Sail boat design

[Aussie]

OK, so this is to prove a little knowledge is a dangerous thing... (stop cringing :rolleyes: and help out the noob).

What is the principal for balancing the centre of effort of your sails to your centre of gravity (righting moment?) of your hull?

How do you figure out the force on the sails and how do you counterbalance it? Is the idea to calculate a reasonable force acting throug the CE due to wind speed and counteract that with so much bouyancy and displacement?

All sensible advice appreciated, but don't let that stop you :D

[mmd]

This is not a subject that lends itself well to short, succinct answers. I would suggest that you pick up a couple of books and read them diligently. Tops on my list would be Skene's Elements of Yacht Design and any good book on the introduction of ships stability; my well-thumbed favourite is Derrett's Ship's Stability for Masters and Mates. This being said, I will try to provide an answer that is short and succinct, and hope that my colleagues and peers who frequent this place with point out my errors and omissions.

Once you have calculated the area and centre of effort of the sail plan, calculate the approximate force of the wind on the sails. For a ballpark number, use about 1.75 lbs per sq. foot of sail area. Now go to the data that you have either calculated from first principles using the linesplan geometry and inclining experiment data, or the stability data supplied by the designer in the stability booklet and design drawing package. Multiply the force on the sail by the distance from the hull metacentre to the centre of effort to obtain the heeling moment. Calculate or use the cross curves of static stability graph that the designer has prepared to find the angle of heel of the hull that creates an righting moment equal to the magnitude of the heeling moment. This will be the stable angle of heel of the hull for that given area of sail.

This is a "quick & dirty" method of getting this information. To get more accurate results with more confidence in the results, you have to look carefully at sail shape, windspeed, sail permeability, hull & superstructure windage, and a plethora of other minutae. Some have made careers of looking carefully at this problem.

[Wild Dingo]

ALL HAIL THE NOOB!! :D :cool:

Would that all noobs of all sorts would stand tall and pronounce their noobiness :rolleyes:

And seein as someone not mentioning names of course Micheal forgot to say gidday and welcome the the noob from downunder let me be the foist to say "Goodonyerson!!" :D

Well seein as youve now recieved some "sensible advice"... let me wade in with the "insensible advice"... nah behave Shane :D :D

Take it easy
Shane

[mmd]

G'day, eh? You too, Shane. :D

[Wild Dingo]

no worrys me ol cobber! :D

Good to see yet another Aussie step forth as Sir Mike of Fields once said so elequently "we will take over! just doin it a bit slow and steady is all" :D

Hey Michael... so do we call ye a Lumberjack? Mounty? Canuk? Canook? or just plain Canadian?... not meanin anythink here mate just wonderin ;)

Take it easy
Shane

[Ian McColgin]

I think you have a couple of concepts scrambled.

The Metacentric height is essentially the distance between the center of mass for the whole boat and the center of bouyancy when floating. the metacentric is generally represented at the center station in hull plans.

The metacentric is a nice dynamic measure as the center of bouyancy shifts as the boat heals over while the center of mass remains in the same place on the boat. This is why you get an area of inverse stability, where the boat is happier completing its capsize and floating upside down. How big that area is depends on many factors but in general, beam increases the area of inverse stability and a deep heavy keel reduces it.

Some of the round the world racers are so stabile inverted, especially if the keel snaps, that they have an escape hatch under the counter so you don't have to swim to get in and out of the boat while awaiting rescue.

[mmd]

Being true to my regional heritage, I guess one would have to call me a Maritimer, or a Bluenoser, or a herring choker. Call me near anything 'cept late for supper.

[mmd]

From Derrett's "Stability for Masters & Mates":

"The verticals through the centre of buoyancy at two consecutive angles of heel intersect at a point called the metacentre. For angles of heel up to about 15 degrees the vertical through the centre of buoyancy may be considered to cut the centreline at a fixed point called the initial metacentre. The height of the initial metacentre above the keel (KM) depends on a ship's underwater form."

From this it can be interpreted that a.) the metacentric height (KM) is the distance from hull keel or baseline to the metacentre, not the distance from centre of mass (gravity) to centre of buoyancy, which is referred to as GB; and b.) KM varies with immersed hull form and angle of heel, so therefore is a dynamic measurement.

Ian, I am not familiar with a standard form of marking the metacentre on hull plans, I have only commonly seen the metacentre indicated as a curve of locums on a graph of static stability curves in a stability package for a specific hull, or as a labelled point in a graphic diagram to illustrate stability concepts. Could you provide me with a reference to an example that I might be able to have a peek at?

[Ian McColgin]

I was so succinct that I completely misrepresented what I had in mind there. Also, since I'm an avid and self-taught plan reader rather than a properly trained naval architect, I take some pretty amusing liberties.

I wish that the centre of bouyancy and center of mass were indeed marked as that would make deriving the stability curve (an all to unreported item) much easier.

Some fully engineered plans indicate it, along with assumptions about hull construction and rig but it does seem that alot of designing is by guess and by gosh.

What I was thinking of when I so badly miswrote was:

You can get a pretty good guess as to the actual stability curve by marking the centers of bouyancy and mass (after you figure them out) at the center station and just graphicly "tipping" that section. It's only an approxomation that gets worse if the boat has a pinched bow and broad stern that creates a kind of raking widest point of beam as the boat heels.

Wouldn't it be nice if all boats were subject (even subjectable) the kind of roll-over test they gave that nifty new Coast Guard 48'er?

[mmd]

Personally, Ian, I'd be happy if all boats came with a rudimentary stability booklet based on calculated weights & centres, or even just a sheet of cross curves of stability - it'd at least give an indication of the boats characteristics. I was quite happily surprised to see stability curves included in new boat reviews in the British mag "Yachting World". I wish all reviews & promo material for new & custom boats would include such data.

[john welsford]

Hi Aussie, a quickie for small boats is to do the sail area, x the wind pressure (the 1.75 lbs per sq ft will do fine) , x the lever arm ( distance from the waterline to the centre of effort or geometric centre of the sailplan) and you will get a figure in foot pounds.

Take your crew weight, x the distance out fron the C/L and that gives you another figure in foot pounds, for a fat boat x that by 1.25 to allow for the shift on centre of bouyancy as the boat heels, 1.1 for a very skinny boat.
This is near enough in small boats!

PS, you can do it in metric too!

JohnW


Originally posted by Aussie:
OK, so this is to prove a little knowledge is a dangerous thing... (stop cringing :rolleyes: and help out the noob).

What is the principal for balancing the centre of effort of your sails to your centre of gravity (righting moment?) of your hull?

How do you figure out the force on the sails and how do you counterbalance it? Is the idea to calculate a reasonable force acting throug the CE due to wind speed and counteract that with so much bouyancy and displacement?

All sensible advice appreciated, but don't let that stop you :D

[mmd]

John Welsford, that is a very elegant approximation method. What is the maximum boat size that this can be applied to and still stay inside of your your "comfort zone", before you resort to more complicated methods? Also, does this approximation method apply to small keelboats or just centreboarders?

[brian.cunningham]

If you want to experiment around.
Try putting a sail on a canoe.
relatively cheap, and and both the sail and the leaboard are easy to move around.
Less of a knockdown when you go over :D

Setting Up Your Sailing Canoe Rig
http://www.paddlin.com/fivelakes/balrig.html

Finding the Cener of effort
http://www.paddlin.com/fivelakes/center_of_effort.html