Ferry Angle Calculations - Beginner Level

[...]

John,

The information above regarding current speed and correction angle explains a lot regarding how trip planning is performed in the San Juan Islands (I suspect in other areas of the world as well). Working with the currents is a really big deal in that location.

In the San Juan Islands, max flood and max ebb can be in the 2.2 to 2.7 knot range. Because of that, paddlers typically use current tables to understand the speed and direction of the flow and then pick the dates and destinations to explore. Here, off the coast of Maine, I typically reverse that process for trip planning.

Your data does clearly communicate how ferry angle adjustments are not very practical in certain circumstances. In those cases, just go with the flow and enjoy the ride.

Warren

[Phil Allen]

The 'space' of possible paddling speeds, current speeds, and angles makes it tough to come up with any easy-to-remember way of taking care of all cases. However, what I keep in my head is the following: most folks paddle at about 3 kts. Most crossings of any consequence are at 90 degrees to the current. Using this - there's a fairly easy 10-20-30-40-50 rule that goes in increments of 0.5 kts

Current speed Correction angle

0.5 kts 10 degrees

1.0 20 degrees

1.5 30 degrees

2.0 42 (40) degrees

2.5 55 (50) degrees

The relation is pretty linear up to 2 kts of current, and then you can see the divergence set in. At 3 kts, there is no solution. But, the 10-20-30-40 rule works pretty well.

...

John- I hate to ask a physicist to check his math, but wouldn't the 3kt current, 3kt paddler case just be 45 degrees?

best

Phil

[leong]

John- I hate to ask a physicist to check his math, but wouldn't the 3kt current, 3kt paddler case just be 45 degrees?

best

Phil

Sticking my neck out for John, no, the best you could do is paddle directly into the current (90-degree ferry angle) and you'd be stationary.

[JohnHuth]

Yup, Leon has it correct.

The assumption is that the course made good is perpendicular to the current direction. As you approach the speed of the current the angle heads more and more directly into the current until you're heading directly into the current and there's no solution.

This revelation hit me quite practically at Woods Hole. At certain times, the current hits 3 knots and there is a huge amount of boat traffic. I was with an instructor who wanted us to do a ferry angle across when the current was running this strong. The resulting angle was so big, that I figured I'd be in the channel for way too long a time, so I just headed perpendicular to the current and paddled like crazy to cross before another boat came through. I then eddy-hopped up the far side. That was quite the lesson.

I was with another paddler at the time, when we both started out with our ferrying and when I saw all the boat traffic and our exposure to it, I said "what the heck, I'm just running straight across, heck with the ferry" and my buddy followed me. I'm not sure whether the instructor took kindly to this, but it seemed prudent given the amount of traffic passing through.

[Phil Allen]

Leon and John,

Got it. Had to do it out graphically for it to make sense (to me).

best

Phil

[EEL]

Any WW paddlers around have a comment technique for and about ferrying is fast current? Seems it is a routine activity in WW although distances are short.

Ed Lawson

[NateHanson]

Hi Warren,

I know different folks have different approaches that make sense to them. Personally the approach of calculating angles and speeds using trigonometry doesn't work as well for me as plotting vectors on a chart. Just the way my brain is wired I guess.

By plotting vectors you can get correct ferry angle and speed over ground using only an orienteering compass. No math. I know there are a number of folks in your area who have a good understanding of BCU 5-star navigation, and one of them might be able to demonstrate this technique for you if you'd like. (Also we can go over it together in May, during the leadership/guide course.)

Nate

[...]

Nate,

I am looking forward to understanding your technique when we get together in May. With outside temps in the single numbers, it is great to think about paddling off Stonington, Maine!

Warren

[JohnHuth]

I agree with Nate that plotting them using vectors is definitely a straightforward and intuitive way to go. A backpacker's compass can be used with a pencil.

Lacking that, there are some rules of thumb that can be used if you assume a 3 kt paddling speed and a current perpendicular to the crossing.

[JohnHuth]

Attached is the graphical method.

1.) First, establish a scale - e.g. half an inch equals one knot.

2.) Draw a vector that has a length representing your paddling speed.

3.) Draw a vector with a length and heading that represents the current speed and direction

4.) Extend a line from the tail of the current vector that represents your desired heading - make it as long as possible.

5.) Take your paddling speed vector - place the tail on the tip (arrowhead) of the current vector.

6.) Rotate that paddling speed vector until it touches the line representing your desired heading.

7.) The angle of the rotated paddling speed vector (touching line) is the heading you need to make *in* the current to make the course made true with respect to land.

8.) To find the actual speed and heading you end up with - extend a vector from the tail of the current vector to the tip of your rotated heading vector. This gives you the heading with respect to land, and the length gives you the speed.

Note: if the rotated paddling speed vector never touches the desired heading line, you're out of luck - there is no solution.

Can all be done with a backpacker's compass and a piece of paper.

Edited January 22, 2014 by JohnHuth

[Phil Allen]

an addition to Johns method if you've got long crossings with multiple current velocities over time is to add all the current vectors (distance and direction) to your start position. The heading from the end of the sum of your current vectors to your destination is your average heading for the duration. Good news is its a lot simpler than dealing with different ferry angles through out the day. Bad news is it doesn't give a good estimation of your local position should you need to report it.

best

Phil

[leong]

John,

I understand and agree with your vector solutions. And I realize that your excellent diagrams were to answer questions in this thread.

However, even given the exact constant current velocity (speed and direction) and a constant paddling speed, the wind velocity can affect the needed ferry angle. The leeway due to wind can be significant because a kayak doesn't have much grip on the water. Another factor to consider is wind-induced current. Wind-induced current might even be in the opposite direction to the normal current.

For the reasons stated above I don’t think ferry angles always do a good job, especially when current speeds and/or paddling speeds are not constant. When possible I believe ranges are more reliable than computed ferry angles. Also, paddling a pursuit curve (always heading towards the target) is not so bad. In the dark or in a fog a GPS might be the only reliable alternative.

Respectfully,

-Leon

[Phil Allen]

Lot of 'beliefs' in that statement for an engineer, Leon ;-) . All these tools have their place and I believe that not one has the ability to most efficiently deal with all circumstances. Being able to use multiple methods keeps you efficient and safe.

best

Phil

[leong]

Lot of 'beliefs' in that statement for an engineer, Leon ;-) . All these tools have their place and I believe that not one has the ability to most efficiently deal with all circumstances. Being able to use multiple methods keeps you efficient and safe.

best

Phil

Phil, my statement just addressed some of those conditions that might affect the accuracy of a calculated ferry angle, nothing more. There were no beliefs or engineering judgements in my statement.

Like you, I believe that various tools have their place. But it’s also important to know their limitations so you can choose the best tool for a particular situation.

I think we agree.

-Leon

[JohnHuth]

I think of navigation as a process of successive approximation - deal with the biggest effects first and then whittle them down as you go along. In some cases, the effects of wind can outweigh a modest current. On the other hand, a strong current can overwhelm wind effects. It's a question of figuring out what the biggest issues are, addressing them and then turn attention to the lesser issues.

That's both in the planning phase and also on the water.

[leong]

I think of navigation as a process of successive approximation - deal with the biggest effects first and then whittle them down as you go along. In some cases, the effects of wind can outweigh a modest current. On the other hand, a strong current can overwhelm wind effects. It's a question of figuring out what the biggest issues are, addressing them and then turn attention to the lesser issues.

That's both in the planning phase and also on the water.

Amen