Nortek Signature VM Operation Manual

Nortek Signature VM Operation Manual Manual pdf 45 page image

45Grounding Point.This should always be submerged !When mounting the sensor in a ship’s hull or on another platform, make sure the grounding point alwaystouches the water.8.2.3 Ship movementsTry to avoid mounting the Signature sensor too close to the bow or to the stern. Instead, the best areas aretoward the middle of the ship. There are two reasons to pick an area toward the middle. First, the verticalmotion due to pitch and roll is minimal toward the middle. Second, the ship-induced flow field has strongerasymmetries toward the bow and stern. These asymmetries will reduce the accuracy of the velocity dataclose to the hull and are best kept to a minimum. Ship induced motion is usually a bigger problem the largerthe ship, the relative water depth is smaller, and the ship is more bulky (less "classic" hull structure). It isusually not a significant problem on small vessels or for Signature Sensors mounted over the side.The tilt angle of the installation should be as small as possible because a level sensor head makes it easierto interpret and analyse the velocity data. However, a few degrees of tilt does not make a great difference inthe accuracy of the horizontal velocity data8.2.4 Ship generated flow fieldsAll ships generate their own flow field. The magnitude of this field is a function of the ship size, shape, shipspeed, and the water depth. It is weaker away from the hull than it is close to the hull and it will be weakeron the side of the ship than it is directly below the hull. The magnitude of the flow field can be analysed bylooking for persistent vertical gradients in the velocity data.a) The flow field is strongest close to the hull and will usually "pull the water along with the ship". As a result,the sign of the velocity gradient will remain constant and it will not change direction as the ship moves in theopposite direction. If the gradient remains constant, regardless of the direction of the ship motion, it is not apart of true current regime, but an artefact of the ship induced flow field.b) When the Signature sensor is mounted close to the bow or to the stern, it will show a net vertical velocityclose to the hull. If the vertical velocity has a strong gradient (i.e. it is not constant with the water depth), it isusually a result of the motion.