Mercator Sailing

[HarmlessWeasel]

We covered Mercator Sailings today, but I'm not sure how the distance ends up being any more accurate than a standard plane sailing. Could anyone enlighten me, please?

Cheers

[Lewis]

Plane sailing treats the world like it's a perfect sphere, in reality it's an oblate spheroid, Mercator sailing takes account of this. For practical purposes it has to be quite a large distance to see any difference.

[HarmlessWeasel]

OK, thanks. I think I was getting confused because I was given the impression that it wasn't another form of plane sailing.

Any chance you could explain why we use D'Long instead of Departure?

[alistairuk]

It's not a plane sailing.

The MP that you use in your calculation corrects for the non-uniformity of the earth.

Plane sailings work to about 200nm as the error is relatively small at those distances - using Mercator for distances under 200 shoul give you near enough same answer - above 200nm the error increases, the greater te distance the bigger the error. The MP figure you take from the tables accounts for this.

Do a plane sailing for a distance of 2000nm (say Barbados to Portugal) then do it as a Mercator - you will have a substantial difference....

Great circles are different and involve following a curve between two points so you travel a shorter distance compared to going in a straight line.

I'm typing this on my iPhone so I hope it makes sense.

[HarmlessWeasel]

You only get an MP value for the start location though don't you? So how does that account for the difference in latitude of your start and end points? And then once you have a course you use plane trig to calculate the distance. Shouldn't we be using spherical if we want to know the true distance of a rhumb line between two points?

edit: thanks for the replies btw, I'm just gonna keep asking questions until I fully understand it though. I like to understands things from first principles as much as possible, rather than just accepting what's said. Takes a little longer and is more annoying, but I'm hoping you guys will put up with me

[Lewis]

no you get both and work out the difference between the two. Be careful if your changing hemispheres

[Lewis]

http://www.amazon.co.uk/American-Practical-Navigator-Nathaniel-Bowditch/dp/0939837544/ref=sr_1_1?ie=UTF8&qid=1382034347&sr=8-1&keywords=bowditch

if you got a spare ?25 will answer just about any question you have on navigation..

[HarmlessWeasel]

oh yeah, forgot that the D in DMP stands for difference, whoops! Thanks.

I've got the really old mates and masters textbooks at home, along with a couple of other nav books, just didn't have any space to bring them down

[CharlieDelta]

Possibly a bit controversial, but I'd steer clear of Bowditch and read the Admiralty Manual of Navigation. Particularly in Warsash.

[Lewis]

Isn't the admiralty manual of navigation out of print?
Why particularly at warsash? I've got image of sailors walking around like an episode of horn blower criticizing colonials :P

[CharlieDelta]

http://www.nautinst.org/en/shop/checkout/shop-product-details.cfm/the-admiralty-manual-of-navigation-vol-1-principles-of-navigation

Ok, so it's a lot more expensive, but it shows the likes of chartwork symbols in Admiralty standard (which, by extension, is generally accepted by the MCA as the way of doing things). Also available in college libraries.

Why particularly at Warsash? Because pretty much everyone involved in teaching navigation subjects at Warsash is either ex-Royal Navy or was taught themselves by one of the ex-Royal Navy lecturers.

[niyall22]

CD, obviously you've used the nautical institute, which I notice has the same adders as Marine Society Sea Cadets, sad I know but so I should after so long, is this part of Marine Society? Can MS Bookshop discount be used do you know?

[CharlieDelta]

Hi niyall, not used them for books but I am an associate member of the NI (?39/yr as a cadet) which gives 30% discount on their books. They're very much a separate entity.

First of all, the chances are if you want an NI book (and most are pretty expensive) your college library will have it. If you really want your own copy, it may be worth checking if the MS bookshop carries the title you want if you have a discount. If they don't and you really, really want a copy then [SHAMELESS PLUG] email [email protected] [/SHAMELESS PLUG]. We have a deal with the NI for books and can order in.

[niyall22]

Ok thanks very much!

[HarmlessWeasel]

Isn't the admiralty manual of navigation out of print?
Why particularly at warsash? I've got image of sailors walking around like an episode of horn blower criticizing colonials :P

My roommate's joined URNU and likes to think he's Hornblower because he's a midshipman...

That book is a heck of a lot more expensive, so I'll take a peek in the library