Knot meter curiousity

[therapidone]

I have a hand-held GPS so having relatively accurate speed readings (over the ground, anyhow, and that's the only measure that truly measures progress) isn't a problem. However, my knot meter is so much out of kilter in its readings than the GPS as to make me wonder why pull the plug from the through hull & put the transponder in when we start every sail.

Yesterday, in very good winds for the Chesapeake in August (in fact, this has been an excellent summer for sailing on the Chesapeake...with our sailing days pretty much limited to weekends, we've had some outstanding sailing conditions all summer long) at 16 - 18 knots & our mainsail reefed at the first reefing point & at the time only about 1/2 the area of our 135% genoa unfurled, we were making 5.8 - 6.3 knots close-hulled (depending upon the size of the waves hitting us) according to the GPS, but according to the knot meter, we were between 7.2 & 8.2 (what a bunch of hooey--I can understand knot meter readings in excess of 6.8 knots if we are surfing, but w/ a 25' 4" LWL that is probably reduced a great deal when heeled over at 20+ degrees, readings of over 6.7 or 6.8 knots when close-hulled just don't make sense, do they? )! Yet, at other times--especially in lighter winds, the GPS and knot meter are surprisingly close.

What would I need to do to determine just how out of touch with reality the knot meter is?

How would I go about fixing it once I have determined the gap from the "fantasy world" of the knot meter and what would be a much more accurate measurement?

Regards,

Ed

 

[rssailor]

Calibration

Ed,
Can you calibrate your knot meter?? My lowrance unit has the ability to insert a correction factor to make the knot meter more accurately. Check out your manual and see what it says? Ryan

 

[therapidone]

Don't recall a manual...

I have all of the materials that I received from the PO on the boat about 50 miles away & while there are all sorts of things in the plastic pouch, I don't remember seeing any manual for the knot meter...in fact, I'm embarrassed to admit that as soon as I posted my question, I realized that a lot might depend on the manufacturer of the instrument...I don't even know that information off the top of my head...maybe I should have waited to post the question until I had more complete information in hand, huh?

I'll recheck the plastic pouch for any sort of info on the instruments--most specifically the knot meter.

Regards,

Ed

 

[Emerald]

Hi Ed,

Your knot meter may be fine. I know I've certainly had enough current and other water action to give the discrepencies you report.

I'd really try to pick a time when you knew there was no current, and go do some runs comparing the two in a controlled area. Just for easy reference, here's some of NOAA's wonderful free info for tides on our area of the world:

http://tidesandcurrents.noaa.gov/ofs/cbofs/wl_upper.html


-David
Independence 31
Emerald

 

[Captron]

Knotmeter

Keep in mind that you're measuring two different things. The GPS, as you state, measures your geographic speed ... speed over the bottom. The knotmeter measures the flow of water past your hull.

The difference in readings is often current. Sailing into a current for example will result in higher knotmeter readings and lower GPS readings. Sailing 'downstream' will give the opposite.

The location of the knotmeter's transducer in the hull can also cause small differences from one tack to the other, depending on the water flow over the paddlewheel.

To calibrate the knotmeter, you use a chip log, but you also need midshipman to heave the log, a marine to turn the minute glass and a bosun to retreive the chip ....

 

[therapidone]

Even with the current?

Greetings Capt. Ron,

Even if I am faced with a current coming directly on my nose, isn't my boat speed on the water's surface limited to the theoretical hull speed of the boat or relatively close to it? I mean, all that stuff about the crest of the wave "trying to get beyond the aft end of the boat & causing the trough & extra drag" thereby limiting the craft, via the laws of physics, from going faster than whatever the lwl limitations are...that stuff doesn't change just because the current is coming towards me, does it (I fully understand the surfing concept allowing me to go faster than that theoretical hull speed)? I'm still going to be limited how fast the boat can move through the water...and having readings in excess of 8 knots while sailing close-hulled w/ one wave after another smacking at a small angle to the right side of the bow shouldn't be anywhere near reality, should it? So the way I figure it, if I were moving at anywhere from 6.6 to 6.8 knots (theoretical hull speed is 6.75 knots) close-hulled & the current is 1.6 knots, I'm really only going 5 to 5.2 knots...and my knot meter shouldn't be reading any faster than the 6.6 to 6.8 because of that theoretical hull speed limitation.

Or am I just too thick to understand all of this stuff?

In the meantime, I think I should go looking for some shapely creatures that can act as a midship-person and a bosun!

Regards,

Ed

 

[G Kiba]

Knaughty Knots

I believe that most knot meters utilize a wheel that spins as water passes by it (a paddle wheel). As your boat travels, the passing water rotates the wheel. If you are traveling against the current the speed of the current additionally rotates the wheel. As a simple test, sail with the current and see what happens. I sail on the Sacramento and San Joaquin river and seeing a difference of 2 knots on my GPS and knot meter in not uncommon.

Grant Kiba
E27

 

[Seth]

Hang on everyone

I am going to suggest that current has no effect on the knot meter, but will obviously affect GPS groundspeed. If you toss a leaf into a body of water with current, it is floating on the surface, not "moving", yet moving over the ground at the speed and direction of the current. A boat should be the same.

The knotmeter senses the speed of the boat through the water. Unless the boat is anchored or docked, the boat (assuming no forward or aft motion by engine or sail) will move WITH the current, with a net speed of ZERO through the water, but it WILL of course have a GPS speed equal to the speed and direction of the current. If you anchor bow into a 2 knots current, you WOULD see 2 knots on the meter, and GPS of zero.

When the boat is underway, going 6 knots into the 2 knot current, the knot meter should show 6 knots (if calibrated of course), but the GPS should show 4 knots. When it is going 6 WITH the 2 knots current, the knotmeter should still show 6, but the GPS will show 8..

class? anyone? Buehler??

S

 

[therapidone]

I'm partially w/ Seth on this

From the standpoint of going against the current, I agree whole-heartedly with Seth...I'm still working on the part about going with the current!

And, from the standpoint of going against the current, I still think that my boat can't go any faster "through the water" than that theoretical hull speed or something close to it or whatever it really is based upon whatever my LWL really is when heeled over...so if Spirit can't go faster than 6.75 kts (let's assume the LWL when heeled over IS the same as the LWL given in the specs) through the water, then if the speed of the current is 2kts, I can only make 4.75 kts actual speed over the ground...and the paddle wheel on my knot meter is showing a speed of 6.75 knots...the 2kts of current that I'm fighting and the 4.75 knots of over the ground speed & then I'm limited by the wave and the trough stuff affecting the theoretical hull speed of displacement water craft...no???

Regards,

Ed

 

[tenders]

I agree with you that the accuracy and portability available with today's GPS systems has rendered the risk and hassle associated with installing and maintaining a separate speed instrument (and its associated hole in the hull) OBSOLETE.

Yes, speed through the water is different than speed over ground. Yes, knowing something about the current can be very important. But you can figure that information out by tacking once or twice through it.

The throughhull, the impeller barnacles, and the hassle of the electronics calibration and maintenance are just not worth the incremental benefit any more.

The depthsounder through hull can also be glassed over if your depthmeter will work with an oil bath set up in a short piece of PVC pipe epoxied to the hull. Another major source of risk, maintenance, and hassle.

tenders

 

[CaptnNero]

theory of theory

I agree with you that the accuracy and portability available with today's GPS systems has rendered the risk and hassle associated with installing and maintaining a separate speed instrument (and its associated hole in the hull) OBSOLETE.
...

The depthsounder through hull can also be glassed over if your depthmeter will work with an oil bath set up in a short piece of PVC pipe epoxied to the hull. Another major source of risk, maintenance, and hassle.

tenders

Tenders, I've been thinking along these lines myself. In fact I ordered a backup handheld GPS last week. I will leave it on in the cabin overnight for anchor watch. On the Chesapeake we use a Fortress which sets nicely but maybe isn't so great for windshifts around varying bottom condition.

We do still use the speed log for the trimmers. The GPS chartplotter is mounted at the helm with it's fancy smancy capabilities.

..I still think that my boat can't go any faster "through the water" than that theoretical hull speed or something close to it or whatever it really is based upon whatever my LWL really is when heeled over...

Ok Ed, back on topic. Let's not forget that little qualifier, "theoretical". You could get the most accurate GPS ever made, and get your speed log calibrated by a board of state and federal engineers complete with a certificate of authenticity to be dead-on-balls-accurate. Then in some conditions I'll bet a case of beer that you'll see some big discrepancies.

I don't remember the name of the originator of the theoretical hull speed formula, but it goes back at least 75 years. In reality there are more factors than just the LWL. I remember having done some careful calibration runs on my Pearson 27 ten years ago. Shortly after that we were in a squall on the West River and the knot log was reading a steady 8 knots on LWL of 22.5 on genoa alone !

On Kokomo I tend the helm most of the time and use the SOG on the GPS to extrapolate when the beer lamp will be lit. Galley_slave uses the speed log to optimize trim. Whether or not the speed log is accurate doesn't matter to her. She just needs relative comparison as she tweaks the sail controls.

 

[therapidone]

I respectfully disagree, sir!

Greetings Cap'nNero,

The only thing that I think is theoretical about the hull speed calculation is the LWL for the boat at the moment one is sailing. If the boat were level & loaded properly so that the specified LWL was actually the LWL & it were traveling as fast as it could in that relatively level position, the vessel (without surfing or other outside forces like being towed by a very powerful, very fast boat) cannot exceed that hull speed...at least not without a huge increase in energy expended (I don't see an extra 15 or 20 knots of wind meeting that definition) and the stern of the vessel practically dipping under water. The use of the word "theory" or "theoretical" in this context does not imply a "wag"...there are different meanings/defintions for the word "theory." And I do not think that a displacement vessel's speed through the water can vary much from the formula of 1.34 times the square root of the LWL (upon completing some additional reading, I've discovered that depending upon hull shape, described as fineness and chunkiness, the multiplier--when using "English" measurements or feet and inches--can vary from 1.18 to 1.42)...certainly not by as much as 2 - 3 kts for a vessels of 20 - 30 feet in LOA, given no planing or surfing is taking place. If one does the multiplication for 1.18 times and 1.42 times the sqr. rt. of 22.5', the results range from 5.60 kts to 6.74 kts (both rounded to 2 decimal places). I don't dispute what your knot log was providing by way of readings...I dispute the actual speed.


Now, this discussion has digressed from what it would take for me to determine if my knot meter is way out of whack (& what I could do about it if it is) to:

a) whether or not an accurate knot meter will read more than the physical limitations placed upon a displacement vessel simply because of variations in current,

b) whether a displacement vessel can go faster than 1.34 multiplied by the square root of the LWL whatever that "real" LWL is at the time the calculation is made...some boats have their sterns cut/designed a certain way to present a longer surface when heeled over, thereby allowing for a longer LWL when heeled...an Ericson 30+ does not present a longer distance from bow to stern when heeled over...so I'm of the opinion that the only thing that happens when I'm heeled over severely (i.e., more than 15 - 20 degrees) is that I've decreased my LWL...thereby limiting how fast I can go. This is why I never bother with burying the rails on Spirit as it means nothing insofar as how fast we're going...in fact, it would reduce how fast we could go...oh, and it scrambles everything that isn't nailed down down below!, and

c) why bother--what with GPS technology--and how I can make the depth sounder work in an oil bath in pvc pipe epoxied to the hull.

However, while researching the whole "theoretical hull speed" subject, I did come across a site in which the author argues that modern day sailboats' hulls aren't "true" displacement hulls (WHAT??? )...at least those with fin keels and the flat-bottomed center-boarders...take a look at the antepenultimate :nerd: paragraph on this page: http://www.cncphotoalbum.com/technical/hullspeed/hullspeed.htm

Also, how many years ago a calculation was determined (without allowing for other factors) should not determine whether it is accurate ...Einstein's "theory" of Relativity, which combines his general "theory" of Relativity or gravity with his special "theory" of relativity or that area of physics formulated by Einstein...were put forth more than 75 years ago...they are called theories...they are not in dispute for what they were applied to.

Regards,

Ed

 

[Jim Mobley]

Check out "The Nature of Boats"

Naval Architect Dave Gerr gives a complete treatment to the subject of hull speed in The Nature of Boats, International Marine/Ragged Mountain Press; 1 edition (September 1, 1995)

A tattered, dog-eared copy of which should live on every boat.

 

[windjunkee]

don't throw away the knotlog just yet. If you want to properly calibrate and use wind instruments, they need boat speed to properly calculate true wind speed and true wind angle.

When we got our new B&G's we went to a measured mile, clicked off the mile at a constant speed and ran the mile 4 times to try and eliminate current and drift in the calibration.

Jim McCone
Voice of Reason E-32-2 hull #134

 

[CaptnNero]

displacing special and general relativity

Yo Ed,

I certainly did not mean to imply that the mere age of a formula should be basis for tossing it, nor that Einstein's relativity work was going stale as well.

Einstein's relativity formulas apply precisely in certain conditions, yet when departing those conditions other parameters and their factors come into play. Hence, after relativity Einstein spent the rest of his life trying to make his superset formula for unified field theory work. He did not succeed at that and the last I heard most of those doing such work are plowing ahead by adding more and more variables.

I don't recall mentioning Eistein in my digressive post, but I do at least have some hope that we can now lay him back to rest for a while .

So back to off-subject, I did mean to suggest that the single factor displacement formula may not be all that is at play when the wind and the waves kick up. Besides planing there may be other factors. For instance in a tub like the Chesapeake and it's estuaries there are not only waves but many, many reflections and underwater currents. Who knows what effect that will have in a particular area at a particular time when it gets windy ? With your mention of "if this" and so and so we are actually approaching the same page.

I have also read recently of some new production boats factoring as high as 1.5 or 1.6 times LWL from a combination of non displacement effects. In your case of course you need to do some opposing direction calibration runs, or at least generate correction factors if you can't calibrate the readout of the knotlog itself before the displacement formula needs to be re-written . Maybe you should put a disclaimer on your knotlog ala what's on those outside rear view auto mirrors these days .

As for the knotlog sensing technology, perhaps there is something else happening in certain conditions, such as turbulence in the area of the knotlog adding to its sensed speed. What if you somehow get an eddy off of the bow and it constructively interferes with that tiny little paddle wheel ?

I have seen large discrepancies between the GPS and the knotlog myself, with error in both directions. I find the discrepancies to be the highest percentage at the slowest and highest speeds.

I'm not actually going to toss my knotlog. These days I look at it more like the tell tales, as input for setting trim. Even then we actually wear the best sensors of all when the thing between our ears can sort out the look and feel of acceleration, speed, and vibrations to achieve a smoothly sailing vessel.

Fair winds

 

[Emerald]

I think Neal may have really mentioned what is truly important - the relative change in speed on any given point of sail and trim change. Be it GPS or traditional knot meter/log, when it comes to getting the most out of your boat, what is really important is looking at the information coming back from the instrument and evaluating it relative to changes in helm, trim, etc.

The formulas are great fun for a dull afternoon in the office, a rainy Saturday, etc., but real life shows that they do deviate. I've had Both GPS and the knot meter show Emerald at something like 7.2-7.5+ knots, which is way above hull speed, and I attribute it not to tide, current etc, but the fact that it was blowing some 20+ knots and we were on a broad reach with full sail and, well, we were flying for her length and displacment (still grining ).


-David
Independence 31
Emerald

 

[CaptnNero]

when in doubt whip it out !

Naval Architect Dave Gerr gives a complete treatment to the subject of hull speed in The Nature of Boats, International Marine/Ragged Mountain Press; 1 edition (September 1, 1995)

A tattered, dog-eared copy of which should live on every boat.

Thanks for the tip. I've still got some room on that shelf and am open to suggestions. FWIW - sailnet.com has a nice thread going on book recommendations.

 

[Blue Chip]

Greetings Capt. Ron,

Even if I am faced with a current coming directly on my nose, isn't my boat speed on the water's surface limited to the theoretical hull speed of the boat or relatively close to it?

I think you folks are making this more complicated than need be, If the boat is on the hard and you aim a hose at the knot meter, it sure does beat the hull speed.

The GPS measures the speed over the bottom. The KM measures the speed of the water...not the boat?


Mark May
Blue Chip

 

[CaptnNero]

it's relative

Greetings Capt. Ron,

Even if I am faced with a current coming directly on my nose, isn't my boat speed on the water's surface limited to the theoretical hull speed of the boat or relatively close to it?

I think you folks are making this more complicated than need be, If the boat is on the hard and you aim a hose at the knot meter, it sure does beat the hull speed.

The GPS measures the speed over the bottom. The KM measures the speed of the water...not the boat?


Mark May
Blue Chip

Mark, I beg to differ. The boat stand analogy doesn't work here. If the boat was tethered to a mooring then yes, a mythical current could make the knot meter scream past hull speed. But since the boat is not tethered then the drag on the hull kicks in and accelerates the boat to the point that the boat is moving at the speed of the current (assuming wind drag effects are negligible), that is until thrust is applied by sail or motor power. Until there is thrust the untethered boat speed relative to the current is zero. Then the good old theoretical hull speed comes into play.

However your jack stand experiment with the focused garden hose is suggestive of what I said about fooling the paddle wheel. When I speculated about eddy effects in the vicinity of the paddle wheel I was trying to find ways to fool the tiny knotmeter without disturbing the macro scale environment around the boat itself.

 

[therapidone]

Ok, I give up! Just shoot me!

Greetings Cap'nNero,

Quoting you: "Yo Ed,

I certainly did not mean to imply that the mere age of a formula should be basis for tossing it, nor that Einstein's relativity work was going stale as well.
I don't recall mentioning Eistein in my digressive post, but I do at least have some hope that we can now lay him back to rest for a while."

I was only using the fact that what Einstein (I had to use an example that included the word "theory" to make my point & Einstein came to mind...I intended at first to refer to Boyle's and/or Charle's Laws about gases, both of which are even older than Einstein's theories--and still apply today, but "theory" isn't part of the terminology there) stated in his special and general theories of relativity took place over 75 years ago & they are still applicable today (and Einstein knew that they didn't explain everything but they did apply to the areas of physics to which he was applying them & they still do...the unified field theory...that was work with which he struggled for an answer for the rest of his life) as a counter to your argument (you say you did not mean to make such an implication...ok, then I made the inference based upon what you wrote) that the theoretical hull speed formula might be deficient simply because it was arrived at 75 years ago. You're right (of course you are, your previous post was there as evidence) that you didn't mention Einstein.

Also, you mentioned that LWL wasn't the only factor...in fact, the salinity of the water (affecting density), the temperature (also affecting density), the resistance of moving through the air, the possible eddying effects in the water are all additional factors & there may be others...but they play really minor roles, to the point of being negligible, in circumstances such as Sunday's.

After painting the bottom of Spirit over the winter & becoming intimately familiar w/ her shape , I'm now thinking that this discussion (about hull speed limitations for displacement vessels) is almost without merit as it applies to Spirit and other similarly designed hulls with fin keels, given the website I cited yesterday (http://www.cncphotoalbum.com/technic.../hullspeed.htm) and the Dave Gerr stuff referred to by Jim Mobley (see: http://potter-yachters.org/manyways/hullspeed/). I've also found that the wording at the cncphotalbum site (at least up to the paragraph that I thought was applicable and ran counter to the theoretical hull speed limitation for Spirit because the hull might not be, strictly speaking, a displacement hull) was lifted word for word, example for example, diagram or figure for diagram or figure from a Steve Colgate discussion on learning how to sail on Sailnet!

I'll just try to find how much the two measuring devices diverge by using both at a time when the winds are relatively calm & the tidal current in the bay at that point in time is not much of a factor and then look for any possible way to adjust the calibration on the knot meter...before posting the question, I did not know that adjusting the knot meter was possible...now I know that it is possible...just not certain if it with my particular instrument nor how to do so.

Thank you, Neal, for your contributions to the thread (and everyone else for that matter). The wealth of knowledge that can be gleaned from this site is one of the reasons that keeps me checking it over almost daily. David's (Emerald) observation that all of this would be interesting while spending a dull day at the office or dealing with a rainy Saturday implies something additional...this would have made for a really interesting discussion during the winter months...we'll just have to come up with something else about which to agree/disagree/embellish/and perfect through synergy then.

Regards,

Ed