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 Boat Design Forums Catamaran Balance

#1
06-19-2012, 12:16 AM
 designz Senior Member Join Date: Nov 2007 Rep: 4 Posts: 18 Location: Sydney, Australia
Catamaran Balance

A Question for the Experienced Catamaran Designers.

I am currently researching the best method to find the 'balance' for a sailing catamaran.

1) One method is the find the 'lead' (pronounced leed). This is the method used since modern sailing yacht design began. A typical sailing (monohull) would have a lead of approx 12%. I understand a typical catamaran would have a lead of 2% - 3%. Is this 2-3% lead correct?

2) Does someone know how to develop the correct balance using the dynamic balance of forces method (i.e. drag verses drive)?
#2
07-01-2012, 01:14 AM
 ChrisSR Junior Member Join Date: Jun 2012 Rep: 10 Posts: 7 Location: Tasmania
Hi designz,
I have found with trimarans that the lead between the CoE of the sail plan and the CoE of the Centre board to be 0%.
If I have missed the point please come back to me.
Cheers
ChrisSR
#3
07-01-2012, 03:14 AM
 designz Senior Member Join Date: Nov 2007 Rep: 4 Posts: 18 Location: Sydney, Australia
ChrisSR,

I do not know why you would consider "the lead between the CoE of the sail plan and the CoE of the centre board".

Normally you calculate the COE of the entire under water lateral plane (called the Centre of Laterial Resistance, (CLR) ) and then calculate the "lead" between that and the COE and the CLR.
#4
07-01-2012, 03:59 AM
 warwick Senior Member Join Date: Jan 2012 Rep: 33 Posts: 275 Location: papakura south auckland new zealand
Under trimaran turning radius thread, Gary Baigent describes the center board location for both a catamaran in one of the replies. I think it was his first post.
#5
07-01-2012, 04:41 AM
 Silver Raven Senior Member Join Date: Oct 2011 Rep: 67 Posts: 437 Location: Far North Queensland, Australia
Quote:
 Originally Posted by designz A Question for the Experienced Catamaran Designers. I am currently researching the best method to find the 'balance' for a sailing catamaran. 1) One method is the find the 'lead' (pronounced leed). This is the method used since modern sailing yacht design began. A typical sailing (monohull) would have a lead of approx 12%. I understand a typical catamaran would have a lead of 2% - 3%. Is this 2-3% lead correct? 2) Does someone know how to develop the correct balance using the dynamic balance of forces method (i.e. drag verses drive)?
Why are you "researching the best method" bla bla ???

What does this have to do with "a sailing catamaran" ???

What is the purpose of your question ???

Please keep it simple - some of us are only 'sailors' eh !!!

Ciao, james
#6
07-01-2012, 10:46 PM
 ChrisSR Junior Member Join Date: Jun 2012 Rep: 10 Posts: 7 Location: Tasmania
Hi designz,
With modern light displacement hulls it appears to work to use the CoE of the keel or centre board. Try it both ways with the CLR of the lot and the CoE of the C/B. I guess there isn't a lot in the profile of the hull fore and aft and the rudder is often a spade, so variable in effect, I don't know. Principles of Yacht Design by Lars Larson has some intersting info and has the calulations. I found they worked for me on my 40' tri.
Cheers,
ChrisSR
#7
07-02-2012, 06:08 AM
 yachtie Junior Member Join Date: Mar 2012 Rep: 13 Posts: 24 Location: queensland australia
Hi Des, obviously you are designing your own cat. It is my case any way. The behaviour of the boat will be much dependent on the profile and shape of the hull under water and type and position of the sails. In may case the mast is sitting on the front beam which in turn is positioned in relation to the cabin. The precise position of the centre bard will remain unknown until trying sailing it. I will do that with a temporary centre board that I'll move back and ford until the best balance is obtained.
Only then I'll cut the slots In the hull to finish the job. This is my plan. Good luck with that.
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#8
07-02-2012, 01:35 PM
 Doug Lord Flight Ready Join Date: May 2009 Rep: 1101 Posts: 7,429 Location: Cocoa Beach, Florida
Here are a series of A Class cats( see pdf below) an AC45 and an Extreme 40. Note that the Extreme 40 uses soft main and the AC 45 a solid wing main. You can print them out and calculate the CE and CLR as a basis for a data file. Experience is the best teacher-copying successful designs works for a start. The type of rig-not just its position is important. The performance level of the boat is important as well. The old rules of thumb in Skenes don't have anything to do with this type of boat. You can read this article which explains how the board can go just about any place you want it to: http://www.aquarius-sail.com/catamarans/arc21/index.htm

Pix daggerboard position-AC 45 and Extreme 40:
Attached Thumbnails

Attached Files
 A class rig position compared.pdf (60.8 KB, 136 views)
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#9
07-02-2012, 01:51 PM
 Eric Sponberg Senior Member Join Date: Dec 2001 Rep: 2745 Posts: 1,648 Location: St. Augustine, FL, USA
Designz,

ChrisSR is essentially correct from my standpoint. Over the years, from the advice I have received from other multihull designers and from studying sailing multihulls, it works out that the best place for the center of the sailplan is directly over the center of the keel, daggerboard, or centerboard. That is, lead is 0%. This has just come down from years of practice and trial and error. It is a different rule from monohulls.

Eric
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#10
07-02-2012, 02:42 PM
 Doug Lord Flight Ready Join Date: May 2009 Rep: 1101 Posts: 7,429 Location: Cocoa Beach, Florida
Eric, it seems to me that if you look at a lot of high performance cats that the geometric center of effort is behind the quarter chord point on the daggerboard. Look at the sketches of the Extreme 40(soft sails) and the AC 45 (wing sail) and the A class cats in the pdf above-the boards are in very different positions-but seem all to be forward of the geo CE of the sails. Do you think there might be any merit in using the quarter chord point of the main, or of the rig as a whole as a point of reference(center of pressure?) rather than the geo C of E?
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#11
07-02-2012, 03:32 PM
 Eric Sponberg Senior Member Join Date: Dec 2001 Rep: 2745 Posts: 1,648 Location: St. Augustine, FL, USA
Doug, you certainly could use any reference point you want, just so long as you are consistent from design to design, and don't mix up the criteria when comparing boats. The lightweight high performance cats are a different breed, certainly, and their centers for sail plan and daggerboard may not behave they way you would expect. Even here, in simply looking at these sailplans, we don't know, really, you and I, how these boats perform, and by that I mean by the little nitty gritty feel of the tiller. They could be balanced, or they could be unbalanced. And how much unbalance is OK for you as compared to me? Who knows? The geometric centers lining up is a good starting point--the midpoints of the quarter chords lining up could also be a good starting point--but the centers will shift slightly one way or another based on refinements and study that come only from actually going out and sailing. It is very hard to predict the proper balance ahead of time by numerical calculation because the forces are so sensitive to minute changes in inputs.

Eric
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#12
07-02-2012, 04:03 PM
 warwick Senior Member Join Date: Jan 2012 Rep: 33 Posts: 275 Location: papakura south auckland new zealand
Would another influencing factor be the use of the catamaran.
a high performance cat or a cruising cat, with a more even spread between main and jib. I would think Eric's comment would be suitable for the later set up.
#13
07-02-2012, 05:46 PM
 cavalier mk2 Senior Member Join Date: Mar 2010 Rep: 148 Posts: 1,298 Location: Pacific NW North America
And don't forget the chicken entrails, Ouija boards and tea leaves......
#14
07-02-2012, 07:40 PM
 Red Dwarf Senior Member Join Date: Jun 2012 Rep: 61 Posts: 205 Location: USA California
Quote:
 Originally Posted by Eric Sponberg Doug, you certainly could use any reference point you want, just so long as you are consistent from design to design, and don't mix up the criteria when comparing boats. The lightweight high performance cats are a different breed, certainly, and their centers for sail plan and daggerboard may not behave they way you would expect. Even here, in simply looking at these sailplans, we don't know, really, you and I, how these boats perform, and by that I mean by the little nitty gritty feel of the tiller. They could be balanced, or they could be unbalanced. And how much unbalance is OK for you as compared to me? Who knows? The geometric centers lining up is a good starting point--the midpoints of the quarter chords lining up could also be a good starting point--but the centers will shift slightly one way or another based on refinements and study that come only from actually going out and sailing. It is very hard to predict the proper balance ahead of time by numerical calculation because the forces are so sensitive to minute changes in inputs. Eric
So I take that as saying balance is still a bit of an art. Could anything be learned by building a large RC test boat? I have played with all forms of RC vehicles for over 30 years and I could make a sailboat on a weekend. The problem is I wouldn't know what to test and how to test. Obviously there is no feedback from servos so I don't see how you get a feel for the balance. In a test tank the model is mounted to a balance so all the relevant forces can be measured and plotted for each configuration. With an RC model on a lake I doubt I would notice much if any difference moving the center board.
#15
07-02-2012, 08:22 PM
 Doug Lord Flight Ready Join Date: May 2009 Rep: 1101 Posts: 7,429 Location: Cocoa Beach, Florida
I've built many rc test boats and you can learn a lot-particularly about balance. Moving the board or rig fore and aft will show up in handling. The best way to test is to set up the rudder servo/trans stick so that you are 100% sure it is on the centerline. Don't skimp on servo size or precision. With the rudder centered and the sails properly trimmed for upwind the boat will head up(what you want) or head down(lee helm). Once you see what it does correct for just a slight weather helm then use the servo trim to see if you can neutralize the movement. Make a precise note of where the trim is when it is neutralized then bring the boat in and measure the angle of the rudder. The balance-weather or lee helm- is easy to determine as long as the rudder can be centered exactly. But the rudder angle at small scale is much less precise but it should be a ball park figure. Build the largest model you can.
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