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

[AntarcticPilot]

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.

 

[ChiPete]

Thanks AP! Nearly there I think. I know all texts say don't worry about the DR! But if it's only "rough", how rough is rough before it impacts the accuracy on the PL??

 

[steve350]

Thanks AP! Nearly there I think. I know all texts say don't worry about the DR! But if it's only "rough", how rough is rough before it impacts the accuracy on the PL??

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".

 

[nigelmercier]

Pretty sure they use speed through the water and heading rather than SOG and COG. [to calculate True Wind]

Raymarine certainly do, they take the signal from the log instrument. One day I will get around to building a log simulator, using the GPS.

 

[Lucky Duck]

Raymarine call the wind speed and direction derived from the GPS as 'ground wind' which some of the later instruments will display in addition to true and apparent.

 

[Woodlouse]

Raymarine call the wind speed and direction derived from the GPS as 'ground wind' which some of the later instruments will display in addition to true and apparent.

Right, which is a different entity. True wind is the wind as you feel it when drifting with the tide on a boat. Ground wind is what you feel when standing still on land.

 

[Donheist]

Sound signals -2 for port, 1 for starboard. Why not the other way around surely easier with each toot being one syllable?

 

[JumbleDuck]

It is not "astro"...it is Celestial Navigation...if you don't mind.

"Astro" is seagoing slang.

It doesn't seem unreasonable to use marine terminology when discussing boat navigation. After all, "halliard" is just seagoing slang for "rope" ...

 

[JumbleDuck]

Right, which is a different entity. True wind is the wind as you feel it when drifting with the tide on a boat. Ground wind is what you feel when standing still on land.

The idea the true wind is anything other than the absolute wind velocity relative to the sea bed is just weird.

 

[JumbleDuck]

I'll offer a confession. I have absolutely no idea what a cunningham (cunninghame?) or a vang are, or what they do, or whether I should care about them.

 

[john_morris_uk]

Sound signals -2 for port, 1 for starboard. Why not the other way around surely easier with each toot being one syllable?

Because one short for turning to starboard is the more frequently used one under IRPCS? Its usually the exception for a give way vessel to turn to port.

 

[john_morris_uk]

I'll offer a confession. I have absolutely no idea what a cunningham (cunninghame?) or a vang are, or what they do, or whether I should care about them.

Cunningham: a means of flattening the mainsail by pulling down on the luff of the sail. (Instinct says that halyard tension has already sorted out luff tension but cunningham tension works for counter intuitive reasons.

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

 

[Stork_III]

Pretty sure they use speed through the water and heading rather than SOG and COG.

Yes, you are probably correct. I thought I remembered my old Stowe DatalineX system utilised SOG if available in prference to STW, but memory is not what it was.

 

[Hydrozoan]

Cunningham: a means of flattening the mainsail by pulling down on the luff of the sail.

And named after Briggs Cunningham, perhaps better known to the wider world for his car racing exploits.

(Dancrane has much to answer for: this isn't a thread, nor even a skein of threads - it's a veritable boaty Bayeux Tapestry! )

 

[LittleSister]

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.

Brilliant! Thank you. Has much unmuddled me on the topic.

 

[Uricanejack]

Because one short for turning to starboard is the more frequently used one under IRPCS? Its usually the exception for a give way vessel to turn to port.

The sound signals are actualy based on the international code of signals, single letter signal flag meanings. and morse code.

Signal flag Echo or E single letter meaning, I am aletering course to starboard. Morse . , dot or 1 short blast.
Signal flag India or I single letter meaning, I am altering course to port Morse .. , Dot Dot or 2 short blasts.
Signal flag Sierra or S single letter meaning, I am using astern propulsion, ... , Dot Dot Dot or 3 shorts blasts.
Signal Flag Hotel or H single letter meaning I have a Pilot on Board, .... , Dot Dot Dot Dot or 4 short blasts
Signal Flag Tango or T single letter meaning, underway making way. -, Dash or 1 long blast.
Signal Flag Mike or M single letter meaning, Underway Not making Way, -- , Dash Dash or Two long blasts.
Signal flag Oscar or O single letter meaning, Man over board, --- , Dash Dash Dash. or 3 Long Blasts.
Signal Flag No5 guess the meaning No Its Not actualy get the F!#$ out the way % short Blasts.

Don't worry I'm not so good at sail trim

 

[Uricanejack]

I wonder what they'll ban next? Marmite, perhaps...because the EEC won't want to encourage hatred, and Marmite seems to incite so much, they even use it as a selling-point.

Here's a sailing question for bargain-end yachtsmen...does any company anywhere still produce and sell salt-water soap?

I think I saw some about 30 years ago awfull stuff, fairy liquid works.

 

[Uricanejack]

If it's ignorance confession time I'm going to start another topic for discussion.

In Secondary Ports calculations, the Almanac gives you the time differences usually at 1200, 1800 etc for you to interpolate between to find the time difference for the secondary port you are interested in, based on the times of high and low water for the relevant primary port. Sometimes these time differences are negligible and sometimes they are appreciable. For the occasional secondary port where the minutes to add or subtract are large they are expressed as +131 or similar. I have never found anyone who can tell me whether that means one hour and thirty one minutes or one hundred and thirty one minutes!

This is slightly embarrassing as I teach and examine this stuff! However I remember asking a Hydrographer and he didn't know either so that makes me feel slightly better. Furthermore when I did my YM theory exam twenty five years ago, I asked the instructor and he mumbled something and changed the subject. Fortunately in real life, the requirement to use the figures for the very occasional secondary ports that give me this potential problem are so few that I've never found it a problem. I'd like to know the answer though...

To be honest I don't know. which is what confessing is about.
I would think 131 minutes. of course i could be totaly wrong.
In order to determine
Sugest looking for a 61 if such time diferance exists it will be 61 minutes or 11 for 1 hour 1 minute which I would have read as eleven minutes.
If it works for 61 up throu 90 in minutes it will continue through 121 up through 181.

The UK hydrographic office produces 2 sets of tide tables or at least it did volume 2 was the rest of the world quite a few of the differances are greater than 1 hour.

 

[Uricanejack]

The times are given in 24 hour notation, HHMM, thus 0113 = 1hour 13 minutes.

Yes makes sense but the post was 131 not 0131. or 1 31
If I saw 131 I would read 131 minutes.
If I saw 0131 I would read 1hr 31 minutes.
If I saw 1 31 I would read 1 hr 31 minutes

truthfully just guessing.
Isn't the main point of posting on the forum to speak authoritvly about subjects you know nothing

 

[Uricanejack]

Thank you to those that have helped explain the difference between True and Apparant wind. I have read the wiki, looked at the vector formulas and understand what is being said.

But... I still don't understand why the heck I should care? Or need to know about this. It must be important as there is a button on my wind instrument to switch between the two.

Surely we all trim our sails as we sail. Adjusting to the wind conditions as they really change or seem to change.

My boat max speed is about seven knots and I could just about understand why tacking for example could change things but I seem to have managed this quite well without pushing buttons on instruments or doing vector equations.

Still confused

Why one should care really depends on ones outlook on life. I tend not to care and go with the flow. I usualy come last in races. This might not be a coinicidence.