SJ23 Tech Tip F38, (Updated 2025-10-07) Bob Schimmel, Greg Overton.

Index

Mainsail Cover - Conventional or Stack Pack.

CONVENTIONAL COVER - A mainsail cover of Sunbrella prevents UV damage to the sail when it isn't used.  It also protects the mainsail from storm damage or bird poop!  Many sailors customize the cover with the name of their boat.  I much prefer to cover the mainsail than remove the sail from the boom after each float.  As the expression goes: "It's definitely the lesser of two evils." 
  • The fabric must be water proof and able to breathe so the sail can dry.  Use Sunbrella or equivalent cloth.
  • The bottom must be open between the fasteners to let the sail breathe.  Having said this, it is not unusual to be sprinkled with rain water or dew when hoisting the mainsail.  I seldom pack the mainsail when wet but sometimes it is unavoidable.
  • The most forward twist fastener should be located about a foot behind the mast to ensure the wind does not open it.
  • The most aft twist fastener should be located at the very end of the cover to prevent the fabric from flapping to death.
  • The cover must be able to stand up to a very strong wind.  Although, a few times I've spiralled a line around the cover to survive a storm.  For hurricane strength wind I would slide the whole works into the cabin; boom with mainsail & mainsail cover on it.  I've had to do this twice with Panache.  It's the quickest and easiest protection to ride out a vicious blow.

DON THE COVER - I place Panache's rolled up cover over the boom, similar to throwing a saddle on a horse.  The top straps are tied around the mast just above the raised spinnaker ring.  The internal flap is folded against the front of the mast to protect the anodized finish from the YKK zipper that zips up the front.  The bottom straps are tied around the front of the mast to off load the zipper during strong wind.  And finally the cover is unrolled towards the end of the boom where the end straps are tied around the boom topping lift to smooth the wrinkles.  Finally the SS twist-locks along the bottom are done up.

I also set the main sheet to starboard, secured with a line tied to the bottom of the starboard stanchion, pulling against the main sheet to keep the boom from swinging.  The boom topping lift keeps the boom just above my head.  I board Panache from port just like a horse. 

DOFF THE COVER - The sail cover is usually rolled up from the forward end to the aft and stored on the forward berth with the jib sleeve. 
 

PANACHE'S CONVENTIONAL MAINSAIL COVER.
 

Fig 1 - The internal flap to protect the anodizing from the #10 YKK zipper.
This top hem is worn and was replaced in 2024.

Fig 2 - The bottom straps under the cleat.  I tie a bow knot at the top and a reef knot at the bottom because the bottom straps are too short.

Fig 3 - End of boom straps.  The twist-lock fastener was moved aft, away from the mainsheet block.  I've always had trouble doing that one up.

Fig 4 - A smooth mainsail cover can withstand lots of wind.  I tend to tension the cover too much.  No need to do that.


Each SS twist-lock fastener is attached to a small square of Sunbrella cloth which is then sewn to the cover.  This technique protects the cover from damage it is fairly easy to move or replace a square with twist-lock.  This technique makes it simple to lay out a final position for all the twist-locks, but it goes without saying that it is best to do it right the first time.

Panache at peace in her slip during the week between crane out and dock out.  Kinda nice to have the place to myself!


 

Fig 5 - Dimensions of Panache's mainsail cover laid out on the ground.
The heavy duty marine grade #10 YKK zipper is 44" long leaving 2" to sew straps on the top and bottom.
Straps should protrude ~18" to tie a bow knot that can be doubled in case of a storm.
 


 

WATER PROOFING (2020)- This mainsail cover has been on Panache's boom since ~1980.  I've added a few bits and pieces over the years to improve it, and repaired loose stitches, but not much else.  It has also received occasional water proofing with silicon spray but I have never seen the need to wash it, thinking that the rain could do that job.  However, after all the years of service I thought it high time to apply a thorough water proof treatment.  First I washed it in a laundry tub where to my surprise a lot of coal dust came out.  This is from the adjacent open pit coal mine that is now closed.  Then I applied NIKWAX TX.DIRECT Spray-on till it saturated the fabric.  The advantage of Nikwax over silicon spray is that it can water proof the fabric and retain breathability.  I like the fact that the bottle has a pump, negating the propellant that can affect the chemical cocktail creating the water proofing.  Nikwax can actually water proof down feathers in a sleeping bag so you stay toasty warm in damp weather. 

AFT STRAPS (2023) - I finally replaced the short strap on the aft end of the cover with a longer reinforcing strap that also straddles the seam where I extended the cover to full length.  The end of this reinforcing strap includes a SS ring to minimize wear from the pull of the tie down strap. 

TWIST LOCKS (2024) - A SS twist-lock used to be installed right where the mainsheet block rests, making for difficult fastening and an unsightly appearance in the slip.  I was reluctant to move it considering the hole that would need to be mended.  Now that I have a heavy duty sewing machine I finally moved the twist-lock aft then added another one forward of the mainsheet block and mended the hole.  This will allow the cover to rest straight and wrinkle free on the boom.  The hem for the top strap around the mast was showing some wear and was replaced.  There were also some small tears from snags that needed mending.  After a good washing and water proofing, the cover is good to go.

Panache's mainsail cover has survived so many storms that I've lost track.  This is the single redeeming quality that keeps me from converting it to a stack pack; besides the fact that I don't feel like changing a perfectly good working system!  But it is showing signs of degradation so I took the time to do some sewing.  Look after yours. 
 

Fig 6 - My finger pointes to where the annoying SS twist-lock fastener was installed and the paper clips mark where the replacement twist-locks will be installed. 


 

Fig 5 - Hole is patched and 2 new SS twist-lock fasteners attached. 


 

Fig 6 - New hem for the top strap.

 
 

UPDATE  NEW ZIPPER (2025) - After 40 some years the zipper at the front finally broke.  They just don't make stuff like they used to!  I was forced to cut through the zipper fabric to release the sail cover after several bottom teeth broke off.  That's frustrating to have to do on the ramp during haul out.

Besides replacing the #10 YKK marine grade plastic zipper, I also replaced the short bottom straps for longer ones.  The softer material should hold a knot well, creating better support to the bottom of the zipper.  The top straps will remain as they are.
 

STACK PACK - A stack pack is an improvement on the conventional mainsail cover since all you have to do is pull the zipper along the top to expose the mainsail for hoisting.  This speeds deployment and more important, speeds dousing for inclement weather.  Its all in keeping with the edict that, "If its quick and easy to do you are more likely to cover the sail quickly to protect it".  The covers stay on the side of the boom while sailing so you don't have to pack it away, eliminating another loose item on a berth.

Some sailors object to the loose cloth hanging along the boom and have figured out how to pucker the cloth up against the boom, leaving things neatly stored and the mainsail fully exposed for reefing.  This clever bit takes some ingenuity.  The better designed stack pack can be pulled tight against the boom so it doesn't flop around when sailing. 

Closing the front was problematic with the early versions as the open sides would catch wind.  Experimenting with different techniques solved this problem with designs that wrap around the mast as shown in the example at right.  Other designs tuck it against the aft face of the mast.  In many cases the portion that goes around the mast is tucked in one side along the boom.  On some designs it is removed and stored in the cabin.

The lazy jacks typically hold up each side of the stack pack with the assistance of a PVC rod through the hem along the top.  This makes catching the mainsail slick and pulling the zipper along the top easy.  The downward slope to the end of the boom sheds rain, albeit into the cockpit.  But that's what cockpit drains are for.

This one looks like a Sailrite design.
 
A LOOSE TOP STACK PACK.
 
 

If you look carefully, the front portion that wraps around the mast, is the only part that detaches.  The remainder stays on the boom as part of the lazy Dutchman system.  Its a clever design that makes it quite easy to don and doff for one person.  No more walking to the end of the boom to stretch the cover to the end.  Only one zipper from what I can tell.

I think the label reads AF Sails.

STACK PACK LINKS -

 

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