Index

Mechanical Boom Vang with Internal Gas Spring.
INDEX- Pneumatic Vang, Fixed Length Vang, Installation, Operation.

I made this variable length mechanical boom vang for Panache to keep the boom above my head and to be the extra pair of hands when reefing or lowering the mainsail.  There is enough force from the internal "gas spring" to offset the weight of the boom so the mainsail can assume its designed shape in light air and the leech doesn't cup or stretch prematurely when the boom is sheeted in.  However, if I use my mainsheet to its full 8:1 mechanical advantage, there is a risk of bending the boom over the end of the fully compressed vang.  So I am very careful with how hard I pull in the main sheet. 
(NOTE - This problem was eliminated with a longer gas spring and an adjustable boom topping lift.  It is an example of why the component dimensions are critical to fit the boat.  I learned this part by trial and error). 
The topping lift is now permanently attached to the end of the boom.  When the mainsail is reefed in heavy air, the topping lift limits the downward travel of the boom, thereby offsetting the mainsheet tension and protecting the leech from stretching. 

In summary, a variable length vang allows you to set a perfect leech shape to match the jib, induce twist in the mainsail to spill wind or to support the boom during reefing, at any angle the boom is sheeted.  But the real advantage is what it does for the shape of a full mainsail in light air.  It pulls very well with the correct draft.  This has to be experienced to be appreciated. 

NOTE 2: It is advantageous to extend the vang control line to the cockpit from where you can see the curve of the leech as you adjust the vang tension.  No more guessing the mainsail shape after crawling off the cabin top to adjust it.  This saves a lot of time when things get dicey out there. 
 

Fig 1 - I intentionally installed a blue vang control line to simplify identification back in the cockpit.

Fig 2 - The vang control line terminates at the black starboard rope clutch and the loose end hangs just inside the cockpit.

Fig 3 - It was a bit difficult to position the turning block at the base of the mast so the vang could maintain the sail shape through the 1800 swing of the boom.  It also had to create a fair lead to the deck turning blocks to minimize friction. I deleted the second block at the base of the mast where the line  rubbed (friction) making adjustment difficult.  The line now goes from the block at the bottom of the vang directly to the deck turning block.  KISS principle at work again.  This is likely my final configuration.

NOTE 3:  Also read Tech Tip F08, Boom Jibe Preventer, as there are many useful benefits to be had with a preventer.
 

1 - DESIGN CRITERIA of a VARIABLE LENGTH PNEUMATIC VANG.

A WORD OF CAUTION - A pneumatic vang equipped with an 80lb. gas spring can exert a surprising amount of force on the mast and boom.  If the gooseneck were to break while you are sailing, it could damage the deck.  So machine all the fittings to the closest possible tolerance and don't scrimp by using underrated components or undersized material.  Also, keep the gooseneck bearings lubricated with a light water proof grease.

FIXED GOOSENECK - The position of the boom gooseneck must be fixed (up/down) to the mast or the vang cannot lift the boom.  There are a number of ways to fix the gooseneck to the mast.  I temporarily inserted a 15" long wood dowel inside the mast kerf (groove that accepts the mainsail slugs) between the vang gooseneck (at the bottom of mast) and the boom gooseneck (15" up the mast).  Then I tighten a lied a line from the bottom of the goose neck to foot of the mast to fix it down against the top of the dowel.  Effectively this connects both goosenecks via a rigid dowel inside the kerf, fixing the distance between the two.  I modified this temporary technique by filing a shallow notch in the mast to accept the mast stopper from sliding.  This has worked so well that I've seen no reason to change it.  Track stoppers are available from West Marine for about $5.95 (US) each.  The SKU is 108340.  See image of stoppers below.

HINT - If you have fixed the height of your boom vang for your mechanical vang and want to install a mast gate as shown in Tech Tip F07, then position the gate so the bottom mainsail slug DOESN'T rest in the gate with the sail hoisted.  The gate is weaker than the kerf of the mast and you don't want it to collapse under load while sailing.

Quinton's Fixed Gooseneck - "I got tired of my track stops getting loose and the goose neck dropping.  So, I drilled two horizontal holes through the track slot to fit two clevis pins and secured each with a split ring.  I installed one above and the other below the boom.  I keep some extra pins on board in case I fumble installing one. To ensure a straight hole I made a wood template and clamped it to the mast to guide my drill bit.  A crooked hole is most frowned upon!  BTW, on my mast the gate for removing the boom and sail slugs from the track is below the goose neck.  This motivated me to install the two clevis pins with split rings instead of bolts.   Quinton Tormanen What a clever and brilliant idea to install the sail gate below the boom instead of above it.  This way the sail slugs never rest against a weak gate.  The difficulty lies in setting the boom on the mast while holding the mainsail up and later setting the mainsail on the boom.  I'm sure that Quinton has a procedure.

FROM THE SJ23 MANUAL - "The length of the mast extrusion is not limited.  However, two 1/2" wide black bands (use electrical tape) are to be maintained on the mast.  The lower edge of the upper band is to be no higher than 26’ above the mast step.  The upper edge of the lower band must be maintained at 2' above the mast step.  The mainsail, when hoisted, shall not extend its topmost portion above the lower edge of the upper band and shall not be down hauled below the upper edge of the lower band."  I hope this is clear as I paraphrased it from the manual.  The bottom line is that if you sail your boat within these constraints, you may as well fix your gooseneck to the mast.

VANG LENGTH - Proportion the length of the vang to fit the mounting locations on your boat.  The length overall of the vang for an SJ23 (bottom mounting bolt to top mounting bolt) is 44" compressed and 47" extended.  The compressed length is set by the length of the internal dowels and the gas spring, fully compressed.  The extended length is set by the control line outside tubes.  In working mode the vang is ~46" long.  The vang is rests at ~30⁰ up from the deck.  When the vang is extended, the boom is raised just enough to open up the leech.  Regardless of how much measuring you do, the adjustable fitting on the track under the boom will absorb and correct your last mistake!  Always build your design with some margin to compensate for error.  You will find an optimum position under the boom. 

PARTS - Shown below is my vang during reconstruction to receive a new 27" long gas spring.  The parts are laid out in order of assembly.  The top fitting was removed from the aluminum tube and is laying on the right, at the end of the dowel.  This is a new gas spring with 85 pounds air pressure and enough stroke for the required operating range.  There were many times when I wanted to lift the boom higher so the extra stroke will provide that.  The extra stroke also allows me to pull the boom in closer with less fear of bending or breaking it on the fully compressed vang.  In the photo below the components are laid out with the gas tube at the top of the vang so the oil flows down to the seal, thereby maintaining the seal for longer life.  This also makes it easy to assemble.  Below are the component dimensions:

Note the aluminum tape wrapped around the black barrel of the gas tube.  The tape increases the diameter so it fits snug inside the inner tube.  This tape does not slide so it doesn't wear out.  It is also at the top of the assembly so it stays dry.  I drilled a drain hole at the bottom of the lower tube because I found water inside.  This spring has 80 pounds of push (pressed into a bathroom scale), which is quite a bit more than my original gas spring.  While the control line will have to be pulled in all the time to adjust the height of the boom, the 80 pounds push is enough to dispense with the topping lift while reefing. 

TUBES - The aluminum tubes are 1/8" thick wall. Use tubes that are at least 1/8" thick to create a solid support for the end fittings.  If you want to use thinner tubing, then increase the diameter to prevent buckling.  The outer tube is 1 5/16" OD and the inner tube fits snug inside, telescopic style.  Aluminum tubing is manufactured in concentric dimensions so adjacent sizes fit, almost machine like.  Strive for a reasonable overlap so the tubes stay in column to minimize jamming and wear.  The bottom tube is the outer one at 37" length and the upper tube is the inner one at 22" length.

GAS SPRING & POSITIONING SPACERS - Use an automotive gas spring (type used to lift a hatchback, etc) inside the inner tube, (see drawing above) to create enough push (~70 lbs.) to offset the weight of the boom with the mainsail folded on it.  If you find a spring that has more push don't worry, you have to compress the vang with an external control line anyway.  Use a gas spring with round end fittings so it centers on the dowel spacers and therefore never touches the inside of the tubes.  If the OD of the gas spring barrel is smaller than the ID of the inner tube, wrap tape around the barrel to center it in the tube.  I used several wraps of aluminum furnace duct tape.  Install the spring with the pressurized black barrel at the top so the oil inside the barrel flows down to the seal to keep it lubricated and retain the gas pressure.

• You can measure how much force the vang has to hold by placing a scale against the mast with the vang pushing on it.  This is largely a function of how much your boom & mainsail weigh.
• Assemble the vang as shown in the photo above.  Positioned as such the gas spring is stationary inside the upper tube.  It takes some experimenting to proportion the dimensions.  I chose maple dowels for spacers because it is light and easy to make.  If you are concerned about humidity weakening them, soak the wood in epoxy to seal it.  Also, cup the end of the dowels that push on the ball ends of the gas spring.  Similarly shape the outbound ends of the spacers to match the end fittings on the vang.  The dowels and spring should remain in contact with each other at all times to minimize impact damage to the spacers.  All internal parts slide freely for servicing.
• With a long stroke gas spring, it doesn't bottom out therefore doesn't destruct itself.

Update FOOD FOR THOUGHT - I received my monthly news letter from RockAuto and it has an interesting article about replacement gas spring liftgate to support a hatch, trunk, hood, etc.  This is when I learned that some are electrically powered with an internal motor and a long threaded rod.  This could be an interesting way to operate a boom vang and eliminate a line across the deck, provided you have the power to drive it.  Just extend the power cord to a 3 way spring loaded toggle switch to raise (extend) or lower (compress) the vang.  I suggest installing the switch in the cabin where you can reach it by standing in the cockpit so you can see the sail shape as the boom is being raised or lowered.  Given how heavy some hatches are, this lift support would have sufficient power to lift a boom. 

TUBE END FITTINGS - Use a metal lathe to machine the end fittings so they fit snug inside their respective tube.  Glue (marine adhesive) and bolt them in place.  The fittings are under some tension but mostly compression loads as they operate.  Make sure they can swing freely, without binding, for the full travel of their function; 90⁰ arc for the bottom joint and 180⁰ for the top joint.  The slightest amount of binding or limited movement will cause metal fatigue and possible failure.

NOTE: There are many different styles of joints and end fittings you can make.  Someone gave me the fittings and I adapted the tubes to them with a machine lathe and a vertical milling tool.  I have a tendency to make things ready for WWWIII.  If you think this is too complicated, pick another design that is within your tooling capabilities.  Just make sure it CANNOT come apart.

BOTTOM FITTING & VANG GOOSENECK - The vang gooseneck must pivot on the same vertical axis as the boom gooseneck.  Failure to do this will result in jamming or varying leech tension as the boom swings.  It must also be very securely attached to the mast.  The greatest strain imposed on the fasteners is when the boom is swung out.  Sailing in a rolling seaway just adds more strain.  So install the gooseneck as close to the mast base as possible.  This is the strongest location and will achieve the greatest mechanical advantage.  I attached my gooseneck to the mast base using the same technique that Clark did.

TOP FITTING & BOOM SLIDE - A 1" sail track under the boom creates a convenient attachment point for the top end of the vang and another degree of adjustment.  I used pop rivets with adhesive along the bottom of a 2' long track.  This length increases the shear strength and creates more adjustment points to lock the vang to.  To create smaller adjustments I drilled a hole between the existing holes in the track.  This is much easier to do on a drill press than upside down under the boom!  The 1" loop slider is manufactured by Ronstan and is equipped with a spring loaded locking pin.  To attach the top end of the vang to the slider I shaped a 3/8" thick piece of flat aluminum to fill the void inside the half ring.  This fitting MUST fit snug.  Any sloppiness here will soon result in metal fatigue and failure. 

2 - DESIGN CRITERIA of a FIXED LENGTH VANG

TUBES - The inner and outer tubes should be made from thick wall aluminum (about 1" OD) that fit snug inside each other (telescopic).  Strive for maximum overlap to minimize jamming and wear.  Observe all the points noted above in GOOSENECK, KEEPING THE TUBES SLIDING and ADJUSTING THE VANG.

DETERMINE THE LENGTH - This is a simple matter of drilling a sequence of 1/4" holes, about 1" apart, and inserting a pin through one of them.  If the pin goes through both tubes, the vang length is fixed.  Some sailors prefer this for simplicity.  If the pin is inserted through the inner tube the vang can be extended up from the bottom stopper pin.  The outer tube will rest on the pin.  Tie the pin to the tube so you don't loose it. 
 

INSTALLATION

Regardless of which vang you make, the installation procedure on the boat is the same. 

1. Screw the vang gooseneck to the foot of the mast. 
   - I store the vang gooseneck attached to the vang for the winter so I don't have to go looking for parts in Spring. 

2. Compress the vang about 2" by pulling in and locking the control line.  Then, while supporting the boom with the topping lift, bolt the top of the vang to the track slider under the boom.  The slider should be positioned in the middle of the boom track. 

3. Release the vang control line to support the boom.  With the weight of the boom resting on the vang, the leech tension will be relieved.  If not, move the track slider forward one notch.  Once you figure out where the slider should be positioned, there should be no need to change it.  So mark the spot on the boom for future installation.

4. Reverse the procedure for removal. 

SAFETY NOTE:  Lock the vang control line when in the compressed position and keep your face away from the end of the tubes.  A gas spring in good condition can exert enough force to knock your teeth out! 
 

OPERATION & MAINTENANCE of a PNEUMATIC VANG.

ADJUST THE VANG - Install a light 3:1 block and tackle around the vang to pull the boom down; thereby controlling mainsail twist.  This is sufficient  mechanical advantage to overcome the push of the 85 pound gas spring with the weight of the mainsail and sail cover on the boom.  The blocks are mounted against the outer tube so they don't interfere with the sliding movement of the inner tube.  Some commercial vangs incorporate an internal block in the top tube to equalize the forces around the sliding junction of the tubes thereby keeping them in column to prevent jamming.  I installed my control lines on the outside, along the bottom, and have experienced no problems with jamming.  The V-jamb block for the control line is located near the top so I can reach it with the boom pulled in while standing on the companionway ladder.  Placing it at the bottom of the vang would mean crawling too far forward on deck to adjust it.  The V-block also facilitates installation and removal of the vang at the ramp.  See NOTE below.

CONTROL LINE TO COCKPIT - In 2022 I installed a longer control line (5/16") to extend it to a cockpit rope clutch on the starboard cabin top.  This is easier to operate than reaching over the cabin.  Because I'm standing in the cockpit, I can look up along the leech, which is a perfect view of the telltales.  The 85 pound gas spring lifts the boom and the vang line works in opposition to lower it.  So all I have to do is release or pull the line to adjust the twist.  Voila, the entire mainsail, top to bottom, now pulls.  However, cockpit control is only possible if the turning block under the vang is installed inline with the turning axis.  See Tech Tip F05.  This ensures uniform line tension and vang length when the boom swings through its full arc.  The diagram above depicts the external control line on the under side of the vang, looking up from the deck.  It illustrates how the blocks are mounted and where the line goes.  The inner (top) tube connects to the boom.  With the combination of the gas spring and the control line in the cockpit, I can readily adjust the vang to have all telltales flying or the top ones stalled to reduce heel. 

KEEP THE TUBES SLIPPERY - After a few years of use, the two aluminum tubes sliding against each other will wear which is evidenced by the black metal dust left on the surface.  The aluminum oxide dust washes off fairly easy with a household cleaner and a rag, but don't touch it or let it contact your just cleaned mainsail.  This stuff can stain with a tenacity similar to high quality printing ink.  It DOES NOT come off.  (Like a lot of things, prevention is better than a cure).  The solution to this problem is to apply a light coat of lithium or synthetic grease between the tubes to minimize friction.  (Lithium & synthetic grease resist washing out with water).  To keep the grease clean (minimize abrasion) and to keep the sails off the vang, you SHOULD add a dust cover over the junction of the two tubes.  I use a black nylon sleeve, slipped over the top end of the outer tube and tied with a light line around the inner tube.  The sleeve hangs down, overlapping the outer tube.  This sleeve is very effective to keep the mainsail off as it flakes over the boom.  My SJ28 buddy uses an automotive rubber boot (bellows) from a rack & pinion steering system to protect the grease.  That's natural, he's a gear head.  Both the top and bottom ends are fastened to the tubes to create a water tight seal.  These boots are made of a rubber like material that resists oil so they should last.  They're available from UAP or NAPA for less than $10.00 Can.  He also assures me that anodizing the aluminum tubes does not eliminate the wear between them but it sure makes them look good.  Lubricate all other fitting with grease so they move freely.
 

CONCLUSION

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