April Propwash 2018 web.pub

High Speed Boat Design - Part 4 By Lohring Miller NAMBA Safety Chairman Flow over objects in water (hydrofoils) is exactly the same as flow over objects in air (airfoils). This is true at moderate speeds with two notable exceptions. These exceptions will be discussed extensively below. The other big difference comes from the difference in density between air and water. Because water is 800 times as dense as air at sea level, the forces on an object in the water will be 800 times as large. If your sponson boom was submerged, it would need to be less than .001” in diameter at the same length to have the same drag. An air rudder replacing a water rudder with 1.5 square inches would need 1200 square inches. A ¾” by 2” water rudder would need to be over 21” by 56” in air for the same effectiveness and drag. These examples show why keeping everything out of the water is critical. Conventional airfoil shapes have the best lift and lowest drag in water at low speeds. However, high speeds cause low pressure areas over conventional shapes. Problems caused by this can be reduced with thinner sections and operation at lower lift. However, eventually the pressure will get low enough to cause problems. These problems are cavitation and ventilation. These have different causes but with similar effects on flow over hydrofoils. In both cases a cavity is formed on the low pressure side of the hydrofoil. With ventilation, air is sucked down from the surface over the low pressure side of the airfoil. With cavitation, the low pressure on the low pressure side of the airfoil “boils” the water leaving a water vapor filled cavity. Often both cavitation and ventilation happen together on our hydrofoils. The faster the flow, the larger this cavity gets. On a conventional airfoil shape, cavity formation from either cavitation or ventilation results in a sudden loss of lift. The supercavitating sections always have some separation, but it starts at the trailing edge with base ventilation. Higher speeds or a higher angle of attack result in complete separation over the top (low pressure) side of the hydrofoil. From The NAMBA I.T. Chairman (Continued from page #7) computer what the start buoy looks like. Then when the race clock fires it snaps a picture at 1/64 of a second and then paints a virtual start line to the buoy to show exactly where everyone is at the start and then instantly displays it to the CD. I am excited about this piece of technology because I really believe “starts” are where so much of the conflicts occur. I have also tried to contribute when I can, sponsoring events through the country. I started a program back in 2012 whereby I purchased between $500 and $700 dollars a year in raffle tickets to events I attend. The prizes that I win from those raffles I then donate to other events for their raffles. This has been an interesting exercise in statistical multiplication. This has allowed me to increase my investment by almost five times in the donated value back to the clubs. An example would be where I invest $100 in raffle tickets and win a motor worth $500 to $800 dollars. However, this would never work without the generosity of people who donate the prizes in the first place. I have also been known to be a meal provider at many of the NAMBA Nationals over the last several years as well as a sponsor for multiple classes each year. Here in my own district I am also very active, having built and paid for a state-of-the-art race trailer using solar panels and highly efficient inverters that never require us to charge the batteries, and frequently contributed prizes and class sponsorships. I have been the Commodore for Rocky Mountain Marine Modelers for over 5 years and we have been putting on two to three district races every year. Here in town my wife and I have been involved in several major charities and made donations in the name of NAMBA International, such as the Stepping Stones food drive and donations to a local family whose small child and father were killed in a tragic tractor accident. I am most proud of the Stepping Stones program because this is another one of the special opportunities to multiply the work we do to help even more people. Stepping Stones provides paid employment opportunities to adults with mental disabilities. These beautiful people are paid to manage and do the day to day work of running the food bank that helps feed needy families in our area. The primary goal of Steeping Stones is to help people with special needs achieve living independence. To see the joy in their hearts as they help someone else is an extraordinary thing. Since returning to model boating, I have felt that it was important to focus my knowledge and experience gained from the Corporate and IT world and apply it to our hobby to the best of my abilities. I am truly proud of the work put into the NAMBA Membership Portal, as it has become an extremely useful tool for our organization. While I am proud of my other contributions, I still aspire to one day be District Director and possibly even make a run for VP or President of our wonderful organization. I know that we all have many attributes that can help us contribute to the future growth of our organization. It only requires that we step forward and work to find ways to engage and involved our membership. I challenge our membership to step forward and be a part of the growth moving forward! Happy boating and I am looking forward to seeing you at the pond this year." PROPWASH Page 8 April 2018 Flow over Hydrofoils