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Bro-ofasailer 03-16-2003 05:43 PM

Easy Question
 
Okay sorry, but why is it that a slight change in material of a sail can affect the overall speed?

Jeff_H 03-17-2003 02:51 AM

Easy Question
 
This is not an easy question but it comes down to two main factors stretch and weight. Even the best sail making materials stretch. When they do the sail cloth alters shape. The more wind the more stretch. Minimizing stretch means that there will be less of a sail shape changed between the gusts (where the sail gets fuller and more powerful which is the opposite of what is desireable) and the lulls when the sail becomes flatter (which is also the opposite of what is desireable). Both over powering and blading out a sail can have major speed robbing impact.

Modern sail cloths are made so that they are stronger and less stretchy in one direction than the other and modern sail construction orients the fabric so that the strong dirction is aimed in a specific direction in order to minimize stretch.

The other problem is weight. If you did not have to sail in light air, sails could be built so heavy that they had negligable stretch, but a sail needs enough wind to blow it into a proper flying shape. In a moderate breeze most sails will assume the intended flying shape but as the wind gets lighter, gravity causes the sail to hang in a flat, non aerodynamic shape. The heavier the sail, the higher the windspeed that it takes to blow the sail into a proper flying shape.

You could make sails very light if you are willing to do frequent sail changes with the changes in windspeed, but for most of us we want a sail that can function in a wide range of winds and so the sailmaker tries to select fabrics and orientations that provide a proper balance between minimizing stretch and weight.

Jeff

paulk 03-18-2003 06:14 PM

Easy Question
 
Besides the weight of sailcloth affecting the strength of the sail, the weight can also affect the sailing qualities of the boat. A boat using lighter sails heels less than a boat with heavier sails. A sail develops more effective force when it is straight up and down - not heeled over. The weight of sails, especially on large boats- can be significant. Masts are made as light as possible for the same reason.

Jeff_H 03-19-2003 02:52 AM

Easy Question
 
You''re right, the weight of sail can significantly raise the boat''s center of gravity, one reason that instrumented full sized cruising boats often have significantly less stability than a simple stactic analysis might imply. As you note this means less stability, both initial and ultimate. It also means more rolling through a wider angle and with that the likelihood of an excitation capsize. Increased rolling also hurts performance by disturbing the airflow across the sail.

Jeff

jcmannone 03-19-2003 05:34 AM

Easy Question
 
What about the effect of different materials on air friction (all other factors the same)? And, will water spray affect the sailing characteristics?

I imagine air friction can reduce your speed by a few knots and may be important for racing.

Thanks
John

Jeff_H 03-19-2003 02:10 PM

Easy Question
 
While masts, sails and rigging offer significant drag in the form of wind resistance, I really have not ever seen a discussion on the impact of differing friction coeficients for different sail cloths on performance. While I am not certain of this, I would suspect that the surface drag of sail cloth has a minimal effect on performance, expecially on smaller sailing vessels.

Jeff

sailingbauble 03-25-2003 08:24 AM

Easy Question
 
Are you saying that, all things being equal, shortening sail just enough to minimize heel should result in equal or greater boat speed?

Sailmc 03-25-2003 08:58 AM

Easy Question
 
If you are talking about excess heel that produces excess weather helm then yes your statement is true. It takes most sailors years to learn that simple fact.

Jeff_H 03-25-2003 02:47 PM

Easy Question
 
Actually more accurately, as wind builds, on almost all boats there is a point where the boat becomes over powered. At that point, the drag of overcoming heel induced weather helm and the reduced efficiency of a heeled sail, results in a drop in speed. Initially, reducing the power of the sails will allow a greater speed. This does not mean shortening sail. It does mean flattening the sails and reducing the angle of attack.

Of course at some point in the wind range, on most boats it is necessary to reduce sail and depending on the boat this can result in an increase in speed.

More modern designs prefer to be sailed with smaller angles of heel, and especially reaching and beating can have really significant gains in speed when properly powered up or down. More traditional designs tend to build gobs of weather helm when overpowered but are less effected speed wise.

Jeff

flicker 07-17-2003 03:28 PM

Easy Question
 
Steve Dashew writes in Practical Seamanship that his 62-foot boat was clipping along at 9.5-10.2 knots in a strong breeze until his wife requested to reef in the main for comfort''s sake and boat speed only dropped 0.3 knot.


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