HI Alan,
there is probably a justification about type B and C.
Being real boats, they use most of the time genua and not jib, therefore the head sail area is counted up to the mast. (only gap, no overlap considered)
In this case the genua surface is larger then a simple jib as on our models. Under such conditions the CE is shifting ahead.
Cheers
ClaudioD
I hope there is, if what you are saying is correct, it would be nice of those who publish such info to make a notation that the formula is only for specific type of sail plan.
Anyway, out of this little exercise I found “my way” of using pendulum method for finding single sails centres that include the odd shapes of roach & head, foot & luff curves and then using the “A method” equation to find the combined C.E’s for both sail combinations, the later is very helpful for quickly looking at how much you need to change the gap between Jib leech and mast to get the C.E you want to start with.
Cheers Alan
…
Hi Alan,
experimented modelers use the pendulum method, simply because is fast and very precise for the purpose.
The two sail form a single piece retained by two small links cut together, see above.
Personally I print the sail plan on A4 format.
Try both methods, pendulum and F and you will see that there almost no difference.
Cheers
ClaudioD
Forget to post the sail plan with 2 the Jibs, max 80 dm2 and min 73 dm2 will be interesting to find out how much more wind range she can take with the 73.
On this next hull I’m also making the keel fin retractable starting out with 30 mm for lighter air (drag reduction) and planning on using two displacement set-ups, will have my hands full getting this baby tuned in.
Wishing everyone a Great Christmas and Healthy New Year !
Cheers Alan
Hi Alan,
I have seen the Sail Plans.
For the second you may have an interest to raise at 80% the fore stay attachment. In spite of an higher CE you will increase the main efficiency with the “wash up” of the jib. Some readings here : http://www.wb-sails.fi/Portals/209338/news/Ad_aerodynamics/index2.htm
http://www.nz.northsails.com/RADUploads/How-Sails-Work.pdf
http://www.uk.northsails.com/uk/SailBetter/GenoaTrim/GenoaTrimIntroduction/tabid/7043/Default.aspx
http://l-36.com/read_html.php?title=Performance%20Racing%20Trim&file=trim3.html
Merry Christmas
ClaudioD
Thanks for the info Claudio,
As I understand there are many factors that interact with sail plan as you have pointed out with up wash and there are many more, but hey I’m just starting out with this exercise and I wanted to K.I.S.S. To start I’ve narrowed it down to two goals 1) Maximum drive in light air and 2) Minimum heeling in heavy air, what I have I know is a compromise.
It is well documented that higher aspect sails produce greater lift when close hauled. I also believe that high aspect (A/R) sails stall much more easily as the angle of attack widens. As A/R gets higher, sails become less and less efficient drive in anything but close hauled, this is why I chose A/R of 5.2 with 1640 mm mast opposed to maximum of 1750 mm.
In looking at jib I’ve have chosen maximum hoist of 80% of main luff length gives A/R of 6 for 80 dm2 plan which is theoretically the most efficient, this fits my goal for light air.
For the 73 dm2 plan I chose a jib with lower A/R of 5.3 to help try and keep the boat’s head out of the wind and give better control and to bring the centre of effort lower, reducing heeling in higher winds. The efficiency at the top of the main is secondary at this point as I like to firstly find heeling limits.
I’m also concerned about handling problems in higher wind speeds due to the fact that a higher aspect rig exerts a greater turning moment on the boat when it is wind flamed. In other words, the taller sail plan provides a longer lever to turn the boat to windward being well forward of the centre of buoyancy depressing the bow and introduces changes in trim…the numbers M-Class numbers you shared with me was big eye opener !
The only way to find out what works and what doesn’t is to have jibs of various sizes; this is just my starting point.
Cheers Alan