SJ23 Tech Tip F09, (Updated 2023-06-09) Art Brown, Bret Hart & Bob Schimmel (UNDER CONSTRUCTION)


Split Backstay Adjuster - A factory design that is easy to operate.
INDEX - Required for Roller Furling, Caution, Second Chain Plate, Safety Wire, Observations, Bret's Installation,
Single Ended Adjuster, Squeeze Adjuster.

Some sailors consider a backstay adjuster a "go fast" gadget for racing while furler manufacturers consider it "essential" for their roller furling.  There is a third category I'd like to call "protection."  A few years ago two of us motored a C&C 27' in really shitty weather.  The weather was just above freezing, with very steep 4' waves in which the aging Autohelm 300 could barely maintain a heading.  We tightened the backstay adjuster to save the mast from pumping itself to pieces.  Tightening the rig like this to save it was a first for me.  Like I said, "Really shitty weather."

So you think you don't need a backstay adjuster because you only cruise your SJ23?  Well think again.  A forestay that sags when the wind speed increases is bad for windward performance because it makes the shape of the jib too full (draft moves aft).  This means greater heeling force and slower boat speed.  It's the reason why the jib and mainsail need to be flat in medium to heavier wind.  A full sail may also be present in a partly rolled jib when the extra sailcloth in the belly adds draft too far aft.  For a roller furler, an excessively sagged (curved) forestay increases the friction, making the sail more difficult to roll up under load.  Although prudence would also have you release the sheet to ease the strain of rolling up the jib.

Don't be tempted to tighten the forestay of a mast head rig to solve this problem.  The backstay can tighten the forestay with less force due to a slightly more advantageous angle to the mast head (greater mechanical advantage).  A tightened forestay relates to approximately 1/2 knot of extra upwind speed.  Not too shabby!
To avoid straining the rigging and jib, ease the backstay adjuster and the halyard when sailing in light wind or when the boat is at the dock. 

Shown at right is a factory 8:1 backstay adjuster kit that was sold by Clark Boats to those sailors wanting to do some racing.  The factory design is quick to set and easy to release.  Some SJ23s were equipped with this option at the factory but most were installed afterwards by the owner, including the second chain plate.  The part numbers shown are early 1980s vintage so you will have to update them to today's equivalent parts.  Listed below are the parts used for Panache.  

Thanks to Art Brown for preserving this document from the Seattle SJ23 Club. 

CAUTION - The Clark backstay adjuster diagram above, shows only one Nicopress sleeve swaged at each eye termination.  I copied this design thinking the 1/8" wire will not experience a heavy load.  In 2023 Panache experienced a sleeve failure on the split wire of this configuration after only 5 years of service.  The wire pulled out of the sleeve below the wire block with a very loud 'thwack', just as I pulled the line to tension the backstay.  This left the mast dangerously close to going over the side.  "I quickly tightened the mainsheet, then the boom topping lift so the mast was safe for the moment.  Later I secured the spare jib halyard to the pushpit as extra insurance.  Now I was safe to lower the mainsail and motor to the slip."  It's unnerving to see the mast arcing against the sky and you must act quickly to save it.  This incident was too close for comfort.  You really don't know how well things are going until shit happens!  Aaaarch. 

POST ANALYSIS - After a thorough investigation of 2 compression tools, types of sleeves, and swaging techniques I used with this installation I finally discovered that the 1/8" wire that pulled out of the single compression sleeve was a hair thinner than Imperial 1/8" wire.  This was confirmed with my dial micrometer.  A sleeve that is compressed on undersize wire with a calibrated tool cannot achieve it rated holding power.  Eventually the two will part company as per this failure.  This difference in diameter is likely the result of manufacturing wire to an Imperial measurement in a metric country, but I can't prove it.

The replacement wire is a true 1/8" thick Imperial measure and is double swaged.   Back to Tech Tip F34a.

TEMPORARY REPAIR - In the repair that followed, each single sleeve was cut off and replaced with two sleeves as recommended by Nicopress.  Doubling the sleeves shortened the wire ~3" so I added a SS strap to restore the length.  This repair allowed me to continue sailing for the season without having to lower the mast.  See Tech Tip f34a for a permanent repair.

Panache BACKSTAY ADJUSTER for ROLLER FURLING (2018-04) - With roller furling now installed on Panache Tech Tip F10 the length of the forestay is fixed, setting the rake of the mast. 
- The forestay tension is then set by the backstay.  This is recommended by all roller furling manufacturers since a furler rotates easier on a tight forestay.  Easy rolling is really important when you need to furl or roll up a jib in a gathering breeze. 
- Many manufacturers also recommend saving the gear by easing the tension on the standing rigging and the jib halyard when at the dock. 
The thread of a standard turnbuckle (IE: factory single backstay) is not designed for continual adjustments for variable sailing loads.  The thread will eventually fail with catastrophic results. 

For these reasons I equipped Panache with an 8:1 backstay adjuster as shown at left.  In this conversion I shortened the factory backstay to 24' and terminated the adjuster portion with 7x19 SS wire crimped with two Nicopress oval sleeves at each eye.  The wire block was bolted to the backstay eye.

I modified Clark's backstay adjuster (shown above) slightly by adding a turnbuckle to the safety wire (shown at left) to set the minimum rig tension.  This way the mast is not solely reliant on the block and tackle for support and should therefore be as dependable as a squeeze adjuster.  The safety wire is connected through two custom fabricated triangle plates to which the 4:1 block and tackle is connected. 

With the turnbuckle parallel to the block and tackle I can remove the barrel and ease the adjuster line out to the stopper knot to lean the mast 2" forward from its normal angle.  This creates the required slack to pin the forestay while stepping the mast.  The stopper knot limits the forward movement.

SPECIFICATIONS - Pay particular attention to selecting the 1.5" wire block over which the split backstay turns.  The breaking strength MUST match or exceed the breaking strength of the 1/8" SS backstay wire.  The profile of the sheave groove MUST also match the 1/8" 7x19 flexible halyard grade wire so the strands are fully supported (can't flatten).  The cheeks of a Harken 304 block can be opened to replace the 7x19 split wire without lowering the mast, a feature that is important for servicing.  Although you MUST secure the mast aft with a spare line during this procedure.  Since a Harken 304 wire block cannot swivel (fixed is stronger than swivel), it was double swaged on the backstay in line with the flexible 7x19 wire to achieve a fair lead over the sheave without torquing the backstay.  While the Ronstan fiddle blocks will handle just under half the load of the backstay wire block, they too must match or exceed the load requirement. 


- If you cut the split wire to length with the turnbuckle twisted to mid position and the mast at the correct rake, it allows for a bit of slack to set the forestay pin during mast stepping. 
- Wrap wire tight around an eye thimble to maintain wire strength.  Match the thimble size to the wire.
- Use zinc plated Nicopress oval sleeves on SS wire. 
- NEVER use an aluminum sleeve on SS wire since the metal is incompatible with SS.  It cannot flow around the strands, i
s too brittle and will fail.
- Crimp the middle of the sleeve first, then the thimble end and finally the wire end. 
- Crimp the second sleeve ~1/8" away from the first.  (2 sleeves = 100% wire strength).
- Leave 2 cable diameters (about >1/4") of wire protruding from a sleeve.  This helps to lock the wire to prevent creep.
- Always crimp with a calibrated compression tool.
 (3 crimps / sleeve = 100% max strength).
- For more info see
How to Select and Crimp a Swage Sleeve.




 24' Backstay wire, 1/8" (1x19) 316 SS  - BL 1780 lbs, SWL 356 lbs.
 7' Split stay wire, 1/8" (7x19) 316 SS  - BL 1670 lbs, SWL 334 lbs.
 3' Safety wire, 1/8" (7x19) 316 SS  - BL 1670 lbs, SWL 334 lbs.
4 Split stay & safety wire eye splices.  - Two 1/8" (3MM) Nicopress oval sleeves (tin-plated or copper) per SS eye thimble.
 1 Backstay wire block, Harken 304  - BL 3000lbs / 1361kg, SWL 1500lbs / 680kg.
 1 Top Fiddle Block, Ronstan S40 RF41500  - BL 2200 lbs, SWL 880 lbs.
 1 Bottom cam/fiddle/block, Ronstan S40 RF41530  - BL 2200 lbs, SWL 880 lbs.
1 clevis, starboard.  - Heavy duty.
 12' Control Line.  - 5/16" Dacron low stretch.
 1 Chain Plate, starboard.  - 316 SS (1/4 x 1 x 10)" strap.


SAFETY WIRE and BLOCK & TACKLE - A good feature of this backstay adjuster is the 1/8" 7x19 SS safety wire that supports the mast in case the tensioning line releases from the cam cleats or breaks.  I reverse engineered Clark's design by considering the safety wire as the primary support and the block and tackle the secondary support to achieve similar safety of a squeeze adjuster.  I don't like compromising safety, strength and endurance for a "go fast" gadget. 
  • The backstay wire block is 6' up from the transom which is 1.5' above the horizontal boom, making it convenient to secure the boom to at anchor or possible to pass through when boarding via the transom boarding ladder.  Be prepared to shorten this bottom wire as it stretches.  Mine stretched 1" in 2 months from new.
  • The top fiddle block is positioned 3' above the bottom block which leaves plenty of line to adjust rig tension.
  • The combined length of the split wires (safety & fixed) was chosen to restore the factory backstay tension with the turnbuckle thread about 1/2 screwed in.  This results in the turnbuckle tightened to the light tension required at the mooring.  Don't be tempted to set the tension less than this as it will promote shock loading in the rigging when the mast flops.  Just millimetres with each roll of the hull can induce fatigue that can break metal rigging with disastrous results. 
  • I incorporated a SS triangular plate assembly at the top and bottom of the safety wire to support the mast.  (SS plates = 2" face with 1/4" spacers at each corner).  The standard nuts shown were changed to nylock and locked with a dab of silicon sealant for extra security.  Using the safety wire is a strong and secure way to support the mast and has shown no problem after 3 seasons of use on the water.
  • The top triangle plate can swivel up/down with the pull of the Dacron line.  It limits the amount of tension the line can exert on the backstay.
  • The bottom triangle plate can tilt fore/aft, duplicating the function of a turnbuckle toggle for stepping the mast.  In practice the bottom block rotates very little.
  • The 14' of control line is low stretch 5/16" Dacron line to prevent shock loads into the rigging.
  • The top fiddle block is a (Ronstan S40 RF41500 at left) and the bottom fiddle block is a (Ronstan S40 RF41530 at right). 
  • The turnbuckle thread is shown almost released, having just stepped the mast.  In sail mode it is tightened with half thread showing.  The turnbuckle is adjacent to the block and tackle so the barrel can be released or removed and still support the mast with the line.  This is a useful technique to pin the forestay when stepping the mast.

RONSTAN block Smart Feature - A sliding post on these fiddle blocks can be pushed to one of two stop detents to fix the swivel pin at 00, 900 or disengaged for free turn.  No tool required.  Its perfect for this application.  These are set to full swivel.

CAUTION - When releasing the control line DON'T just let it go.  This shock unloading, plus forgetting to tie a stopper knot, is a recipe for loosing the mast over the bow.  Instead, ease the line out with your hand for a soft landing at the stopper knot.

INSTALL SECOND CHAIN PLATE (2018-04) - I've been wanting to do this job for a long time but held off because the cockpit drain tubes blocked access to the starboard side of the transom.  It was too cold to twist off the port drain tube so I cut it.  But I still couldn't quite reach the top of the starboard transom.  So I used an offset box end wrench with a dab of butyl rubber on the nut to hold it against the bolt so my buddy could start threading it from the outside.  (I never dropped a nut this time and wish the technique always worked this well).   The alternative was to install a 6" inspection port in the aft end of the cockpit as per Tech Tip D03.  Something I didn't want to do.

  1. Note the location of the port chain plate then inspect the inside of your starboard transom (under the cockpit) for a similar clear area.  Mark the location through the top of the transom for the second chain plate. 
    - If your SJ23 has a structural reinforcement running almost vertical on the inside of the transom, install the second chain plate beside it. 
  2. Drill and smooth a hole through the top of the starboard transom, similar to the port plate.  The chain plate should fit a bit loose so sealant can go down the hole to do its job. 
  3. Determine the location of the bolt holes through the transom then mark and drill the holes through the chain plate.  I drilled 3/8" holes for 5/16" bolts, allowing for a bit of misalignment in the installation. 
    - The top 2" of the chain plate should be bent forward about 100 to be inline with the backstay.  Bend this in a bench vice. 
    - Mark the plate so it is installed leaning forward!  Don't say I didn't warn you!!
  4. Temporarily place the chain plate in the transom hole at its intended angle and use a ruler to transfer a parallel line from the plate to the outside of the transom as in fig 2 below.  With the chain plate held against the outside of the transom (a spacer at the bottom of the plate will stabilize it parallel to the hull) and lined up squarely to the top hole, draw a line on the transom.  Mark the 3 mounting holes on the transom with a transfer punch. 
  5. Drill the holes slowly so it doesn't punch out on the inside.  All measuring and drilling must be done perpendicular to the transom so the holes line up.
  6. Lower the chain plate through the transom hole till the 3 bolt holes line up.  Insert the bolts and large washers coated with butyl rubber sealant behind each head. 
  7. On the inside, tighten each nylock nut to just prevent movement.  This is definitely a two person job.  It helps to have the arms of an Orangutan!
  8. Seal the deck hole and wipe away any excess.  Make it look pretty.  Then vacuum the drilling dust from under the cockpit.
    - Its not necessary to add sealant between the chain plate and the fibreglass but it will prevent movement.  Your choice.


Fig 1 - Panache's transom showing the planned second chain plate in red.  This is closest to a symmetrical installation without a huge amount of rework of the thru hull fitting.  The installed 1/4" thick SS chain plate actually extends 6" below the red line so the mounting bolts would be well below the thru hull fitting.  Unfortunately the nut of the thru hull fitting interfered with where the plate was installed so a 1/4" thick spacer was required around the fitting.  I used a hole saw to drill through 1/4" thick vinyl "puck board" creating a doughnut spacer to slip over the fitting.  This created a uniform flat surface to tighten the nut against.  Problem solved. 

By the way, that is ice nestled in the orange tarp at top right.  Fortunately Mother Nature
blessed us with "warmer" temperature the next day.  This is February.


Fig 2 - I drew a pencil guide to outline the chain plate on the hull.  This guide is required to position the plate for an exact alignment to mark the bolt holes for drilling.  Use a center punch so the drill bit doesn't "walk".  After all, this exacting work.


Fig 3 - Mounting holes drilled but the nut of the thru hull blocked the 1/4" thick SS chain plate.  This is where I slipped a 1/4" thick spacer over the thru hull fitting and tightened the nut against the common surface of the chain plate & spacer.


Fig 4 - The finished installation sealed to the deck.  It's quite a reach to the top of the transom but my box end wrench holding the nut with butyl rubber did the trick.  It helps to drill the chain plate holes 1/16" larger than the mounting bolts.



FIELD OBSERVATIONS of BACKSTAY ADJUSTER - At the end of the first season of use I now feel it is fair to judge the performance of this backstay adjuster.  I installed a lot of things at the same time and had some growing pains as a result, hence the delay.  Keeping in mind that Panache has a split pushpit, I have noticed the following since the installation:
  • It is now more difficult to board the boat via my transom hung boarding ladder.  While "little people" can go through the split bottom wires, "big people" step over the pushpit.  Dock or dinghy boarding on Panache is usually done through a cockpit side gate.  Tech Tip B21.
  • In the beginning I found it more difficult to reach the side of the outboard to push the starter or kill switches.  I've learned to lean overboard between the wires and its better.  Although I added the extension handle on the shifter for the delicate task of motoring into a slip.  Tech Tip D07.  I've finally learned to remove my hat and take stuff out of my pocket before bending over the transom!
  • While I haven't tried yet, it would be more difficult to pull start the engine.  With a single backstay it was relatively easy.
  • There is less forestay sag from an observation I made from the pulpit, although I've never actually measured this while I had hanked jibs.
  • The boat sails faster with the backstay tightened.  In fact it sails very well with the 110% working jib that was converted to roller furling.
  • I'm happy that I moved the safety wire with the turnbuckle to the same side as the block and tackle.  It allows me to release the barrel so the mast can lean further forward with control while I pin the forestay when stepping the mast.  Pinning is more arduous with the weight of the furling system drooping the forestay.  See Tech Tip F10. 
  • The flexible backstay wire over the wire turning block is still very healthy after several seasons of use.  I originally thought the groove in the sheave might not be deep enough to support the wire but it supports it well with no flattening.  In addition, there isn't even a bend in the wire showing where it went around the sheave so the radius is correct for the wire size. 
  • All good stuff.  So go ahead and install a backstay adjuster.



Q - "I do a little racing now and then and I noticed that I have some head stay sag that is affecting my upwind performance. I've thought about installing a backstay adjuster and would probably use the factory design you've posted here.  I wonder how well this would work with a mast head rig and deck stepped mast.  It seems to me it could put a lot of extra downward force on the deck and compression post and I wonder if you see it as a potential problem more than a benefit.  I have a friend who has a deck stepped fractional rig and the pull results in bend rather than a downward force.  I have seen some SJ23's with adjustable backstays, in a split configuration, and it didn't look like the deck was damaged or anything so I wonder what the factory design looks like.  I tend to worry about the small things too much so maybe it's only a quick answer for you!  Your advice is greatly appreciated."  Bret.

A - "If you want to be successful in racing or fast cruising, you must have a backstay adjuster to tension the forestay.  It's one of the best gadgets for pointing upwind as high as possible.  For downwind sailing you slack it off and away you go with fuller sails to grab the wind.  This adjuster works even better on the SJ7.7 with its fractional rig.  To understand this you have to realize the mechanical advantage that makes it possible to bend a fractional rigged mast with less effort.  On a mast head rig it is less beneficial, but still useful.  I added one to my previous Macgregor Venture 222 with mast head rig and it helped a lot going upwind.  I also added a baby stay adjuster that could flatten the mainsail by bending the mast forward.  A baby stay can bend a mast very easily so use a stopper knot to limit the pull.  Don't overdo it." 

If I were to install a backstay adjuster on an SJ23, I'd install the Clark design shown above.  An adjuster can increase the loading on the bottom of the mast, as you suggest, but this force is NOTHING compared to the load when the boat is knocked down on her side.  Panache is one of the earliest hulls out of the mold and she survived two knock downs in 2000 without damaging the original standing rigging.  So don't worry about damaging the deck.  The forces are well distributed through the tabernacle and supported by the compression post under it.  If you are concerned about the deck see Tech Tip F03."  Bob.

Construction - Bret modified the original Clark design somewhat by installing a new 5/32" backstay with Sta-Lok fittings for the wire block.  This is the smallest wire size a Sta-Lok can fit to.  He ran the free end of the control line to a cam cleat so it is easy for the helmsman to pull or release the line when heeling.  Below are the photos of his installation on Cosmo.

Fig 1 - The control line lead directly to the helmsman.  Speed of operation isn't that critical but convenience for the helmsman is.

Fig 2 - Looking aft from the cabin.  I like the contrasting coloured line.  Makes it easier to identify in the flurry of activity.

Fig 3 - The all important, extremely strong Ronstan wire turning block held with a Sta-Lok fitting at the bottom of a new 5/32" backstay.  The free shackle is probably a good place to clip the end of the boom for storage.

Fig 4 - The overall view. Very nice work.

"In future I will change the purchase ratio from 3:1 to 4:1 as shown in this Tech Tip.  I can add a lot of tension with the 3:1 but the pull gets pretty hard towards the end.  I use 1/4" Yale Warp Speed for the running line since it is very strong and low stretch.  The safety wire around the blocks will be added shortly as backup."  Bret.




A variation of the Clark design is to terminate the shortened backstay to an 8:1 block and tackle on the factory port chain plate, albeit without the important backup safety line.  This variation eliminates the task of installing a second chain plate on the starboard side which is a tad difficult with the cockpit drain tubes blocking access under the cockpit.  On the other hand, you could install an inspection port on the starboard aft end of the cockpit.  See Tech Tip D03.  An access hole that you can also use to inspect the outboard bracket bolts.  "Don't store stuff inside here!  Geez."

An advantage of this design is that the second wire doesn't block access to the outboard or passage over the transom slot.  Something to consider if you board the boat here.




For ultimate strength and peace of mind the split backstay adjuster system that squeezes the two split wires together with a couple of wire blocks is the strongest and most dependable.  While there is enough space on the top of the transom to fit a 2" wide pad eye just inside each chain plate, the 2 pad eyes must be reinforced from below to offset a potentially weak fibreglass joint under the black corner moulding.  It would be a daunting task to fit an angle reinforcement plate and tighten the nuts from below, considering the tight space.  In any case, the squeezer design works best when the 2 split wires are about 300 apart which is impossible to install over the narrow spacing of the SJ23 transom chain plates.  To give this design a fighting chance it should have ball bearing blocks to roll along the 2 bottom wires.  For these reasons I rejected this system.  Just thought you should know. 

NOTE - A backstay adjuster is not cheap since the hardware has to be strong enough to replace the bottom 5' of the backstay.  Verify the strength of the blocks, etc when you buy.  The two systems discussed here cost about the same with each having its pros and cons.   See Tech Tip F10.

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