Anonymous asked in Cars & TransportationBoats & Boating · 8 years ago

Which hole should I use in the clew of my dinghy jib in strong or light winds?

I have a small sailing dinghy. The clew on my jib has three holes in it so I can thread the sheet through either the upper, middle or lower hole. I think this is so that I can adjust for strong or light wind conditions. Which hole should I use in strong winds and which in light winds?

2 Answers

  • 8 years ago
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    The reason there is a choice in clew point on a headsail is to allow you to change the rig tune and set the shape of the sail in order to gain maximum acceleration of the air through the slot and thus improve the flow of air across the back of the mainsail. It has much more to do with mast rake, which itself will depend on the conditions as to how the boat is tuned for that race than simply as a primary adjustment - which it is not.

    There could be a simple advice to use the furtherest forward in strong conditions to twist off the leech and open the slot, and to use the rear one in lighter conditions to control the leech better, while the middle setting will be a fair compromise in medium conditions. While there is some basic truth in that, it is far too simplistic to say that in practice. Rigging and tuning your boat relies on your sail condition and cut, mast rake and spar bend, tensions both fixed and adjustable, the sea state and a multitude of other variables. As long has your sailmaker has done his job, twist and head sail shape are controlled by the sheet tension - in general you set the sheet angle to give close to equal tension to the leech and foot of the sail and then sail it. Unless conditions drastically change there is no need to fiddle with that - and if it goes mental you won't be able to....

    The head sail and mainsail must be considered as a single combined foil, not two separate foils - in fact the combined shape when looked at from above is considered as an airfoil as far as the way the air reacts and responds to the sails. Either sail on it's own is unable to point as high or separately produce the thrust of the combination. You need many potential adjustments in order to gain the most effective venturi effect through the 'slot' that is formed between the leech of the head sail and the depth of the main sail. If the slot is too great the mutual support in accelerating the air flow is lost, both sails revert to individual foils and performance drops remarkably. If the slot is too close the airflow stalls and leeward flow is stopped - as leeward turbulence builds the whole leeward air stream effectively stalls. You can see that as the luff of the main increasingly backwinds and inverts - in high winds there is so much air trying to get through the slot that you need to let the leech of the head sail twist off, after all you are dumping the main to keep the boat flat in the gusts and if that's the case you have way more power than you can use anyway.

    In general the only time I will consider moving the connection point to a jib clew plate is in the stronger conditions, where I probably cannot hold the boat without having to dump power. All other conditions I just use sheet tension to get maximum drive. Most classes allow you to move the cleat and fairlead while you are sailing, the basic choice is made when rigging up as to where you connect the sheet to on jibs fitted with a clew plate or multiple cringles.

    In smaller classes you tend to find clew plates with multiple holes from larger sailmakers providing a generic average sail suited to a standard mast bend or stay sag - as there can be a variation in individual spar and rig choices they allow the sail to work across many variations - sails made specifically for a certain mast section and rig tension often have a single cringle for the sheet, as there will be ample adjustment via the cleat/fair lead track. Unless it is a very simple class with no sheet tracks allowed, and the only possible adjustment available is in the clew plate.

    Unless you fit Leech ribbons to your sails you cannot see if you are choking the slot, or leaving it too open. There is no point fitting twenty or so, but an absolute minimum of two for the head sail and three for the main, plus at least as many flow tufts on the power points of the sails respectively shows when you have it right, or need to change things.

    Unless you can see the airflow with your eyes, in which case you are way better than I am.... :)

  • 8 years ago

    You will need more area of sail in light winds and less in strong winds ,if you have to ask this as a

    serious question I suggest you learn properly BEFORE going on the water.

    Source(s): sailing since 1950 ,still sailing safely
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