|Topic Review (Newest First)|
|10-17-2011 02:18 PM|
The thing with 'knotmeters' is that they are 'calibratable', however that doesn't mean they've been accurately calibrated. Out of the box they should be OK but the only way to be certain is to run a measured distance in the absolute guarantee of no influencing current.. time the runs and do the math.
Not many boaters go to that effort..
|10-17-2011 01:15 PM|
Originally Posted by emoney View Post
Long answer: When you look at the way SOG is calculated from GPS positions, there is some error at slow speeds because the instantaneous positions have some error. But that would show up as random noise in your readings, which is easily damped out by good software. As a practical matter, if your GPS is showing a stable speed to the first decimal (x.x), without frequent changes after the decimal, then your signal is good and/or your software is damping it. I would consider the GPS to be highly reliable, and far more reliable than the pitot tube, which I believe could be severely affected by turbulence, salinity level, and temperature. In fact, I think its potential non-random bias probably exceeds the tidal currents in most waterways.
I'm not familiar enough with pinwheel-type speedometers used on sailboats to make a comparison, but I doubt they would be more accurate than a GPS for SOG.
|10-17-2011 11:14 AM|
Originally Posted by emoney View Post
BTW - you should be able to change to the units of measure from miles, knots, kilometers, etc. on both the GPS and the speedometer.
|10-17-2011 10:43 AM|
On my powerboat, I checked my GPS vs. the speedometer (hey, it's measure in mph so I couldn't say knotmeter) and there was a 5mph difference betwixt the two. Question was, which one was correct?
I'm sure there's a lot of technical reasoning in this debate, but ultimately I'd surmise two things: 1)don't 'just' trust your GPS for actual speed 2)don't care about speed, just go as fast as you can.
|10-17-2011 10:12 AM|
Newport... you hit it right on the head. Most people want GPS for navigation, and ETA, not for speed perse' But knowing SOG is essential for ETA, and travel times. It is just as significant for boating, however, Speed over Water, is essential for all of us as well because it tells us our best trim settings for a given wind (assumes steady wind).
You are now starting to get JUST how complicated sailing can be (about as complicated as you let it)... for those of us (most of us, even the so-called cruisers only) who are after the "perfect trim," it is a life-long unachievable goal. You are catching it now... Don't let it rule you, but don't give up on it either. It'll make you a better sailor. It will make ALL of us better sailors.
Don't get "instrument i-tus" either. Gadgets are cool, but remember they are a modern phenomena. The old-salts sail without them, and can smoke some of the young punks using all the gadgets. A knot-meter, and a depth sounder are pretty essential gadgets though, and a great start.
I am with you though, I JUST have a GPS (actually my phone running navionics)... my goal this winter though is to get a knot and a depth gauge to get me better information on my sailing. Will I still use my GPS (absolutely), but those hard instruments I feel are pretty essential for me going forward.
|10-17-2011 10:02 AM|
Originally Posted by jackdale View Post
|10-17-2011 01:07 AM|
|NewportNewbie||Ok...great discussion, so let me get this...I understand that there is a difference between boat speed which may be aided or impeded by currents, but just raw speed GPS should be far more accurate. How long does it take to get from point A to point b and figure out is speed in knots based on that. That should never change...right? So a knot meter on a boat will tell you how fast the boat is moving (or more exactly how fast the water is moving past the boat) and will give you instant results in regard to sail trim. A GPS device should give you the actual speed, not boat speed.|
|10-17-2011 01:03 AM|
Until your post I didn't understand how a boat's waterline could be the only "boat dimension" variable that could affect speed...
My Sea-Doo boat is a great example of what you're saying...putts along at idle nicely...give it a little throttle and it hits the wall...now I can bore my crew with the reason why. =)
|10-17-2011 12:35 AM|
Originally Posted by NewportNewbie View Post
|10-17-2011 12:00 AM|
The quotes from Gary Mull pretty much sum up why "hull speed" shouldn't be called "maximum hull speed". Let me just add a bit to why hull speed works as a concept. As a displacement hull moves through the water it creates a wave, which boaters/sailors usually call the "bow wave" or "bow wake". This bow wave initially has a celerity equal to the velocity of the vessel. The celerity of a wave actually slowly increases with time, but we only need to worry about what happens when the wave is first created. The wavelength of a surface wave is proportional the celerity of the wave; the faster the wave, the longer the wavelength. There is actually a fairly complicated formula for this relationship, involving the celerity, the force of gravity, and the density, viscosity, and surface tension of the water. But, since everything is pretty much constant except celerity and wavelength, the whole thing boils down to C = 1.34*sqrt(wavelength).
Now, what exactly does that mean? As the boat moves through the water the wavelength of the bow wave gets longer as the boat moves faster. The first crest is always at the bow, with the next crest being one wavelength aft of the bow. When the speed of the boat (and hence the celerity of the bow wave) increases to the point that the next crest is near the stern of the boat this crest is often called the "quarter wave", but it is really just the second crest of the bow wave; the stern quarter isn't causing the wave, it's being cause by the bow. The velocity at which this crest is right at the stern is called the "hull speed" or HS, and is described simply by substituting HS for C and length at the waterline (LWL) for wavelength in the above equation, to get, HS = 1.34*sqrt(LWL). The important thing here is what happens when the boat is going fast enough (and hence the bow wave has a high enough celerity) that the wave has a wavelength longer than the dynamic waterline of the boat? Now the stern of the boat will no longer be riding on the second crest of the bow wave (or on the quarter wave, if you prefer), and the boat will have to pitch up slightly. This takes a power input to maintain speed and so the power/speed ratio of the boat increases due to this added factor. The power/speed ratio has been increasing with speed all along due to other factors, but now this increased pitch of the hull and the forces generated by the bow having to push "through" more of the initial crest of the bow wave causes a sharp increase in the relationship.
So, it always takes more power (energy/time) to make a boat go faster, but above hull speed the relationship between power and boat speed gets steeper and steeper. This is essentially because above hull speed the boat is forced to climb is own wake. Next time you're in a planing boat you can actually demonstrated this. Give the boat a bit of gas and initially the bow will pitch up, the stern will squat down, and it will feel like the boat is trying to climb out of a hole. Give it enough gas and the boat will "jump" onto a plane and the rules will suddenly change; once the hull is actually planing you can ease the throttle back a bit (lower the power) and stay on the plane fairly easily. But, kill the engine (or go to idle) and the boat will plane for a bit then suddenly slow as it can't maintain a plane anymore and has to crawl up its bow wake, until it gets below hull speed again and more easily glides along as a "true" displacement hull again.
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