sailing dingy design another question

[Daren]

hi all;
i'm now waiting for some books to show up. meanwhile, i'm doing a lot of surfing the web and thinking. if i understand it correctly, the centerline of the centerboard and the geometric centerline of the sail should, theoretically, be in line with the hull's center of lateral resistence. is this assumption anywhere near the mark??
thanks loads;
daren......

[Hwyl]

I'm afraid it's much more complicated than that Daren. What books are you getting? Any should help.

The super technicians will tell you that the Centre of Effort and Centre of Lateral resistance relationship have nothing to do with balance. The pragmatists will tell you that the CE should be ahead (Lead) the CLR by about 10% of the boat length.

[mmd]

Daren, if you were pushing the boat sideways through the water, you would be correct and the boat would be balanced. However, most of us would like to make at least a little headway as well as leeway. ;)

In the simplest of terms, as a hull moves through the water, the water exerts pressure on the hull unequally over the length of the hull. Most of this pressure acts on the bow area. How much pressure and where on the hull depends on the shape of the hull and how fast it is moving through the water. This changes the centre of lateral resistance of the hull (called the CLR) from at the centroid of the hull underwater profile to somewhere a bit farther forward. To compensate for this imbalance, the designer moves the centre of effort of the sail plan (the CE) forward of the at-rest CLR by some amount. The distance between the at-rest CLR and the CE is called the lead (leed, not led). How much lead is necessary is one of the "black arts" of boat design. There are rules of thumb gained from long experience for different hull types and sail plans, but it is not an exact science. Quite often even the best of designers have to tweak the rig or re-cut sails to shift the lead around a bit.

To repeat Hwyl's question, what books are you waiting for?

[Daren]

thanks guys;
to answer your questions, the books i'm waiting for are Classic Small Craft You Can Build and CLassic Small Craft by John Gardner, and, Boatbuilding Manual by R.M. Steward. just fyi, the boat that i'm trying to rig for sail is the Swallow by Andrew Stransky.
daren...

[Don Maurer]

It seems to me, from a purely practical standpoint, it doesn't really matter how much lead or lack thereof you have in a sailing dinghy. The crew weight makes up such a huge proportion of the total mass that shifting the crew forward or aft will have a much greater effect than changing the center of effort. If you have too much lee helm, move the crew forward. If you have too much weather helm, move the crew aft.

[mmd]

There are many niceties of small craft design that one could do without and still be able to sail, such as centreboards or blocks, but with them the sailing experience is so much more convenient and pleasant. A well-balanced rig is one of those items, I think. Besides, not everyone sails with self-propelled portable ballast (crew).

[Bruce Hooke]

Aside from providing some good models to follow, I'm afraid those books are not going to help much with things like understanding the balance between sail area and the underwater area of a boat. You might want to look for a basic book on boat design. I've heard good things about, but never really looked at, a basic book on understanding boat design by Ted Brewer. It's called Understanding Boat Design and it is available from the WoodenBoat store.

Regarding your initial question, the centerboard is just one part of the underbody that factors into where the CLR is actually located.

One of the better ways to decide how much lead you need between the CE and the CLR is to look at other SIMILAR designs that haven proven successful and calculate the lead on those boats. Of course what you may find is quite a range of values and then the art is in figuring out why there is a range and where in that range your boat should fall!

Edited to add: the reason the books you have on order are not going to help that much with understanding CLR and CE (other than as a resource of examples of related designs to look at) is that they are all basically boatbuilding books and design catalogs that start with the assumption that you have a completed design in hand.

[ 04-09-2005, 01:41 PM: Message edited by: Bruce Hooke ]

[Wild Wassa]

If you get a chance to crew a dinghy with a jib, hand hold the jib sheet and move the clew to the windward side of the centre line, this will upset lateral resistance. If the Skipper can hold the boat ... you will laterally move to windward.

Just a little trick for when the wind changes ... and you find yourself not standing the mark. It can save doing a tack then having to tack back on track, as you vector to windward. Playing with the VMG is where it is at.

Warren.

[ 04-09-2005, 02:23 PM: Message edited by: Wild Wassa ]

[Daren]

Classic Small Craft You Can Build was reccomended in my first thread on this topic. it was mentioned that there is a section on balancing small sail rigs. i understood that the books i have ordered are not going to teach me to be a boat designer. as to portable ballast.. i'll be sailing the dingy solo, just for fun. when the little boat is used for it's intended purpose (yatch tender) it will be rowed.
daren.....

[Ian McColgin]

For an empiracle method, try making the board a bit long on the top end so's you can pass a line through it, across the top of the gunnel and secure to something like the seat riser or whatever. Have some different wedges that you can temporarily clamp to the board down maybe half way to the normal waterline so it can be made to hand straight and in line with the keel. You'll have to hold it in place as you get moving but once in motion the pressure will keep the board steady enough long enough to get a really good idea as to just how far abaft the mast it needs to go.

The advantage of some kind of pivot for the final board is that the center can be moved fore and aft as needed for the conditions.

G'luck