How accurate are boat speedo's...

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Franki

Lieutenant Junior Grade
Joined
Feb 16, 2002
Messages
1,059
Hi Guys..<br /><br />I have a teleflex speedo, (brand new) and I just bought a brand new pitot (the bit that goes in the water) and hose for it..<br /><br />going to rig it up tomorrow and hopefully see how fast the boat goes next weekend..<br /><br />Anyway, I was wondering how accurate these things are? I know the reasons they aren't, (moving water for one.) but just how far "out" can a speedo be???<br /><br />and if I get a reading going with the tide. should I turn around, go the other way, and then average the speed to get the real figure???<br /><br />Is there likely to be a huge amount of difference???<br /><br />I don't yet own a GPS, and I wanted a reasonably accurate speed. <br /><br />regards<br /><br />Frank
 
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Jakabite

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Joined
May 3, 2002
Messages
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Re: How accurate are boat speedo's...

Frank,<br /><br />I just put one on myself. They work on pressure. You want to try to mount it to where the probe sticks into the water at lease an inch or so. You want it to to be in clear path of on comming water. It also has to be stright, the cylider where the water goes through the probe needs to be perpendicular with the force of the water. They are pretty acurate in lakes, but the ocean, its going to be off just a hair if your not going with the tide. If there is any other questions, let me know... Have a great one, and good luck..<br /><br />-JOE
 
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Franki

Lieutenant Junior Grade
Joined
Feb 16, 2002
Messages
1,059
Re: How accurate are boat speedo's...

Thanks Jakabite <br /><br />I knew how to install it.. I was just curious as to how accurate the are.. I use the boat mostly on a river, which has fairly strong tides..<br /><br />No biggie, I'll find out the old fashioned way.. I'll go and get a speed reading from the speedo and beg/borrow/steal a GPS from somewhere so I can get a speed reading from that.. compare the two.. since teleflex is one of the better brands around, I imagine that its probably as accurate as anything based on water pressure can be.<br /><br />Thanks<br /><br />regards<br /><br />Frank
 
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blifsey

Senior Chief Petty Officer
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Jan 17, 2002
Messages
769
Re: How accurate are boat speedo's...

Well, mine reads 0 no matter what my speed...So it reads perfect when sitting still :D I already had a handheld GPS so I've not bothered w/speedometer. From what I've read on other fresh water lake boats, the speedo are *usually* 3 to 8 mph faster than GPS reading.<br /><br />BL...
 
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Franki

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Feb 16, 2002
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1,059
Re: How accurate are boat speedo's...

hi BLifsey <br /><br />Thanks for the tip..<br /><br />Which brings me to another point.. GPS's<br /><br />What are good hand held brands/models???<br /><br />I have seen alot going really cheaply in the paper over here. (second hand.) unfortunatly I don't know good from bad and don't want to waste money.. so if anyone has tips.. I'd love to hear them..<br /><br />regards<br /><br />Frank.
 
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DJ

Guest
Re: How accurate are boat speedo's...

Franki,<br /><br />You'll see a HUGE difference going against and with the tides and/or currents compared to a GPS.<br /><br />On the GPS=go Garmim. They also sell some reman. units at bargain prices with warrantees!<br /><br />Do a search, on the net, with key word: Garmin
 
Bob_VT

Bob_VT

Moderator & Unofficial iBoats Historian
Staff member
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May 19, 2001
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26,071
Re: How accurate are boat speedo's...

Frank even the cheapest GPS unit will give accurate speed. I agree go with Garmin and get a handheld for the versatility. The more pricy models just have more whisltes and bells (and the newer ones are more accurate) Hit the Garmin web site and just browse. I currently use a GPS 12 by Garmin which is plent accurate for my hunting, hiking, and fishing.<br /><br />Bob
 
rolmops

rolmops

Vice Admiral
Joined
Feb 24, 2002
Messages
5,522
Re: How accurate are boat speedo's...

Hello Franki<br />Your teleflex is very accurate.....relative to waterspeed.<br />You will never be able to measure your speed relative to land simply because your measurements are done relative to the element you are in -water-.<br />A GPS will give you variable speed readings for the same stretch because it measures your speed relative to land and does not take water speed relative to land in account.<br />Just remember.<br />You'll get there when you get there.
 
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Terry H

Lieutenant Commander
Joined
Sep 25, 2001
Messages
1,862
Re: How accurate are boat speedo's...

On checkin boat speed...gps vs. speedometer last week I found the speedo read 28mph and gps at 33mph. New pitot and glassy lake. On other ocations I have found that there are diferences both high and low...never found a speedo on a boat to be closer than 2mph...usually more. <br />As far as handheld gps...I agree with the Garmin. If you are a first time buyer I would start with one like the Etrex...very inexpensive and simple to use and figure out. I wouldn't buy a used gps, especially without the manuals...just my Thoughts
 
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DJ

Guest
Re: How accurate are boat speedo's...

Boat speedo's and tides/currents work the same way as aircraft airspeed indicators and wind.<br /><br />Example: <br /><br />If an aircraft is indicating 300 mph. and is flying straight into a 100 mph headwind, it will show an airspeed of 300 but is only covering "ground" at 200 mph. The reverse would be true of a tailwind. That is why "easterly" flights on airliners are typically faster than "westerly" flights. They always fly at close to the same "indicated" airspeed, yet headwinds or the jetstream can greatly help or hinder progress.<br /><br />That is why aircraft use to use ground points to determine speed. As electronics came into play, they used LORAN and now GPS.
 
F

Franki

Lieutenant Junior Grade
Joined
Feb 16, 2002
Messages
1,059
Re: How accurate are boat speedo's...

Wow, thanks for all the info guys.. much appreciated..<br /><br />I knew there would be some difference, just didn't know how much..<br /><br />Thats why I asked if going in both directions and averaging it out would give a more accurate reading.. I figured that going with the tide would give a false "slow" reading, and going against it, would give a false "high reading" so if I added them and divided by 2, then I should have the average.. would that work do you think?<br /><br />As for the GPS, I will only by a 2nd hand model if it has the books and I verify it works..<br /><br />And thanks to you guys, I'll now make sure is a Garmin...<br /><br />Thanks for all the help..<br /><br />regards<br /><br />Frank
 
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DJ

Guest
Re: How accurate are boat speedo's...

Franki,<br /><br />It MIGHT work ,if you can count on tides going the same speed, in and out??
 
F

Franki

Lieutenant Junior Grade
Joined
Feb 16, 2002
Messages
1,059
Re: How accurate are boat speedo's...

Hi Djohns..<br /><br />I think I might have explained myself wrong.. <br /><br />I didnt' mean wait for the tide to change direction...<br /><br />Here is an example..<br /><br />you find a nice straight stretch of river, maybe two or three miles long.. I go WOT all the way down and write down my highest speed.<br /><br />Then I turn around, and go back at WOT again writing down the highest speed.. the tide hasn't changed direction or speed, only I have..<br /><br />Does that make sense?? I should then in theory be able to add both the speeds together, and halve the result to get the average, and thereby the land speed. (takin water speed/direction out of the equation.<br /><br />hope that explains what I mean better..<br /><br />regards<br /><br />Frank
 
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