Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 16-03-2015, 15:08   #151
Registered User
 
Sailor Doug's Avatar

Join Date: Jan 2010
Location: Lake Erie
Boat: H36
Posts: 384
Re: speed through GPS versus old fashioned Paddle Log

I am just having fun whaling for the ice to melt.


Sent from my iPad using Cruisers Sailing Forum
__________________

__________________
Sailor Doug is offline   Reply With Quote
Old 16-03-2015, 15:49   #152
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: speed through GPS versus old fashioned Paddle Log

Quote:
Originally Posted by Sailor Doug View Post
Wow goboatingnow are you a sailboater and have you ever raced? No instance speed with that gps, do you new know the scan time? If you sailed a lot would notice false gps speed when the boat is not moving and latent acceleration. What credentials do you have to cling to your sifi statements.


Sent from my iPad using Cruisers Sailing Forum

There's are a series of reasons why sometimes you get false speeds at zero etc

And yes I race ( beneteau 8metre, & J 24 )

On what basis do you accuse me of si-fi. Would you like to describe how GPS works or advance an alternative theory as to the basis of inferometry as it applies to GPS position determination.


Sent from my iPhone using Cruisers Sailing Forum
__________________

__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 16-03-2015, 16:21   #153
Registered User
 
Sailor Doug's Avatar

Join Date: Jan 2010
Location: Lake Erie
Boat: H36
Posts: 384
Re: Speed Through GPS Versus Old Fashioned Paddle Log

It appears you may know that a knot log is better if you are looking for small changes of speed in a short period of time, GPS is not so good at this but will give a very accurate time over bottom. I realize GPS uses complex information based on time differential and Doppler shift to calculate position and velocity on a complex plane. This is no where in the same accuracy of a doppler knot log.


Sent from my iPad using Cruisers Sailing Forum
__________________
Sailor Doug is offline   Reply With Quote
Old 16-03-2015, 16:34   #154
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Speed Through GPS Versus Old Fashioned Paddle Log

Quote:
Originally Posted by Sailor Doug View Post
It appears you may know that a knot log is better if you are looking for small changes of speed in a short period of time, GPS is not so good at this but will give a very accurate time over bottom. I realize GPS uses complex information based on time differential and Doppler shift to calculate position and velocity on a complex plane. This is no where in the same accuracy of a doppler knot log.


Sent from my iPad using Cruisers Sailing Forum

I would argue GPS speedo in the absense of significant tide is much more accurate then a paddle wheel.


Sent from my iPhone using Cruisers Sailing Forum
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 16-03-2015, 16:55   #155
Moderator
 
sailorchic34's Avatar

Cruisers Forum Supporter

Join Date: Feb 2011
Location: SF Bay Area
Boat: Islander 34
Posts: 4,815
Re: Speed Through GPS Versus Old Fashioned Paddle Log

A doppler speed log has an accuracy of 1% of range according to manufacturers data. http://furunousa.com/ProductDocument...20Brochure.pdf

With a 40 knot range, the accuracy would be 0.4 knots.

A non-differential GPS has a speed accuracy or .2m/s or 0.44mph as tested by NCBI. Accuracy of non-differential GPS for the determination of speed ove... - PubMed - NCBI

Pretty close to the same in my book.

__________________
sailorchic34 is offline   Reply With Quote
Old 16-03-2015, 16:57   #156
Registered User
 
Sailor Doug's Avatar

Join Date: Jan 2010
Location: Lake Erie
Boat: H36
Posts: 384
Re: Speed Through GPS Versus Old Fashioned Paddle Log

I said change of speed not stable speed. The length of the algorithm is very complex and takes time for gps to process so changes in speed are slower than a simple counter. This has been my point of pointing out why a knot log is very usefully. All so some of the disinformation on how a gps functions.


Sent from my iPad using Cruisers Sailing Forum
__________________
Sailor Doug is offline   Reply With Quote
Old 16-03-2015, 17:14   #157
Moderator
 
sailorchic34's Avatar

Cruisers Forum Supporter

Join Date: Feb 2011
Location: SF Bay Area
Boat: Islander 34
Posts: 4,815
Re: Speed Through GPS Versus Old Fashioned Paddle Log

Quote:
Originally Posted by Sailor Doug View Post
I said change of speed not stable speed. The length of the algorithm is very complex and takes time for gps to process so changes in speed are slower than a simple counter. This has been my point of pointing out why a knot log is very usefully. All so some of the disinformation on how a gps functions.


Sent from my iPad using Cruisers Sailing Forum
Honey, Accuracy is for all intents the same between GPS and a Doppler log, NCBI testing was not at static speed but in real world testing against a calibrated wheel speedo.

At typical cruising sailboat speeds, there is no appreciable difference in accuracy. Yes a GPS is doing calculations, so is a Doppler log and the doppler log is per the manufacture information, accurate to 1% of range, which on the unit I listed is 40 knots. Distance measured is also to within 1 percent.

The log may look more accurate with the 0.1 knot resolution, but manufacturer only lists a 1% accuracy so it could be off by .4 knots. If it was better I'm sure the marketing folks would have put it in the data.
__________________
sailorchic34 is offline   Reply With Quote
Old 16-03-2015, 17:21   #158
Moderator
 
sailorchic34's Avatar

Cruisers Forum Supporter

Join Date: Feb 2011
Location: SF Bay Area
Boat: Islander 34
Posts: 4,815
Re: Speed Through GPS Versus Old Fashioned Paddle Log

Signet also lists a +/-1% for their speed log, after calibration.

http://www.signetmarine.com/products...7A/SL267A.html
__________________
sailorchic34 is offline   Reply With Quote
Old 16-03-2015, 17:57   #159
Registered User
 
Sailor Doug's Avatar

Join Date: Jan 2010
Location: Lake Erie
Boat: H36
Posts: 384
Re: Speed Through GPS Versus Old Fashioned Paddle Log

Please let's not get into Doppler log. This post has been into the function of a simple knot log verses gps. My reference to a doppler log was that it uses only Doppler shift to calculate speed. The argument being that the latent speed change of a gps is a disadvantage and it's accuracy at slow locale speed is often wrong. Also long argument on how a gps calculates speeds.
Thanks sweetie I am only having fun.



Sent from my iPad using Cruisers Sailing Forum
__________________
Sailor Doug is offline   Reply With Quote
Old 16-03-2015, 18:14   #160
Nearly an old salt
 
goboatingnow's Avatar

Cruisers Forum Supporter

Join Date: Jun 2009
Posts: 13,649
Images: 3
Re: Speed Through GPS Versus Old Fashioned Paddle Log

Quote:
Originally Posted by Sailor Doug View Post
I said change of speed not stable speed. The length of the algorithm is very complex and takes time for gps to process so changes in speed are slower than a simple counter. This has been my point of pointing out why a knot log is very usefully. All so some of the disinformation on how a gps functions.


Sent from my iPad using Cruisers Sailing Forum

I'd have to disagree with this, in practice gps instantaneous speed is one thing and is not a metric we use much on a boat from navigation. Distance travelled per time sequent is what most people use.

If you did want to mark up EPs using a gps calculated log , it will be in the most part more accurate then a paddle wheel.

Personally I have a beautiful walker log I use from time to time !! , I bought it some years ago for 50 from a friend.

Dave


Sent from my iPad using Cruisers Sailing Forum
__________________
Check out my new blog on smart boat technology, networking and gadgets for the connected sailor! - http://smartboats.tumblr.com
goboatingnow is offline   Reply With Quote
Old 16-03-2015, 18:15   #161
Registered User
 
transmitterdan's Avatar

Join Date: Oct 2011
Boat: Valiant 42
Posts: 4,024
Re: Speed Through GPS Versus Old Fashioned Paddle Log

Almost any method of measuring speed using Doppler is more accurate than a paddle wheel. And Doppler speed measurement is very fast. So if you want accuracy and fast updating then Doppler is the way to go.
__________________
transmitterdan is online now   Reply With Quote
Old 16-03-2015, 18:37   #162
Registered User
 
Sailor Doug's Avatar

Join Date: Jan 2010
Location: Lake Erie
Boat: H36
Posts: 384
Re: Speed Through GPS Versus Old Fashioned Paddle Log

I once got stuck on a sand bar, I revved the diesel to try to free my boat. Garmin gps read 0.3 knots so I kept trying. Finally I spat over the side and found I was not moving.
Instance reading does not equal real change in velocity, all of our computers take time to calculate and display value.


Sent from my iPad using Cruisers Sailing Forum
__________________
Sailor Doug is offline   Reply With Quote
Old 16-03-2015, 21:31   #163
Registered User

Join Date: Jul 2011
Location: Atlantic ICW 29N/81W
Boat: Beneteau Oceanis 36CC, now sold
Posts: 817
Re: Speed Through GPS Versus Old Fashioned Paddle Log

Quote:
Originally Posted by goboatingnow View Post
I'd have to disagree with this, in practice gps instantaneous speed is one thing and is not a metric we use much on a boat from navigation. Distance travelled per time sequent is what most people use.

If you did want to mark up EPs using a gps calculated log , it will be in the most part more accurate then a paddle wheel.

Personally I have a beautiful walker log I use from time to time !! , I bought it some years ago for 50 from a friend.

Dave


Sent from my iPad using Cruisers Sailing Forum
Funny that is what I sold my last one form Maybe we knew each other ??. In later years I mainly used mine to calibrate or check the paddlewheel log but suitably mounted on a nice piece of varnished teak it would make a nice table ornament too.
__________________
Robin3 is offline   Reply With Quote
Old 16-03-2015, 21:48   #164
Registered User

Join Date: Jul 2011
Location: Atlantic ICW 29N/81W
Boat: Beneteau Oceanis 36CC, now sold
Posts: 817
Re: Speed Through GPS Versus Old Fashioned Paddle Log

Quote:
Originally Posted by goboatingnow View Post
I would argue GPS speedo in the absense of significant tide is much more accurate then a paddle wheel.


Sent from my iPhone using Cruisers Sailing Forum

Indeed, and If We all only ever sailed on ponds or lakes where there are no currents then there would be no discussion needed but then we would have missed over 150 contentious replies
__________________
Robin3 is offline   Reply With Quote
Old 16-03-2015, 22:13   #165
Registered User

Join Date: Apr 2011
Posts: 2,243
Re: Speed Through GPS Versus Old Fashioned Paddle Log

My paddle wheel is actually quite a decent measure of whether I need to get the bottom cleaned, it is so sensitive to water flow over it. Immediately after cleaning it over-reads by 10% or so. It is spot on after a month, and 10% low after a month or so more. Presumably this is because it is very close to the hull, in the laminar flow region.
__________________

__________________
Bristol 31.1, SF Bay.
MarkSF is offline   Reply With Quote
Reply

Tags
gps

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

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


Similar Threads
Thread Thread Starter Forum Replies Last Post
Old Log will be moved to opencpn.log.log PHD1026 OpenCPN 26 19-05-2015 14:01
Log paddle wheel kit ancor Marine Electronics 4 19-10-2014 20:11
Speed Log Paddle Wheel Won't Spin in Water Reefmagnet Marine Electronics 11 12-09-2012 05:43
Log Speed vs GPS [SOG] Speed? Sandyh Navigation 57 07-06-2009 23:22
Any "old-fashioned" looking boats besides Tayana 37? coyotewrw Monohull Sailboats 30 07-08-2008 20:22



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 13:05.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.