Not to mention old timers at the New York Yacht Club. Boats with planing hulls are designed to rise up and glide on top of the water when enough power is supplied. Of course, attention must be given to mathematic calculations to arrive at a satisfactory design but such calculations seldom create an understanding of the nature of the phenomena or lead to innovation. As a result, my methods may appear antiquated, but they are the way I like to do it. Meeting our economic goals requires that we take the lower weight path. Every definition of planing, semi planing and displacement is flawed because every boat has lift from buoyancy and dynamic lift from movement (which can be positive or negative – it actually varies through the length of the hull. Many boarders use … So a 16ft boat would be at hull speed, an 8ft boat would be truly “planing” (it’s ok to use the word descriptively, but not technically) and a 25 footer would be well below hull speed. Hull speed is not a place where the boat goes from “displacement mode” to “planing mode”. For scientific objectivity we can say that their total might be positive or negative. So why doesn’t a light dinghy or powerboat or multihull bog down and why can it beat this barrier. Boats with a planing hull are typically unstable in choppy water. However, their speed is mostly limited to planning on the water surface and not cutting the waves like the popular V-hull boats. If the boat carries a lot of its weight in the mid section … like a heavy commercial boat or older yacht … it sinks down substantially. Of course, a boat is moving so the wave pattern is different but the forces are the same. ... and is said … It cannot interact with the water at all. I finally put this together on a drive with and old school friend, Graham Murray, on a trip to see the Newick designed trimaran he’s been building for a few years. 5 Basic Steps to Trim a Boat. She caught a monster wave. The reason for this extra drag is that the boat now has a trough under most of the hull … it is the also the part of the hull that is supporting the weight. V-shaped hulls, whether shallow or deep, combined with a flatter underwater section aft are signatures of planing hulls. There are many established boat design parameters such as prismatic coefficient, displacement/length ratio, power/weight ratio, center of gravity (in all coordinates), center of floatation as well as many others that are important to designers of boats. … because if it did … the boat would not be touching the water at all … so could not develop “planing lift” from the water. Unlike the crappy normal explanation which is widely repeated with authority. Trim angle of a planing boat will always be that which is necessary to provide the required lift for the weight of the boat and the speed at which it is traveling. Hull technology. It seems to me a boat … as soon as it is moving is developing some degree of lifting forces … either in a positive or negative direction. A semi-displacement hull displaces water at low speeds, but is able to semi-plane at cruising speeds. Even with small waves, most planing hulls will be incredibly bumpy and wet. Boats with planing hulls can skim along at high speed, riding almost on top of the water rather than pushing it aside. Someone experienced in observing these wave patterns might infer the angle of deadrise just from looking at the waves generated by the boat. There is also a generality about what sorts of boats can get past hull speed more easily. Well Graham did everything actually. It takes less time for the boat to move a given distance at higher speed and so the work done in supporting the boat per unit of distance must be less at high speed than at lower speed. Such a formula is only useful in thinking about a narrow range of boat shapes. While I generally ignore the concept of “hull speed” as being irrelevant to planing boat design, it is still a fact that all boats do start out from rest as displacement craft before accelerating to their operating speed and hull speed may be important in getting onto plane in the first place. Above a certain speed, hydrodynamic forces lift the forward portion of the hull out of the water. Be Careful not to remove or destory this number. Planing lift + water displaced = weight of boat, gear and crew. An example drawing of this exit angle is provided in the spreadsheet along within the output calculations. Most powerboats look like flat-bottom boats but use a shallow V-shape that helps the boat to handle better at higher speeds. Running at high trim angle allows the water to first hit the boat bottom where it is flatter and results in uncomfortable pounding. One major goal of my design was to have a boat that planed at low speed so that cruising could be pleasant and economical in the desired speed range of 10 to 20 MPH. Trim angle also affects the comfort level in the boat. But here we have a wave preventing the boat from sinking down and she can greatly exceed her normal speed. Higher displacement is detrimental to economy of operation is a planing boat, so that consideration is wedded to all other design decisions. This is about 20ft. In addition to the above factors, Bluejackets have high lift surfaces in the form of wide chine flats added under the stern. The aft buttocks are straight (monohedron) from the transom to about station 6 and rise forward to a sharp entry with deadrise of 26 degrees at a point 25% aft of the waterline entry. When added together they equal the weight of the boat and crew etc. As long as nothing moves in the boat, the CG remains fixed while the CB moves (usually aft) as well as being combined with dynamic lift to give a new center of vertical force. However, in order to reach speeds in excess of maximum displacement speed, a vessel must have the appropriate aft quarter butt exit angle. Trim angle of planing boats. This translates to the need for only a low trim angle when getting on plane and less power to do so. This has proven to satisfy all the design goals and handles a reasonable amount of chop and waves with ease. I used to slip and unslip that boat years later – it was pretty cool! We should not care a fig what it is called anyway, but rather expend our efforts in improving the planing ability of a boat intended for that service. The boat is also inclined by the crew so that the bow is higher than the stern during planing which also acts to lift the boat out of the water; much like when a stone is skimmed across a pond. For instance, at one end of the hull shape spectrum, the 16’ Hobie Cats and other multihulls can go fast enough to be considered planing by the speed/length definition, but they are achieving practically no lift from dynamic forces and are not even capable of planing on their knife blade hulls. They are generally too heavy for their bottom area to allow easy transition from displacement speed to planing speed. I do not intend that this discussion should diminish the importance of any of these or other mathematical operations and my only purpose here is to aid the understanding of the physical forces that allow a boat to plane or allow one boat to plane better than another. And just to show that the categorisation of multihulls into non planing is also arbitrary and incorrect … I offer this picture … if you see a smooth hollow behind ANY boat … the force that pushes the water down behind the boat equals the force that pushes the hull up. It is a spiral that has only been overcome by the availability of high power engines and cheap gas. Such boats are only happy at very low or very high speed and are dinosaurs to many designers, including me. That is a deliberate omission. Immersed volume of  the hull + dynamic lift. That term, “dynamic lift”, is all-important and, to be called planing, a boat should be getting a significant portion of its lifting force from dynamic sources in addition to the buoyancy force. They require considerably more horsepower to get the boat up but they can attain much higher speeds from the reduced friction of moving on top of the water rather than through the water. I have never gotten into computer design programs. Some planing involved! In light trim and ideal conditions with only one aboard, the Bluejacket 24 will actually clear the transom and appear to start to plane at about 9 1/2 mph. This is the dynamic force vector of planing, also normal to the plate surface. A planing hull, however, rides on top of the water. To repeat, deep V means greater weight which is unacceptable to our goals. These are often used for fishing boats on small bodies of water. Another negative to high trim angle is that forward vision is restricted over the bow. The generation of these waves by a planing boat represents work done in supporting the weight of the boat. And can it be explained without breaking monohulls and dinghies into two groups. These boats may operate like displacement hulls when at rest or at slow speeds but climb toward the surface of the water as they move faster. That leeward hull is getting significant lifting forces. Boats that run at this angle will usually have a far higher trim while getting onto plane. They are consistent terms right through the speed range. But “planing” is something that people say “suddenly appears”. . Also avoided is any hint of longitudinally convexity in the aft planing bottom as a performance robber. What about when a truly displacement boat surfs on a big ocean wave. The center of longitudinal gravity (CG) and the center of longitudinal buoyancy (CB) will always coincide on a boat at rest. Just be sure not to raise the engine so much while planing that it begins to cavitate. Dynamic lift is proportional to both speed and trim angle. It occurs when planning across smooth water. The flat hull offers amazing stability in calm waters. The reality is that there is a bow wave and then the second wave will be the right distance behind the first according to the equation by Froude. But what we have is different boats that can move through the hull speed more or less easily. For other than purely displacement or semi-displacement boats, hull speed is only of marginal interest. Behind a flat bottom hull, these standing waves follow along directly behind the boat. In my opinion some boats have dangerously limited forward vision. Handles Poorly. The ocean wave cancels out the trough that would normally be under the hull. In smooth water lake boats, like classic runabouts, the negative properties of bow section concavity are not always apparent since such boats are for pleasure use only and not required to operate in poor water conditions. These chine flats have a trim angle that is greater than the rest of the aft bottom surface which provides extra lift aft and tends to hold the bow down and the stern up. Planing definition at Dictionary.com, a free online dictionary with pronunciation, synonyms and translation. In reality, the water is doing no such thing. Whereas that of the displacement vessel is typically round bilge to minimise the wetted surface drag as its slim hull slices through the water virtually ignoring wave drag. Here we will go to Issac Newton’s fundamental principles of motion and try to think like a water molecule. An accurate painting by artist Alexander Creswell makes the point. As the boat goes faster the second wave after the bow wave moves further back. Like. Lesser values of bottom loading is better in this regard and explains why the great majority of commercially available powerboats expend so much power and fuel in getting onto plane. With less of the hull contacting the water the vessel becomes laterally unstable, which might seem like a bad thing, but this instability allows for the hull to pivot along its length and managed to stabilize turning at high speeds.