I am begining to develop a new patented folding multihull sailing boat here in the UK (see www.fourhulls.com). It is 5m long and 3m wide and weighs around 70Kg, similar dimensions to a Hobie 16, and is designed to go on a roof rack.



It will take me quite some time to get around to making it availbale to the American market and so was wondering if anyone knows of companies that might be interested in making it in the USA under licence.

Wow, sorry not to be able to help you with your real question--you might try Performance Catamarans, though.



How does your creation sail? How much sailing have you and others done with it? This looks like just what I've been looking for: something easy to get into and out of the water and compact!



John

Thanks for the interest.



How does it sail? Thats the big question, I hope to have the first prototype on the water at the end of this month.



Even though it is not designed as a performance boat it should sail very well. The hulls are pure aerofoil sections so it will have very low drag, it is very low weight (~70kg) and has a large righting moment as it is 3m wide.



I will keep you posted as to its performance.

I like the concept, just one question- Why does it fold exactly like how it's likely to "collapse" as soon as the mast is loaded up (ie. all beach cats have high compression loads at the mast base)? It seems if it folded opposite to this then the compression load of the mast would serve to keep the whole structure "open" rather than "closing" it--

But hey, I'm no engineer-



Kirt

What a funky concept to have each hull subdivided into a front hull and an aft hull. Is there a drag advantage or penalty for this? How about making each hull rotatable about a vertical axis? Or a longitudinal axis? That would keep the hulls perpendicular to the water, even with some heel. Maybe the articulated hull has arrived.



David Ho

TheMightyHobie18 1067

It does look that way but the direction it folds in does not make a difference to its strength as the current design is locked when unfolded at the joints by a sleeve wich slides over the internal hinge see attachment . So when unfolded the hinge does not take any load.



The main reason it folds that way is for compactness.



Keep the questions coming as this is still a concept design and someone may spot something I have not!

Theoretically there may be a drag disadvantage, however the hulls are pure aeofiol shapes so they should have lower drag than a normal catamaran hull. Also they should not need a keel which creates drag and may not need a rudder. Also with four hulls there is an opportunity for wave cancellation, which I have not yet investigated.



As for the articulation it is all in my original patent, I have not shown it on any of the pubished concepts as I think it may be a bit much to take in. Though it does mean that the front hulls can pivot to avoid pitchpolling and the rear hulls could possibly be to steer the boat.



All of these will add cost and complexity but I intend to try out all the possibilities over the coming months.

When the choice comes down to those two boats. Please take them as contructive critizism.



Lower drag of the hulls ?



Unlikely. In fact a good case can be made that the wave making drag has been significantly inceased. Reason; by diving up a full length hullin two shorter hulls you have lost displacement. Ergo you need to make the little hulls fatter to get back at the required displacement again. The fattening of the hulls will make the prismatic ratio of the liitle hulls to be considerably greater than that of a full length hull which can be made narrower. Your wave system which is greatly influences drag at higher speeds may well be significantly greater that that of a conventional.



Furthermore; by adding two more bows and stern section your ratio of volume to wetted surface area may also be greater than that of a conventional hull. Wetted surface is very important in determining drag at lower speeds.



The fact that your hulls more closely resemble a "more efficient" NACA shape is something alot of people will take to but NACA never optimized these sections to function best on a transition plane from water to air = the watersurface. The researchers develloped the optimal shapes for an object completely and sufficiently surrounded by a substance in one state = 100 % gas (NACA aerofoil section ?) or een 100 % liquid (at lower speeds where compression of air is not a factor).



In short the NACA were never developped with a wave system in mind and this system is cause to a large portion of the overall drag of a sailing craft.



Not needing a Keel ?



Any sailboat needs a item the limits sideways moved or else the sails can not propel the boat forward on other course than downwind. Now a rounded hull gives some resistance to sideways motion but not alot; hence the requirement of either a daggerboard or a keel/skeg. These parts are adsolutely required for sailing, it is the core of the sailing design. Yes other methodes are possible but I don't see any the proposed design.



Crossing beam on a 3 mtr. platform.



With the righting moment you can develop you are looking at something like 1000 kg's of maststep pressure. Your design features two beams of about 5 mtr. length. When each beam takes 500 kg load in bending your dolphin strikerless beam will look like 0,15 mtr. diameter beam with 2 mm wallthichness at the minimum. And these beams will deflect about 50 to 70 mm in the middle (2 to three inches). Your proposed joints like to weak and the whole platform will be very flexible instead of stiff. The weight of 2 such beam will combined will be 22 to 23 kg's. Compare this to 7,5 kg's for two beams on a conventional 16 footer. That leave only 50 kg's for all the rest the mat will be 10 kg's at least and the sail will be some 4 to 5 kilograms at least. that means that you are left with 35 kg's for four hulls and the remainder of the beams on the side. Personally I don't think you can build 4 hulls and three meore beams (totalling 11 mtr. for 35 kg'.



But than ofcourse I'm only a human and I would love to see a novel design like yours work it is what brings new technology into cats sailing. Very important. Just make sure you wear a helmet when you sail it the first time, okay ?



Wouter



(Naval and Mechanical engineer)

Go Wouter, first you state all the logical and tactical reasons this will not work (I agree with most, OK all your stated facts) when your great close... go for it, but wear a helmet. Wonderful positioned.

Hi Wouter,

Thanks for your concern over the structural integrity, stiffness and drag of my boat, I can assure you that your concerns are unfounded. 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.



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



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



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.



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.



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



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.

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

Wouter,



I don't think I will ever be able to convince you of the merits of my design through this forum.



I hope to be at Texel again next year, perhaps I will take the boat with me and I will let you see it for yourself.



Unlike yourself I am not a naval engineer only a mechanical engineer.



You sound like you are quite an expert, I would be interested to see some of your designs to see how it should be done.



I will keep you posted as to the final production weight.



All the best



Gareth

Grob,



Thing is that others gave me the same work over, pardon my French. I know the feeling well. That is why I started my earlier post with a reference to constructive criticism.



I've been aware of your webpage for a while and have kept an eye on it and I on it to see what you come up with. Coz lets face it, only designers going to extremes come up with really new stuff. And that is exiting.



Personally, I bombed out with a extremely small main beam which was intended to function solely as a compression bar while the dolphinstriker was intended to take all the mast step load. In theory it all fit. Solved the buckling problem and the beam was even under less stres than bigger beam. There was a very remarkable phenomenon at work there. I was convinced it was a closed argumentation and that it would step up to the expectations.



Than I picked up my mast at Saarbergs and discussed beams. Pieter told me albout the A-cat experience in small beams and said in a mnay words : "Small beams never worked, platform lost to much stiffness to work properly; we (A-cat sailors) don't go for small beams anymore rather to larger ones."



I was still convinced of my own design, figuring that a large portion of the stiffness was given by the rigging. Than I sailed my own cat with only the inside bolts of the rearbeam unscrewed by as little as 2 rotations. The boat walked over the waves like nobodies business, instead of sailingover or throught them. She was an absolute dog to sail in waves.



And than I realised exactly what Pieter meant. My stress theory was probably right no arguments on that, but I forgot to figure in stiffness of the platform and grossly overlooked the fact that that is an very important issue in the general behaviour of the boat. My main beam design was likely to be inadeqaute to resist torsion and probably would have all but lead to very a flexible platform which was greatly outweighing the gain made by the 4 kg weight savings.



As you can see, you are not alone in this; I'm also quite sure that the greats like Saarberg travelled much the same route of critisisme, testing, Failure and succes.



On my own deisgns ;



Personally I choose not to design everything myself from scratch but to take what is proven to work and redesign things I know that can be done better. With a lot of help from others I must add. These discussions with others do indeed weed out the bad from the good idea's quickly although one has to swallow ones pride from time to time.



Anyways, good luck in the competittion (and I do really mean that, no crack)



Wouter