# Trolling Speed GPS vs. Engine Gauges



## Reel Rowdy (Oct 4, 2007)

Which is more accurate? I actually have 3 speed read outs on my boat, the Honda Gauge, Raymarine C120 speed and speed over ground displays. None ever read the same and usually differ by as much as 5-7 mph. 

Can someone explain the difference and which is better to use?


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## Mike W (Jul 11, 2011)

I'm pretty sure your C120 is most accurate. I mean GPS calculated by multiple satellites down to within a foot or so vs. the engine gauge which replies on water pressure in a tube entering the lower unit...the C120 is the easy winner haha

Although the speed readout is there on our Yamahaa Command Link gauges (and they're always within 1 mph of the GPS speed...for me at least), I never look at the speed on any engine gauge. I only use the GPS speed.


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## MSViking (Oct 11, 2007)

They are all three a little different and not are perfect. The gps can be tricked if you are wobbling as opposed to a straight line as it wants to take a distance / time measurement and the distance can be skewed if your course is not exactly straight, but overall its very accurate.

Engine speeds are determined by a small pitot (sp) tube that is facing forward. It measure water pressure caused by the water pressure of the boat moving forward, current and seas can skew that as well.

Other common devices use a small paddle wheel that is encased in a housing under the boat but it can be skewed just as the pitot tube.

I use them all for trolling but ultimately rely on how my lures look and are performing.

Robert


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## RMS (Dec 9, 2008)

Due to currents and wind/wave action, there will always be some difference between speed relative to the water's surface and the GPS speed over ground.

A good obvious example is trolling in or around a pass during large tidal movements.


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## Reel Rowdy (Oct 4, 2007)

Thanks for the input guys. RMS you hit the nail on the head! GPS speed over ground is what has me confused. I understand the engine speed reading comes from the pitot tube which can become clogged and not be accurate at all. The C120 "speed" reading comes from the paddle wheel on the airmar transducer. RMS I'm thinking this is the correct speed relative to the waters surface and the GPS "speed over ground" is taking into account wind, waves, and variation in course like Robert said, correct? 

Robert, I'm like you, it all boils down to how they look and run but I was always confused over the speed readings. Now I know a little more! Thanks


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## GWally (May 16, 2009)

Driving in water is like flying, ground speed vs true airspeed. If there is zero air movement (head or tailwind) the ground speed and air speed will be equal. If you are in an area with a current, say part of the gulf with 3 knots and you are traveling with the current and the GPS says you are going 8 knots (straight line assumed) your water speed (what the lures see) is actually 5 knots. If your paddle wheel or pitot senser are accurate, they should show 3 knots slower than the gps or 5 kts. If you turn around and head the other way, the pitot/paddle wheel sensors will still indicate 5 knots, but the gps should indicate 2 knots. 
I have yet to get out, project boat in the finishing stages, but looking at charts, it looks like if you are running east or west in say the nipple area, you are going to be in a current and GPS readings are going to vary dependant on heading.
It has been way too long since I have run a boat offshore (looking forward to it again, GPS is so much better than wet compass and a stop watch ), but what we did "back then", not having any speed indicators, would be to go by engine rpm once we were happy with how whatever we were dragging was running. A given rpm will give you consistent "water speed".

This is sort of timely for me. I finally decided to go with my shoot through 260 transducer for my primary fishfinder and was going to just use my Raymarine for GPS (though I like the idea of my iPad for a second readout and control,Raycontrol app works great). Now, as much as I hate to hang something off the stern, I may install my second transducer so I will have a paddle wheel sensor and backup fishfinder. I had been planning on using the GPS speed readout, but hadn't considered the current effect.

Thanks for bringing this up.

Sorry for being so longwinded. Hopefully something in all that is useful.


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## Reel Rowdy (Oct 4, 2007)

Thanks for the great explanation Gwally!


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## RMS (Dec 9, 2008)

Your good to go now, Reel Rowdy.


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## jim t (Sep 30, 2007)

Trolling lures are also subject to tide and the direction you are traveling. In the bay, trolling Stretch 30's WITH the current at 4 to 5 knots of ground speed on your GPS will make for nice action on the lures on light trolling gear. But 4 knots groundspeed AGAINST the currents will have line coming off the reels at the same drag setting if the tide is running fast. Into the current you may want a gps groundspeed of 1-2 knots to keep the drags from slipping on your lighter gear.

It really is more about how your trolled baits and lures are running.

Jim


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