Real Navigation - Running Fixes (Tutorial... sort of)

[LemonA]

I would like to ask how "Doubling Angle on Bow" works?

[Karl Heinrich]

No problem, LemonA.

Again, "doubled angle on bow" is generally less accurate in coastal nav than two point (or more) cross fixes. But if you only have one landmark, it can be useful like running fixes (Although I prefer the latter).

Again the most difficult thing is finding a clearly identifiable bit of land.

The Actual Explanation

Basically, you take a bearing of the landmark when it's at a nice round number (not essential of course) and under 45 degrees, e.g. 20 degrees. Plot this on the map across your dead reckoning course/track line. Note time and speed as always.

Proceed along the set course and keep an eye on this landmark, when it reaches 40 degrees in this example, note the time and plot another position line. Consulting speed/distance charts calculate the distance travelled. In this case, say it's 12km.

This works because it generates an isosceles triangle, so the distance between your boat and the second bearing taken is the same as the distance travelled. So 12km down the second bearing position line is where your boat is. If you're on couse this should pretty much be where the second bearing position line and your intended course line meet.

The alternative is the Four Point Bearing - same procedure, but the first bearing must be taken at 45 degrees and the second at 90 degrees.

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Apologies it's a bit of a rough explanation, I'll do a proper one with pretty pictures soon - for some reason the screenshot function in SH3 wasn't working before but is now.

I'll put together some revised guides shortly, this one was a little more haphazard than intended. If I can get FRAPS working I'll do it as a video tutorial.

If anyone knows to what extent wind affects position it would be appreciated, need to do more tests on this to calculate "set and drift".

I know time compression affects speed as wel (but only in rough seas it seems), I recall Vanjast suggesting it was around 15% increase, and my single test so far has shown it to be 14% (so near enough).

I'm also planning a world and map mod that adds more notable landmarks (lighthouses etc). to the landscape - assuming this is feasible. The intention is to focus on a relatively small area first. Where possible these will be historically accurate

[LemonA]

How does it works here?

Course 303 t,
speed 8 kts
1. bearing to landmark: 281 rel

calculation...
281 * 2 = 562 = 202.
But waiting for 2. bearing 202 rel is not successfull.

[LemonA]

Originally Posted by Karl Heinrich

I'm also planning a world and map mod that adds more notable landmarks (lighthouses etc). to the landscape - assuming this is feasible. The intention is to focus on a relatively small area first. Where possible these will be historically accurate

This!

[Karl Heinrich]

Apologies, my explanation wasn't clear, I rushed it.

You wouldn't be able to take a bearing at 281 as that's a relative angle of 79 degrees (if you found yourself seeing something for the first time at that bearing, then you could use the Transferred Position Line in the main post )

For Doubled AOB it needs to be less than 45 degrees for it to work - and no less than 10 degrees ideally - best results are at around 30 degrees I find.

So on the port side, you can take the first readings from relative bearings 316 up to 350 and on the starboard side 10 to 44.

So it's not the bearing that you need to double but the angle. I know it's my fault for not making that clear before. Part of the confusion being what is meant by relative bearing.

So a relative bearing of 330 is an angle of 30 degrees, so it's double is (obviously) 60 degrees which is a relative bearing of 300.

Pretty Pictures and Example

(I find it much easier to explain with images)

So in this example, I plotted your course the same as your example, 303T.



As always it's important to keep your speed and heading constant (I know you already know this LemonA).

Through the UZO or 'scope, you spot a lighthouse ahead. You wait till you know you will get a good bearing for a fix.

At 330 relative bearing on the UZO/Scope you plot the position line on the map and note the time - 0900 GMT. As this gives you 30 degrees AOB to the lighthouse, you keep checking till you have 60 degrees AOB to the lighthouse - relative bearing of 300 (on the UZO/scope reading).

Travelling at 8kts, this takes you 30 minutes - consulting the charts tells you that you have travelled 7.41 KM. You've plotted this on your map and this gives you a nice isosceles triangle - so you know you have two lines of equal length - we know the distance from bearing #2 to your position is the same as the distance travelled along the course.

Check bearing line #2 against your log, you know you've travelled 7.4 km so check with the ruler that bearing #2 is 7.4 km. If you've stayed on course this should be where line #2 and your course line meet. If not, then you are roughly where your ruler line ends at 7.4km...

I feel like I've made this even more complicated now, I really hope it makes sense.

You can also do it where the first bearing is 45 degrees AOB and the second 90 degrees AOB, that's slightly more accurate generally.

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As I said, I planned to do a proper tutorial for this too... Trying to translate certain RL things to SH does need a bit of tweaking doesn't it, it's almost impossible to take compass bearings as you would IRL unfortunately.

With this it's much easier to work out the "relative bearing" without ambiguity.

On the plus side of course we know in SH magnetic north = true north so no need to calculate for magnetic deviation.

[LemonA]

Found "Double the Bow" formula:

AoB_2 = AoB_1 + (180 - (2*AoB_1)) , if AoB_1 < 90
not available , else

with AoB_1 is first AoB shot and AoB_2 is second AoB shot.

Example:
AoB_1 = 13
AoB_2 = 13 + (180 - (2*13)) = 167

[Karl Heinrich]

That seems needlessly complicated lol. That looks like a TDC targetting formula. We just need to count down or up from 360/0.

IRL you'd point a compass at the lighthouse and subtract the compass bearing from your heading to get the appropriate relative bearing.

So on a heading of 303T we point the compass at lighthouse, compass says 283.
303 - 283 = 20. So we know we need a second reading when it's 40, working backwards 303 - 40 = 263. Doesn't work quite the same in SH but the principle is the same of course.

[LemonA]

Originally Posted by Karl Heinrich

it's almost impossible to take compass bearings as you would IRL unfortunately.

Thats so true. On the other hand the SH3 bino and/or UZO are sort of replacement tools.

[Karl Heinrich]

Originally Posted by LemonA

Thats so true. On the other hand the SH3 bino and/or UZO are sort of replacement tools.

Exactly, but that's when the terminology of "relative bearing" gets mixed up. It's good to get your feedback though, it helps me to see what bits need to be clearer when I get round to doing the video tutorials.

I have tried doing vague bearings from the compass on the bridge though, good for confirming bearings but bloody useless if you want anything precise lol.

[LemonA]

Originally Posted by Karl Heinrich

I have tried doing vague bearings from the compass on the bridge though, good for confirming bearings but bloody useless if you want anything precise lol.

As I have said already...

Maybe you add to your list also "Danger bearing", a fundamental coastal navigation method.