Pitot tube knot meter

[chasmains]

I’m am always interested in scientific measurements. Lately, I have become engrossed in finding my boat true speed through water. So their maximum speeds are likely 15 knots or less most of the time. These seem to me to be especially useful for a racer to make sail adjustments “on the run” to gain speed. They eliminate set and drift from the true speed through water. So, there are pitot tubes and kits out there. But there are no gauges that have a scale in the range of the boats I sail on, specifically, monohull cruisers. The lowest ranges I’ve seen on line are 5-35 mph. Does anyone know of a pitot tube knotmeter that will accurately indicate 0-15 mph? (And 0-5 knots should also be reasonably accurate, I think.)

[jt11791]

Paddle wheels work better at low speeds, that's why you don't see pitots used on sailboats. Why not use a paddle wheel?

[chasmains]

Thanks, JT. I will consider paddlewheel sensors. I have experience with air velocity sensing using pitot tubes. My primary need is a gauge that reads in the low range. I don’t see any at all. You would think there would be some.

[Calif.Ted]

I've had pitot tube speedos on powerboats and they do not register below 5 mph, some fishfinders offer a transom mount paddlewheel speed transducer that still only registers to 2 mph. And it should be noted are in mph not knots.
https://www.westmarine.com/buy/hummi...67?recordNum=4

[Calif.Ted]

Just remembered a device that may suit your needs, a knotstick, basically a spring scale with a towed "chip" like in a chip log. You could probably cobble your own from a fish scale if you could figure a way to calibrate it.
Knotstick Sailboat Speedometers

[pcmm]

Originally Posted by chasmains

Thanks, JT. I will consider paddlewheel sensors. I have experience with air velocity sensing using pitot tubes. My primary need is a gauge that reads in the low range. I don’t see any at all. You would think there would be some.

I think you larger issue will be keeping growth out of the pitot tube. Water is different than air. stuff loves to grow in it.

[Scaramanga F25]

Knotmeters that give you almost instant 1/10 knot variants are available.

[chasmains]

I think a paddlewheel sensor is likely to clog very easily to yield false speed or no speed. Pitot tubes maybe not so much, since tubes pointing into the water stream reach a static pressure point. This would be possible even with bio growth in the tube because the growth cannot resist pressure changes. A pitot tube does not have flow, only changes in internal pressure due to water momentum into the tip of the tube.
With respect to Scaramonga’s note, GPS instruments can give 1/10 knot readouts but GPS gives only speed over land, not speed thru water.
I checked out the Knotstick. A very cool and clever instrument. It may require the user to deploy it occasionally, and not permanently installed.

But my real question is: What gauges are available? I’m willing to take either paddlewheel sensors or pitot tube sensors. The sensors are easily available. The gauges I’ve seen have ranges that are too high for cruiser sailboats.

[HeywoodJ]

I suspect there are a number of reasons, some mentioned above, why pitot tubes are not used on slow speed boats. Which means you’re probably in the “build it yourself” world.

Engineering Toolbox has a convenient calculator for dynamic pressure, although you will have to adjust it for the density of salt water, which you can do here. End result, 6 knots ~ 3m/s ~ 500mm dynamic pressure. To be really accurate you will also need to measure salinity and water temperature to calculate fluid density, but for relative measurements (did that trimming just increase or decrease speed?) you could largely ignore those- although in my tidal waters we sail in and out of pools of varying density, both temperature and salinity, all the time.

So you need a square root extracting gauge in the 500-1000mm range. Dwyer, as an example, makes an IP 66 rated flow display that does the square root extraction. It will all cost you the same as a boat instrument, but if you want to go down that road it should be quite doable using industrial instrumentation and would be capable of displaying in knots (with a freely scalable instrument). The Dwyer units are 24V, but you could likely find a wide voltage input unit in someone’s catalog that would take 12V directly.

[team karst]

Airmar tri sensor, using ultrasonics. No moving parts, no tubes to grow animals inside.

[mvmojo]

I delivered a 72' Deerfoot yacht many years ago from Trinidad to Ft. Lauderdale. It had some sort of ultrasound speed sensor that, if memory serves, had two thru-the-hull ultrasound transducers mounted fore and aft several inches apart. The electronics would "read" the flow of particles and air bubbles passing beneath the transducers and compute the boat speed. It was extremely fast-acting and I think it may have even had 1/100th of a knot resolution. It's been a long time ago but you may want to try searching on ultrasound knot meters...

[boat driver]

Originally Posted by team karst

Airmar tri sensor, using ultrasonics. No moving parts, no tubes to grow animals inside.

Caution on the joy of the Airmar sensor- depending on the install, if shooting through the hull/the material must be pure.
On the outside, the hull must be clean. Growth and rough surfaces will alter the readings.

BTW, unless really drifting or reason for the accuracy, paddlewheels are probably the most efficient bang for buck. If you get below 1 knot, then most efficient will be dropping a stick in the water and measuring how fast it passes the boat.

[JBP]

Other options include a taffrail log and I seem to remember a tube you could stick in the water to measure speed but I can't find it.
https://harborshoppers.com/taffrail-log/

[Nord Sal]

I measure flow in rivers for my day job and have used acoustic doppler current profiling devices. These are basically a high tech tri-axial sonar and is probably what mvmojo used on the yacht delivery they mentioned. The devices I used was mounted in a tiny boat which we'd tow across the streams on a tether to collect data.

People are using these instruments in permanent fixed installations to measure flow in flumes and canals. Here's an example of one: https://in-situ.com/us/doppler-ultra...elocity-sensor
Something similar could be used to measure boat speed and perhaps someone is marketing one geared towards this application.

I suspect the pitot tubes marketed for boats referred to don't work well at lower speeds because they aren't equipped with a precise enough pressure transducer. The ones I use are calibrated to 1/100th foot of (fresh) H2O, and accurate to ±0.1% full scale (0.00001 ft H2O). That stagnation pressure corresponds to a velocity precision of ±V = sqrt(2gH) or ±0.025 ft/sec (±0.015 kts).

One problem I see with a pitot tube is the pressure changes as the boat is heeled or loaded since the transducer will change position in the water column. You'd need to be able to account for this to be accurate in your measurements. The doppler devices wouldn't have this problem.

[waterman46]

If you like electronics, a fun project is to design and build a pulse counter for distance log and pulse rate indicator for speed with paddlewheel as input. Done that, a long time back.