Evan Gatehouse wrote:

Ships can come over the horizon in about 10 minutes to your position
if moving at say 22 knots.

AFAIK the horizon is 22 odd miles away. A ship moving at 22kts would
therefore take about an hour to get to you.

Dan wrote:
Evan Gatehouse wrote:

Ships can come over the horizon in about 10 minutes to your
position
if moving at say 22 knots.

AFAIK the horizon is 22 odd miles away. A ship moving at 22kts would
therefore take about an hour to get to you.


Sort of. A bunch of common misconceptions revolve around the "horizon".
Your horizon, the water perimeter you see will be around 2-3 miles
depending on your height. The ship on the other hand being much higher,
and with lights high up at night, will have much longer horizon of its
own. Couple those to get the distance at which you see it, and you are
looking at 15+ miles.

On the other hand, take one of those bloody cross channel cats at 40
Knots and it get s a bit more interesting.

PyroJames

The horizon depends on the height of your eye - the dipping distance
tables in the almanac give the distance off for height. You will see
objects above sea level 'over the horizon' according to their height so
as the ship approaches, you will see more of it.

It is all in the yachtmaster shorebased course - possibly day skipper too!

Nick

Dan wrote:
Evan Gatehouse wrote:


Ships can come over the horizon in about 10 minutes to your position
if moving at say 22 knots.


AFAIK the horizon is 22 odd miles away. A ship moving at 22kts would
therefore take about an hour to get to you.

Nick wrote:
The horizon depends on the height of your eye - the dipping distance
tables in the almanac give the distance off for height. You will see

objects above sea level 'over the horizon' according to their height
so
as the ship approaches, you will see more of it.

70 foot high ship viewed from 3 metres high. I can't be arsed to do the
maths but I bet it isn't *far* off 22 miles.

What would you estimate?

Dan wrote:
Nick wrote:

The horizon depends on the height of your eye - the dipping distance
tables in the almanac give the distance off for height. You will see


objects above sea level 'over the horizon' according to their height

so

as the ship approaches, you will see more of it.


70 foot high ship viewed from 3 metres high. I can't be arsed to do the
maths but I bet it isn't *far* off 22 miles.

What would you estimate?


Not quite that far. Dipping distance(nm) is 2.08 * SQRT(height in
metres). 3 metres gives a horizon at 3.6 nm. The 21 metre ship has a
dipping distance of 9.5 nm. So at 13.1 nm you will see the tops of the
masts and at 3.6 nm you will see the hull down to waterline.

Nick wrote:

Not quite that far. Dipping distance(nm) is 2.08 * SQRT(height in
metres). 3 metres gives a horizon at 3.6 nm. The 21 metre ship has
a
dipping distance of 9.5 nm. So at 13.1 nm you will see the tops of
the
masts and at 3.6 nm you will see the hull down to waterline.

So it aint gonna hit you in 10 mins.

On Tue, 1 Mar 2005 11:52:34 UTC, "Dan"
wrote:

: Nick wrote:
:
: Not quite that far. Dipping distance(nm) is 2.08 * SQRT(height in
: metres). 3 metres gives a horizon at 3.6 nm. The 21 metre ship has
: a
: dipping distance of 9.5 nm. So at 13.1 nm you will see the tops of
: the
: masts and at 3.6 nm you will see the hull down to waterline.
:
: So it aint gonna hit you in 10 mins.

If it's a HSS it'll have time to hit you, circle the area
machine-gunning survivors, nip back home and come back with a wreath
in ten minutes.

Ian

Ian Johnston wrote:
On Tue, 1 Mar 2005 11:52:34 UTC, "Dan"


If it's a HSS it'll have time to hit you, circle the area
machine-gunning survivors, nip back home and come back with a wreath
in ten minutes.

In which case keeping a watch wouldn't have done you much good anyway!

On 1 Mar 2005 01:24:24 -0800, "Dan"
wrote:

AFAIK the horizon is 22 odd miles away. A ship moving at 22kts would
therefore take about an hour to get to you.

=========================================

That's way optimistic, even for a good radar. Good sized boats
disappear from view on my flybridge at around 8 miles, small boats at
2 to 4 miles.