Making sense of mainsail trim

Bill Roosa · Apr 9, 2009

twist, camber and entry angle

Twist is used to accommodate the increase of wind speed with altitude. If you suspect that the winds "at altitude" are higher then the sail will require twist to keep the angle of attack the same "down low" and "at altitude"
Camber is used to develop power. In low winds you need more power than in high winds soooooo more camber is use in lower winds than in higher winds.
Entry angle is the angle that the relative wind makes with the forward portion of the sail (luft). A negative angle of entry will result in back winding all other things being equal. Jib and main interactions can get you some advantages here. The jib can create a change in the relative wind the main "sees." A 0 angle of entry will allow even a full (lots of camber) sail to "draw wind" efficiently. A high angle of entry will stall the sail. Stalled sails are not necessarily a bad thing. If you are trying to sail dead down wind then the sail is usually stalled. This is the reason sailing dead down wind is not the fastest way to get dead down wind. 3-5 degrees of positive entry angle is normally considered best. A windex if VERY helpfull here!
These concepts all work together to produce thrust in the forward direction. They can also all work together to produce "thrust" to heel the boat!

The trick is to get the thrust to be aimed as much to forward as possible. I tried to trim sails for 2 years without some sort of speed instrument and was frustrated at every turn. Once we got a GPS we discovered that a 2" pull on the sheet was enough to change our speed by 0.5 knots. A couple of more 2" pulls on different lines and we where all of a sudden doing 3 knots faster!!!!!

If you really want to learn then get out and decide to figure out how to set the sails for xx degrees relative wind. CHANGE THE BOAT HEADING TO KEEP THE RELATIVE WIND C-O-N-S-T-A-N-T!!!!!
Start with the all the sheets let out so the sails are almost "a-slant." Sheet in the jib (always trim sails fore to aft) a little and note the change in speed. Pull the traveler in a little and note the change in speed. Repeat this till you see the speed either not increase or decrease. Sheet/traveler in and out 2" at a time till you get a max speed. You may have to wait up to a minute to see the small changes in speed.
All through this we did not change the jib sheet lead or the main sheet. Observe the jib tale-tails, are they all streaming correctly? Adjust the jib sheet lead till you have the jib twist correctly. Re-trim the sails. Observe the main tale-tails, are they all streaming correctly? Sheet in the main till they are and re-trim the sails.
Getting tired? With practice, practice, practice you will get a feel for how the sails look for beating, and reaching. You will then just know where to put the jib sheet lead, travler and sheets and start to play with the Cunningham and main outhaul and halyard tension to get that extra 0.25 knots to win the race!!!

Stu Jackson · Apr 9, 2009

Gee, getting recommendations to go out on your boat more often

to learn is GREAT!

Really, Bill's right. Each day is different and sometimes the rules change, for instance in your comment: "...I've read that the main should be full to develop power and I've read that it should be flat since very light air has difficulty flowing properly over a deep sail. Which is it??"

Without telling us what condition your sails are in, and whether you have hi-tech or, like me 22 year old getting-to-be-baggiy" sails OK, OK, they're cruising sails!, your experience may vary. I've found different days require different approaches to the "bag it" or "flatten it" question, mostly because wind strengths change and it's hard to use your example because I've never had what one could call consistent winds, even here on San Francisco Bay. It keeps changing and so should your sail shape. Heck, sometime I simply say: "If it looks good, it's workin'!"So buy Don's excellent guide and chart right here from this forum, and go sailing. Who can beat that recommendation?

Stu Jackson · Apr 9, 2009

Clark said:
I have an engineering background (narrow minded?) and get frustrated when terminology and descriptions aren't consistent.

I'll keep trying, maybe it'll sink in after awhile.

Clark,

I'm an engineer too! It took me years to learn to have the heart of an artist.:dance:

Keep practicing. One thing no one ever mentions (psst, psst): It'll NEVER come to you, you have to go to it!

So un-engineering-ish...

Don Guillette · Apr 10, 2009

Clark: Here it is in a nut shell for the complete range of closehauled wind conditions for your main and jib. The following is an over simplification.

In very light conditions you want the sails as flat as possable because the wind does not have enough force to get around the sails. Test this for yourself - start out with a flat sail and induce some draft depth and see what happens - you'll stop the boat. Inducing belly into the main and jib in light conditions is a mistake most middle to back of the pack racers make and that is one of the reasons they are where they are. Watch how the front runners trim for these conditions. Cruisers make the same mistake.

As the wind starts to pipe up you gradually induce draft depth (belly). What you are doing is powering up the sail. As the wind increases you induce a bit more belly. When the wind is in say the 10 to 12 knot range you start REDUCING the belly gradualy. By reducing the belly your depowering the sail. A flat sail is a less powerful sail. The flatter you make the sail the less power it develops. In the 15 knot range you would want to induce some twist in both sails. What your doing is spilling power out of the top 1/3 of the sails thus depowering them.

Twist is the last step before reefing. I don't like to reef until I have to and that is why I induce twist. I'm trying to keep the powerful set of the sails for as long as I can.

Sooo, you start out flat in light conditions and at the top end of the wind range your back to flat again. This keeps your baot under control.

It is hard to explain this on a chart - a chart is just that , a chart. If I chocked it full of info it would be a book!! What you should do is take this basic info I've provided and fool around with it on your boat and see if it works for you.

Nicky17000 · Apr 11, 2009

re;draft\camber

This is just my humble observation so if I got it wrong please correct me.
It is generally assumed thet more draft corresponds to more power. I think that a little too much draft is counter productive becouse the back half of the sail will create turbulence and drag, so the extra power goes in heeling the boat not driving forward.
So, if this holds true, it leads to another question. What are the guidelines to evaluate if there is enough or too much draft? When the jib is backwinding the belly of the main?
When the main leech telltails are flying to leeward?
Thanks

shemandr · Apr 11, 2009

In trying to learn how much is enough draft for a particular wind on your boat watch your telltails (telltales!). They can tell the story reasonably well. If the leech telltails aren't flying there isn't flow in the right direction over them and its time to change something. Like if its very light wind and the sail is trimmed in you may have to flatten it to allow the wind to flow over the sail without detaching and causing turbulence. A light responsive boat (Like a dingy) speeds the learning curve.
A lot of boats I've sailed will have backwinding on the main from the jib - especially with large overlapping sails. I wouldn't use that metric so much. Its usually the leech that counts. If there is flow there, the sail is working.
When the main leech telltails are hooked around to leeward the flow of air is turning around the leech and heading forward on the leeside. It is a bubble. I think the main could be overtrimmed creating that leeward turbulance. Try letting the main out to re-establish flow backward on the leeside and that telltail should start to stream again.
You can't have all of your telltails flying all of the time in all winds and wind angles. But for gauging how much trim, or twist or camber, they can show how the wind is flowing over the sail.
I hope this is helpful

Don Guillette · Apr 11, 2009

How much draft depth is enough? I can't give an exact answer that will fit each and every boat plus it also depends on your point of sail and wind conditions. Those two elements determine how much belly is enough. Draft depth and draft position are measured as a percentage. The max draft depth is about 15% and that would match a DRAFT POSITION of about 50% to 55%. I know that is meaningless to some listers but that is how it is measured and if you ever want to understand sail trim you have to spend the time to understand these terms.

Draft depth (belly) does provide the energy to power the boat and one would think that the more belly you induce the faster you go. It is not true and the only way I can prove it to you is for you to experiment with draft depth on your boat. Let the outhaul out all the way and see what happens. Try the same thing in very light air and see what happens. Let the outhaul out gradually in small increments and give the sail time to power up before you let out some more - sort of like adjusting the hot water on your shower in your bathroom. DO NOT forget the jib adjustment at the same time.

Sail trim is not rocket science. Additionally, you can not just focus on draft depth - there are 3 other items you have to deal with at the same time and they are draft position, twist and angle of attack.

When I was racing in So Ca in light air I would watch the boats in front of us with a large amount of draft depth and think to myself it is just a matter of time before we pick them off. Skippers think they are harnessing the power of what little wind there is but if the wind can't get around the sails, no matter what its spped, the boat is going to stop because you've stalled the sails.

Bubble in the mainsail - this is another situation that a lot of sailors get wrong. The bubble is caused by the jib backwinding. Over the years a that I've been involved with the sail trim forum a few sailors have told me they like to sail that way - whatever floats their boat!! Here's how some or most masthead rig skippers who don't like to sail with the bubble in the main correct the situation - they ease the jib. This is exactly the wrong move. On a masthead rig the engine is the jib not the main. The correct adjustment is to TRIM IN the main until the bubble disappears.