Hull Speed? - Page 3 - SailNet Community
LinkBack Thread Tools
post #21 of 23 Old 10-17-2011
Senior Member
jackdale's Avatar
Join Date: Dec 2008
Location: Calgary, AB, Canada
Posts: 9,044
Thanks: 27
Thanked 59 Times in 56 Posts
Rep Power: 7
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)
jackdale is offline  
Quote Quick Reply Share with Facebook
Sponsored Links
post #22 of 23 Old 10-17-2011
Senior Member
TakeFive's Avatar
Join Date: Oct 2009
Location: Swarthmore, PA
Posts: 3,216
Thanks: 8
Thanked 88 Times in 78 Posts
Rep Power: 6
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.

1998 Catalina 250WK Take Five (at Anchorage Marina, Essington, on the Delaware River)
1994 Mason 44 Firefly on loan from my BFF (West River, Galesville, MD)
1991 15' Trophy (Lake Wallenpaupack)
1985 14' Phantom (Lake Wallenpaupack)
TakeFive is offline  
Quote Quick Reply Share with Facebook
post #23 of 23 Old 10-17-2011
Just another Moderator
Faster's Avatar
Join Date: Sep 2005
Location: New Westminster, BC
Posts: 16,713
Thanks: 112
Thanked 327 Times in 311 Posts
Rep Power: 10
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..


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)
Faster is online now  
Quote Quick Reply Share with Facebook

Quick Reply

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


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

Email Address:


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

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

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

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