Upgrade Your Rig With a DIY Adjustable Backstay

At some point when you get serious about sail trim, whether for racing or just high performance cruising, you’re going to want an adjustable backstay. Most C-22s and similar daysailers were rigged at the factory with fixed length backstays that are only slightly adjustable with a turnbuckle. They’re not intended for adjusting to different wind conditions. You set it and forget it.

Consequently, you only have one setting for mast bend and headstay tension. That’s fine for casual cruising. Set it for the conditions that you usually sail in and it will usually be close. But an adjustable backstay gives you a range of trim positions to optimize the mainsail and headsail shape for any conditions, which are what you can encounter when racing or when you’re no longer just a fair weather skipper. 


In other rigging posts on this site, I’ve described how to add DIY controls for each of the three sides of a mainsail:

Each of them secondarily affects the middle or belly of the mainsail a little bit but an adjustable backstay primarily affects it and completes the sail trim picture. Genoa car placement affects the leech and foot of the headsail and halyard tension affects the luff. An adjustable backstay also affects the belly of the headsail. The cool thing about an adjustable backstay is that it affects the belly of both sails at the same time. It’s a two-for-one control that improves performance both upwind and downwind.

How an adjustable backstay improves sail shape

Your mainsail might have been designed and built with a slight outward curve in the luff specifically to take advantage of bend in your mast. With a fixed backstay or a loosened adjustable backstay, the mast (and consequently, the luff) is relatively straight. This lets the mainsail form a more rounded shape in its belly when it’s filled with air, which adds power and is just what you want in light air. The extra fabric width in the middle of the sail has to go somewhere, so it fills to leeward.

When you tighten an adjustable backstay, the mast curves slightly aft. This fits the curve in the mainsail luff, which flattens and depowers the mainsail, just what you want in a strong breeze. Even if your mainsail has a straight luff, the effect is the same. The mainsail is more efficient and the boat will heel less. Hence, you might not need to reef the mainsail as early or at all.

Similarly, your headsail was probably designed and built with straight luff but it can take advantage of an adjustable backstay as well. With a fixed backstay or a loosened adjustable backstay, the forestay should be tuned with several inches of sag in it. Like the mainsail, this lets the headsail form a more rounded shape in its belly when it’s filled with air, which is just what you want in light air.

When you tighten an adjustable backstay, since it pulls the masthead slightly aft, it also increases the tension on the headstay and pulls the sag out. Then it is straight and matches the luff, which removes some of the belly of the headsail and flattens it, just what you want in a strong breeze. Together with the mainsail, it too becomes more efficient.

Incidentally, a tighter headstay can also make your headsail furler work better. An adjustable backstay can also make trailering easier without the need for a quick release lever on the forestay. It lets you slacken the forestay, which can make disconnecting it to unstep the mast easier, especially if you have a furler. If you need just a little more slack, pull the mast forward with one of the halyards.

The simple animation below illustrates this simultaneous flattening of the mainsail and headsail. An adjustable backstay widens the middle of both sails a few inches.

Mast bend intentionally exaggerated for illustration purposes

Another benefit of an adjustable backstay is that after a day of sailing with a tight backstay in a strong breeze, you can slacken the backstay to let the rig relax and release tension on the hull while your sailboat is moored.

Direct vs. indirect backstays

Adjustable backstay designs fall into two types: direct and indirect. With a direct adjustable backstay, the adjuster (typically a tackle system) is integrated into the backstay. The adjuster directly controls the length of the backstay and bears the full load on the backstay. This is the type of system that I’ll describe how to make in this post.

The advantages of a direct system are that it is simpler and therefore, more economical to make. It’s also more mechanically efficient compared to indirect systems, as I’ll explain in a moment. The disadvantage of a direct system is that if any part of the adjuster breaks, the entire backstay can fail. That’s not likely to happen except under extreme conditions and it can be safeguarded against by adding a safety wire or strap to back up the adjuster in case of failure.

With an indirect adjustable backstay, the adjuster (also typically a tackle system) is not integrated into the backstay and it doesn’t carry the full load of the backstay. The adjuster indirectly controls the length of the backstay, which can function without the adjuster. The advantage of an indirect system is that it is more fail-safe. If the adjuster breaks, the backstay can continue to work, albeit without adjustment ability. The disadvantage of an indirect system is that is more complicated and therefore, more expensive to make and maintain.

The adjustable backstay that was installed on C-22s at the factory is an indirect system that looks like this;

Original Catalina Yachts adjustable backstay

Tightening the tackle system pulls the center ring down, which pulls the bridle wires together and shortens the overall length of the backstay. Another disadvantage of this design is that the more you tighten the tackle, its mechanical advantage decreases.

The angle of the line through the center fiddle block increases and the angle of the bridle wires through the wire blocks decreases. Both of these effects increase the amount of force required to shorten the backstay. The end result is, it’s easier to adjust at the beginning of the adjustment range and harder to adjust at the end of the adjustment range. It gets hardest in strong winds, right when you need it most. That is why most modern backstays are direct designs.

DIY materials list

Following are the parts and materials you’ll need to make the direct adjustable backstay shown. I used a 5:1 tackle system because that’s what I had on hand but you could substitute a 4:1 tackle (two double blocks, no triple block) instead. It’s important that the breaking load of each part is equal to or greater than the breaking load of the backstay wire. You don’t want the adjuster to be the weakest link.

  • Harken #304 1-1/2″ wire block or equivalent
  • Harken #94 29mm triple block with cam cleat or equivalent
  • Harken #85 29mm double block with becket or equivalent
  • 1/2″ x 13 tpi SS eye bolt. The older C-22s used nearly identical eye bolts for the backstay, keel cable attachment, and the chain plate bolts. They’re readily available and inexpensive on eBay. However, the chain plate bolts are not threaded the full length of the bolt to the flange and need spacing washers. The backstay and keel eye bolts are fully threaded, do not need spacing washers, and are preferred for this project.
  • 1/2″ SS washers (4-6 required if the eye bolt is not fully threaded)
  • 20′ x 1/4″ New England Ropes Sta Set double braid
  • 22′ x 1/8″ 1×19 SS wire w/swaged eyes on both ends. This is the main, non-adjustable part of the backstay.
  • 10′ x 1/8″ 7×7 SS wire w/swaged eyes on both ends. This is the adjustable part of the backstay. Do NOT use 1×19 wire for this piece, which is not designed for use with wire blocks.
  • SS shackles to attach the backstay to the eye bolts

For tips to help you decide whether to make the wire parts of the backstay yourself or to have a rigger make them for you, see How to Replace Your Standing Rigging for Less.

Installation instructions

To assemble and install the direct adjustable backstay shown:

1. If your sailboat already has an eye bolt installed in the port side of the transom, skip to step 2. If your sailboat does NOT have an eye bolt already installed in the port side of the transom, continue with this step. If your sailboat is not a Catalina 22, modify these instructions to provide adequate transom reinforcement.

A. Drill a 1/8″ starter hole through the top of the transom 2″ outboard of the traveler bar (6″ from the port side of the tiller cutout). Place the hole in the middle of the transom thickness. There is a 5/16″ thick brass bar embedded by the factory in the top of the transom for this purpose. Drill completely through the bar.

B. Redrill the hole to enlarge it to 3/8″ or 27/64″ (preferable if you have that bit).

C. Chamfer the fiberglass down to the brass bar with a countersink bit.

D. Tap the hole to 1/2″ x 13 tpi. The finished hole should look like this:

New tapped hole for the port side eye bolt

2. Test fit the 1/2″ eye bolt in the hole to decide how many washers you need for a tight fit. The tab of the eye bolt when tightened must point toward the cockpit like the picture below.

3. Apply a 1/4″ cone of butyl tape around the bolt and the underside of the lowest washer so that it will fill the countersink in the transom and squeeze out a little.

4. Apply blue thread locker to the eye bolt threads and install the eye bolt snug.

Do not overtighten the bolt or you might strip the brass threads.  If you do strip the threads, then you will need to drill the hole out to 1/2″ and add washers and nuts on the inside of the transom, which is very difficult just to see, let alone work on. This is also a possible workaround if your sailboat is not a C-22.

In that case, most owners end up cutting access holes in the front of the transom to install the nuts and then cover the holes with access plates or vents. To make matters worse, the back of the transom has a wood core and is thicker, the front of the transom has no core and is thinner. With the eye bolt centered on the transom, the threaded end of the bolt barely clears the core inside the transom. You will have to cut into the core to create clearance for the washers and nuts. To avoid all this, don’t strip the eye bolt threads.

Salvaged chain plate bolt reused for the adjustable backstay

5. Unstep the mast and, if necessary, move it so that you can work on the masthead.

6. Remove the existing backstay and attach one end of the 22′ wire to the masthead in its place.

7. Step the mast and reconnect the shrouds.

8. Reave the 10′ wire through the wire block and attach the wire block to the loose end of the 22′ wire like this:

Wire block arrangement

9. Use a shackle to connect one end of the 10′ wire to the transom eye bolt on the opposite side (typically the starboard side) from where you want the adjuster cam cleat to be located (typically the port side). 

10. Attach the double block with becket to the loose end of the 10′ wire like this:

Adjuster tackle attached to the backstay bridle wire

 11. Reave the 1/4″ double braid line through the double and triple blocks.

Use a double luff reaving order like shown below.

Start from the becket on the double block, reave the line through the sheaves on one side of both blocks, through the opposite sides of both blocks in the opposite direction, and exit through the middle sheave of the triple block and the cam cleat. Do not spiral reave the line through the sheaves. Leave a long tail in the line until after the backstay is installed and the rig tuned.

12. Use a shackle to connect the triple block to the remaining eye bolt (typically on the port side) like this:

The completed installation should look like this:

Completed adjustable backstay

13. With the adjuster slack, check the mast rake and prebend and the standing rigging tuning. If you’re not sure how, refer to the Catalina 22 Tuning Guide from North Sails. They have guides for other sailboats here. If you don’t have a tension gauge, consider purchasing one after you read Product Review: Loos Tension Gauge.

14. Tighten the adjuster just enough to take the slack out of the backstay and so that it won’t interfere with the boom when the mainsail is raised. This will be the minimum backstay tension setting.

15. Tie a stopper knot in front of the cam cleat to prevent the adjuster from being slackened any further.

16. Trim the excess adjuster line to leave about a 1′ tail. Tie another stopper knot on the end to give a better grip on the line.

17. Tighten the adjuster to 25% of the breaking strength of the main wire or the bridle wire, whichever is less . This should bend the top of half of the mast aft a few inches. This will be the maximum backstay tension setting that you should not exceed.

For example, if the breaking load of the main wire is 1587 lbs and the breaking load of the bridle wire is 1350 lbs, calculate 25% of 1350 lbs, which is 337.5 lbs or a setting of 25 on a Loos PT1 tension gauge.

18. Mark the adjuster line in front of the cam cleat with a permanent marker. Do not tighten the adjuster beyond this mark when you are sailing.

Now go out and practice adjusting your new backstay in various wind conditions to optimize the headsail and mainsail shape and maximize your pointing ability and speed. When you’re done for the day, slacken the adjuster to the minimum setting.




4 Comments Add yours

  1. Ed in Gig Harbor says:

    Nice description of the why and how to add the direct backstay adjuster. If one doesn’t have a transom with the brass structure to mount to, I assume one would want to use the typical metal backplate when installing that eyebolt? Cheers, and thanks for some fresh inspiration to work on my project boat!

    1. Yep, treat it just like you would a chain plate bolt because it’s doing much the same job.

  2. Mike Bracket says:

    I had an indirect system when I got my boat and it functioned (minimally) as you describe. I couldn’t tell any difference. Then I installed a system as you outline here. sailing on a close reach in light air and flat water, I could actually FEEL the boat accelerate as I eased the backstay and I could FEEL the boat slow down as the sails stalled when I tightened it up. I played with it for half hour like this just appreciating the noticeable difference from the “indirect” method I had previously uses. This is one of the best modifications you can make to your boat.

    1. Thanks for sharing your experience, Mike!

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