Directions, Rotations, Translations (Heading/Course/Drift)

[Pieter Boelen]

Compasses point North. Everybody knows that.
But do they? Really?

A Magnetic compass points to Magnetic North.
Which hangs around somewhere in the islands of Canada.
It also slowly moves around.

Plus the Earth Magnetic field doesn't point straight to it but there's Variation depending on where you are.
And if you've got a Magnetic field of your own (metal in your ship) then you get an extra Deviation too.

Not so easy then...

 

[Pieter Boelen]

A Gyro compass is more predictable.
Still doesn't point to True North, but instead makes a 90 degree angle with the direction of the Earth's rotation.
Which is to say... the Earth's rotation that it feels.
If your ship starts moving, the Gyro compass starts pointing somewhere else too.

This difference can be calculated given your direction of movement (and it's a compass so it already knows that), the speed of that movement and your latitude.
On modern ships, the GPS for speed (over ground; see later) and latitude is fed directly into the Gyro compass.
This helps it to very nearly point to True North.

Once it's booted up, that is. But literally "taking it's bearings" can take a good couple of hours.
That's why a Gyro compass is generally kept permanently on because you don't want to go through it's start-up process more often than you need to.

Also worth being careful about what happens if the GPS starts acting up and feeding wrong position/speed information into the Gyro.
I've seen this happen and as all other bridge equipment (rate of turn indicator; RADAR's) was linked to the Gyro, literally everything starts going wrong at the same time.
Of course this happened at night in the Chilean Fjords, which is probably one of the worst places for this to happen.
No lights on the shore, very deep until you hit a cliff-side, strong currents; generally not an awesome situation and the ship ended up sailing completely blind for a while.
With over 1000 people on board at the time.

Extra word of caution for the use of the word "Gyro".
Mistakes are easily made, but there's a massive difference between a Gyro CHIP and Gyro COMPASS.
The chips are small, cheap and measure only (rate of) rotation. Basically what's found in any mobile phone these days.
A compass on the other hand is large and bulky and made to show a direction rather than a rate of rotation.
Completely different animal then.

The ones I've seen were cylinders half a meter in height or so. Not applicable on small ships (or models) then.
Especially since also rolling/pitching affects a Gyro and will make it point in other directions than it's meant to.

 

[The Nameless Pirate]

Once more, a great read.
Thanks for making these threads!

 

[Pieter Boelen]

Communication lessons to take away so far:
- When mentioning a direction of North, include the type of measurement: Magnetic, Gyro or True.
- The word "Gyro" is ambiguous. Differentiate between direction (compass) or rotation (chip).

And since we're discussing ambiguity and rotations, it's also worth mentioning what is rotating.

For example, in model tests, 'rate of turn' often refers to the (maximum) speed with which the rudder(s) can rotate.
The default is 35 degrees one side to 30 degrees the other in 28 seconds, or (35+30) / 28 = 2.32 degrees per second.
This is the minimum speed for one steering pump as per the International Maritime Organisation (IMO).
To avoid confusion, I like to call this the Rudder Turn Rate (RTR) as this quickly and intuitively communicates what exactly we're talking about.

There should always be a backup pump and so the rudder(s) can also move at double speed (2 x 2.32 = 4.64 deg/s).
Switching to two pumps gives a bit more responsiveness, but more often than not the reason for enabling double pumps during manoeuvring is simple backup 'just in case'.
The difference between 1 or 2 pumps is practically negligible, but you really don't want to have to deal with the difference between 1 and 0 pumps.

In other words, where models tests focus largely on the rudders themselves, this gets overseen once the full-size ship hits the sea.
While somewhat unfortunate, it also makes sense. Rudders and their speeds are but a small element in a much larger whole.
There can easily be several minutes between rudder orders and so even a 'slow' rudder has time to catch up on the requested angle most of the time.

Much more relevant is the rotation rate of the ship herself, which is a pretty crucial piece of navigational information.
This value divided by the speed gives a close approximation of the radius of the turn.
Plot this in a chart (or use a Curved Heading Line on the RADAR screen) and it becomes relatively simple to predict where the ship is going to end up in the future.
Nice thing to know as 'not hitting anything (in the future)' is a pretty major part of the business.

Clearly this deserves attention and therefore the "Rate Of Turn" or ROT-indicator is given a place of prominence.
As a large indicator at the centre of the ceiling display, it is easily visible; pretty much wherever you're standing on the bridge.

So far, so good.

 

[Pieter Boelen]

But... the name 'ROT' conveniently misses out the word 'ship' and therefore technically doesn't mention what is rotating (in defiance to my recommendation above).
Seafarers understand that term obviously refers to the entire ship, while people involved in model tests understand equally clearly that the term relates to the rudders.
This is where the practical and theoretical people start to lose each other and, even without it being the intention, two sides of the same coin are now talking (slightly) different languages.

It gets worse when inserting units into the mix. The official SI unit for time is the second.
But a second is a very short period for a human being; and especially for a large ship moving.
A ship rotation rate given in seconds therefore leads to very small values that are difficult to interpret intuitively.
This is why 'ROT' at sea is given in degrees per minute instead. That gives large numbers, easily differentiated; and easily remembered.

Better yet, ROT [deg/min] divided by speed [knots] is approximately equal to radius of turn [nautical miles].
Very practical. Very non-SI. And by now, scientists and seafarers have thoroughly lost each other.

Completely unintentionally, but simply by approaching the same reality from two quite differing points of view.
Even though these are all people working towards the common goal of safe, efficient shipping.

 

[Pieter Boelen]

The way I see it, these sort of divides are quite unfortunate as they stand in the way of effective cooperation.
Seafarers being seafarers, they're a bunch of stubborn individuals and, I'm sorry to say, not always the most open-minded bunch.
They like to keep things simple and, given the real-life risks they deal with on a daily basis, that's probably wise.
Therefore shaking up terminology and units on their end is not going to happen; and neither should it.
There's hundreds of years of experience in the real-life navigation and ship handling and this is not something to throw away; or just ignore.

My question then to the world: would scientists be willing to (somewhat) let go of their hard maths and adherence to SI units?
For the sake of matching with historical and practical truths that would help them communicate more effectively with the people 'out there'?
And that could (and probably would) allow for effective cooperation between very different people who, in the end, do actually want to accomplish the same things?

Only time will tell...

 

[The Nameless Pirate]

My question then to the world: would scientists be willing to (somewhat) let go of their hard maths and adherence to SI units?
For the sake of matching with historical and practical truths that would help them communicate more effectively with the people 'out there'?
And that could (and probably would) allow for effective cooperation between very different people who, in the end, do actually want to accomplish the same things?

Only time will tell...

As you said, time will tell.

But I think that it will probably happen, after all who wants to risk lives fir this?

 

[Grey Roger]

Compasses point North. Everybody knows that.
But do they? Really?

A Magnetic compass points to Magnetic North.
Which hangs around somewhere in the islands of Canada.
It also slowly moves around.

Plus the Earth Magnetic field doesn't point straight to it but there's Variation depending on where you are.
And if you've got a Magnetic field of your own (metal in your ship) then you get an extra Deviation too.

Not so easy then...

Don't ships' magnetic compasses have magnets on them to compensate for the ship's magnetic field? I'm fairly sure that's what the red and green spheres on these two are for:

 

[Pieter Boelen]

Don't ships' magnetic compasses have magnets on them to compensate for the ship's magnetic field?

Absolutely correct.

Those need to be tweaked in such a way that they compensate the ship's magnetic field as much as possible.
I've once been involved with just that on the Maasdam.
We had an engineer on board to do it and we took the ship for two full 360 degree turns in Halifax harbour.
"Swinging the compass" it is called.

The result is an improved compass along with a Deviation card.
It's not possible to get this inaccuracy down to zero, but we got it below a maximum of 2 degrees.
That's pretty respectable I'd say and more than accurate enough to be useful.

Ship was a big lump of metal though.
I suppose on a wooden ship the inaccuracies should be much less.

Compass on my own boat has no compensation.
And though I haven't actively used it for pinpoint navigation, the numbers seem pretty accurate.
Polyester (plastic) boat. Not much metal around to mess things up.

This applies both for the regular and the 'fluxgate' one.
That second measures the magnetic field eletronically.
Useful to tie into digital onboard systems.

I now have a special compass on board that has a gyro chip also built in.
This measures the ship motions so it can subtract those from the compass heading.
That makes quite a difference on a small, bouncy ship.

It took me some substantial discussion with the supplier to gather how it worked.
They made it sound like a Gyro compass, but that seemed too unlikely to me.
Rightly so as it turned out to be a gyro-stabilized Magnetic compass.

That makes a difference!
Because if you don't know what kind of compass you've got, you might start applying the wrong type of corrections.
And obviously that doesn't do any good...

Especially 'entertaining' when I was reading an official cabin sailboat handbook and it made mention of Gyro compasses.
I'm really curious if those even exist at all for (what I call) small boats.

For me, it just doesn't make sense...
They're too big. Too unweildy. Too heavy.
And too affected by vessel motions.

By the way, in addition to those two balls, there should also be a long, thin cylinder in the middle.
Probably on the other side of those photo's.
That's also part of the magnetic compensation.

 

[Mad Jack Wolfe]

The way I see it, these sort of divides are quite unfortunate as they stand in the way of effective cooperation.
Seafarers being seafarers, they're a bunch of stubborn individuals and, I'm sorry to say, not always the most open-minded bunch.
They like to keep things simple and, given the real-life risks they deal with on a daily basis, that's probably wise.
Therefore shaking up terminology and units on their end is not going to happen; and neither should it.
There's hundreds of years of experience in the real-life navigation and ship handling and this is not something to throw away; or just ignore.

My question then to the world: would scientists be willing to (somewhat) let go of their hard maths and adherence to SI units?
For the sake of matching with historical and practical truths that would help them communicate more effectively with the people 'out there'?
And that could (and probably would) allow for effective cooperation between very different people who, in the end, do actually want to accomplish the same things?

Only time will tell...

Their desire to keep things as uncomplicated as possible is completely understandable. The last thing you want to deal with during an emergency is a 150-page manual for a single instrument. And as you said, practical knowledge refined over the course of centuries is an easier thing to trust than a relatively "immature" technology.

Even with all that knowledge and tech, and doing absolutely everything right, the sea can still kill you. But if you're a very good navigator, you'll know exactly where you were when you died.