First mate Daniël Pruijmelaar noted down data on navigation and determining the position of The Unity in the logbook. The direction of the wind as well as the wind speed was noted throughout the day and also partly during the night. He also recorded maintenance or repairs on the ship, trade transactions, deaths and other notable events, such as changes in the colour of the water.
Hourglasses, compasses, sounding lines with deep sea lead and a box with special first mate’s tools comprised the common navigations tools that, according to the ship’s inventory, were given to The Unity. Supplementary tools were taken along by the captain and the crew members themselves, but these were private possessions.
The list of possession from a MCC captain who died in 1767 at sea lists some of these navigation tools:
- a set of handwritten books
- 3 sea boos
- a set of maps
- 1 Ruijte map
- 1 cross staff with accessories and 1 compass
- 1 octant with accessories (new)
- 1 quadrant with accessories
The above-mentioned captain is Daniël Pruijmelaar. During the 3rd voyage of the ship the Unity he was first mate, but after he would be promoted to captain.
Coastal and sea navigation
The position of the ship was noted down in the logbooks of MCC ships as accurately as possible. This was done using two different methods: coastal navigation or sea navigation. For coastal navigation, the position was determined using visual points of recognition at the coast. No longitude or latitude was noted down. For sea navigation the position at sea was determined using dead reckoning and by means of navigation tools. The longitude and latitude were noted down in the logbook.
The longitudes that were used in the 18th century do not match with the current ones. Dutch map makers used the Ferro meridian which ran over the Canary Island El Hierro. The Ferro meridian differs approximately 18 degrees with the current Greenwich meridian.
Position at sea – dead reckoning
The position at sea could be determined using dead reckoning. The following data are necessary to make a dead reckoning :
- The speed. The speed could be calculated with a log and an hourglass. The log was used to determine the speed of the ship through the water. The log consisted of a small lath of wood that was shaped like a circular sector. The side of the bow was made heavier with a piece of lead. The lath was put overboard and the logline would unwind. The logline was marked with set distances and by counting those marks, which unwound in either 14 or 28 seconds, the speed of the ship could be calculated.
- The degree of leeway. This could be calculated by letting a buoy attached to a rope float and calculating the angle of leeway with a compass. Compasses could be used to follow a pre-determined, steady course.
- The distance. The average distance that a ship made per hour was used as measurement, in miles.
- The course. The course was marked with a compass and a ruler on a map on which lines were drawn between recognizable points.
Along the coast and in sea straits, the speed of the sea currents could be estimated reasonably well and taken into account for the calculations. At sea, the different speeds of sea currents could not be taken into account, which is why the end result could sometimes show a significant difference from reality.
The dead reckoning was determined based on the sailed course, the measured speed, if possible the estimated current speed and the sailed timespan. It was then drawn on the ship’s map. Only in 1762 when John Harrison invented an accurate chronometer did it become possible to determine the latitude more precisely.
Position at sea – Instruments
To determine the position at sea the longitude and latitude had to be calculated. A celestial globe was used to find the location of the stars that stood above the horizon at a specific moment.
The Pole Star lies directly overhead when viewed from the North Pole, at a 90º angle at the horizon. At the equator it lies just above the horizon, at a perceptible angle of 0º. On the northern hemisphere the latitude could therefore be calculated at night by measuring the angle, since the height of the Pole Star from earth is virtually equal to the latitude.
On the southern hemisphere, where the Pole Star could never be seen, the height of the sun during the day could be used for calculation. By means of tables the measurement was converted to the latitudinal position. With instruments to measure angles, such as a quadrant, a nautical astrolabe, a Jacob’s staff or a sextant, the height of the sun during the day or that of the Pole Star during the night could be measured. The instrument showed the number of degrees latitude or longitude where the ship was located.
Determining the east and west meridians remained problematic until the late 18th century, mostly because the necessary tools were lacking. Because of the rotation of the Earth, it was impossible to stick to a fixed point, such as the Pole Star. There were different ways of approaching this issue.
The longitudinal position could be calculated through compass variation (declination): the difference between the magnetic north and the geographical north is perceived as compass variation at different longitudes. Plancius made tables for this, which could be used to calculate the local variation at full sea, based on the geographical distance between where the sun rose and where it set. As time went on the magnetic north pole moved which made the method less reliable.
The difference in time per 15º over a circle of latitude is one hour. Tables existed that gave the time of sunrise as compared to the Amsterdam time. However, using these required an accurate clock that remained functional throughout the entire voyage.
Towards the end of the 17th century tables were made in which the changed position of the moon as compared to star constellations (which are perceived in the same manner at every position on Earth) was noted down. The required observations were hard to carry out with the instruments that were used at the time.
Navigational instruments in the 18th century
The quadrant is a navigational instrument that is made from wood or bronze and is shaped like a quarter of a circle. The cathetus of the instrument was aimed at the astronomical object and the plumb-bob showed the measured angle. The quadrant was less suitable for measurements during bad weather, because the swaying of the ship also made the plumb-bob sway.
The astrolabe was more refined than the quadrant, but it worked in the same way. The turning board, the alidade, was aimed at the astronomical object after which the height could easily be read. The astrolabe was also used to determine the time. The instrument was adjusted to the date of perception and with the alidade the positions of several stars was determined. From this one could determine the hour: from the hour angle the sidereal time was determined which could be calculated into the solar time. The astrolabe was followed by the Jacob’s staff and later by the octant and sextant.
The cross-pieces of the Jacob’s staff were adjusted in such a way that the horizon could be aligned with the lowest points and the astronomical object with the highest points. The staff contained a logarithmic scale division on which the height of the sun in degrees could be read. The disadvantage of the above mentioned instruments it that the measurement had to be done by looking straight into the sun. The Davis quadrant (or backstaff) avoided this drawback.
The first to follow the Jacob’s staff in the 18th century was the octant (1731) and later the sextant. Not only were these instrument more accurate, but because of the indirect measurements via two mirrors and small protective glass one did not have to look directly into the sun.
The octant had a framework the size of one-eighth of a circle (45º) and had to be held in a vertical position. This allowed the horizon, the opening in the horizon vane, and the line in the arc to aligne. The alidade (turning board) could then be adjusted so that the reflection of the sun projected in the middle of the horizon mirror. De observation angle could be read on the intersection of the alidade and scale on the arc. The sextant was developed from the octant and had a framework the size of one-sixth of a circle, increasing the range from that of the octant.
Navigational instruments on board
The first mate on board of the Unity was Daniël Pruijmelaar from Middelburg. From the list of instruments that Pruijmelaar has in his possession in his function as captain, four years after our voyage with the Unity, shows that he and his mates had access to:
- 1 octant with accessoires (new)
- 1 quadrant with accessoires
The seafaring Daniël Pruijmelaar had a cousin, who was also called Daniël Pruijmelaar, and who held a special position in Zeeland. He was:
- the examiner of the commanders and lieutenants of the College for Admiralty in Zeeland
- the examiner of the steersmen of the East India Company chamber, Zeeland
- the gauger for the West India Company chamber, Zeeland
This examiner was mentioned by his contempory, the autor W.A. Willems, in his translation of and notes to Benjamin Martin’s publication ‘Theory of Hadley’s Quadrant Demonstrated’. In this publication Willems writes that the octant is very accurate in measuring horizontally and that he came to this conclusion when he examined the octants with his friend, the examiner of steersmen for the East India Company, Daniel Pruijmelaar.
On basis of this and the family relations it can be assumed that the seafaring Pruijmelaar, at the time of the voyage of the Unity, was working with a Hadley’s quadrant.
- Naamregister (…) Admiraliteit alsmede (…) Oost- en West-Indische Compagnie in alle steden (…), (Amsterdam 1781), via: Early Dutch Books online.
- Willems, W.A., ‘Het Engelsch Oktant …’ in: Uitgezogte verhandelingen (…), (Amsterdam 1763), pag. 150.
Sea charts on board
Because the charts on board were the property of the officers they were not preserved in the archives of the MCC. As mentioned above, when Pruijmelaar was captain he had in his possession:
- some handwritten books
- 3 rutters
- a variety of charts
- 1 Ruijte chart
- 1 cross staff with accessoires and 1 compass
The books undoubtedly contained the tables which could be used to calculate the position of the ship. See above for ‘dead reckoning’ and ‘navigational instruments’.
The three rutters are probably three sections of the atlas Zee-fakkel (Sea torch). This atlas with charts appeared for the first time in the 17th century, edited Johannes van Keulen (1654-1715) and published in Amsterdam. At the time of the voyage of the Unity an improved version of the atlas was issues by his great grandson Gerard Hulst van Keulen (1733-1801). The charts in these atlases all had the same measurements of 50×100 cm (20 by 40 inch.).
The variety of charts probably consisted of different types of charts, ranging from charts covering large areas of the Atlantic Ocean to more detailed charts of, for example, the coast of Africa. It may also have contained charts made by French or English cartographers, who were at the forefront of the development of charts in the 18th century because they used the hydrographic maps of the French and British navies. These contained more data on water depth, water and seabed composition,, the tide, the waves, and the currents. The charts in the Van Keulen atlases were based on less thorough surveys along unknown coast by captains and skippers.
The Ruijte chart was probably based on the work of the draftsman and land surveyor Levinus Ruyte (ca 1553-1601) from Zierikzee. During the second half of the 16th century he made maps of areas in Zeeland. The chart that he made of the Sloe, the throughway between Walcheren and the Bevelanden, was copied for centuries, with the last copies dating from 1763 and 1792.
source: Donkersloot-De Vrij, M., ‘Repertorium van Nederlandse Kaartmakers 1500-1900’ (Utrecht 2003)
The cross staff with accessoires and the compass are self-explanatory. They are the ruler and tools for plotting a course on the chart. As first mate Pruijmelaar noted in the logbook a week after departure of the ship the Unity from Flushing: “we estimated our dead reckoning position”.
Compass on board
According to the inventory list the ship the Unity carried the following compasses on board:
- 1 bearing compass
- 4 round compasses
- 1 hanging compass
The compass is an important navigational tool with a rotatable pointer that aligns itself with the North Pole. The dial or compass card is divided up into 32 streaks and/or 360 degrees. When steering, the steerman relied on these streaks for determining the compass variation of degrees. Using a compass, one could therefore follow a certain fixed course.
On the northern hemisphere the needle of the magnetic compass points to the (magnetic) north. However, because of the Earth’s magnetic field the compass does not always accurately points north. These deviations were known in the 18th century. The compasses on board were frequently adjusted which was also mentioned in the logbooks. On 4 November 1761, while sailing on the Atlantic Ocean, the first mate of the Unity wrote: “we reset the compasses from 16 to 10° northwest”.
A bearing compass, or a ship’s compass with a pelorus, an upstanding disc or pen, enables the determination of the direction, based on the position of an astronomical object or a recognizable point ashore. The bearing compass can be used to support the magnetic compass.
The ‘hanging’ compass is most likely a compass that was gimbaled in a wooden house of chest.
Compass of Kakelaar
The compasses on board of the Unity were all new and delivered by compass maker Henrikus Kakelaar in Middelburg. One of his compasses has been preserved in the planetarium, also managed by the Zeeland Archives. This planetarium was built by the influential regent J.A. van de Perre in Middelburg in the 1860s. The planetarium is located in the former home of Van de Perre, the current housing of the Zeeland Archives, and it is owned by the Koninklijk Zeeuwsch Genootschap der Wetenschappen (the Royal Scientific Society of Zeeland). At the end of the 1860s Kakelaar was to become chief accountant of the MCC.
The depth of the waterway at the coast could be plumbed with a plumb line, which was marked at regular intervals. The unit of such an interval was called a fathom.
The crew onboard the ship the Unity had access to two large plumb lines, three small ones, and three leads, weighing 24 to 26 pounds, 16 pounds and 5 pounds.
The lead was cylindrical and had a cavity at the bottom. Soil material would stick on hard grease that was smeared in the cavity. The depth of the water and the condition of the soil were then used to determine the location of the ship, and its suitability for anchorage.