In this video Harry joins Reuben to explain why a longer board is faster than a shorter board. Harry has a Masters degree from Plymouth University in Marine Technology and is a Chartered Engineer through the Royal Institute of Naval Architects. So he is the man to tell us why!There are many things that affect the glide of a board including: the tail shape, nose shape, outline, width, rocker, trim, flexibility, fins, weight and length. But length is the biggest factor.In fact the length of a board makes such a big difference on the top speed of the board that if you enter a race and your board is over the length criteria you will be disqualified.At first thoughts it may seem counter intuitive that a longer board is faster as there is more area of the board touching the surface of the water so there should be more drag and you are right there is more viscous drag. But there are a number of other factors at play and one in particular that makes a big difference.The first reason many people will say a longer board is faster is because of its tracking. But this isnt the main reason. Tracking is how much of the energy of each paddle stroke is put into forward motion as opposed to turning the board. And its easy to see how a longer board has better tracking as they are much harder to turn. When you do a step back turn the objective is to reduce the length of the board in the water to decrease the tracking and allow the board to pivot.But this isnt the main reason why a longer board is faster its all down to the different types of resistance.There are a number of different types of resistance which affect a board on the water. The main three are viscous, air resistance and wave making.Viscous resistance is about how much water the board drags along with it and this increases proportionally with length. So a longer board has more viscous resistance than a shorter board.Air resistance is fairly minimal on a paddle board, but becomes very noticeable when paddling into a head wind.Wave making resistance is actually about 70% of the total drag at higher speeds so it has by far the biggest affect.Any object moving through the surface of the water will generate a wave pattern that travels at the speed of the object in the direction the object is travelling.The bigger the wave pattern the greater the resistance. But also there are other factors to consider.The wavelength generated (thats the distance between two peaks of the wave) is the same for any object travelling at the same speed through water. i.e a paddle board and a duck traveling at the same speed will create waves of the same length.As the speed increases the wavelength increases. So the faster you go the longer the wave length. If you can measure that wavelength you can determine the speed.The worst case scenario is when Wave length is longer than the board and the trough of this wave is aligned with the tail of the board.Therefore the optimum or hull speed of the boards is when wavelength = waterline length and for a 10 board this is about 4.2kts and for a 14 board it is about 5kts.Any faster than this the wavelength will be longer than the board and the tail of the board will dip into the trough of the wave. At this point you are essentially paddling up hill up the back of the wave.The worst case is when the tail aligns with the trough of the wave For an 10 board this speed happens at about 4.9knots (A wave length of 4.6m) For a 14 board this happens at about 5.8knots (A wave length of 5.75m)Therefore it will take far less energy to paddle a 14ft long board at 5kts than it will an 116 board and extremely difficult for anything under 10ft to reach 5 knots without additional sources of power.The properties of this wave and the relationship between waterline length and speed can be calculated by something called the Froude Number.Where u = speed, g = gravity and L is waterline length.The greater the displacement of the board (total weight) the bigger the waves will be and therefore the steeper the waves. This means this effect is even more noticeable for heavier riders or if you are carrying a lot of equipment. Also the less efficient your board shape the bigger the waves it will generate.It is possible to go above this speed, but you will probably need more energy than you can produce by paddling. Only the top sprint racers can get the board on the plane but only for a limited amount of time before they run out of energy and any sort of longer distance will see this speed limit enforced.For example on the ICF world champs 200m sprint they had a strong tail wind the whole way and the winner averaged 8.8kts. in the 16k race the winner averaged 5.5kts on a 14ft board remember the speed limit for a 14ft board is 5.8kts.But you can break this limit with some help Additional sources of energy include waves and wind. So surfing or downwinding you will be able to get the board over the crest of the wave and start to plane.Planing is essentially when you are then riding down the front of the wave generated by your board and the board is supported by hydrodynamic lift rather than buoyancy, this happens when the Froude number is above 1.2, but thats an explanation for another video.Also any tide or current in your favour will increase your speed over ground. But you speed over the water will still be limited by wave making resistance. There are some things which can improve the situation. When we talk about the board length we mean the length thats touching the water. So a board with a lot of rocker will have a shorter waterline length than its overall length. And if you havent trimmed your board right you will be reducing the waterline length of the board.There are also methods which can reduce the trough of the wave at the stern and so reduce the wave making drag.This is done on some boards by the use of tail wedges. This wedge at the tail creates lift in the tail which counter acts the trim caused by the wave pattern. I dont have the figures for paddle boards, but on naval ships such as frigates and destroyers they have been proven to reduce the power requirement at their top speed by 6%. (however on ships it usually requires a considerable amount of engineering input and the effectiveness is very sensitive to other aspects such as speed, trim, displacement so the effectiveness of these on iSUPs for this purpose is questionable).Therefore for our challenge to industry is why limit the length of boards to 14ft if not all boards are going to be raced?And, should industry be considering different shaped boards for sprint races where the top athletes are able to get the board on the plane? In surfing the Mini Simmons style surfboard is designed to get on the plane early and have little resistance. Could this work in a sprint race?So to summarise a longer board is faster because of the relationship between the length of the board and the waves it generates. The faster you paddle the longer the wave becomes and when that wavelength becomes longer than the board you essentially start to paddle up hill which stops you from paddling any faster.I hope that you enjoyed this video and if you did then please keep an eye out for more in this series where Harry explains other technical aspects of paddle boarding.The post Why are longer boards faster? / SUP Tech talk appeared first on SUPboarder Magazine.