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Nancy and Bud



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PostPosted: Sat Oct 29, 2016 12:37 pm    Post subject: To what does this tube connect in my lower unit and why is Reply with quote

there not one on each motor????


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Sea Wolf



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PostPosted: Sat Oct 29, 2016 1:06 pm    Post subject: Reply with quote

That's probably a pressure tube to connect to a speedometer (if you have one). There's an opening on the front of the lower unit's leg to let the pressure of passing water get into the far end of the tube. Blow into the tube's upper end (shown) to find out where the opening below is. To explain:

Some simple type speedometers for boats use what is called a Pitot Tube to measure speed. The Pitot Tube extends out into the passing fluid, and takes a measure of the pressure created by the fluid on the front of the tube. The faster the travel the greater the pressure, and a pressure gauge can then be calibrated into speed, e.g., mph. This is also used in airplanes.

Paddle wheels can also be used to measure speed in a fluid (air or water), but speeds beyond a certain point make the use of a paddle wheel inaccurate.

Better yet, use a GPS to measure true speed, irrespective of current flow of the fluid, to get true speed over ground.

"there not one on each motor????" Probably not on counter rotating twins, as only one speedometer would be used/available.

Pitot Tube

Joe. Teeth Thumbs Up

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dotnmarty



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PostPosted: Sat Oct 29, 2016 1:49 pm    Post subject: Reply with quote

Here's a previous discussion
http://www.c-brats.com/viewtopic.php?t=18139

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Nancy and Bud



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PostPosted: Sat Oct 29, 2016 3:20 pm    Post subject: Reply with quote

Thanks! Laughing
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journey on



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PostPosted: Sat Oct 29, 2016 5:52 pm    Post subject: Reply with quote

The counter-rotating single motor on Journey On has the speedometer tube. However, it's run up to the dash, but there's no speedometer.

Boris
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chimoii



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PostPosted: Sun Oct 30, 2016 12:00 am    Post subject: Reply with quote

Sea Wolf wrote:
That's probably a pressure tube to connect to a speedometer (if you have one). There's an opening on the front of the lower unit's leg to let the pressure of passing water get into the far end of the tube. Blow into the tube's upper end (shown) to find out where the opening below is. To explain:

Some simple type speedometers for boats use what is called a Pitot Tube to measure speed. The Pitot Tube extends out into the passing fluid, and takes a measure of the pressure created by the fluid on the front of the tube. The faster the travel the greater the pressure, and a pressure gauge can then be calibrated into speed, e.g., mph. This is also used in airplanes.

Paddle wheels can also be used to measure speed in a fluid (air or water), but speeds beyond a certain point make the use of a paddle wheel inaccurate.

Better yet, use a GPS to measure true speed, irrespective of current flow of the fluid, to get true speed over ground.

"there not one on each motor????" Probably not on counter rotating twins, as only one speedometer would be used/available.

Pitot Tube

Joe. Teeth Thumbs Up


That's absolutely correct but let me add a couple of comments. A couple of years ago Boating Magazine did a comparison test of the pitot measurement on a leg vs a paddle wheel. The pitot won hands down for accuracy.

In my working life we looked at an air pitot (think commercial aircraft) vs gps. We weren't in C-dory class but looking at high speed craft up to 170 mph. The accuracy difference for SOG was very close. For a water pitot we had to allow for current.

So what does this mean for a C-Dory? For SOG your gps is a great tool. For speed through the water, go to you pitot based readout off your motor, not your paddle wheel. If you are concerned about having enough fuel to reach destination think about SOG and gph not the mpg shown on your motor readout. That shows mpg through the water.

The difference between SOG and speed through water is the current or headwind helping or against you. If you are doing 8 knots through the water against an 8 knot current you are going nowhere, just burning fuel. Ask a sailboat about this stuff.

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Sunbeam



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PostPosted: Sun Oct 30, 2016 9:00 am    Post subject: Reply with quote

In a much less interesting and detailed way than the last post (thanks for that - good read), I'll say that I also like to always have speed-through-the-water in addition to speed over ground. The two in combination can be very useful. So I wouldn't give up the speed through the water and only use GPS. I'd keep (or get) them both.
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hardee



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PostPosted: Sun Oct 30, 2016 1:16 pm    Post subject: Reply with quote

For those of us that travel on salt (tidal current) waters, having both SOG and water speeds can be very useful: figuring an ETA, Determining current speed and direction, confirming current and tidal chart correlation and even projecting fuel consumption. For that reason it is a good idea to be sure and calibrate your water speed indicator by whatever method the manufacturer recommends. Running on a lake or crosswise at the end of a bay at slack tide are good chances to compare the WS reading with the SOG reading from the GPS.

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BrentB



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PostPosted: Sun Oct 30, 2016 1:19 pm    Post subject: Reply with quote

M2cw speedometer tube
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Kushtaka



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PostPosted: Mon Oct 31, 2016 12:23 pm    Post subject: Reply with quote

I have the same tube and have always wanted to connect it but have not done so yet. While my GPS does give good SOG, I like to know my speed through water while trolling cos you catch more fish that way!

The currents around these parts get fairly rippin'. I've produced trolling while moving about 1.5kt into the current (I usually like to troll with the current, but on this day I was making passes through a canyon) but going backwards over ground.

It's just fairly awesome to have both.

Does anyone know what gear it takes to get the speedo up and running? I'm guessing more tubing and a speedo?
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Sea Wolf



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PostPosted: Mon Oct 31, 2016 5:16 pm    Post subject: Reply with quote

Kushtaka wrote:
(Some deletions)

Does anyone know what gear it takes to get the speedo up and running? I'm guessing more tubing and a speedo?


My experience with this type of speedometer is that it's not really accurate at displacement speeds (under 10 mph), although that experience was gained when I was a teenager (1955-1960) (!)

At speeds under 10 or so mph, a paddle wheel is much more accurate, and if you're wanting to measure trolling speeds under 2 mph, such as in freshwater lakes, there are special trolling speedometers that have weighted lever arms that read out on a scale as the boat passes through the water.

Take a look here: Boat Speedometers (discusses troubleshooting and incidentally includes how they work)!


And Here: Trolling Speedometer

Have fun!

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Kushtaka



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PostPosted: Mon Oct 31, 2016 6:23 pm    Post subject: Reply with quote

I guess that's what I was asking. I should really phrase things more carefully.

I'm looking for a digital speedo. Most of the speedos available go to 60mph and start at 5mph. so no good. And I agree that the physics of a pitot tube speedo make it more accurate at high speeds, as doubling velocity squares pressure.

But yamaha makes digi speedometers for their outboards. Maybe there is something for honda?
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rogerbum



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PostPosted: Mon Oct 31, 2016 8:12 pm    Post subject: Reply with quote

Kushtaka wrote:
I have the same tube and have always wanted to connect it but have not done so yet. While my GPS does give good SOG, I like to know my speed through water while trolling cos you catch more fish that way!

The currents around these parts get fairly rippin'. I've produced trolling while moving about 1.5kt into the current (I usually like to troll with the current, but on this day I was making passes through a canyon) but going backwards over ground.

It's just fairly awesome to have both.

Does anyone know what gear it takes to get the speedo up and running? I'm guessing more tubing and a speedo?


If you want speed through the water, the best way to get that is to use a gauge you already own - the tachometer. At a fixed RPM, you'll get the same speed through the water regardless of current. You just need to "calibrate" the tach (e.g. learn what RPM correlates with what speed over water). You can do this a couple of ways. Go someplace or measure sometime where/when there is no current. Then SOG = speed through the water. OR - measure your speed over ground with the GPS, turn 180 degrees (at fixed RPM) and do it again. In one direction, you'll get speed over ground + the current. In the other direction, you'll be speed over ground - the current. Average the two (sum/2) and you get speed over ground without the current = speed through the water. Make a little chart of a few measurements and keep it by the helm (or safely tucked away in your brain).

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Wigeon



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PostPosted: Tue Nov 01, 2016 3:01 am    Post subject: Reply with quote


What a luxury it is to constantly have ground speed from your GPS & if you know your waterspeed or airspeed, you can instantly know if you have a tail current, headwind etc. On the water or in the air, when practible, you can change course to get out of a tide rip or get back in the jetstream to pick up that 200 knots you had on the tail, but estimated ground speed is the number you'll need for calculating your estimated time enroute or estimated time of arrival & of course tide/current tables or winds aloft are carefully checked. And if we know our ETE & fuel burn rate, fuel estimates can be made. So yes, waterspeed/airspeed is important, but groundspeed checked often will be your best friend.

Larry
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rogerbum



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PostPosted: Tue Nov 01, 2016 1:06 pm    Post subject: Reply with quote

Wigeon wrote:

What a luxury it is to constantly have ground speed from your GPS & if you know your waterspeed or airspeed, you can instantly know if you have a tail current, headwind etc. On the water or in the air, when practible, you can change course to get out of a tide rip or get back in the jetstream to pick up that 200 knots you had on the tail, but estimated ground speed is the number you'll need for calculating your estimated time enroute or estimated time of arrival & of course tide/current tables or winds aloft are carefully checked. And if we know our ETE & fuel burn rate, fuel estimates can be made. So yes, waterspeed/airspeed is important, but groundspeed checked often will be your best friend.

Larry

Unless you're a fisherman. Speed through the water is what's most important since that's what affects the action of the lure/flasher. Once one has caught their limit, then ground speed matters on the way home.
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