Transducer MPH versus GPS MPH

Pseudomind

Pseudomind

Active Member
Jul 1, 2008
2,122
Jacksonville, FL
Boat Info
2011 Hurricane with Magic Tilt Trailer
Engines
115 HP Yamaha Four Stroke
I have seen some threads where users post their speed in MPH stating it is from the GPS.

What is the difference, example my Raymarine Tridata will have maybe 16.xx KTS and the GPS will have 21.xx KTS, about a four KTS difference.

Which is closer to the true actual speed?

Thanks

:thumbsup:
 
"The transducer is not reliable" -> a true statement, but nonetheless for many applications the transducer speed is of importance for a boater. For instance, when travelling in a channel that has a significant current, knowing that the boat is moving at 7mph in the water and is at the point of producing wake IS relevant, just like the fact that speed over ground of 2mph is relevant (which makes you gun the engine anyway)
 
The other important facter to keep in mind is if you are trolling for fish you have to rely an the transducer as to the speed of pulling your bait thought the water.
 
The transducer speed can also be inaccurate because of flow turbulence, paddle wheel friction, and boundary layer flow.

Sounds like a job for Dr. Fluid Dynamics!

Best regards,
Frank
 
sfergson727 said:
GPS is the only accurate way to judge your speed over the earth.
Click to expand...

There are other means of accurately measuring Speed Over Ground (SOG), including radar, though between a mechanical means such as a paddle wheel or a knot log, and an electronic means such as GPS or radar, trust the GPS (or radar).

The speed through the water is useful, in addition to the reasons cited previously, in case your power goes out (or batteries in your handheld GPS die), in which case you can use dead reckoning techniques to combine knowledge of external factors (current, tides, wind, etc.) with your boat's speed through the water to fairly accurately determine your SOG as well as your position.
 
:smt021GPS =+oR _ 30 FEET THAT IS 60' ERROR:smt021
sfergson727 said:
GPS shows your speed over the ground. The transducer shows your speed over the water, but doesn't account for wind, current, etc. GPS is the only accurate way to judge your speed over the earth.

For example: You are operating at 21 (transducer) mph directly into a 5 mph current. Your actual speed over the ground as read by GPS is 16 mph, even though your analog spedo shows 21 mph.
Click to expand...
 
Wow, this weekend I was asking the same questions to the people on my boat. Last week I bought my first GPS, Garmin Nuvi 765. I was excited because I always wanted to know how fast my boat was. So the first first time ever we went camping at Charlestown State Park (IN) which is on the Ohio River. I had never been boating on the river. It was a lot of fun to cruise down to Louisville.

Disappointed because I only got to 38 MPH with the current. But was wondering out loud to my friends why on my GPS it said Maximum speed was 43.7 MPH. The whole time I was watching and the high was 38 MPH. Then one of my friends suggested that the GPS was stating the "true" speed, boat speed plus the current. He used to fly planes.
 
You must log in or register to reply here.

Forum statistics

Threads
113,162
Messages
1,427,568
Members
61,071
Latest member
TellurideBoater
Back
Top