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/ ***************************************************************************Received on Thu Dec 12 2002 - 12:20:23 PST
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