Junk Rig Conversion, Structural Considerations.

Installing an unstayed mast on a wooden boat.

A junk rig conversion is possible for almost any boat

However, as it works best with an unstayed mast there are hull structural considerations to take into account.
Why convert?
Mast position
Structural considerations
Deck layout
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MignonneBefore you begin designing the sail plan you need to check the existing structure of your boat to determine where the mast or masts can be sited.

If you are converting from a bermudan rig the new mast will want to be set further forward, so it will require a suitable supporting structure.

Once you know where it is possible to place the mast, then work on a sail plan to suit.


Why convert?


junk rigBoat Books on-lineIf you have read this far I guess that you have already conceived some ideas of why you think a conversion is right for you.

So I will only enumerate some of the advantages.

Junk rig is the easiest rig to handle particularly if you are short handed.

Reefing easy and quick with all the control lines running back to the cockpit.

The sail is self tacking and self stowing when dropped.

The structural loads on the hull are less than for a stayed bermudan rig.

However, the junk rig is a cruising rig, if you are into racing forget it, unless you can compete against other junk rigs.

Boats with a deep forefoot and or long keel can be slow in stays, however this can be over come with a better rudder or using a foresail for backing.

You might be able to allay some of the cost of conversion by selling the old rig and sails, especially if it is one of the popular rigs.

And you can use the old sail material if it is in reasonable condition.


Mast position



Tai ShanBecause of the nature of the rig a junk sail works best on an unstayed mast.

And with most junk rig conversions the mast will need to be positioned forward of the bermudan mast position.

So, before you start to design the sail you need to check your boat for a suitable position mast, then design the sail plan to suit.

Positioning a junk sail is about getting the sail’s CE (Centre of Effort) positioned in front of the hull’s CLR (Centre of Lateral Resistance).

Too far aft and it will develop too much weather helm too far forward, lee helm.

The CLR will be fixed by the shape of the hull.

The CE will depend on the size and shape of the sail plan.

The sail plan can vary enormously, depending on the aspect ration, number of sails etc, and of course it will depend on the mast position.

Having said all that, it is rare for a junk rig to suffer from much lee helm, weather helm is much more common, so the further forward the CE the better.

PhaHowever, this is not an exact science, the CE of a sail will alter as the wind strength alters and as you trim or reef the sail.

The CLR of the hull will alter somewhat as the boat heels and the underwater shape alters.

It is because of the desirability of having an unstayed mast that the junk rig conversion of catamarans can be problematic, though some have gotten around this by having a twin rig with a mast in each hull.

And shrouds can be used though this will greatly reduce the versatility of the rig.

Tabernacles have been used on some smaller boat junk rig conversions but they still need as much bury between the pivot and heel as for fixed masts, this calls for an intrusive ‘slot’ extending below decks.

The CE can be moved forward by raking the mast forward.

Forward rake is seen on many multi-masted vessels but with a single masted junk rig conversion there is a danger that this may cause too much downwards press on bow.

For a single masted rig the recommended rake is 'plum'.


Structural considerations



Junk Mast BuryThe first consideration for a junk rig conversion is having sufficient ‘bury’ to support the mast.

An unstayed mast is only attached to the boat at the mast step and at the partners, the distance between these is known as the ‘bury’.

As a rough guide this should not be less than 1/10th of mast height above the deck or the length of boat.

The next consideration is how the loads on the mast will be transferred and spread to and from these two positions.

Even though loads on a junk mast are less than with a bermudan rig they can still be considerable.

So, the mast partners and the mast step need to be strong, affixed to main beams, bulkheads or floors and able to spread the loads without causing undue strain on the hull shape.

First check that there are suitable floor timbers across which to fit a ‘keel step’.

This should span and be fixed to at least two floors, preferably more.

The mast step should be strong enough to take loading in any direction, particularly sideways, towards aft and vertically down.

Deck layoutThen have a look at the deck beams to see if there is a suitable position to place the ‘mast partners’ directly above the ‘keel step’ position.

The partners support the mast at deck level and spread loads across the deck.

They should be supported between two main deck beams and two carlines and reinforced with tie bars and logging knees.

Once you have established where you can put the mast then you can design the sail plan to suit your junk rig conversion.




Deck layout



practical junk rigBoat Books on-lineBefore you begin any alterations consider the deck layout.

Moving the mast forward might necessitate moving the fore hatch, so you need to plan where this will go, whether it will snag any lines etc.

This brings us on to those control lines, where they will be routed etc.

The lines coming vertically down the mast need to change direction through ninety degrees so they can be lead to the cockpit, this requires a block for each and somewhere secure to anchor the block.


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Plan the position of blocks sheet and halyard leads to reduce the number of ‘corners’, the fewer changes in direction the less will be the friction.

Consider also the helm position and where to place cleats, winches etc.

And the position of the sheet horse or traveller is another factor which might affect the design of the sail plan.

Sheets drooping into cockpit can be a nuisance and even a hazard if they get caught during a gibe, on the other hand the sheets need sufficient lead.

It is also worth bearing in mind even at this stage that there will be times when there will be a lot of rope in the cockpit.

Being able to stow it all neatly safely and quickly needs some thought not just about the means of stowage but the position of the lines.

Take your time planning your junk rig conversion and try to consider all aspects in relation to the whole.



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