Back in December I reminded myself what ‘too close for comfort’ feels like. After 18 days at sea, my crew and I finished an Atlantic crossing and parked up in Rodney Bay, St Lucia, with around 5lt of usable diesel left – barely a couple of hours run time for our 60hp Volvo.

You could look at that and consider it an example of exceptional seamanship and precise fuel management – a perfect plan. Barely a drop in the tanks more than needed!

Or you may judge it a reckless ‘near-miss’. Just one unexpected variable such as a messy sea state or dirty fuel, and we’d have been stuck, windless and embarrassingly fuel-less, in sight of land.

Whether you judge it good or bad; two things hold fast. 1) Cut it fine and get away with it and you have a great story in the bar. 2) Dig deeper, and there’ll be lessons to learn.

So, was it luck? Or a masterful plan that came together?

A month earlier, I joined friends of mine on their new home: a 45ft catamaran. The boat was brand new. They’d sailed her straight out of the factory to the Canaries. By the time I joined them they had a few thousand miles under their belt, and the very short list of warranty issues meant a crossing before Christmas was viable. Good news!

We prepared together for almost a week, at which point a brilliant weather window awaited us. As is my routine on joining a boat, I gathered data.

Sailing across the Atlantic into the setting sun

On the fuel specifically: the crew had been monitoring rpm and engine hours from new. From always running the engines in gear at 2,000rpm, and then cross referencing their logged engine hours with the diesel they added to the tanks on their first fill up in Tenerife, they knew the engine was burning 3.8lt/1gal per hour, which matched the fuel consumption curve as per Volvo’s manual.

So, we made an educated assumption that the graph could be used to accurately predict fuel consumption for lower rpms and assumed a 2.2lt/hr burn at 1,800rpm.

At 2,000rpm the boat made 6.2 knots in a flat sea; at 1,800rpm it was 5.4 knots. The power generation was similarly significantly more efficient at lower rpm. On a cloudy day at sea, the power draw on the batteries required five hours of running the engines at 1,800rpm to bring the batteries back up to 100%.

According to the manufacturer’s manual the boat had two diesel tanks of 250lt each and 90% of the fuel in each tank was usable in practice. So, we had 450lt of usable fuel. We then added four 20lt jerry cans as an emergency reserve – which would offer us 125 miles range at 2,000rpm or 160 miles at 1,800rpm.

Assuming a 28-day crossing in which every day was cloudy, we’d need 308lt [28x5x2.2] for charging, and would have 142lt remaining [450-308] for motoring. That would mean 40 hours, or just over 200 miles of range.

Article continues below…

Most of our life on land we spend hiding from ourselves. But at sea there is no escape – Nikki Henderson

Ahoy there, from halfway to the Caribbean! This is my third attempt at writing this column. The first go was…

‘Boat ownership is a wildly impractical love affair, and gets complicated when you’re smitten’ – Nikki Henderson

Being a professional sailor, I realise how stupid this is, and I really should know better. From working for owners…

As each day of the crossing progressed, we recalculated these figures to take into account our real-time progress, the reduction of charging time needed due to solar generation, and the future weather forecast.
But despite our meticulous planning and monitoring we encountered a few surprises. Does anything ever go completely to plan at sea?

During a bilge inspection on day 14, we discovered a stamp on each of the fuel tanks showing their capacity was 230lt and not the 250lt stated in the boat manual. Lesson: check the tanks match the manual.

On the penultimate day, our starboard engine stopped as we’d run its tank ‘dry’. So despite the manual stating that 90% of the fuel is usable, in reality it was 84%. Lesson: find out what the real-time ‘bottom’ of the tank is before you face a wind hole on day 17 of an Atlantic crossing.

We’d been running our two engines in sync, to avoid one having too many more hours than the other. So, when one tank reached bottom, the other wasn’t far off and there was no scope to change strategy. Lesson: avoid stressing all the systems at the same rate.

The jerry cans saved the day. We hadn’t expected to use them at all. Lesson: the extra weight of the odd jerry can of fuel can save a whole load of stress.

So to sum up, assumptions and ambiguity are a sailor’s worst nightmare. Test everything, never guess. And have a back-up plan.

If you enjoyed this….

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