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Kit Guide

The Science Behind Planing

By 30th September 2020
A lot of people have of course experienced planing before. The feeling is brilliant when a boat’s real potential is unleashed and let rip. We thought it might be interesting to give an engineering view on planing. On a hull there are 3 modes of drag: - Displacement mode - Forced mode - Breakout and planing Displacement mode A hull supports its weight by displacing its own weight of water. This means that when a boat has moved a boat length it has moved its own weight of water which, as expected, makes this mode the slowest. This mode is seen in most boats until they reach their hull speed (normally around 5 knots, see below). Some boats which are quite heavy or too slender always sail in this mode! If you look at a boat in displacement mode, you will notice a bow wave and a stern wave as shown in the diagram below. If you look at the boat from the side (also shown below) you can see that the boat sits in between the 2 wave peaks which act to slow the boat down. In water the speed of a wave depends on the distance between 2 wave peaks. This corresponds to the hull length for displacement sailing. The derivation is quite nasty but it boils down to: For a laser which is 12.5 feet long, the hull speed is 4.67 knots(just before planing speed). For an oppie which is 7 feet long, the hull speed is 3.5 knots. This is a slightly simplified process with a few exceptions but otherwise it’s very accurate! Forced mode This is where the heavy boats begin to stop accelerating and the high performance boats speed up. As soon as the hull speed is exceeded the boat enters forced mode. The boat is trying to break through the bow wave, mentioned earlier, and as a result there is much greater drag on the hull. This can be quite a pain in intermittent conditions where the wind speed is just too low to give the boat enough power to blast through the bow wave. This largely depends on a boat’s power to weight ratio. However if it is windy enough, a simple pump and adjustment to the boat trim can help you to power through the bow wave and into the third mode, planing. A pointy nose (bow) can help getting through this wave. This is why high performance boats tend to have pointer bow than other dinghies. A comparison can also be made between fighter jets which also have a pointy nose to break their wave (at speed of sound in air) whilst slower planes tend to have more rounded noses. Planing Providing the bottom of the hull is correctly shaped, planing is the most efficient and fastest mode of sailing. In planing your stern has overtaken your bow wave which can now be found around the middle of your boat. The bow wave acts to lift the hull out of the water providing there is enough surface area at the rear of the boat. This too is seen in many high performance boats which go from a pointy bow to a flat stern. Other non-planing boats go for a rounded nose and a rounded stern. The faster you go, the greater this lift from the bow wave and the further the boat rises out of the water. This reduces the wetted area and drag hugely allowing the boat to go much faster! 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. However, be wary as too much inclination of this kind can act to slow the boat down. Lastly, here is a video of us sailing in breezy conditions last Sunday! Some planing involved! httpv:// Sean and Tyner

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