The perfect kite

Sail Trim

In this new column, Jordan Spencer discusses techniques and concepts specific to dinghy sailing. In this issue he looks at spinnaker trim for off-the-beach boats.

Obviously sailing is sailing and there are many similarities between off-the-beach and big boats. But there are some unique characteristics small boats bring, and that definitely applies to spinnaker trim.

The critical skill for a great spinnaker trimmer is understanding apparent wind and it’s effect on trimming technique. The need to understand this concept is heightened in a dinghy because apparent wind changes far more quickly.

Apparent wind is a fundamental concept. It is the wind you feel on your face or body when you are sailing. It is the wind your sails "see" when underway.

Apparent wind is a combination of true wind, the wind you feel when you remain stationary, and headwind, the wind you create by forward movement. If there was absolutely no true wind and you were moving forward at 5 knots, you would feel 5 knots of headwind from the direction in which you are travelling, whichever direction you decide to travel.

So logically, if you travel faster for some reason, your apparent wind will move forward and if you travel slower your apparent wind will move aft. Also, if you get hit by a gust of wind, initially your apparent will move aft, and if you get a lull in wind speed, initially your apparent will move forward. Of course, your apparent wind will slowly return to it’s original direction as the boat accelerates or slows due to the change in wind speed.

Flying on the edge

When we are trimming a spinnaker, we are flying the sail on it’s very edge. We are trying to trim so that the luff is just curling. When we have a spinnaker trimmed in that position, we know there is full air flow over the sail.

We also know that if we ease sheet, the spinnaker will lose power and quickly collapse. This is something you want to avoid at all costs, because it shakes the rig, so not only do you lose flow of the spinnaker, but also the main and jib as well. Additionally, because it is a deep sail, it takes a few seconds to get the wind flowing over the sail again, so you will lose several boat lengths each time the kite flogs.

Over-sheeting

If we sheet on from a position where the luff is just curling, we increase the angle of attack for the wind, making it more difficult for the wind to flow around the sail, especially because the spinnaker is a deep sail.

There are several consequences of this. The air flow doesn’t travel around the length of the sail, so we get laminar separation. This means less lift from the sail, so reduced acceleration, increased drag, which reduces speed, higher sheet loads, so the trimmer tires quicker, closing of the slot between the spinnaker and the main, so less power from the mainsail and therefore less speed, an increased lee helm for the skipper, who then responds by pushing the rudder, increasing drag from the foils and reducing speed again.

This means an over sheeted spinnaker can cost a significant amount of distance on each downhill.

Gusts and lulls

So you are sailing downhill, spinnaker trimmed perfectly. You are a good spinnaker trimmer. A gust hits, are you still trimmed correctly? Only if you eased sheet as soon as you felt the gust!

The gust of wind has brought your apparent wind aft and remember you are trimming to apparent wind. So if you didn’t respond by easing sheet, you are over sheeted and suffering the consequences mentioned above.

Obviously, if the wind lightens, you want to trim on. But it has to be a controlled trim, because you are going into lighter wind and you can quickly over-sheet.

Putting it together

Conceptually now, we can think through various scenarios and how to respond with our trim. Slowing boat, ease trim, apparent will move aft.

Accelerating boat, trim on!

If you fall off the back of a wave, or the bow buries into the back of a wave, you need a big ease.

As you catch a wave trim on, but gradually ease as you get to the bottom of the wave and the boat starts to slow.

Pumping? Ease the spinnaker prior to the pump so you don’t over sheet as you pump the sail.

All you have to do is think of a scenario, think of the effect it has on your apparent wind and trim to suit. Then you will be a great trimmer.