It is cheaper and easier to build that way, plus it allows easy fitting of the rudder pintles. Ever tried to allign rudder pintles on a curved catamaran stern, I did !

My point here is of course that such shapes can also be the result of pretty non-spectacular considerations. A while back we covered the reason for the rakes back bows and that too was pretty mondain.

But I have no desire to endlessly discuss any virtual hull theories, they are all simply wrong. The reason for it that the pressure at the stern plane is still very much the ambient airpressure. As a result there is no significant difference whether the hull has a virtual extension or not. Again I refer to aerospike rockets engines to see a real example of how a virtual lengthening should look like. Here the zone between the exhaust jets is a (gass) spike of much increased pressure pushing against the flat between the rocket engines. In effect the rocket base is entlarged with a spike made up from compressed gasses. Theoretically this spike can also be made of metal but this works just as well. Actually it works better as you have less weight in the engine and less trouble preventing the metal from melting. This is a true example of a virtual extension. Water cleanly seperating from a stern is really not. I may have some secondary drag reducing effects but nothing major. It certainly doens't involve Froude'a law much. I leave it at that.




How do you know that they are not ?

Additionally, maybe the dynamic pressure zones around a cat hull are too low to result in significantly lifting or suction forces. Afterall the hulls are very long and slender and water is only accellerated out of the way relatively gently. This can also be seen in Gareths plots. Overall wave-making drag is not a very big factor. If you make plots like that for mono-hulls you'll see the wave-making drag to be alot more dominant. Rememder Miss Nylex crew found a wave-making drag to be only 15 % of the total boat drag, the pressure forces will be accordingly small. Besides on a sail boat you also have a mast and sail trying to push the boat over its bows with a leverage that requires two people to be at the very back of the hull. If anything, the suction downward at the stern is helpfull !

later !

Wouter

Wouter

earlier in this thread you quote drag due to centreboards as 21%, is this the figure an average for upwind and downwind or is it the upwind drag.

Seems to me that this drag must be mainly caused by the leeway a boat going upwind makes so that the boards are slanting through the water, I would have thought that the downwind component induced by boards must be far less than the upwind since the board is travelling parallel to it's centreline(pretty much).

This post intents to show that there exist an optimal hull length for a given displacement. This optimal point is defined in the way of overall drag ; wave-making and wetted surface (c.q. form drag and frictionous drag)

If such an optimimal point is found then the immediate result will be that some boats will be better off with shorter hulls and other with longer hulls when only looking at overall drag.

Do this mind experiment.

Stretch a flat plate to such a length that its volume is equal to the required displacement. Its crossection area is extremely small while its hull length is extremely long and so the wave-making drag (form-drag) will be negligiable. Hull speed laws (Froude's law) will predict very high hull speeds. But the wetted surface area will be inmensely large and so to the wetted surface drag. It will be even be much much larger then the overall drag of say a normall sized catamaran hull of the same displacement but with much less wetted area.

As a counter experiment place the same plate perpendicular to the movement. Now we have an extremely small hull length with a inmensely large crosssectional area. The wave-making drag (form-drag) will now go through the roof while wetted surface drag in the direction of the movement will be infinisemal small. Again the overall drag is much much larger then that of a normal catamaran hull. Try to pull this plate through the water and you'll experience the truth of the last statement.

Clearly both extremes are more draggy then a normal hull, these three points can never be linked up by a straight line, ergo the connecting line is a curve of some shape and will have at least one minimum (drag) value. This point corresponds to a single hull length where the overall hull drag of the boat will the smallest. This will then be the optimimal hull length from an overall drag perspective.

This is also the direct counterproof against the rule : "Longer hulls always have less hull drag (and are faster)"

The same proofs that "shorter hulls always have less drag" is wrong as well.

Simply put the use of "always" is wrong.

For us it will be interesting to find where this optimal hull length is located. Is it inside the 6.5 - 4.5 mtr hull length spectrum of beach cats or out side of this range ?

That is why I asked Gareth to produce the plots for several hull lengths.

It is my opinion that for some bouyancy levels the optimal hull length is to be found inside this normal hull length range. Prime candidates are the 105 kg bouyance (M20, eagle 20, F16) and the 75-100 kg singlehanded bouyancy boats as with A-cats and F16's. Boats being significantly heavier like
that will have optimal hull length laying beyond 6.5 mtr and are therefor always better longer hulled. That is when limiting our hulls to the range 6.5 - 4.5 mtr.


I hope this is clear enough for all

Best of enjoyment on your steps into boat designing science !

Wouter

Stern,




Actually I haven't yet, I've only pointed to the fact that drag graphs are curves and that therefor an optimal point can be found. This directly implies that a longer hull isn't always better from a drag point of view. Of course the same can be said for a shorter hull. The trick for a boat designer, among others, is to find this point and balance it against other considerations.

The true advantages of shorter hulls are in fact :

-1- it is easier to build a lighter hull
-2- it is easier to build a stronger hull
-3- it is easier to build a stiffer hull
-4- the boat feels more responsive to steerage
-5- can be build light, strong and cheap with plain materials.

Actually points 1 to 3 are strongly dependent on length of the hull and the size of the crosssectional area. The fact that the M20 despite Marstrom best (carbon autoclave) efforts can't hold its weight down to 108 kg while homebuiders can made dependable 108 kg F16's from ply shows this in undeniable terms.

There are of course also drawbacks. But others covered those already.





That is not entirely true. The overall drag will ONLY increase when the increase in wave-making drag (absolute sense) is more then the decrease in wetted surface drag. If it does not then the overall drag will actually reduce when making the hulls shorter. But because of the curved nature of these dependencies you will find that reducing hull length beyond at certain hulls length will increase drag again.

Also you can see both things happen at different boat speeds, go take a look at Gareths plots again. You'll see the 5 mtr hull having less overall drag at low and high speeds when the 6 mtr has less overall drag in the mid range of hull speeds.

By now we have enough data to proof that any rule using the word "always" is wrong. Therefor a longer hulls is not *always* faster because it may not always have less overall drag.






I never said it was automatically an optimally solution, that is only the way you intepreted it.






No it is not, you are trying to insert the word *always* again.





No, the Eagle 20 carbon is, same weight as the M20, same hull length but it has a jib as as such it is faster over a significant spectrum of windspeeds. The only exception being the light winds as here the jib is not of much help.

Wouter

This data comes from miss Nylex C-class design article. The text with the table says :

Approximate drag values for the following conditions :

Boat speed - 12 mph
40 degrees from true wind
true wind speed 15 mph
apparent windspeed 25 mph

They have more or less optimized the Miss Nylex cat for these conditions as they found the C-class boats on the C-class course to spend at least 43 % of the time on such a leg. Therefor most gains were to be had with improvements here.

Also remember that Miss Nylex had a solid wing sail and those rigs produce more thrust for a given drag and side force. Therefor you can expect the dominance of the rig to be larger on soft cloth beach cats this means that the drag numbers given for hulls drag etc (precent wise) probably can be considered as upper limits for those found in beach cats.





I expect as much as well.

But also note that boats under spinnaker achieve higher speeds then when going upwind. Wave-making drag becomes increasingly less important relatively to wetted surface area drag with increasing speeds. As such downwind legs can actually favour shorter hulled boats more then upwind legs, meaning that when a shorter hulled boat is found to be favoured on the upwind (which is not a certainty) then it is logical that under spinnaker it will be favoured even more.

I hope this helps.

All very interesting stuff really.

Wouter

What speed range would we want to optimise a beach cat for, As I said in an earlier post at 100kg displacement shorter cats are only better above around 15knots (according to drag prediction from Michlet). My guess is that the average speed of a cat around a race course would be in the 8-12knot range.

Also it should be noted that Michlet does not take induced drag (leeway) into account. And the induced drag effects the hull drag as well as the rudder/centreboard drag. So I would expect a different story going upwind, but I am not sure how to quantify it.



In simple terms you can look up the drag bucket plots for the foil section and estimate it that way.

But in broad terms upwind has higher drag than down wind for lots of reasons.

Gareth

It seems as these arguments tend to forget that form drag consists of two components, and wave drag is one of them. Even submerged submarines and airplanes at lower speeds are subjected to form drag even when no waves are produced. Hence, increasing cross-sectional area increases form drag even when wave drag is (almost) constant.


I have been trying to interpret your arguments that the shorter 16-foot hull should produce less drag than a 19-20 foot boat (that this thread was about).
I cannot agree that introducing the observation that sailboats almost always tend to be at maximum class length, or mentioning 'planing' or 'semi-planing' (either defined as transition between modes or as part of the boat being dynamically lifted), can be brushed away as simplistic absolutes: "always" or "never".
Rather to me it seems that absolete denial of the possibility that beach cats are subjected to some dynamic lift, or that 'semi-planing mode' does not exist, classifies as absolutes.

I am not at all an expert in boat design. Furthermore, the texts that I read tell me that several different mathematical models/algorithms are being employed to model different phenomena of hydrodynamics and boat design.

Hence, I fully agree that we as amateurs should be humble and careful when trying to discuss these matters.

This is a great forum, and the possibility learn is great because people like Wouter try hard to help and explain. We have to live wih disagreements and uncertainties of what is really correct until somebody comes along and brings us hard empirical data.

Stein

Quote [If anything, the suction downward at the stern is helpful!]

To "drag" your transoms on any point of sail is always the slow way. There is always a better compromise. (Besides, "suction", doesn't exist as such, it is a layman’s descriptive term for the effect (in most cases) of a head of air exerting a positive equalising pressure)

Quote [But also note that boats under spinnaker achieve higher speeds then when going upwind. Wave-making drag becomes increasingly less important relatively to wetted surface area drag with increasing speeds. As such downwind legs can actually favour shorter hulled boats more then upwind legs, meaning that when a shorter hulled boat is found to be favoured on the upwind (which is not a certainty) then it is logical that under spinnaker it will be favoured even more.]

Personally I would have thought that the fact that boats going downwind go faster than they do upwind was due more to the fact that they are carrying much more sail (with their spinnaker), rather than any alteration in the effects of any drag inducements or hull lengths. IE drag coefficients become much less important to overall speed than actual sail area/efficiency (assuming that the hulls are relatively efficient to begin with) Or perhaps I missed your point?

so a smaller vessel must absorb a lot more energy to create speed then a larger one ie: sardine=tuna. The sardine is capable of the same speed as the tuna but cannot travel the distance the tuna can when the tuna is not in a can.Peregrine= chicken ,if the chicken is not protected it will probably be hit like a "Pinata".(this is a poor example). thank you ..the Zissou society.

BUT put a spinnker on the sardine then------

You missed my point

Wouter

That is one of the reasons why I don't the identifier "form-drag".

I'm very strict in the wording of definitions if it is ambiquous then I consider it useless.



All hulls have aerodynamic drag as well and we haven't included that one yet either. Gareth mentioned also excluding induced drag (because of the sideways slipping). We can think of a bunch of other factors not included, this still doesn't mean that they are large enough compared to wave-making drag and wetted surface drag to really factor in in the overall picture. Any scientist will have to simplify his models at some level, in this thread we implicetly decided to only look at the wave-making drag and wetted surface drag factors as they are the largest ones.

But still my original statement applies even for these simplified models. If the wetted surface drag decreases by a larger amount then the wave-making drag increase then the overall drag of our (simplified) hull will decrease.

The question we are trying to find out in this thread is whether this is found to happen in the hull length and weight ranges that are associated with beach catamarans. I'm on record stating that I think that for some lightweight craft this may be the case.




I did not really state it that way. I said that I thought that a really lightweight (105 kg) 20 footer may be better off with shorter hulls like for example 5.7 mtr long hull. I later refered to the F16 design to show how a smaller boat with less sail area can still run with the larger F18's by having been made both lighter and shorter. It does so over a wide range of conditions. This was achieved, in my opinion, by both reducing wave making drag and wetted surface drag by equal percentages, in this shortening the hull was a way to achieve this.




I'm not sure whether I did that but I do know that I brush away descriptions like "semi-planing" because they are not well defined and so allow everybody to read something else into it. As such these "definitions" are useless and can even be confusing.




I did not deny that. And I argued that some hull shapes will not have a planing mode no matter how fast they travel. Therefor the definition of semi-planing is increasingly useless as it implicetly assumes that all hulls plane at some speed. Submarines are great counter examples, so two are ships with fully submerged floats where only the skirts penetrate the watersurface.




Correct.



True

Stein

Quote
[I'm not sure whether I did that but I do know that I brush away descriptions like "semi-planing" because they are not well defined and so allow everybody to read something else into it. As such these "definitions" are useless and can even be confusing.]

Please outline your "description/definition" and source of the term planing as applied to waterborne craft.

It would seem that "semi planing” is the more accurate description, as although all/most sailers feel that they know what planing is, that term when applied to boats is, in reality, a misnomer. No vessel under horizontal motion and in contact with the water is ever FULLY planing it is instead in a state of "semi plane". The only thing that varies between what is called, when in motion, "full displacement" and what sailers vaguely interpret incorrectly as "fully planing" is the degree/percentage to which the hull is affected by the "lift" generated by their forward motion. If a boat were “FULLY planing” it would be, as the name implies, flying (as per a ‘plane, aeroplane, aircraft) I fully realise that most people commonly apply the term planing to waterborne craft, but that does not necessarily make it correct, especially if/when precise descriptive analogies are being asserted.
Mind you if the terms aquaplane or hydroplane were used instead the meaning would be a little better defined, but that still doesn’t alter the application of the terminology “semi plane” as being the most accurate.

Quote
[I did not deny that. And I argued that some hull shapes will not have a planing mode no matter how fast they travel. Therefor the definition of semi-planing is increasingly useless as it implicetly assumes that all hulls plane at some speed. Submarines are great counter examples, so two are ships with fully submerged floats where only the skirts penetrate the watersurface.]

EVERY and ALL shape(s) of object/boat WILL plane (by your definition of planing) IF its velocity is adequate (even a submarine). To say otherwise is to deny a fundamental physical truth.

“When "lift" generated due to the velocity of any object when in contact with a fluid is greater than the “weight” of that object, the object will then (aqua) plane”
(Plane by your definition of plane)

There are too many examples of this principle to fully list I.E just a few being. An automobile is not a very hydrodynamic shape yet it still commonly “hydroplanes” across a puddle of water on the road when travelling too fast for the prevailing conditions (same principle). A spacecraft re-entering the earths atmosphere will skip (plane) across the outer earths atmosphere before it’s velocity is reduced enough to safely re enter (same principle). Large bombs were “skipped” across the water to destroy German dams during the second world war, calculated to lose enough of their velocity (and their “lift” due to their velocity) so that they would sink at the base of the dams before they exploded (same principle), etc ad fin idem.

One of, if not the key foundation stones of mechanical engineering have to be precision and accuracy, particularly with definitions and principles, otherwise conclusions are nothing better than subjectivity

Back to the original question...

I have personally seen some very light boats forced to shore because of "fragility" (if that is a real word)thin carbon parts snapping, dismasting, etc....

At this point in time, are the carbon 20's smokin' the T boat?

nope.

w,
Can you enlighten me on how heavy the big 18teens were?
Boats like the last colour 7 before they were banned?

S