Sail, Mast Questions

I have a few questions regarding sails and their shapes.

The flat panel sails that are stocked on the SeaWind that I sail have a curvature built into the luff. From what I have found, this is to add fullnees to the otherwise flat shape of the sail. So that leads me to my question. Do seamed paneled sails also have this curvature to the luff? And if so, what is the purpose for it? Still added fullness?

The standard mast setup that most of the boats in my club use is to keep the mast as straight as possible and to bend the mast only in stronger winds to easily get more twist in the upper part of the sail. So here is another question. For seamed panel sails do you have to introduce mast bend in order for the sails to set correctly? Somewhat linked to the question above.

Thanks in advance for your comments.

Tom
Seawind #80

I’m going to out on a limb and take a shot at this. If I’m wrong will someone please correct me and then I will learn something too. Luff curve will add fullness to the sail but it adds more fullness to the middle of the sail. (This is assuming an even curve, I’ve never tried more curve at the top and bottom and flat in the middle(note to self)).Broadseaming can add varying shape from top to bottom. ie sails that are flatter at the bottom and fuller at the top. I believe that if a sail is broadseamed that the luff curve is mostly used to make up for unavoidable mast bend. That said-luff curve on a broadseamed sail will add even more fullness on a straight mast. I found that for trying to understand all this stuff that if you sit down with some old printer paper , scissors and some scotch tape you can try putting together some sails. Try broadseaming, using a very slight curve on the edge of one piece of paper(maybe 1/16" in 8")and tape it edge to edge to a straight piece. You will very soon learn to appreciate how accurately this curve has to be cut and taped to make the difference between a 5% and a 10% chord. Cut a luff curve in a piece of paper and then tape it to a straight line on a piece of wood (or the counter-top if your wife isn’t watching) You can see the effect. Something to play with when the wind isn’t blowing and the new boat is finished. To answer you question- No,if the sail was cut for a straight mast. How you determine if the sail was cut for a straight mast is beyond me. It seems very strange to me but trying to get information about sailmaking is like pulling teeth. Hope this is all correct and of some help.

Thanks
Don
Vancouver Island

Tom -

The amount of “luff curve” found at the mast side of the main sail allows the mast to be bent (usually using backstay tension) which in turn allows the sail to be made “flatter” - reducing the “power” in the sails for heavy wind. The fuller the sail (usually provided by the amount of camber in the individual sail panels when assembled) in turn provides “power” to punch through waves.

Often flat sails are referred to as “speed sails” while full sails are referred to a “torque” or “power” sails. Thus, you can have a fairly full, cambered sail, but by bending your mast using backstay, you can "flatten the camber by pulling the forward part of the sail forward to depower. Very basic explanation.

Paneled sails do not need to have the mast bent for sails to sit properly - but that is dependant on the sailmaker and how he has cut and laid out the panels. There are some who like some pre-bend to the mast in order for sails to sit well. Something to discuss with the person who made the sails - as to what they were thinking. Aalso the stiffness of the mast you are using will influence the luff cut too. Sail adjustments consist of a variety of different adjustments.

You have downhaul, outhaul, vang tension, mast bend, sheeting location and in some cases, mast rotation, spreaders and diamond wires, and shrouds. If you look to the simple, very basic one-design like a laser - out haul (end of boom) down haul (front of boom) vang (under boom controlling how high the boom rises) and the main sheet.

Don,

You are mostly right, which is to say you are a little wrong…

Panneled sails will still have luff curvature. You can look at it as either being an artifact of the design (in other words if you built a molded sail on a perfectly shaped mold with a perfectly straight luff and then took it off the mold and layed it flat on the table there would be a curve to the luff) or you can look at it as part of the design (your small offsets in the seams coupled with some luff curvature give you the proper shape). In reality, it does not matter where you think of it as coming from - it is there.

Most sail makers of any repute, will add an allowance to their sail shape to account for mast bend. So if you normally sail your boat with 1/4" of bend in your 60 inch mast, the sail maker will add this 1/4" of extra curvature to the sails he cuts for you in addition to the curvature he has designed into the sail otherwise.

Tom, you are right that the curvature gives you shape - especially in the middle as Don noted (see Don, you were right about that…). But the amount of shape you can add this way is somewhat limited (by the bias of the cloth) and more importantly, the characteristic of the shape is virtually uncontrollable. In other words by adding shape in that way, you cannot control the draft position with any precision. You will also find it difficult to control the draft and twist independently (with a flat sail, adding twist will take out draft and vice versa whereas with a panneled sail you can add twist and shape at the same time when you cut the pannels).

So a good panneled sail will be cut to match your nominal mast bend and give you good camber and twist for that mast bend. Tuning that sail for other conditions involves adding mast bend as well as outhaul and luff tension. Adding a bit more bend than nominal will “pull out” some of the shape especially in the middle of the sail. It will also add a bit of twist to the sail. So even with a well cut sail, you do not always want your mast perfectly straight. But the amount of mast bend required to make effective sail shape changes tends to be a bit smaller with a well cut panneled sail than a flat sail.

Don,

As far as learning about well cut sails, there have been some excellent articles over the years in the AMYA magazing “Model Yachting”. In the most recent issue (the Victoria issue) there was an excellent article by Rod Carr where he showed the effect of adding a little bit of mast bend to change the camber and twist of the sails. There have been several other great articles in recent years as well…

  • Will

Will Gorgen

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by wgorgen

Don,

You are mostly right, which is to say you are a little wrong…

Panneled sails will still have luff curvature. You can look at it as either being an artifact of the design (in other words if you built a molded sail on a perfectly shaped mold with a perfectly straight luff and then took it off the mold and layed it flat on the table there would be a curve to the luff) or you can look at it as part of the design (your small offsets in the seams coupled with some luff curvature give you the proper shape). In reality, it does not matter where you think of it as coming from - it is there.<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Agreed- althought I tend to think of luff curve as something that the sailmaker adds on. The luff curve you’re refering to here is a result of the broadseaming. Just symantics- we agree

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”> Adding a bit more bend than nominal will “pull out” some of the shape especially in the middle of the sail. It will also add a bit of twist to the sail. So even with a well cut sail, you do not always want your mast perfectly straight. But the amount of mast bend required to make effective sail shape changes tends to be a bit smaller with a well cut panneled sail than a flat sail.<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

I kind of asked this before but why is that I never here of anyone using reverse curve to put more shape into a sail?
<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>
Don,

As far as learning about well cut sails, there have been some excellent articles over the years in the AMYA magazing “Model Yachting”. In the most recent issue (the Victoria issue) there was an excellent article by Rod Carr where he showed the effect of adding a little bit of mast bend to change the camber and twist of the sails. There have been several other great articles in recent years as well…

  • Will

Will Gorgen
<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

Sounds like a sales pitch for joining the AMYA
[:D] which I think I will do. Thanks for the input. I can’t seem to get enough talking about sails. Kind of art and craft and science all rolled into one.

Thanks
Don
Vancouver Island

EDIT- I finally learned how to chop quotes into little pieces- Proud of myself I am.

Don,

I think if you added any measurable amount of reverse bend, it would cause the sail to crease when you applied any amount of luff tension. I think it is better to design your sails to accomidate a bit more nominal bend and then when you straighten the mast you will have the same effect.

Of course there is a limit to how much shape you can add and still get more performance. If you add too much camber, you will loose pointing ability (the incidence angle will get worse). If you try to add too much camber in light wind, the sails will tend to stall. So there are very few reasons I can think of to add more camber than a well designed sail already has…

You are right about art and science, although it it leaning more toward science these days (another trickle down effect from the IACC development programmes but that is another thread). CFD has really come into its own for sail design in the last few years and you now need a PhD in fluid mechanics to compete in the sailmaking arena… Take a look at what North and some of the other bigger lofts are up to these days. Imagine if we could do anything close to that for our little boats!!!

  • Will

Will Gorgen

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by wgorgen

Don,

I think if you added any measurable amount of reverse bend, it would cause the sail to crease when you applied any amount of luff tension. I think it is better to design your sails to accomidate a bit more nominal bend and then when you straighten the mast you will have the same effect.
<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

Will
I wasn’t thinking of running that way for any length of time. More like- I don’t think I’m getting enough power from these sails so I put a little reverse bend and all of a sudden my boat is a rocket. Time to make sails with a little more fullness. Kind of a testing thing.

Thanks
Don
Vancouver Island

Regardless of the type of boat you are making sails for the main thing is to get good shape. The head needs to be workable. When making roachy sails, the luff curve is greater in the top third of the sail as that roach needs to be held up. I have found that the easiest way to go about building sails is to cut a convex curve into the top of each panel, (start with about 1/8") then stick that curve down to the bottom straight edge of the next panel, allow an overlap of about 3/16" and proceed up the sail doing the same thing on each panel.

Getting the luff curve just right depends on several factors, sail cloth used, mast bend, mast stiffness, normal wind conditions, mast rake and rig size, not to mention the type of sail, pinhead or roachy.

Most panelled sails are built with horizontal panels, we are starting to play with vertical panelled sails. So far in testing they are setting better, but are much harder to build. Once the method has been worked out i’ll pass it on, unless someone is already doing this, then I would appreciate any ideas to build them easier.
Peter

Thank you very much for all your comments on this subject.

I am buying my daughter a SeaWind and her request is to have black sails. The “Black Pearl” is going to be the name of the boat. [:-pirate]
In the next month or so I will be making a few sets of sails. The first set will just be a duplicate of the existing sails, however I have enough material to start playing with paneled sails. I’ll let you know how it turns out. [:D]

If she keeps getting the 4.0 grades this is going to be a very expensive hobby for me.[:-graduate]
Thanks again,

Tom
Seawind #80