I have a Hunter 30 1989. I recently sailed under over 30 knots wind. I want to see how fast the boat can go. At a broad reach, the speed meter reached 10.5 knots. With correction, the water speed should be around 10 knots. Then suddenly, the boat lost control and turn into the wind. After it happened a few times, I give up and started to reef. Anybody has similar experience? How fast have you ever sailed? Frank
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GPS
A GPS is going to be more accurate than your knot meter. Multiply 1.34 times the squre root of your waterline for your maximum theoretical hull speed. The boat was rounding up because you were over powered--chances are if you had reefed you could have maintained the same speed with less heel and more control. I tend to doubt the 10 knots on a broad reach--maybe if you were running downwind and surfing down waves.
A GPS is going to be more accurate than your knot meter. Multiply 1.34 times the squre root of your waterline for your maximum theoretical hull speed. The boat was rounding up because you were over powered--chances are if you had reefed you could have maintained the same speed with less heel and more control. I tend to doubt the 10 knots on a broad reach--maybe if you were running downwind and surfing down waves.
Lake vs Salt Water
One of the main difference between salt water and lake sailing is salt water has tides and currents. In the case of Puget Sound there are currents everywhere, rivers within a river. Don't know which day Frank was out but if it was on last weekend, January 4th or 5th, the max current off Bush Point on Saturday (the 4th) was 2.1 kts flood and 0.6 kts ebb (during the daylight hours). Even if Frank was sailing in the main channel around Shilshole with a slightly reduced current it is still significant. Under these situations a GPS cannot be used for measuring your speed through the water. I've been through Seymore Narrows north of Campbell River in my H-35 doing over 20 knots per the GPS! Boat speed through the water was probably 6 to 6.5 knots - the rest was current! 10+ knots in a H-30 is doing really good! Obviously Frank likes sailing on-the-edge. What was the wind chill?
One of the main difference between salt water and lake sailing is salt water has tides and currents. In the case of Puget Sound there are currents everywhere, rivers within a river. Don't know which day Frank was out but if it was on last weekend, January 4th or 5th, the max current off Bush Point on Saturday (the 4th) was 2.1 kts flood and 0.6 kts ebb (during the daylight hours). Even if Frank was sailing in the main channel around Shilshole with a slightly reduced current it is still significant. Under these situations a GPS cannot be used for measuring your speed through the water. I've been through Seymore Narrows north of Campbell River in my H-35 doing over 20 knots per the GPS! Boat speed through the water was probably 6 to 6.5 knots - the rest was current! 10+ knots in a H-30 is doing really good! Obviously Frank likes sailing on-the-edge. What was the wind chill?
Unreliable GPS-based boat speed readings
John is right in pointing out that GPS cannot be used to calculate boat speed if there is a bit of current running. Even if there is no current, however, GPS is usually not the proper tool for precise boat speed measurements; the exception being the use of a rapid response Differential GPS system at relatively high vessel speeds. The reason is simple: GPS can only determine boat speed (as well as compass heading) by "subtracting" two or more consecutive position measurements. The unavoidable errors in these position measurements tend to lead to an even larger error in the calculated value. For instance, assume a boat speed of 5 meter per second (i.e. 18 kilometer per hour or roughly 10 knot) and a time difference between the two measurements of 20 seconds. In this time interval our imaginary vessel has traveled 100 meter. If the position uncertainty at each point is +/- 10 meter (roughly 30 ft) a little sketch will show that in the worst case scenario the GPS might determine the distance between these 2 points to be as little as 80 meter (corresponding to a calculated velocity of ~8 knot) or as much as 120 meter (i.e. ~12 knot). Fortunately, such worst case speed values can only be expected to occur if the measurement error at point 2 would be independent of the error at point 1, whereas in reality the two errors tend to be correlated. In fact, within the imaginary 20 meter diameter error circle the measured position tends to wander around the center (i.e. the true position) rather than to jump from one point to the other. As a result one is likely to see the speed reading wander up and down past the true value, even if boat speed is constant. This wandering can be stopped effectively by choosing a much longer integration time than 20 seconds. This indeed produces a much more stable and reliable "average boat speed" reading but also means that transient changes in boat speed, e.g. while surfing a wave, will be missed completely. Have fun, Flying Dutchman
John is right in pointing out that GPS cannot be used to calculate boat speed if there is a bit of current running. Even if there is no current, however, GPS is usually not the proper tool for precise boat speed measurements; the exception being the use of a rapid response Differential GPS system at relatively high vessel speeds. The reason is simple: GPS can only determine boat speed (as well as compass heading) by "subtracting" two or more consecutive position measurements. The unavoidable errors in these position measurements tend to lead to an even larger error in the calculated value. For instance, assume a boat speed of 5 meter per second (i.e. 18 kilometer per hour or roughly 10 knot) and a time difference between the two measurements of 20 seconds. In this time interval our imaginary vessel has traveled 100 meter. If the position uncertainty at each point is +/- 10 meter (roughly 30 ft) a little sketch will show that in the worst case scenario the GPS might determine the distance between these 2 points to be as little as 80 meter (corresponding to a calculated velocity of ~8 knot) or as much as 120 meter (i.e. ~12 knot). Fortunately, such worst case speed values can only be expected to occur if the measurement error at point 2 would be independent of the error at point 1, whereas in reality the two errors tend to be correlated. In fact, within the imaginary 20 meter diameter error circle the measured position tends to wander around the center (i.e. the true position) rather than to jump from one point to the other. As a result one is likely to see the speed reading wander up and down past the true value, even if boat speed is constant. This wandering can be stopped effectively by choosing a much longer integration time than 20 seconds. This indeed produces a much more stable and reliable "average boat speed" reading but also means that transient changes in boat speed, e.g. while surfing a wave, will be missed completely. Have fun, Flying Dutchman
GPS speed determination
A GPS is the best thing since white bread, for navigation and for simple speed determinations. A GPS gives speed over the bottom. This fact allows a calibrated knot meter to be read along with the GPS speed determination to estimate current running. A GPS speed determination would jump around as Henk's arcane analysis indicates if a speed display based on each calculation were made as he outlines. Not the case though since even my old Garmin GPS III calculates position at one second intervals but displays a velocity based on a running average of the distance/time calculations. Displayed result is a smoothed velocity that does not fluctuate erratically but smoothly ramps up and down as speed varies. Anyone having a GPS that does fluctuate in velocity display as Henk describes can fix it by buying a current generation of the instrument.
A GPS is the best thing since white bread, for navigation and for simple speed determinations. A GPS gives speed over the bottom. This fact allows a calibrated knot meter to be read along with the GPS speed determination to estimate current running. A GPS speed determination would jump around as Henk's arcane analysis indicates if a speed display based on each calculation were made as he outlines. Not the case though since even my old Garmin GPS III calculates position at one second intervals but displays a velocity based on a running average of the distance/time calculations. Displayed result is a smoothed velocity that does not fluctuate erratically but smoothly ramps up and down as speed varies. Anyone having a GPS that does fluctuate in velocity display as Henk describes can fix it by buying a current generation of the instrument.
Thanks
Terry, I could not figure out what Henk was talking about as our GPS gives a very accurate speed over the bottom. We have tested it with the loran also.
Terry, I could not figure out what Henk was talking about as our GPS gives a very accurate speed over the bottom. We have tested it with the loran also.
Exact vs Estimate
One of the points that Henk makes is that due to the way the GPS works there is no way it can provide an EXACT speed. What it provides is an estimate - which can be a close estimate or one that is less close. Even surveyors who use GPS know the coordinates they get are not exact and they have far better setups than we have on our boats. One needs to realize there is a "circle of confusion", that area where it is likely, to some probability, that the boat (GPS antenna) is. In the case of a land surveyor this circle of confusion can be measured in centimeters but with our marine GPSs it is likely many feet if not yards (or meters in the rest of the world). Many surveryors still do not use GPS due to inaccuracy and cost. The fact that the GPS speed does not jerk around does not necessarily mean it is accurate, and in fact, may mean just the opposite. If the numbers don't vacillate much it may be because the speed over ground is not changing much, or, it may be the smoothing algorithm is doing it's job. Manufacturers of GPSs know that customers don’t like to see bouncing speed numbers so that’s why they developed algorithms to smooth out the numbers. The end result is the figure you see is an approximate or estimated speed over ground - not exact - but one the customer can/might feel good about.
One of the points that Henk makes is that due to the way the GPS works there is no way it can provide an EXACT speed. What it provides is an estimate - which can be a close estimate or one that is less close. Even surveyors who use GPS know the coordinates they get are not exact and they have far better setups than we have on our boats. One needs to realize there is a "circle of confusion", that area where it is likely, to some probability, that the boat (GPS antenna) is. In the case of a land surveyor this circle of confusion can be measured in centimeters but with our marine GPSs it is likely many feet if not yards (or meters in the rest of the world). Many surveryors still do not use GPS due to inaccuracy and cost. The fact that the GPS speed does not jerk around does not necessarily mean it is accurate, and in fact, may mean just the opposite. If the numbers don't vacillate much it may be because the speed over ground is not changing much, or, it may be the smoothing algorithm is doing it's job. Manufacturers of GPSs know that customers don’t like to see bouncing speed numbers so that’s why they developed algorithms to smooth out the numbers. The end result is the figure you see is an approximate or estimated speed over ground - not exact - but one the customer can/might feel good about.
Done 28 knots
I have done 28 knots on my NACRA 6.0 catamaran running a 700 sqft masthead chute. I probably went faster but after a while I turned my GPS off. I figured if I didnt know how fast I was going it would reduce the tendancy to chicken out and back off the throttle. I was in a distance race of 54 miles. It took me 2 hours to sail it and I still came in 3rd. grrrrrrrr. Ted
I have done 28 knots on my NACRA 6.0 catamaran running a 700 sqft masthead chute. I probably went faster but after a while I turned my GPS off. I figured if I didnt know how fast I was going it would reduce the tendancy to chicken out and back off the throttle. I was in a distance race of 54 miles. It took me 2 hours to sail it and I still came in 3rd. grrrrrrrr. Ted
13kts on a Capri 30
I was crewing on a Capri 30 during a race in light and varible winds. On our last leg we were in the middle of the pack when we noticed a strong wind coming across the bay. We were able to get our heavy spinnaker up just prior to about 30kts of wind reaching us. We quickly came up on a plane and maintained a steady 13kts. Since we were the only boat light enough to plane, we were quickly moving to the head of the pack. All was great until we heard a few choice words from the helmsman. We took our eyes off the spinnaker and turned to see the helmsman holding up a broken tiller. Obviously the 8in stub remaining was not sufficient to maintain our course and we quickly rounded up. Luckily we were all able to grab something as the boat laid on its side with spreaders in the water. This made for quite the story at the post-race party since the entire fleet had been watching us zoom past them. Besides a torn spinnaker and broken tiller, no other damage occurred. Oh the joys of racing and going as fast as possible!
I was crewing on a Capri 30 during a race in light and varible winds. On our last leg we were in the middle of the pack when we noticed a strong wind coming across the bay. We were able to get our heavy spinnaker up just prior to about 30kts of wind reaching us. We quickly came up on a plane and maintained a steady 13kts. Since we were the only boat light enough to plane, we were quickly moving to the head of the pack. All was great until we heard a few choice words from the helmsman. We took our eyes off the spinnaker and turned to see the helmsman holding up a broken tiller. Obviously the 8in stub remaining was not sufficient to maintain our course and we quickly rounded up. Luckily we were all able to grab something as the boat laid on its side with spreaders in the water. This made for quite the story at the post-race party since the entire fleet had been watching us zoom past them. Besides a torn spinnaker and broken tiller, no other damage occurred. Oh the joys of racing and going as fast as possible!
GPS speed reading issue
Terry, John's explanation is correct. Unfortunately, the underlying math and physics are pretty straightforward, leaving not a whole room for manufacturers of "current generation" instruments to pull rabbits out of their hats. If you really have a high end GPS with quite a few functions you should be able to reduce the integration interval and see exactly the phenomenon I talked about, i.e. the "wandering" of the calculated versus the true position reflected in wandering speed numbers, even when your vessel is going at a stable speed. Sure, it is very simple to "smooth" that velocity and get a stable reading. However, now the velocity shown is by definition some kind of average between your current velocity and your velocity one or more minutes ago. Not exactly what you would like to see if you are trying to gain .2 knot by adjusting the tension of the boomvang...... Therefore, serious racers know they cannot use GPS speed readings to help them win races, unless they happen to have a fast DGPS system (just check the discussions on their websites). You are welcome to dream on, of course, but this is the reality. ;o) Flying Dutchman
Terry, John's explanation is correct. Unfortunately, the underlying math and physics are pretty straightforward, leaving not a whole room for manufacturers of "current generation" instruments to pull rabbits out of their hats. If you really have a high end GPS with quite a few functions you should be able to reduce the integration interval and see exactly the phenomenon I talked about, i.e. the "wandering" of the calculated versus the true position reflected in wandering speed numbers, even when your vessel is going at a stable speed. Sure, it is very simple to "smooth" that velocity and get a stable reading. However, now the velocity shown is by definition some kind of average between your current velocity and your velocity one or more minutes ago. Not exactly what you would like to see if you are trying to gain .2 knot by adjusting the tension of the boomvang...... Therefore, serious racers know they cannot use GPS speed readings to help them win races, unless they happen to have a fast DGPS system (just check the discussions on their websites). You are welcome to dream on, of course, but this is the reality. ;o) Flying Dutchman
GPS speed II
GPS is the best thing available for over the ground speed determinations. An easy test for building confidence: With your favorite 12 channel hand held gps receiver on and locked in with velocity showing and placed on the dash of an automobile, accelerate onto a level limited access highway having mile post markers. As the car accelerates, compare speed readings with the speedometer. (It will lag little or none, indicating that the averaging time for the gps is on the order of seconds, not minutes). Now with the speed on the gps held at 60 mph (one mile per minute) start a stopwatch or simply have a passenger note the second hand of a watch as the first mile post comes exactly abreast. Keep the speed at 60.0 mph as closely as possible on the gps and note the split time or second hand position each time a new mile post marker comes abreast. This will soon get monotonous, for the mile marker always draws abreast at the 60 second interval. The test will also provide a quick determination of the speedometer error, awfully helpful in preventing speeding tickets. Velocity determination exactly the same when the gps is on a boat so confidence in velocity output with it or a similar instrument sitting on a boat exactly the same. A knotmeter, on the other hand assigns a distance for each revolution of the impeller and then digitally displays a velocity using a computed relation between distance and time, (a gps does this too) A fundamental difference though is that whereas the gps determines the necessary distance component through the arcane and accurate computations within the black box, the distance assigned to an impeller revolution is subject to error, e.g. from differences in water flow around the impeller on different boats, differences in the constant for different boat speeds, and changes in the constant coming from crud on the impeller or changes in the impeller bearing. Thus for any kind of accuracy on a knotmeter, it must be periodically calibrated. The gps can serve well for this calibration since the knotmeter calibration probably calls for running an accurate distance and then changing the knotmeter trip reading to agree with the accurate distance. Criticism of the gps velocity measurement, most especially in light of the normally used alternatives is puzzling. Tradition dies hard I suppose. At any rate, the knotmeter still has a legitimate though secondary role, that of determining speed through the water which a gps cannot do and thus, (by comparison with the gps speed) give means for estimating water current components in the direction of boat movement. Another important remaining use of the knotmeter is for dead reckoning position estimates in the event of gps failure or non availability. As for me, if I didn't already have a hole in my boat for the knotmeter, I wouldn't put one there now. GPS velocity readout is the way to go for over the bottom speed. Period!
GPS is the best thing available for over the ground speed determinations. An easy test for building confidence: With your favorite 12 channel hand held gps receiver on and locked in with velocity showing and placed on the dash of an automobile, accelerate onto a level limited access highway having mile post markers. As the car accelerates, compare speed readings with the speedometer. (It will lag little or none, indicating that the averaging time for the gps is on the order of seconds, not minutes). Now with the speed on the gps held at 60 mph (one mile per minute) start a stopwatch or simply have a passenger note the second hand of a watch as the first mile post comes exactly abreast. Keep the speed at 60.0 mph as closely as possible on the gps and note the split time or second hand position each time a new mile post marker comes abreast. This will soon get monotonous, for the mile marker always draws abreast at the 60 second interval. The test will also provide a quick determination of the speedometer error, awfully helpful in preventing speeding tickets. Velocity determination exactly the same when the gps is on a boat so confidence in velocity output with it or a similar instrument sitting on a boat exactly the same. A knotmeter, on the other hand assigns a distance for each revolution of the impeller and then digitally displays a velocity using a computed relation between distance and time, (a gps does this too) A fundamental difference though is that whereas the gps determines the necessary distance component through the arcane and accurate computations within the black box, the distance assigned to an impeller revolution is subject to error, e.g. from differences in water flow around the impeller on different boats, differences in the constant for different boat speeds, and changes in the constant coming from crud on the impeller or changes in the impeller bearing. Thus for any kind of accuracy on a knotmeter, it must be periodically calibrated. The gps can serve well for this calibration since the knotmeter calibration probably calls for running an accurate distance and then changing the knotmeter trip reading to agree with the accurate distance. Criticism of the gps velocity measurement, most especially in light of the normally used alternatives is puzzling. Tradition dies hard I suppose. At any rate, the knotmeter still has a legitimate though secondary role, that of determining speed through the water which a gps cannot do and thus, (by comparison with the gps speed) give means for estimating water current components in the direction of boat movement. Another important remaining use of the knotmeter is for dead reckoning position estimates in the event of gps failure or non availability. As for me, if I didn't already have a hole in my boat for the knotmeter, I wouldn't put one there now. GPS velocity readout is the way to go for over the bottom speed. Period!
Good analysis Terry, except...
I think that at highway speeds the ratio of expected error to distance travelled would be much lower than at typical sailboat speeds. If the GPS computes your location every second then your locations will be much further apart at 60 mph than at 5 knots. I would think this might dilute some of the affect of the positional error and make the GPS more accurate at higher speeds? You also have to be carefull calibrating your knotmeter to your GPS since they really measuring two different things: speed over ground vs. speed though the water. With no current you'd be ok. I do agree though that the GPS is great to have especially in an area with big currents, I wouldn't trade mine for anything. Not even throwing wood chips off the bow and timing how long they take to reach the stern! Enough nitpicking, I gotta go sailing!
I think that at highway speeds the ratio of expected error to distance travelled would be much lower than at typical sailboat speeds. If the GPS computes your location every second then your locations will be much further apart at 60 mph than at 5 knots. I would think this might dilute some of the affect of the positional error and make the GPS more accurate at higher speeds? You also have to be carefull calibrating your knotmeter to your GPS since they really measuring two different things: speed over ground vs. speed though the water. With no current you'd be ok. I do agree though that the GPS is great to have especially in an area with big currents, I wouldn't trade mine for anything. Not even throwing wood chips off the bow and timing how long they take to reach the stern! Enough nitpicking, I gotta go sailing!
Rounding up
I have a 1990 H30, the fastest ive had it was 9.6 according to the Knotmeter and Ive had the same problem when on a reach of any kind in strong winds. Ive also found that letting both sails out more than you think neccessary stoped this.
I have a 1990 H30, the fastest ive had it was 9.6 according to the Knotmeter and Ive had the same problem when on a reach of any kind in strong winds. Ive also found that letting both sails out more than you think neccessary stoped this.
Where does one sail in Denver (Colorado for that m
Not related I know but I am curious where a 20 ft boat could sail let alone a 30 ft.
Not related I know but I am curious where a 20 ft boat could sail let alone a 30 ft.
sailing in Colorado
Chatfield, a small pond, little over two miles long has a 32', three 30's. We all wish we had more watter. Pueblo reservouir, just about seven miles long has, yes, a 46' benneteau, 47' older Catalina and several 40' boats.
Chatfield, a small pond, little over two miles long has a 32', three 30's. We all wish we had more watter. Pueblo reservouir, just about seven miles long has, yes, a 46' benneteau, 47' older Catalina and several 40' boats.
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