If you set the GPS
to go to a way point or route
it sends the sentences that tell the autopilot what the course to steer and is. You need to check both the GPS and autopilot to see what ones they send and receive. The GPS only sends. There is nothing in the autopilot except it can tell the current
heading over ground over time via the fluxgate
and the memory in the auto pilot. It also knows the rudder
angle from the sensor. Wheel
pilots are no so good at rudder
. There is an extra level of assumption with them.
Where autopilots get different is how they use that data they have and can get from the GPS. If you compute course over time plus add rudder angle the course computer can determine how effective the rudder is over time and determine how far to turn the rudder for how long to make a course correction. They all do this differently and don't need the GPS just the fluxgate
and rudder indicator. Based on the various sentences they accept and what the course computer does can vary. Better auto pilots can learn the sea state and others need to be told by you. You can be pointed due east yet heading NE. Crossing the Gulf Stream
it can happen on smaller boats.
If you have a wheel
pilot - don't bother. Set your GPS and monitor
the results it shows you and adjust the wheel pilot accordingly. Based on the GPS compass
compared to the magnetic compass
you see the difference. What no auto pilot can do is compute set and drift. When the boat speed is slow compared to the current
the autopilot can fail to get you there totally or greatly increase the time. In a strong current where you are crossing 90 degrees and the boat speed through the water
is less than the cross current the auto pilot will eventually have you going head
into the current going back wards and not be able to do any better. The extreme example shows the problem.
Using set and drift you compute the straight line across the current and add the offset from the current based on the time to get across. Steering
below the destination
will let you end up where you want to be over the time it takes to cross the extra distance. In a current, the boat will always follow the current direction at the speed the water
ins moving over ground because the is in the water but it also follows the powered rudder direction too. You add them up to see where you end up over the same period of time. This is what the autopilot can't do.
It is because of this that you need a magnetic and a GPS compass and why they don't always agree. The autopilot doers not know why the two don't agree and assumes it is because of the "effective" rudder vs sea state. For power boats they add a gyroscope to measure the acceleration but in a slower boat like anything under sail it really does little.
To get all the way there you would need a ring laser gyro. Very cool gadgets that run about $80,000.00. With that you could have the ultimate course computer. They can subtract the earths rotation out of the equation too and compute the sum of the changes in velocity to determine exact X,Y, and Z position over time given an exact location. They call it a cruise
Personally I never do it that way. I plot out my course before I leave and load the GPS with a basic route. I manually set my autopilot and watch it and make some adjustments knowing if there is current to compute or not. The idea is you study all this before you set off so you don't do it on the fly. On the fly navigation
can be quite dangerous because you can make one mistake and send yourself off the edge of the world. It's easy enough to do it many time correctly but that one time it may be an OMG moment - we are not supposed to be here!
Not all auto pilots can do all of this. I know how straight a course mine can steer and it is enough for when I need it. Once you know your own limits you can use it in a safe navigation