Hull Speed? - Page 3 - SailNet Community

   Search Sailnet:

 forums  store  


Quick Menu
Forums           
Articles          
Galleries        
Boat Reviews  
Classifieds     
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Charts
Clothing
Electrical
Electronics
Engine
Hatches and Portlights
Interior And Galley
Maintenance
Marine Electronics
Navigation
Other Items
Plumbing and Pumps
Rigging
Safety
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here






Go Back   SailNet Community > General Interest > General Discussion (sailing related)
 Not a Member? 


Like Tree4Likes
Reply
 
LinkBack Thread Tools
  #21  
Old 10-17-2011
jackdale's Avatar
Senior Member
 
Join Date: Dec 2008
Location: Calgary, AB, Canada
Posts: 8,926
Thanks: 27
Thanked 52 Times in 49 Posts
Rep Power: 6
jackdale will become famous soon enough
Quote:
Originally Posted by emoney View Post
On my powerboat, I checked my GPS vs. the speedometer (hey, it's measure in mph so I couldn't say knotmeter) and there was a 5mph difference betwixt the two. Question was, which one was correct?
Assuming no current, your speedometer might need to be calibrated. If there is current, it is hard to tell. Also they are measuring different things boat speed (through the water) versus SOG.

BTW - you should be able to change to the units of measure from miles, knots, kilometers, etc. on both the GPS and the speedometer.
__________________
__________________
ISPA Yachtmaster Offshore Instructor Evaluator
Sail Canada Advanced Cruising Instructor
IYT Yachtmaster Coastal Instructor
ASA 201, 203, 204, 205, 206, 214
As I sail, I praise God, and care not. (Luke Foxe)
Reply With Quote Share with Facebook
  #22  
Old 10-17-2011
TakeFive's Avatar
If it's blowin' I'm goin'
 
Join Date: Oct 2009
Location: Swarthmore, PA
Posts: 2,649
Thanks: 6
Thanked 50 Times in 47 Posts
Rep Power: 6
TakeFive will become famous soon enough
Quote:
Originally Posted by emoney View Post
On my powerboat, I checked my GPS vs. the speedometer (hey, it's measure in mph so I couldn't say knotmeter) and there was a 5mph difference betwixt the two. Question was, which one was correct?
Short answer: I'd trust a satellite before I'd trust a powerboat's Pitot tube.

Long answer: When you look at the way SOG is calculated from GPS positions, there is some error at slow speeds because the instantaneous positions have some error. But that would show up as random noise in your readings, which is easily damped out by good software. As a practical matter, if your GPS is showing a stable speed to the first decimal (x.x), without frequent changes after the decimal, then your signal is good and/or your software is damping it. I would consider the GPS to be highly reliable, and far more reliable than the pitot tube, which I believe could be severely affected by turbulence, salinity level, and temperature. In fact, I think its potential non-random bias probably exceeds the tidal currents in most waterways.

I'm not familiar enough with pinwheel-type speedometers used on sailboats to make a comparison, but I doubt they would be more accurate than a GPS for SOG.
__________________

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

Formerly posted as "RhythmDoctor"
1998 Catalina 250WK Take Five (at Anchorage Marina, Essington, on the Delaware River)
1991 15' Trophy (Lake Wallenpaupack)
1985 14' Phantom (Lake Wallenpaupack)
Reply With Quote Share with Facebook
  #23  
Old 10-17-2011
Faster's Avatar
Just another Moderator
 
Join Date: Sep 2005
Location: New Westminster, BC
Posts: 14,933
Thanks: 76
Thanked 214 Times in 206 Posts
Rep Power: 10
Faster has a spectacular aura about Faster has a spectacular aura about Faster has a spectacular aura about
The thing with 'knotmeters' is that they are 'calibratable', however that doesn't mean they've been accurately calibrated. Out of the box they should be OK but the only way to be certain is to run a measured distance in the absolute guarantee of no influencing current.. time the runs and do the math.

Not many boaters go to that effort..
__________________
Ron

1984 Fast/Nicholson 345 "FastForward"

".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
Reply With Quote Share with Facebook
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools

 
Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On


Similar Threads
Thread Thread Starter Forum Replies Last Post
C&C 24 hull speed? PaulinVictoria General Discussion (sailing related) 7 06-17-2010 11:41 AM
Hull Speed? PalmettoSailor General Discussion (sailing related) 38 08-10-2007 04:14 AM
Hull Speed timangiel General Discussion (sailing related) 35 07-24-2007 09:08 PM
LWL and Hull Speed flicker General Discussion (sailing related) 2 12-03-2003 07:41 PM
Hull Speed Sailormon6 General Discussion (sailing related) 11 08-03-2002 08:16 AM


All times are GMT -4. The time now is 04:23 AM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) Marine.com LLC 2000-2012

The SailNet.com store is owned and operated by a company independent of the SailNet.com forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.