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Spinnaker Trim

January 6th, 2011

Our good friends at Elliott/Pattison Sailmakers put together the following primer on downwind sail trim:

Spinnaker trim really falls into two categories; sails flown on a pole and sails flown on a fixed sprit. There are also a very few boats that fly asymmetrics on an articulating sprit but in general those articulate through a small enough range that flying them is nearly the same as flying a sail on a fixed sprit. However what you are trying to accomplish is the same in all cases. For that reason this article will focus mainly on trimming spinnakers flown on a standard spinnaker pole with some ideas on how to incorporate the same ideas for sprit sails.

There are four main controls you use to obtain the shape you want; pole position vertically, pole position horizontally, sheet tension, and sheet lead position. Basic wisdom has always been to fly the spinnaker pole pulled aft so that it is perpendicular to the wind, fly it high enough so both clews are level (on symmetric sails), then ease the sheet out until the luff of the sail just starts to curl over. While this will get you close it will also leave you lagging behind the good trimmers that take full advantage of all the controls at their disposal.

For pole position fore and aft, once the apparent wind is aft of 90 degrees, the pole should be pulled back just far enough that when the sail is sheeted so that the luff is just starting to curl the sail maintains an even shape from the middle down to the foot. If the pole is too far aft the trimmer will have to sheet the sail in tighter to keep it from breaking and you will see the foot of the sail is stretched out straighter than the middle of the sail is. If the pole is too far forward when the sheet is eased until the luff just starts to curl you will see the foot of the sail is much deeper than the sail is in the middle. The other thing to look for is that the luff goes straight up nearly vertically from the end of the pole to the middle of the luff but this doesn’t work for asymmetric runners on a sprit. These sails are usually designed with a lot of luff round so that as the sheet is eased the sail will rotate well out to weather of the end of the sprit and let you sail deeper.

Setting the pole so that both clews on a symmetrical spinnaker are flying at the same height is a good starting point but the more important point is to get the sail to fly square so that it takes the shape it was designed with. I like to look at the horizontal seam where the head is sewn on and the vertical seam down the center of the sail. The pole height should be adjusted so that the head seam is level and the center seam goes straight up and down; they should be at 90 degrees to each other. This technique is very important when sailing on a boat that uses asymmetric spinnakers on a standard pole since the tack and clew will never be at the same height. It also works fine and well for a properly shaped spinnaker flying with the wind aft of 90 degrees, but that is not always what you have. By changing the pole height you can also change the location on the luff where the sail breaks first, you can pull the draft further forward which also opens the leech, and you can change the amount of horizontal camber. You want the sail to break first on the shoulder, just above the head seam where the luff profile has the most curve. If the pole is too high the upper leech will twist open and break well before the rest of the sail; if the pole is too low the upper luff will be stretched too tight and will break later than the lower luff.

What about on reaches you might ask. Well since the pole height determines how tight the luff is stretched it acts something like the halyard on your headsail. As your lower the pole and tighten the luff it pulls the draft forward in the sail and also opens the leech. This is an advantage if you are sailing on a leg that is a tight reach because the open leech lets the air exhaust without closing off the slot between the main and the spinnaker as much which will give you better speed. However if you are sailing on a reach that is very tight, where you can barely hold the spinnaker you need to remember you high school math and the Gauss-Bonnet Theorem. It says that the total amount of curvature in a surface remains constant. If you increase curvature of a surface (your spinnaker) in one direction curvature in other directions has to decrease. So sailing down a reaching leg you start out with pole slightly lower to open the leech and go faster but as you approach the mark you find the leg is getting tighter and tighter and you need to keep the spinnaker up because the next leg is also spinnaker leg. By raising the pole up in this condition you increase the vertical camber in the sail which decreases the horizontal camber which will let you sheet the sail tighter and point slightly higher. You will also go a little slower so this technique should only be used for short distances. The rest of the time it is better to sail slightly lower and keep the spinnaker working at maximum efficiency for a little longer then the switch to you headsail and come back up for the remainder of the leg.

In positioning the spinnaker sheet lead block there are two things to consider. First, the further aft you have the lead the wider the sheeting angle becomes, which helps keep the spinnaker spread as wide as possible when running; and second, the correct lead angle for reaching. It is best to sail the boat on a tight reach with the apparent wind just forward of the beam and check to see how the lead is. In this condition you adjust the lead much the same as you would for a genoa. When the sail is sheeted in tight to the point where the foot is just pulled out tight the leech curve should follow the twist in the mainsail. If the lead is too far forward by the time the foot is in tight the sheet will be pulling down too much on the leech which closes off the slot between the main and spinnaker. If the lead is too far aft the leech will be twisted too open and you lose power from the top of the sail. If this position ends up being near the transom then you can get by with just the single lead; however in most cases you will find that you want to sheet the spinnaker quite a bit further forward for tight reaching. The simplest way to accomplish this is by using tweakers, basically a block that the spinnaker sheet runs through before it gets to the sheet lead block. The tweaker block is attached to a line that runs down through another block fixed to the rail about half way between the transom and the shrouds, and then to a cleat. By pulling the tweaker block down you effectively move the lead position forward. Tweakers also give you the ability to move the lead aft while reaching if you are getting overpowered (just like moving your jib lead aft to depower) and when running in heavy air conditions you can move the lead well forward which chokes the spinnaker down and helps prevent oscillations.

Three Steps to Better Starts

April 23rd, 2010

Here is another nugett from our friends at Elliott/Pattison Sailmakers: 

There are three basic considerations for getting a good start. First is determining which end of the starting line is favored, second is deciding which side of the course is favored, and third is positioning yourself relative to other boats. For the purpose of the article we are going to assume a windward start where the first leg is a beat.

Figuring out the favored end is pretty straight forward, in the absence of other considerations you want to start at the end that is furthest upwind. The most accurate way to do this is to sail past the leeward end of the line and then tack around so that you are sailing straight back towards the weather end of the line on port tack; line up your boat so that you are on the course that will take you directly through both ends of the line and record the compass heading. Next go head to wind and record that heading. Subtract 90 from the bearing of the line and if that number is bigger than the bearing of the wind then the left end is further upwind, if it the number is smaller than the bearing of the wind then the right end is favored. For example if the bearing of the line is 325 degrees on port tack and the bearing of the wind is 225 degrees you subtract 90 from 325 and the result is 235. The wind at 225 is 10 degrees to the left of perpendicular so the left end of the starting line is further upwind. If the wind is at 235 then the line is square and both ends are equal. If the wind is 245 it is 10 degrees to the right of perpendicular so the right end is favored.

Get your bearing on the line as soon as the committee has the line set. That way as you continue to take wind readings before the start you will always know which end is favored in the current shift. It is important to get a good wind reading as close to the actual start time as possible because often the wind will shift and as it does the end of the line that you thought was favored may change.  As a side note, if you are sailing on a course where they use a leeward gate you can use the same procedure to determine which end of the gate is favored for rounding.

Step two, picking the favored side of the course, is the hardest which naturally also makes it the most important. There are three factors that are important in making your decision, wind shifts, wind velocity, and current. Local knowledge can be a big help here but never let it be your overriding consideration; I have seen people sail off to one side of the course all too often just because that is “what always works” only to find out that it doesn’t.  Since you were sailing around well before the start recording the wind direction and the time of day each time you checked, you now have a good starting point. In general wind shifts fall into three types; oscillating where the wind directions shifts back and forth, persistent where the wind gradually shifts in one direction throughout the day, or a combination where you have oscillating shifts but the overall picture is still that the winds moves further and further in one direction as the day goes on. By looking at the data you recorded you should be able to see a pattern of what the wind shirts have been doing, and the general time period of the shifts. In oscillating conditions very often the shifts will occur at about the same interval. Remembering the #1 rule for upwind sailing, sail into headers, you want to plan your start so that you are sailing towards the next header or at least have the ability to tack and go in that direction without much interference. If you think the next shift will be to the left you want to start on starboard tack as far down the line as possible so that you will be one of the first boats to get the header, and that when you do will gain more advantage from it that anyone that is to weather of you. If you think the next shift will be to the right then you want to get onto port tack as quickly as possible so that you lead the fleet into that shift. Depending on how big you think the shift will be, and how much the line is favoring one end or the other, it is very likely that you may not want to start all the way at the favored end if it is the end away from the direction you want to go. For example if the left hand end of the line in favored by a couple of boat lengths but you are expecting the next shift to be 5 or 10 degrees to the right, then starting right at the pin is probably not going to work very well. The further you are towards the left end of the line the more boats you will have to weather of you on starboard tack that will keep you pinned down unable to tack towards the shift. And when you do you are much more likely to have other boats tack on top of you plus as the wind shifts to the right you will be headed down towards the transoms of the boats that sailed into the shift first even though they started at the un-favored end of the line.

4 Steps to Sail Downwind Like a Pro

April 21st, 2010

Here’s a really good article to improve your downwind perfromance written by Harry Pattison at Elliott/Pattison Sailmakers in newport Beach, CA. 

 Okay, really there are 5 but the first one is more like homework, you do it before you go sailing; Know Your Targets! If you want to sail downwind efficiently you have to know the target speed and target wind angles you should be sailing for different wind speeds. With luck you can get these numbers from the designer of your boat or from US Sailing if your boat, or one of the same design, has been rated for ORR. Without one of those options you may be able to find targets for a very similar boat you can use as a starting point and modify over time as you collect real data from sailing your boat; or you may have to totally develop your own by observing and recording your performance over time. In the absence of tactical decisions you will ALWAYS sail downwind faster if you sail your targets!
Step 1: Choose the side of the course you want to sail towards initially before you get to the weather mark. If the wind is oscillating you should know if the wind is in a left phase of a right phase as you approach the weather mark. If the wind is to the left of average you will be expecting the next shift to be to the right, so with a normal port rounding on starboard tack you will be sailing towards the left side of the course (I always look at side of the course relative to upwind so downwind you may be sailing towards your right side but it is still the left side of the course, this makes it easier and more consistent to view the wind shifts). This means you will be sailing towards a lift which will allow you to gybe over sooner and sail a header to the leeward mark. Once you have chosen which way you want to go after the weather mark that will make your decision on what type of spinnaker set you want to do. The options are a normal bear away if you want to continue on the same tack, or a gybe set if getting to the other side of the course is called for. If you want to get to the other side you have two options; the classic gybe set or a normal set followed quickly by a gybe. A normal set and gybe are quicker than a gybe set if there aren’t other boats close by that could gybe inside of you, or if it isn’t necessary to get to the other side of the course immediately.
 

Your rounding leads into Step 2, Controlling you Competition. On a downwind leg the tactical advantage goes to the boat behind, the wind shadow and its use are powerful tools. In a bear away set, if there are other boats around, you usually want to get deep quickly by sailing very low during the set. This will gain you control of boats that have rounded just in front of you because they won’t be able to gybe until you do, and it will defend your lane from boats  behind so that they can’t get inside and prevent you from gybing when you want to. If you are behind and can keep the boat in front from gybing you can hold them there until you are on the layline for the leeward mark. When you both gybe for the mark they will be behind your wind shadow and you will be able to sail down in front of them. To make this most effective you have time your gybe to get maximum benefit from your wind shadow. If possible you want to sail just slightly passed the layline so that the boat in front has no chance to gybe back away to find clear air. As you approach the leeward mark if there is other traffic around it is best to plan your approach to be on the inside at the mark rounding, with port roundings this will also set you up to be on starboard tack at the mark. As when sailing upwind there will be crossing situations with other boats. In each case you want to force the other boat to sail towards the un-favored side of the course while you sail towards the favored side
Step 3: Sailing downwind the rule is the opposite of sailing upwind, you always want to be sailing on the headed course towards the next lift. Upwind you tack on headers, downwind you gybe on lifts. Just like sailing upwind you also have to be aware of persistent shifts, current, and wind pressure in making your decisions. After sailing the initial upwind leg you should have a good feel for what the wind and current are doing; is the wind oscillating back and forth or is it consistently shifting in one direction. In oscillating shifts you want to gybe on the lifts so you are always sailing on the header. In a persistent shift you want to sail towards the lift but sail into it far enough so that when you gybe you will be close to the layline and will still gain on any further shift as you get headed down to the mark. If you sail too close to the layline and there is any further shift you will risk overstanding and losing distance to the boats that gybe inside of you.