OK, let me hope I can write this down so it can be understood. I want to paddle across a channel between mainland and an island, distance about 4 miles. There is a current going from my right to my left at about 3 miles per hour, sometimes stronger. So I pick out an object on the far shore that is to the right from the point I want to land and use as a reference point BUT that means I will end up paddling almost parallel to shore when i get close to the landing. INSTEAD should I not pick a heading that is to the right of my landing (about 5 degrees)and keep that heading. This way I do not head towards a point but keep the heading to counter the current?
Hope that is clear 
Holding a 5 degree heading
– Last Updated: May-24-07 9:03 AM EST –
..is difficult in a kayak. If holding a fixed compass heading and it takes you one hour to cross the channel with a three mph current you will end up 3 miles left of your destination.
I would use a landmark 3 miles to the right of your target landing and use that as a heading.
Good luck..
Or…
Pick two points on the opposite shore and try to maintain a constant a relationship between them as possible. It might give you better feedback on your comparative angle across the current.
Ranges
Trying to aim at the object 3 miles to the right will not work for the reason you stated “…end up paddling almost parallel to shore…”. Taking a bearing on this object and paddling on this bearing is on the right track but its not as simple as that (for a start, despite the crossing being 4 miles it will take more than 1 hour paddling at 4mph) and paddling on a bearing will work but can be tricky, I would only use this method if visibility was very poor.
Also I’m sure the the current is not of equal velocity for the whole crossing, with this speed of current there may well be eddys in the opposite direction.
Using a range is reliable, most books will tell you how, remember you don’t need two points in front. Two behind or one on each side will work just as well.
However with this current velocity I would be looking at a smarter way of doing it. E.g. wait for slack, or start up current and drift on to the object while paddling.
Or be a nerd about it…
How far you point to the right of your goal is dependent on your straight line, flat water paddling speed (s).
The angle would be arcsin(3/s)
If, for example, you can hold a steady 5 mph, you would have to point 36 degrees to the right. If the destination is straight North (bearing 000) you would need to keep your boat pointed NorthEast (bearing 036). To paddle the 4 miles across you would end up paddling 5 miles through the water.
If you could only hold 4 mph in flat water, you would have to point 48 degrees to the right and you would end up paddling 6 miles through the water.
An extreme example would be if you were only able to hold 3 mph in flat water, then the best you could do is sit in one place and paddle against the current never making any progress across the channel.
The other option would be to start “upstream” and paddle straight across, expecting to get pushed downstream to your goal.
Isn’t trig fun?
Learn to use natrual ranges
– Last Updated: May-24-07 1:03 PM EST –
And as Flatpick taught "Get High and Stay High". Meaning try and stay on the upstream (high) side of the line across. I am assuming you are not talking about crossing a river, so the current speed and strenght will vary with time. If visibility allows, find an object near the surface and another above it on the opposite shore and keep them lined up. You will be drifting off course in whatever direction the object higher up is moving. As an example, just hold a finger from each hand out in front of you in line and separated by a foot or so. Now keep your fingers still and move your head from one side to the other. Observe how the finger further from you moves. Your kayak will be doing the same thing. On a crossing the lenght you are talking about, you will likely be changing your heading numerous times as the current will not be the same all the way accross.
If you have to cross a shipping channel, do it at right angles (fastest way). If you know there is a channel, then get high and stay high before the channel so you don't loose too much ground. Out here the channels are usually 1 mile wide. So figure you'll be in the channel for about 20 minutes.
Knowing how to adjust your ferry angle using ranges comes in handy in a lot more places than just crossing a channel. In the San Juan's there are lots of islands to camp on. When approaching one being carried in a strong current, and you need to camp on the left side, you need to use natural ranges to make sure you don't get swept around the right side. If you don't use ranges, it is very easy to think you are heading the left side of the island only to be swept around the right side and have to go around the entire island and fight the current back up the left side. Of course that never happened to me :)
Beginners on group paddles up there always ask why we are paddleing in one direction when we want to land at another.
Have fun, this is what it's all about!
simple math
– Last Updated: May-24-07 12:56 PM EST –
For setting ferry angles when crossing straight across the current, we can use the formula:
ferry angle = current speed/paddling speed x 60.
Example: 1 knot of current, divided by 3 knots paddling speed, times 60, equals 20 degrees.
You can either remember this formula, or simply remember 20 degrees for every knot of current when paddling at 3 knots. When paddling at 4 knots it becomes 15 degrees for every knot of current. At 5 knots, it's 12 degrees.
As others have said, use ranges to make sure your on track.
Sounds good but in practice …
On my first hairy crossing I learned that accross a good stretch of water the current is not uniform in the crossing, and winds can be quite variable and alternatively augment or substract the current effects, so you have to adjust your ferry angle. Nice to have a GPS and a compass if you are in an area that is foggy or poor visibility of your target.
In spots with interesting topography the tide flows can cause water to be moving in both directions during your crossing. Another complication can be heavy swell with steep waves coming through the channel at various angles. Gives you some sense of the seamanship of early people who made routine crossings accross large expanses of nasty waters like in the British Isles, Polynesia etc.
a bit more complicated math
For the nerds among us…
C- current
S- padding speed
a- ferry angle
C/S=sin(a)
sin(a)= a -a^3/6
Approximate sin(a)=a
C/S~=a;
a is in radians, A is in degrees
A= a/Pi180 == a/3.14 180~= a60;
Then, C/S = A/60; or A= C60/S;
Absolute error turns out to be a few degrees, which for a kayaker is good enough.
It will take some research
but that may have been the first post in the history of p-net to have both “Pi” and “radians” in it.
Nice work.
D
THANKS
Thanks to all, and to clarify the route is from mainland to an offshore island here in the Gulf of Mexico. The first time I was told to pick a point to the right and paddle towards it BUT I ended up as I said almost parallel to the beach when i got across the channel/bay.
Thanks to all!
Ah Hell
– Last Updated: May-25-07 9:08 AM EST –
Getcha a GPS and it'll draw a line for ya.
(uhhhh... no I've never used it but I know it will do it.)
Simple math…
If you’re doing a straightforward crossing, simply do the following:
- calculate the time it will take to cross as if there were no current.
- determine the speed of the current.
- based on the speed of the current and the time you’ll be on the water crossing, you can figure out how far you will drift.
- Draw a line the same distance as 3 INTO the current (up stream), and the bearing…that’s the bearing you’ll paddle. All things being equal, you’ll end up directly on the spot you want to hit.
Based on your example, it’d be like this:
- You have 4 nautical mile crossing from Point A to Point B. You paddle 3 knots, so it’ll take you about 1.25 hrs to cross from point A to point B.
- You don’t specify direction of the current, so lets say it runs E to W for the purpose of this example. If the current is 3 knots, you’ll drift 4 nautical miles to the west during your 1.25 hr crossing, and end up at Point C.
- Simply draw a line 4 nm to the east of Point B (your intended target) and then calculate the bearing (E) to the new point (D) from your departure point (A).
- Paddle bearing E and you’ll travel straight across and end up at B.
D C
\ | /
\ | /
\ | /
\ | /
\ | /
E \ | /
\ | /
\ | /
\ | /
\ | /
\ | /
\ | /
|/
A
This assumes a consistent current, and a beam current. If the current is diagonal, it will also either speed up or slow down your time on the water, which will change how much you are set by the current.
Another option is to leave at slack current, or about 40 minutes before…you’d then drift E for 40 minutes, then west for 40 minutes when the current changes direction, which would set you back at B.
GPS can be misleading
It only draws a line from where you are now to where you want to go. As you drift with the current, it readjusts, but you may wind up paddling a great many more miles than you planned.
Best bet is to use a compass and ranges and a little math to figure out the correct ferry angle. The GPS is basically good for letting the CG know where you are when you get in trouble. Stick with the ded reckoning. It will get you where you want to go and it’s a lot more fun than trying to read it on a gps screen.
I Don’t Think So
My buddy’s unit would draw a line from the original starting point to the destination and show your tracks in relation to that line. Very simple.
It is very simple
You start out following the GPS heading. As you drift down current, the heading will change, but you will wind up paddling many more miles than if you set a ferry angle and then even if your heading should have been 270 degrees but you point your boat at 300 you’ll still hit your mark.
If you follow the initial gps heading of 270, as you drift down current, your gps will tell you to change your heading - however, you may be paddling at 3 knots, but you are NOT making 3 knots progess towards your destination.
If you find this concept difficult to accept. Try it out. You’ll be surprised at how much further you wind up paddling.
Many people on their first crossing will make this same mistake.
Slideslip… A Tweak Factor.
In a cross wind a kayak will slide sideways. A very hard chine boat will hold a much truer line than a softer chine hull.
Yep, too much information…
Course and Bearing
I just checked my MAP76. It provides a bearing line. Like you said, it points toward your destination from wherever you are. My unit also provides a course line. A straight line from waypoint to waypoint. If you follow the course line you’re travelling a straight line. Those of us who are bad at math just buy batteries and follow the course line!
Your losing sight of the fact
that your boat will be moving sideways with the current as well as in the direction that the bow is pointing.
I too use a GPSMap76, but on a crossing with currents, I use it to check how much off my intended course I’m drifting.
It’s not like your car that only moves in the direction its pointed.
We’ve done this exercise with people who didn’t believe it. They paddled the GPS course, while others figured out the correct ferry angle. The total distance paddled according to the GPS was quite a bit shorter for those who used a ferry angle - and the GPS can’t figure those out for you.
simple is best and safest.
all the math and calculations are great and interesting. the serious trig stuff is bollocks for real life kayaking though.
re-read Slowcoach’s post, that’s the wisest advice. you don’t calculate strict angles, come up with a strategy, given the many variables. basically, the start high, finish high concept, nails it.
have fun.