Tom Tom speed

[tuffshot]

Went to the local lake this morning to do a little fishing.

My boat is an 89 bass tracker 16 ft fiberglass with 90 hp Johnson. The pitot was broken off when I bought it so speedo doesn't work. I borrowed the wifes Tom Tom and took it out on the lake with me..

Cool it worked as well as any GPS, the picture even showed where I was on the lake. I opener her up and got 40 MPH.

 

[25thmustang]

Re: Tom Tom speed

I did the same thing on mine and with 4 people and 3/4 tank of fuel I got 35. I was told they are very accurate, and my number is about what I think the boat should have done.

 

[PWH1]

Re: Tom Tom speed

I recently did the same...

The speedometer in my boat was reading 38 MPH at WOT. The GPS read 42 MPH at WOT.

 

[metriccrescentwrench]

Re: Tom Tom speed

The speedometer in my boat was reading 38 MPH at WOT. The GPS read 42 MPH at WOT.

Boat speedometers are notoriously innacurate, and they're usually very optimistic.

 

[Philster]

Re: Tom Tom speed

A GPS can be way, way, way off.

If there is a current, or tide, and you head in that direction at 45 MPH as speed over water (pitot tube style will read around 40), and the current/tide is heading out at 5mph, the GPS reports 50 MPH.

Turn around against the tide/current and the pitot tube style will read about 50 MPH and the GPS will read 40 MPH.

Make GPS runs in different directions and always take the average. With the current and tide? Pitot reports lower, GPS reports higher. Against current and tide? Pitot reports higher, GPS reports lower.

 

[GatorMike]

Re: Tom Tom speed

I reckon it depends on whether you are talking "true speed" or "indicated speed". If you are talking true speed which would be used for navigation purposes the GPS is more accurate. Indicated speed which takes into account the current and is not relative to a fixed point is given by the speedo.

 

[Philster]

Re: Tom Tom speed

For figuring out how a boat performs, you need to know actual speed over water. Period.

Different than indicated speed. Different than GPS true speed used for navigation purposes.

 

[JB]

Re: Tom Tom speed

A GPS can be way, way, way off.

If there is a current, or tide, and you head in that direction at 45 MPH as speed over water (pitot tube style will read around 40), and the current/tide is heading out at 5mph, the GPS reports 50 MPH.

Turn around against the tide/current and the pitot tube style will read about 50 MPH and the GPS will read 40 MPH.

Make GPS runs in different directions and always take the average. With the current and tide? Pitot reports lower, GPS reports higher. Against current and tide? Pitot reports higher, GPS reports lower.

Completely inaccurate. The GPS will always give an accurate, over the surface of the earth, speed.

The pitot will usually give a useful approximation of through the water speed, which is useless for piloting or navigation purposes.

 

[JB]

Re: Tom Tom speed

Hmmmm. I guess we have different ideas of what is a useful measurement.

I concede that if you are only interested in how fast you think your boat can go, then Philster's comments are correct and the pitot speedo estimates are useful, and you have neither need nor use for a GPS.

 

[Bob_VT]

Re: Tom Tom speed

My handheld and my car GPS will both give average speed as well as highest speed attained.

 

[RickJ6956]

Re: Tom Tom speed

A GPS can help you calculate speed at a specific RPM by using Philster's method. On a calm day in the Niagara River the current is about 4 mph. At 1100 RPM in my boat the GPS reads 4 mph going upstream and 12 mph going downstream. In the protected harbor with no wind or current the GPS reads 8 mph at 1100 RPM.

(And I don't think Philster meant the GPS will read lower than ground speed. I interpreted his post to mean that it will read lower than the speedo when going against the current.)

 

[GatorMike]

Re: Tom Tom speed

Sorry guys, I didn't intend to start a debate. I just didn't want somebody to think if their spedo read 10 miles per hour upstream against a 5 mph current that they would arrive at their destination 10 miles away in an hour. As an ex pilot indicated air speed was just a tool used to calculate true air speed. This was in the days before GPS of course.

 

[PWH1]

Re: Tom Tom speed

My GPS test was on an inland lake (my profile pic) with optimal conditions. Low wind and smooth as glass...

I believe the GPS reading to be accurate.