Another useless inquiry. Speedometer

[tjm]

Any inaccuracy in Pitot tube speed would not be in relative water speed nor relative airspeed if in an airplane; the  key is relative water speed does not necessarily coincide with actual land speed, currents add to or take away from relative speeds when compared to surface travel or land speed.

[Devan S.]

38 minutes ago, fishinwrench said:

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.  

Most analog speedometers don't have enough resolution(in the face of the gauge) to be accurate to 1 mph let alone .1 mph. Sure the speedometer itself is going to repeat within a tolerance but your actual resolution you can read is likely far less than that tolerance. 

I don't disagree with you though in 99.999% of situations either/or is more than enough and most of the time neither is needed. For me, it came down to the keeping junk out of the pitot tube vs. turning the GPS speed display on my graph on(the graph is there and already on).

 

[fishinwrench]

If you're referring to the speedo port of an outboard lower unit...yeah they suck, and a tiny leaf fragment will put you out of business.   An actual pitot though has an oblong port and will still read accurate when partially blocked, since regardless of the size of the port pressure remains the same.  Unless you smack something big enough to kick it up or break it then it always reads, and if it reads at all....then it reads accurate and is instantaneous when speed changes.  

Of course none of this matters at all 🙄 I just like to point out that old school simple technology is often the most reliable.   It will definitely read 1mph, and .01 mph just flat out doesn't matter unless you are a powerboat racer trying to set a record.

[jdmidwest]

Sometimes the Pitot tubes don't kick up.  If you back them into a stump, they break at the mount and become worthless.

Then you buy a replacement and it has a different bolt pattern, you give up because you don't want to drill more holes in a perfectly good boat and rely on your GPS.

I use the GPS when trolling and it seems to work well at the 1-2 mph range.