I was playing with some math today figuring out the best way to use my E6-B flight computer for marine applications.   Somehow my mind went to log lines and sand timers. 

I got the idea that with todays watches, all of which incorporate a second hand, working a log contrary to the old way, which took counting knots to determine speed after a certain time, typically 28 or 30 seconds, a predetermined length of log line might easily be timed to determine speed. 

The following is a list reflecting time in seconds vs speed in knots for a log line extending 60ft:

1 kt   35.5 sec
2       17.8
3       11.8
4       8.9
5       7.1
6       5.9
7       5.1
8       4.4

So you take a fishing line 60 feet long on a cuban rig fishing spool.   Attach a log device to throw into the water which pulls the line.  Throw the log into the sea and start your timer.  When the line pulls taught stop the timer.  Determine your speed over the water using the chart above.  If the spool is held any more than 4 or 5 feet off the water accuracy may suffer.  With high cockpits a longer line would be better; just refigure the knots vs time table.

The math is thus:

1knot = 6076ft ÷ 3600s = 1.6878ft/s

You could have a log line 6076 feet long and spool it off for 3600 seconds which would tell you your speed is 1 knot.  But there's a better, faster way.

60ft of line ÷ multiples of 1.6878 gives time in seconds

Ex: 
60 ÷ 1.6878 = 35.5s for 1 knot
60 ÷ (2 × 1.6878) = 17.8s for 2 knots ...  etc.

You can help me out here and let me know if I got the figures wrong.  Any length of line would do beyond a certain minimum.   I took 60 because in an emergency 10 arms spread would about equal 60ft.  You can kinda sense here that line length doesnt get critical until you reach the higher speeds.

Have fun and give it a try next time you're out on the lake and dont forget to tell us what happened.

Cheers

Its 22 14 00 gmt dec 27.  That's 06 23 00 local.  I  was up before nautical twilight to shoot venus, spica, and perhaps regulus, but there was complete overcast.  Now that rainy season is over we're getting all the rain we missed during rainy season.  Let's hope the rest of the day isn't as frustrating... Maybe I'm getting too old for this business.  Naw. You're only too old if you can't see the sun through the sextant.   I have a hankerin to go to sea... anyone wanna go?  Who's gotta boat I can borrow?

Cheers

PS 21 09 00 now and still overcast... just not my day for CN. I wonder if at sea how long I could go without a fix before I'd get nervous. I suppose that might depend on how close I thought I was to some obstruction - like a continent.

Ever wonder why we still see haversine tables in some navigation books?  Me too, until I read this article about them:

https://plus.maths.org/content/lost-lovely-haversine

The main point is this:

"The reason why the haversine function has come out of fashion is that with the help of calculators and computers it’s easy enough to work out the distance straight from formula (2). That’s why you don’t find a haversine button on your average calculator. "

Here is a quote from another pub describing haversine as it relates to the Gunter versed sine line :

"The scale title V * S means versed sines, but it does not reflect our current versine which equals (1 -cosine). It actually represents, in modern terminology, the logarithm of (1 -haversine), or 1/2 (1 + cosine). The haversine function (half-versed-sine) was introduced for computational reasons in the formula that calculates the angular distance between two arbitrary points on the globe. This formula could be calculated between the NUM, the SIN and the V * S scales on the Gunter rule."

So that's pretty much why we have haversine tables.  If you've never used the haversine table give it a try... you'll get a good feel for what our forefather navigators went through to chart their ships. Where once about every navigator knew the meaning of haversine, now you can consider yourself a member of a select few who still do :)

Cheers

I had a dream last night that my crew and I were sailing into dangerous waters.  My neighbor's chickens woke me up so I didnt find out what happened,  but it made me think of the great voyages of discovery during the sailing ship era.

If you had a chance to go back in time to witness one of those great voyages, which would it be?

What would you say to Cook, or Magellan,  or Darwin, Columbus,  Ericson,  or a hundred other voyagers?  Would you even want to be a sailor during those times.  If you do then what would you prefer; whalers, china tea cutters, general merchants, Navy ships, or something else like a British barge or a pirate ship.

I think I would have enjoyed the china tea runs or perhaps a coastal merchant.  If I was to witness some great adventure it might be Darwin on the beagle, or one of the polar expeditions.  

It almost goes without saying though that I wouldnt want to work as an ordinary seaman.  I'd have to be a mate or navigator or captain or better yet - owner.

Anyway, I'd be interested in your perspective.

Cheers

Ok, so this is kinda difficult to admit, but I've never taken a shot of Polaris.   I know, it's the most basic thing one can do with which to navigate, but I've sort of shied away from polars until recently.... I will soon give it a go. 

I do have, though, a preliminary question.

What ever happened to the A0, A1, A2 reduction?  All I see now is some Q method ive not yet grasped.  Please tell me what IS the current prevailing Polaris reduction and what tables are required.

Thanks in advance
Stupid me

Hello esteemed members of this forum.
I heil from Southern Coastal Florida and I'm dusting off my Celestial Nav abilities  to keep my brain going.
Merry XMass to all.
JP

The American Practical Navigator (Bowditch) 2019 version is on the site.

It's perfectly organized by Sections and Chapters allowing you to easily find and download the material you want.

You can download the entire work also.


Get it here- 2019 Bowditch- American Practical Navigator

Hello fellow CN freaks,

Its been 2 years since I logged in last.  I hope some of the old shipmates are still breathing.

I was wondering if any of you guys have become more active in celestial navigation since the covid hoax began. 

I started taking more shots with an artificial horizon and with no little difficulty.  For some reason my lops are always 4 or 5 nautical miles short of my gps fix.

I've gone over the math a hundred times, checked my time, checked the sextant, checked everything. 

Recently I began to wonder if my sun shots are wrong.  My habit is to bring the sun down to the AH then touch the two suns with the AH sun on the bottom.  Do I have this backwards?

I add index error, divide by 2, then add altitude correction.  Am I missing something?  should I be using SD, semi diameter,  instead of altc, or is there a step of which I'm unaware?  I dont add dip to AH shots.

Anyway,  I hope to hear from some of the old gang; including admin.

Merry Christmas

Does anyone have experience with bubble sextants?  I recently acquired one, but am finding it difficult to move the Slow Motion Knob.  When I first inspected it, the knob seemed to moved freely.  Then I mistakenly turned the winding crank only one click.  After it wound down, the knob was extremely hard to turn, almost as if it is locked so the user can get a reading?  Maybe there is a way to unlock it?  

Any help is greatly appreciated!

Mike

I found an Ebbco sextant online for a really great price and had to have it. Everything on it is in top shape except a couple of the shades were darkened to the point of uselessness or someone replaced them with opaque material. I have some theatrical gels so it was simple enough to fix. 

I could probably make some adjustments to the mirrors but the error has remained consistent over several days. Its probably best to just subtract from the sighting, rather than go messing around, loosening things ups that might not tighten again.

Yesterday was my first attempt at a sight reduction and I found it works quite well. I rather like it. Given the poor conditions, wind blowing the artificial horizon around and pushing tree limbs into the line of sight, the accuracy was well within expectations especially for a new piece of gear. I have a Davis Mk15 and the Ebbco (so far) seems a little more consistent and accurate. The error varies more with the Davis.