Sea trials with the Sea Stallion from Glendalough

When we sail a new reconstruction for the first time, we have to rediscover a technique more than 1,000 years old. No modern human beings possess the same basis of experience that the Vikings had, neither on land nor at sea. But the boatbuilder's sense of materials, form and aesthetics, and the sailor’s desire to know his ship, can be found in the sagas, skaldic poems and, in particular, the five Skuldelev ship-finds.

We learn more about the people and culture of the Viking Age by testing the reconstructed ships under realistic conditions and by sailing with the same seriousness and dedication as the Vikings did.

In our sailing research, we distinguish between trial voyages and sea trials. The two forms of experiment each have a specific focus and give different results.
On trial voyages, we examine the ship's seaworthiness, manoeuvrability, speed, and the performance of the crew. The skipper and mate register a ship’s travel speed from A to B, and the crew tries out various sleeping arrangements, the storage of supplies, night communications and organisation on board.
In sea trials, we test a ship’s sailing abilities under various wind and weather conditions. The crew investigates a ship's ability to beat to windward, to tack and veer, and what it is like to row and hoist and lower the mast. The trials are repeated several times under varying conditions in order to obtain an average result.

The experiments illustrate the importance of a Viking ship’s loading capacity and the sailing capabilities that were needed to facilitate transport and communication in Viking Age society. They give us a glimpse of the possibilities and limitations that a merchant or a warship commander faced a 1,000 years ago.

Background and objectives

In the Roar Report, Max Vinner described the sailing characteristics of the Skuldelev 3 reconstruction Roar Ege. The report was primarily based on two series of sea trials, carried out in the autumn of 1984 and 1985 respectively. The purpose of the trials was to "investigate and document how a Viking ship similar to Skuldelev 3 performs fully laden under varying natural conditions at sea" (Roar Report page 236). In practice, a series of standardised beating trials were performed, where ship speed was recorded while sailing at various angles to the wind in differing wind strengths in order to determine "speed roses" for the ship. The report also describes rowing trials and various sail-setting manoeuvres.

Since the Roar Report was published, a distinction has been made between "experimental voyages", where the overall performance of the ship and her crew is investigated and documented in terms of travelling speed, and "sea trials", where individual factors are isolated in order to investigate the ship's sailing characteristics under various conditions.

A major objective of sea trials is to allow comparisons to be made between the ship in question and other vessels. In the Roar Report, Max Vinner made several such comparisons. The results achieved with Roar Ege were thus compared with other Viking/square-sail ships (Saga Siglar and Rana), a modern yacht (X 99) and working vessels from various periods of history. The comparisons were mainly concerned with the ships' beating characteristics and speed roses, but also examined specific manoeuvres like tacking, wearing and reefing.

The Sea Stallion from Glendalough is special because, like other longships, she is designed to be powered by either sail or oar. Longship hull design was largely determined by the rowing requirement and it is therefore important to establish why rowing was so important to the Vikings. Was it primarily to allow longships to travel up rivers? Was it to increase their sphere of operation? Was it simply to become independent of weather conditions? Or was it tactical manoeuvrability that was crucial – the ability to travel short distances against the wind in order to gain a strategic advantage?

While crew fitness and performance are inseparable from the results of the experimental voyage, the skill and ability of the crew will be isolated as far as possible from the results of the sea trials. In practice, this is of course an unachievable ideal, but the chances of approaching the ideal are greatest in connection with the experimental voyage to Dublin as the crew have had three years of practice in handling the ship and will be able to fine tune their skills along the way.


Basically, the sea trials are designed to answer the following questions:

1) How fast can the ship travel at various angles to the wind in differing wind strengths?

2) How does the ship respond when beating against the wind in differing wind strengths? How much leeway is there? How close to the wind can she sail? And what speeds are achievable, calculated as velocity made good (VMG) into the wind?

3 How do the oars benefit ship performance, if at all?

While concentrating on these main issues, the sea trials will also investigate such specific factors as the following:

1) Differences in ship (and crew!) performance when rowing or beating upwind under comparable conditions

2) Rowing under conditions where sailing is not possible, e.g. on calm seas or rivers.*)

3) The influence of the number of oarsmen on speed under various conditions

4) The influence of lowering the mast on rowing speed

5) The time taken to tack and wear

6) The time taken and distance travelled while raising and lowering the sail

7) The influence of reefing the sail on speed and leeway

8) The conditions necessary for the ship to break clear of leeward coasts

*) We cannot be certain that actual conditions will allow us to investigate these factors during the sea trials to be held this summer, but we will seize the opportunity should it arise.


As mentioned, speed rose and beating ability are important concepts when describing a ship's sailing characteristics. In principle, the data required to quantify the two can be collected in two different ways during the trip to Dublin. Firstly, the data collected during the experimental voyage itself can be analysed in relation to the conditions experienced along the route. Secondly, data can be collected in isolated sea trials performed whenever there is spare time on the voyage and in the period between our actual arrival in Dublin and the official reception.

In addition, all relevant weather data (wind direction and speed in particular) will be collected by the Viking Ship Museum from the Danish Meteorological Institute and other sources in order to allow comparison with the weather actually experienced. Such data comparisons may allow us to predict how similar voyages would have been under differing weather conditions.


Observations en route (i.e. while travelling specifically towards Dublin)

The navigation equipment on board will continuously record and electronically save the following data:                  

Date, time and position

Speed through the water and VMG

Intended and actual course

Wind speed and direction

Readings from the instruments will also be recorded in the ship's logbook. In addition, the route followed (and thus also course and speed) will be tracked using independent pocket-size GPS devices. And finally, the ship's position, speed and course will be sent to Denmark via satellite and published on the Internet. One important parameter, sea conditions (wave height, swell, etc.), will only be recorded in the ship's logbook in the form of observations made by individual navigators.

The number of oarsmen in action will be recorded whenever the ship is rowed.

The time taken to "raise sail" and "lower sail" will be recorded separately by noting the exact time at which the manoeuvre is commenced (i.e. on the "well rowed" and "lower sail" commands respectively) and completed (i.e. when the ship begins to make headway in the desired direction by sail or oar respectively).

Speed roses

If, during the voyage, the ship sails at many (or even all) angles to the wind, it will subsequently be possible to calculate speed roses on the basis of a large number of recordings. In comparison with the speed roses prepared during the sea trials with Roar Ege, inaccuracies caused by changes in wind direction during data collection will be largely eliminated. On the other hand, however, the data may contain "holes" as not all angles to the wind will necessarily be sailed in all wind strengths. It will be possible to determine the extent of such missing values by simply keeping track of logbook entries for "angle to the wind" and "wind strength". If and when the opportunity arises, any "holes" in the data can then be filled.

Rowing and mast lowering

Rowing trials will be performed under similar conditions to the beating trials in order to allow comparison between the two and to quantify which is fastest. In practice, rowing trials will be performed immediately before and/or after the beating trials. Ideally, the rowing trials should be performed before the beating trials, but this would require that the sail be raised and lowered an unreasonable number of times.

In each wind strength, rowing trials will be performed with the maximum number of oars and with reduced numbers (every second oar and approx. 20 oars). The ship will be rowed for a set period of time, 10 minutes for example, with each number of oars. The oarsmen will row strongly, but with a cadence capable of being maintained for prolonged periods. It is thus "cruising speed" rather than "top speed" which will be recorded.

It is possible to lower the Sea Stallion's mast, thus reducing wind resistance while rowing. The rowing trials will therefore include trials with both raised and lowered mast.
The time taken to lower and raise the mast will be recorded and the practicalities involved will be described and assessed, including any inconvenience to oarsmen.
As this manoeuvre is particularly risky, safety aspects will always be taken into account and the mast will only be lowered in suitable wind and sea conditions and once the crew has become sufficiently confident with the necessary procedures.  

Poul Nygaard

Sea conditions

Wave height and characteristics will be continually assessed on the Beaufort scale and entered in the logbook. Sea conditions will also be described in terms of wave length (the distance between successive wave tops) and wave speed (in m/s).
The skipper and mates will assess the way in which sea conditions affect the ship, especially under exceptional conditions (e.g. strong currents, flat seas and high waves), and will describe their observations in the logbook.


Poul Nygaard 25/6-2007

True sea trials – held during stops en route and on completing the voyage

While it is likely that sufficient data will be collected to prepare speed roses during the voyage to Dublin itself, it is doubtful whether enough data will be collected on the ship's ability to beat against the wind as all previous experience shows that it is better to wait for favourable conditions than to beat into a headwind. The true sea trials will therefore primarily concern the ship's beating ability, but will also include rowing and mast lowering trials. The ship's ability to tack and wear will also be investigated and recorded.

Beating, tacking and wearing

En route, it will be noted in the logbook whenever the ship sails close hauled (full and by), i.e. when not following a set course or landmark but keeping as close to the wind as possible. The data collected on such stretches will help describe the ship's beating ability.

In the true sea trials, the ship will always start by beating to leeward towards a predetermined "target", a buoy or randomly selected waypoint. Date, time and position will be recorded along with wind strength and direction. The date and time will be synchronised with the GPS equipment.

As far as possible, tacks will be selected which are free of currents and other unusual characteristics.

Several short beating trials, each covering 1-2 nautical miles from start to finish, will be preferable to one long.

Whenever the ship is turned, the time and position at which the "go about" command is given will be recorded (rather than the time and position when the "ready to go about" command is given as the time elapsing between these two commands will vary for many reasons). The time at which the "going about" command is given will then be noted and the wind direction and strength immediately recorded. Finally, the time and position at which the ship is once again fully trimmed and making headway on the new tack will be noted. As the number of turns greatly influences the duration of the trial, it is important that the time taken to change tack is accurately recorded so that it can be deducted when calculating the ship's VMG. During the trials, the trial leader will note in the trial log any special circumstances which may affect our interpretation of the results.

In the subsequent analysis, the recorded data can be used in several ways to calculate results. The simplest method (which requires none of the recorded electronic data) is to calculate the time and distance from start to finish or between successive tacks.

It will also be possible to analyse or, if necessary, omit individual tacks in a trial on the basis of the electronic data collected. Using GPS data, it will be possible to determine the time taken to change tack as the time taken for the ship to cross its own wake. This will tell us what the tacking manoeuvre "cost".

We cannot master the wind! It is therefore pointless to decide in advance the best conditions for beating trials. It would, however, be particularly interesting if trials could be performed in winds of about 10-12 m/s, where reefing the sail is usually considered. Performing trials in such winds with both full sail and reefed sail would allow the effect of reefing to be determined.

The results previously obtained in beating trials with Roar Ege show considerable variation in VMG. Many trials will therefore be necessary to obtain a reliable result. Whether we have performed sufficient trials can be determined by plotting wind speed against VMG in a coordinate system. If a close relationship between the two exists, sufficient trials have been held.

Sea conditions

Wave height and characteristics will be continually assessed on the Beaufort scale and entered in the logbook. Sea conditions will also be described in terms of wave length (the distance between successive wave tops) and wave speed (in m/s).
The skipper and mates will assess the way in which sea conditions affect the ship, especially under exceptional conditions (e.g. strong currents, flat seas and high waves), and will describe their observations in the logbook.