Expert advise needed. If I calculate it correctly. My maximum hull speed is 6.01 knots. (20.1 LWL) (240 O'day) Sunday we had LOTS of heavy wind and decided to see what we could do. According to our new Garman Map 76 GPS, We constantly hit 5.8 to 6.1 knots with the wind to our port. But on starboard we constantly hit high 6's and even as high as 7.2. I did not figure our theoretical hull speed until we got back. Now I'm a little confused. Did I figure wrong? Are the GPS's not that accurate for speed? I thought that theoretical hull speed was pretty much the max.
hull speed
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GPS is not water speed
GPS is land speed, not water speed. And even then it's not amazingly accurate in real time. There are other factors, IE: current, wave action, wind etc that affect water speed a great deal. If you're at hull speed 6 knots with a 1 knot current then you're at 7 knots GPS ground speed. Or if you're surfing a good swell and have the current you could top 9-10 knots momentarily. Get a paddlewheel for accurate water speed measurement.
GPS is land speed, not water speed. And even then it's not amazingly accurate in real time. There are other factors, IE: current, wave action, wind etc that affect water speed a great deal. If you're at hull speed 6 knots with a 1 knot current then you're at 7 knots GPS ground speed. Or if you're surfing a good swell and have the current you could top 9-10 knots momentarily. Get a paddlewheel for accurate water speed measurement.
GPS measures...
the doppler shift on different channels from different satellites. Speed is determined by that shift, regardless if you are walking, flying, driving, sailing, running, falling... get the idea? Speed is distance over time. Again, regardless of the media one is using. A paddle wheel under you boat measures how fast the water is passing under the boat. If the boat is moving in a direction and the wind is pushing the boat in the same direction and the water is moving in the same direction, and all are equal, then your paddle wheel is not moving and therefore, you are not moving. Right? GPS is more accurate than a paddlewheel. Just my opinion.
the doppler shift on different channels from different satellites. Speed is determined by that shift, regardless if you are walking, flying, driving, sailing, running, falling... get the idea? Speed is distance over time. Again, regardless of the media one is using. A paddle wheel under you boat measures how fast the water is passing under the boat. If the boat is moving in a direction and the wind is pushing the boat in the same direction and the water is moving in the same direction, and all are equal, then your paddle wheel is not moving and therefore, you are not moving. Right? GPS is more accurate than a paddlewheel. Just my opinion.
a bit of terminology
the knotlog measures speed through the water (STW) the gps measures speed over ground (SOG) the paddlewheel propels steamboats up the Mississippi assuming that all instruments are properly calibrated, if the knotlog and the gps don't agree, it's either because of current or because your bottom's dirty. (YBD)
the knotlog measures speed through the water (STW) the gps measures speed over ground (SOG) the paddlewheel propels steamboats up the Mississippi assuming that all instruments are properly calibrated, if the knotlog and the gps don't agree, it's either because of current or because your bottom's dirty. (YBD)
We seem to be beginning to rely too much
on our instruments and not enough on our minds. If you should find yourself in a flat calm, and you are drifting with the bubbles, your GPS will show the speed and direction of your movement. That is the speed and direction of the current at that time and place. If you consult your tide and current tables they will display speed and direction of ebb and flood and will also give the times of slack water before ebb and before flood. Just as the plots for the tides seem to describe a sine wave, constantly changing the velosity of the current at ebb and at flood will constantly change. All speed records require that the runs be made in opposite directions within one hour. If you sail for one mile from north to south and then turn and sail east for one hour and plot your DR position against your GPS fix, you will quickly find the effects of the set and leeway. Of all the methods available today GPS will provide the most information, most accurately of all of the systems affordable today.
on our instruments and not enough on our minds. If you should find yourself in a flat calm, and you are drifting with the bubbles, your GPS will show the speed and direction of your movement. That is the speed and direction of the current at that time and place. If you consult your tide and current tables they will display speed and direction of ebb and flood and will also give the times of slack water before ebb and before flood. Just as the plots for the tides seem to describe a sine wave, constantly changing the velosity of the current at ebb and at flood will constantly change. All speed records require that the runs be made in opposite directions within one hour. If you sail for one mile from north to south and then turn and sail east for one hour and plot your DR position against your GPS fix, you will quickly find the effects of the set and leeway. Of all the methods available today GPS will provide the most information, most accurately of all of the systems affordable today.
I don't know about your boat
but mine will plane given enough wind as it only weighs 1300#'s and basically has a planing hull. I've seen 9.7 with GPS on a lake with NO current in 20+ winds. Twas a wild ride for sure but a whole lotsa fun, especially singlehanded. How can ya tell your're planing? When ya exceed hull speed and you're pounding over the wave tops like a power boat sending spray plumes over your head.
but mine will plane given enough wind as it only weighs 1300#'s and basically has a planing hull. I've seen 9.7 with GPS on a lake with NO current in 20+ winds. Twas a wild ride for sure but a whole lotsa fun, especially singlehanded. How can ya tell your're planing? When ya exceed hull speed and you're pounding over the wave tops like a power boat sending spray plumes over your head.
I hear what you're saying, Ross, but....
...the modern racer is more reliant on the knotlog than ever because you can't sail your polars without one, and if you can't sail your polars the other guys are going to beat you nine times out of ten. And, back to the original question, everyone who has raced a bunch knows that on most boats it's easier to hit the polars right on one tack than another. Although this can be a simple matter such as having the fairleads wrong on one side, it can also indicate a tuning problem. Or, it can simply happen because the chop is set up to slow you down 1/10th of a knot more on starboard than on port. That said, every boat I've ever owned seems faster on port tack than on starboard, which probably means the guy at the helm needs a bit of tuning.
...the modern racer is more reliant on the knotlog than ever because you can't sail your polars without one, and if you can't sail your polars the other guys are going to beat you nine times out of ten. And, back to the original question, everyone who has raced a bunch knows that on most boats it's easier to hit the polars right on one tack than another. Although this can be a simple matter such as having the fairleads wrong on one side, it can also indicate a tuning problem. Or, it can simply happen because the chop is set up to slow you down 1/10th of a knot more on starboard than on port. That said, every boat I've ever owned seems faster on port tack than on starboard, which probably means the guy at the helm needs a bit of tuning.
Joe and Charlie
Joe: Theoretical hull speed is based on a theoretical bath tub. That is, no wind, no waves and no current. So your theoretical hull speed can be exceeded if the current is going with you. For instance, you are at THS of 6 knots, the current is pushing from behind at 5 knots,then you are actually moving at 11 Knots. The THS is a calculation made based on a sine wave pattern of a real wave that your boat makes going through the water. If you were to apply enough power, what would happen is that your bow would ride up on top of its own bow wave, and your stern would squat in the low part. YOu would then actually exceed the THS, but with a disproportionate amount of extra power. You could for example triple you power and fuel consumption and only go maybe 10 to 15% faster. These are not actual figures, but thats the general idea without gettingb too technical. Charlie: Theoretical Hull Speed only applies to displacement hulls. A planing hull has no maximum THS based per se. The top speed of a planing hull would be limited to horse power,hull shape and how much stress it can take before it shakes itself apart. These statements may not be totally correct, but i'm trying to keep it as non-technical as possible Tony B
Joe: Theoretical hull speed is based on a theoretical bath tub. That is, no wind, no waves and no current. So your theoretical hull speed can be exceeded if the current is going with you. For instance, you are at THS of 6 knots, the current is pushing from behind at 5 knots,then you are actually moving at 11 Knots. The THS is a calculation made based on a sine wave pattern of a real wave that your boat makes going through the water. If you were to apply enough power, what would happen is that your bow would ride up on top of its own bow wave, and your stern would squat in the low part. YOu would then actually exceed the THS, but with a disproportionate amount of extra power. You could for example triple you power and fuel consumption and only go maybe 10 to 15% faster. These are not actual figures, but thats the general idea without gettingb too technical. Charlie: Theoretical Hull Speed only applies to displacement hulls. A planing hull has no maximum THS based per se. The top speed of a planing hull would be limited to horse power,hull shape and how much stress it can take before it shakes itself apart. These statements may not be totally correct, but i'm trying to keep it as non-technical as possible Tony B
tunning
I have no doubt that I need lots of tunning as this is my first sail boat and we have only had her wet a few times. All this input is great and helps with the learning curve. This is one of the best blog sites out there.
I have no doubt that I need lots of tunning as this is my first sail boat and we have only had her wet a few times. All this input is great and helps with the learning curve. This is one of the best blog sites out there.
I like to keep my GPS on when racing
and recently installed one of those 'paddle-wheel' thru hull transducers for a knot meter. When the two instruments give a difference in reading then I have an idea that the current and/or wave action may be slowing me down in terms of SOG (speed over ground) in the Hudson River. The GPS will show peaks of over 'theorhetical' hull speed due to wind gusts and waves. Use the GPS and forget about 'theorhetical' hull speed but know about the tides and currents in your area. You will figure it out.
and recently installed one of those 'paddle-wheel' thru hull transducers for a knot meter. When the two instruments give a difference in reading then I have an idea that the current and/or wave action may be slowing me down in terms of SOG (speed over ground) in the Hudson River. The GPS will show peaks of over 'theorhetical' hull speed due to wind gusts and waves. Use the GPS and forget about 'theorhetical' hull speed but know about the tides and currents in your area. You will figure it out.
The formula Vh=C(LWL)^1/2 (hull speed = a constant times the square root of the LWL) is just an estimate as the constant "C" has to be the right one for your boat. If memory serves me correctly C=1.3 for an average boat. If your boat is "fair" and not so beamy then C goes up to 1.4 and if your boat is beamy and not so "fair" it goes down to 1.2. There are lots of other combinations too. So the best you can do is use it as an estimate since no one knows the exact value of C except that it is 1.? One significant digit!!!! so taking it to 7.01 is kinda invalid as it could just as well be anything from 6.01 to 7.99.
Hull Speed/Actual Speed
The 'Theoretical Hull Speed' is for a 'Displacement Hull' and is based on the formula 1.34 (the accepted average for "C")times the square root of the LWL (length of the water line). That is the maximum speed through the water, not over the face of the earth. This theoretical limit can momentarily be exceeded by 'surfing' on the face of a wave, and by planing by some lighter displacement boats reaching under spinnaker. The GPS is measuring your speed over the earth. It has nothing to do with 'hull speed'. If your speed log (the paddle wheel thing) is calibrated it should agree with the GPS when there is no current. Waves and other influences on performance will affect SOG as well. So you cannot use a GPS to tell if you are reaching "hull speed". What your GPS was telling you is that something (fairlead, mast, knot meter location, traveler, etc.) is different on each tack.
The 'Theoretical Hull Speed' is for a 'Displacement Hull' and is based on the formula 1.34 (the accepted average for "C")times the square root of the LWL (length of the water line). That is the maximum speed through the water, not over the face of the earth. This theoretical limit can momentarily be exceeded by 'surfing' on the face of a wave, and by planing by some lighter displacement boats reaching under spinnaker. The GPS is measuring your speed over the earth. It has nothing to do with 'hull speed'. If your speed log (the paddle wheel thing) is calibrated it should agree with the GPS when there is no current. Waves and other influences on performance will affect SOG as well. So you cannot use a GPS to tell if you are reaching "hull speed". What your GPS was telling you is that something (fairlead, mast, knot meter location, traveler, etc.) is different on each tack.
theoretical hull speed....
...well boys, we're at it again! Tediously discussing the worn out topic of "theoretical hull speed". There is no more glaring evidence for Phil to create a FAQ section on this website than this subject. Joe... you can't measure theoretical hull speed with a GPS. also... theoretical hull speed is... a theory.... it is not physical law..
...well boys, we're at it again! Tediously discussing the worn out topic of "theoretical hull speed". There is no more glaring evidence for Phil to create a FAQ section on this website than this subject. Joe... you can't measure theoretical hull speed with a GPS. also... theoretical hull speed is... a theory.... it is not physical law..
Hull speed for a given hull is determined by
physical laws. However there are so many variables involved that most of us don't have the means or the mind to determine them all and then do the math. I think that it is enough to say that a boat witha 36 foot water line length will have a hull speed approzimately equal to 6x1.34 or about 8 knots. It may be a little more or a bit less but it really means that you can plan a passage using 9 knots as a predicted speed.
physical laws. However there are so many variables involved that most of us don't have the means or the mind to determine them all and then do the math. I think that it is enough to say that a boat witha 36 foot water line length will have a hull speed approzimately equal to 6x1.34 or about 8 knots. It may be a little more or a bit less but it really means that you can plan a passage using 9 knots as a predicted speed.
Hull Speed ..... one more time.
Hull speed is the speed of WAVES that support the bow and the stern. A boat moving AT or less than hull speed will be supported by (at least) TWO waves - one at the bow and one at the stern .... and the boat will be somewhat LEVEL. If a 'displacement' hull goes faster than the hull speed (the speed of the waves the moving boat generates) the bow wave will shift back under the bow thus raising the bow AND the stern wave will move aft from the boat and no longer be UNDER (supporting) the boat and the stern will squat in the trough. Hull speed is NOT the maximum speed of a boat but the speed of the WAVES that supports the boat and keeps it level. Its not a theory, its a rock solid principle/fact of 'wave mechanics'. If a boat 'moves' it produces WAVES and the hull speed is the maximum speed of the WAVES. It has NOTHING to do with the maximum speed of a BOAT. Indirectly, once a boat moves faster than the 'hull speed' (the speed of the waves), its now 'going uphill' and if it has enough POWER to climb that incline it can certainly go faster than the 'hull speed'.
Hull speed is the speed of WAVES that support the bow and the stern. A boat moving AT or less than hull speed will be supported by (at least) TWO waves - one at the bow and one at the stern .... and the boat will be somewhat LEVEL. If a 'displacement' hull goes faster than the hull speed (the speed of the waves the moving boat generates) the bow wave will shift back under the bow thus raising the bow AND the stern wave will move aft from the boat and no longer be UNDER (supporting) the boat and the stern will squat in the trough. Hull speed is NOT the maximum speed of a boat but the speed of the WAVES that supports the boat and keeps it level. Its not a theory, its a rock solid principle/fact of 'wave mechanics'. If a boat 'moves' it produces WAVES and the hull speed is the maximum speed of the WAVES. It has NOTHING to do with the maximum speed of a BOAT. Indirectly, once a boat moves faster than the 'hull speed' (the speed of the waves), its now 'going uphill' and if it has enough POWER to climb that incline it can certainly go faster than the 'hull speed'.
Waves and Hull Speed
OK hull speed is caused by the stern falling into a hole and the bow climbing a hill. The more power you add the deeper the hole or the higher the hill. Hull speed is in fact the speed of the wave. But if you are sailing in waves and the waves are traveling faster than your hull speed the following wave lifts your stern out of the hole so you can go faster. Often this is referred to as surfing. In flat water conditions a boat with overhangs will increase it's waterline and therefore it's hull speed. Of course some displacement hulls have flat areas that allow the boat to plane under some conditions. My old Chrysler Bucaneer 18 was a displacement boat until the wind got up to about 15 knots and then it would plane. My hull speed is a little over 7 knots and I've exceeded it several times and this is usually the result of "surfing" . Remember hull speed is the result of digging a hole in the water that you can't escape without an extraordinary amount of force. Theories in scientific terms are pretty absolute. When an exception to a theory is discovered that the theory can't explain the theory is changed. Like the theory of gravity..no exceptions have been discovered. Evolution is still called a theory even though it explains life on earth and is the best supported theory in Science. In common usage theory is often equal to a guess but to a scientist it is a different word.
OK hull speed is caused by the stern falling into a hole and the bow climbing a hill. The more power you add the deeper the hole or the higher the hill. Hull speed is in fact the speed of the wave. But if you are sailing in waves and the waves are traveling faster than your hull speed the following wave lifts your stern out of the hole so you can go faster. Often this is referred to as surfing. In flat water conditions a boat with overhangs will increase it's waterline and therefore it's hull speed. Of course some displacement hulls have flat areas that allow the boat to plane under some conditions. My old Chrysler Bucaneer 18 was a displacement boat until the wind got up to about 15 knots and then it would plane. My hull speed is a little over 7 knots and I've exceeded it several times and this is usually the result of "surfing" . Remember hull speed is the result of digging a hole in the water that you can't escape without an extraordinary amount of force. Theories in scientific terms are pretty absolute. When an exception to a theory is discovered that the theory can't explain the theory is changed. Like the theory of gravity..no exceptions have been discovered. Evolution is still called a theory even though it explains life on earth and is the best supported theory in Science. In common usage theory is often equal to a guess but to a scientist it is a different word.
A bit more terminology
A paddlewheel is and has been for some time the term most commonly used by experienced sailors and boaters of all nature to refer to the SENDING UNIT format which I recomended, which may or may not be connected to a knotlog and in many cases is connected to a GPS via NMEA bus.
A paddlewheel is and has been for some time the term most commonly used by experienced sailors and boaters of all nature to refer to the SENDING UNIT format which I recomended, which may or may not be connected to a knotlog and in many cases is connected to a GPS via NMEA bus.
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