BOAT BACKING OPERATIONS
By Butler Smythe
Yes, I know - you just bought a feathering propeller and the individual who sold it to you said that it will solve all your backing problems - guaranteed. Well, I can probably tell you what company said that because not all companies will, and for good reason. One of the reasons we can back-up as we would like is mental. It's the new outlook on life that came to us with our new, albeit expensive, propeller. Well there is something else to consider. It's your boat and the effects of your propeller. Winds, current and other environmental factors are not specifically covered here and can impact any boat, but in different ways. They do need to be taken into account by the operator, but please read on.
First, I want you to do something for me as well as yourself. Go to your boat and start your engine(s). Hopefully more than one of you does this hence the "s." Center your rudder and lock it. Put the boat in gear - in reverse (at the dock and tied up please). Now comes the test - and a small survey. With the boat in reverse, give it some throttle (about 2000 just for grins). Look at both stern quarters and the water there. You will probably notice that the starboard (right) side has a lot of disturbance ("high pressure" or H). The port (left) is smooth and relatively undisturbed ("low pressure" or L). These pressure differences are a result of the increased disturbance/water pressure or lack there of. If you had 2 identical boats next to each other (try your buddy in the sailboat next to yours or any boat that has a different propeller (fixed vs. feathering) and compare the difference. I think you'll find that the comparison of the disturbed water, fixed vs. feathering, will surprise you. They're about the same.
Note: The disturbance and resulting pressures generated will vary depending on the proximity of full or partial docks, piers or seawalls. The closer and fuller the structure, the more pronounced are the effects. Have you ever been swimming in a pool and passed by a flowing water source? Pushes you around doesn't it. Same effect a dock can have on a boat as a result of your own propeller or that of another.
Next - throttle back - go to neutral and do the same in forward. This time - look at the disturbance that results on both sides of the rudder. See a difference on either side? Now move the rudder until the two sides are equal in terms of the disturbed effect and note the wheel position. Try it with a higher RPM and note the amount the wheel has to be moved to compensate. This effect shows the propeller induced water flow over the rudder that can affect your ability to control/steer the boat. You are looking at something similar to the weather or lee-helm you may need to compensate for when sailing. Please note that there is some of it whether you have a fixed or feathering propeller.
Also think of your rudder as a flow control device that can direct a flow of water to enhance your maneuvering in tight places. Tied up to the dock you can move your boat about by simply steering/directing the water flow over the rudder. It's not quite a jet ski, but some of the same effects can be observed. Try it. Note also that too much rudder can have a negative effect (stall). That's why you should NEVER back the boat using full rudder without first establishing good water flow over both faces of the rudder. You'll stall the rudder much like an aircraft wing will stall if there is not sufficient airflow over both upper and lower surfaces. The hull, rudder, keel, and all the sails help or hinder the boats sailing/motoring abilities. So can the dodger and bimini. Most real racers don't use or remove the latter for both safety as well as performance.
Some PROP FACTS:
1. EVERY propeller imparts torque to the shaft and thence to the boat.
2. EVERY propeller will cause the boat to walk to port or to starboard, to varying degrees, depending on the rotation (fwd. or rev.), rotation (left hand (LH) or right hand (RH)) as well as (to some degree) blade pitch orientation (more identifiable with fixed props).
Note: The feathering props many are purchasing these days will actuality help you for two main reasons:
They will decrease drag under sail and - by design - they facilitate a "more optimum" blade orientation in both forward and reverse (some better than others). Notice how I didn't say optimum, just "more optimum." This is because the blade shape and leading edge orientation are the same in both directions because the blades reverse themselves. Each design has its positive and negative traits.
Other point(s) of fact:
RH turning props will help move the boat's stern to starboard in forward and to port in reverse (bow just the opposite and it moves faster) for three main reasons:
1. Water density makes the bottom of the blade stroke more powerful than at the top (hence the relative walk). The upper blades proximity to the hull also decreases their effectiveness.
2. Blade shape and orientation creates a driving pressure that wants to screw the prop through the water. When the prop is unable to move the boat, the torque is allowed to move the stern in the respective direction. Blade shape also affects its performance, and in reverse, a fixed blade's best bite is not at the back of the blade where it ideally should be.
3. The prop, especially in reverse, creates a high-pressure (prop wash) area that pushes (lifts) the hull or rudder face towards the opposite side of the boat that has a lower pressure area. The aircraft wing is a very close example. In reverse, the high-pressure area gets trapped against the full keel. There is a more dramatic effect at slower speeds because the boat sits for a longer period of time in the wash that helps to push the hull to the left.
Note: The effects of #2 will be lessened by a feathering prop due to its ability to reorient the blade's true leading edge, reversing itself more optimally and lessening the other negative effects. The fixed prop has more "walk" than the feathering because of the blade's "bite."
I think you saw an example of that high-pressure area with the boat in the slip.
Things you can not change:
1. Water density - ALL boats will have a tendency to walk to one side or another.
2. The hull - which traps more of the higher pressure when in reverse (less of that effect with a fin-keel but they have other "negative" traits).
Heavier displacement full keel boats have a tendency to react a bit slower to the effects of the high and low pressure areas than their fin keel brethren. But even differences will occur between various full keel boats, due in no small part to the hull's shape which can trap various pressures for longer periods of time. Boat speed and throttle will help as well as HARM you in your "slip dance."
There is a sailboat manufacturer, that up until recently, had their engine offset in the main cabin (under the sink). The shaft actually exited the boat well off centerline. Imagine what difficulties people would have with this arrangement when using their engine.
Insight on backing out of or into a slip:
1. Get initial momentum first (with rudder centered!) by pushing/propelling the hull, without using the propeller, so that the rudder has the driving effect (bite) over the prop. Engine in gear with throttle at idle works as well. The prop wash/pressure is distributed over a greater underwater area faster with less negative effect.
2. Do NOT look at the stern. The bow will move sooner/faster (boat will pivot) and it will be more evident long before the stern will display a reaction. Use the bow as your guide! You will be able to observe and correct earlier by using the gear shift lever (neutral – rev or fwd) or the wheel).
Note: You can counteract the bow's movement, to some degree, by counter steering. As an example: When moving astern to starboard and you want to kick the stern to starboard more, move the throttle to neutral and then forward. Give the boat some throttle and steer the boat to port (no need for a hard over helm) even though you may have just had the helm to starboard. The kick of the prop combined with the stronger flow of the water from the prop aft and over the rudder's surface will direct the flow to port. It's your cheap jet drive.
In addition:
1. Should you want to go to one direction or the other (i.e. back to starboard), you can assist the process by putting your crew on the opposite side of the boat (port). The additional heel releases the higher pressure created by the prop on the starboard side, which is allowed to reach the surface faster due to reduced entrapment. In addition, the heel to port will force to stern to "head up", much as healing a boat under sail does when going to weather.
If the boat gets away from you just stop, use the force of the prop's effects (use neutral, forward or reverse, with the appropriate amount of power) to get you back to where you want to be and then start again. Too much movement of the stern to port can be countered by shifting to forward and too much to starboard by a shift to reverse. You can actually sideslip the boat using the throttle and the wheel, much as you would parallel park a car. It is not easy, but practice (try it) will help you know your boat under power that much better.
Speed is not necessarily "life" and rushing in and out of the slip does more to hurt than help you. Of course if the current or wind across the slip necessitates this, more proper use of lines to the dock may be warranted. Try and avoid pivoting on filthy and potentially damaging pilings at all costs.
The Big Picture (from the dock)
In forward or reverse the boat will go in a straight line when there are equal relative pressures on either side of the keel and rudder. Change that equilibrium with current, heel, pitch, wind or rudder, and the boat will turn one way or the other.
Those that have a heel angle at the dock will have a harder time sailing and powering because you have "negative" helm inputs even before you leave the dock. Ever been bugged by crew moving about the deck when you are trying to enter a slip or are sailing in light winds? Think we've all been there. It will affect the boat's performance. This is very evident the more sensitive the boat is to crew weight.
Remember, under sail or power you are working with your hull form, keel, rudder and propeller. All can be manipulated to some degree. Reducing the variables makes operating under power that much easier (no wind and current effects are discussed here).
Try to imagine two surface planes - the keel and the rudder. Both in this case, have high pressure on the starboard side and low on the port. The entire boat will move to port. With the reverse situation the boat will move to starboard. In most cases you will see a combination of the two since the effects of the hull and rudder are not perfectly equal.
With the propeller spinning in reverse there is more high pressure to starboard of the keel, the torque of the prop (walk) pulls the stern to port and the rudder has water pulled from the port side of the rudder creating a low pressure area. You want higher pressure on the rudder to port to compensate for the keel and propeller. You can't help but get a difference in pressure due to the prop and other effects. To get that equalizing pressure you need water flow over both rudder surfaces. The rudder is your sail and if you steer it too high/far it will stall..
Now what happens when you try to go in reverse to starboard and you goon it (i.e. the bow doesn't go where you want it to (i.e. to port)?
Cause - you probably have over steered the rudder and stalled it. (You may have seen this when you have the rudder full starboard or port and the boat is not obeying the command. Start to reverse the wheel and all of a sudden you regain command, not necessarily in the direction you want.
Solution – Center rudder, give FWD throttle to kick stern to starboard (use port helm if necessary). Regenerate a more equal flow of water over the rudder, reverse again and slowly turn rudder to starboard from a relatively neutral position.
Note: It is highly advisable that you mark the wheel for a centered helm (both visual and touch). Do not use a visual on the rudder itself. You'll loose track of bow's movement.
To practice, go into open water on a calm day and put something in the water (your dinghy, a float, or a buoy attached to an anchor. Try starts from the object on the bow, attempting to control the boat by your commands to the helm and the engine. Try using people on board as well. In addition, you can practice backing up to this object, simulating your dock, without the damaging potential.
Here are a couple (3) common backing situations and solutions (in brief).
In a straight fairway with helm to starboard, bow to port but moving in a straight line. You try to correct by moving the wheel to port but now the boat increases it's turn to starboard. (Process: Idle - Neutral - FWD - apply throttle as necessary with helm to starboard until bow centers on fairway - idle - neutral - REV - watch bow and steer appropriately. The water flow in FWD, over the rudder, will counteract the walk of the prop to port.)
In backing practice you have a heavy helm and release the wheel. The wheel goes hard over and you have a difficulty time regaining control. (Process: Idle - neutral - FWD. Aft wards flow of water will now ease the strong pressures of the boat moving in reverse (you have a barn door back there). Now regain control by slowing the movement of the bow with the wheel, neutralize it and then get it moving in the direction you want and then REV the boat again. You now have control and the boat may still be moving backwards.)
At the dock, while trying to backing out to starboard, the bow starts to move to starboard. (Process: Idle (should be there) - FWD - apply throttle to force water/pressure to the bow - steer lightly to port - as bow centers and passes to port - idle - REV - reverse helm to starboard, just past neutral, and try again, watching the bow).
Bottom Line:
FACTORS THAT EFFECT STEERING
Helmsman
Hull shape
Balance of boat including heel and fore and aft balance (pitch)
Keel type and shape
Rudder shape and type
Propeller
Area around boat including bulkheads and docks
Wind, current, etc.
If the stern is heavy the bow will be affected by wind and current more easily.
If the bow is heavy, the effects of weight placement will be more evident (i.e. weight on the port side will make the boat head to starboard as if the boat was heeled over by the wind while sailing and it heads up to weather.
A very good but simple reference to much of this is in Royce's Sailing Illustrated (see balance variables, rudder/sail balance, sailboat speed factors and prop torque ideas). As far as I'm concerned, every sailboat sold (new or old) should have one aboard!
