Downwind Lift

yeah tom does say something along those lines in his book… i’d have to go back and re-read it to be able to quote it perfectly…

however, what andrew says is i fear, true. if you are looking to raise the bow by increasing the lift of the sail, it is going to have to be done by raking the mast back. look at moth-class foilers and windsurfers to get a feel for just how far to windward and aft they are raking those rigs… i don’t know that easing the outhaul would help with downwind lift… downwind efficency yes, but lift, i am inclined to disagree…

Here are several points to consider:

  1. Raking the mast of your McRig aft to lift the bow requires the rig’s boom to travel uphill as the sails are let out. Therefore in inconsistent conditions (are there any other kind?) gravity will be working against wind filling the sails. The sailor holds the boom of the windsurfer sail in relation to the wind. If the sail loses airflow he or she pumps the rig to reestablish it. Neither function is practical to control on a Footy, although the automatic “spring action” of the McRig comes close to a pump.
  2. The desired effect we are going for here in this thread is to impart more twist in the sail’s leach for offwind work. Just increasing camber will not necessarily result in improved performance.
  3. The McCormack rig’s main selling points are light weight and ease of construction. As a rig type it most closely resembles a lateen or crabclaw. These rig types are not particularly close winded, they are more efficient offwind. The marconi or bermudan rig, which is the standard in the other r/c development classes, is the most effective rig to windward. Although in triangular course racing the straight line distance is longer in the offwind legs, going upwind requires tacking back and forth to fetch the windward mark making the actual distance traveled physically longer. Since in a race the emphasis is on windward sailing the McRig theoretically is not the ideal choice.
  4. The light weight of the McRig is a factor in its favor. You don’t want excess weight carried aloft. The McRig also has incorporated in the design the “spring action” that spills overpowering gusts from the rigging thus helping to keep the boat in control.
  5. The commencement of “spring action” comes at mother nature’s discretion. So this discussion on how to best “control” the twist in the McRig’s sail is really counter to the purpose of the McRig in the first place. Adding gizmos to control features that are designed to be under auto control only adds complexity and weight to a rig that is supposed to be simple and lightweight.
  6. A more fruitful direction to pursue would be to focus on cambered sails. Most of the photos of McRigs floating around out in cyberspace show single panel sails. Any curve that is imparted seems to come from mounting a sail with a curved luff to a straight mast spar. This practice puts the sail’s camber too far forward to be efficient for upwind work. Most of the photos also show Mcrigs trimmed with a lot of camber. But without camber throughout the sail, from top to bottom, the mast deflection which makes the McRig unique not only dumps wind but slacks the leach off and wrecks any shape in the top of the sail. The rig must spring back fully for flow to reestablish over the sail. A fully cambered sail would retain its airfoil shape better through the “spring action” and recovery.
  7. One final thought, for those of you who have to tinker, boom rotation. The McRig is made up of a Z bend metal rod, one end functioning as a mast pivot and the other fitting inside the mast spar. A boom extension is joined to the horizontal span of the metal Z.
    My suggestion is to uncouple the boom extension from the horizontal span and have it pivot around the mast pivot part of the z bend. The part of the boom that extends in front of the mast pivot would have its throw limited by a line so that it might only rotate a few degrees either side of center. The hole in the boom that would allow the mast pivot part of the z bend rod to pass through could be angled slightly so that the aft end of the boom would be slightly higher than the opposite forward end. The main sheet would pass through a loop on the mainboom, then through an eyelet in the forward end of the boom, and finally attaching to the horizontal span of the Z bend rod.
    If you read my earlier post about placing the gooseneck pivot aft of the mast you will realize that what I’ve described here is the same idea with one twist. Trimmed in the mainsheet closes the angle between the horizontal span of the Z bend rod and the boom. Once the sails are let out the angle opens up, shortening the distance between the mast and the aft end of the sail imparting more camber. Also, because the boom is angled up the distance between the mast tip and the aft end of of the boom is shortened thus slacking off the leach a bit. Killing two birds with one stone as they say.

Enough said - enjoy your winter projects!

Neil - you have provided us with a plethora of good ideas. Meanwhile, I have found that my proposed idea will allow the foot to bulge forward by about 4 inches, but unfortunately a strong wind will also cause the top of the mast to go forward, which defeats part of the objective. So it is back to the drawing board, perhaps with the emphasis now shifting to the iceboats.

Perhaps we need to consider setting the mast back, behind amidships, with a small main and a very large jib. A bowsprit may be needed too. I’m sure I’ve seen a photo of a German design of the 1930’s with something similar.

Perhaps we can use the fact that the so called “McCormack rig” bends forward down wind …attach a "sail"to the top of the mast…like a kite…parrallel to the water.the “sail” or “kite” will then be angled upwards when running( mast leaning fwd) …hence providing lift.
upwind it will act as an endplate.
might work…