Accurate speedometer for trolling with down riggers.

[The Dread Pirate Fisherman]

So I'm in a canoe with a 55lb thrust trolling motor. I set it to the lowest forward setting (1) and use a GPS speedometer app on my phone that quite frankly was a waste of time. I take a look at my deeper sonar readout and it says I'm doing 6mph. I want to be going 1.5 to 3 mph. That being said can anyone recommend a solution to this issue? Preferably a inexpensive one that will accurately give me the correct speed I'm going in calm to windy conditions and be easily installed because my hull is plastic 

[Further North]

What is the difference between your "deeper sonar readout" and your GPS app...and where is the sonar reading coming from?

[casts_by_fly]

A gen 2 Humminbird helix 5 that is gps only (no down or side imaging) will be pretty cheap and should be plenty accurate.  A small battery pack and you’re in business. No transducer needed.  You should be able to find one for under $200 and you’ll have lake mapping included. It will also have a transducer for sonar imaging if you want it in the future. 

[Tennessee Boy]

I would take a look at some different apps for your phone.  The GPS in any smartphone is accurate enough to give an accurate speed.

[The Dread Pirate Fisherman]

3 hours ago, Further North said:

What is the difference between your "deeper sonar readout" and your GPS app...and where is the sonar reading coming from?

Deeper sonar says 6mph. Phone app says 1mph, trolling motor set to 1  but when I put it up to 2 the speed on the phone app jumps to 4mph and the speed on the deeper sonar jumps to 10mph. Which is way off. Digital read out GPS speedometers seem like a solution but I'm hesitant because of reviews. Any good digital read out GPS speedometers that anyone knows of or has used that work? 

41 minutes ago, Tennessee Boy said:

I would take a look at some different apps for your phone.  The GPS in any smartphone is accurate enough to give an accurate speed.

So far all three GPS apps I've tried all did the same exact thing. Two were free the third was 5 dollars and all of them read 1mph. Could be that I need better control of the trolling motor using a PWM speed controller. 

[Tennessee Boy]

I use the Humminbird One-Boat Network app on my I-Pad to look at LakeMaster maps while I’m in my recliner.  I turned on the GPS on the I-Pad and walked around the house and the speed readout seemed pretty reasonable between 0 and 1.5 MPH and it changed as I changed the speed I was walking.  
 

What type of phone do you have?

[The Dread Pirate Fisherman]

1 minute ago, Tennessee Boy said:

I use the Humminbird One-Boat Network app on my I-Pad to look at LakeMaster maps while I’m in my recliner.  I turned on the GPS on the I-Pad and walked around the house and the speed readout seemed pretty reasonable between 0 and 1.5 MPH and it changed as I changed the speed I was walking.  
 

What type of phone do you have?

Iphone 

[Further North]

9 minutes ago, The Dread Pirate Fisherman said:

Deeper sonar says 6mph. Phone app says 1mph, trolling motor set to 1  but when I put it up to 2 the speed on the phone app jumps to 4mph and the speed on the deeper sonar jumps to 10mph. Which is way off. Digital read out GPS speedometers seem like a solution but I'm hesitant because of reviews. Any good digital read out GPS speedometers that anyone knows of or has used that work? 

I asked the question incorrectly:  Where are you getting the sonar reading from?

A fish finder? Or some other device you are using?

 

[The Dread Pirate Fisherman]

6 minutes ago, Further North said:

I asked the question incorrectly:  Where are you getting the sonar reading from?

A fish finder? Or some other device you are using?

 

Yes a fish finder but it is portable. Look up Deeper Sonar pro plus. I use an android tablet for the read out. 

[Further North]

56 minutes ago, The Dread Pirate Fisherman said:

Yes a fish finder but it is portable. Look up Deeper Sonar pro plus. I use an android tablet for the read out. 

Got it, sorry I didn't get it sooner, I've seen those.

If there's a GPS in there, it should be accurate.

...but trolling motor props are speed limited to around 4.5 - 5 MPH by the props they use, regardless of thrust or weight of the watercraft.

 

https://minnkota-help.johnsonoutdoors.com/hc/en-us/articles/4413536408343-Calculating-Speed-and-Determining-Required-Thrust-
 

Quote

 

Determining M.P.H.

Given motor R.P.M. under load, and Minn Kota’s 4" prop pitch, the approximate speed that a motor will push/pull a small boat can be calculated with the formula:
prop pitch in inches, times motor RPM, times 85%, (factor for prop slippage), yields calculated inches per minute. This value divided by 12 equals calculated feet per minute. The feet per minute times 60 equals calculated feet per hour. The feet per hour divided by 5280, (number of feet in a mile), equals the calculated miles per hour.
Example: ((4 x 1540 x .85) / 12) x (60 / 5280) = M.P.H.

 

 

...so I kinda doubt your trolling motor is pushing you to 6 MPH.

 

 

 

[Further North]

MK has shared that 1540 is the top end for their motors, so
 

[casts_by_fly]

Based on the 6 and 10 mph reading coming from the sonar unit, I would put zero faith in its ability to tell you speed.  You won't hit 6 mph on the highest speed setting with an electric motor as calculated above.

 

The phone reading at 1 mph on the lowest setting makes sense.  Getting up to 4 mph in a light canoe is more than doable, though I don't know if 'setting 2' is enough to get you there since we have no way to know what that means in real life.  An iphone GPS should be more than accurate enough to give you speed, so I'd use one of the apps you have now and do a speed test.  Pick two points on the lake that you can measure their distance.  If you have a rifle range finder then pick two points that are a couple hundred yards apart.  Use a stopwatch on your phone and make a run between the two points at a constant speed setting.  if you don't have a rangefinder, then use google maps and mark two distinct points.  Go as long of a distance as you can to minimize error. 

 

In practical terms, expect to get up to around 4-4.5 mph, maybe a touch more if you're loaded lightly.  Higher thrust motors (and I'd put 55 lb in that category for a light canoe) will get you there faster with a lower setting, but the top end will still be basically the same.

[Further North]

1 hour ago, casts_by_fly said:

Higher thrust motors (and I'd put 55 lb in that category for a light canoe) will get you there faster with a lower setting, but the top end will still be basically the same.

Yep.  Even if there was a way to rig a 36 volt, 112# thrust Minn Kota on a canoe, that's still where you'd wind up.