Matt Broze wrote: > [Lots of good information about hull speed and wave-making.] ... > Holding a heavy > boat up against gravity means that a lot of energy is being used to do so ... While I commend you on a good description of hull speed and the transition between displacement and planing regimes, I do have a quibble with the above sentence. It takes energy to initially lift the boat against gravity when it starts to plane, but no further work is done against gravity once the boat is at a constant level while planing. In the same way energy is required to lift your boat up onto some sawhorses, but the sawhorses do not expend any energy while holding the boat up at a constant height. The energy required to move a planing boat goes into wave formation, surface friction, air resistance, and some losses to turbulence at the propellor (or paddle) to water interface. Adding up all of these will give the total energy with no additional amount needed to hold the boat up against gravity. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On Wednesday, December 11, 2002, at 10:13 AM, Peter Rathmann wrote: > Matt Broze wrote: >> [Lots of good information about hull speed and wave-making.] ... >> Holding a heavy >> boat up against gravity means that a lot of energy is being used to >> do so ... > > While I commend you on a good description of hull speed and the > transition between displacement and planing regimes, I do have a > quibble > with the above sentence. > It takes energy to initially lift the boat against gravity when it > starts to plane, but no further work is done against gravity once the > boat is at a constant level while planing. In the same way energy is > required to lift your boat up onto some sawhorses, but the sawhorses do > not expend any energy while holding the boat up at a constant height. > The energy required to move a planing boat goes into wave formation, > surface friction, air resistance, and some losses to turbulence at the > propellor (or paddle) to water interface. Adding up all of these will > give the total energy with no additional amount needed to hold the boat > up against gravity. > I agree with you. It does not take any energy to maintain a boat at a constant elevation on the face of a wave. But there is something called "slope drag". This is the force that cause a boat to surf when running with the waves. It does require a force to overcome the desire of the boat to slide backwards down its bow wave. Since this counteracting force is developed by the paddler sticking his paddle in the water and pulling, it does require an expenditure energy to fight gravity and stay on your own bow wake. However, just because there is some energy required to stay on the wave, I don't think it is appropriate to say that at hull speed, you can't go any faster because your are "climbing" over your own wake. The wake is the effect of displacing the water around the hull. This displacement of water adds energy to the water. The energy in the water is dissipated by means of waves. The energy applied to the water increases with the square of your velocity. So the faster you go, the higher the energy you are applying to the water and the harder you have to paddle. Hull speed is when you reach a point on the velocity/energy curve where the energy required to go faster starts to climb steeply and your additional power does not add much additional speed. The fact that you have to use additional power to stay up on your wake is a tertiary effect. The boat moving fast adds energy to the water, this energy makes waves and thus you need to stay on the wave. The primary reason you can't go faster is the kinetic energy you are imparting to the water is more than you can supply by paddling. The slope drag just makes it that much harder. Nick Schade Guillemot Kayaks 824 Thompson St Glastonbury, CT 06033 USA Ph/Fx: (860) 659-8847 http://www.guillemot-kayaks.com/ *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Nick Schade wrote: > The energy applied to the water increases with the square of your > velocity. So the faster you go, the higher the energy you are applying > to the water and the harder you have to paddle. Hull speed is when you > reach a point on the velocity/energy curve where the energy required > to go faster starts to climb steeply and your additional power does > not add much additional speed. Great explanation. The speed power curve is a quadratic function and it'll get vertical very fast. Just plot 5 points on a piece of graph paper (1,1) (2,4) (3,9) (4,16) and (5,25) and you'll be convinced. John *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
On Wednesday, December 11, 2002, at 10:13 AM, Peter Rathmann wrote: > Matt Broze wrote: >> [Lots of good information about hull speed and wave-making.] ... >> Holding a heavy >> boat up against gravity means that a lot of energy is being used to >> do so ... > > While I commend you on a good description of hull speed and the > transition between displacement and planing regimes, I do have a > quibble > with the above sentence. > It takes energy to initially lift the boat against gravity when it > starts to plane, but no further work is done against gravity once the > boat is at a constant level while planing. In the same way energy is > required to lift your boat up onto some sawhorses, but the sawhorses do > not expend any energy while holding the boat up at a constant height. > The energy required to move a planing boat goes into wave formation, > surface friction, air resistance, and some losses to turbulence at the > propellor (or paddle) to water interface. Adding up all of these will > give the total energy with no additional amount needed to hold the boat > up against gravity. > I agree with you. It does not take any energy to maintain a boat at a constant elevation on the face of a wave. But there is something called "slope drag". This is the force that cause a boat to surf when running with the waves. It does require a force to overcome the desire of the boat to slide backwards down its bow wave. Since this counteracting force is developed by the paddler sticking his paddle in the water and pulling, it does require an expenditure energy to fight gravity and stay on your own bow wake. However, just because there is some energy required to stay on the wave, I don't think it is appropriate to say that at hull speed, you can't go any faster because your are "climbing" over your own wake. The wake is the effect of displacing the water around the hull. This displacement of water adds energy to the water. The energy in the water is dissipated by means of waves. The energy applied to the water increases with the square of your velocity. So the faster you go, the higher the energy you are applying to the water and the harder you have to paddle. Hull speed is when you reach a point on the velocity/energy curve where the energy required to go faster starts to climb steeply and your additional power does not add much additional speed. The fact that you have to use additional power to stay up on your wake is a tertiary effect. The boat moving fast adds energy to the water, this energy makes waves and thus you need to stay on the wave. The primary reason you can't go faster is the kinetic energy you are imparting to the water is more than you can supply by paddling. The slope drag just makes it that much harder. Nick Schade Guillemot Kayaks 824 Thompson St Glastonbury, CT 06033 USA Ph/Fx: (860) 659-8847 http://www.guillemot-kayaks.com/ *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Peter Rathmann prathman_at_attbi.com wrote: Matt Broze wrote: > [Lots of good information about hull speed and wave-making.] ... > Holding a heavy > boat up against gravity means that a lot of energy is being used to do so ... >>>>>>While I commend you on a good description of hull speed and the transition between displacement and planing regimes, I do have a quibble with the above sentence. It takes energy to initially lift the boat against gravity when it starts to plane, but no further work is done against gravity once the boat is at a constant level while planing. In the same way energy is required to lift your boat up onto some sawhorses, but the sawhorses do not expend any energy while holding the boat up at a constant height.<<<<<<<< If this is true can we also infer that no energy is being used to keep a plane in the air that is flying straight and level (other than that required to overcome friction-or the sound wave barrier)? To remain at a certain height against gravity an equal and opposite force must counteract the downward acceleration due to gravity. The ground provides this equal an opposite force. So do those sawhorses solidly resting on the earth. The water provides an equal and opposite force for a floating object only once it has displaced enough water to equal the weight of the object. Crawl under the sawhorses and lift the kayak another 6 inches off of them and hold it up there for a few hours. The kayak didn't move once you lifted it. Are you providing any energy to hold it up? Damn right, you had to take over the roll of the sawhorses to provide the equal and opposite force to the acceleration downward due to gravity and you will expend energy to do so just not as much as in lifting it higher. A planning boat is no longer displacing enough water to totally do that job. The energy that powers the boat is providing the difference it takes to hold itself up (the inclined plane--due to the density of water and the speed of the boat) against gravity. >>>>>>The energy required to move a planing boat goes into wave formation, surface friction, air resistance, and some losses to turbulence at the propellor (or paddle) to water interface. Adding up all of these will give the total energy with no additional amount needed to hold the boat up against gravity.<<<<<<<<<< Because the planing boat is not so deep in the water as it was before it was planning the waves are much smaller (water waves are also a manifestation of gravity but I won't go into that here). Certainly friction and air resistance go up with speed although the area of wetted surface goes down with increasing speed so the frictional drag is no longer climbing at near the square of the speed. Certainly there was extra energy used to lift the boat up on to the plane (just like when you first lift the boat off the sawhorses) but you have to keep providing plenty of energy to it to keep it up there. Matt Broze http://www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Matt Broze wrote: > > Peter Rathmann prathman_at_attbi.com wrote: > > Matt Broze wrote: > > [Lots of good information about hull speed and wave-making.] ... > > Holding a heavy > > boat up against gravity means that a lot of energy is being used to do so > ... > > >>>>>>While I commend you on a good description of hull speed and the > transition between displacement and planing regimes, I do have a quibble > with the above sentence. > It takes energy to initially lift the boat against gravity when it > starts to plane, but no further work is done against gravity once the > boat is at a constant level while planing. In the same way energy is > required to lift your boat up onto some sawhorses, but the sawhorses do > not expend any energy while holding the boat up at a constant > height.<<<<<<<< > > If this is true can we also infer that no energy is being used to keep a > plane in the air that is flying straight and level (other than that required > to overcome friction-or the sound wave barrier)? Yes, that is correct. If you calculate the power required for level flight it is just based on the amount needed to overcome the drag forces on the plane (power of level flight = speed times horizontal drag force). There is no extra component for overcoming gravity unless the plane is rising. > To remain at a certain height against gravity an equal and opposite force > must counteract the downward acceleration due to gravity. The ground > provides this equal an opposite force. So do those sawhorses solidly resting > on the earth. The water provides an equal and opposite force for a floating > object only once it has displaced enough water to equal the weight of the > object. All of that is correct, but requiring a force does not in itself constitute a need for energy. The work, or energy, is calculated by the vector dot product of the force and the distance through which it acts. If the force and the movement are in the same direction then the dot product is equivalent to simple multiplication, but if they are in different directions then it gets multiplied by the cosine of the angle between them. In the case of a boat moving horizontally, the force to overcome gravity is acting straight up but the movement is horizontal or at 90 degrees to the force. Therefore the dot product of the force and movement is zero since cosine (90) = 0, and the energy required is also zero. > Crawl under the sawhorses and lift the kayak another 6 inches off of > them and hold it up there for a few hours. The kayak didn't move once you > lifted it. Are you providing any energy to hold it up? I would not be providing any energy to the kayak since I'm acting on it in exactly the same way the sawhorses or the ground did - none of which imparted any energy to the kayak. It's true that my muscles will get tired, but that's because of internal energy losses in my body from pumping blood around, etc. to keep the muscles functioning; it has nothing to do with doing work on anything outside my body. Similarly you can stand and push against a brick wall and get tired doing so, but you are not imparting any energy to the wall. All the energy you are expending is going into internal processes in your body and no external work is performed. Again, the work performed is determined by the vector dot product of the force and the distance through which it moves. You may be pushing on the wall with great force, but if it doesn't move then no work has been done *on it*. > >>>>>>The energy required to move a planing boat goes into wave formation, > surface friction, air resistance, and some losses to turbulence at the > propellor (or paddle) to water interface. Adding up all of these will > give the total energy with no additional amount needed to hold the boat > up against gravity.<<<<<<<<<< > > Because the planing boat is not so deep in the water as it was before it was > planning the waves are much smaller (water waves are also a manifestation of > gravity but I won't go into that here). Certainly friction and air > resistance go up with speed although the area of wetted surface goes down > with increasing speed so the frictional drag is no longer climbing at near > the square of the speed. Yes, there is a point just after the boat gets up on plane where the waves created do get smaller and the boat can be kept on plane at this point even with a reduction in motor power. To initially get up on plane takes considerably more power than to maintain the planing state after it's been achieved. Conversely, if the motor power is kept constant after you first get up on plane then the boat will accelerate to a higher speed until the product of the velocity and horizontal drag forces equal the available net power from the motor. Gravity only had to be overcome when first lifting the boat up on plane. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. 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On Thu, 12 Dec 2002, Peter Rathmann wrote: > > If this is true can we also infer that no energy is being used to keep a > > plane in the air that is flying straight and level (other than that required > > to overcome friction-or the sound wave barrier)? > > Yes, that is correct. If you calculate the power required for level > flight it is just based on the amount needed to overcome the drag forces > on the plane (power of level flight = speed times horizontal drag > force). There is no extra component for overcoming gravity unless the > plane is rising. Does the term "induced drag" mean anything to you? It's the drag that is created as a direct result of producing lift. And you need lift to overcome gravity. It's not drag in the convential sense of friction - that is called "parasite drag" in aerodynamics. Induced drag is small at high airspeeds, but can get very large at low airspeeds, near the stall, much larger than parasite (friction) drag when slow. Induced drag is *precisely* the extra component necessary for overcoming gravity, even in level or descending flight (not free fall). *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Nick Schade wrote (in regards to Peter’s post disagreeing with what I said about planing, energy expenditure, and gravity): >>>>>>>>I agree with you. It does not take any energy to maintain a boat at a constant elevation on the face of a wave. But there is something called "slope drag". This is the force that cause a boat to surf when running with the waves. It does require a force to overcome the desire of the boat to slide backwards down its bow wave. Since this counteracting force is developed by the paddler sticking his paddle in the water and pulling, it does require an expenditure energy to fight gravity and stay on your own bow wake.<<<<<<<<<< Normally Nick and I are in agreement on these things so I was expecting him to back me up here. No such luck this time (or we are looking at this in quite different ways even though we may be saying mush the same thing). Aren’t the first two sentences in the quote above being contradicted by the rest of that paragraph? By the end you seem to have concluded that the boats “desire” to slide back down the wave is due to gravity that must continually be overcome by paddling to stay up some on the slippery water slope (rather than at its bottom). >>>>>>>However, just because there is some energy required to stay on the wave, I don't think it is appropriate to say that at hull speed, you can't go any faster because your are "climbing" over your own wake. The wake is the effect of displacing the water around the hull. This displacement of water adds energy to the water. The energy in the water is dissipated by means of waves. The energy applied to the water increases with the square of your velocity. So the faster you go, the higher the energy you are applying to the water and the harder you have to paddle. Hull speed is when you reach a point on the velocity/energy curve where the energy required to go faster starts to climb steeply and your additional power does not add much additional speed.<<<<<<<<<<< First, I said the boat climbs out of the hole in the water. I specifically did not say that it climbs over its own wake (as this is more in dispute--even though I’m not yet sure it is wrong). What makes the energy needed start to “climb steeply”? “Climb steeply” is a very interesting choice of words here given the context. I think you may have stumbled onto something. I think the reason the energy expenditure gets to the point where it “climbs steeply” (for awhile and then is not required at such a rapidly increasing rate above that speed range—at least for light fast craft) is because this is the point where the boat must climb out of the trough that has formed (because of the water the boat is pushing through is incompressible it must go upwards and gravity is pulling it back down --but the momentum overshoots its former level making a hole, that hole is overfilled by the water filling in the depression but piling up too high again from the momentum of the filling—in other words, waves) and to go faster the boat must climb to a higher level out of that hole in the water (of its own making--either by displacing it or by making a wave trough by pushing against it). You seem to be saying there is a smooth continuum of increasing drag caused by friction and wave making, but that is not at all the case. The frictional resistance curve is a smooth continuum (increasing at the 1.84 power) but the wave making drag curve is kind of wavy because at different speeds below hull speed the waves generated from different parts of the boat interfere with each other and magnify or cancel each other out to some extent. When the waves are canceling each other out the drag is less than when they are magnifying each other. Once hull speed is reached the main waves begin magnifying each other one last time and provide a formidable barrier (probably somewhat akin to the sound barrier) that must be overcome with much extra energy expended during that time until the boat is “up” on a plane. The rate of energy needed to go any faster is still increasing rapidly, but not at the same high rate of increase that was required to climb up that steep slope against gravity to get out of the hole in the water. The water that had previously totally supported it once it sank deep enough. All that energy expenditure is still continuously required to maintain the planing speed (to overcome gravity) and more energy must be added to it to go any faster. It is just the rate of increase of the energy required that has been reduced by planing not the total amount of energy needed. Lastly, I think the energy you impart to the water is ultimately dissipated in turbulence and that waves are just a manifestation of the energy transferred to the water. Waves are very efficient at transporting energy to a distant shore where it finally dissipates into randomness (turbulence/heat). >>>>>>The fact that you have to use additional power to stay up on your wake is a tertiary effect. The boat moving fast adds energy to the water, this energy makes waves and thus you need to stay on the wave. The primary reason you can't go faster is the kinetic energy you are imparting to the water is more than you can supply by paddling. The slope drag just makes it that much harder.<<<<<<< Since a powerboat can get beyond this slope drag and find a shallower slope at higher speeds we know this slope is just a temporary hump that must be overcome to get to the shallower slope beyond. This shows that the wave-making curve is not continuously rising until it goes nearly straight up. Just watch a powerboat picking up speed. At first it is a pure displacement hull and rides level. As speed increases it angles upward at the bow to a higher and higher angle and seems to labor at the steepest angle for awhile (and the engines start to roar). After running at full throttle for awhile the boat slowly rises up and then levels off quite a bit (but not totally since it is still running up that inclined plane of water under its inclined hull in order to stay up there against gravity). Could the tertiary effect you speak of be gravity? Isn’t the slope drag also gravity? Isn’t lifting some water up in order to push its “incompressionableness” aside also doing work against gravity? Maybe we shouldn’t call it wave drag at all since waves are only a manifestation of the (and a means of measuring) energy expended in the fight against gravity. Maybe we should start calling wave-making drag “Gravity Drag*” so we don’t keep thinking it is the waves causing the drag. It’s “The Big G*”. Remember you heard it here first. Matt Broze http://www.marinerkayaks.com <http://www.marinerkayaks.com/> *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. 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On Thursday, December 12, 2002, at 03:53 AM, Matt Broze wrote: > Nick Schade wrote (in regards to Peter’s post disagreeing with what I > said > about planing, energy expenditure, and gravity): > >>>>>>>>> I agree with you. It does not take any energy to maintain a >>>>>>>>> boat at > a > constant elevation on the face of a wave. But there is something called > "slope drag". This is the force that cause a boat to surf when running > with the waves. It does require a force to overcome the desire of the > boat to slide backwards down its bow wave. Since this counteracting > force is developed by the paddler sticking his paddle in the water and > pulling, it does require an expenditure energy to fight gravity and > stay on your own bow wake.<<<<<<<<<< > > Normally Nick and I are in agreement on these things so I was > expecting him > to back me up here. No such luck this time (or we are looking at this > in > quite different ways even though we may be saying mush the same thing). > Aren’t the first two sentences in the quote above being contradicted > by the > rest of that paragraph? By the end you seem to have concluded that the > boats > “desire” to slide back down the wave is due to gravity that must > continually > be overcome by paddling to stay up some on the slippery water slope > (rather > than at its bottom). I've been thinking about this for a while, and still don't have an answer I am completely satisfied with. So I am arguing the best model I can think of with the knowledge it has some deficiencies. If you glue a kayak to a ramp it doesn't take any energy for it to stay there. It does take force. Glue is pretty good stuff so it can maintain that force without expending any energy. The force required can easily be calculated based on the slope of the ramp (we will call this the slope drag). There is no energy transfer anywhere. Now remove the glue so the kayak can slide down the ramp. If you have some handholds, you could probably hold the boat there almost indefinitely. It doesn't really take much energy to hold the kayak in place, but it does take energy to maintain your grip. You will eventually loose your grip and slide down the hill. The force is the same as when the boat was glued to the ramp, it is just the slope drag. The energy expended is because the human body is inefficient at creating forces. If you glued your hands to the hand holds, you could stay there forever. Now pour water down the slope. The kayak will have some frictional drag so it will be harder to hold the kayak in place. The more water you pour, the higher the drag, and as you increase the velocity of the water, the drag will increase. The force to hold the boat against the slope of ramp hasn't changed, but there is a lot more drag to overcome so the energy required is greater. The force now is the drag caused by the water, plus the slope drag. There is now some energy transfer. The boat slows down the falling water, creating waves and frictional heat. Now try paddle up the ramp with water flowing down it. Since you are generating your propulsive force by pushing against a fluid with the resulting inefficiency, you need to apply even more power and use more energy. But the force you are overcoming is still just the drag caused by the moving water plus the slope drag. The slope drag does make the paddler have to work harder, but the boat does not climb so. If you could somehow glue yourself to the slope it would take no energy to stay in place on the wave. The increased work is due to the inefficiency of the means of applying power - a human powered paddle. The only reason you need to use energy to stay on a slope is because the means of generating the force is inefficient. The reason I don't like the climbing-out-of-the-hole model is it implies that the slope drag is the primary source of drag at hull speed. There is an expenditure of energy associated with the fact that the waves create a slope that the boat would like to slide down, but the slope is created by the waves which are a result of the boat pushing the water. It takes a lot of energy to push the water, and not that much to stay on the slope. > > Since a powerboat can get beyond this slope drag and find a shallower > slope > at higher speeds we know this slope is just a temporary hump that must > be > overcome to get to the shallower slope beyond. This shows that the > wave-making curve is not continuously rising until it goes nearly > straight > up. Just watch a powerboat picking up speed. At first it is a pure > displacement hull and rides level. As speed increases it angles upward > at > the bow to a higher and higher angle and seems to labor at the steepest > angle for awhile (and the engines start to roar). After running at full > throttle for awhile the boat slowly rises up and then levels off quite > a bit > (but not totally since it is still running up that inclined plane of > water > under its inclined hull in order to stay up there against gravity). > Could the tertiary effect you speak of be gravity? Isn’t the slope > drag also > gravity? Isn’t lifting some water up in order to push its > “incompressionableness” aside also doing work against gravity? Maybe we > shouldn’t call it wave drag at all since waves are only a > manifestation of > the (and a means of measuring) energy expended in the fight against > gravity. > Maybe we should start calling wave-making drag “Gravity Drag*” so we > don’t > keep thinking it is the waves causing the drag. It’s “The Big G*”. > Remember > you heard it here first. In the transition from displacement mode to planing the elevation of the boat climbs. There is an expenditure of energy to create the change in elevation. Once a planing boat gets up on a plane, it is no long displacing as much water. So the energy applied to the water is less. The boat does not create as big waves because it is not putting as much energy into moving water out of the way. It does still require energy to stay in place because the means of applying force is pretty inefficient. The propellor needs to push water which is not that good a means of generating a force. If the water were to suddenly freeze when the boat gets up on a plane, it would require no additional energy to keep the boat at the higher elevation. But the question is, is the energy required to raise the elevation of the boat from displacement mode to planing mode sufficient to explain the increased energy needed to go faster when at hull speed. I don't think it is. I think the explanation for the increased energy at hull speed is more related to trying to force a clock pendulum to swing at a different rate. A pendulum has a natural frequency that it likes to swing at. At this frequency, it takes almost no effort to make it swing. It is possible to make it swing at a faster rate, but it takes a lot of energy, and it just tends to pull back to the natural frequency. At hull speed the water is moving at its natural frequency. The waves rise up and drop down over the length of the boat when the boat is going the natural velocity of a wave with that wavelength. When you try to go faster, than the natural velocity of the waves the boat produces, it is like trying to force a clock pendulum to swing at a different frequency. It takes a lot of power. You are over driving the pendulum. The natural response is for the waves to just get bigger without actually changing their frequency or speed. Nick Schade Guillemot Kayaks 824 Thompson St Glastonbury, CT 06033 USA Ph/Fx: (860) 659-8847 http://www.guillemot-kayaks.com/ *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
I got the feeling we got about all the theories we need. What I'd very much like would be some graphs, some figures, some statistics, on the force needed to move a displacement hull, to make the transition to a planing hull, and to keep the hull planing. Matt, I've seen your homepage, you've done a lot of testing on hullshapes. Any measurement on planing hulls there? Anyone else? Something on jetskis maybe? Powerboats? Surfboards? *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Nick wrote: <SNIP>>>>>>The reason I don't like the climbing-out-of-the-hole model is it implies that the slope drag is the primary source of drag at hull speed. There is an expenditure of energy associated with the fact that the waves create a slope that the boat would like to slide down, but the slope is created by the waves which are a result of the boat pushing the water. It takes a lot of energy to push the water, and not that much to stay on the slope.<<<<<SNIP> The more energy the boat applies to the water to go faster the higher the crest, the deeper the trough, and the longer the wave-length of the wave the boat is making. The wave barrier is being made larger in because of the process used in trying to climb out of the hole or over the wave. It is a vicious circle. Just climbing up against a force that wants to accelerate you down at 32 ft/sec./sec. for every second of every day takes a hell of a lot of energy. Especially when you have only a very inefficient and low power propulsion system with which to do it. To then also have to push increasing amounts of water up (why water is so hard to push aside/up is also due to gravity "pulling" it down) against that same 32 ft/sec./sec force just so your speed can increases to the point your hull can use the waters density as a ramp to climb up out of the trough also take a hell of a lot of energy. The heavier you , your boat and your load is the harder it will be to do. >>>>>>>>At hull speed the water is moving at its natural frequency. The waves rise up and drop down over the length of the boat when the boat is going the natural velocity of a wave with that wavelength. When you try to go faster, than the natural velocity of the waves the boat produces, it is like trying to force a clock pendulum to swing at a different frequency. It takes a lot of power. You are over driving the pendulum. The natural response is for the waves to just get bigger without actually changing their frequency or speed.<<<<<<<<<< A strange but interesting analogy. Both frequencies are the result of the acceleration due to gravity and the mass of the objects. Since you can't easily get a grip on a wave unlike on a pendulum waves are probably going to propagate at their natural frequency no matter what you do short of overpowering huge masses of water. The water is always in the way of a hull and the more of it you have to lift (displacement weight) and the less time there is to lift it (as your speed increases) the more energy must be applied in a given time to push it aside (and thus displace it upwards against gravity). Gravity is "pulling" things (including the water) back down at a constant 32 ft./sec/sec. and this rate is what is determining the constant speed to wave-length ratio of a wave (speed in knots is equal to 1.34 times the square of the wavelength in feet). Using the right terms you will find the acceleration due to gravity somewhere in the formula used to arrive at the 1.34 etc. Once the wave has gotten so long that the boat is no longer level but on the slope of that lengthened wave. The way to go faster is to take on gravity in a more direct way by climbing out of the hole (in addition to the energy to keep pushing up waves that is also still rapidly increasing as you climb the wave slope). It is the fact that the wave gets longer with increasing speed that eventually leaves the boat with no way to go but up (or some say through the wave with a very narrow hull like a catamaran-but I think there are those who dispute this). Niels Blaauw niels.blaauw_at_wanadoo.nl wrote: Subject: Re: [Paddlewise] Excessive Weather Cocking / Energy, force and work >>>>>>>>I got the feeling we got about all the theories we need. What I'd very much like would be some graphs, some figures, some statistics, on the force needed to move a displacement hull, to make the transition to a planing hull, and to keep the hull planing. Matt, I've seen your homepage, you've done a lot of testing on hullshapes. Any measurement on planing hulls there? Anyone else? Something on jetskis maybe? Powerboats? Surfboards?<<<<<< If you go to the downloads page of our website you can call up (the one ending in XLS or download the ones ending in ZIP) the drag prediction spreadsheet (MS Excel 2000 or 5.0) that I put together to automate John Winters method of trying to mimic my method for calculating drag (used for the other Sea Kayaker magazine drag prediction method) by using formulas and data taken from the graphs (rather than plotting parameters directly on the graphs like I do). Both methods use Gertler's "Reanalysis of the Original Taylor Standard Series" graphs to predict the drag (derived from tank testing of ship models which were varied a little bit at a time in consistent ways about a century ago). Although they use a few different parameters both methods are similar and have been tweaked slightly to bring their predictions closer to Sea Kayaker magazines 1986 towing tank test results for several sea kayaks. I plot from Gertler's graphs and John used a formula and data from Gertler's graphs to try to get the same results. We are usually reasonably consistent out to 4 knots but if they differ significantly it is most likely due to the limitations of the "worm curve formula" in the spreadsheet (by not being wormy enough). The data is calculated out to 8 knots (where possible) so that usually brings them over the wave hump and up to at least semi-planning speeds. You will notice on the drag graph that at no point does it get easier to go faster, only the rate of the increase with speed is reduces (the drag curve starts to shallow in slope not turn down). This is also true for the residual (mostly wave-making) drag component of the total drag. The drag curves are further broken down into frictional drag and residual drag (anything other than what friction would be on a smooth thin flat plate of equal wetted surface-wave-making is by far the biggest component with normal hull shapes). The new Excel 2000 spreadsheets I just uploaded a few days ago also have all the Sea Kayaker magazine results (for the 75 kayaks reviewed so far with this method) archived below in the spreadsheet in a way they can be copied and pasted on to row 9 of the spreadsheet to also look at their results in graphic form. You can enlarge the graph size for more detail by pulling the corners or sides. I'm sure there are some methods that are optimized for predicting drag on planning hulls and John Winters might know which ones to look for or where to find them. Are you out there John? Matt Broze http://www.marinerkayaks.com *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
Matt Broze wrote: (about planing hulls) > You will notice on > the drag graph that at no point does it get easier to go faster, only the > rate of the increase with speed is reduces (the drag curve starts to shallow > in slope not turn down). Surprising! I have to admit I have been in powerboats a couple of times and, in my memory, I was able to throttle down the engine once the boat was planing. That only shows that memory and observation can be deceptive: I have no doubt that Matt has the correct data. About the spreadsheats: I would really like to have a look at them, but since my computer is microsoft-free, I'll have to go to an internet-cafe to do that. I will! Niels. *************************************************************************** PaddleWise Paddling Mailing List - Any opinions or suggestions expressed here are solely those of the writer(s). You must assume the entire responsibility for reliance upon them. All postings copyright the author. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************
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