Sail Balancing / Placement ?

[notjustsomeone]

I'm looking for a little information on the placement of sails in a boat with a keel.
To start I should say that this pertains to questions raised in sailing and continued modification of my dinky 10' boat I made to learn more about boat-building and sailing.
I know that when using a fin (leeboad in my case) the C.E. needed to be placed right above or slightly behind the C.L.R., which produced the desired result of the boat tending to turn into the wind when unattended-I think the term is weather helm?..... Anyway I got a bit tired of the leeboard and tried bolting on a shallow keel which slightly increaced surface area below the water and moved the C.L.R. more towards the rear of the boat. Now the C.E. is slightly foreward of the C.L.R. and generally produces the same results. I'm wanting to continue to mess about with the rig and different sails but I'm not sure what that'll do to handling the boat.
Does anybody know a good source to the explination of this or some general guidelines on C.E. / C.L.R. placement/relationship for boats with a fixed keel?

[JimD]

Google turns up plenty of hits: http://www.answers.com/topic/center-of-effort

http://www.myrc.org/Library/centeref.htm

http://books.google.com/books?id=VLg6Lx5yRP0C&pg=PA49&lpg=PA49&dq=center+of+effort+of+sail+plan&source=web&ots=jwjCMjt46o&sig=8OfFYMUzhoJLk35ljjNfKCU0TpY&hl=en&sa=X&oi=book_result&resnum=7&ct=result

[Ian McColgin]

I think it's Ted Brewer has a good primer on yacht design to answer these and so many other questions.

You want a little weather helm for positive helm feel, safety, and improved lift to counter the leeway you normally make.

The center of lateral resistance moves forward with speed and heel, the latter being more pronounced, which is why there are so many rules of thumb about how much "lead" to give the center of sail area. Fat boats with long low sail plans, like catboats, have the center of sail area effort out over the lee side of the boat and as they heel an incredibly asymetric water line and underwater shape, further exagerating the weather helm.

Since water is so much denser than air, moving the CR has more impact per inch than moving the CE. Especially with small boats, live ballast (crew) is a huge issue. Lots of dinks that can sail just fine are disappointing to their owners because the sailer plants his or her stern on the stern sheets causing the poor boat to drag her butt and have a really hard time tacking or even pointing up.

Check out how the St Lawrence River skiffs are sailed sans rudder, bearing up and off and tacking or gybing entirely by sail trim and crew placement. Somewhat similarly, windsurfers are steered by sail trim in three dimensions - in/out, fore/aft & athwartships.

Incidentally, leeboards have two advantages over keels or centerboards: They gain working surface as you heel, and they can be shaped and angled to provide added lift for each tack. If hinged as well as pivoting, you don't even have to pull the weather board up while short tacking as it will just float along the surface like a broken wing and will set itself when you tack.

On small dinks that also serve as a tender and are easily sailed flat anyway, you can get by just fine with one "sideboard" on the dink's starboard side (since you normally land on the starboard side of the mother ship) that's pinned to pivot enough below the gunnel that it stays in place on either tack. This is also nice when rowing in a wind to keep it easy to row down or off the wind, cut leeway across the wind, and in a really high wind allowing you to tack, as it were, rather than trying to row straight into it.

G'luck

[Peerie Maa]

I'm wanting to continue to mess about with the rig and different sails but I'm not sure what that'll do to handling the boat.
Does anybody know a good source to the explination of this or some general guidelines on C.E. / C.L.R. placement/relationship for boats with a fixed keel?

As your current sail plan works for the boat, all you need to do is to design your new sail plan so that its centre of area is in the same place as your old suit. JimD's links tell you how.

[JimD]

Here's a link to Ted Brewer's on line design primer: http://www.tedbrewer.com/yachtdesign.html

[BrianY]

Also see John Gardner's discussion of balancing a small boat rig in his book "Classic Small Craft You Can Build"

[slidercat]

A nifty compendium of small boat design resources has been compiled by Gavin Atkin:

http://home.clara.net/gmatkin/design.htm