Speedo accuracy? Do they work?

[W_Guy]

Long story short, I've been told that the boat I just bought registered 40mph with 4 200 pounders in it. Other people told me it would be lucky to do 30. Just how accurate are these do-hickys? The photo shows the sensor on the transom. Do those gauges just work by water pressure in the tube? If so, I'd guess the angle of the pick-up has a lot to do with accuracy. Should I pull the tube off the back of the gauge and blow the whole line out? How accurate are the gauges themselves?
I'm low tech, so don't suggest I use a gps. I still have a rotary phone in my house.

 

[5150abf]

Re: Speedo accuracy? Do they work?

that is a pitot tube speedo and they are notoriously inaccurate.

Yes it works on water pressure so if the smallest thing gets in the tube it makes it register wrong, and yes, pull the tube off the back of the speedo and blow through it to clear it.

And as much as you don't want to hear it a GPS is the only way to get an accurate speed, they aren't complicated, pretty much if you can straighten your finger and push a button you can run a GPS.

You have managed to work a computer and they are way more complicated.

 

[W_Guy]

Re: Speedo accuracy? Do they work?

Thanks for your confidence in me
Is it possible, looking at this other photo that the water flows through the bottom of the sensor, creating a venturi effect whereby vacuum is applied to the gauge?

I'm never going to be on a lake big enough that I'll need a gps to get home.

 

[Gary H NC]

Re: Speedo accuracy? Do they work?

It works on pressure not vacuum.
Any kinks in the line or zip ties squeezed too tight from the pitot to the gauge can affect it also.
I have been in brand new boats that were off as much as 15mph according to the GPS.
This is the newer style pitot...

http://www.iboats.com/Universal_Pit...52246600--**********.643789825--view_id.70963

 

[cribber]

Re: Speedo accuracy? Do they work?

At lower speeds pitot tube speedo's are accurate enough... Mine starts to drift away from what the GPS is recording around 20-25 MPH. My pitot tube speedometer on the boat will register 48 MP when GPS is showing about 42. I use the boat speedo to let me know when I'm trimmed out at WOT and getting the most bang for my buck on a WOT run. I don't use the map function on my Garmin because we travel the same water every weekend. I'm usually using the trip functions to record distance and speed so I can get a feel for my GPH and MPG when we're out on the lakes. An afternoon running 60 plus miles is a blast!!!

 

[QC]

Re: Speedo accuracy? Do they work?

I have had two standard through leg pitots that were/are almost dead nuts accurate. Weird, I know, but I had an Alpha that was within 1 MPH of GPS from 25 - 60, and same with my current Bravo . . . I've never had good luck with pitots below 20ish.

And just to continue the clarifications . . . they actually work on air pressure created by water being jammed into the tube.

 

[W_Guy]

Re: Speedo accuracy? Do they work?

Thanks. Then I'll just treat the gauge as a "novelty" item and just enjoy the ride, whatever speed it is......

 

[Mark42]

Re: Speedo accuracy? Do they work?

The pitot speedo on my MFG was withing a mile or two of accurate when using GPS at 28mph. That's close enough for me.

 

[skargo]

Re: Speedo accuracy? Do they work?

Thanks. Then I'll just treat the gauge as a "novelty" item and just enjoy the ride, whatever speed it is......

That sounds like a great plan to me.

 

[cribber]

Re: Speedo accuracy? Do they work?

Every boat's setup is different... my accuracy starts to drift northward as I start to trim my Volvo I/O SX drive around the 20 MPH mark... My main concern is my WOT RPM so I don't ruin a good day out by over revving my little 3.0 Volvo. If it gets near 4800, I throttle back since I'm already 200 over reccommended RPM range.

 

[windsors03cobra]

Re: Speedo accuracy? Do they work?

I just tell the passengers matter of factly what the gauge reads, haven't had any disputes.
My Harman Kardon sat nav 810 does not indicate MPH.

Curious about the wood boat......

 

[Ram58]

Re: Speedo accuracy? Do they work?

Mine reads 3-4 MPH slower than the GPS reading.

 

[kenmyfam]

Re: Speedo accuracy? Do they work?

Mine is 2 mph slow at 40mph GPS verified.

 

[Mike Robinson]

Re: Speedo accuracy? Do they work?

.....Curious about the wood boat......

Me too! Show us yur boat!

PS: My wife picked up a used handheld GPS the other day at a thrift store for 5 bucks

 

[CheapboatKev]

Re: Speedo accuracy? Do they work?

can you borrow a GPS for one outing to see what the diff may be and do the math from there?

 

[W_Guy]

Re: Speedo accuracy? Do they work?

Well, since you asked, here she is. She's a 1960 Thompson Bros (Peshtigo, WI) Sea Coaster Special 16 footer. All orignial, single family owned. Came with the original Gator trailer (needs paint) and a '58 Evinrude Four Fifty with electric start & generator that runs great. I've posted other photos of the motor and asked questions on the E-J motors forum. I'm very glad I found this site. It's very active and full of well informed people who have helped me a lot so far.

 

[cbavier]

Re: Speedo accuracy? Do they work?

SWEET..................!!!


Yea GPS read mine 6 mph faster than my spedo reads and my speedometer doesn't register until I'm doing about 10 mph.

 

[floatingwoody2006]

Re: Speedo accuracy? Do they work?

does the Gator have the square axel on it? Mine does, and even though i have no reason other than surface rust not to trust it.. I find myself worried about it while in route..

 

[mike64]

Re: Speedo accuracy? Do they work?

Nice boat! I love those old wood boats, and yours looks like it's in great shape.

As for the accuracy issue, do any of those 200 pounders riding with you have an iPhone on them? I believe iPhones come with gps software on them. Could be wrong though.

 

[22E6441]

Re: Speedo accuracy? Do they work?

Ditto.....nice boat!

I was wondering about accuracy too, so I took my running gps on the boat and it showed 28 mph while the speedo showed 30. Pretty close.

However, at lower speeds it was way off. Low and high at times. Weird. I think a lot of it depends on the water conditions.....smooth water would give a more reliable reading. Any way, GPS is the way to go for sure. I wish my boat had a tachometer rather than a speedometer.....