Sunday, June 24, 2012

Winter ~ along the Sydney cliffline

Sailing with a light westerly to North Head at dawn.

With one eye on building a new single-masted sail this is a continuing experiment with the behaviour of a couple of different rig designs, as referred to in the previous post. A double-masted V-sail and a slightly weather-beaten single-masted Tasmanian rig are being used in the photo above.

Having sailed to the base of North Head we re-set our course 180 degrees and managed to pick up a breeze to take us southwards along the cliffs. Above are the V-sails on Tony and Derek's kayaks. Not withstanding the findings expressed below on the limitations of the V-sail, this rig works well in light breezes - picking up every breath.of air and giving the kayaker a free knot or so of momentum.

Breakfast of ocean trout at a favourite cliff spot - only accessible by nimble kayak in slight seas. And then, with our kayaks perched on the rock platform, we snorkelled amongst crayfish crevices and into deep blue drop-offs along the cliff's edge.

A humpback whale singing its way north.


Monday, June 11, 2012

Kayak sail

Just a limited observation on the historial precedents of sail design for modern sea kayaks ...

1. Observations of an Inuit's kayak sail:

- Single mast
- Single sail batten
- Single sail sheet
- No mast stays
- Sleeved / socketed mast housing
- Fairly low sail area, say 0.6 to 0.75 sqm
- Low centre of effort in the sail, say no higher than the paddler's eyeline
- Mast housed off centre-line of the kayak
- Mast step located well forward of typical paddle stroke
- Limitations on engaging the sail at sea (or at least I'm uncertain how the sail is engaged at sea. Presumably by a paddling partner)
- Sail and mast can be dis-engaged at sea by a deft flick of the paddle and retrieved by hauling in the sail sheet
- Sail is stored off the foredeck (presumably on the rear deck or within the cockpit).

 Above: Sailing by the Manitounuk Islands, 1927.
(L.T. Burwash. Public Archives of Canada)
Source - Inuit kayaks in Canada (1987) - E.Y. Arima

Above: Inuit sketch of kayak sail in elevation. (Same publication as above).

It is unclear whether this kayak sail has ancient origins or if it is a relatively late innovation based upon exposure to European and American sailing vessels.

The observations of this sail design include that the intent was to:

- keep the sail simple (one mast, one batten, one sheet),
- minimise the risk of overturning (low overall sail area with low centre of effort, and
- maintain a clear foredeck with the sail stored elsewhere when not operational.

The Inuit sail shown above is not too dis-similar in profile to many modern kayak sails.

However, unlike the Inuit kayak sail, modern kayak sails are typically stored in one of two ways: -

(i) folded down and lashed to the foredeck (where the sail is exposed to catching waves, accumulating water and impeding the paddler's performance); or
(ii) housed within a sleeve on the foredeck.

2. Observations of a Maori canoe sail:

- Double mast (V-shape sail)
- No sail battens
- Multiple sail sheets.
- Single forestay and twin side-stays
- Large sail area, say greater than 1.5 sqm
- High centre of effort in the sail, higher than the paddlers' eyeline
- Mast housed on centre-line of the canoe, but able to be inclined to leeward to allow spill.
- Mast step located forward of typical paddle strokes
- With an open cockpit the sail is simply engaged at sea by the forward paddler
- Sail can be disengaged by the forward paddler
- Sail is stored within the canoe's cockpit.

Above: Nouvelle ZĂ©lande - Pirogue du canal de l'Astrolabe.
Hand coloured engraving by De Sainson (1833)

The observations of this sail design include that the intent was to:

- maximise the opportunity to catch air movement above the paddlers' bodies (high centre of effort in sail) , and possibly
- be able to trim the sail setting by adjusting the angle of the leading mast edge (as in a windsurfer's sail).

The above Maori canoe sail is also not too dis-similar to some modern sail designs that are marketed for kayaks.

Summary from the above observations ...

The Inuit kayak sail is well suited to the low moment-resisting profile of a kayak. Its sail area is relatively small and contributes to topping up the hull speed of the down-wind journey of the kayaker.

The Maori canoe sail - or V-sail - with its high centre of effort, is not as well-suited to the low moment-resisting profile of a kayak. In a canoe the combined weight of the mutiple paddlers leaning out to windward can compensate for the over-turning effect of the V-sail - this is difficult to compensate for in a kayak with a single paddler. The V-sail is also prone to a yawl-effect when travelling directly downwind.
And when this type of V-sail is mounted onto the deck of a kayak it has a further disdvantage - the foot of the sail will generate a stalling effect on hull speed when the kayak's bow overtakes the wave infront and gets partially buried within the wave on its way through - which is often the case in steep seas. 

Obvious conclusion:
The smaller profile of the simply rigged Inuit kayak sail rig holds multiple benefits for the sea kayaker as compared with the V-sail design of the Maori canoe.

Other thoughts:

In designing a modern version of a sea kayak sail rig the designer would do well to arrive at a solution that;

(i) limits the use of multiple mast stays (more lines on deck = additional risk of entanglement),
(ii) has a rig (mast foot + mast + sail + sheet) that can be simply and quickly deployed and retrieved by the paddler,
(iii) has the capacity to store the sail in such a way as to minimise the negative effects of catching seawater within the folds of the dis-engaged sail (i.e within a deck sleeve), and
(iv) has its mast footing positioned well forward of the paddler's stroke.