GPS determined speed - Page 3 - SailNet Community
 10Likes
Reply
 
LinkBack Thread Tools
post #21 of 28 Old 08-15-2019 Thread Starter
Senior Member
 
SanderO's Avatar
 
Join Date: Jul 2007
Location: Westchester, NY
Posts: 4,513
Thanks: 2
Thanked 113 Times in 113 Posts
Rep Power: 13
 
Re: GPS determined speed

Sailing boats move pretty slowly... and their motion is impacted by gusts, waves, currents, wakes and heel. Sails can be trimmed to get the last bit of speed out of a sail plan. Speed changes from trimming can be very subtle. The sampling rate or the precision of GPS is capable of producing accurate speed through the water. Current alone is invisible to GPS. Sailing in LIS I know there is current impacting SOG almost all the time. I don't believe current charts and tables are precise and accurate either.

The only way to know how fast your boat is moving through the water is with an accurate, precise and calibrated speed transducer.

Speed thru the water may not matter much to boaters because ultimately it is the real world speed over the ground which determines when you will reach your destination or mark. I don't imagine racers use or rely on GPS speed very much. But I am not racing and I wouldn't know.

pay attention... someone's life depends on it
SanderO is online now  
Sponsored Links
Advertisement
 
post #22 of 28 Old 08-15-2019
Senior Member
 
Join Date: Feb 2010
Location: Narragansett Bay
Posts: 21,106
Thanks: 82
Thanked 593 Times in 569 Posts
Rep Power: 12
   
Re: GPS determined speed

SOG, or better VMG, are what I use for efficient navigating. Deciding whether to tack/jibe, take wave action on the nose or beam, move to better current, etc. You are right, current tables are, at best, accurate at a specific spot. Move closer to land, or a turn, or deeper/shallower water, and itís different. Perhaps the only thing I believe is fairly accurate across the spectrum is the direction of current.

I only use STW to see the efficient result of sail trim, given wind speed and direction.
SanderO likes this.


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Jeanneau 54DS

In the harsh marine environment, something is always in need of repair. Margaritas fix everything.
Minnewaska is offline  
post #23 of 28 Old 08-15-2019
Moderator
 
Arcb's Avatar
 
Join Date: Aug 2016
Location: Eastern Ontario
Posts: 4,030
Thanks: 215
Thanked 202 Times in 198 Posts
Rep Power: 10
 
Re: GPS determined speed

There are some pretty good speed tracking apps out there that track both speed and acceleration in graph format with multiple updates per second. All the ones I am familiar with are in cell phone apps, but I assume the better chart plotters must have something similar?

Check out some of the apps out there for performance cyclists and kayakers. They not only give reallly good speed tracking but in some instances cross reference it with other info like heart rate, strokes per minute etc.

I think the technology is there for speed from GPS, I just don't know how much its being utilised by most sailors.

I know personally, if I am concerned about performance tracking and analysis, I am using cell phone apps.

Curious what the Americas cup guys use? Surely not paddle wheel speed logs on foiling boats?
Arcb is offline  
 
post #24 of 28 Old 08-15-2019 Thread Starter
Senior Member
 
SanderO's Avatar
 
Join Date: Jul 2007
Location: Westchester, NY
Posts: 4,513
Thanks: 2
Thanked 113 Times in 113 Posts
Rep Power: 13
 
Re: GPS determined speed

Quote:
Originally Posted by Arcb View Post
....

Curious what the Americas cup guys use? Surely not paddle wheel speed logs on foiling boats?
Of course they have lots of data.... but they DO use transducers for boat speed... just as they do for wind speed and depth.

I don't know that for certain... but I'd bet on it.

pay attention... someone's life depends on it
SanderO is online now  
post #25 of 28 Old 08-15-2019
Moderator
 
Arcb's Avatar
 
Join Date: Aug 2016
Location: Eastern Ontario
Posts: 4,030
Thanks: 215
Thanked 202 Times in 198 Posts
Rep Power: 10
 
Re: GPS determined speed

Looks like they use intertial navigation systems.

https://www.ixblue.com/news/fiber-op...-greatest-race
Arcb is offline  
post #26 of 28 Old 08-15-2019
Senior Member
 
fallard's Avatar
 
Join Date: Nov 2009
Location: Mystic
Posts: 2,349
Thanks: 20
Thanked 80 Times in 79 Posts
Rep Power: 11
 
Re: GPS determined speed

Quote:
Originally Posted by SanderO View Post
I happened to notice that my old Ray C80 and a newer, but old B&G Zeus T7 were not reporting identical SOG yesterday. They were never more than .1 knots different... but what do I attribute this to?


old technology in the C 80?
rounding algorithm?

I have several other GPS including smart phone which I did not check. The difference is small and of no consequence...

Just curious... any thoughts?
There could be a difference in the sample rate in your data busses, assuming the Ray and B&G units use different busses. When I did a wholesale change out of my electronics a number of years ago the installer made a point of connecting my flux gate compass directly to the autopilot computer to facilitate a higher sample rate (than Seatalk supported) for more responsive seakeeping via autopilot. He indicated that it the compass data would then be distributed to the rest of the network via Seatalk to the other instruments and via the HD buss that connected my C90W chartplotters.
fallard is offline  
post #27 of 28 Old 08-15-2019 Thread Starter
Senior Member
 
SanderO's Avatar
 
Join Date: Jul 2007
Location: Westchester, NY
Posts: 4,513
Thanks: 2
Thanked 113 Times in 113 Posts
Rep Power: 13
 
Re: GPS determined speed

Quote:
Originally Posted by fallard View Post
There could be a difference in the sample rate in your data busses, assuming the Ray and B&G units use different busses. When I did a wholesale change out of my electronics a number of years ago the installer made a point of connecting my flux gate compass directly to the autopilot computer to facilitate a higher sample rate (than Seatalk supported) for more responsive seakeeping via autopilot. He indicated that it the compass data would then be distributed to the rest of the network via Seatalk to the other instruments and via the HD buss that connected my C90W chartplotters.
These are more or less stand alone. The Ray has DSC and AIS and compass inputs and exports to cockpit repeaters. AP is stand alone with its own compass.

pay attention... someone's life depends on it
SanderO is online now  
post #28 of 28 Old 08-15-2019
Senior Member
 
Join Date: Oct 2011
Location: Nanaimo B.C.
Posts: 4,563
Thanks: 5
Thanked 141 Times in 140 Posts
Rep Power: 9
 
Re: GPS determined speed

Slightly off drifting. Crewed on a vessel to the Andamans. At the helm,, the only GPS. Used for steering. (no compass) For a number of reasons we'ed lose way. ...into irons cross sea or just bad steering. Nether the skipper nor the pompous 'expedition leader' understood that course displayed was a calculation based where we had been and not where we were pointing in the dark and confused sea. Numbers can be misleading.
SanderO likes this.
Capt Len is online now  
Reply

Quick Reply
Message:
Options

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.
Click Here to view those rules.

Register Now



In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.


User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in











Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may 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 On
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome