Another useless inquiry. Speedometer

[liphunter]

I apologize if this has been done before, but I couldn't find what I was looking for in search. Which do you feel is more accurate for real speed, Your boats speedometer or GPS? I have always assumed GPS, but I really don't know.

Mike

[MrGiggles]

I have always trusted that GPS is more accurate. Reading now, GPS is supposed to be accurate within .1-.5 MPH.

My boat does not have a speedometer (my graphs use GPS for speed) so I do not have a side by side comparison.

[fishinwrench]

Believe it or not, water pressure analog speedo's are very accurate.  The only problem with them is that the water pickup on most outboard lower units gets clogged easily.  The fix is to install a standard speedo pitot to the transom and move the hose over to the pitot.  They will kick up if you strike anything so if the speedo stops working you'll have to reach over and snap it back down.   

GPS is a little flakey unless you are running at a certain RPM for a 1/8-1/4 mile.  Analog speedo's record speed changes immediately.

[tjm]

4 minutes ago, fishinwrench said:

Believe it or not, water pressure analog speedo's are very accurate.  The only problem with them is that the water pickup on most outboard lower units gets clogged easily.  The fix is to install a standard speedo pitot to the transom and move the hose over to the pitot.  They will kick up if you strike anything so if the speedo stops working you'll have to reach over and snap it back down.   

GPS is a little flakey unless you are running at a certain RPM for a 1/8-1/4 mile.  Analog speedo's record speed changes immediately.

How does that work with current?

[fishinwrench]

6 minutes ago, tjm said:

How does that work with current?

Good question, I don't know.    

Hold the boat stationary in the current and see what your reading is, then do the math.  Probably be a 3 mph (or less) variance.  

[tjm]

My understanding is GPS is measuring land speed by incremental fixes and will be more accurate percentage wise at higher travel speeds. Pitot tubes measure relative movement between hull and water or air and give continuous or analog readings of that movement as speed. In other words they are measuring different speeds.  Not having experimented with boats, my guess (based on effects of current on  ships speed and gun bombardment ) is that across the lake the pitot analog should be very accurate  and that going upstream it would show faster than actual speeds  and going down stream it would show slower than actual speeds. I think the answer to the op question is "yes"- meaning that both are sometimes more accurate than at other times. At sea the GPS, checked against a sextant would be my choice, on river or  lake I can't see it being of great importance- never far from shore and land marks.

[snagged in outlet 3]

I have two apps on my phone for GPS.  Speedbox and Speedometer.  I've used them in my truck and they dead on to the truck and each other.  Coincidentally they are dead on to my cycling app that tracks speed too.

[fishinwrench]

I have a pitot speedo in my bass boat just because I have them here and it didn't cost me anything to install it.....and there was already a hole in the console anyway.  Otherwise I couldn't care less how fast I am going.  My 18'Ranger/150 Evinrude tops out at 62 mph with just me in it. 58 mph with a passenger.  Oddly enough the livewells being full/empty or the amount of fuel I'm carrying doesn't measurably effect the top end speed, but does effect the hole shot.  

[Devan S.]

Pitot tubes in and of themselves are instantaneous. The real loss in accuracy comes into how they translate the actual pressure change.

GPS actually have an incredibly small delay(that most would never notice) and accuracy can be effected by the number of satellites available to ping.

Regardless I have had far less troubles with GPS than I ever had with the analog speedometer and frankly the delay on GPS is so small most people that aren't flying commercial or military aircraft will never know the difference. 

[fishinwrench]

7 minutes ago, Devan S. said:

Pitot tubes in and of themselves are instantaneous. The real loss in accuracy comes into how they translate the actual pressure change.

Analog guages that measure pressure are inherently consistent.  Tire pressure guages, compression guages, and fuel/water pressure guages will almost always read exactly the same.  The mechanics of that are just really simple.