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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #1  
Old 07-15-2008
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Mainsail Trim

Not sure this is the right area for this question, but here goes.

I'm an experienced sailor that has owned several yachts but I've come across an problem I've never seen before.

I have a 33ft crusier. Nothing special but a nice little yacht. She has furling job and in mast furling. However there is something going very wrong with the main sail trim. I'll explain the situation.

Sailing close to the wind in say a 3-4 she sails wonderfully. Nicely balanced, sails quite close to the wind (for a crusier), all seems very happy.
HOWEVER
As soon as the wind come round to the beam i.e. any sort of reach she slows right down. Obviusly I trim the sails accordingly but she just goes slower. As well as slowing down she doesnt feel right. She's thrown off by the smallest wave, isnt balanced ect just feels horid.
NOW HERES the kicker. I was playing with her mainsail trim last weekend, I had a 3-4 on the beam and she was playing up again. So just as a laugh I let the main out until it started to luff (back of the main flapping like crazy), and what would happen SHE SPEEDS UP. Yes I know ?? she actually sped up with the main luffing. I test this over and over again and the same result. Somehow the back of the sail seems to working against itself on a beam reach? I've never seen this before. True I've not had much experience with in mast furling (hate them really), so has anyone out there got ideas about what's going on?

I suspect the rigging is out somehow but looking at all my book about rigging nothing mentions this situation.

Any advice / ideas welcome

Thanks

Matt
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Old 07-15-2008
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Smile Mainsail Trim

Matt,

There are so many possible variables .. but let me take a stab at it.

First is helm, your underwater rudder surface and its angle. When going to windward, you want to have a slight weather helm thus requiring you to have the wheel over to leeward slightly to counteract it. Rudders being symetrical airfoils .. only produce lift efficiently at low angles of attack and start getting very draggy quickly when those angles increase.

Typically, you want your rudder to have an angle between 2-4 deg's.

Anything more and you're overpowered and slowing the boat down.
Anything less and you're not getting the lift from the rudder to windward.

On my boat .. I've actually measured the angular displacement of the rudder (mine is on a skeg) and know the ratio between rudder angle and wheel angle. Mine is ~9:1. So, I like to sail with my wheel knot around 30-45 degs to leeward, which equates to about 4-5 degs at the rudder. Anything more and I need to remove main. Anything less .. need to add main.

As you go off the wind and ease the sheets in higher winds .. the weather helm will naturally increase just from the headsail .. forcing you to use more wheel to leeward and ease the main. If you find that you are luffing your main and speeding up .. you must be slowing your boat down with the rudder.

The solution is to reduce headsail by RF'ing it. This will allow you to re-engage your main without introducing helm that will slow you down. Depending on the cut of your headsail, you need to move your jib-car forward as you furl it. I've actually marked furling spots on the foot of my jib so that I can furl it to the same spots each time and thus know where to put the cars.

Hope that helps.

Angelo
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Old 07-15-2008
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Angelo mentions some good things to look at. On reading the OP, my first thought was jib lead locations and my second was position of the traveler for the main's angle and my third was was whether pulling the vang down to flatten the main would provide the improved speed and improve sail shape and reduce weather helm.

I seem to recall a book that I have stuck away somewhere which is a collection of articles from SAIL called the The Best of Sail Trim (or something like that) that covers sail shape and speed for all sails and conditions. It was worth looking at - some review, some new stuff.
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Old 07-15-2008
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... another thought ...

Next time you are out .. do the opposite. When you are off the wind .. furl your jib completely and sail under main alone. Work your outhaul and vang to maximize your speed (as Jim suggested). Once done .. then start adding jib and see what happens.

I think you'll learn a lot about your boat's off wind performance and balanced combinations if you do that.

Ang
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Old 07-15-2008
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Maybe with in-mast furling, the extra thickness of the mast is stalling out the smaller (no roach) main until you ease it so far the main and jib work together to create a single foil. How much shape does the main have?
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Old 07-15-2008
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One good piece of advice I learned a long time ago is that most sailors tend to over-trim the mainsail. So ease it out until it starts to luff and then harden up on the sheets a tiny bit... that will keep the mainsail powered up properly.
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Old 07-15-2008
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All good suggestions here. Interesting problem. The boat that I am currently sailing is a 20' C Scow. It is all main sail. I race it. Something I have found on a few legs that started out as a run and ended up at nearly a broad reach, is that the leach cord needed to be release completely. It seems that the sail needed to breath more. The sail needed to have a clean exit. I also let out my rear traveler as far as it will go. Great Speed and balance on the helm resulted.


You might want to try flattening your main when on a reach and see if that helps. I would put on some vang and cunningham, make sure you have the out haul tight. Watch the trailing edge and see to it that it has a clean exit. something to try anyway.

Last edited by 6string; 07-15-2008 at 01:27 PM.
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Old 07-20-2008
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Quote:
Originally Posted by mpwildeman View Post
NOW HERES the kicker. I was playing with her mainsail trim last weekend, I had a 3-4 on the beam and she was playing up again. So just as a laugh I let the main out until it started to luff (back of the main flapping like crazy), and what would happen SHE SPEEDS UP.
I know I'm relatively new to this and all, but I'm confused as to why anybody finds this surprising . Aren't those the common rules: "When in doubt, let it out" and that maximum performance out of the main is when it's just short of luffing?

Random thoughts from my studies and what little hands-on experience I have...

What happens when you trim the main back in just a bit--just to the point it stops luffing?

Is the sail actually luffing, or is it simply blown and that's the leach chattering? Does it have a leach line?

I've no idea how this works with in-mast furling, but on any sort of reach in the air you're talking about, I would expect little vang or mainsheet tension, light on the outhaul, hmmm... light on the hayard and little backstay . A very full main with lots of twist. (I'm sure somebody will correct me if I've any of that wrong .)

Perhaps the jib is too closed and/or too close-in and it's back-winding the main? (At 3-4 kts? *shrug*) As angelog suggested: Try on just the main alone.

As for the balance: Do you have any mast rake? Too much? Too little? You say "she doesn't feel right" on a reach, but in what way does she feel wrong?

In a couple places you mention "3-4." As in 3-4 kts of wind? Seriously? You have more patience than most .

Jim
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Old 07-20-2008
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Quote:
Originally Posted by 6string View Post
All good suggestions here. Interesting problem. The boat that I am currently sailing is a 20' C Scow. It is all main sail. I race it. Something I have found on a few legs that started out as a run and ended up at nearly a broad reach, is that the leach cord needed to be release completely. It seems that the sail needed to breath more. The sail needed to have a clean exit. I also let out my rear traveler as far as it will go. Great Speed and balance on the helm resulted.


You might want to try flattening your main when on a reach and see if that helps. I would put on some vang and cunningham, make sure you have the out haul tight. Watch the trailing edge and see to it that it has a clean exit. something to try anyway.
I suspect that therein lies your answer. You have a battenless mainsail and therefore the leech line has that much more effect. With too much leechline tension you're creating turbulence at the leeward end of the sail. Also make sure you have proper halyard tension. In light air you might want some horizontal wrinkling of the sail to give it more curvature or belly, as wind increases you want a flatter sail.

And SemiJim is correct in that letting it luff and then sheeting in just until it stops isn't a bad way at all to trim your main.
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Old 07-22-2008
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I agree, the problem is most likely no roach, to tight of a leech line. An easy indicator is to put tell tales about every 10 feet up the leech of the main. big ones, easy to see. When they are flowing aft, you have proper flow over the main, easy to see and easy to adjust. I also agree most people under sheet their headsails and over sheet their mains. Tell tails on the luff of the jib and on the leech of the main are very simple very easy indicators and educators.
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