Ship Handling 101 – Propeller Forces

After more than two decades of service in the United States Navy, many of these spent driving, better known as conning, or managing the ‘deck’ I’ve learned a thing or two on how to handle ships. These experiences covered a variety of ships including aircraft carriers. Understanding the effects of weather, the characteristics of ships themselves, and an interaction of forces is vital to safely navigating any vessel on the water. This is especially true for anyone handling large ships as they are much less responsive than smaller recreational vehicles. Given all this experience and the many hours spent learning these concepts, I thought I’d try to distill some of this into easily understandable hubs hoping it assists someone!

One of the first forces to understand when guiding a ship is the impact of the propeller. I’ve driven vessels one, two, and four propellers with each configuration having peculiar tendencies. Often referred to as right- or left-handed, the first thing you need to figure out is which directions your screws turn. To determine this, pretend you’re standing behind, or astern, of the vessel watching the props turn. If it turns clockwise, it is right-handed. This means the vessel will tend to “walk” in that direction if no other force is applied. Many dual propeller ships have one left- and one right-handed propeller that you would think would cancel out the effect of this “walking,” but in reality one will usually overpower the other due to mechanical imperfections and as a ship handler you must recognize and identify which direction your ship will move. Fortunately, this ‘walk’ in dual, counter-rotating props produces a very slow movement of the ship’s stern that can actually be used in close aboard (near other objects) situations to position this ship.

The way a propeller works is by sucking water through the screw and then discharging it astern. Oddly enough, this is known as suction and discharge screw current. This process will produce three forces; horizontal, vertical, and prop thrust. Without getting into too many technicalities, the pressure differentials surrounding the propeller blades will produce a “lift” effect that is resolved into the horizontal and vertical thrust components. The horizontal thrust produces movement in either a forward or backwards (aft) direction while the vertical lift is actually normally not noticed. Due to the design of most surface vessels, the vertical lift that tends to lift the stern out of the water is offset by the tendency for the prop, deep in the water, to also lift the bow upward. Unless the prop itself is aimed toward the sea floor, the two effects will normally not produce a visible effect. Anyone, however, who has travelled a high rate of speed in a pleasure craft will have noticed the bow lift out of the water as you increase in speed. This means the vertical lift of the rather small prop and effect of the prop’s depth, being fairly shallow on most small boats, produced more bow than stern lift. Even in the larger vessels of the Navy, bow lift can be observed if you get the ship moving fast enough! The third force, prop thrust, is harder to quantify and explain. Basically the propeller’s rotation means the discharge screw current is not flowing in a straight line out of the prop. Instead, it has a bit of a twist that acts on the prop shaft casing an equal but opposite torque. This phenomenon imparts directional force acting somewhere between the vertical and horizontal planes. These three forces produce the ship’s movement.

Now that you’ve created movement, there needs to be a way to control direction. On multi-shaft vessels, you can control direction with the engines. Placing one engine in a forward position and one in reverse (astern) will cause a rotation on the vessel, but it is also very hard on the ship! The next ship handling lesson will cover rudder forces – a very common method of directional control, but there are also variations on standard propeller geometry that can affect speed or direction. A Kort nozzle is a shroud around the propeller (imagine putting a fan in a metal can open on both ends) that will provide greater efficiency at lower speeds. Think of it as a funnel that helps eliminate the lost efficacy from prop thrust. These are especially useful at lower speeds, but do not have much affect at high speeds or while moving astern. Bow thrusters are often placed on larger vessels because they are extremely hard to control. These provide lateral control to move the ship sideways or assist in low speed turns. Similar to Kort nozzles, they tend to lose efficacy at high speeds. Unlike the nozzles, they remain just as effective while using astern propulsion. Finally, an inboard/outboard (I/O) or straight outboard engine can often be turned. These are often mounted on a swivel and manually turned using a handle near the boat’s operator. By changing the direction of the prop, the vessel’s direction is changed without the need for a rudder.

For those that need a rudder, the next hub will explain rudder forces. Eventually, you’ll understand exactly how that ship you just saw on the evening news became grounded or collided with another vessel and wonder just which mistake crew made!