The big Speedometer

[Crikey]

So many posts dealing with sail trim, ballasting, methodology etcetera - etcetera - etcetera ..... all to squeeze that extra half knot or so (particularly while racing another synaptically challenged skipper!).
I would like to work towards the big digital cockpit readout to help out my old career burnt eyes (retired), and be able to tell at a glance what is helping, and what is not. Problem is there are three methods that I can see that produce this data - stern paddle wheel, gps and sonar transducer.
Should I have all three for some kind of joint switchable coverage? Or does one kind give a more immediate, and consistent picture of what's going on than the others?

Challenged

[pokerrick1]

Excellent question and probably idea

Rick

[Crikey]

I left out pitot tube!

[Trouts Dream]

If its for relative speed on the same heading
I would guess that GPS would be sufficient. Maybe I misunderstood but it seemed that you were looking at what changes resulted in increase or decrease in speed.

[mastreb]

I don't know of a fantastic way to get accurate real-time speed information about a boat.

For determining relative difference in the boat's speed over ground, GPS is a little too delayed to notice immediately what's going on. GPS calculates your speed after you've moved a multiple of the distance that is its maximum accuracy (10'), so you've got to go 50' or so before you have a reading about where you were. It's continuous, but what you're looking at is the speed a few moments ago, not the current speed. Besides the delay, it's the most accurate and lease environmentally affected way to obtain speed.

The problem with a paddle wheel is that it includes the effects of current and wakes, which if you're on the same heading is usually fine but can vary pretty dramatically if you turn. They also foul easily. You'll typically read at least 1 knot difference on a paddle wheel when going the opposite directions in San Diego bay. Since we're normally talking about "that last knot", this kind of inaccuracy is too high in my opinion.

Transducer speed is more accurate than paddle wheel but is also measuring speed through the water, not speed over ground, and so is affected by currents.

Pitot give you wind speed, not boat speed. By the time you were done doing the math, it would have changed.

In theory you could use the accelerometer in your iPhone or Android device with an accelerometer app to compute your current speed based on the phone's own motion, but you have to start from a dead stop (like at the dock) and run it continuously and never move the phone from a fixed mount. If the app is interrupted the zero baseline is lost.

Perhaps you could buy something like a G-Tech automotive accelerometer and use that? They're used for drag racing, but they can keep their calibration and they are a dedicated device. I've got one, perhaps I should test it on the boat. You program it with your weight, and it calculates accumulated forces to determine speed. They're astonishingly accurate on a motorcycle, but I've not used one for hours on end.

Probably your best bet is to use the GPS, and wait for the speed to "settle" after making an adjustment. It's certainly cheaper than buying anything and not any less accurate.

[Crikey]

Good stuff there!

I should study more.... I thought the depth transducer iterated the speed over ground by reading the bottom. My thought was monitoring the immediate affect of sail/trim changes, so by that reckoning the wheel or through-hull should provide the most noticeable timely evidence of change. I haven't settled on a final choice of depth system yet so I'm tempted to go with both as the paddles are nmea available, and cheap. I already have the gps and that can probably log a continuous curve somehow regardless of the delay using some type of pc app. I suspect the inclusion of the forthcoming Galileo gps will up the accuracy, and minimize the delay factor appreciably (?).
For the purpose of real time changes monitoring I don't suppose subtraction or addition of current is too important in the scheme of things and occasionally cleaning a paddle wheel can't be too difficult. We clean our baby's bottoms, don't we?
I thought pitot's were made for water as well (?) - but they are probably analog only, like the airborne versions.
Ideally, having control over adding everything together would be the best of all worlds.
Your accelerometer information is intriguing and I would like to hear more of your experience with it when you have it.

Ross

[vizwhiz]

This is interesting!

I have a paddle wheel on my boat. I know it works, because I can turn it on and spin the wheel, and something shows up on the screen. How accurate it is has yet to be determined, but at least I know it works.

And I would tend to agree with you that the paddlewheel or something "instant" is going to be the best way to tell that "pulling that rope dropped my speed"... You would need to check the speed just before and just after the change you make (and only change one thing at a time!) to see the impact, but over the course of the 30-60 seconds you're testing something new, your environment shouldn't impact the speed that much. Also, when I was looking through the manual for my old paddlewheel, I believe it said you could change the sampling rate, meaning you can adjust how quickly the display updates with new information from the paddlewheel...so you can tune it to give you a more instantaneous readout, which you wouldn't want during "regular" sailing.

The GPS would definitely be the better way of determining whether you are being consistent with your sailing...good sail trim for 20 minutes, then you messed something up, you were wandering and not maintaining a steady course, etc.

And don't forget, the admiral can ALWAYS tell when you've messed something up!


By the way, yes, pitot tubes can be used for water (for any fluid really), but they and the readout have to be calibrated because the specific gravity of the fluid makes a difference in the accuracy (and they have little holes that can get plugged up). There are also other methods of measuring speed, by using devices designed for flow measurement (check out Dwyer Instruments' website) such as thermal dispersion, etc. However, they are designed to measure in feet-per-second or something like that, which you'd have to convert. I still think the paddlewheel is the easiest choice for instant gratification...

[bscott]

Trim the tell tails, sails look beautiful, and we all know that looking beautiful makes us feel good, and when we feel good we are fast--no need for a speedo

Bob

[Divecoz]

X 2 ......

Trim the tell tails, sails look beautiful, and we all know that looking beautiful makes us feel good, and when we feel good we are fast--no need for a speedo

Bob

[Ixneigh]

I use a gps. It's good enough to correct my estimations on speed that are always about a knot off.
If I need to slow down I just turn it on and my speed magically drops from five to four.

Ixneigh

[snotnosetommy]

Knotstick works well, too. http://www.knotstick.com/ (Speed log) Hangs off the stern quarter, easy to read, pull it in when done. Instantly shows change in speed.
Yes, it too would be affected by current, but as long as you're on the same heading moment to moment, the log will still instantly show changes in speed.
Speed over ground, speed over water...... what we want to measure (for this narrow purpose) is change in speed relative to what it was a few seconds ago.

[Crikey]

Trim the tell tails, sails look beautiful, and we all know that looking beautiful makes us feel good, and when we feel good we are fast--no need for a speedo

Bob

Amen brother! Except the speedo could be very necessary when passing a boat with a load of 'Peaches' on board.
http://www.google.ca/imgres?imgurl=http ... &dur=16049
For that, my desire for an accurate speedometer might come in handy.
I sail with my tell tails every time I go out and they are the only way to see if the sail motor is near optimum. What they can't do is show the difference between optimum-fast and optimum-faster. I suppose it is all hypothetical really and just another figment of my 'can't leave it alone' mentality, but sometimes boy's must have their toys.
Seriously, seat of the pants is how we operate most of the time whether boat or car or ski's, but with so many variables happening on the water I think using these things as a training aid, is not too far out in left field.
I think the paddle is the best low speed immediate solution talked about so far, and for logging average over time there is gps, which we all have. It would be helpful to my tech smitten brain to be able to glance at a flowing speed line, or graph, instead of perusing a table of log sampling's but I'm still looking for that.

This must be a curse I've inherited from a childhood of staying up past midnight over many years, watching the Americas Cup races.

Ross

[mastreb]

I thought the depth transducer iterated the speed over ground by reading the bottom.

It's much simpler than that. Speed transducers work by having two identical transducers next to one another, fore-to-aft (actually it's a single sender and two listeners). The sonic "signature" (just a simple waveform) returned by the forward one will match the signal returning from the aft one after some delay. The length of that delay corresponds exactly to the speed at which reflections in the water from bubbles, particles, etc. pass the two receivers.

It's measuring only within a few inches of the hull, and doesn't reach the bottom. They're much more precise than paddle wheels (within 0.1 knot) and require no calibration, nor will anything jam them. Unfortunately, they're measuring the boat's speed through water, not its movement over ground.

It's almost certain that a speed transducer is the most accurate and immediate way to get feedback about sail changes. While it is affected by current, there's not a better existing mechanism I'm aware of.

[Crikey]

Matt, Whoaa! Are you meaning a speed transducer as different from a depth transducer? I thought one of the functions of the latter was the ability to provide the former, as well? Don't most Depth transducers compute the depth data, over time, and from that come up with a speed result - or not....

Totally fu@&*d up now!

[mastreb]

Matt, Whoaa! Are you meaning a speed transducer as different from a depth transducer? I thought one of the functions of the latter was the ability to provide the former, as well? Don't most Depth transducers compute the depth data, over time, and from that come up with a speed result - or not....

Totally fu@&*d up now!

Woops, sorry to cause confusion. A true speed transducer is indeed different from a depth transducer.

That said, many head units with depth transducers also provide speed, but the speed calculation is done in the GPS head unit rather than in the sensor itself and is delayed even more than GPS speed, and in these cases the transducer must be compatible with the head unit. The depth transducer isn't really doing anything related to speed sensing, the GPS head unit is calculating it by comparing old depth data to new. It's delayed, low accuracy, and can become wildly inaccurate if the bottom picture is cluttered.

Actual speed transducers (also called ultrasonic speed sensors) give you instantaneous speed within milliseconds and are much higher accuracy than speed data calculated from depth traces by the head unit.