Motors and Propellers

A motor drives a boat by turning a propeller which displaces and redirects water underneath the boat. Much like our earlier discussion of wind aerodynamics, this movement of water creates forces that are reacted by the boat, propelling it through the water. The movement of the propeller primarily creates a flow of water directly behind the propeller, known as prop wash. Similar to sailing against the wind, a less desirable side force, called prop walk, is also created as a byproduct.

Inboard vs Outboard Motors

There are basically two types of motors used on sailboats, inboard and outboard. An outboard motor hangs over the transom and can be turned just like a rudder (using a handle that acts like the tiller). Outboard motors steer the boat by directing prop wash, replacing the rudder. In fact, when using an outboard motor, the rudder should be taken out of the water.

Inboard motors, on the other hand, are housed inside the boat. Only the propeller protrudes from under the boat, just forward of the rudder. The propeller is in a fixed position, always directing the prop wash straight aft. The direction of the boat is controlled by the rudder moving through the water (or sometimes the prop wash acting on the rudder), but not by the prop wash direction alone, as in an outboard motor.

This distinction is the important contrast between inboard and outboard motors. Inboard motors rely on the rudder to control the boat, prop wash only acting to propel the boat forward, but not to change its direction. Outboard motors use prop wash to control the boat directly. Accordingly, outboard motors provide greater maneuverability. However, since most larger boats use inboard motors, it’s important to understand how to operate inboard motors as well.

Fueling

Filling up your gas tank on a boat is slightly more complicated than your car, but not by much. The main dangers of fueling are fire and carbon monoxide poisoning. Common sense will reduce these dangers. Don’t smoke or have any open flames or sparks when fueling. Keep enclosed spaces well ventilated by opening windows, hatches, etc. Follow the Coast Guard’s fuel safety checklist and you should have no problems.

Fueling Safety Checklist

Engine Controls

Controlling a boat engine takes some getting used to. Typically, inboard engines are controlled by two levers. One lever switches between gears – forward, neutral, and reverse – while the other level adjusts the engine speed.

As you can imagine, operating these two levers, plus the wheel, takes some coordination, and can be tough if you’re in a tight spot and aren’t completely sure what you’re doing. All the more reason to become comfortable with the concepts and maneuvers covered in this section and practice whenever you have the opportunity.

Cavitation

When you find yourself in a tight spot, drifting the wrong way, the first impulse is to crank up the throttle to try to change direction. More often than not, this doesn’t help. Either the boat doesn’t respond in the way you expect, or it doesn’t respond at all. All you hear is a high pitched whine. Unfortunately, if you rev the engine too quickly, you can cause a phenomenon called cavitation.

When the propeller moves too quickly through the water, small bubbles form all around the propeller face. When these bubbles pop, tiny water jets impact the propeller, causing damage. And of more immediate concern, the propeller basically spins freely, through the air bubbles, not creating any prop wash to drive the boat.

Cavitation is most likely to occur when the propeller is close to the water surface, which is easy to prevent with an outboard motor. Just make sure the propeller shaft is dropped as far into the water as possible. With an inboard motor, there’s not much you can do to change the propeller depth.

Usually cavitation is rare unless the engine is too powerful for the propeller size. Just be aware, if you hear a high pitched squeal, your propeller is not doing the work you’re intending. Ease off the throttle a bit, and if possible, drop the propeller deeper into the water.

Controlling the Boat Under Motor

As we alluded to earlier in the discussion on inboard vs. outboard motors, a motor boat is controlled primarily by redirecting the thrust of the propeller, or prop wash, either by directly turning the propeller shaft on an outboard motor or by using a rudder on an inboard motor. Prop wash, the side force byproduct of propeller spin, can also be used constructively.

Propeller Forces in Forward Gear

Unfortunately, the rudder is only effective when the boat is travelling forward, or there is a large burst of prop wash. This fact is the main reason that maneuvering a boat in tight quarters is tricky. There are no brakes, and you need to be moving to have any control at all using the rudder.

A rule of thumb for delicate maneuvering is to use the minimum power required to maintain rudder control. There’s no need to go any faster, but go any slower, and you’ll be at the mercy of the current and wind.

Prop walk on the other hand, does not require headway, but it is much more pronounced when the propeller is going in reverse. This, and the fact that it acts perpendicular to the boat, makes prop walk ideal for making sharp turns. However, it only acts in one direction, so it must be used carefully.

Prop wash, can also be effective without headway, but only in short, strong bursts. A burst of power creates a quick jet of water that can be redirected sharply by the rudder to create a side force, similar to prop walk. Since the rudder is aft of the propeller, this method only works with the engine in forward gear.

Accordingly, one method for turning sharply is to alternate between quick forward and reverse engine burst to take advantage of both prop walk and prop wash, as we’ll discuss later.

Propeller Forces in Reverse Gear

Wind and Current

Aside from the propeller and rudder, which you can control, there are also outside forces acting on the boat that you can’t control. It’s important to be aware of how wind and current will turn the boat, so that you can best counteract them or use them to your advantage.

Yes, wind will act on a boat even when the sails are down. The wind will push on the side of the hull – most prominently on the bow, since it has the highest profile – tending to turn the bow away from the wind. The current, on the other hand acts mostly on the stern, since it is the deepest part of the boat, turning the bow into the current.

It’s difficult to predict which outside force, wind or current, will dominate. Suffice it to say that if you aren’t actively controlling the boat using the propeller and rudder, one of these two forces will take over. Fortunately, while you’re inside a marina, much of the current and wind will be blocked, but you should still keep them in mind when planning your maneuvers.

Pivot Point

One big difference between boats and cars is the pivot point, which is the point around which the object rotates during a turn. All boats are unique, but as a general rule, the pivot point on a sailboat is about one third of the way back from the point where the bow touches the waterline. The pivot point on a boat is the same whether you’re moving forward or backwards.

A car’s pivot point, on the other hand, depends on the direction, due to the fact that the car has two sets of wheels. When going forward, the pivot point is near the middle of the car, roughly even with the driver. When moving in reverse, a car’s pivot point is between the rear wheels, behind the driver.

The “feel” of driving is determined by this relationship between the locations of the driver and the pivot point. So fundamentally, a boat feels different because the driver is located well aft of the pivot point, while on a car, the driver is right on top of the pivot point. Of course, that difference applies when moving forward.

When driving a boat backward it feels more like a car (going forward) because the pivot point is behind the driver (who’s now looking backward). For this reason, it’s sometimes more comfortable to move forward of the wheel when going backwards in a boat.

It’s also important to understand the pivot point, because boats are much longer than cars. At the helm of a boat, you must always be aware of movement of both the bow and the stern. It’s easy to be looking one way, behind the boat for example, focusing on the stern, and forget that the bow is swinging around behind you possibly colliding with a dock or another boat.

Differences in Pivot Point between Cars and Boats

Common Maneuvers

The figures and most of the discussion in this section assume a standard “right-handed” propeller, which turns clockwise when going ahead and counterclockwise when going astern. For a left-handed motor, all directions will be reversed, but the concepts will still apply. The orientation of the propeller can be checked by driving it in reverse with the wheel and rudder pointed dead ahead. If prop walk moves the stern to port, it’s a standard, right-handed propeller. If the stern moves to starboard, you’ve got a left-handed propeller.

For a normal, right-handed propeller, prop walk acts by pulling the stern to starboard when the engine is in forward gear. When in reverse, prop walk pulls the stern to port and is much more pronounced, as mentioned earlier.

When making sharp turns, prop wash can be used in short bursts to turn the bow either way. However, this action mainly rotates the boat, but doesn’t move it forward.

To turn the boat while moving forward, normal rudder control must be used, which requires headway and more room to maneuver.

These considerations along with the ideas discussed earlier lead to some general guidelines for maneuvering under power.

Guidelines for Maneuvering Under Power

Sharp Turns

The best way to turn the boat sharply in tight space is to use a combination of prop walk and prop wash to rotate the boat with minimal forward or backward motion. The rudder is set hard to starboard and held in that position. Then, the engine is alternated between short, powerful bursts in forward and reverse gear.

The forward bursts use prop wash to produce a quick thrust, turning the bow to starboard. The reverse bursts create a side force using prop walk that moves the stern to port. Both of these actions turn the boat in the same direction without much headway or sternway. The reverse bursts will produce some sternway, though minimal, so it’s best to begin this maneuver with a bit of headway to avoid drifting too far backwards.

Sharp Turns

Holding a point

A similar method is used to hold a point, i.e. a heading, which may need to be done while mooring or taking down the mainsail. To hold a point, short forward and reverse engine bursts are used as above. Here however, the rudder is set hard to port.

The reverse bursts still produce prop walk, acting to send the stern to port. The forward bursts, however, now send the bow to port, acting opposite the movement created by prop walk. In this position, you simply need to watch the bow and keep it pointed at an object in the distance (or into the wind using a wind vane atop the mast). When the bow moves too far to starboard, give the engine a quick forward burst. When the bow moves too far to port, give the engine a burst in reverse.

Again, there will be a slight tendency for the boat to drive backward, but this can be corrected by holding the engine in a forward gear with low power and moving forward slowly.

Holding a Point

Leaving and berthing

When leaving and berthing a slip, you have very little room to maneuver and must end up in a spot not much wider than your boat. The more you can think through your maneuvers in advance, the more you will be prepared to correct from minor deviations without panicking.

Generally, you’ll be backing out of a slip. A rare few boats will be berthed stern-in, but the concepts here can be adjusted accordingly without much difficulty.

The basic idea that you must keep in mind is that the boat can be turned much more sharply in the clockwise direction (see the section on sharp turns). Therefore, it makes most sense to turn that way when backing out of a slip. You’ll always begin by turning the stern to port once you clear the slip.

Leaving a Slip to Starboard

The next steps then fall into place based on which direction you need to go to exit the channel. When you’re leaving a slip to starboard, you go forward upon completing the initial rotation. When you’re leaving a slip to port, you continue backwards until you have more room to maneuver.

Leaving a Slip to Port

When berthing to a slip, the sharp rotation to port is used only as a final corrective maneuver to slow down and/or sharpen the turning radius into the slip. When approaching a slip to starboard, you simply make a smooth, controlled turn into the slip, preferably in one motion. A turn that is too sharp can be corrected with bursts of reverse, which will swing the bow around to port. If the turn is too wide, there’s no way to correct, because any reverse engine thrusts will cause prop walk, further exacerbating the situation. In that case, it’s better to simply back out of the slip as in the leaving procedure and head out to the main channel to start over.

Berthing a Slip to Starboard

When approaching a slip to port, you may be able set yourself up with the slip to starboard by reversing into the channel. In that case, once you’re past the slip, you simply proceed as above tuning to starboard. Alternately, if there isn’t room behind the slip, or you’re simply not as comfortable going in reverse, you can approach the slip going forward turning to port. In this case however, prop walk is less helpful for making corrections. While logic would suggest that a turn that is too wide could be corrected with a burst of prop walk to swing the stern back to port, the bow is likely to have bumped the dock by that point.

Berthing a Slip to Port