Maneuvering a ship, particularly a large
one is both a science and an art. A science because
some of the forces are going to exert their influence, inexorably in accordance with the scientific laws of physics and vector analysis of forces, regardless of what we, as a skipper do. Yet it is an art, because the end result involves the "prediction" of what the wind, the current and the net result of ALL the forces combined will end up doing with the ship. Let's list the forces we are dealing with:
1. The thrust of the engine, as exerted by the propeller.
A. Fairly straight forward if simply going that way.
B. The forces are not so straight forward when we are backing.
C. But what if the ship was moving in reverse and we then apply forward thrust of the engine?
2. The propeller, in turn, going in a circular direction, imparts a circular flow to the water, which, particularly, in reverse, hits one side of the hull, creating a force in that direction.
3. How much force can the rudder exert? Going forward; relative to propeller forces? Going backward to port? Going backward to starboard?
4. The center of mass of the boat, as the equivalent point for the effect of gravity and momentum over the entire boat; and the laws of Physics that a mass at rest tends to stay at rest and when moving tends to continue moving in the same direction.
5. The center of lateral resistance: The point, below the water line, where there is equal area, in front of and behind, so the boat tends to turn around that point.
6. The center of wind resistance. The same point above the water line, that the wind tends to turn the boat around. The angle that the wind hits the ship, having a particularly significent role, in how much of the wind's force is going to effect the ship, at a given moment. And what happens if #3, #4 and #5 are in various different allignments?
7. We are use to our car, moving instantaneously, where we want it to go. Not so with a ship, in the fluid medium of water, without the traction of tires on a highway. There are several delays in the response of the ship. It turns slowly at first. And even then, tends to go "sideways" rather than track in the turn, going more forward than the track would indicate. When we apply the brakes by reversing the propeller, it takes far longer to stop, than a car; and the sideways force of the propeller, moving the stern, can at times, be considerable.
8. These delays mean we must decide well in advance, what controls to change.
9. That also means that by definition, the hazard we are trying to avoid or the point we desire to get to, is at some distance from us; giving us difficulty in judging position and distance, whether conning the ship from the bridge or on shore with our radio.
10. The human determination of distance, is far more often a matter of the relative size of objects, and FROM EXPERIENCE, "how they appear at various distances and at various angles, at those distances", than it is from "binocular range finder principles." Seldom, in our normal lives, do we have THE EXPERIENCE OF working with objects in the water. Therefore, a great deal of the difficulty we experience with perception in guiding our ships, is due to lack of data of what various things look like in relationship to each other, at various distances on the water. Thus the need for practice, ie: increasing our data bank of such things!
In the next article, we will go over how these forces act on a
single screw ship; since most in the club have these types of ships.
Particularly, in relationship to docking; perhaps the ultimate challenge
for a skipper, full size or model. It is certainly impressive to
have the skill to zip into a dock; briefly back it down and have the stern
gently touch the dock and stop; particularly as compared to the alternative
of repeatedly having to go backward and forward and probably ending up
hitting other nearby boats or the wind taking us away from the dock.
It takes a great deal of judgment, experience and skill, to apply the correct
force at the correct time, to combine with other forces we do not control;
to give the desired result. And also to recognize when "other forces"
are such, that we may have to find "some other way" to get into or away
from the dock, because such forces make the "direct approach" very hazardous.
PRECISION MANEUVERING OF A SHIP
The second assumption we will make is that the propeller when creating a forward thrust is moving: COUNTERCLOCKWISE. Note: this is the opposite of the way full size boat propeller's, most often go, when moving forward. Which can cause confusion, if you are reading about full size ship maneuvering. But most model's are rigged that way, because gas engines, designed to go into model airplanes rotate in the conventional way, ie: "clockwise" when viewed from the rear of the airplane, as do electric motors. When we turn them around to face to the rear, to attach to a drive shaft in a boat, then their most efficent operating direction moves the shaft in a counterclockwise direction, which we want to be the forward direction, since that's usually the mode in which the boat operates.
But either way, the FIRST RULE OF MANEUVERING, you should picture and commit to memory:
Rule #1: The rotating propeller tends to move the stern, in the direction that the TOP of the propeller is moving.
Or, perhaps easier to remember, picture the boat moving in very shallow water and the propeller is hitting the bottom. Moving forward, the propeller is going to "walk" to the LEFT as its hits the bottom going from left to right. In reverse, it is going to tend to move the stern to the RIGHT. Convince yourself of it and memorize it. And once again, for a full size ship, the effect is reversed.
That means going forward, it is a little easier for the ship to turn to the right, than it is to the left.
It is so, for two primary reasons. 1. The density or pressure on the top blade is less the pressure on the bottom blade which is deeper in the water. Thus the bottom blade exerts more force than the top one. And under the laws of physics when the propeller exerts a force one way, it causes the stern to go the other, right? 2. The propeller produces a, horizontal, rotating path of the water coming from it. The effect forward is minimal, as the twisting water is behind the ship's hull. But the force when IN REVERSE is considerable ............. because this rotating water is "leaving the hull on the right side [again, we are going in reverse, remember] and coming under the hull and up and around, hitting the left side of the model hull; which pushes the stern to the right.
Thus we have a two part RULE #2 for maneuvering.... when going backward primarily. It comes in two parts because remember, the rudder only creates force when the ship is moving. Therefore part A is: When the ship is not moving backwards and reverse thrust is created by the propeller uneffected by the rudder force ..... and B: when reverse thrust is applied and the ship is moving in reverse and the rudder is also appling its force.
The result is quite logical:
In "A" the propeller is the only force and therefore the initial movement of the stern will be because of propeller forces alone ..... which are ..... to the right, right? [The top of the propeller is going to the right, when in reverse] Generally, with a single shaft ship, YOU CAN EXPECT, WHEN YOU FIRST GO INTO REVERSE ..... THE STERN IS GOING TO SWING TO THE RIGHT, whether that happens to be the way you want to go or not!
In "B" as the ship begins to move backward and the rudder starts to create a force: we have to stop a minute and consider which way the stern tends to go, when GOING IN REVERSE. If we move the rudder to the right, does not the rudder tend to make the stern go to the right, WHEN GOING IN REVERSE? I think so. So now you have both the propeller and the rudder creating force in the same direction ..... so the stern is now really going to move to the right!
On the other hand, with "left rudder" they oppose each other. The rudder tries to move the stern to the left, while the propeller keeps trying to move the stern to the right. THE RESULT? When you first start MOVING IN REVERSE, the propeller, usually has the upper hand, so the stern is going to the right. The rudder force in the opposite direction, loses out. As you pick up reverse speed, the rudder becomes more effective. Depending on the below water line configuration of the hull; IF YOU HAVE GATHERED REVERSE SPEED SLOWLY, so as to not create a lot of sideways turning momentum of the stern to the right, [and perhaps if the wind will help] the rudder may ultimately over come the propeller and move the stern to the left. But if you speed up quickly, in reverse, not only do you have the stern momentum moving to the right ..... but the bow developes a momentum to the left; which if you understand airplanes tends to result in a "tail dragger's accelerating ground loop type of forces;" and the rudder force may not be sufficent, to ever overcome the opposite momentum.
Well, what if you really want the stern to go to the left? Try this: start slowly in reverse. Just as you begin to move backward, shift to hard right rudder and full forward. The thrust will tend to get the momentum turning the ship's bow to the right and stern, of course, to the left. That's what we wanted right? Now, before you get forward "way" on [as sailors speak of it!] go gently into reverse and FULL LEFT RUDDER; until you pick up once again some reverse speed. You may have to repeat this process, more than once. Ultimately, you will have enough momentum of the stern moving to the left [by "kicking forward to the right" that the momentum, together with the rudder will allow the stern to keep going to the left, going backward, inspite of the propeller forces to the contrary. And with enough reverse speed, you can then neutralize the rudder and "steer" while going backward ...... and among other things ..... back right into a dock! But do not make large rudder movements, at this point, as the mometum of the bow, once given a good start is hard to change or neutralize.
Now let's discuss the effect of the wind. Look at your ship, out of the water. Picture the "area" above the water line, effected by the wind; versus the "mass" of the hull" below the water line, tending to resist the force of the wind. Is it like my tug or cargo ship. That is a relatively low stern to the water and a much higher bow? If so ..... what is the effect of the wind going to be? It is going to exert more of a force pushing on the bow than on the stern. The same can be said of significent waves. That have a much greater force with the bow than the stern.
So you are coming up to a dock. If the wind is blowing across the dock, that's great. As the bow gets close to the dock, the wind will tend to push the bow and rotate the ship until you are parallel to the dock. But if its coming towards the dock, then as you nose the bow into the dock, the wind is going to push the bow further towards the dock; which, of course, the stern is pushed away from the dock. That's not what we wanted. The answer "may be" to turn away from the dock sooner and harder in your attempt to get parallel; swinging the stern into the dock, but the wind will stop it by pushing on the bow. Presto, everything stops, right when you are parallel to the dock! And remember the stern wants to swing to the right in reverse. So, if you have a choice, or perhaps you might want to insist, that you "dock" with your starboard side to the dock, so at the last minute you can go into reverse; which will also tend to make the stern go the the right and help the bow fight the wind.
This is, of course, just a couple of the many variations in forces, you may come up against and just a couple of the ways we might combat them, to con our ship to where we want it to go. But perhaps it gives you an idea of the thought process. I have a book of "Naval Ship Handling" that discusses many types of problems and docking situations of all sorts of ships from Battleships to Distroyers to smaller landing craft and explains the particular problems each has. If you would like to borrow it, please let me know.
Let's close this article with a review of what most of you know. When you are moving forward through buoys, WITH A WIND, start far enough away, so that you can determine the "track" of the ship through the water; which will tend to be ...... not where the bow is pointed ...... but rather with a certain amount of sideways slip. What many tend to do, is point the bow through the buoy openning and then surprisingly hit the leeward buoy, none the less, because the ship was not travelling in a straight bow line, but rather somewhat sideways ...... down wind. Depending on the angle and how "balanced the forces are on your particular hull" you may well have to "aim" for the windward buoy, or perhaps even more; to end up going through the buoys, without touching them.
In the next article, we will have fun dealing with the much greater flexibility that two shafts and independent speed controls for each, can given you for maneuvering! It/s all not that hard. It's just a matter of gaining the experience and the perception data bank, to more accurately interpret what we are seeing. I encourage you to get out on to the pond and practice; just as sail skippers and race driver's have to do.
PRECISION MANEUVERING OF A SHIP
Robert Enos and I have a more than one discussion on the relative merits of a ship with one shaft versus one with two. It's hard to argue with the outstanding performance of his Dumas P.T with one Astro 40 and one large prop. Performance wise, he may be right. Less problems, less expense and better performance. But if precise maneuvering interest you, then in my view, the addition of a second independently controlled shaft, is worth the trouble.
There are several maneuvering advantages: With two counter rotating propellers, the sideways force we have referred to in previous articles, can be cancelled out. Or can be used to advantage, by using only one, at a time.
With the shafts off set from the center line, together they provide a "straight thrust" but when used independently they provide considerable turning thrust. Compounded, of course, if one is used forward and the other turned in reverse. In fact, with most model ships; by doing so, they can be turned in their own length. More importantly, when maneuvering to dock; the ability to "turn" without gain appreciable headway or sternway, is a distinct advantage.
The two shafts, definitely make backdown, a far more controllable operation and one is seldom if ever, caught in the bind of the inherent tendency of the stern going to the right, in backing a single shaft ship, even tho you want to go the other way.
And since the two shafts counteract each other's sideways force; the rudder is equally effective in both directions, when backing down, both shafts.
And finally, you can usually move the ship directly sideways, when needed. Simple. Run the propeller on the side you wish to move toward, backwards. Run the other propellor forward. AND turn the rudder away from the direction you wish to move toward. Remember, this will move the STERN toward the direction you wish to move toward. Then it is simply a matter of adjusting the propeller turning in reverse, to just counteract the bow wanting to turn in the opposite direction as well as balancing the two propellers to keep from getting any headway or stern way on.
Backing a single screw ship, is always, partly "pot luck", particularly in relationship to the effect of the wind; at best. Perhaps, it is like flying a tail dragger airplane. It is the ultimate in knowing precisely what forces are effecting your ship and how best to deal with them. On the other hand, after struggling time after time, and having to often accept something less than what we intended as an acceptable single screw docking; it's a lot of fun and still a challenge, to use a twin screw ship, so one can, in fact, do precisely what one wants to do, without discounting the inherent limitations that confront single screw skippers. Incidentally, it is helpful as well as realistic to have a transmitter with the two throttle sticks, side by side, in a vertical direction. The one transmitter that comes to mind with that configuration is the ACE NAUTICAL COMMANDER, seven channel, which was out of production for awhile, but someone told me they recently saw it advertised again. It is also possible to modify a Futaba to do that, WITHOUT DOING ANYTHING TO THE ELECTRONICS. The joy stick comes completely apart and it is simply and matter of reconstructing it, mechanically, only, for two vertical side by side sticks.
In any event, I encourage you, to consider following through on the precision with which you have made your boat, to precision in its operation, maneuvering and docking. Woody's Port of Litchfield, is a perfect environment in which to practice. It is a constant challenge, with no two dockings exactly the same; particularly when the wind is present. And sometimes, tho conservatism, is generally a good rule, when docking; there is no substitute put to apply hard rudder and a hefty shot of power and/or speed at just the right time, to be successful; where as to continue to be "conservative" may allow the wind or current to get the upper hand, with disastrous results, [at least in real ships] as they end up controlling the result, rather than you the skipper.
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