Speedo vs. GPS....data

[will941s]

Went out this morning and ran my boat with a speedo, against my GPS in different conditions to see how accurate the speedo would compare to the GPS, here are the specs and the data collected.

Tested @ Bushy park (freshwater resevior side and salt water side) in Goose Creek South Carolina. 74 degrees outside with 70% humidity, water temp @ 63 Degrees. Wind speed was 1-3mph.

1998, 14.5' BayRayder fiberglass side console V-hull with 1998, 25hp Mercury, 2-stroke running with a 3 blade, 13p Aluminum BlackMax prop. Motor is 114 pounds, boat is 650pounds, im 220, gas tank was 34 pounds, 2 batteries @ 81 pounds total and 38 pounds of gear. 10 gallon live well filled with water.

Faria Speedo with Teleflex pitot tube. Tube is 10' long from pitot to guage. Against Garmin Explorer GPS handheld.

Results: Fresh water lake at WOT (5,900 RPM's) with no water movement was 32 MPH speedo, and 32.1-3 MPH garmin

Fresh water @ Cooper River junction with 13,000 CFS running into the river from the lake. Current speed was 2 MPH (confirmed by drifting W/ GPS): Upriver: 32.0 MPH GPS and 32-33 MPH speedo (light water pressure on pitot tube). Downriver: 32.1-3 MPH GPS and 32 MPH Speedo.

Salt water @ Cooper River with tide moving out @ 5 MPH (confirmed by drifting w/GPS). Upriver: 31.0-1 MPH GPS and 32-33 MPH Speedo (spiked when hitting current pockets), Downriver: 33.0-2 GPS and 32 MPH Speedo

Goose Creek @ Cooper River (Salt Water) with tide going out @ 12 MPH (confirmed by drifting w/GPS) Upriver: 28-29.8 MPH GPS and 32-34 MPH Speedo. Downriver: 32-33.8 GPS and 32 MPH Speedo.

I think it's safe to say that if a speedo is properly hooked up it will read just as accurate as a GPS if you figure a 3 MPH gain or loss depending in conditons. Obviously this data may only be able to used on a rig the same style and speed as mine, as im sure hull design, lenth, weight, OB or I/O size, and speed all have different impact on the accuracy of the data, as well as temperature and altitude.

 

[Brewman61]

Re: Speedo vs. GPS....data

You seem to have a pretty accurate speedo. Mine is relatively close to my GPS, but I didn't test it anywhere as thoroughly as your did.

 

[QC]

Re: Speedo vs. GPS....data

My speedo is accurate too, also a Faria, but Bravo 1 internal pitot. My problem with your data is there is not enough discrepancy between the current affected pitot numbers and the GPS. If I am in 2 MPH current (Colorado River, Parker AZ) my GPS and speedo are off 2ish MPH each way. When I run on the lake, they are within 1% . . . Seems like yours should be the same i.e. 2 MPH and 5 MPH apart respectively . . .

 

[JB]

Re: Speedo vs. GPS....data

A pitot speedometer can only accurately measure speed through water. If the water is moving, too, the reading will be inaccurate by at least the amount of the water speed, as QC points out.

That makes the pitot useful for comparing boat performance with changes in rigging, power, loading, etc. unaffected by current.

For navigation, however, the GPS is superior.

 

[will941s]

Re: Speedo vs. GPS....data

I mounted my GPS right above the Speedo on the console for the test so I could see both at real time while I was running. All the tests were done while at WOT. The GPS speed fluctuated alot more then the speedo, so thats why the lows and highs are in the data, everything was written down right after each test. I expected the speeds to change alot more than they did, but the higher current creek had the biggest impact. I would expect if the hole in the pitot was bigger, or a different design it may cause more real time fluctuation in the speed (maybe hull design at the keel of the boat had something to do with it). I would expect different makers may produce different results. I hope to come across some more pitot tubes and run different makers to see if it makes a difference, also I would like add a paddle wheel to my Depth Finder to see how that stacks up. Someone else should do this with a higher HP faster boat and see what happens, I think it would be interesting to compare data and give people a guideline on what to expect from a speedo.

 

[fishinandbeer]

Re: Speedo vs. GPS....data

I tested my speedometer against my GPS awhile back just to see how accurate it was. It's pretty much on the dot up till 50mph. Then when it says 60 the gps says 56. And when the speedometer hits 65 it lines blow off it so I can't measure any higher than that. Usually about the time I hit 65 though the gps is reading about 60. Top speed by gps 73mph, but I'll never do that again. Too fast for a 19 ft boat.

 

[mommicked]

Re: Speedo vs. GPS....data

If you try that test entering Oregon inlet N.C. at the wrong time you will get drastically different readings.But nobodys looking at the instruments when its that bad!!!!