| Rudder for an Inflatable Canoe |  |
I really enjoy a good outdoor activity, that's why I usually buy gear for such activities and that's why I bought an inflatable canoe from Sevylor. Such boats have a hard and tough body when they are inflated. Also such boats are very convenient as they can be stored very easily under a bed or in a small storage room when deflated. This is a much cheaper option to normal boats as well.
But as I experienced myself and also heard from some friends, it is not easy to steer such boats. Their floating direction can go easily out of control. Let's see why. In boats such as canoes, it is the paddling that steers the boat around. Paddling from one side, steers the boat to the opposite side. This fact is shown in the figure below.
Therefore if you wants to go straight, it is best that there are two people paddling each on one side of the boat at the same time and with the same power so that the boat keeps going straight. The other way is paddling once on one side and once on the other side to correct the floating direction. The third way is a little bit tricky, one only paddles on one side but after each stroke, they use the oar as a rudder to steer. It goes like this: you paddle once, but you keep the oar in the water. The friction of the oar with water results in a force against the moving direction that straightens the boat. This way wastes some of the forward energy of the boat and slows it down. In all three cases one will need to paddle backwards to correct the way at some point, wasting some of the energy.
What I experienced with my boat, which seems common especially in inflatable boats, is that when the boat moves relatively fast, if it starts turning, it is almost impossible to steer it back. This happens more when the weight on the boat is concentrated on the rear of the boat. This example explains it better. Take an empty plastic bottle. Stick a rock or any weight at one end of the bottle. Then throw it in the air. What you see is that the bottle turns until the side with the weight runs in front of the rest. The reason for this act is the inertial force along with the friction with air. Assuming that the bottle has a uniform shape looking from the side, the friction against the side of the bottle is uniform too. But the side with more inertial force, which is the heavier side, resists against the change in speed more than the light side. Therefore the rest of the bottle which is lighter turns under friction with air and the heavy side goes to the front. Similarly in a boat, when the weight is on the back, which usually is the case, the boat turns out of control to bring the heavy side to the front under the friction with water. This is demonstrated in the figure below. You can see from the figure that a uniform friction against the boat is provided by water. But the friction force minus the inertial force results in a much larger force pushing the boat back on the lighter side compared to the heavier side where the person sits. This turns the head of the boat back. Especially in higher speeds where there is a larger friction force, the force is so large that paddling won't help to make the boat straight. The boat should slow down to reduce the friction for a better control.
I found it quite frustrating to paddle like this. The boat would go out of control frequently and a lot of power was wasted for a short distance to go. So I tried to find a way to solve it, and I found a rather easy way: adding a rudder to the boat. Boats such as kayaks have rudders controlled by pressing paddles inside the boat by foot. Figure below shows how a rudder works:
In
this figure the rudder has an angle to the moving direction of the boat. This
results in water pressing against the rudder in the direction of the
water flow relative to the boat with a force shown as 
. To understand how this force
effects the rudder, it is divided into two forces 1 and 2 where = 1 + 2.
As you can see 2 is parallel to the rudder, which results in the change of the water flowing direction parallel to the rudder. 1 on the other hand is right angle to the rudder and presses against the
rudder. Again 1 is divided into '1 and '2 where 1 = '1 + '2. '1 is
parallel and reverse to the boat moving direction on water, a force against the flow
that slows down the boat. '2 on the other hand presses the tail of the boat, here in this example to
the right side of the moving direction. These means the head of the boat turns left and therefore the
entire boat turns left. Similarly when the rudder turns to the
right side, the entire boat turns right. Note that we usually use a
stick attached to the rudder to control the rudder. To turn left for
example, we press the stick to the right side, resulting the rudder to
turn to the left side and the entire boat to the left and
vise-versa.
Not all the rudders are moving. There are fixed rudders as well, usually in surfing boards and some other boats as a fixed part attached to the body. Such rudders are always parallel to the object body, otherwise the object would always turn in one direction. You may ask what is the point of such rudder. A fixed rudder is used when there are other means to turn the boat. A fixed rudder on the tail of a floating object acts exactly like a tail in air plains, rackets, arrows, darts or badminton balls. No matter how you throw them, the tail goes back and the object moves straight and remains straight. This is also explainable from the forces figure above. If the fixed rudder has an angle to the movement direction, there is a force from the gas or liquid in which the object is floating in to the rudder, which presses the rudder and turns the entire object. This happens until the rudder is parallel to the movement direction and therefore no force is presses against the rudder. As the rudder is parallel to the object body, it makes the entire object to move straight, parallel to the movement direction.
Now having such a fixed rudder, the movement of the object is very well controlled. In the case of a surfing board, the surfer turns his or her body and this movement results in an inertial force that turns the board. Or in a boat, a paddle stroke applied force in one direction that turns the boat to the other way. Searching the web I realized that they already have a fixed rudder for my boat, I assume for the same purpose of better direction control:
I assume that this rudder is effective. But in my opinion a very small area of the rudder goes under water and therefore small force is provided to control the boat. Also it sounded expensive and I didn't want to try and see if it really works. So I decided to design a fixed rudder for my boat for a lower price. Why fixed? First because a moving rudder requires more mechanical complexity, which is not quite applicable on a canoe where it has a pointy tail, not to mention that it is harder to mount such device to an inflatable boat. Second, it is not practical to paddle and control the rudder at the same time. So I went for a simple design, a fixed rudder with a shape similar to the figure below:
Now the only problem was how to attach it to the inflatable boat. There is no room to screw things to an inflatable boat. But fortunately my boat had features that helped me achieve the purpose. As you can see from the picture of the boat above and partially below, there are to covers on the head and tail of the boat, suitable to put gears and food underneath to avoid them from getting wet. These covers are mounted by ropes, passing through many holes provided on the cover and the boat body.
Therefore I decided to strap the rudder to the holes. Two of the holes were on the softer fabric. Therefore to avoid them from tearing apart I secured them with metal rings made for the this purpose. To strap the rudder, I used four of those tough elastic bands with hooks at the ends, used mostly to secure loads:
I made the rudder out of cedar wood. I attached a wooden plate right angle to the rudder at its top, which would sit on the boat bottom. Four elastic bands would be connected to this plate with their hooks around the four screws on the plate. Figure below shows the rudder and the parts:
Finally the rudder goes on the boat like the picture below, firmly mounted on the bottom of the boat with four tough rubber bands holding it in place. It is important for the rubber bands to have symmetrical pulling forces so that the rudder is held in the middle and straight to the boat.
So I tried the boat with the rudder I made for it this time, and it was great! You can't believe the improvement in steering of the boat you have with a rudder mounted on the boat. The only thing we needed to do was to paddle forward. Therefore there was a much higher speed and the ride was much more joyful. Below is a picture of me and my wife riding the boat, here without the rudder! I couldn't find a picture with rudder on the boat. You wouldn't be able to see it anyway as it would be under the water! Just remember that there is a huge improvement in the boat performance using a rudder.
| Written by: Mehdi Sadaghdar | |