Luff Tension and Frestanding Rigs

I understand that a frestanding mast bends to a varying degree depending on wind force, and that the ability of the sail to adjust to these changes is important.

My question is, what differences are made to a sail plan to allow for a freestanding mast to bend freely? IOW, how does the sail design differ between conventional stayed masts and freestanding?

Thanks!

 

Gooday Charly. Great question & it sure needs an experienced person to answer it.

Maybe Bob Pattison of Neil Pryde Sails (USA) might care to drop in & give us an informed answer ??? Sure hope so - cause 99% of other answers would be pure quess. I M H O Ciao, james

 

For freestanding cat rigs, check with Eric Spoonberg (sp?).

 

Hi Charly,

In drawing a sailplan for a free-standing rig, I don't do anything different than if I were drawing a conventional rig. That is, the drawing follows normal drafting principles. However, the sails are built differently, but these differences don't show up on the sail plan.

In a conventional rig, the mainsail and jib are both given some curve to the luff so that when they are set, they actually have some camber to them. The mainsail usually has more camber than the jib, relatively speaking. The reason for doing this is because both the mast and the headstay are fixed, yet the sails have to set to varying angles of attack. The luff curve on the mainsail tends to improve the airfoil shape of the mast-sail combination, kind of fooling the wind into seeing a more efficient shape than is actually there. On the jib, the headstay is nice and tight, so it can support a cambered sail easily--the shape is nice and clean on many angles of attack.

On a free-standing rig, particularly a rotating wingmast rig, the mast rotates, and so on many angles of attack the airflow is always very good and there is no fooling of the wind necessary. Since the mast controls the shape of the sail camber, the mainsail itself must be cut very flat, with practically no, or at least very little luff curve. In design, we can calculate the amount of mast bend in any given wind condition, and the sailmaker can use that information for planning the layout and cutting of the sails.

For the jib, because the mast bends, the headstay is not really very stiff, and it will sag. The headstay sag tends to destroy sail camber, so instead of a positive luff curve, as we may call it in a normal rig, the free-standing jib will have some negative luff curve, or hollow, cut into it to account for the amount of sag. A good sailmaker will take all this into account and usually come up with a good first pass at sail panel layout and cutting.

None of this fine tune sail shape is shown on the sailplan. Even when I draw the panel layout on a sailplan, that is only for show, it is not necessarily how the sails are built. The sailmaker has final say on how to build the sails. About all he gets off my sailplans are the overall shape of the sails, the positions of the battens, and the positions of the reefs. Other than that, what the sailmaker recommends is what is usually provided.

A lot of sailmakers have a really hard time understanding that free-standing sails have to be cut very flat, so when you get a good sailmaker that understands that, you tend to hang onto him. Some sailmakers that I have found reliable are Hood Sailmakers in Middletown, RI, and Doyle Sailmakers down here in Stuart, FL. On my design Saint Barbara, Elliott Pattison sailmakers in Newport Beach, CA, according to the owner, made him a beautiful set of cuben fiber sails that worked marvelously pretty much right out of the box. I did not deal with them directly, but the owner was very happy with them. Saint Barbara has a free-standing wingmast sloop rig.

I hope that helps.

Eric

 

Very helpful information! Thanks Eric.

Also I was wondering if the tension of the luff affects the bending characteristics of the mast. A freestanding mast would get the same amount of tension when hoisting the sail as a conventional mast?

 

Charly,

Yes, the tension of the jib luff does affect how the mast bends. It is a point load near the top of the mast. If the jib were not there, the mast would react to the load of the main only, bending to the side and back, generally along the plane of the sail fabric. With a jib, the tension of the luff wire pulls the mast forward, with the resultant that the mast actually bends more sideways and less back. Most sloop rigs with free-standing masts are 3/4 or 7/8ths rig; the headstay does not go to the masthead.

A conventionally rigged mast experiences only compression loads which are brought on by the tension in the wires. A free-standing rig experiences bending loads which results in a high tension stress on the windward side of the mast, a high compression stress on the leeward side, and a high shear stress in between. The shear load above the deck is relatively low, but the shear load between the deck and the mast heel is extremely high. In both styles of rig, the compression created by the halyards is measureable, but generally is of low magnitude compared to the compression created either by the shrouds (stayed) or by the bending of the mast due to the mainsail (free-standing).

Eric