Some MC-Scow tuning guides recommend a lot of cunningham (downhaul) in heavy air. You may have to pull very hard to get the maximum cunningham tension possible. This post describes an MC-Scow cunningham modification that gives you more power and better control. Thanks to John Porter for this idea!
Why the Cunningham is Important
Adjust Position of Maximum Draft
As the breeze builds, the position of the maximum draft of the sail tends to move aft away from the designed 45%-50% position. Increasing vang tension also moves the maximum draft position aft. Tensioning the cunningham moves the draft of the sail forward. There’s a nice video from Captain John showing this effect.
Open the Upper Leech
The cunningham also helps open the upper leech of the sail to reduce power high in the sail. This effect is quite noticeable in C Scow sails, and although less effective in an MC, will still help, especially if you pull hard. Laser sailors also recognize the effect of the cunningham on the upper leech, as discussed in this post from MySailing.com.
MC-Scow Cunningham Modification
The MC Scow cunningham has a 4:1 purchase using blocks in the backbone. Above the deck, the cunningham attaches to a grommet in the sail with an S-hook. You can increase the purchase of the cunningham by simply running the above-deck line through the sail grommet to an attachment point below on the mast. This makes the purchase 8:1.
Parts and Tools Needed
- Five feet of single-braid spectra or dyneema line, 2.5 – 3 mm in diameter. (Use spectra or dyneema for abrasion resistance and low stretch.)
- Lacing knob, such as these from Amazon.
- One 10-32 x 1-1/2″ stainless screw
- Drill and tap for 10-32 screw
- Spicing fid
- Stopper ball
- Use the fid to make an eye splice in one end of the spectra line. The eye of the splice should be just big enough to fit snugly over the lacing knob. Don’t make the eye too big, or you risk having it come off the knob while sailing. Here’s a video showing how to make the splice.
- Tie a figure eight knot in the spectra line about 32 inches from the eye splice and thread the stopper ball onto the line from the un-spliced end. The knot and stopper ball prevent the new line from being pulled through the deck.
- Drill and tap a hole for the 10-32 screw in the mast groove about three inches below the gooseneck fitting.
- Install the lacing knob with the 10-32 screw.
- Remove the hook from the current cunningham and untie the knot in the end of the line. Secure the end of the current cunningham line to the free end of the spectra line with thread.
- Reach under the forward portion of the deck into the backbone and find the block that is attached to the current above-deck cunningham line. Pull the line down through the deck, bringing the new spectra line with it.
- Separate the new spectra line from the old cunningham line.Tie the new spectra line to the block that was attached to the old cunningham line. You will have to experiment with the length to get the right range of motion. Test range of motion by hoisting a sail and running the line through the grommet to the lacing knob. This will require a few tries.
- When satisfied, remove the old cunningham line from the block.