# Piloting a Boat

 Standards A 5, 6, 15 B 3 C 3 D 1, 2, 3 Concepts F = MA Inertia Friction

Most Alaskans live or hunt on rivers. On almost every Alaskan river there are a few people who really know how to pilot a boat. They are river scientists who operate by basic principles the rest of us need to learn.

Once we are able to read a book, we have the skills to read other books. Once we are able to read a certain river, we discover that we have the skills to read other rivers also. The signs and patterns are common to all rivers. Repeated travel helps us learn the river better. However, a good pilot can travel a completely strange river and sight read it the same way you would read a new book.

With identical boats, an experienced pilot can go from one place to another faster than an inexperienced pilot. How is this possible? He understands the forces working for and against his boat, for and against the river.

Three Main Forces

There are three main forces working on the water in a river.

• Gravity pulling the water downward towards the sea. The steeper the incline, the greater the velocity of the water.
• Momentum of the water goes around a bend. Inertia causes most of the water to go to the outside of the bends.
• Friction with banks and river bottom.

The river channel is often deeper on the outside of bends.

On the inside of the bends, the current is slack.

Look at the following drawings of a typical bend in a swift river. The two forces, gravity and inertia, are sometimes working together and sometimes competing. In the first drawing, the slope of the river isn’t too steep, so inertia carries most of the water to the outside of the bend, creating a swift deep channel there.

1.         2.

In the second drawing, the water is flowing down a steep slope. Inertia is working to carry water to the outside of the bend, but the drop is so great, gravity forces the water to fall to the shortest distance which is on the inside of the bend. Gravity overrides inertia.

If the pilot understands these forces and how they interact, he can understand almost all river situations in Alaska.

Islands Formed

When the current slows, and its ability to carry sediment also decreases, an island or sandbar forms in the middle of the river. The channel goes to either or both sides of the island.

Other islands are formed when the river channel erodes its way through the banks, separating large land segments.

Pilot’s Priorities

As the pilot looks at the river while going upstream, the priorities are:

1. Water deep enough for his boat and motor
2. Shortest distance
3. The least resistance from the current

The pilot looks at the stretch of water in front of the boat. Math and science blur together as decisions are continually made figuring the best path to travel. Where are the snags, eddies, slack current, straightest paths, and shallowest waters?

The Current Varies

The current is slower close to the riverbank than in the middle of the river. Friction with the riverbank and bottom of the river in the shallow places slows the current. If you measure the current across a river, you will find that it varies tremendously from place to place.

Salmon are aware of this and travel the path of least resistance in their journey upstream. They prefer the shallows at night because they feel safe, but swim on the bottom in the middle of the river during daylight in order to avoid detection.

The current might be six miles per hour (mph) in the middle, eight mph five feet from the cutbank, two to three mph right against the bank, two mph on the sandbar side of the river in the shallow places, and minus two mph in the eddy.

A good river pilot takes advantage of all slack current. Against a cutbank, the water swirls backwards due to friction with the bank. Sometimes the stumps and brush keep a good pilot away from those back currents, but traveling close to the cutbank adds several miles per hour to the boat’s speed as it goes upstream.

Eddies

The water swirls backwards in eddies. The current is actually going in the opposite direction from the rest of the river. Good pilots search for eddies.

Fish rest in eddies and use the current to assist in the upstream effort. The opposing currents hold a net in perfect position to catch the fish as they head upstream. Because of this, good pilots watch out for nets as they travel the eddies.

Using Rocks and Snags

The current swirls in low pressure areas behind rocks and snags. A good pilot enters a place with slack current behind a rock, gains speed, and shoots out before crashing into the rock.

In very shallow water, an experienced pilot knows that the water is deeper right beside snags and rocks, as the water intended for the snag is diverted aside, creating a mini-channel deep enough for the boat.

Efficiency

A pilot must decide which path upriver gives the least resistance. Obviously it isn’t worth crossing a wide river to get a little help from an eddy. There is a constant give and take in this regard, constant decision-making by the pilot.

Ground Effect

A boat lifts when traveling in shallow water. The pilot hears the motor speed up as the boat lifts. The shallower the water is, the more lift there is, but there is also a greater the chance of hitting bottom. This is why the pilot pays close attention to the sound of the motor.

When the boat is in deep water, the water is pushed down and away from the bottom of the boat. The boat sinks to some extent.

In shallow water, the water is pushed downward and away from the bottom of the boat, but it cannot move downward because the bottom of the river is solid. The boat pushes down. The water and bottom push up.

## Waves

Some waves are caused by the current moving around underwater obstructions.

Surface waves are created by wind blowing over the water’s surface. When the current and wind are moving in the same direction, the waves are small. When the current and waves go against each other, the combined velocities can be great, causing large waves. This explains why some sections in a river can have no waves, and another section of the same bend can have whitecaps. On one section of the river, the wind blows directly against the current. On another section it is crosswise. In another section, the wind is blowing with the current.

This is particularly distressing when rafting logs or firewood. The pilot feels totally safe on one section of the river, rounds a bend, and gets into huge whitecaps that threaten the safety of the boat and raft.

Wind generated waves are smaller against the bank. Why is this? The current against the bank is slower than it is in the middle of the river, and therefore the combined velocities of the wind and water is less. The waves in the middle are bigger because the combined velocities of the unobstructed wind and current going against each other are greater.

Bow down—
high friction

Bow up—waves
pound bottom

Jon boats have ridges
that trap air under the boat

## Friction

Another consideration of piloting a boat concerns the friction of the boat against the water. The more surface that is in contact with the water, the more friction there is.

A boat that has the bow too low will have a lot of surface to create friction.

A boat that has the bow too high will plow the water, and pound in the waves. A good pilot adjusts the angle of the boat by shifting the load and changing the tilt on the engine.

## Trapped Air

Waves trap air under the boat. The air, once under the boat, becomes flat bubbles that greatly reduce the friction with the water. This effect is very noticeable when coming from a small creek onto a larger river on a windy day. The boat accelerates when it hits the small choppy waves and breaks free from the drag of smooth water. The boat decelerates considerably when it comes from a river with small choppy waves and enters a smooth creek.

It is very hard to read a river when it is windy. Whatever sign might be presented by the snags and rocks is masked by the waves from the wind. It is like trying to read a book in the dark.

It is also very hard to read a river when there is little or no current. Some of my greatest disasters have been on windy days or in dead water.

When there is adequate current, shallows, rocks, and stumps all give sign of their presence and are easy to read.

## Upstream vs Downstream

Going upstream in a swift river is easy because all the obstacles are obvious and the boat is traveling slowly against the current allowing time to look and think.

Going downstream is another story. The rocks and other obstacles are covered with water. The boat is going so fast there is little reaction time. Going downstream is more difficult and dangerous.

## Lining Up

Years ago, people pulled their boats up the river. The tops of the banks were cleared of brush. They were lined with hard packed trails. Even today, people who break down while downriver from their village sometimes line up to get home. With one rope, the boat is very hard to steer. Two ropes easily steer the boat in and out around snags and obstacles.

## Travel at Dusk

Good pilots learn to protect their night vision, as it is quite easy to read the channel in twilight after the sun has gone down. The glare from the surface of the water highlights all of the signs the pilot is looking for. It takes most people fifteen to twenty minutes to develop night vision.

Artificial lights are a hindrance unless they are extremely bright. The glare from the water makes visibility worse and ruins night vision. It is important to have running lights, not for the pilot’s vision, but so boats will not collide.

Years ago a boatload of people ran over a swimming moose at night. Five people drowned. Travel in full darkness is not wise at all.

## Activities

1. Watch a video about a swift river or go to a swift section of a local river. Discuss the route you would go to take advantage of all the areas with slack current. Draw a map of a section of that river. Ask one of the elders in your village which way they would travel if they were piloting a boat on that stretch of river.
2. Imagine that you are a salmon going upstream in that river. Color the path you would swim during the day in blue. Color the path you would swim at night in black.
3. Draw a typical stretch of river in your location, or where you go to hunt. In your imagination, estimate the current in different parts of the river.
4. If it is possible, measure the current in a cross section of that river. If you have no way to accurately measure, release a stick on the sandbar side, timing how long it takes to pass a certain point 100 feet downstream. Do this again, releasing the same stick in several points across the river from the original release point. Measure the seconds it takes to go 100 feet. Compare their results. Tell students to divide the number 68 by the number of seconds it takes a stick to float 100 feet. The result is miles per hour.
5. At each of the above points measure the depth of the river. The easiest way might be to put a weight on a string, putting a knot at every foot in a string. Counting the knots as they slip through the fingers will give the depth in feet.
6. Observe islands in your river. Do you think the river widened, depositing the islands, or did the island occur because the river cut a new channel? Either might be the case.
7. The next time there are waves on your river caused by wind, note the bends they occur on, the direction of the wind, and the relationship of the wind to the current. Where are the biggest waves? Are the waves as large by the shore?
8. If possible, drive the boat from a river where there are choppy waves into a creek where the water is flat. Can you feel the difference in the speed of the boat?
9. While piloting a boat in deep water, set the throttle so the boat is barely on step. Cruise to the sandbar side of the river and notice the increase in speed of the boat and motor. Be careful not to hit bottom!
10. Listen and watch closely the next time you are in a boat. Hum in tune with the motor. Does the pitch of the motor get higher when you pass through shallow water? In and out of eddies? Do you think this effect is more noticeable with a planing or displacement boat?
12. Pour water out of a teapot that has a spout. Observe. Where is the strongest flow of water? Which is stronger, gravity or momentum?
13. Put a piece of plywood on a slant. Pour water from the teapot across the top end. Observe and mark where most of the water flows. Tilt the plywood up and down changing the angle, again observing and marking the greater flow of water. At what angle does gravity exert the greater force, pulling the flow of water downward instead of yielding to momentum? Try to keep the water flow and pressure the same while changing the angle.
14. Design a boat that would trap air under the boat so it will travel on a cushion of air.

Plywood not.
Momentum is strong. Water pours out

Plywood.
Gravity is stronger pulling
water down more

## Student Response

1. What are the three main forces working on the water in a river?
2. Draw a typical bend in a local river. Identify the deep and shallow places. Estimate what the current will be in five places on the river.
3. Why is the current next to a river bank slower than the current in the middle?
4. In the picture to the right, tell how fast you think the water might be going in the different places circled if the current in the middle is 6 mph.
5. What are the three priorities a pilot operates by when traveling upstream?
6. Draw a picture of a typical bend in a river. Draw a big rock in the middle of the river. Draw the path a boat might take.
7. Draw a picture showing a boat in ground effect and another in deep water.
8. Why are the waves caused by wind larger in the middle of the river than on the sides of the river?
9. Draw a picture of a boat that is traveling at the best angle for waves.
10. Draw a picture of a boat that is traveling at the best angle for calm water.
11. Why do small choppy waves help a boat travel a little faster?
12. What are three things to remember or do when traveling at dusk?

## Math

1. Pete can travel the from the store to his cabin in 3.5 hours. His son can make the same trip in 4 hours. If gas is \$3 per gallon and the motor uses 4 gallons per hour, how much more does it cost his son to make the same trip?
2. A boat travels at 16 mph relative to the water. The river’s current averages 9 mph. How long will it take to make a round trip of 22 miles each way? What is the total time of the 44 mile trip?
3. A boat travels at 16 mph. How long will it take to make a round trip of 22 miles each way across a lake? What is the total time of the 44 mile trip?
4. Compare the trip in current and the trip on the lake. Why do you think there is a difference?
5. An outboard motor uses 4 gallons per hour. It can go 21 miles per hour. How many miles per gallon does it use?
6. Another outboard uses 3.2 gallons per hour, and goes 18 miles per hour. Which outboard is more economical?
7. Which of the above outboards is more economical going upstream for 72 miles on a river with an average current of 12 miles per hour?