How Hydrofoils Work

believe it or not hydrofoils were invented over a century ago with the first patent dating back to 1869. while several inventors were working on hydrofoils in the early 20th century the first successful use of a hydrofoil is credited to alexander bell the same guy who invented the telephone the development started in canada back in 1908 with the first successful prototype ready by 1911 called hd1 which is short for hydrodome 1. the first hydrofoil achieved speeds of 72 kilometers per hour by 1919 the fourth prototype hd4 achieved speeds of 114 kilometers per hour which was more than double of the fastest steam ship of the time that only traveled 48 kilometers per hour hydrofoils are underwater wings which function similar to airplane wings as the speed increases hydrofoils produce lift which equals the weight of the boat which lifts the hull out of the water thus greatly eliminating friction allowing for faster speeds over the past century there was a lot of research into hydrofoils by the soviets and americans for both military and civilian uses examples of this include the soviet-built riketa which is a passenger riverboat with a maximum speed of 70 kilometers per hour and the american uss pegasus capable of achieving 89 kilometers per hour while foil born during more recent years hydrofoils have entered the recreational market for boating the advantage is clear fuel efficiency adding a foil wing to a hull can increase fuel savings by over 35 as is the case with this power catamaran from aquila for surfing hydrofoils mean you can serve endless waves and for wind sports enthusiasts the options are endless ranging from kites to wind wings now whether it's an airplane wing or a hydrofoil there are a handful of scientific explanations on how the wing actually produces lift depending on who you talk to or what article you read you may get a different answer so we searched the web we talked to aerospace engineers and we watched university lectures on fluid mechanics and we finally got to the bottom of how the hydrofoil actually works let's now take a look at three typical explanations of lift the most common goes something like this any wing always has a leading edge let's call it point a and a trailing edge let's call it point b if you look at the upper surface of the wing the distance between points a and b is longer than on the bottom side of the wing because there is a larger distance across the upper surface of the wing in order for fluid to reach the trailing edge at the same time for both the top and bottom parts of the wing fluid traveling along the upper surface has to travel faster than fluid on the bottom side of the wing back in the 18th century daniel bernoulli discovered that if fluid speed increases its pressure decreases this became known as bernoulli's principle since the fluid travels faster on the top side of the hydrofoil following bernoulli's principle it would result in low pressure at the top of the wing and high pressure at the bottom of the wing which would result in lift force a way to visualize this if you grab a piece of paper and blow on it the air over the upper side of the paper will travel faster while the air below is still this creates a low pressure zone on the upper side of the paper and a high pressure zone on the bottom causing the paper to lift while the previous explanation of why wings generate lift is the most widespread it's incomplete at best as it doesn't explain how airplanes can fly upside down or why flat wings can generate lift furthermore countless experiments have shown fluid particles do not arrive at the trailing edge of the wing at the same time as fluid on the top side of the wing reaches the end faster than the fluid at the bottom side of the wing an alternative theory which explains lyft is based on newton's third law which states that for every action there's an equal and opposite reaction this particle kinetics theory states that incoming molecules are deflected downwards by the fluid and consequently the foil feels a force upward unfortunately this theory is also wrong as it doesn't match reality as it fails to account for the interaction of molecules with each other and fails to explain pressure differences on both sides of the wing a third theory relies on venturi effect which states if you constrict the fluid's flow it will move faster based on bernoulli's principle we know that if fluid travels faster it produces low pressure and due to the pressure difference between the top and the bottom it produces lift again this theory doesn't explain how planes can fly upside down or how a flat wing can generate lift as you can see so far each explanation of where lyft comes from fails to explain it in one way or another from our research it seems that the scientists are still debating the extreme specifics of it however they do all seem to agree on the following explanation lift is a reaction force experienced by the wing due to its turning of the flow downwards any object such as the hydrofoil or an airplane's wing traveling through water or air must have some force pushing the fluid out of the way be it engines sail or momentum if more of the fluid is pushed downwards than upwards by that object such as the hydrofoil then the difference is called lift it is important to note that you don't need to worry about pressure difference in order to explain the lift of a hydrofoil you can simply explain lift by saying that the wing turns the flow downwards and appeals to the conservation of momentum you turn the flow down and you get force upwards as fluid receives downward momentum the hydrofoil receives the same amount of upward momentum if you're driving a car and you stick your hand out and turn it up and down you can feel it yourself how turning the flow downwards creates lift fluid flows like a continuous material over the wing meaning that velocity and pressure are affected over a wide area pressure difference can be explained by the curved streamlines that are pointing down as the wing changes the direction of the fluid you can see some streamlines are pointing more downward than others and that is due to the pressure gradient if we look at a single stream line in order to push the fluid downward we must have high pressure on top and low pressure on bottom the tighter the turn you get the larger the pressure difference if we look at streamlines around a hydrofoil wing high pressure below the wing is explained by the streamlines pointing down as that's what the hydrofoil did to the flow the streamline closer to the bottom of the wing will have the highest pressure with each consecutive streamline having lower pressure before the pressure drops to free flow level at the top of the wing the low pressure can be explained by the fact that in order for a hydrofoil to change the direction of the flow downwards it must have lower pressure than free flow pressure with each consecutive streamline above the top side of the wing the pressure will rise up to a free flow state to summarize be it hydrofoil or airplane wing the lift comes from the turning of the flow downwards as the wing introduces curvature of the flow around it high and low pressure areas develop which leads to higher average velocity of the fluid on the top side of the wing due to bernoulli's principle according to research hydrofoils are most efficient when the angle of attack is three to four degrees with a lift to drag ratio of about 20 to 25 to one if the angle of attack is higher than 15 degrees stall can occur depending on the wing design overall compared to airplane wings hydrofoils use a smaller angle of attack due to the water's increased density and viscosity there are two main hydrofoil designs surface piercing hydrofoils and fully submerged hydrofoils surface piercing hydrofoils are v-shaped as a portion of the wing rises above the water while foil borne they are simpler cheaper and less prone for up and down motion as the v-shaped wing rises gradually out of the water the big downside of the surface piercing hydrofoils is that they don't perform well in waves as they can pop out of the water fully submerged hydrofoils are often shaped like an inverted tee and are the most popular within the water sports community the biggest benefit is that they are less prone to the effects of the wave but require a better stabilization system on surfing foil boards this means a rear stabilizer is required and on boats that use fully submerged hydrofoils a variety of sensors computer and flaps are required to keep the hydrofoil system stable so there you have it this is how a hydrofoil lets you glide effortlessly above the water be it on a boat or a surfboard if you enjoyed this video make sure to like and don't forget to subscribe if you'd like to help us out with our video production make sure to check out our patreon page as every little bit helps as always thank you so much for watching and we'll see in the next one [Music] bye

Channel: Our Kite Life

Views: 488,952

Rating: 4.8659663 out of 5

Keywords: hydrofoil, foil, kitesurfing, windsurfing, surfing, bernoulli principle, hydrofoil boat, how wing generate lift, how wing works, bernoulli, kite foil, surf foil, foilboarding, foiling, hydrodynamics

Id: fX-r0COL-xA

Length: 8min 8sec (488 seconds)

Published: Fri Sep 25 2020