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Trimming and Steering

"The most prominent job of the helmsman is to steer the ship after wind and sails, but it is the duty of the watch officer to trim the ship in a way that she follows the wheel."  (from: Handbook of STS "MIR")

A museum ship in port is normally having the yards braced in a right angle. Same on paintings and postcards, that normally show square riggers with such a position of the yards. But to conclude that this is the normal way the yards are positioned on these ships is wrong.  It is surely the most impressive way to show a tallship as is demonstrates the size best, but when sailing the square position of the yards is rather unusual. It is only one possible way to gain movement from the wind and actually not even the best one.

The positioning of the square sails and fore-and-aft sails in the best possible angle towards the wind to gain the under the given circumstances most possible propulsion is called trimming of the sails. With a good trim the ship follows the helm easily and fast, lies calm and without heavy rolling but with a certain list in the water. Sailing is safe and the movements of the ship are in a way predictable. 


First we must keep in mind that on ships we always deal with two different kinds of wind - the "true wind" which we can see on the flags in port and which the weather report tells us. This wind is important for the planning of the route. For the trimming of the sails it is useless. Here the only wind that is of interest is the "appearant wind". This is the wind how we feel it on board, which we can see on the flags and wimples on board and which we use for steering.  Therefore in this text is always meant the "appearant wind" when the term "wind" is used. It is a vector of the "true wind" and the head winds. It relies on the angle between the course we steer (aka the ship's axis) and the "true wind" and also relies on the speed we have.

Let us first imagine we have only one single mast with one square sail. With wind from astern falling into this sail the ship will move forward. That is logical. The only problem is that at very most occasions the wind is coming from another direction. So there must be other possibilities. Furtheron sailing with winds from astern is not really perfect as the wind looses speed in the same way as the ship gains speed forward. So even if you forget about streamings and waves this means that a ship sailing with winds from astern can maximum reach half of the speed of the true wind that hits the ship. On a ship with more than one mast things are even worse as the sails of the last mast would take all wind away from all other sails which would become absolutely uneffective. So which possibilities do we have to use the wind best with a fullrigged ship?


For a good trim the yards get braced into a position that cuts the angle between the wind and the ship's axis in half. That way it is guaranted that all sails are filled from the wind and help move the ship forward. Unfortunately this is terminated by a yard position of 30° towards the ship's axis as now they already lay on the shrouds. This means that in such a way you cannot sail a square rigger nearer to the wind than 60°.


A ship of the size of the MIR now provides 2 further possibilities to optimize the work of the square sails. The first is fanning of the yards. This means that the yards get braced like a fan. The higher the sails are over deck, the tighter the yards get braced. This is possible through 2 facts. First the shrouds are not hindering the upper yards as much as the lower ones. Second is that the direction of the wind changes in accordance to the height above the water. Means the wind in 40m above water comes more from behind than on the deck. Therefore with courses of less than 90° the sails have to get trimmed fanwise.


Another way to optimize the work of the square sails is the additional use of fore-and-aft sails, especially when sailing by the wind. Sailing with winds from astern the fore-and-aft sails are of not much use and as a rule most of them are not set. With courses of more than 110° they add only little to the speed of the ship. More interesting  are fore-and-aft sails in halfwind or by-the-wind courses. To understand why we first must look how these sails work. A fore-and-aft sail, e.g. a staysail has almost the shape of a wing of an aeroplane. The aerodynamic powers are also comparable. On the curved lee side of the sail the wind flows faster than on the aerodynamically flat windward side. Thus in lee you find low pressure that tows the sail outward. This pressure produces the forward movement of the ship.


If we have several parallel fore-and-aft sails (e.g. the head sails) standing in relatively small distance to each other, they produce a special effect, which is particularly important for a good trim. With an optimised trim the wind gets pressed with high speed through the gaps between the sails. This speed leads to a lower pressure between the sails compared with the air streaming along any sail on its own. This low pressure produces propulsion as it practically sucks the ship forward. To achieve it, it is necessary that the leeches of the headsails stand absolutely parallel. In the luffs there shall not be any curvature, as this would produce disturbing turbulences. It is also necessary to check that the downward winds produced by the headsails or staysails do not hinder the work of the square sails standing behind them. As a rule a compromise that suits the very situation must be found. Normally the staysails need correction from time to time. 


If we cannot brace the yards any tighter, how can it work then that the MIR can sail 30° by the (relative) wind? What else can we change?  All would be very easy if we had only fore-and-aft sails. Then we could sail MIR like a schooner. Unfortunately the sail area of these sails alone is not enough to achieve some speed by the wind. So what to do? One could try to integrate the square sails into the system of fore-and-aft sails. Thus we brace the yards as tight as we can. Means they stand in a 30° position towards the ship's axis. Now we trim the headsails and the stay sails into an almost parallel position towards the square sails. Now we steer carefully higher to the wind until the wind starts streaming between the fore-and-aft sails and the square sails. Due to the low pressure the square sails get curved forward and the ship gets sucked forward. The effect hits the entire rigging. Now the helmsman has to steer very carefully in a way that the yards now do not cut the angle between the wind and the ship's axis in half but stand almost parallel towards the wind. The higher the ship sails towards the wind, the faster the (relative) wind becomes and the faster the ship gets. Attention: if the wind falls into the sail in an angle of less than 10°, the ship slows down again as now friction and drift grow.
As sailing that hard by the wind leads to a heavy list, as a rule the royal sails and the courses get clewed up. Taking away the royals lessens the list. This is very necessary if you keep in mind that the underwater lines of the ship produce the more friction the more the ship heals over. To take the courses away is mainly a precaution to avoid damage if they  suddenly come aback due to an unexpected squall or a mistake of the helmsman.


Most important for the sailing capacities of a ship are movements along the vertical axis - the yawing. Only if the sail area before the pivotal point of the ship is the same as the sail area behind it, the ship moves forewards. (When tacking or wearing the ship you alter this to make the ship turn.) The sail area here does not mean the square metres of cloth, but how much of it actually work to move the ship forward. If due to bad trim the forces are unequal, the ship gets a tendency towards or away from the wind, which has to be corrected by the helm permanently. However, any movement of the rudder slows the ship a little bit. Means if possible the wheel should be turned as little as possible. It is better to trim the ship in a way to keep it on course without much effort. Sometimes the trim is that good that helming can be done with quarter turns of the wheel to either side ( 1/8 degree rudder position!). As a rule the ship is trimmed in a way that makes her turn slightly towards the wind.


With a good trim the helmsman can steer the ship after wind and sails. This means he does not receive the command to steer a certein course on the compass nor is he told to keep a certain position towards the wind, but he watches the sails. The highest sail on the main mast is the steering sail. If it is completely filled with wind the position of the ship towards the wind is perfect. When steering by the wind, it is possible to steer higher until the windward side leech slightly starts flapping. Leewards the sail then still stands full and drives the ship forward. The angle towards the wind is now between 30 and 35°. The ship almost finds her way alone. It is practically not necessary to check the angle towards the wind on the instruments as one can feel by the movements of the ship if everything is still okay. A change in the sounds of the wind or if the speed goes down - those are signs for a changed angle towards the wind. This can mean that it becomes necessary to fall off a little and then steer higher littly by little again.


The ship lies better in the water when the wind streams along the staysails. The sails now get trimmed parallel to the direction of the wind - even if the wind comes directly from the front. They work like stabilizers on big passenger ships - only that they are not below the waterliine.


Even then it is necessary to brace the yards to keep the centre of gravity of the ship directly above the deck. It also helps to reduce the working surface for the wind and to save fuel.

this page was updated 10/07