Deviation Table from GPS

[Pagan]

Originally Posted by GordMay

If there’s a difference between experience & theory, one of them is wrong. Often, the error is a misinterpretation of experience, by one who doesn’t understand the theory.

I couldn't have put it better myself!!

I have been sailing (professionally and for leisure) since 1964 I have a host of certificates, and so as they say: -

You have to know the rules before you bend them. and

Rules are for the guidance of wise men and the obedience of fools!!

I smell an overload of platitudes, so I'm off to make coffee!

[Jim Cate]

G'Day All,

An interesting discussion here, some of which has addressed the OP's query.

I would like to make a couple of observations, neither of which relate closely to that query:

First, while discussion of steering by compass while cruising is interesting and somewhat contentious, I do not believe that many cruisers actually pilot their boats this way, at least while at sea. Rather they establish a distant waypoint, determine the approximate course to steer, and then direct their vessel in that direction. The course may be followed by hand steering, by autopilot or by a wind steering device. Then later they compare their actual position (as determined by GPS usually) with the course line -- usually displayed as "cross track error". If the boat has drifted off the desired line, a course change is entered into whatever system is steering. Repeat as required until reaching the destination. Nowhere in this process is absolute accuracy of the ship's compass required.

And yes, I do realize that the process fails if the GPS fails. At this point, one reverts to celestial position finding. Now you discover your position less frequently, but still follow a similar plan, adjusting the course steered to aim at the desired destination. Again, absolute accuracy is not required.

Now, as to constructing a deviation table -- one technique not mentioned yet is using a simple device (name unknown to me) consisting of a vertical pin which casts its shadow upon a plane marked off with 360 degrees (like a plotting sheet). By placing this in the sun, the shadow shows the relative bearing of the sun. The sun's azimuth can be determined accurately from the almanac, or by calculation, and thus the ships heading can be derived without reference to charted landmarks or even knowing the ships position. Quite some years ago Latitude 38 published (in the Max Ebb column) a short program written in Basic for determining the azimuth of the sun. Made the process pretty easy.

Cheers,

Jim and Ann s/v Insatiable II lying Towlers Bay, NSW, Oz

[Ziggy]

Originally Posted by Jim Cate

I do not believe that many cruisers actually pilot their boats this way, at least while at sea.

It may be true that "at sea", i.e, with no hazards within a few hundred miles radius, you could get by without absolute heading accuracy. Where it becomes critical is in coastal areas, especially in reduced visibility (night or fog).

[Jim Cate]

Originally Posted by Ziggy

It may be true that "at sea", i.e, with no hazards within a few hundred miles radius, you could get by without absolute heading accuracy. Where it becomes critical is in coastal areas, especially in reduced visibility (night or fog).

In the situation that you describe -- inshore, poor visibility -- I depend far more on the GPS's COG display than the ships steering compass, and I will frequently inspect the cross track error if I have established a target waypoint. When close to shore, currents are often strong and (despite the nice arrows on the current charts) rather unpredictable on the micro scale that is of interest in avoiding hazards. I have found that especially in poor viz conditions it is very difficult to estimate the set that one is experiencing, and without that knowledge steering by ships heading is a dangerous practice.

Before the advent of GPS we all did our piloting by this method, and I can say from personal experience (mostly on SF bay) that I often ended up rather far from my intended destination. I would have been pretty happy to know what my actual course over the ground was then.

I'm not suggesting that one shouldn't swing ones compass, not at all! But to worry obsessively about a couple of degrees on a small yacht is counter productive IMO.

Cheers,

Jim and Ann s/v Insatiable II lying Church Point, NSW, Oz

[Don1500]

Of course you could always use this little gizmo:
Method and apparatus for determining and recording boat compass deviation - US Patent 4241511 Description

[Bruce626]

OK, but it seems like all that is needed is a chart of where one is and some calm shallow water, and a few buddies. Perusing the chart would allow one to establish an accurate position (or use the GPS) and then locate the appropriate true directions. A sighting arrangement seems like it would make the person's job at the helm (where the compass is) easier. Your buddies use ropes and if necessary anchors to position the boat on the visual heading direction and then the person at the helm records the compass reading while the boat is being held, fixed, while the compass settles and the reading recorded. Even if this is done for a number of directions, it shouldn't take too long. This should remove all the cross track errors from whatever source as it seems like having the boat moving makes things more difficult. The result should show the difference between true directions and individual compass directions for that particular location. Sorting out the difference between true and magnetic north could likely be derived from the chart information and accounted for. Admittedly this is going to be easier in a boat of relatively shallow draft such as a multihull or a power boat where the self righting apparatus (keel) does not cause complications. Since I am new to all of this, is there some reason that there is so much preference to doing this while the boat is traveling?

[lagune]

i'm relatively new to all of this and the effective techniques look very difficult to me, on a 22ft sailing boat mainly used in the lakes in holland and a bit coastal sailing as well is it ok to use COG to make the deviation table?

[psneeld]

Originally Posted by lagune

i'm relatively new to all of this and the effective techniques look very difficult to me, on a 22ft sailing boat mainly used in the lakes in holland and a bit coastal sailing as well is it ok to use COG to make the deviation table?

It can be done but there are a few intricacies that you should really understand...if you don't and have a good eye for on the fly corrections/backup/etc...probably better to do it the old fashion way if understanding that way is easier to get your head around.

Using a GPS to do all the old school compass swinging is actually pretty easy if you know the tricks and understand where they are and what can go wrong with your calculations. The compass installation manuals that
I have used lately explain the procedure in my mind pretty simply...then again...I have a good grasp of the old ways too.

[hellosailor]

I would suggest using COG to make a deviation table is not a good idea, since COG does not tell you if the boat is making leeway (crabbing sideways) and the course over ground may not match the bearing. Obviously you could correct for that but then you've just more a more complicated solution, more chances to make errors.

[cal40john]

Originally Posted by hellosailor

I would suggest using COG to make a deviation table is not a good idea, since COG does not tell you if the boat is making leeway (crabbing sideways) and the course over ground may not match the bearing. Obviously you could correct for that but then you've just more a more complicated solution, more chances to make errors.

When I did it I was on a lake on a windless day with the autopilot steering a constant course long enough to see it settled out. Of course you need to see if the engine running or not has an effect.