Water Speedometer Pressure

[leeschaumberg]

Hello
Back in the good ole days people could tell some one how much pressure (psi) diff equaled 10 -50 mph. I can't find this info today! I want to make a chart showing this. GPS works but if your on a river flowing at 6 mph it means nothing. Thank You

 

[slasmith1]

Re: Water Speedometer Pressure

gps will show correct sog (speed over ground) regardless of wind or current. it is the pitot style speed o's which are notoriously wrong.

 

[Silvertip]

Re: Water Speedometer Pressure

You don't need a chart. If you are going upstream against a 3 mph current and the speedo registers 30 MPH you are actually going 27 as the current is "adding" 3 mph to the reading. Conversely, if you are going downstream in a 3 mph current and speedo registers 30 MPH you are actually going 33 MPH as you have to "overcome" the current. In other words, if you anchored INTO a 3 mph current the speedo would read 3 MPH. If you are anchored AWAY from a 3 mph current the speedo would register zero.

 

[dingbat]

Re: Water Speedometer Pressure

If you're traveling up river at 6 mph into a 6 mph current, how long will it take you to travel 3 miles?

 

[fire7882]

Re: Water Speedometer Pressure

If you're traveling up river at 6 mph into a 6 mph current, how long will it take you to travel 3 miles?

8 laying down

 

[SeanT]

Re: Water Speedometer Pressure

My speedo doesn't even register anything under 10mph anyway...

 

[45Auto]

Re: Water Speedometer Pressure

7. If adjustable air pressure is available, standard calibration speedometers should react to the following pressures:
5 PSI = 20 MPH; 11 PSI = 30 MPH; 19 PSI = 40 MPH; and 30 PSI = 50 MPH.

From the Teleflex site:

http://www.downloadingfiles.co.uk/teleflexfiles/gauges/Pressure (Pitot) Type.pdf

If you're traveling up river at 6 mph into a 6 mph current, how long will it take you to travel 3 miles?

Got to specify whether the 6 MPH you're traveling at is water speed or ground speed ........

 

[leeschaumberg]

Re: Water Speedometer Pressure

[/FONT]
Hello
The many varied responses to this question prove that an acurate chart is needed. A river flow speed of 6 mph was given. Then going upstream and seeing 35 mph on the GPS your boat speed is 47 mph. 6 times 2 plus 35 mph! What happens when the river goes 8 mph? The GPS will register31 mph! River speed changes from time to time and some people would think thier boat is slow. More acurate than the pitot pressure system is the paddle wheel. But a good diferential presure gauge is OK. Even cheaper and most acurate is the water tube monometer. Thanks for the Teleflex site which is a good approximation of pressure. So if the river is going X mph and the GPS says 36 mph how fast is the boat? Thank You

 

[jdlough]

Re: Water Speedometer Pressure

lee,

Not quite. There's no need to double the river speed.

Think it through.

Suppose the river speed is 6mph. You're facing upstream, motor running, and GPS says you're going zero. Meaning, with respect to the river bank, you're not moving at all.

This must mean that you're moving thru the water at 6mph, but the water is moving downstream at 6mph, so you end up not moving (with respect to the river bank)

No need to double anything.

 

[Moody Blue]

Re: Water Speedometer Pressure

GPS works but if your on a river flowing at 6 mph it means nothing Thank You

What are you trying to measure? GPS gives you absolute speed relative to the ground completely independent of any other factors. Does not matter whether river is moving at 3mph or 20 mph. The speed of the boat relative to the ground (not the moving water) is what the GPS provides. 30 mph is 30 mph no matter how you look at it. There is no more accurate measurement of absolute speed. If you are trying to determine speed of the boat relative to the moving water I'm not sure how you can do that with any degree of accuracy. You would have to measure the speed of the water on a constant basis.

 

[45Auto]

Re: Water Speedometer Pressure

Not real complicated.

Going downstream, boat's absolute ground speed is boat water speed PLUS current speed.

Going upstream, boat's absolute ground speed is boat water speed MINUS current speed.

If the boat is doing 35 MPH in relation to the water, the pressure speedo will read 35 no matter whether it's going upstream or downstream. The river current will have no effect on the speedometer reading.

The GPS will give the true ground speed in either case. If the boat is heading DOWNstream with a 6 MPH current with the pressure speedo reading 35, the boat's true ground speed (what you'd see on a GPS) is 35 + 6 = 41 MPH. The boat is going 35 MPH over the water and the water is carrying it in the direction of travel at an additional 6 MPH.

If the boat is heading UPstream against a 6 MPH current with the pressure speedo reading 35, the boat's true ground speed (what you'd see on a GPS) is 35 - 6 = 29 MPH. The boat is still traveling 35 MPH over the water, but the water is carrying it in the opposite direction at 6 MPH.

 

[slasmith1]

Re: Water Speedometer Pressure

the problem with all this discussion is the fact that it doesn't account for wind. so just look at the gps and be done with it.

 

[Lion hunter]

Re: Water Speedometer Pressure

The hardest variable to fiqure out is the current speed. I don't know if I do it right but I figure that if I run upstream at 15mph and down at 20mph, my avearge speed is 17.5. And the river current is 2.5 mph. Is this correct? I don't have a speedo only gps.

Wouldn't you have to know the river speed 1st to use a chart?

 

[slasmith1]

Re: Water Speedometer Pressure

The hardest variable to fiqure out is the current speed. I don't know if I do it right but I figure that if I run upstream at 15mph and down at 20mph, my avearge speed is 17.5. And the river current is 2.5 mph. Is this correct? I don't have a speedo only gps.

Wouldn't you have to know the river speed 1st to use a chart?

yes and you also need to know how much drag effect the current has on your hull this will be different for every hull and even for different loads in the same hull.

 

[Fl_Richard]

Re: Water Speedometer Pressure

Or drift downstream and look at your GPS it will tell you the speed of the drift. Hense the speed of the current....

 

[dingbat]

Re: Water Speedometer Pressure

If you're SOW is really that important (dragging a lure) use a paddle wheel to compensate the GPS speed as I do while trolling into or against a current.

Or.......you could go across the current and don't worry about it.

 

[45Auto]

Re: Water Speedometer Pressure

yes and you also need to know how much drag effect the current has on your hull this will be different for every hull and even for different loads in the same hull.

You don't need to know anything about "drag effect" on your hull. It's the same no matter which direction you go. The boat speed is relative to the water. If it'll go 35 relative to the water downstream, it'll go 35 relative to the water upstream (although actual groundspeed will be different in each case). No difference in drag from the water. All the moving water does is provide a moving plane of reference to relate your speed to. You do realize that the Earth is rotating at about 20,000 MPH? Do you take that into account when you're calculating your boat's speed???

Maybe go look up the old "Airplane on a treadmill" thread. Some pretty funny theorys in that one about relative motion too!