In dash GPS speedo?

[harringtondav]

Be Aware that the GPS will be giving you a Ground Speed, not a Water Speed. If there is a Current, you will find the boat going faster in one direction, and slower in the opposite.

Yes, when making the 'speed runs' to verify WOT speed, it is advisable to make a pass in both directions to be able to net out any current.

I had to ponder over @jimmbo 's remark, but I get it. I'll be doing speed runs/prop slip checks in the Miss. River, so I'll take the data points up and down river, and average the results. Per my OP my pitot speedo was significantly different than my phone's GPS. So I reckon a GPS speedo will give me the most accurate results. I'll be comparing slip between my 3 blade 22P SS prop and a 4 blade 22p aluminum prop. The calculated slip may not be dead nuts, but the comparison should be valid.

 

[JimS123]

I’m in the measurement business. You would be amazed at how many customers (experts) spend big money for instrumentation with far better performance than needed to met their need with little to no ROI

In that line of thinking…given the wide range of recommend WOT rpm and that most props are only available in 200 rpm increments, how accurate do the speed and rpm readings need to be to prop a boat?

What speed device do you recommend to met the accuracy criteria knowing “over the water” speed devices are notorious for speed and linerity issues?

Don’t you need a anemometer input as well?

Would you not remove or minimize any effects of current by simply running your test across the current?

From a practical stand point, what does a “performance curve” buy you other than a nice graph to display?

In the olden days I ran a performance curve as baseline data on a new boat. My reasoning was twofold. First to optimize the prop pitch, and secondly to review data annually to see if I was losing performance, and thus needed a tuneup.

Back then I was lucky enough to live where boat races were common, and had a measured mile. I DID have a tach, accuracy unknown. Another 26 mile course with a 6 gal tank would allow me to check fuel usage every time I made a change.

The measured mile course was run up and downstream, and then averaged. The water wasn't wide enough to go across current.

OK, all rudimentary "equipment", but it was all I could afford. Today, GPS for speed, VesselView for fuel usage and rpms. We've come along way, baby....LOL.

I do measuring stuff too. Back when I was a young Engineer we used an analog watch and a bucket to calibrate plant systems. "Control" was made by sight and feel, usually by manually turning a water faucet handle. Was it good enough? Maybe, we made and sold product. Now we have PLCs and everything is computer controlled. ROI? You betcha.

Is a $300 VesselView cost effective? I dunno. However, the very first time I used it, just as I was looking at the dial the temp spiked and I shut her down in a hurry. Turned out I picked up a floating plastic bag. Raised the outdrive and the bag fell off. In my mind, THAT made the sensor worth every penny.

 

[jimmbo]

Plastic Bags have ruined many engines.

 

[Renken2000Classic]

Be Aware that the GPS will be giving you a Ground Speed, not a Water Speed. If there is a Current, you will find the boat going faster in one direction, and slower in the opposite.

That's a most interesting point that I hadn't thought of. So that applies to WOT runs, or at a set RPM. Probably negligible where I boat, which is maybe why I haven't noticed it. I don't remember seeing more than 1mph on the GPS while drifting with the anchor up. Not that it doesn't happen.

Just like an airplane going into or with the wind, just not as dramatic a difference as that can be. Only the boat's going through the water and the air, so some effect from that too.

 

[jimmbo]

I was just pointing out that GPS is not the Godsend when it comes to speedometers. On land, Going up/down inclines will provide inaccurate readings, usually in numbers lower than the actual speed, so GPS will be less accurate than the Cop's Doppler Radar, used to give you a Ticket

 

[Renken2000Classic]

so GPS will be less accurate than the Cop's Doppler Radar, used to give you a Ticket

There's no telling what they have now, but back in the day, the more angle off between you and the radar gun, the lower the reading. So the further off the side of the road the cop was if he was parked, the better that might work for you, heh. I'd imagine there's stuff now to take care of that prob, but don't know.

 

[jimmbo]

Simple math and the actual speed can be determined with Ground Radar

 

[JimS123]

I was just pointing out that GPS is not the Godsend when it comes to speedometers.

The "best" speedometer I ever had was on my last boat. It was a paddle wheel on an Eagle Sonar Unit. My river has a 1 mph current. If I was going upstream at 30 GPS, the downstream was (obviously) 32. The paddle wheel showed 31 (actual water speed) no matter which way I went.

My Yami PWC has a built in paddle wheel speedo. It is only good to about +- 1 mph.

 

[harringtondav]

I just received this Faria Beede GPS speedo from one of their retailers. Faria tech help was great. I sent a pic of my dash and they gave me the correct model. I'd picked the wrong trim but they set me straight.
Cloudy day, so I'm going down to the lift to install this.

 

[jimmbo]

I just received this Faria Beede GPS speedo from one of their retailers. Faria tech help was great. I sent a pic of my dash and they gave me the correct model. I'd picked the wrong trim but they set me straight.
Cloudy day, so I'm going down to the lift to install this.

Don't think the a GPS Speedo is Accurate all the Time. Oh, it will be correct if you want to know how fast you are moving across the bottom of the Lake/River, but will be inaccurate as to the speed on the water, if there is any Current in that Lake/River

 

[harringtondav]

Don't think the a GPS Speedo is Accurate all the Time. Oh, it will be correct if you want to know how fast you are moving across the bottom of the Lake/River, but will be inaccurate as to the speed on the water, if there is any Current in that Lake/River

I agree. ...you or someone else mentioned this above. We're 100% on the Mississippi river. Always current. If I want to split hairs I can drift and get my handicap . ....This time of year about 2-3 mph.
Either way it will be better than the original pito speedo. It shows 20 mph in the lift. When new I checked it against my phone's GPS app. 56 mph vs 52 GPS.
...guess I've got money to burn on 'wants' vs. 'needs'.

 

[QBhoy]

If it’s an mpi engine you have. Get a VesselView mobile.
Pitot speedos don’t really start showing speed to nearly 20 mph I’d think. Good to look at when showing off to passengers though . I can get mine off the clock showing way past 70 odd..but the gps will be less optimistic at a couple mph under 70 in reality.
No doubt the old pitot dreamometer has been to blame for decades of unrealistic bragging claims at the dock…but the reality and modern days of gps can usually result in the reality and often disappointing truth, haha.

 

[harringtondav]

No doubt the old pitot dreamometer has been to blame for decades of unrealistic bragging claims at the dock…but the reality and modern days of gps can usually result in the reality and often disappointing truth, haha.

Yea, my pal bought a generic in-dash GPS speedo to replace his kaputt pitot. I told him prepare to be humbled. ...he's always been a bit of a wind bag topper, so I reckon he'll still lie.

 

[jlh3rd]

Plastic Bags have ruined many engines.

...I'd like to see the stats on that....my guess, like yours, is that it is no where close to other "ruined engine" causes....I've yet to see one plastic bag in the 5+ years I've operated my boat on the river and lakes I've been on. Plenty of logs,....

I finally got tired of unplugging my stock speedo and fitted my pontoon with a gps speedo. It matches my phone speed tracker. The stock speedo was accurate also, just a nuisance.....

 

[tpenfield]

I always use the GPS for speed, don’t even hook up the pitot speedo.

 

[nola mike]

I always use the GPS for speed, don’t even hook up the pitot speedo.

My pitot got ripped out when the boat swamped. Was going to replace it until I realized that gps was more accurate and I could fill my hull penetrations from the pitot.