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. Submissions: PaddleWise_at_PaddleWise.net Subscriptions: PaddleWise-request_at_PaddleWise.net Website: http://www.paddlewise.net/ ***************************************************************************Received on Thu Dec 12 2002 - 00:50:27 PST
This archive was generated by hypermail 2.4.0 : Thu Aug 21 2025 - 16:31:01 PDT