is less than 2.0 metres in height, moderate when it is between 2.0 and 4.0 metres in height, and heavy when it is more than 4.0 metres.

Swell may even be moving in a different direction to the prevailing wind.

Waves travel on top of swell. While the distance between sets of waves can be short, the distance between the sets of swell can be large.

The danger here for boaters is when a wave hits the top of a swell. A 2.0-metre swell and a 1.5-metre wave means the boat will need to travel 3.5 metres vertically to cross the combined wave and swell. 

The other thing to be wary of here is that those numbers are an average of what you can expect to experience on the water.

Statistically, roughly every seventh wave a boat encounters will be higher than the “significant average” wave height that you will see in a weather forecast. As well, roughly one in every 2000 to 3000 waves – around three a day – will be around double the size of the significant wave forecast, creating the so-called “freak” wave that can capsize an unwary vessel. 

Going back to our 2.0-metre swell, If the weather forecast is for 1.5-metre waves, three times a day a freak wave could theoretically hit 5.0 metres in height.

A great way to assess swell and waves is by looking at how easily your boat can move through the water.

As conditions get tougher, the skipper has to reduce the boat’s speed to match the conditions.

If you’re in a small tinny, conditions should be good up to a wave height of about 0.5 metres. Stretch that out to around 0.8 metres, and things are starting to get uncomfortable, with significant spray even at a reduced speed, and even some water coming aboard. 

At around 1.0 metres of wave height, a tinny has hit its operational limit for safety. Progress through the water will be extremely slow and there is a serious risk of being swamped or capsized.

If you’re venturing out in a small trailer boat of around 4.3 metres, you should still be able to cruise comfortably at a wave height of up to 0.8 metres without the need to reduce the boat’s cruising speed, although you can expect the ride to be a bit bumpy and a bit of spray.

Step up to 1.0 metres and the trailer boat will be slowing down, with the nose potentially plunging into “green seas” – the name given to a solid body of water breaking over the bow.

At 1.2 metres, the sea state will have the skipper backing way off the throttle to make the ride more comfortable. At this point, there will be plenty of spray kicked up and making its way into the cockpit.

The limit for a 4.3-metre boat is theoretically waves of around 1.5 metres. At this point the boat is crawling along, and the bilge pump will be running to try and clear the water coming in over the bow or the sides. 

What about bigger boats? Like all the others here, a bot of up to 6.0 metres in length should be able to run at cruising speeds in waves of up to 0.8 metres with relative comfort, although some banging and spray.

The advantage of waterline length is that while a 5.0-metre boat will still be able to plough on at half speed through a 1.0-metre sea in relative comfort – although with green sea breaking over the bow – a 6.0-metre boat won’t hit the same limit until the wave height reaches 1.2 metres.

The difference for waterline of larger boats comes when cponditons get really tough. In a 5.0-metre boat, the skipper will be driving from wave to wave, setting the throttle and steering for crossing each one, when the waves hit 1.5 metres. In a 6.0-metre boat, that number will be around 1.6 metres.

The same goes for operational limits. A 5.0-metre boat will struggle in 1.8-metre waves, while a 6.0-metre boat will handle a 2.0-metre sea, but once again be right on the edge of its ability to stay afloat, even with the bilge pump running.

Learn how to read a weather forecast, because while conditions may not be suitable on larger stretches of water, enclosed waters and inlets may be well protected from the wind, waves and swell. For instance, you may be well-protected from a strong westerly wind tucked in behind land on the western edge of an inlet.

Be careful, too, with tides. Wind against tide is a phenomenon where the wind comes from the opposite direction to the way the tide is running, creating short, choppy and confused waves that tend to stand taller as the wind stalls the top of the wave, but the current drags it forward.

Plan trips so that if you go out in the still of the morning, by the time you come back in the afternoon the tide is running in the same direction as the building breeze.

The simple answer is to turn around. Sometimes you can have perfect conditions on the run out along a channel only to find that as you get to the larger body of water, conditions are challenging.

Pick your moment to turn around. Look for a low wave or a long trough, and whatever you do, complete the turn before the next wave hits the boat. 

There’s no shame in calling it if conditions look too challenging. Don’t be caught out, too, by the bravado of other skippers speaking from experience when in reality, they too would have preferred to turn around.

If you have family onboard, be very conservative about conditions. One rough day on the water could potentially be enough to discourage someone from heading back out with you for a second trip.

As always, the best advice is to talk to people about where you want to go, and when. If someone else has already visited the site, they may have valuable insight inot where to go, when, and under what weather conditions.

National Safe Boating Week runs until Friday, October 7. Friday is wear your lifejacket to work day.