SJ23 Tech Tip F09, (Updated 2007-04-15, Bob Schimmel & Bret Hart)

Factory Adjustable Backstay Kit.

Think you don't need a backstay tensioner because you only cruise your SJ23?  Think again.  A head stay that sags when the wind speed increases is bad for windward performance  because it makes the shape (draft) of the headsail fuller.  Wind flowing across a full sail translates to greater heeling force and less forward drive which means you can't point as high.  This is why you need the jib to be flatter in medium to heavy wind.  This effect may be magnified in a partly rolled headsail when the extra sailcloth in the belly adds draft too far aft.  In addition, an excessively curved (sag) forestay increases the friction inside a roller furling foil, making the sail more difficult to roll up. 
Don't be tempted to tighten the forestay to solve this problem.  The backstay can tighten the forestay with less force due to a greater mechanical advantage (larger angle) to the mast head.  A tightened head stay relates to approximately 1/2 knot of extra upwind speed.  Not too shabby!  To avoid straining the rigging, ease the tensioner when the wind drops or the boat is at the mooring.

Shown at right is a factory 4:1 backstay adjuster kit that was sold by Clark Boats to those sailors wanting to do some racing.  Some boats were equipped with this option from the factory but most were installed afterwards by the owner.  This design fatigues less and releases easier than a back stay tensioner that is squeezed together by a couple of rolling blocks.  It is also more compact and takes up less transom space which is important on the narrow transom of an SJ23. 
NOTE - Rolling blocks work best on a split backstay where the 2 bottom wires are about 30 degrees apart.
The part numbers shown are early 1980s vintage so you will have to update them to current vintage.  The diagram gives you a good start to search for equivalent parts.  A good safety feature of this design is the stainless steel safety strap to support the mast if the Dacron line breaks.  The length of the safety strap can be set for minimum backstay tension required for sailing downwind or to relax the rigging tension while at the dock.  Don't be tempted to relax the rigging tension too much as it will promote shock loads in the wire as the mast flops (millimetres) with each roll of the hull.  Metal fatigue will break rigging components with catastrophic results.  Use low stretch Dacron for the control line to prevent shock loads into the rigging.
  Clark's design requires halyard grade wire to be used over the large Scheafer 33-13 block.  The wire could be replaced with 1/2" Dacron line and matching block to make it softer.  Each design has its pros and cons.  Just in case you have the heeby creebies about shortening your back stay and installing a compression fitting, check out this product.

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

NOTE 1: This diagram shows the control line on the preferred port side to avoid clutter around the engine.  This is the same reason why the factory installed the single fixed length backstay on the port side.  When installing the starboard chain plate, place it outboard a couple of inches from a structural reinforcement running almost vertically on the inside of the transom.  Click here for the parts list.

NOTE 2:  A variation of this hardware configuration is to replace a lower portion of the back stay with a 6x1 or 8x1 block and tackle fastened to the existing port chain plate.  This is potentially weaker so you should install a safety line around the block and tackle as a backup to failure. 

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 an adjustable back stay tensioner and would probably use the 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 would be greatly appreciated."

A - If you want to be successful in racing or fast cruising, you must have a backstay tension adjuster. It's one of the best gadgets for pointing as high as possible upwind. For downwind sailing you slack it off and away you go.  This tensioner works really well on the SJ7.7 with the fractional rig.  You have to understand how a fractional rig is supported to understand the mechanical advantages that make it possible to bend the mast with less effort.  On a mast head rig it is less beneficial, but it's still useful. I added one to my previous Macgregor Venture 222 with mast head rig and it helped a lot going upwind.  It also had a baby stay that could bend the mast quite easily.

If I were to install the back stay tensioner on an SJ23, I would use the design recommended by Clark as shown in this Tech Tip.  I realize it will increase the loading on the bottom of the mast, but this force is NOTHING compared to the load on the bottom of the mast 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.  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 really concerned about damaging the deck under the tabernacle, add some high quality marine bedding compound like Sikkens Sikaflex or butyl rubber.  Panache's mast step has Sikaflex.  Remember to clean the deck area with acetone, coat the surface liberally with the adhesive and then push the tabernacle down on the deck to squeeze out as much extra sealant as possible.  Make a fillet from what oozes out to seal the deck to tabernacle joint.  Remember to seal all the bolt holes as well.

Construction - Bret modified the original Clark design somewhat by using a Sta-Lock fitting for the block at the bottom of the back stay.  He ran the free end of the control line into the cockpit to a solid cam cleat so it is easier for the helmsman to pull and release the line when heeling hard.  Below are the results of his installation on Cosmo.

A view of the control line lead to the helmsman.
Looking aft from the cabin.
The all important turning block with the Sta-Lock fitting at the bottom of the back stay.
The overall view. Very nice work.
"In the future I will change the purchase ratio from a 3:1 to a 4:1 as drawn in the tip. I can add a lot of tension with the 3:1 but the pull gets pretty hard when I get to the end of it.  I also used 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." Bret.
The safety wire is required to back up this style of adjuster, just in case the tensioning line breaks due to UV rot or overloading. The last thing you want is for the rig to come tumbling down. Never compromise safety, strength and endurance for a "go fast" gadget.
A variation of the Clark design is to terminate all the block and tackle lines on the port chain plate.  This eliminates the task of installing the starboard chain plate which can be difficult with the cockpit drain lines blocking access to the starboard side of the hull underneath the cockpit.  That is, unless you install an inspection port on the starboard side of the aft cockpit.  See Tech Tip D03.  Now this makes access to the starboard side much easier.
BABY STAY - Another go fast gadget you could try is a baby stay from the spreaders to a deck fitting, forward of the hatch.  Equip the bottom end with a six foot long roller tube over an 8:1 block and tackle.  I installed one of these on my previous Venture 222 and it worked wonders for flattening the mainsail.  What makes this gadget work so well is that it doesn't take much effort to pull the middle of the mast forward to flatten the mainsail.
- A couple of words of caution though, you MUST reinforce the underside of the deck with a substantial cross beam to support the loads under the baby stay.  In addition, you MUST have a stopper knot on the line to limit the amount of pull.  There are lots of deck apes out there who don't know their own strength and under the rolling motion of the boat it is easy to overdo an adjustment like this.  The stopper knot makes the operation idiot proof!  This is no reflection on the apes!

Return to Techtip Index. . . . . . . . . . . . . . . Have a Question?