Grob,



>> You describe yourself as a Naval and Mechanical Engineer, I am the latter myself.





Well, in that case we can talk business.



> I can assure you that your concerns are unfounded.



I'm not quite sure how to interpreted this; I understand the words but the basis of the meaning of the phrase eluded me.





You describe yourself as a Naval and Mechanical Engineer, I am the latter myself. Indeed I have 15 years of postgraduate experience, 5 of which in structural analysis of automotive and aeronautical structures, and 3 years as a computational fluid dynamicist. The remainder as a designer. So I am no amateur myself.



(wouter) Noted.





>You start by saying that thinner hulls are faster hulls.



No, I actually didn't. I said that hulls with a lower prismatic ratio will create a smaller wave system. This would lower the contribution of wave making drag in the total amount of drag. I did not say that thinner hulls are faster. I do quite well understand that thinner hulls also have a bigger wetted surface area which is cause for more drag and that faster hulls can only be created by a balancing act between lowering wave drag and not increase the frictional drag to much while doing so. You have rephrased my comment into something that you can easily rebut , but also that I never said to begin with.



My main point made in my last post that I believe that your design may well have overdosed on the shortening of waterline length , placing the hull outside the bathtub curve that is associated with hulls that function on the watersurface. Bathtub curve : To short a waterline means high drag because of a huge wavesystem; to long a waterline means high drag because to much wetted area giving high friction. Optimum is somewhere between those to extreme states. Please comment on this issue and not the false issue that you attributed to me, please.





>>>You will often hear that thinner sections or hulls produce less drag, that is partly true at 0 degrees angle of attack but not between 3 and 7 degrees where a catamaran operates, here it is better to have thicker sections. In fact for low drag catamaran hulls are too long and thin. If you don’t believe me compare the drag coefficients of two identical aerofoils of say 10% and 20% thickness ratio at 5 degrees angle of attack, you may be surprised by the results.





I'm quite sure that you facts are right but my second point in my mail was that the hulls can not be assumed to function on the condition under which a lab test AEROfoil was tested. The fact that a hull needs to operate on the transition level between a liquid and a has make application os aerofoil test very shaky. Also Aerofoils like NACA were very often develloped under the assumption that the foils were long enough to be able to neglect tip effects. Ergo the foils need to be much longer than their cord or width. This holds true to some extend for boards and rudders, which are typically 5 time taller than wide or even more. But this is very much not true not for a hull that is actually alot longer (cord) than it is tall. In effect a hull is nothing more than to tips with a very short winglike section between them. Hence the tip effects will be much, much greater and likely to completely do dominate the characteristics of the hull than.



To give a nautical example that everybody will understand. Modern submarines are very round and have a full torpedo shape. When fully submerged at some minimal distance to the watersurface this resembles a 100 enclosure by water and gives the lowest drag possible. Ever seen a modern submarine "sail" at the surface ? It greates a HUGE wake and is extremely draggy. Now look at the older submarine of the two world wars. These have typically a long and slender hull shape with a very noticable shiplike bow section. These are also far more rectangular in shape compared to the modern rounded hulls. These hulls were reasonable draggy underwater when compared to modern submarines but had alot better surface characteristics. Didn't the old designs what they were doing ? Well, yes they did, they optimized the hulls for surface level sailing as these old subs would be sailing on the ocean most of the time rather than under it. This is my prime example of when a low hydro or aero hullshape would not give cause to lower drag when used on the watersurface or even close to it.



You must be aware of it.





>I agree that having double the number of bows and sterns is detrimental but I hope I have made up for this in my other measures.





(wouter) Okay, no further comment on this.





>I disagree with your implication that an aerofiol shape is not an efficient shape at the transition plane, I have never come across this argument before and would be interested to hear on what evidence you have based this statement.





(wouter) see my earlier example of the submarines. But also I wish to underline that I didn't say that an earofoil shape isn't a efficient shape. It is better than say a cube; although this doesn;t make it top dog of hulldesigns. My point was the use of NACA or other aero foil data to explain why such hull section should be more efficient than most if not all other hull designs. I would say that aerofoil section would be in the uppersection of the low drag spectrum of all possible shapes, but that it isn't in the top part of the spectrum.





>>ter all catamaran rudders use aerofoil sections and they are surface piercing.



And that is the reason why rudders spray up water at speed, loose attached flow well before the section pressure reaches the cooking pressure of water at 20 degrees celsius (overrotating rudders).



All surface effects that are not encoutered when such boards (foil sections) are fully submerged under say 2 mtr of water.



The fact that these sections are so small compared to the overall hull make their contribution to the overall drag rather small. This does not mean that these rudders are at the optimal design. In fact, when possible, designs place rudders under the hulls and perferable at some depth. How many rudders of oceanliners and freighters are surface piercing ?



As a nautical engineer you must surely be aware of this fact.



I would like to underline again that NACA section are not ineffecient sections and often the best we've got but in the case of hull design better shapes have been developped. And rudders work to a greater depth from the surface than hulls do. The last point is very important.





>>I am well aware that hulls need something to limit the sideways movement (by creating lift), my hulls should generate enough lift on their own without the need for any additional keel, daggerboard or skeg. I stand by my statement but may prove myself wrong in the coming months.





(wouter) Okay, no further comment.





>>Now for the easy bit, your assertion that my boat is not stiff enough and is too weak is based on you making some wrong assumptions. You have calculated the deflection of a simply supported aluminium beam of some assumed dimensions. Whereas in reality the boat does have a structural mechanism that works in the same way as a dolphin striker and the cross beams are carbon fibre.





(wouter) Okay, I accept that explanation. With a dolphinstriker the load and stresses situation changes. I didn't see it in the picture of the beam hinges and the other pics so I assumed you wanted the boat to be without.



But I'm sorry to introduce the next point. Buckling, adding a dolphinstriker to your beams will put them under alot of compression as the dolphinstriker will take lion share of the mast step loads and convert it into compression. Your beams are rather long and that is bad for buckling resistance. You will need to arrive at a high stiffness of your beams. But then again you can design with this in mind. I will need to get my calculator out to give a new prediction of the weight of the beams.



Carbon. An interesting aspect. Carbon sounds light but I found that alot of people overestimate the savings of carbon use. In most mechanical engineering carbon gives no more savings in weight than 25 %. Structural limits prevent designs from reaching the theoretical limit of 50 %. A good example are A-cat beams Peiter Saarberg quoted me that I should expect to gain more than about 0,5 kg's when I replace my 3,5 kg alu beam for a carbon beam. This is a saving of less than 15 %. As designers, we know that fibres like carbon are just as strong and stiff as alu and steel (Yep, not stronger and stiffer, just lighter for the same stiffness/strength) but that they dislike pointloads and often need more material to stabilize the elements or reinforce locally than what would be need to take the modelled loads.



We all know that you can't make beams of 1 mm wallthickness. These beams would just be as fragile to denting, punctiuring and buckling of the wall as 1 mm alu (same strenght remember). That is even though 1 mm wallthickness could be enough to take the modelled loads.





>>The mast, hulls and beams are all carbon fibre that is how the weight is kept so low.



(wouter) Okay, no further comment on this apart from my request to mail me the final weight of the boat in its production version.





>>As for the crack about needing to wear a crash helmet when I sail it, you are not the first and will not be the last person to criticize my design, but I have also had plenty of positive feedback too. The design is already a finalist in the International Concept Boat 2002 competition which is judged by a panel of respected naval architects. Winners will be announced in Jan 2003.





(wouter) It was not just a crack , it was very much an advice for caution will all the right intentions. And with respect to critizing your design, that is how better designs get devellopped, I know I've walked the same route. Although I will say that your reply didn't convince me just yet.



With kind regards,



Wouter