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About this blog

answers and questions about boat aerodynamics

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looking back on importance of aerodynamics

well no questions or comments yet apparently so we shall continue. In the mean time i upgraded my computer system and rechecked some things and I got to a point that i think i can make a better claim about this comment I made earlier.   many people have this impression and i to have this impression but is it true? well to find out we first have to quantify all major drag components of our boat. to do this the famous aussie Ken Warby, holder of the current water speed record at 317.596 mph, gave us this example Mr Warby's calculation from his website of what is suppose the calculation of a boat similar to his boat just under the current record. well from this if we take the ratio of each drag component (combined by source, i.e planning surface, rudder and aerodynamic) over the total drag we find: planning surface 14.0% of total drag rudder surface 51.8% of total drag aerodynamics 34.2% of total drag   So for this case the claim still appears true. now eventhough Mr Warby presents a clean looking calculation when I wanted to do a similar calculation for our hulls i found i needed to change the input a bit. for instance the planning surface area ratio is constant according to this calculation therefore the lift coefficient remains constant and thus the lift increases with change in speed. right you might say? well no in the case of a boat planning at speed lift force can change but in turn this will mean the boat will come up higher out of the water (considering no sudden weight change and no aerodynamic lift). this means the planning surface area changes, thus the lift coefficient changes etc. well to keep things simple there will be a point where lift force equals the lift coefficient belonging to a certain surface area ratio. since the width of a sponson does not change just the wetted length changes and this equilibrium can be found iteratively.   the other thing i changed to do the calculation was the aerodynamic drag coefficient. Ken claims a Cd 0.2 but for a small boat with big round surfaces (tubes, engine etc) this is highly optimistic. from my 1/2model simulation i found a value for Cd of about 0.47. that's about 2.3 times more. the rudder i kept the same expect for filling in the proper values, the result was surprising.   planning surface 31.8% of total drag rudder surface 10.5% of total drag aerodynamics 57.7% of total drag   now in my simulations an improved model gave me a new Cd of around 0.37 and since power is equal to the total drag times the speed this meant the boat needed 10.6% less power to sustain the 100mph speed. in my opinion 10.6% is a great improvement. and suddenly it doesn't seem half bad to play around with aerodynamic improvements although they are still more difficult to solve then for instance gaining 3% less power taken by cutting about 10mm length from your rudder.   right so next time as i promised before a bit more on round shape drag. but i forced myself to go back over the calculations and clean improve there validity to present them better.

Tom v.d Brink

Tom v.d Brink

 

what was found initially

Right so as explained a preliminary simulation was done on a cad modelled outrigger. the outrigger resembled the hull as I had run in 2003 or better explained a model near an eagle SG with just details changed to better suite saw performance. Off course a cfd simulation is always just an approach to reality. the software needs a lot of computer power to run even a simplified model. therefore the model had to be simplified and some things could not be simulated completely. The concessions that were made to suite the cfd software were: -the simulation assumed a model to have a symmetry plane in the longitudinal direction. * As we simulated on the rudder side this means a hull would pretend to have two rudders * The carburettor only had a fuel intake side. * No fuel and water lines were included in the model - the software did not take into account any interaction between a liquid and a gaseous fluid. * chosen was that no water would be simulated only the airflow. * the water surface would be represented by a moving flat surface - no surface roughness was applied. * this means that all boundary layer flow will likely remain laminar for a longer length on the model before attempting separation. it will reduce the model's air resistance but as it is applied to all surface the component drag scale to the overall drag will still be valid. - the model was mainly tested at the target 100mph speed it would have to achieve. only a few lower speeds were run but they shall not be discussed here.   So a lot of things did not yet scale to reality yet. despite that effort was made to enhance and study in more detail the components that were included in the simulation when discussing simulation results the question is always how detailed was the mesh of the model. so great effort was made to detail the mesh round surface and straight edges. the simulation was run in various mesh settings to achieve optimal convergence at the end of the calculation. this was a long and lengthy process but does give the project proper validation.   after several simulation runs some primary conclusions were made to describe the components of the model causing aerodynamic drag. the total component of the aerodynamic drag was made up out of three major components besides the tub. in following order they were the round sponson boom 37% the open engine 21% the sponson 20%   this was with the traditional sponson shape. however another run included a different sponson shape and the boom resistance as well as the sponson resistance increased to 42% and 24% respectively. The amount total aerodynamic drag is as many have already claimed not as high as a potential hydrodynamic drag. but to people who are trying to get the all the speed potential from their boat i would say it is significant enough to notice.     well now that we know what our major players are for aerodynamic drag next up will be a study of these parts individually and their relation to the drag and stability of the boat. starting with the biggest component the booms.    

Tom v.d Brink

Tom v.d Brink

 

blog purpose

Welcome Readers and fellow boat fanatics,   When starting high school I had no idea how interesting this hobby was going to be when a friend brought me to the lake the first time. I started with a little electric that was great fun. what was supposed to be just a hobby has become a big part of my life. I thank a lot to all the very experienced boaters who shared their knowledge so freely on the web. it started as the listbot and has now become many forums which daily have good information for all boaters from beginners level up to the more seasoned boaters. In this blog I plan to give back to the web what it has given me when I needed knowledge and learned (and still learning) from the experience of others. I plan to write some interesting articles about the aerodynamics of our model boats. the topic will probably not suit a beginner level boater that much and I do advise every one who reads this to improve your basic knowledge first before you embark on the aerodynamic path. In the articles I will give evidence to the improvements but also point out the sand traps along the way. but first a bit of what I have done so far:   well in 2000 I started to be really interested in SAW running with my .45 outriggers and since then I looked at a lot of aspects of running a boat at high speeds. in 2004 I had some help from a Dutch company specialized in simulation software who ran some cfd-analysis (computational fluid dynamics) on the cad models I presented them with. It would be an understatement to say that it was just interesting. From what was learned by the simulation I build a boat that ran well but gave additional problems to the list of problems I was already facing. that is why I build another boat taking a step back to the basics. this basic boat helped me learn again to get the simple things right and brought me an official back-to-back pass of 103.57mph. The aerodynamically improved boat still holds my one-way record at 105mph. So the step back has proven to me that it is necessary to get your basics right and then try to get that last bit from the aerodynamics. however it will show that at high speed a little change can make a big difference.   So what can be expected in the blog further? well first of all feel free to ask a question and if I have any knowledge on the subject I will talk about it. And second I will start with the basics as it is also a learning path for me to really write down the plain facts. by writing it down it forces me to think about it harder and understand it a bit better yet again. and finally I hope it can be something that every one enjoys reading.   As a final word of caution I am not a aerodynamics expert. By trade I am a junior engineer working on strength and stress analysis (this is also a topic that can be nice to discus perhaps). My personal interest is fluid mechanics and in that context I will write here.   so next topic will be some results from the first basic study.

Tom v.d Brink

Tom v.d Brink

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