Hi everyone,
I have been told that on a beach cat The centre of effort of the rig needs to be balanced with the centre of lateral resistance of the hulls, if the CE is in front of the CLR the boat will have lee helm, if its the other way round it will have weather helm. Most people like a bit of weather helm, you can tune it a bit with the rake, and of course by changing jib and main sizes. I have have also been told that the center of lateral resistance is the positioninhg of the daggerboards. Is this true? If the center of effort is 3.7m, do you measure from the bows 3.7m for the posistioning of the daggerboards?

The CLR is the point on a hull around which the hull will pivot in the water. With centre boards it is actually an artificialy induced pivot point (by the boards). If the centre of effort of the sails (the combination of all the sails) is in front of the CLR then there will be lee helm, in fact it only has to be a very moderate way in front to induce quite large moments of lee, but due to the resistance of the rudders to sides way movement (the larger the rudders the greater the sidesway resistance) the C/boards and the rudders act together to counter the weather helm as the centreboards are positioned further aft, so there is a quite large degree of latitude in the positioning of the boards aft. The further aft the centre boards and the larger the rudders (within reason) the higher the hulls will go to windward. If the balance is right with sloop rig and then a spinnaker is added the theoretical result would be increased lee helm, but in practice, with most cats today, due to the low resistence to forward movement through the water shape of the bows (fine entry and relatively deep bow), by depressing the bow deeper into the water, this tends to counter most or all of that increased lee helm.

where do you measure the center of lateral resistance from? (e.g bows, the water line at the bows, stern etc.)

Hi
I,ll see if a simpler version is helpfull -

The CLR is basically the side underwater profile area ,-
calculated to its center including underwater hull area board area and rudder area , in a cat times 2 hulls,-

The CE of the sailpan is the center of its area point
on most standard type sails take a point from the tack and clew up diagonally 2 /3 of the way up on the luff and leach ,---where they intersect is close to the CE -
generaly that is somewhat balanced with the CLR to provide slight weather helm ON MULTIHULL DESIGN ,-,-
Other factors are the hull fullness. Fine entry bows TRACK and hold a line to weather more than fuller flatter hulls .Also change in hull area as force and speed increase and hull area submersion changes differently in different hull design dependant of deeper fine entry taller bows that submerge more under sail forces than say a very full type hull bow design that may submerge more evanly along its length as sail forces and speed increase.--that of couse changes the hull area underwater profile differently in the two types of comparative designs ,--
Two existing design examples would be the Inter 20 with flat wide 16 in width hull design by station 3 back as compared to a Tornado type hull with very fine deeper bow hull sections .
Also how ce TO clr ,sail design ,--how full or flat the main is and type of mast section --a non rotating fatter mast will cause more air flow turbulence and along with fuller sail move the effective CE back in real terms much more than it calculated area as compared to a rotating wing mast type section and flat sail ,--as per A class cat design for example ,-

Exactly how a design will balance out in helm is always a matter of fine tuning --mast rake adjustment -rudder rake ,ect .

to calc your CLR -take your designs entire underwater hull area -board area and rudder area ,-diagaram it ,-then calc its center .--Sometimes other existing cat designs and models are the best references to determine board placement on a similar design .

this article might be helpfull
http://www.steamradio.com/JSYD/Articles/NESTalk.html

hope that helps
Carl

Darryl / Carl,

Virtually all designers use the centroid (center of area) as the CE. Since in a beach cat the sails work as arfoils in a laminar flow almost all the time, I imagine that a more precise position of the CE could be calculated using airfoil theory, like when designing a wing.

Likewise, since the CLR position is important mostly when the underwater shape is working as a foil, it is also not exactly in the centroid and could be calculated more or less the same way.

Do you think this is wrong or only too complicated for practical puroses?

Thanks,

Luis and Carl, every one is right, but to keep it simple if the geometrical centre of each sail is taken (for convenience sake this will be accurate enough for the purpose in hand) by scaleing a drawing of the boat and the sail plan, mast etc, and then bisecting the angles at the head, the tack, and the clew, then by projecting a line in towards the centre of each sail, where they bisect will be the approximate centre of effort of each sail, not only the fore and aft but also vertically, if these points are joined between the different sails and a point is taken scaled relative to the percentage difference in area of the different sails, we will then have the combined centre of effort. Project that point down at right angles to the hull and it will then give the fore and aft C of E at the hull.
For the centre of lateral resistence, if the boat has centre boards, then assume that the point of maximum thickness of the board is the centre of lateral resistace. If it is something like a Hobie 16 we found years ago that, although the actual centre of lateral resistance of the Hobie will vary quite dramatically depending on the attitude of the hull in the water (ie if the bows were depressed then the CLR would move forward and if the bow was raised in the water the CLR would move back), by sitting the hulls on the ground without any mast rigging fittings rudders or rudder cases, then the point at which the hulls balanced was as near as damit to the CLR, (not very scientific, but it seemed to work OK). When it comes to measuring the point of CLR on the hulls, it doesn't matter if you measure from the bow or the stern, it is still the centre of lateral resitance! I don't quite understand why you ask that question????

That is unless you want to know where to position your centre board cases on a new design? Then you position the cases, (after you have determined where the centre of effort of the sails is) just behind the centre of effort of the sails.

Thanks guys ,always enjoy design and reading different perspectives on multihull design ,-

On a practical application note ,-one aspect for the update of numerous existing cat designs is the adoptation of new sq top mainsails ,--which moves the CE back ,--and conversly also adding a new spin snuffer system on cat designs not originally designed for them ,--but sail very well with the addition of them .The 20s have been an area of interest towards a Formula 20 rule for N A ,-The interest for next years Tybee/Atlantic 1000 will be as a Formula 18 and NA Formula 20 EVENT !!which will open up numerous possible boat types and combinations of sail plans and board rudder coresponding combinations ,--based largely on the CE to CLR design aspects. Also I very much like the idea of allowing adjustable stays for distance racing ,--as the 60 tris use now,--They seem to be sailing away from the Parlier cat design -http://www.thetransat.com/en/default.aspx
This of course allows adjustment by mast rake for and aft to help achieve a balanced CE TO clr -a balanced easy to control helm so important in high wind and sea conditions on the Atlantic. The adjustable stays also allowing ease of righting if capsized ,-evan possible by one if needed in an emergency .

The larger sq top mains adapted to existing designs in windward mode {no spin up} require the CE move back or rig forward {not always desirable in seas} --the design solution by Bill R on the ARC designs of shared lift ,using a larger rudder and allowing some loading on it seems ideal in getting more area aft in calc to balance the added sq top aft sail area . It allows better control also .

The recent change to the inter 20 larger rudder configuration is indicative of this thinking and allows a little more mast rake which is faster with the spin up ,mainly due to better balance and helm control .

tHE TRICK IS OF COURSE the changing CE with the 270 or 350 sq ft chute up set forward on a pole moving the CE way forward ,--while conversly in windward mode with larger sq top main and small jib moving the CE back --way back -then how to achieve some type of reasonable balance between the two .--The adjustment of stays seems an ideal add on as well along with all other design factors effecting CLR to CE ,mentioned earlier .-board placement rudder area ,hull ,sail plan -aspect ration -reef capabilities etc .
This could be applied to numerous existing designs -
H-20S N-60s -all 20s . for example -A Nacra 6/0 with the Inter rudder system or Arc rudder system would allow it the same inter 20 type sail plan and mast ,plus spin snuffer and most likely be potentially faster in numerous conditions .
An ARC 20 version -based on the ARC 21 -would be a great ocean racing Formula 20 cat design ,-though I,d like to experiment with a canted hull version and think a CF mast is needed ,---that mast weight reduction being one of its main speed features.

We may see a number of Formula 20s for NA with the larger rudder shared lift design feature to help solve the CE to CLR ,-adding a safe easy adjustable stay system may be the next innovation . I recall the 87 World 1000 ,-the Hurricane cat design entered did use adjustable stays ,-nothing new here ,-just time for it to be refined and used more in distance racing .

The design challenges and putting together a wider variety of 20s to compete always increases the development and speed potential as well as seaworthy features of design .
Hope we see this in the seasons ahead and in major race events like the 05 Tybee Atlantic 1000 ,-plan to be there on a NA fORMULA 20 !!with these features and hopefully a good CE to CLR balance for good positive steering -handling and seaworthy craft that allows us to finish well.-again

Carl

Hi Boss,
I see that several sailors interested in boat design have repeated what they read in various yacht design books, classical yacht design and ship design. I would point that there is one area in the CLR calculation where beach cat design differs from classical yacht and ship design.
Ship design and sailing yacht design, displacement mode, take into account in the CLR calculation the vertical projected area of the side of the hull. This is because the side of these ships is a very large underwater area, like 100ft long and 20ft deep for example. This large steel wall underwater can certainly generate much side force. It takes large tug boats at full thrust to spin these ships/yachts around or push them sideways becuase of these large underwater walls along the sides of these vessles.
A beach cat hull differs from this concept of a ship's CLR.
Our beach cat hulls sail, much more so than a ship, on top of the water and have a very round hull cross section shape from water line to water line. Our hulls, those designed for centerboards, slip sideways very fast with boards up because they are poor shapes at generating side force. Just try sailing to windward or on a close reach with boards up. The hulls slip sideways rapidly in an effort to find a speed where the lift from the hulls equals the side force from the sails. This point of sail can be found and the boat will also move forward but slowly because there is also so much drag in this condition. The important point here is that for these round bottom hulls to work at generating side force, they must be moving across the water sideways at a significant sideways speed. Now, put the daggerboard down and this sideways hull speed becomes very very small. Therefore the sideways lift from the hull is very close to zero. So, take this term out of your equation when sizing daggerboards for round bottomed lightweight boats, guys.
In the case of a boardless beach cat hull design like the H16 or P16 you have the significant vertical wall and a sharp corner along its bottom edge. This underwater hull shape is designed to not slip sideways and it is the total underwater lifting body of this sailing system so certainly take it into account in this case.
Good boat designing, guys,
Bill

Hi,
I'm in the process of designing a new 20ft cat and would very much like to know how much aft of the CE you should place the cases (CLR)?
The foils will probably be assymetric. I don't know if this has any impact on this.

Also I wonder if there are any examples of the "shared lift" theory employed on the ARC-21. How does that work more precisely. ( Board placement, areas, rudder sections aso.)

All input appriciated!

Hi Patrik,
The shared lift concept is covered at the Aquarius-Sail web site under ARC21. BTW, the ARC21 can become an ARC20 by moving the transom forward one foot. No other changes; mmm... shorter tillers.
Classical yacht design criteria tells you to put the CLR 3% in front of the CE. This results in a slight weather helm sailing to windward.
Bill

Hi Patrik,
I did not answer your second question about the ARC21 in my previous response so here goes.
"Also I wonder if there are any examples of the "shared lift" theory employed on the ARC-21. How does that work more precisely. ( Board placement, areas, rudder sections aso.)"
The ARC21 employs "shared lift" to a very large degree. The daggerboard is in front of the main beam. At this forward location it is exposed to only 70% of the side force from the sail plan. Therefore the daggerboard is downsized to 70% of the size it would have been if it were located in the classical position, roughly at the shroud chainplate, carrying 97% of the side force from the sails. The area that was removed from the daggerboard has been added to the rudder. The distribution of carrying the side force from the sail has been changed from 97% CB and 3% rudder to 70%CB and 30% rudder. For the CB to carry 97% of the side force from the sail, the CB trunk is located near the shroud chainplate. For the CB to carry only 70% of the side force from the sail, the CB trunk is located just in front of the main beam. The final "center of laterial resistance" from these two systems is the same location along the hull. Sailing on main and jib, the boat doesn't know the difference.
The advantage of having the daggerboard so far forward is that the center of effort from the sail plan "with a full size spinnaker up" will not migrate to infront of the daggerboard and cause leehelm which is additional rudder drag and therefore slows the boat down.
Bill





All input appriciated!


[/quote]

I see a couple of additional advantages:

- The shorter daggerboard reduces the maximum draft, which is good for sailing in shallow waters.

-Moving the daggerboard forward allows a less disturbed flow of water to reach the rudders, increasing their efficiency.

The Catri is another boat which uses the same concept. Interestingly, it was conceived using a totally different engineering background - former Soviet Union technology.

Cheers,

Hi Bill...I have a question on your shared lift concept. Is there a benifit form having a relatively large percentage of the lateral resistance carried by a foil, the rudder, that is operating at a slightly higher angle of attack than a centerboard? Thanks Dan