Reply
 
Thread Tools Display Modes
 
Old 02-08-2016, 08:08 PM   #41
Guru
 
tpbrady's Avatar
 
City: Anchorage/Wrangell
Country: USA
Vessel Name: Silver Bay
Vessel Model: Nordic Tug 42-002
Join Date: Sep 2012
Posts: 505
I have the chart plotter to set to show knots/NM and Coastal Explorer set to show mph/SM. If called I respond with the units that are appropriate. It's also a good training tool for visitors on the boat. Now all I need is a calibrated line with knots in it to measure speed.
__________________
Advertisement

tpbrady is offline   Reply With Quote
Old 02-08-2016, 08:12 PM   #42
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,705
It's simple.
Knots on the water and statute miles on land.

When I see mph in boat adds I assume it's nautical mph unless it's an add by a private owner .. then it's questionable.

As an aside Wm Atkin lists mph for his boat designs. I think it's statute mph.

I don't much care what other people do. For me it's knots on the water and statute miles on land. And it's silly to make an issue out of it in the pleasure boating world.
__________________

__________________
Eric

North Western Washington State USA
Nomad Willy is online now   Reply With Quote
Old 02-08-2016, 08:40 PM   #43
Guru
 
Moonstruck's Avatar
 
City: Hailing Port: Charleston, SC
Country: USA
Vessel Name: Moonstruck
Vessel Model: Sabre 42 Hardtop Express
Join Date: Nov 2008
Posts: 7,848
Quote:
Originally Posted by djmarchand View Post
Knots is a nautical term and of course it is nautical miles per hour. A nautical mile is exactly (well maybe not quite, since the earth isn't perfectly round) one minute of longitude.

A good but maybe trivial use is measuring vertical distances on Active Captain. You can use the Path feature, but it takes time. I put the cursor on one feature and then note the longitude and scroll up or down to the next. The difference can easily be calculated: degrees * 60 + minutes equals nautical miles.

David
David, I believe that would be one minute of latitude. Longitude does not work because the circles get smaller as you move north or south of the equator. Concentric circles so to speak. I always go the the latitude scale at the side of the chart to measure distances. I'll bet that you do too.
__________________
Don on Moonstruck
Sabre 42 Hardtop Express & Blackfin 25 CC
When cruising life is simpler, but on a grander scale (author unknown)
http://moonstruckblog.wordpress.com/
Moonstruck is offline   Reply With Quote
Old 02-08-2016, 10:20 PM   #44
Veteran Member
 
Paul Swanson's Avatar
 
City: Ventura CA
Country: USA
Vessel Name: Proud Mary
Vessel Model: Pacific Trawler
Join Date: Apr 2015
Posts: 77
Baker is correct that a nautical mile is one minute of longitude at the equator. That is the definition of a NM. However, one minute of latitude is not exactly one NM since the diameter of the Earth is slightly smaller through the poles as compared to the equator. For practical purposes, to measure the distance on a paper chart, put your dividers between the two points you want to measure and then compare it to the latitude scale on the edge of the chart. The number of minutes of latitude is the distance in NM.

I am also always disturbed by the use of the improper term “knots per hour”. I am also disturbed that one of the posters used time=miles/mph but didn’t appreciate that time = NM/knots.

How many know the origin of the term “knots”?

Paul
Paul Swanson is offline   Reply With Quote
Old 02-08-2016, 10:50 PM   #45
Guru
 
Moonstruck's Avatar
 
City: Hailing Port: Charleston, SC
Country: USA
Vessel Name: Moonstruck
Vessel Model: Sabre 42 Hardtop Express
Join Date: Nov 2008
Posts: 7,848
Quote:
Originally Posted by Paul Swanson View Post
Baker is correct that a nautical mile is one minute of longitude at the equator. That is the definition of a NM. However, one minute of latitude is not exactly one NM since the diameter of the Earth is slightly smaller through the poles as compared to the equator. For practical purposes, to measure the distance on a paper chart, put your dividers between the two points you want to measure and then compare it to the latitude scale on the edge of the chart. The number of minutes of latitude is the distance in NM.

I am also always disturbed by the use of the improper term “knots per hour”. I am also disturbed that one of the posters used time=miles/mph but didn’t appreciate that time = NM/knots.

How many know the origin of the term “knots”?

Paul
You, sir, and David are correct. My error. I shall be dusting off my parallel rules and dividers.
__________________
Don on Moonstruck
Sabre 42 Hardtop Express & Blackfin 25 CC
When cruising life is simpler, but on a grander scale (author unknown)
http://moonstruckblog.wordpress.com/
Moonstruck is offline   Reply With Quote
Old 02-08-2016, 10:52 PM   #46
Guru
 
BandB's Avatar
 
City: Fort Lauderdale
Country: USA
Join Date: Jan 2014
Posts: 13,111
Quote:
Originally Posted by Paul Swanson View Post

How many know the origin of the term “knots”?

Paul
Wifey B: I do...I do...

It's also where we get the term "log." They would throw a log over the transom with a rope attached. The rope had knots all along the way, evenly spaced. You let the log pull the rope out for a fixed period of time and then how many knots went through your hand doing it was your speed.

That's all I know about it.
BandB is online now   Reply With Quote
Old 02-08-2016, 11:03 PM   #47
Veteran Member
 
Paul Swanson's Avatar
 
City: Ventura CA
Country: USA
Vessel Name: Proud Mary
Vessel Model: Pacific Trawler
Join Date: Apr 2015
Posts: 77
You get an A+! It's also called a taffrail log since that is where is was thrown overboard.

Paul
Paul Swanson is offline   Reply With Quote
Old 02-08-2016, 11:17 PM   #48
Guru
 
koliver's Avatar
 
City: Saltspring Island
Country: BC, canada
Vessel Name: Retreat
Vessel Model: C&L 44
Join Date: Nov 2007
Posts: 2,165
Until the mid-19th century, vessel speed at sea was measured using a chip log. This consisted of a wooden panel, attached by line to a reel, and weighted on one edge to float perpendicularly to the water surface and thus present substantial resistance to the water moving around it. The chip log was "cast" over the stern of the moving vessel and the line allowed to pay out.[6] Knots placed at a distance of 8 fathoms - 47 feet 3 inches (14.4018 m) from each other, passed through a sailor's fingers, while another sailor used a 30-second sand-glass (28-second sand-glass is the currently accepted timing) to time the operation.[7] The knot count would be reported and used in the sailing master's dead reckoning

So says Wiki.
koliver is online now   Reply With Quote
Old 02-08-2016, 11:25 PM   #49
Veteran Member
 
JNandJN's Avatar
 
City: Green Cove Springs, FL
Country: United States
Vessel Name: Sea Gypsy II
Vessel Model: 2007 Great Harbour N37
Join Date: Jan 2014
Posts: 45
Quote:
Originally Posted by Tom.B View Post
I use MPH and Statute Miles 100% of the time because Knots and Nautical Miles don't relate the distance to time. If I need to go 60 miles and I go 20 MPH the entire way (-ish), I can very easily figure out the time it will take to make the journey. It just takes a step out of the process of estimating time over distance.
Knots and nautical miles work the same way as MPH and statute miles. It takes 1 hour to travel 1 nautical mile at 1 knot.
__________________
John and Jen
Sea Gypsy
JNandJN is offline   Reply With Quote
Old 02-08-2016, 11:48 PM   #50
Senior Member
 
City: Owings, Md
Country: US
Join Date: Jan 2016
Posts: 224
Quote:
Originally Posted by Paul Swanson View Post
Baker is correct that a nautical mile is one minute of longitude at the equator. That is the definition of a NM. However, one minute of latitude is not exactly one NM since the diameter of the Earth is slightly smaller through the poles as compared to the equator.

Paul
360 degrees x 60nm/degree = 21,600nm (24 856.836 077 mile [statute]) is closer to the polar circumference (24,860) than the equatorial circumference (24,902) so even at the equator, using latitude is more accurate.
Gdavid is offline   Reply With Quote
Old 02-09-2016, 12:09 AM   #51
Senior Member
 
Nsail's Avatar
 
City: Benicia CA
Country: USA
Join Date: Mar 2012
Posts: 498
Quote:
Originally Posted by kthoennes View Post
Well of course I'd agree with you there, in some scenarios different units do indeed matter, like that one. But if we get a weather report about wind gusts to 40 mph or 40 knots, either way I'm going to stay tied to the dock.
For others that don't understand, koliver is not referring to knots per hour vs miles per hour. He's referring to kilometers per hour vs knots per hour and miles per hour. Big difference.

If you're in Canadian waters and listening to certain weather broadcasts, it's imperative to know the conversion factor.
Nsail is offline   Reply With Quote
Old 02-09-2016, 02:16 AM   #52
Member
 
City: British Columbia
Country: Canada
Join Date: Feb 2016
Posts: 10
And hence the term, log book or captains log
Bigfoot4 is offline   Reply With Quote
Old 02-09-2016, 06:13 AM   #53
Senior Member
 
Jetstream's Avatar
 
City: Tasmania
Country: Australia
Vessel Model: Old Town Loon
Join Date: Jul 2015
Posts: 175
Quote:
Originally Posted by ulysses View Post
So I don't guess anyone is using cables. 1 nm = 10 cables
Yes I do. Ive always used cable as a description of distance at sea. Its a handy distance. Traditionally it about 100 fathoms or 200 yards but if you want to get more precise its 608 ft for the imperial measurement or 720 ft in the US navy. You'd have to wonder why we could never standardize such a measurement.
Jetstream is offline   Reply With Quote
Old 02-09-2016, 06:57 AM   #54
Guru
 
City: Sydney
Country: Australia
Join Date: Jul 2015
Posts: 1,348
Thank god for KPH
gaston is online now   Reply With Quote
Old 02-09-2016, 07:11 AM   #55
Guru
 
City: Satsuma FL
Country: United States
Vessel Name: No Mo Trawla
Vessel Model: Hurricane SS188
Join Date: Jul 2014
Posts: 1,672
Quote:
Originally Posted by gaston View Post
Thank god for KPH
As in Kilometers Per Hour!
Donsan is offline   Reply With Quote
Old 02-09-2016, 07:15 AM   #56
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 16,515
Offshore where Lat & Lon are used K makes the most sense.

Inshore where lubber miles are the norm MPH works best.

Selling the boat MPH gives the best numbers for speed and MPG,

although counting in Klicks would give even better numbers !
FF is offline   Reply With Quote
Old 02-09-2016, 07:20 AM   #57
Guru
 
psneeld's Avatar
 
City: Avalon, NJ
Country: USA
Vessel Name: Freedom
Vessel Model: Albin 40
Join Date: Oct 2011
Posts: 15,861
Quote:
Originally Posted by Gdavid View Post
360 degrees x 60nm/degree = 21,600nm (24 856.836 077 mile [statute]) is closer to the polar circumference (24,860) than the equatorial circumference (24,902) so even at the equator, using latitude is more accurate.
I agree, I have never hear NM related to longitude. It's not how a NM was originally calculated according to these and dozens of other references.

What is the difference between a nautical mile and a knot?

A nautical mile is based on the circumference of the earth, and is equal to one minute of latitude. It is slightly more than a statute (land measured) mile (1 nautical mile = 1.1508 statute miles). Nautical miles are used for charting and navigating.

https://en.wikipedia.org/wiki/Nautical_mile
A nautical mile (symbol M, NM or nmi) is a unit of distance, set by international agreement as being exactly 1,852 meters (about 6,076 feet). Historically, it was defined as the distance spanned by one minute of arc along a meridian of the Earth (north-south), and developed from the sea mile and the related geographical mile.




I also believe the name "taffrail log" didn't become popular till logs that were left overboard for periods of time....previously know by the naes previously described.




http://educators.mysticseaport.org/a.../taffrail_log/


This type of log is known as a taffrail log. The taffrail is the aftermost railing around the stern of a ship, to which a log could be mounted with a clamping mechanism (thus resulting in the name taffrail log).


https://en.wikipedia.org/wiki/Chip_log




More modern logs and replacements[edit]


A patent or taffrail log


Mechanical speed logs called patent logs or taffrail logs,
psneeld is online now   Reply With Quote
Old 02-09-2016, 07:22 AM   #58
Guru
 
City: Sydney
Country: Australia
Join Date: Jul 2015
Posts: 1,348
Quote:
Originally Posted by Donsan View Post
As in Kilometers Per Hour!

Yes Kilometers Per Hour!
Metric makes life so east
gaston is online now   Reply With Quote
Old 02-09-2016, 07:31 AM   #59
TF Site Team
 
Peter B's Avatar
 
City: Brisbane
Country: Australia
Vessel Name: Lotus
Vessel Model: Clipper (CHB) 34 Sedan/Europa style
Join Date: Jun 2008
Posts: 6,669
Send a message via Skype™ to Peter B
Quote:
Originally Posted by Baker View Post
So "knots" are not a measure of speed?
What he means is, a knot is one nautical mile per hr, so if you say knots per hr, it is saying nautical miles per hour per hour...but you knew that, right..?

Coming back to why you like knots etc, it's probably because in aviation you also use those terms, and the raison d'ętre of the nautical mile is that it is the distance the subtends one minute of longitude at the equator, so has a navigation validity no other measurement has. A lot of folk don't know that. Also, although you do drive to mph in the US, being in the aviation field, you would see more of other countries, and are probably more aware than most USians, that apart from the US, maybe Canada, and the UK, the rest of the world use kph..?

https://en.m.wikipedia.org/wiki/Nautical_mile

PS. Sorry I just repeated a lot that had been said already. I didn't realise so much traffic on this had occurred while we southern hemispherians were kipping...
__________________
Pete
Peter B is offline   Reply With Quote
Old 02-09-2016, 07:43 AM   #60
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 16,515
Knots were used by navigators with a "chip log".

A triangular piece of wood was dropped overboard , and a sand glass was started.

When the sand ran out the number of knots that had gone overboard was logged as boat speed.

The line slipped out of one corner of the chip log so hauling aboard was eased.

If you want to make one the Navy Chip Log Pattern has good info.

Chip log - Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Chip_log


Wikipedia


A chip log, also called common log, ship log, or just log, is a navigation tool ... All nautical instruments that measure the speed of a ship through water are known ...



Chip Log pattern - Navy & Marine Living History Association

www.navyandmarine.org/planspatterns/speedlog.htm




The Speed Log --History, Construction and Use. by Allen Mordica, TMLHA. Introduction- One of the three main aspects of navigation is dead reckoning. Simply ...



How do sea navigators measure their ships speed?

www.physlink.com/Education/AskExperts/ae400.cfm




Chip Log Rope had knots tied at equal distances along the reel. Sailors would throw the wood ... In fact that is the origin of the nautical speed unit: the knot.



Chip Log Explained - An Ancient Navigation System

www.brighthubengineering.com › Marine Engineering › Seafaring




Dec 31, 2009 - Chip log is a navigational tool that was used during the ancient time to ... Marine Navigation: Understanding basic maritime terminology · Ship ...
__________________

FF is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





All times are GMT -5. The time now is 04:13 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012