The present invention addresses the problems of wave for- mation produced by ships, which has acquired immediate importance because of the increased use of high speed catamaran ferries in internal waters. The problems con- cerning wave formation from ships and the impact of ships on the environment altogether, however, are not of recent date. In some waters, such as coastal waters, fiords, in- lets and the like, the natural conditions dictate a natu- ral navigation route and a lower speed. These natural limitations may moreover be accompanied by restrictions in the form of marked navigation routes and restrictions on the speed to reduce the impact on the environment as much as possible.

The creation of such restrictions is typically the result of collected observations over an extended period of time from the area concerned, performance of model tests, etc.

The object of the invention is to open up the possibility of a more direct monitoring of the impacts of the wave formation from a ship on the environment.

The invention provides a method which is characterized in that the speed of the ship is recorded together with the water depth, and then the Froude number is calculated ac- cording to the formula:

and that the Froude number is optionally also calculated according to the formula: where Vs is the speed of the ship, h is the water depth, Lw is the length of the ship in the waterline, and g is the acceleration due to gravity, thereby making it poss- ible to monitor the wave formation after a ship under way. Since wave formations in coastal shallow water areas are of particular interest, the depth-based Froude num- ber, Fr, is basically used.

Where monitoring of the wave formation in deeper water is additionally desired, the Froude number Fn is used. It has been found expedient to change from the depth-based Froude number Fr to the Froude number Fn, when the depth- based Fr is smaller than 0.6-0.7 and vice versa, i.e. to change from Fn to Fr when Fr is greater than 0.6-0.7.

To avoid large waves from the ship, the speed of the ship may be adjusted in dependence on the Froude number so as to maintain a speed at which the ship just generates waves of a specified acceptable size.

The speed of the ship may thus be reduced or increased if the Froude number is equal to 1 or approximately equal to 1, i.e. the master may either decide to reduce the speed of the ship or to accelerate to a higher speed if condi- tions so permit.

More specifically, the speed of the ship is reduced or increased if the Froude number is greater than 0.8-0.9 or smaller than 1.1-1.2, which gives a suitable margin around the critical speed of the ship where the wave for- mation is greatest. The largest waves are typically

formed at a Froude number about 1, where the ship sails at the same speed as the speed at which the waves propa- gate on the water depth concerned.

Instead of changing the speed of the ship, the course may be changed to deeper water, if possible, so that the wave formation will be smaller, even though the ship keeps the same speed.

The water depth is expediently determined with an echo sounder known per se, but the water depth may also be de- termined in another manner of course. The echo sounder is attractive, because, generally speaking, it is standard equipment on most ships and measures the depth with an acceptable accuracy.

The speed of the ship is expediently determined with a precision log, e.g. a GPS or DGPS navigator known per se, which is likewise standard equipment on most ships, gen- erally speaking, and which has the required precision.

Speed may also be determined in another manner of course.

The invention also provides an apparatus which is charac- terized by comprising: a unit for measuring the speed of a ship a unit for measuring the water depth a calculating unit for calculating the Froude number ac- cording to the formula: and optionally also according to the formula:

a unit, preferably a visual or/and an acoustic unit for direct or indirect signalling of the result.

The GPS or DGPS of the ship is conveniently used as the unit for measuring the speed of the ship, and the echo sounder is used as the unit for measuring the water depth. The rest may be embodied in an instrument intended to be placed on the navigating bridge of the ship so that the master has it within his field of vision. The appara- tus as such may be constructed as a card for insertion into an existing computer.

The signalling unit is expediently formed by a display, e.g. a digital display which currently shows the calcu- lated value of the Froude number, or it may be an acous- tic unit which emits a warning signal if the ship ap- proaches the critical speed, or in the form of light in- dicators.

The signalling unit expediently comprises indicating means which, as a function of Fr and/or Fn, indicate at least a lower and an upper permissible range as well as an intermediate critical range and optionally also a for- bidden range in the critical range.

In an embodiment the indicating unit comprises a set of indicators, preferably in the form of light-emitting di- odes which indicate: speeds lower than 0.8 x critical speed speeds in the range 0.8-0.9 x critical speed critical speed speeds in the range 1.1-1.2 x critical speed, and speeds greater than 1.2 x critical speed.

Expediently, the apparatus also comprises a recording unit which currently records at least the speed of the ship, the water depth and the Froude number. The record- ing may take place on an electronic medium or is cur- rently printed out on paper.

An apparatus according to the invention will be described more fully below with reference to the accompanying draw- ing, in which: fig. 1 shows an instrument seen directly in front view, and fig. 2 shows the instrument seen directly from one side.

The instrument shown in the drawing contains a computer known per se, which calculates the critical speed with respect to the instantaneous speed of the ship and the present water depth on the basis of the water depth based Froude number, Fr.

The instrument is connected with a DGPS (Differential Global Positioning System) navigator which calculates the position as well as course and speed extremely accu- rately, and an echo sounder which shows the instantaneous water depth, and the computer gets its input from there.

The three devices are connected by means of NMEA 183 (standard for maritime electronic devices and instru- ments) inputs and outputs.

When the computer receives the above-mentioned informa- tion, the critical speed is calculated according to the water depth based Froude number, Fr.

On the basis of the calculations, it is indicated on the front plate of the apparatus by means of a plurality of light-emitting diodes where the speed of the ship is at present with respect to the critical speed.

Fourteen light-emitting diodes are used, as follows: a) 4 green ones - indicate speed <0.8 x critical speed b) 2 yellow ones - indicate 0.8-0.9 x critical speed c) 2 red ones - indicate critical speed d) 2 yellow ones - indicate 1.1-1.2 x critical speed e) 4 green ones - indicate speed >1.2 x critical speed.

The master can then readily see whether he approaches the critical speed. If so, he has three options of distancing himself therefrom: a) increasing the speed so that he gets above 1.2 x critical speed, b) reducing the speed so that he gets below 0.8 x criti- cal speed, c) changing the course to deeper water to sail on.

In addition to the above-mentioned light-emitting diodes, the apparatus has an ON/OFF button and a DIMMER to dim the light at night.

The instrument is powered by the 24 volts DC power supply of the ship.

The instrument thus gives the master a current overview of the wave formation from the ship in the form of the speed of the ship with respect to the critical speed, and he can clearly see when the ship approaches the critical speed where the largest waves are formed. The master can then act in good time by increasing or decreasing the speed or changing the course to deeper water.