• iainmacneil

Week 10 - A Ship Nerd's Guide to Crossing Big Blue (with interesting bits for the rest of you!)


This extract from the US Routeing chart for this region of the Pacific, shows a 'simple' representation of the winds and currents.

As we set out from Valparaiso to cross the Pacific, we all felt that this really was the 'big one'.. a body of water of 4,400+ miles, in a boat of just under 24 metres (75ft) that had to be fitted with fuel tanks on the fore and aft decks to ensure we can make it.

I remember the words of broker John Clayman, of Seaton Yachts in Newport R.I. saying “You’ll have to turn any boat below 24 metres into a tanker to get her to cross the Pacific”. At the time I laughed it off as I was focused on getting the most capable boat available, but it appears he may well have been right!

Astra has the following elements all dedicated to the cause of maximum mileage (and they sound pretty tankerish to me.....Ed!):

  • 20 individual tanks extra fuel tanks and 2 slop tanks

  • a cargo transfer pump and a stripping pump

  • a common vent line to all tanks of the same grade and a set of reducer couplings for fuel transfer

  • Local liquid level gauge, hydrostatic liquid level gauges and electromagnetic liquid level gauges

  • Manifold savealls (things that catch drips under the valves....Ed)

In addition, all fuel movements are carefully checked in the stability computer before being undertaken.

At the time, we glossed over the tank cleaning we had to undertake after the dirty batch of fuel we loaded in early January, which meant we had to take time at anchor (off hire!) while we moved fuel around in preparation for the next cargo and cleaned tanks. It was very stressful.


As we set off from Valparaiso, we thought about the weather we had gone through around the Horn and how much we were now looking forward to crossing the Peaceful Ocean. Looking back we realised that of the previous 60 days, we had recorded winds of Force 4 or more in 56 of them. In the good old days, that would have been enough for the charterers to accept a reduction in the vessel's speed and not add time penalties for a slower journey (although Kat seems disinclined to agree!)




The Pacific Ocean is the largest and deepest of the world's oceans. It covers ~ 60 million square miles and contains >50% of the water on Earth and all of the world's continents could fit into the Pacific basin. The Pacific Ocean has more names than any other, including:

  • "The Great Southern Sea" (Mar del Sur) Vasco Núñez de Balboa (Spanish explorer), 1513

  • "Mar Pacífico" (peaceful sea) Ferdinand Magellan, Spanish Cirumnavigation, 1521.

  • 太平洋たいへいよう literally translated as "peaceful ocean".

  • The Great Pacific Garbage Patch' Unfortunately, its most recent name covers the area of the North Pacific from Japan to the West Coast of North America. There are actually two areas, the Western Garbage Patch, located near Japan, and the Eastern Garbage Patch, located between the U.S. states of Hawaii and California. While, these are areas that we won’t go through, Luke (engineer) has a very vivid recollection of going through the garbage patch on a Teekay tanker and he described the debris as being like in a disused swamp and it lasted for 700 miles!

  • ‘Big Blue’

The area surrounding the Pacific Ocean is known as the ‘Ring of Fire’ and most of the active volcanoes on Earth are located on its circumference. While the Tonga earthquake of January 2022 made world news, there are mini tremors in the Pacific on every day of the year and any one of them would be mainstream news if it happened in Europe


Routes across the Pacific

At the top of this update you will see the routes described. We identified three routes across the Pacific Ocean:

Great Circle (4,205 miles) This is route type used by planes and is the shortest distance between two points on the earth’s surface. Ships use them to cross oceans but, in our case, the apex of the great circle would be too far south, towards the South Pole, which would draw us in to opposing winds and currents. Rhumb Line (4,300 miles) This is the direct route along the same compass heading, which crosses each meridian of longitude at the same angle. However, this would pass through the usual centre of the South Pacific high pressure and would not guarantee us favourable winds and currents.

Dogleg North (4,410 miles) This route takes us >250 miles north of the direct rhumb line route and places us in the main body of the anti-clockwise rotation of ocean currents to provide us with more favourable winds that are almost directly astern of us. While this route is 100 miles greater than the rhumb line and over 200 miles further than the great circle, we only need to pick up 0.25 knots of additional speed for it to pay off. The other routes also were likely to produce 05-0.75 knots against us, an additional strain on our fuel and endurance calculations.


The main consideration for our crossing of The Pacific is fuel economy. Iain ran his calculations on a worst case scenario, based on unfavourable winds from ahead at Force 4 (which we have seen plenty of!) for 75% of the crossing. A great benefit of the long range forecasting is that we could get a sense of Pacific for 14 days ahead on the day that we left Valparaiso, giving us comfort on the best speed for the 1st half of the crossing.

Technical Description Alert!

Measuring fuel range and autonomy: Astra's fuel range is probably the question we get asked most. When looking at listings of boats for sale, they are typically targeted at the owner in terms of amenities etc and

detail on the particular model of the main engine and its associated machinery is light. Fuel consumption figures are based on still water conditions and bear little reality to the open ocean sailing.

A considerable amount of time was spent assessing Astra's fuel figures, including 3 days on a quiet and long Scottish Sea Loch in August 2021 to assess baseline fuel consumption figures in still water conditions, which allowed Iain to create tables and curves of performance. This provided firm data that Astra’s fuel consumption performance could be checked against each day at 12:00hrs. The most economical fuel speed setting for ASTRA is 600 RPM with 60% pitch setting on the controllable pitch propeller. This delivers a speed of 6.53 knots at a fuel consumption of 28 litres per hour. To this figure we need to add 3 litres/hr for the shaft generator or the Cummins Onan generator, which gives us a figure of 31 litres/hr delivering 6.53 knots in still water conditions.

Iain’s experience has shown him that obsessing with litres per hour on trawler yachts and expedition yachts is wrong you need to gross those figures for the more 'normal' experience. On ASTRA, in open seas of variable conditions, seas of varying heights of up to 3.0 m (10ft), winds of up to Force 5-6 (17-27 knots) and with variable currents and varying engine speeds for weather and periods with stabilisers on etc, we consistently require between 8,400 – 8-600 litres of fuel per thousand miles, which equates to 8.5 litres/Nautical Mile. While on some voyages we can be under this figure and on others we can be over, particularly if it’s a short voyage and we are pushing hard.


Therefore, on the basis that Astra’s full fuel lift is 43,000 litres, the extremes of fuel range are:

  • In ideal conditions: 43,000 / (31*24) = 57.8 days, which at 6.53 knots is a range of 9,058NM.

  • In typical mixed ocean conditions: 43,000 / 8,500 = 5.06 x 1,000 = 5,060NM.

So it is fair to say that depending on sea states etc, but with a full load, Astra's range is somewhere between 5,000 to 9,000 nautical miles. (However, in practice, we like to keep 4,000 litres of fuel as reserve, which on the basis of economical steaming, gives us a reserve range of 800NM+).


Strategy for crossing the Pacific

The first 3 days after sailing from Valparaiso we experienced Northerly winds blowing along the coast of Chile. We set off at the most economical speed (600 RPM, 60% Pitch) and really had the sense of settling into a marathon! However, because we are about 20 tonnes heavier than when we did the sea trials, due to the additional fuel weight, our effective speed was ~5.75 knots. After 3 days, we increased the RPM to 700 and adjusted the fuel consumption to 1,000 litres/day. At this point our speed was ~ 7.3 knots.


However, while we are ‘slow speed steaming’ we have to de-coke the engine to remove the carbon deposits that build up and choke the valves and exhausts at these comparatively low RPM. This is done by increasing the RPM and Pitch at intervals to create a load on the main engine that brings the exhaust gas temperatures >350°C.


Once we settled at 1,000 litres/day, the strategy was to run like this for 3 full days to get an accurate set of figures to maintain the reserve of 4,000 litres, which is then deducted from the total fuel onboard as Iain calculates how much he believes he can safely consume on the remainder of this leg. Erring on the side of caution with each adjustment made the other key factor is that each day the weight being carried reduces by at least one tonne (2,240 lbs), so we are also getting a little faster each day as we get progressively lighter.


By day 70 (Tue 08 Feb) we had the engine set at 750 RPM, pitch at 71.5% and we are

making a speed of 8.3 knots (200 nautical miles per day).

In early January Iain estimated that Astra would cross the Pacific in 23 days at best and 27 days at worst, so we used 25 days for the ETAs for forward calculations. However, we now look like we will arrive into Tahiti on Monday 21st Feb, with an outside chance of making it in the evening of Sun 20th Feb, which suggests we could make the Pacific crossing in 22.5 – 23 days!


Weights & Ship Stability

Astra was built to the Classification standards of Det Norske Veritas (DNV) and is provided with detailed stability information for specific loads and distributions, typically:

1. Lightship condition

2. Departure from port with Fuel at 100%

3. Mid-point ‘at sea’ condition

4. Arrival back to port with Fuel at 10%.


Astra's design makes her extremely stable and so:

- Astra has no point of vanishing stability (ie she rights herself)

- She was built for the carriage of 6 tonnes of cargo on deck

- In addition to cargo on deck and, as a former rescue vessel, she was designed on the basis of 96 persons having being rescued and stood on the main deck so it is important the deck does not immerse.

96 persons at an average weight of 75kg = 7.2 tonnes + 6 tonnes (deck cargo) = 13.2 tonnes.

17,500 litres of fuel = 14.35 tonnes, which showed that these rough figures were close to the parameters that had already been tested. However, such a distribution of weight in the open ocean would still have to be very carefully monitored.


Using Astra to cross oceans is obviously not what she was originally intended to do and, while she has substantial reserves of inherent stability and buoyancy, we still need to check each weight movement onboard to ensure we remain stable and that we are optimally trimmed. There are too many permutations to allow them to realistically be pre-calculated and presented for use in an easy-reader pre-tabulated format for use onboard. Therefore, the only solution was to have all of Astra’s trim, stability and hydrostatic data compiled into a ship stability computer program suitable for use on small ships.

End of Tech Alert!


Ship Spotting

As we cross the Pacific we are making a special of any ship sightings in the logbook. We travelled 7 days from Valparaiso before we sighted our 1st ship, which was a handy sized geared bulk carrier on passage from Portland, Oregon to Maputo in Mozambique. Her route was taking her round Cape Horn - a whopping distance of 11,600 Nautical Miles!

However, the likelihood of seeing another ship in the next two weeks is very low. To detect other vessels, we have 3 radar sets on the navigational bridge, the most powerful of which can detect the echo of another merchant ship at 20 miles, which is very good for our comparatively low height of eye. Astra was fitted with an Automatic Identification System (AIS) ‘A’ unit, rather than the ‘B’ class that is more typically fitted to leisure craft. The AIS is a transponder and, to the average boater, looking at ships on an AIS tracking website, what you are seeing is terrestrial coverage when the vessels are near a land mass. While users at home can pay extra to see the satellite received data for a vessel substantially beyond the shore, Astra has her own continual satellite link to monitoring equipment ashore that allows those with monitoring roles and the close family of the 5 crew members onboard to see exactly where we are and what course and speed we are doing at any time of the day!!


Our route this week

On sailing from Valparaiso, we had Robinson Crusoe Island 150 miles on our port side and between Alexander Selkirk Island and Easter Island we are 800 miles from civilisation!

To put this in context, the International Space Station, orbiting 220 miles above the earth is our closest contact to humanity for over 3,000 miles of our crossing of the Pacific, until we reach the British island cluster of: Pitcairn, Henderson, Ducie and Oero.


On Day 67, Sat 05 Feb

We completed the first transfer of fuel of 7,000 litres from our tanks on deck down in to our main holding tanks (wing tanks), which are capable of holding 11,000 litres. At that point Paul then set up the fuel oil purifier (centrifuge) to polish that batch of fuel, before it was transferred through to the day tank to directly supply our main engine. Our day tank is 2,300 litres capacity and, when the diesel oil level drops to 1,250 litres, it automatically fills the day tank up to 1,950 litres from the nominated wing tank.




.....And Finally

When Iain was growing on the Isle of Barra in the Outer Hebrides, in his grandparents house, there was a smoothly polished wooden pigeon that was hand carved with the inscription ‘Made by …. who is a descendant of Fletcher Christiansen, Pitcairn Island’. Pitcairn Island was made famous by the Mutiny on the Bounty and Iain’s father had sailed there in the early 1960s on a Bank Line cargo ship and had traded a can of condensed milk for the wooden pigeon. When planning the voyage and looking at a route across the Pacific, Iain thought that Pitcairn would make a good stop. However, when he read up about it, with a population of only 65 people and a comment that while fuel is available to passing vessels and is brought out in 20 litre jerry cans, he realised he needed to look for a larger, better equipped stop-off in the Pacific and settled on Papeete in French Polynesia!



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