From around the world

Around the World

 

This area has been set up to capture material from other Unicorn sailor’s around the globe, there is quite alot of activity on a number of continents. The most frequent contributors are based in Canada and South Africa where fleets are still sailing or are being restored to their former glory.

There is much happening in Canada, the campaign being led by Jim Helps who has developed a laser cut kit for those planning to build a new boat. Jim is also looking at sourcing all the other parts needed to complete the kit and is developing a construction manual. With Jim’s permission I have posted a few pictures of one of his boats ‘in build’ and also a stimulating article produced by Jim. He can be contacted at This email address is being protected from spambots. You need JavaScript enabled to view it.

 

 

 

 

 

UNICORN SAILING DYNAMICS

Explained for new...and some old… sailors.

Understanding exactly what is physically happening to the sail, mast and hull structure of a Unicorn while underway will assist in optimizing your sailing performance.

Modern catamarans have multiple systems. The simplest way to understand this mess is to analyze one unit at a time. This should include all of the components involved, sail, mast, hull and controls.

The Unicorn mast is not just a convenience used to hoist the sail and secure the luff. The mast is dynamic and not the semi-static pole found on so many other classes. The mast is designed to bend under load and that fact is used to great advantage.

Mast bend is imposed fore and aft by both the boom vang and the main sheet.

Mainsheet bend involves a force triangle roughly formed by a line from the masthead to the mainsheet attachment point, then to the mast heel and back to the masthead.

The mast portion of the triangle is being held in position by the standing rigging at the hound and the mast heel.

When the rear portion of the triangle is moved downwards by hauling in the mainsheet, a load is applied at the masthead. Because the mast is being held at the heel and part way up the mast at the hound, mast bend is induced.  There are secondary loads applied to both the mast and the hull.

Besides forcing the mast to bend, an axial load is placed on the mast. The axial load is imposed by a rough force triangle formed by a line from the forestay attachment point to the masthead, to the mainsheet attachment point and back to the forestay attachment point. The force applied to the hull structure consists of a downward load on the main beam at the mast step and an upward load on the rear beam at the mainsheet attachment point. These are substantial loads.

Because the hull beams are being bent in opposite directions, main beam down, rear beam up, loading has led to cracking in the rear beam at the lower inside bolt hole This effect  can be minimized by strengthening the main beam with a dolphin striker which will help eliminate main beam bend. This will minimize beam bend differential and reduce rear beam cracking loads.

There is a major torsional load imposed on the hulls by the beams which are bending in opposite directions.  The hulls appear to have sufficient strength to absorb this load. The weak spot remains in the rear beam and this probably should be left alone. Strengthening the rear beam could lead to failure in another area. Rear beams are easily replaced are relatively inexpensive.

The main beams are being forced to seat in the beam chocks by the load on the mast step. The hulls are being held in position by the standing rigging.  There are no tension loads on the beam bolts. They only need to be drawn up snug.

The rear beam is being pulled upwards and nothing is gained by drawing the bolts up drum tight.  They should just be drawn up snug as well.

Hull racking occurs when the windward hull is lifted out of the water and the leeward hull is being held in a semi-static position by floatation.  Because the skipper is at the aft end of the windward hull, his weight induces a downward load on the stern. This load changes longitudinal hull alignment.

The windward hull is being held in a semi static position by the tension in the standing rigging and the wind load on the sail.  The lee hull is being forced to move up and down by wave action. This affects hull alignment.

When the mainsheet traveler is at the lee end of the track, mainsheet load is carried through the standing rigging to the bow chain plates.  This lifts the lee stern causing a change in hull alignment.

The changes in longitudinal alignment induce a torsional load on the main beam.  This induces shear loads on the beam bolts which will ultimately cause the bolt holes to elongate as the boat ages.  You may notice that when moving the boat on shore it is not quite as stiff as it was when it was new.

This should be of no concern as the boat is strong enough to absorb the loads. Drawing the beam bolts up tighter will accomplish nothing.

These movements also affect the rear beam by imposing radial and torsional loads which may contribute to rear beam cracking.

In addition to the mast’s axial load imposed by the mainsheet, the Cunningham adds to it.  Remember the halyard is usually a two part tackle and a four part Cunningham will give you an eight part advantage in applying the total load.  This additional load remains static while the Cunningham is cleated.  This does not add to the load on the main beam.

Boom vang forces remain static when once applied.  Mainsheet forces are static only when the mainsheet is cleated. Main sheet forces can exceed vang forces because of mainsheet geometry.

The vang force triangle is formed by a line from the mast heel to the gooseneck, to the boom attachment point and back to the mast heel.

When you shorten the lower leg of the triangle by hauling in the control line the boom is pulled down and the forces imposed are similar to that imposed by the mainsheet. There are also secondary forces. A static vang load triangle creates a major stress point on the mast at the gooseneck position.

Some monohull classes run a strut from the hull to the gooseneck to control this load and can use the strut to induce a mast bend pre-load on a semi-static mast.

Lower shrouds were designed to prevent over bending the mast because over bending could precipitate a mast failure. Don’t ever sail without lower shrouds.

If a fully bent mast is set with the lower shrouds up snug, it creates a resistance to mast rotation.  Because of the mast bend, you have an arm equal to the distance between the static mast center line and the axis of the bent mast. If this arm is held in position by snug lower shrouds, the mast resists rotation.

To eliminate this problem, install blocks on the chain plates and cross connect the lower shrouds.  Use eyes at the lower ends and join them with a lashing.  The lashing can be used to adjust the length of the lower shrouds.  Set the length of the lower shrouds to limit the mast bend that best suits the cut of the sail.

There is one more load that bears looking into.  That is lateral load placed on the mast by sail wind load.  This load, if extreme, shapes the mast into a modified “S” when viewed from the front.  I gather this has not been a problem.

In other classes this problem has been eliminated by utilizing flat top sails. The new Unicorn sail has been cut to closely match the advantages of a flat top, a full roach at the top that is supported by battens.

If the major axis of the mast section is kept in line with the luff of the sail there should be no lateral bend problems. The alignment can be achieved by utilizing mast rotation.

Another effect created by vang tension is halting complete mast rotation.  This is caused by boom end load on the gooseneck and the knuckle joint effect inherent in gooseneck design.

A force triangle is formed by a line from the mast axis to the vang attachment point, to the pivot on the gooseneck and from the pivot point to the mast axis. This triangle remains static as long as vang forces are being applied.

If you haul in the mainsheet until the vang goes slack, it will help remove the boom end load on the gooseneck and the mastshould rotate on its own. Mast tiller set-up will also assist in breaking the lock up.  As a last resort, push the side of the boom just behind the gooseneck.  This should break the lock up.

The way the sail and mast interact has presented a huge advantage.

Classes with semi-static masts have to force the mast into the shape that suits the sail.  Extended length spreaders induce mast bend. High rig tension (Fireball - 500 pounds; “505“- 800 pounds), high vang tension, high luff tension, all are used to force the mast into the shape that suits the sail.  The only advantage gained is a tight jib luff.

The Unicorn sail forces the mast to comply with the shape it wants in the following manner:

Imagine a sail reacting like a quarter of a bicycle wheel. When you pull the clew (wheel hub) out with the clew outhaul and down with the main sheet and vang, the radial lines from the clew (spokes) pull and bend the mast (wheel rim) until the radial lines (spokes) are under tension. The mast now matches the sail shape. The amount of force applied to the clew controls the camber in the sail from full to flat. The sail has set the bend in the mast. No spreaders, struts or high rig tension forcing the mast to bend to suit the sail shape. Another interesting thing happens when you pull on the clew.  The mast bends and the distance from the sail headboard eye to the eye in the tack decreases. This lowers luff tension and can cause wrinkles in the sail luff.  Extra downhaul pressure will remove the wrinkles in the sail and induce additional mast bend. Because mast bend can be controlled sail shape can be controlled while in a static position or under full wind load.

The key to success is a sail cut to suit the characteristics of the mast.

Because the Unicorn mast bends under load, a change in helm can be induced.

Because the bulk of the sail area on a Unicorn is positioned between the tack and the hound, the center of pressure is located in this area. When sailing in heavy air, full mast bend is used to flatten the sail. The luff of the sail is pulled forward by the bend and it carries the center of pressure with it.  This movement can induce lee helm.  Because the sail area above the hound is moved aft by the mast bend, this offsets the forward movement of the center of pressure, but only in a minor way.

This will explain why mast rake has to be adjusted to maximize boat speed when on a beat in heavy air. Because mast bend is decreased in light air to set more camber in the sail, this moves the center pressure back which can induce weather helm.  In light air the forces are much lower and changing your position on the hull should correct any problems.  These forces do not apply when sailing off the wind.

Shroud tension should only be tight enough to stop the mast from moving around.  Lower shrouds should be set to prevent mast over-bending. Luff tension should be tight enough to remove sail wrinkles.  Do not over tension the luff except under extreme conditions. Extreme luff tensions can lead to sail damage.

Let me describe what excessive mast axial load could do:

One method we used to check sail batten stiffness was to stand a fairly long batten upright. Place the lower end of the batten on a low capacity household scale. Reach up and press downwards on the top of the batten until the batten suddenly bends. The force measured by the scale at that time is the tipping point. This is a direct indication of batten stiffness.  The higher the number, the stiffer the batten.

Apply this to a mast:

You will notice while testing battens that as you reach the tipping point, very little additional pressure is required to continue bending the batten until it fails.  If you reach the tipping point on a mast and try to correct it….you are usually too late.

Because the Unicorn mast bend is controlled by sail shape and bend is being limited by the lower shrouds, we seem not to have had mast failures caused by excessive axial loads.

Lateral loads are imposed on the main beam by the load triangle roughly formed by the mast, the windward shroud and the main beam from the windward side to the mast step. This force triangle is always static and any beam loads are applied by an arm extending from the mast step to the lee end of the beam. The lee end of the beam is static and is supported by the floating hull.

By outlining the dynamics of the Unicorn, I have tried to explain why unusual things happen while under way and possibly how to correct these problems.

Jim Helps

Edmonton, Alberta Canada

(KC 15 circa 1967)

This email address is being protected from spambots. You need JavaScript enabled to view it.

 

 

 

 

 

 

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