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Re: Looking for references on "aiming off"
From: Hewitt Schlereth
Date: 2010 Sep 10, 20:35 -0400
From: Hewitt Schlereth
Date: 2010 Sep 10, 20:35 -0400
There are some more general sources for aiming off, often quite good reading, just as a good read. I'm thinking of Harold Gatty who was Wiley Post's navigator in the Lockeed Vega (?) Winnie Mae. There he details how he made sure he found the re-fuel sites by aiming off so as to always know which way to turn when he reached a known land feature - often a river.
Then there's Erskin Childer's novel The Riddle of the Sands, in which aiming off plays a crucial part.
I have a notion the technique is well know to hikers, and the lore of woodcraft or any outdoor activity would be replete with examples. This may be a case of an embarrassment of riches.
One of Gary's WW II flight manuals has an example of a flight across the Atlantic to Dakar that uses it. I've used it in fog by aiming to find a specific fathom curve well to one side of a harbor and then following the curve to the entrance buoy.
Hewitt
Then there's Erskin Childer's novel The Riddle of the Sands, in which aiming off plays a crucial part.
I have a notion the technique is well know to hikers, and the lore of woodcraft or any outdoor activity would be replete with examples. This may be a case of an embarrassment of riches.
One of Gary's WW II flight manuals has an example of a flight across the Atlantic to Dakar that uses it. I've used it in fog by aiming to find a specific fathom curve well to one side of a harbor and then following the curve to the entrance buoy.
Hewitt
On Fri, Sep 10, 2010 at 7:53 PM, George Huxtable <george@hux.me.uk> wrote:
Irv Haworth shows a touching faith in my abilities to explain these
matters, and I'll try not to let him down.
Trouble is, Tom Temple has told us very little about his initial knowlege;
for a starting-point, but judging by his question "Has anyone heard of
off-aiming in maritime navigation, perhaps under a different name?", I'll
assume he is starting near the beginning.
The most dangerous phase of an ocean passage is usually the landfall;
particularly so when a vessel's position is not well known.
Gary has pointed to an important example of heading off, of pointing the
vessel in some other direction than directly toward the destination or next
headland, in the days before longitude could be determined. The aim was to
arrive at the latitude of the destination, long before the destination
itself was reached. Then sail down the latitude line, and the observer
could be pretty sure he would arrive at his destination, even if he had
little idea when that would be.
It can go spectacularly wrong. The most famous occasion was in Anson's
circumnavigation in his flagship "Centurion" in 1741, after his fleet were
scattered in a hard Westward rounding of the Horn. He waited off the island
of Chiloe, but none turned up, so he set off for the next planned
rendezvous, the island of Juan Fenandez, way out in the Pacific, with the
crew suffering badly from scurvy.. Having reached the right latitude,
without sight of the island, all that was necessary was to sail down that
latitude line until it showed up. But which way, Eastwards or Westwards?
With only dead-reckoning to rely on for longitude, Centurion's master
decided that any longitude error would have put her too far West, so
plumped for sailing East until the island appeared. It only took a few days
to travel downwind, until in a dreadful moment, the high mountains of the
(hostile) Chilean mainland showed up. The guess had been wrong. Then it was
a matter of beating Westwards, back to the island, which took over a week,
in which time, the ship's company were dying like flies. That episode cost
hundreds of men's lives.
But there's another stage in a voyage at wich it could be useful to head a
bit away from a destination. Say a vessel is heading down a latitude line
towards an intended harbour, but near the end of the voyage, cloudy skies
have put the ship's latitude in some doubt. The deep-sea log is used from
time to time, to discover when she has reached soundings. The shore line
comes into sight, but it may be undistinguished or unfamiliar. So, which
way should the vessel turn, along the coast, to seek her destination? The
trick was, to aim for a latitude that's a bit North, or a bit South, of the
destination, but not the destination itself. Then, on reaching the
coastline, it's clear which way to turn; North or South.
In tidal coastal waters, a situation in which I'm much at-home, with a
vessel that's little faster that the strength of the tidal streams, other
considerations come in. If travelling cross-tide to a destination, you have
to make due allowance for tidal offset, for which it's necessary to make a
good guess at when arrival will be. But then, on top of that, the sin is to
find yourself down-tide, and having to fight the current, in the final
approach. So the common trick is to note which direction the tide runs in
that final approach; then head-off appropriately to make quite sure you end
up up-tide of the destination.
I haven't answered Tom Temple's request, because he has asked for
references, and I've provided none; just common seamanship. No doubt it's
written up in texts somewhere, but not the ones on my shelves. Sorry about
that.
George.
contact George Huxtable, at george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
Sent: Friday, September 10, 2010 7:46 PM
Subject: [NavList] Re: Looking for references on "aiming off"
Hi George H.
While I do not want to burden you with extra work , I "know"?(from your
past posts) that you are able to give this chap more practical techniques
that you have used in your sailings...say to France, in order to get him
moving in the right direction. (Tidal currents, wind age factor, seamanship
, fixes, reliability of charts use of Pilot Charts etc.
( You might also want to touch on the easting-westing routine )
Hope you don't mind my 2 cents worth .
Kind regards
Irv
-----Original Message-----
From: navlist-bounce@fer3.com [mailto:navlist-bounce@fer3.com] On
Behalf Of George Huxtable
Sent: Friday, September 10, 2010 10:15 AM
To: NavList@fer3.com
Subject: [NavList] Re: Looking for references on "aiming off"
contact George Huxtable, at george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
----- Original Message -----
From: "Gary LaPook" <glapook@pacbell.net>
To: <NavList@fer3.com>
Sent: Friday, September 10, 2010 6:07 PM
Subject: [NavList] Re: Looking for references on "aiming off"
| "Aim off" is also known as "single line of position landfall
| procedure" or simply "landfall procedure."
|
| gl
|
| On 9/10/2010 6:20 AM, mjans wrote:
| >
| > I'm a PhD student and am working on a paper quantifying when and by
| > how much to aim off. Can anyone point me to any references on the
| > subject? For instance from [NavList 10886]:
| > "Noonan could be expected to have known this technique since he knew
| > Weems and Chichester had popularized in 1931."
| >
| > How did they do that?
| >
| > Has anyone heard of off-aiming in maritime navigation, perhaps under a
| > different name?
| >
| > Thank you very much.
| > Tom Temple,
| > Ph.D Candidate,
| > Laboratory for Information and Decision Systems,
| > Massachusetts Institute of Technology
| >
| > At the moment, my results are still preliminary but I would be happy
| > to share them with anyone who is interested.
| >
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