SJ23 Tech Tip F09, (Updated 2018-01-29, Bob Schimmel, Art Brown & Bret Hart)

Back Stay Adjuster & Second Chain Plate - A factory design that is easy to operate.
INDEX - Consideration for Roller Furling, Bret's Installation, Baby Stay, Removable Inner Stay.

Some sailors consider this a "go fast" gadget for racing and others consider it "essential" for fast cruising.  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.  It was just above freezing, with very steep waves that the old Autohelm 300 could barely deal with.  We tightened the back stay adjuster to save the mast from pumping itself to pieces.  Tightening the rig to save it was a first for me.

So you think you don't need a back stay 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 main need to be flat in medium to heavy 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 have you release the sheet and halyard a bit to ease the strain of rolling up the jib over a tight forestay.

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

Shown above is a factory 2(4:1) back stay adjuster kit that was sold by Clark Boats to those sailors wanting to do some racing.  Thanks to Art Brown for preserving this document from the Seattle SJ23 Club.  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 shown in the diagram.  The part numbers shown are early 1980s vintage so you will have to update them to today's parts.  The diagram gives you a good start to search for equivalent parts.  Below are the parts I used.

UPDATE A CONSIDERATION for ROLLER FURLING - When roller furling is installed on an SJ23 the length of the forestay is fixed and its tension is set by the back stay.  Most roller furling manufacturers recommend sailing with a tight forestay as this allows the luff foil to rotate more easily which improves the overall performance; particularly important when you need to roll up cloth in a gathering breeze.  They also recommend easing the rig tension at the dock and to release the halyard tension to save the jib.  The thread of a standard turnbuckle (back stay) is not designed to be adjusted during varying sailing loads.  It will eventually fail.  Therefore it makes sense to install a back stay adjuster equipped with a safety wire. 
I've decided to add a turnbuckle to Clark's design to set the minimum rig tension using the safety wire as the primary support line.  This way the mast support is not reliant on the line or the cleat of the bottom block.  To be installed in Spring 2018.


Pay particular attention to the 1.5" wire block (Harken 304) where the back stay splits.  It MUST match or exceed the breaking strength of the 1/8" SS back stay wire.  The sheave groove MUST also match the 1/8" 7x19 SS wire and it should be possible to open the block to easily replace this wire without lowering the mast.  Clark's design requires 1/8" 7x19 flexible halyard grade SS wire to be used over this block.  While the block and tackle (fiddle blocks) will handle just under half the load of the back stay block, they too must match or exceed their load.  All wire ends will be swaged with a Nicopress copper sleeve and eye thimble.  Never use an aluminum compression sleeve for standing rigging.



 Back stay, 1/8" (1x19) 316 SS wire  BL 1780 lbs, SWL 356 lbs.
 Split stay, 1/8" (7x19) 316 SS wire  BL 1670 lbs, SWL 334 lbs.
 Safety wire, 1/8" (7x19) 316 SS wire  BL 1670 lbs, SWL 334 lbs.
 Back stay wire eye splices  A 1/8" copper Nicopress sleeve with a SS eye thimble,
 3 crimps per sleeve = 100% strength of wire.
 Back stay wire block, Harken 304  BL 3000lbs / 1361kg, SWL 1500lbs / 680kg.
 Top Fiddle Block, Ronstan S40 RF41500  BL 2200 lbs, SWL 880 lbs.
 Bottom cam/fiddle/block, Ronstan S40 RF41530  BL 2200 lbs, SWL 880 lbs.
 Control Line  5/16" Dacron low stretch, ? ft.
 Chain Plate, starboard  316 SS (1/4 x 1 x 10)" strap.

NOTE - If your back stay block does not swivel, then swage it in line with the split wire to achieve a fair lead over the sheave.

SAFETY WIRE and BLOCK & TACKLE - A mandatory safety feature of this design is the 1/8" 7x19 SS safety wire that supports the mast in case the tensioning line releases from the cleat or breaks.  You should consider it as the primary support and the block and tackle as secondary.  Never compromise safety, strength and endurance for a "go fast" gadget. 

  • The back stay wire block is positioned just above the boom to make it convenient to secure the boom at the mooring. 
  • The length of the safety wire is cut to take up the slack with the turnbuckle thread about 1/4 engaged.  Then the turnbuckle is tightened to the minimal tension required at the mooring.  Don't be tempted to set the tension too low as it will promote shock loading in the rigging when the mast flops (millimetres) with each roll of the hull.  Fatigue breaks metal rigging components with catastrophic results.  (Length to be determined at installation).
  • I'm adding a double triangular SS plate at the top and bottom of the safety wire to support the mast when the block and tackle is relaxed at the mooring.  (Plate = 2" SS face with 1/4" gap & spacers between).  I think this is a cleaner, stronger way to support the mast.  Time will tell.  It is easy to redesign.
  • The 2 plates will swivel towards the helmsman and the bottom one will tilt fore/aft for stepping the mast, duplicating the function of a toggle at the bottom of a turnbuckle.
  • The top fiddle block for 5/16" line will swivel with the bottom block.  (Ronstan S40 RF41500 fiddle block, BL 2200 lbs, SWL 880 lbs).  Smart feature - The 2 sliding posts are pushed in to 2 stop detents to fix the swivel pin at 00 , 900 or disengaged for free turn, all without tools.
  • Bottom fiddle block for 5/16" line will swivel towards the helmsman to pull line.  (Ronstan S40 RF41530 cam/fiddle/becket, BL 2200 lbs, SWL 880 lbs).  Smart feature - 2 sliding posts are pushed in to 2 stop detents to set the cleating angle without tools.
  • Use low stretch 5/16" Dacron for the control line to prevent shock loads into the rigging.  (5/16" Dacron low stretch).
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 going forward.  Instead, ease the line out with your hand with a soft landing at the knot.

SECOND CHAIN PLATE - There is no getting away from it, the starboard chain plate is difficult to install due to the tight space below and the cockpit drain tubes.  However, I removed the port cockpit drain tube to make it possible to reach the top of the transom for this installation.  I will likely have to fabricate a super reach tool that can hold a nut without dropping it.  Failing that you can take a lesson from Bobby and Bill in (Tech Tip D03) to install a 6" diameter inspection port in the aft end of the cockpit.  Its fairly easy to install.  I just have to protect the gel coat from the jig saw using some tape.  Some SJ23s have a structural reinforcement running almost vertical on the inside of the transom.  That being the case, install the new chain plate outboard from it.  Once you determine the location, drill and cut the hole through the top of the transom, just inside the corner moulding similar to the port plate.  Then hold the chain plate against the outside of the transom, line it up to the top hole, and mark the 3 mounting holes with a transom punch.  Drill the holes.  Slip the chain plate through the top hole, line up the holes, apply some sealant to the fibreglass side of the plate and insert the bolts with a washer and sealant behind each head.  Slip large washers behind each nylock nut and tighten.  Two people make this job a lot easier.  Its not really necessary to add sealant between the chain plate and the fibreglass but it will prevent vibration movement.  Seal the deck hole and wipe up any excess.  Make it look pretty.  Shown below is Panache's transom with the planned second chain plate.  It will extend down further than the red line shows to get away from the weaker through hull fitting.

  • Chain plate - (SS (1/4 x 1 x 10)" 5/16" turnbuckle hole, three 3/8" mounting holes, same as factory chain plate).



For ultimate strength and peace of mind the split back stay 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 back stay adjuster is not cheap since the hardware has to be strong enough to replace the bottom 5' of the back stay.  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.

Bret Hart's Installation
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 back stay 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 back stays, 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 would be greatly appreciated."  Bret.

A - "If you want to be successful in racing or fast cruising, you must have a back stay tension adjuster.  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 really well on the SJ7.7 with its fractional rig.  To understand this you have to realize the mechanical advantage that makes it possible to bend the 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 bend the mast and flatten the mainsail.  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 back stay 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 mould 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 Sta-Lok fitting for the block at the bottom of a new 5/32" back stay.  This is the smallest wire size 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.

The control line lead to the helmsman.
Looking aft from the cabin.  I like the different colour line.
The all important, extremely strong wire turning block held with a Sta-Lok fitting at the bottom of a new 5/32" back stay.  The free shackle is probably a good place to clip the end of boom line for mooring storage.

The overall view. Very nice work.

"In the future I will change the purchase ratio from a 3:1 to a 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 back stay and 4:1 block and tackle on the port chain plate.  This eliminates the task of installing a new chain plate on starboard which can be difficult with the cockpit drain lines 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.  "No, don't store stuff inside here!  Geez."

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