Confessional: admit to boat-related things you've never quite understood...

[vyv_cox]

I aways remember 'No Port Left'.

I can never remember the one for Starboard though.

When we first began cruising I thought up 'S = single, P = plural' for these sound signals. We still remember them.

 

[Keen_Ed]

Exactly how it works Using SOG and Track, or STW if no GPS signal.

Not using SOG/COG. Calculated using AWS, AWD and boatspeed and heading. Systems which give TWS/TWD to any useful degree of accuracy are more generally found on racing boats, and are highly sensitive to calibration. An estimate is made for leeway.

Using SOG/COG gives ground wind, which is of little use outside of checking actual conditions vs the forecast.

 

[laika]

True Wind is what you would feel if your speed log was reading zero. If you are in a current, you would still be moving.

It's a kludge invented before it was possible to measure Ground Wind, which is the absolute wind passing over the ground. To calculate this, you need a GPS.

Kludge? not sure. I think we want to know what the true wind is *right now* more than we need to know what the ground wind is. We need to be aware of the concept of true wind to know what things are going to be like when we change heading, what kind of tacking angle we'll need to keep up speed when close hauled etc. The main way we need to think about ground wind is in terms of how apparent wind will change with the tide, e.g. if I'm close hauled with the tide on my lee bow, I might be headed as the tide abates and turns. Of course I think most of us are happy with a simple readout of apparent (or nothing for the purists) with just a general idea of what will happen to apparent when we turn (ie true wind) or the tide does (ie ground wind). I'm not interested in true wind other than conceptually: What I actually want to know when I change heading is what the apparent wind on the new course will be and I can roughly work that out from what apparent is on the old heading without going through an intermediate "true" stage. But I reckon for people that want something other than apparent displayed, true is more useful than ground.

People making electronics probably give you additional information "because they can" rather than "because you need it". Traditionally they couldn't do ground wind but they could do true so you got true and apparent. Now you can get all three (allegedly, though I've still got ST50s

 

[AntarcticPilot]

Here's what Moitessier had to say about position lines, "It's worth noting that to draw a position line, you can begin your calculation with an assumed position that is way off: the position line will still determine the boat's position. For fun, I sometimes chose an assumed position 600 miles in error. In two calculations, the boat took it's true place on the chart".

I presume that Moitessier calculated a first PL from his wildly wrong position, then used that PL to correct his DR, and then did another PL based on the updated DR?

Basically, my understanding is that the DR position is in principle only used to provide an azimuth (direction!) for the PL; the PL's distance from the nadir point is measured directly by the elevation measured by the sextant. An assumed position 600 nm away from the actual position would only change the azimuth of the PL by 10 degrees or so (less the further away from the nadir point you are), so the first PL would still not be so bad, and if used to update the DR, a second calculation would get it pretty close.

 

[AntarcticPilot]

Vang: just another word for the kicking strap. Sometimes ascribed to be an Americanism. A means of controlling the twist in the mainsail.

I think "vang" originated on Thames barges and similar craft, where it is:

"VANG. One of a pair of wires rigged from the sprit end to the deck, controlling the sprit (port and starboard). The vang fall is the tackle rigged on the lower end of the vang, whose lower blocks are mounted near each end of the main horse. Rolling vangs are preventers led forward to complete the control of the sprit in heavy weather, in order to keep the sprit out to leeward. (Pronounced 'wang.') " (from http://www.thamesbarge.org.uk/barges/bargeglossary.html)

I don't know where it's use for the good old kicking strap has come from; it seems to be a new usage that certainly wasn't around when I was a lad.

 

[AntarcticPilot]

One of us is muddled quite possibly me. Muddeling my navigation or reading and comprehension I'm not sure.

Astro or Celestial Navigation as opposed to Terrestial. I suppose NASA must be into Exta Terrestial Navigation to find the Moon, Mars and other planets I believe voyager has now left the Solar System, Wondering what kind of navigation it is.

According to me.
If a "star" is at your zenith exactly 90 alt your position line is a dot. Not a line. you are at the stars terrestial position.
If your star is within about 1 or 2 degrees of your zenith your position line is definitly a circle and can be plotted using a drawing compass. Plot stars position measure the true zenith distance. If 1 deg you are 60 minutes from the stars position which can be drawn or plotted using lat scale and compass.
Theoreticaly you could also calculate time and azimuth and plot bearing and get a "fix" but unless you are at the N Pole the Stars move to fast. 15 deg per hour or 4 minutes per degree or 15 minutes per minute. 4 seconds per minute.

Hopfully I have now muddeled everyone.

If you have a big enough map (or better, a globe!) the Position Line is always a small circle (i.e. a circle defined by the intersection of a plane through the surface of the earth but NOT passing through the centre of the Earth) centred on the nadir point. The DR position is used to determine the azimuth of the nadir point, and hence the local azimuth of the position line.

It nearly always helps if you visualize these problems with a globe. The problem is three-dimensional, and diagrams on flat paper can be misleading.

As you say, in theory you could measure the azimuth of the body being used and hence get a fix. However, it wouldn't be very accurate a) because the bodies move quite quickly, as you say, and b) because you couldn't measure the azimuth with enough accuracy. An error of one degree in Azimuth would give errors of 0-60 miles (depending on how far from the nadir point you were), and I'd be surprised if you could measure the azimuth of a celestial body with anything like that accuracy from a moving boat! I reckon I'm doing quite well to measure the bearing of something at sea-level to a couple of degrees with a hand-bearing compass; measuring the bearing of a body at (say) 45 degrees elevation would be much harder.

I'm trying to get my head around whether you could use a method of successive approximations, as the post about Moitessier suggested, but I can't get it clear in my mind at the moment. I suspect that it would work some of the time, but that there might be pathological situations where the solution could diverge, or converge on the wrong point.

 

[Woodlouse]

I think "vang" originated on Thames barges and similar craft, where it is:

"VANG. One of a pair of wires rigged from the sprit end to the deck, controlling the sprit (port and starboard). The vang fall is the tackle rigged on the lower end of the vang, whose lower blocks are mounted near each end of the main horse. Rolling vangs are preventers led forward to complete the control of the sprit in heavy weather, in order to keep the sprit out to leeward. (Pronounced 'wang.') " (from http://www.thamesbarge.org.uk/barges/bargeglossary.html)

I don't know where it's use for the good old kicking strap has come from; it seems to be a new usage that certainly wasn't around when I was a lad.

I suspect gaff vangs predate Thames Barges as we know them. Any one of Nelsons ships would have them on their mizzen gaff sails.

Nowadays a vang does the same thing as a kicker, but a kicker is normally a block and tackle affair, whilst a vang is a solid piston of sorts.

 

[alant]

I was taught D (-..) = Keep Clear. (or danger, keep clear).

Surely, you mean U --_ (you are running into danger) even the sound signal seems to give some urgency.

 

[prv]

I suspect gaff vangs predate Thames Barges as we know them. Any one of Nelsons ships would have them on their mizzen gaff sails.

The Gaff Rig Handbook thinks that the spritsail rig predates the gaff, which evolved from it by progressively moving the heel of the sprit up the mast. Maybe these early spritsails didn't have vangs (or didn't call them that), but they were there before Nelson.

Pete

 

[symondo]

Most if not ALL of it.

I will always refer to trim tabs as boat flaps.

 

[Woodlouse]

The Gaff Rig Handbook thinks that the spritsail rig predates the gaff, which evolved from it by progressively moving the heel of the sprit up the mast. Maybe these early spritsails didn't have vangs (or didn't call them that), but they were there before Nelson.

Pete

I've always thought that it was the lug rig that was the gaff rigs immediate predecessor. Still, my point was about Thames barges, not their rigs which I'm sure are much older in concept.

 

[alant]

Astro in a nut-shell!

At any particular moment, any celestial body (Sun, Moon, planet, star) is directly over a specific point (the nadir point) on the Earth's surface. If you know this point (which requires an ephemeris and time), and measure the elevation of the object using a sextant, in principle you know that you're on a circle centred on that point, whose radius is 90 - the measured elevation in degrees. You can use that circle (which because you know roughly where you are from DR, you can regard as a line perpendicular to the direction of the nadir point) as a position line in exactly the same way as any other position line.

Of course, there are a lot of other things to take into account, such as the effects of your height above sea-level, atmospheric refraction and (for the Moon) parallax errors, but in principle it really is as simple as that. Of course, the mathematics of spherical trigonometry is a bit tricky, but people have kindly worked out rule based methods of side-stepping that bit.

This was originally explained to me, using a ceiling light.

If you are directly under it, the angle measured with a sextant (using the floor as a supposed horizon) is 90 degrees.
If you step away from it by 1 degree, that sextant angle, now becomes 89 degrees.
If you step away from it by 10 degrees, that sextant angle, now becomes 80 degrees.
Therefore, there is a direct relationship between the sextant angle & the distance from the GP (geographical position of the heavenly body). This distance is 90 degrees - sextant angle & is called the zenith distance. Usually measured in degrees/minutes, but can be converted to miles. However, because the earth is round, these zenith distances are difficult to plot accurately on a flat paper chart.
The 'trick' used, is to assume a position, using a known lat/Long (which can be plotted. This 'known position', enables you to calculate backwards, to give a corresponding sextant angle (thereafter known as the calculated angle).
Comparison with the actual (observed) angle, will show a difference. It is then possible to easily calculate the difference & starting at the position used for the calculated angle, plot where you were when taking the actual (observed) sextant reading. This will give a position line (plotted as a straight line, but effectively part of a large circle, whith its centre at the GP of the heavenly body). Several position lines, when plotted, will hopefully give your exact position.

All the difficult sums, have already been worked out for you, giving the information in a Nautical Almanac. You access this information, using the GMT of your sight as a starting point, then apply various corrections (which is the really difficult bit, if not used to ready reckoners). You normally use a pro-forma, to set all these 'tricky bits' on paper.

If you go on an RYA course, as yachties, you are taught to use 'air sight reduction tables', rather than use the RN/MN reduction tables, which use several volumes to look up the same basic info.

Hope the above is correct & makes sense, cos after my course, I still didn't know what/why.

 

[reeac]

Danke schön. It would appear that I have a vang, then, but no cunningham to worry about.

A very experienced dinghy sailor once told me that the only time that she tightened the Cunningham was when she saw someone about to photograph her boat.

 

[alant]

A very experienced dinghy sailor once told me that the only time that she tightened the Cunningham was when she saw someone about to photograph her boat.

Did it make her sit up straight?

 

[IJL]

Sails, my boats simple a Jib, Main and Mizzen.

Whats a Code zero, a gennaker, a solent etc etc.

 

[smeaks]

On screws, one must also bear in mind the existence of the Phirrips head screw.

These are commonly used on items assembled in China and notable for the fact that there is not a screwdriver in existence which will fit properly, with the exception of the unique one used by the person who installed the screw in the factory.

absolutely brilliant! i have not stopped chuckling for an hour!

Thankyou

 

[Keen_Ed]

Port is red is left is short.
Starboard is green is right is long.

Also holds true for match racing. Port entry is blue, starboard is yellow

 

[Keen_Ed]

Sails, my boats simple a Jib, Main and Mizzen.

Whats a Code zero, a gennaker, a solent etc etc.

Strictly speaking, a code zero is a flat sail, with a rope luff, that measures as a spinnaker. Looks like a genoa with a roachy leech. Zero because it's for sailing higher than an A1.

In spinnaker nomenclature, odd numbers are for reaching sails, even for running. A/S 1,3,5 are reaching, A/S 2,4,6 running. Higher numbers for higher windstrength.

Gennaker- another name for an assymetric spinnaker.

Solent- working jib. Non overlapping jib.

 

[Swanrad2]

Why nobody ever told me years ago that when thinking about right of way - if the light you see/could see if it was night is red, you don't have right of way. Green you do (other fiddly bits around this as well, windward, overtaking blah, blah, I know - but that's a good start.

Secondary port tidal calcs - I can do them, but can count on the fingers of no hands how many times hanging off a bit and having a cup of tea didnt save the hassle.

Engines. All of them.

Cheers

 

[reeac]

Did it make her sit up straight?

There's always one. I carefuly wrote "the Cunningham" not "her Cunningham".