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Tuesday, 26 August 2014

Sail Evolution

Okay, first off I do have to profess I'm not a life-long sailor (I've only sailed with Jim for the past couple of years), so I may get some of the terms wrong, but I hope that this post clarifies what I've been trying to do with the sails and why.

So, my first thing was to forget I was dealing with a sail and think in terms of reference I could understand. I started by thinking of the sail like an aeroplane wing stuck up on end. An aerofoil shape generates lift by directing the air over the top of it along a longer path than the air under it. A nice chap called Bernoulli found out that if a volume of fluid (air) travels at a higher speed, it has less pressure than a volume of fluid at a lower speed. On an aerofoil, the wind directed over the longer path has to move faster and so there is less pressure over the top of a wing than below. It gets sucked into the air.

In a boat the low pressure effect on a sail makes the boat heel over on its side and if its working efficiently then it generates lots of lift, which propels the boat.

A wing designed for slow speed is very thick. A baggy sail makes an efficient low-speed aerofoil, but at higher wind speeds the air flowing over the sail starts to separate from the surface of it and that causes turbulence at the rear of the sail which causes drag and drag slows you down or counteracts the lift the sail is generating.

If you want to sail in faster wind more efficiently you employ a thinner aerofoil shape with your sail, which causes less drag. A baggy sail works well in light wind, but as soon as you try and sail in higher wind speeds with it, the boat tends to roll over on its side and not go very fast. You get the pressure difference an aerofoil shape generates, which makes the boat heel over, but the drag holds it back. Its this lack of "drive" that I experienced when I first took Sprite out.

Look at the "airfoil" (American spelling...) entry on Wikipedia to see low and high speed aerofoil shapes.

Here's a picture of the sail when I first hauled it up:
First haul of the sail, a little wind, no tension and as baggy as an old sock.
About the only flat area is between the battens. The rest of it is loose and baggy with no tension in it.

Here's another wider picture:
A picture of a hugely baggy sail earlier this year.

This is from the first trip I had out of the harbour this year. The wind was behind us blowing us out of the harbour so I poled out the jib and winged the main sail. Nice and easy because the baggy main caught the wind fine.

However getting back in was a different matter. No matter what we did, when facing anywhere near upwind the boat would just tip over and the sails wouldn't drive the boat. It also generated an alarming tendency to just veer off where it wanted (so-called "weather helm"). In the end we had to resort to motoring back into harbour, such was the poor performance from the sails.

So that's when I started reading up on the internet about improving the performance of the sail, which mainly entailed reducing the bag in it. A few sites pointed to the fact that the sail isn't stretched out enough along the luff, which is the part of the sail that runs along the mast. Suggestions included releasing the bolt rope along the luff, so before I went to that extreme I tried to have a look and see if I could put some more tension into the luff using the down haul. This is where I hit a snag because even if I hauled the sail as high as I could, I couldn't put any tension into the sail via the down haul as the down haul stopped short of any sort of tension. After checking the mast I found a broken slider which stopped me pulling the boom down by an inch. I removed it and at the same time put a shorter shackle in the top or head of the sail, which gave me a couple of inches more travel.

Surely I could put a bit of tension into the sail now... But no, it still looked baggy. If I pulled on the main sheet the boom was higher at the mast and lower at the cockpit. If its set right the boom should be relatively horizontal. So it meant the front bottom corner of the sail, the tack, was too high. If you look back at the second picture you can see the boom is where the cleats are on the side of the mast.

So several more days of thought and a couple of trips out to the boat made me home in on the thing that was stopping the boom move down any more: a pin in the track of the mast. Its there to stop the boom coming down too far, but I assume its only needed when not using the sail. The pin was seized into the mast and it took a couple of days worth of wd40 and persuasion with pliers to release it.

Now I pulled on the down haul and the boom went further towards the bottom of the mast. Although I felt that a bit more tension would flatten the sail even more. That's when I hit on the idea of using smaller pulleys on the down haul, which would give me another inch or so of travel.

The down haul seemed to be sorted but again on the next trip out, the force of the wind bowing out the sail pulled against the kicker. No matter how hard I hauled on the kicker, the boom would not come down to the horizontal position. That's when I had the idea of using multiple pulleys to "gear up" the kicker. I'd have to wait until the next boat jumble to buy them as they are not cheap brand new.

So at the next boat jumble as reported last weekend I bought pulleys and rope and fitted them to the kicker.

Here's a new picture of the new flatter sail shape:
A picture of the same sail transformed, last weekend.

If you look at the mast at the bottom right of the picture, you can see that incredibly the boom now pulls down a foot or so lower than the cleats on the mast. That's a lot of slack sail taken up! There is almost no effort in pulling the sail down that far, so its possible it can go even further.  This is something I'll looking into trying next. There's a couple more tweaks I can put into the down haul to get maybe another inch or so of travel. Although now with the boom so low headroom above the cockpit is an issue. After this final tweak the only other option would be a longer mast. Or a shorter sail!

Of course with the tack of the sail so much lower, the boom is lower at the front so when I haul on the kicker it pulls the boom into the horizontal position tensioning the whole sail. The downside is you do now have to duck when tacking or gybing, as the boom is not much higher than the cabin roof!

Now the sail works like a thin, high speed wing. The sort of wing fitted to a jet, which is designed to work at higher speeds with less drag. Now I can put the boat into the wind and the sails work with minimal tipping, less drag and a lot more lift, or drive. Of course if the wind drops, I can release the tension and make the sail baggy again.

This weekend's test was only with light winds, but already I can see a difference in the boat's behaviour, hence why I'm so enthusiastic!

In contrast, the jib sail (I suppose its technically a Genoa), has been fairly easy to sort out. The main problem was it was too low on the furling system, so it caught on the pulpit rail. Using a few shackles did that job, but it means that the clew of the sail is higher than it was and the track on the side of the boat is in essence too far forward now.

However there's a fix for that. If I raise the position of the point where the jib sheet is held by the track, it balances everything out. I can do that with a couple of small shackles and a couple of pulleys. Guess what; I have a couple of old dinghy pulleys I got off Freecycle which will do the job nicely. All I need are the small shackles.

So when they're added, all should be rosy when it comes to the sails on Sprite. I just have to take her out in some stiffer breezes and see if the work has been successful. I've already done a bit of work on how I'm going to reef the main sail. in the really windy stuff, but that's for another blog post.

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