How far away is a ship?

[pugwash]

Lots of talk on a previous post (Yachts can be too careful) about what to do when a ship is three miles away, or two miles, etc. But forgetting radar for the moment, is there a rule of thumb for judging the distance by eye? For instance, when you can see the stem cutting the water it's one mile, or something similar. When you can read the name it's too damn close. I'd like to see a graduated table for different distances. Any thoughts?

 

[Skyva_2]

A bit difficult, depending on the size of the ship. But I have always assumed that from a yacht you can't see any ship which is more than 7 or 8 miles away. Is that right?

 

[sarabande]

What about a pair of binos with "mil grid" in one eyepiece ?

Or these ?

http://www.cheaperthandirt.com/SCP003-1027-1395.html

There must be lots of ex-military optical rangefinders on the market, as laser designator/rangefinders seem to have taken over.

 

[MoodySabre]

I recall reading somewhere that a ship sitting on the horizon is three or four miles away. No idea where I saw that unless it was Yachtmaster stuff.

 

[Talbot]

that is a hopelessly inaccurate estimate. The only way to estimate is to try to work out how big the ship is, and then estimate distance from that. It needs a lot of experience and even then you can get it hopelessly wrong if you have no idea of the size of the ship.

 

[davierobb]

Taken from the distance of the sea horizon table in Nories Nautical Tables.

From a height of eye of 2 metres Horizon is 3 NM so you would be able to see the complete structure of a small yacht at 3NM, assuming your eye is 2 Meters above sea level. On the other hand the top of the superstructure on a ship such as the Emma Maersk at 30 Metes high would just become visible at a distance of 11.9 NM, assuming height of eye 2 metre.

Other values,

Height/Horizon

2/3
5/4.7
10/6.6
15/8.1
20/9.4
25/10.5
30/11.5
32/11.9

 

[Richard10002]

Pugwash,

I think you are asking generally so, when I say "you", I mean people in general, rather than you - if you see what I mean.


The fact is that you dont know by merely looking - period. You can guess...... but that's all.

You dont know how big a ship is until it gets close, so you might be seeing a big ship a long way away, or a small ship closer. I have seen what I thought were 2 ships on the horizon but, as it got closer, it turned out to be one very big ship appearing over the horizon. I have seen what I though was 1 ship, but as "it" got closer, it was 2 fishing boats... and so on. With this in mind, a graduated table isnt possible.

With radar, you can establish distance reasonably accurately, as long as the vessel you can see visually is the one you can see on the radar... usually, but not always the case - the rules talk about scanty radar information.

What you can usually do, with some experience, is establish when it is time for you, or them, to take some action... i.e. if it feels like it's time, it's time. If it's you who should give way, make the alteration. If it is them who should give way, and they havent, and you think they should have, make your alteration. As long as it's bold, and an appropriate action, you shouldnt come to any harm, and earlier is better than later.

There will be times when you think you could have done it differently, and that will add to your experience for next time.

 

[Guest]

The worst example is a laden oil tanker verses a empty one. The bow wave is the best give away. When you can see it then it is your horizon distance.

Some ships look very similar in shape but can be many times the size.

If you can read the name then its too close.

In general never look at the ship. Look at the water between you and their water line.

This is also the case in anchorages. Never try to judge the distance by comparing sizes of yachts or you will end up too close to the small ones. Look down at the water and compare the distance without looking at the yacht.

 

[Cruiser2B]

Determine your height of eye in your cockpit; go to previously-mentioned Nories and figure out distance to the horizon from your perspective. That gives you a good rough guide for ranges of anything below the horizon, through interpolation. Anything over the horizon is much more of a rough guess, assuming you can judge the height of the other vessel with reasonable accuracy. Wherever you can, confirm your estimates - either by radar or measurement on charts, etc. As you gain experience with this, your estimates will become more accurate.

 

[ChrisE]

As other have said, you can see the bow wave of a big ship at about 2-3 miles off which, by happy coincidence, is when it is worth worrying about whether they are going to cause you any bother.

 

[doug748]

It may also be of use:
Large buoy visible - 2M Colour can just be seen - 1M
Small - 1 1/2M Shape """""" - 1M
Figure can be made out - 1M Legs and arms show - 1/4 M
Face can be made out - 2/300 m
Coastline:
Trees and large buildings can be made out at 4M
Windows will appear as dots - 2M

 

[Andrew_Fanner]

[ QUOTE ]
It may also be of use:
Large buoy visible - 2M Colour can just be seen - 1M
Small - 1 1/2M Shape """""" - 1M
Figure can be made out - 1M Legs and arms show - 1/4 M
Face can be made out - 2/300 m
Coastline:
Trees and large buildings can be made out at 4M
Windows will appear as dots - 2M

[/ QUOTE ]

Aha, that's not what you get with -11 and -11.5. Nowt, nowt, nowt, blob, bigger blob, big bigger blob, bit bigger coloured blob, "Feck me we just missed that"

 

[VictorII]

As I replied in the mentioned post, at night when you see the reflection of the lights in normal swell, you are between half and one NM away. In heavy swell, half a Mile or less.

 

[savageseadog]

If you can hold a conversation with the ship's captain it's too close. Yes it's happened to me.

 

[johnalison]

As I said yesterday, it's a good idea to practice by studying ships at a known distance which, if you haven't got radar, you can establish by GPS from a charted object such as a buoy which the ship is passing. You can make this a game with the crew.

 

[tome]

Judging by eye is quite difficult and requires years of practise. The general tendency in my experience is to under-estimate distances

Using radar is great for training and one reason why you shouldn't only have it on when viz is poor. After several years on the bridge of survey ships (where the radar is always on) i've become reasonably good at judging distances

Visibility of navigation lights is also a good clue - again observing radar will help develop your skills

 

[FlyingSpud]

There is the old thing that if you close one eye, stick your arm straight out and shut one eye an line it up with the object, if you then keep your arm still, but change the eye you are using, the movement of the object along the horizon is one seventh of the distance between you and it.
Or something like that (also depends if you have arms like gorillas)

 

[longjohnsadler]

Erk..