How far can you see?

Brian · 16 August 2002, 17:11

A multiple choice thread, this, for anyone who went to school and was awake during maths.
1). On a perfect day, from the helm of your RIB, how far can you see before the earths curvature obscures your view of your fellow ribster?
2). If you climbed up your yacht's mast (the one you keep back at the Marina and don't tell anyone on this Forum about), to 10m. How far can you see then?
3). I ask because a) I am an anorak and b) because my house is 100m exactly above sea level with views across Torbay. Out of idle interest, while sipping my G&T on the verandah, I wondered how far I could possibly see.
There is a formula, isn't there?
Anyone bright spark know what it is?

sirzap · 16 August 2002, 17:43

"On a clear day, you can see forever"...

Brian · 16 August 2002, 18:03

Yers..thank you Neil. I just knew the US contingent would be able to offer SOMETHING useful to this highly technical discussion.
I actually ask the question to cover the embarrasment of my wife saying to me "Brian, how far away do you think that ship is on the horizon over there?"
And me having to answer "Erm, paralax, Darling, earths curvature and all that malarky..Oh look, my glass is empty, must get a refill". In the hope that when I get back she will have forgotten all about it. Trouble is she is going to insist on an answer one day and she thinks I'm quite bright. Don't want to disillusion her just yet.

Alan · 16 August 2002, 18:07

What yer need to do Brian is fit a Radar scanner on the roof then you can look respond to Christines question with "23nm its steering 090 at 24kts and its CPA is 18miles"!!!

Charles · 16 August 2002, 21:09

I was always led to believe that one can see approximately 25 statute miles?!

Ribald · 16 August 2002, 22:56

Brian,

From page 37 of the current Macmillan Reeds:

Ht of eye of 1.5m = horizon of 2.5M
Ht of eye of 100m = horizon of 29.4M

Of course these are dependant on the quantity of G&T's consummed and whether you are horizontal or vertical at the time of sight; ht of tide; viz, etc etc

Peter (anorak-in-waiting)

badbaws · 19 August 2002, 09:27

It also depends on where u are I work on the bridge of a ship and due to refraction and other phenomena have seen land that is 40-60 miles away.....
But generally on a clear day...20ish miles from the ship's bridge and in a RIB about....5-8 miles.....

alex

Alan Priddy · 19 August 2002, 21:23

Brian
Why worry about it? if you are there you are, if not you're not. Sometimes it is better to have poor visability, this way you do not have that longing of "are we nearly there"when you hit the harbour wall!

A piece of really good news is Allen C from the Isle of Man is back in circulation so watch out when he starts posting items on this site

The best answer when the wife / girlfriend ask's how far is lie about it!

Keith Hart · 02 September 2002, 09:05

Hi folks

Here is an interesting link on Boaters World Wide Browser:

http://www3.ns.sympatico.ca/leeh/default.htm

There are all sorts of useful calculators, including one to find the distance of the horizon. Put it in your favourites list. You need to scroll down the main page a bit and look in the box on the right.

Keith (web surfer extraordinaire) Hart

Brian · 02 September 2002, 09:26

Nice one Keith.
I especially like the geographic calculator.
Many is the time when we have been chugging along and said to ourselves "first one to see The Eddystone gets a free beer-should be just off our port bow". Now I know that if the Eddystone is x metres high, Wont be able to see it until I am y miles away. Bloody useful that.
To take Alan's point. Alan I find I get soooo bored that just about anything gets my interest as a point of discussion. Counting down the miles, can I see xyz headland, I wonder what that yacht bears from us, how does MARPA work, what the nearest port, what do you reckon the current is running at, ANYTHING !!!
For example, I pose the following:
I want to calculate how busy a motorway is from a fixed point, this is usually measured in number of vehicles per hour passing said point. Well, if the motorway is mega-choker then the number of vehicles PASSING the fixed point drops to a low number, doesnt it? So the motorway isn't very busy then is it? But we all know it's choker. So how do they do it.
Usually shuts the crew up for a bit whilst their brains slowly unfreeze!

Allen · 02 September 2002, 10:50

Wow Brian - I'm pleased not to have crewed with you !

It has taken me many years to perfect the technique of thinking about nothing other than the bare essentials when at sea, indeed nothing could be further from my mind than motorways

Brian · 02 September 2002, 15:17

Allen
Now would this be Allen The Fred Flintstone of Peel, sometimes seen breaking stones in a quarry? or perhaps......
Allen The Navigator, the only man who drives in a dead straight line from the Isle of Man to France and swerves round Lands End AND the Lizard and goes into Fowey? or perhaps.....
Allen The Engineer, who drives up to you when you are in a boat being propelled by a 2-stroke, outboard, petrol engine and asks you if you have any spare Yanmar diesel engine fan belts on you?
Obviously not...
It must be Allen Fat Chance.
Fat Chance of him getting a job as a navigator or engineer on my boat !!!
Cummon clever Dick, whats the answer to the motorway question then? Better hurry now, I sense BGLS (Brtains Greatest Living Sibster) is preparing a learned tome on this selfsame subject as we speak.

Allen · 02 September 2002, 16:08

No Brian you've got the wrong Allen - I'm the one who went the scenic way to France, but having now been to Fowey will not bother again, and I am the one who asked you did you know where the belt (power steering) might be available somewhere close on the south coast. I am also the Allen that gave up calling 'Cyanide' on the VHF that day, AND above all else I am the Allen that has managed to get 650 hours (and still counting) out of a Bravo 3 gearbox

Despite not having motorways on the Isle of Man, I do know the answer, but I can't resist waiting for Keith's reply .......

Alan Priddy · 02 September 2002, 16:14

And you are the Allen who is my best mate

Bittentobuggery · 02 September 2002, 18:56

Brian,

Only just discovered this thread and had to larf.

I know you don't need to work financially. but GET A JOB - even if it's only a part time lollipop man or a bit of basket weaving, just something to stop you chuntering away maniacally.

I'm sure your missus would be delighted to pack you up some marmite sandwiches and hold the front door open for you.

As for anybody who has known you for so long thinking you are "quite bright", I think there are 2 factors at play here.
1. Delusion on your part.
2. Confusion on hers. It must be all those moths fluttering in your beard.

All the Best,

Brian

P.S. You never answered my question ! Did you really eat that enormous pile of chips in the Channel Islands.

Keith Hart · 03 September 2002, 07:23

Brian

Quote:

I want to calculate how busy a motorway is from a fixed point, this is usually measured in number of vehicles per hour passing said point

Your problem here is that you are looking at the wrong measurement.

There are two different criteria. 1. Vehicles per hour - volume of traffic - , 2. Average speed of vehicles - rate of traffic flow.

As you are talking about CONGESTION then it is actualy the latter criteria that applies. If you are on a motorway and are doing 70 mph, but the motorway is full of vehicles, no problem as the flow of traffic is unhindered. If you are crawling (the average speed is low) and stop start and the motorway is full because the flow of traffic is held up, then you have congestion.

Both amount of vehicles per hour past a given point and average speed of vehicles past a given point are measured.

Both can then used to calculate an overall view of traffic usage.

For example 1 vehicle per hour @ average 10 mph = some idiot on the motorway who has fallen asleep! 100 vehicles per hour @ average 10 mph = traffic congestion.

This is further complicated by the fact that the number of vehicles over a given area of motorway is calculated at different points. This shows how many vehicles are going into the system. This can be compared with the number of vehicles leaving that stretch of motorway. It is then possible to calculate the number and rate of vehicles leaving the area. This will show if vehicles are being detained in the area and therefore there is traffic congestion.

Well Brian, there you have it.

Keith (must get out on Quicksilver again soon) Hart

Allen · 03 September 2002, 09:34

Quote:

Originally posted by Alan Priddy
And you are the Allen who is my best mate

Alan, you say the nicest things - I'll cook you your favourite meal when you get home tonight

Charles · 03 September 2002, 10:37

Ooooh, this is getting cosy, shall I invite the Village People?

Allen · 03 September 2002, 10:59

I thought they were already here ....

Brian · 03 September 2002, 18:48

Allen
I knew I had got it wrong!
You must be the Allen then that owns that beautiful Scorpion cruiser that is both a joy to behold and darts through the seas with the greatest of ease, and when alongside in harbour it's skipper is immediately surrounded by a throng of admiring, swooning women and teeth-grindingly envious men.
That Allen?