While sailboats rely on the wind to propel them through the water, they also have some similarities with power boats, the propeller. A sailboat powered by a propeller will handle very differently as compared to the way it will handle under sail.
First, all the power is hidden from sight. When sailing, you can easily see how each of your sails is behaving. Is it full? Is something blocking it? Is something impairing it's ability to work? Is one sail overpowering the other? With a propeller, all of these factors still come into play, but they occur under your hull and hidden from view.
Propellers are composed of blades which are arranged to push water in a certain direction, and they do this very well. They come in all sorts of shapes and styles, purpose built and designed to perform their best at the given task.
While all propellers are purposefully designed for their selected task, one thing they all have in common is they all work best in forward. Propellers are usually used to drive a vessel forward for many miles, and only used in reverse for short periods of time while maneuvering. This means that your propeller is designed to move you forward with ease while sacrificing reverse performance, making close quarters maneuvering all the more interesting.
One of the most "interesting" thing about propeller powered vessels in reverse is a phenomenon called propeller walk, or prop walk for short. When you put a sailboat in forward, it will move forward; but when you put it in reverse, you will move sideways! To a beginner, this might seem like evil magic that has cursed the boat as he tries to enter his slip. The curse that has come over his boat as he tries to maneuver is called prop walk, and it can work against you or be used in your favor once you figure out what is going on.
In the simplest of explanations, prop walk is the lateral result of the propellers rotation. In forward, the water is being thrust over the rudder, giving you forward motion and steerage. A slight and unconscious adjustment of the helm will cancel out any of these effects, making it feel like it doesn't happen in forward, but it does. You typically put the boat in forward and head off to a distant destination, so any wavering from your course due to prop walk goes unnoticed. When you put the boat in reverse, you tend to be very close to your destination, so any lateral wavering is greatly noticed and not appreciated.
The reason it feels more pronounced in reverse is the rudder has no effect on steering at slow speeds, so the lateral movement from the propeller goes unchecked. As you begin to waver from your course, you frantically turn the wheel trying to correct your course, but to no avail! Tempers rise as the boat seems to have a mind of its own.
To help you understand why your boat is doing what it is doing, lets look at what causes prop walk in the first place.
Prop walk is amplified by:
- Deeper location of the propeller
- Enclosures near the propeller
- Angle of the propeller shaft
- Speed that the propeller is rotating
- Hull shape and keel style
Deeper water will amplify prop walk because deeper water will provide more propulsive force, conversely, shallow water will provide less propulsive force. The lower blades of the propeller will provide more thrust from the propeller, while the upper blades will provide less. This difference in thrust will produce a bias which results in a lateral component to the propeller spinning. If the propeller is in very shallow water, prop walk is reduced because the entire propeller is not producing much thrust. In deeper water, the propeller will produce significantly more thrust and this difference in thrust from the upper blades to the lower blades will be amplified, resulting in more noticeable prop walk.
The water around the propeller will also play a huge role in the amount of prop walk. A propeller that is exposed to clean and clear water will show fewer signs of prop walk when compared to a propeller that is quite restricted. A propeller inside an aperture will produce more prop walk because of the apertures tendency to cause water to shoot laterally from the aperture. The top of the aperture will shoot a weaker jet of water because it is in shallower water while the bottom of the aperture will shoot a much stronger jet of water because it is in deeper water. The same propeller will have significantly more prop walk when placed inside an aperture as compared to one that has nothing around it.
You can see the different angles of the blades on these angled shafts. The blade in the background is nearly vertical while the blade in the foreground is quite aggressive. The propellers on this boat are set in opposite rotational directions to help cancel out any prop walk.
An angled propeller shaft will cause more prop walk simply because it will have an adverse effect on the angle of the propeller blades. If the propeller shaft is set on an angle, one side of the propeller will have blades that are significantly less effect, as they will be nearly vertical and with negligible thrust and nearly horizontal on the other side with significant amounts of thrust. At low speeds, the vertical blade will have no effect and the horizontal blade will push laterally, unopposed by the vertical blade on the other side; causing a lateral push which will cause prop walk. A horizontal shaft with no angulation will reduce this problem, as the propeller will be set straight with the blades on both sides of the propeller working at similar levels of effectiveness.
Knowing that a horizontal prop shaft is ideal, why don't more boats use this style? Because the engine needs to be mounted at the other end of a straight propeller shaft. Deep keeled sailboats can mount the engine low in the bilge and have the propeller located aft of them on a horizontal shaft, but all other boats with a straight shaft will need to angle the system to allow the engine to be mounted at a higher position than the propeller. The only way around this is to use a sail drive which we will talk about later.
Speed of the propeller has a huge impact on prop walk. At slow rotational speeds, prop walk is not as apparent because there is less turbulence created by the spinning propeller. There is also less thrust produced, so any difference in thrusts will be less notable and therefore less apparent to your maneuvering. A slow moving propeller will gradually move the boat in a set direction without causing much in the way of secondary effects. A fast moving propeller will generate a greater amount of thrust and a whole host of secondary effects, most notably, prop walk.
Hull and keel shape will be the final component in the generation of prop walk. A shallow bilge sailboat with a thin fin keel will have much less prop walk when compared to a deep bilged full keel sailboat. The shallow bilge sailboat will not interact with water coming off of the propeller as much. Whatever water from the propeller that does reach the shallow bilge will have much less thrust, meaning that the water interacting with the underside of the boat will be much less powerful. Since the thrust form the propeller will have less effect on the hull, less prop walk will be noticed, allowing the sailboat to reverse more predictably.
A deep bilge hull will interact with deeper water where more thrust is present and the effects of prop walk will be more prominent. Deep bilge hulls tend to have full or long keels, which will actually separate the thrust from a center line propeller. This means that thrust from the ascending blades with their greater thrust will be pushed down one side of the hull and the descending blades with their weaker thrust will be pushed down the other side of the hull. This stark difference of great thrust on one side and little thrust on the other side will result in significant prop walk!
Source: http://www.x-yachts.com/range/xp/xp-50/#PhotoSwipe1471987985724
To tie all these individual factors together, the ideal propeller situation for minimal prop walk would be:
- Shallow bilge
- Shallow propeller
- Low RPM
- Propeller free and clear of all obstructions
- Propeller mounted horizontally with no angulation
Source: https://upload.wikimedia.org/wikipedia/commons/f/f3/Saildrive-First-40.7.JPG
If you have a shallow bilge, how can you mount the propeller with no angulation? The sail drive! A sail drive will mount the propeller free and clear of any obstruction with no angulation. This is one of the reasons sail drives are so popular on sailboats, they provide a clear and unobstructed propeller mounting location where the engine can be located higher than the propeller with no angulation issues.
The ideal set up for the worst possible prop walk would be:
- Deep bilge
- Full keel with the propeller set in an aperture
- High RPM
- Propeller mounted close to the rudder post
- Propeller mounted on a steep shaft angle
This setup would yield a boat that would move forward and sideways!
If you have significant prop walk, running your engine at lower RPMs would help alleviate the issues. If you find yourself wishing to back into a slip, try using light bursts of low throttle. This low throttle will keep prop walk to a minimum while moving the boat astern. Once you pick up a bit of speed (which will be light since you are not giving it much throttle), put the motor back into neutral, allowing it to drift backwards without any influence from the propeller. This will allow you to drift backwards in a predictable manner without suffering from the lateral thrust of prop walk. Learning how to use prop walk to your advantage may be a better solution.
Prop walk will reliably push your stern in the same direction. Right Hand propellers will push your stern to port, Left Hand propellers will push your stern to starboard. If you have significant prop walk, you can use it as a stern thruster. If you find yourself on a pier or sea wall and need to get off of it, applying hard throttle will actually push your stern off the pier (if you are set up to prop walk away from the structure) like a stern thruster, but without all the complexity or added maintenance of an actual thruster.
Once you know which way your boat will walk, and how strong the prop walk is, it is important to note one additional feature of this phenomenon. Prop walk will do the opposite if your boat is moving forward with speed. This means that if you have a LH propeller and walk to starboard when you put it in reverse from a standstill, you will actually rotate the stern to port if you put it in hard reverse while moving forward with speed.
Not all boats do this, so be sure to test it out in open waters so that this little feature doesn't catch you off guard during close quarters maneuvering. This phenomenon is caused by the confused water rushing past the propeller as the propeller is spinning fiercely in the opposite direction. The disturbed flow will cause the boat to pivot in the opposite rotation from the norm when not moving forward.
Before you begin cursing your boat for making you look like a landlubber who "tried to park his boat" when you attempted to back into your slip, figure out which way it wants to walk and learn how to use it to your advantage!
