>>Well I am sorry to disagree, but from my own calculations based on figures and quite thorough practical testing conducted by C.J.Marjac.

Marchaj and Bethwaite both looked at this and have done many years of research and are both highly respected authors with heaps of proven credit. Yet their views are (apparently) 180 degrees opposite of eachother (according to you). They can't both be right so at least one of them is wrong. Personally I've read Marchaj and I do not recognize that Marchaj wrote anything that would place wave-making drag over wetted surface drag in lightweight beach catamarans. I have the new, updated, 1990 version of his work. I do have some serious misgivings about how Marchaj compares boats. Here he picks information that eventually will suit his own prejudice regarding the fact that no serious improvement in design have been made over the last 100 years. But still I think it is you who is intepreting the results differently.

For lightwind the contradiction is easily proven away. Below theorectical hullspeed (Froude law = 1.54 * sqrt (waterline length), wave making drag is of no serious importance in boats. This is a well established fact in Naval engineering. Hullspeed for 5-6 mtr boats (16 to 20 feet)is about 6 to 7 knots. So when a boat is travelling at 3.3 mtr a second (11 feet/sec) the wavemaking drag is less than a fraction (15 % or so) of the total hull related drag. This means all the way up too medium sailing conditions when sailing upwind and downwind without a spi in 0-10 knots of wind. The reasond for this is because the energy is given back to the hull by the bow wave rising back again before it reaches the sterns. This is so fundamental in Naval engineering that it must be common knowlegde. So my light to medium calcs must be right.

As Froudes law suggest above theoretical hull speed the drag of a hull rises drastically and very quickly (on heavy monohulls) as a result of the wave-making drag increasing drastically. This is a result of the fact that the bowwave system is now laying at the stern or behind it and the hull can't recouperate the energy that is lost in it. Now here our views start diverging I'm sure. I definately belong to the camp where the opinion is that Froudes law doesn't accurately describe what happens to catamarans and skiffs when they pass the theoretical hull speed. The fact that both skiffs and lightweight beachcats reach multiples of their theoretical maximum hullspeed in stable conditions is at least a clear sign that something different from what Froude's law predicts happens. But you can also see it. How big is the wave system thrown up by a A-cat / F16 or even Tornado at 7 knots boatspeed when compared to say a 20 foot monohull day/weekend cruiser ? Then the cruiser stays there with increasing winds while the skiffs and cat accellerate to 15-20 knots with increasing winds. This at least indicated that weight per length has a big role to play as both designs have the same the theoretical hull speed. Arguably the wetted surface of both boats can't explain this sudden divergence at 6 knots, only differences in how the wave-making drag is behaving can explain this. Froude's law accurately described what happens to heavy yachts so the fault can not be found there therefor it must be wrong predicting what happens to truly light weight yachts.

But you ahve not refered to Froude's law and only to your won experiences :

>>many years ago, when it comes to the practical application of drag factors for craft of small dimensions, of which can be included all of the sort of cats that we refer to roughly as "off the beach", wetted surface area produces such a minimal difference in drag between ANY/ALL of these sized cats as to not be of relative consequential variation to be used for the "static" calculations of differences in hull performances.

Than please explain how your results allow for a 14 foot catamaran with less sailarea to outsail larger boats as you have claimed yourself about your own 14 ft designs. This boat has nothing going for it according to you ; less sailarea and less waterline length. If anything this boat should be slower. Still these perform the same. How can that be ? According the theorem of less waterline length it should be slower. Unless the generally accepted waterline length theorem is simply wrong in these cases; as is long known to designers of military fregates and destroyers as well. These do breach theoretical hull speeds by multiples as well and with limited engine power.


>>Whereas there is a huge difference in drag (and therefore potentual performance) between cats with hulls of equal length, weight, and having the same "driving" power applied, but of different "wave making drag" design efficiency or lack there of. The practical "observations" of this are many and plain for all to see.

Boats of same length, weight and driving power can have huge differences in drag ? Now I think 90's A-cats and 00's modern wave-piercer A-cats look very different in their hulls but their performance seem to be within 5% of eachother. Look at Inter 18 hulls and Tiger hulls, very different yet about the same speed during the 90's. Where is the huge difference in drag ? Same applies to pretty much all Formula classes. And what happens when an F16 hull is just a scaled down F18 hull ? Surely non existant differences in hull design can't by used to explain anythig away. Remember we now have a Blade F16 and a Blade F18 designs that are performing the same as other F18's. What explains the equality when the hulls are arguably similar and the 16 has less waterline length and less sailarea ?

The answer can only be weight. You probably say that this reduces wave-making drag considerably while I say that this mainly reduces wetted surface drag. For a long time this divergence was still possible. We only need to find a counter example that disproofs this. I present F16's to F18's. Actually the prismatic ratio of F18's and F16's hulls is identical (strongly linked to wavemaking drag) with the F16's being shorter. Even the rigs are a scaled down version of eachother. An F16 hull is nothing more than a scaled down F18 hull meaning it has exactly the same wedges and pushed water a side just a quickly as an F18 hull at the same speeds. Only the F18 pushes more aside as its hull is wider and goes deeper. Still these boats perform equally with the F16 being noticeably shorter. Which has the higher wave-making drag you think ? This can only result in the situation where the F16 is having less (hull)drag together with less waterline length. A direct contradiction with the common accepted waterline related drag believe which states that more waterline length reduced (wave-making) drag. Now I can see how wetted surface is reduced by making a hull shorter leading to less hull drag of such a hull, but there is no easy way to explain how making a hull shorter will make wave-making drag less as well. There are only two ways out of this situation :

-1- Wetted surface related drag is much more dominant than wave making drag in these designs. So that any increase in wave-making drag is completely dwarfed by the reductions in wetted surface drag. The descreases in one more then compensates for the increases of another.

-2- Wave-making drag is NOT directly linked to only waterline length but to some other ratio like Weight/length (which I used in my other post) or Width/length where reductions in wave-making drag are possible when a hull is shortened.

Combinations of both are possible as well as I used in my earlier post.

So Seriously Darryll, I fail to see how Marchaj or any other person with experiments can disproofs what I've done in my other post. My theory explains what happens in a few very important cases where their theories simple predict a different outcome.

We can still decide we believe one system over another knowingly that it fails in certain key comparison but than we enter the realm of believe systems instead of science.

I hope this explains the background to my earlier post a bit. There is way more but it will take far too long to write all that down.

Best thing I can say is that no-body believed the F16 claims in the beginning. With regard to equality to F18's. Among which the US rating committees. Now we know better. Daniel won his class at Westland cup in 2004 at an F18 rating, Jennifer won a first in first-in-wins-racing in 2004 in a mixed F16/F18 fleet and is also Alter Cup qualifier Area D and Spitfires are all in the top 20 at Texel 2004 with a 104 Texel rating (F18 = 102). All things not possible under commonly accepted waterline length reasoning.

With kind regards,

Wouter