Sail Trim for a Cruising Catamaran
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Last Updated on January 17, 2021 by Amy
Cruising catamaran sail trim can be a fairly technical issue with differing opinions about how to best trim the sails to optimize boat speed and balance. I most certainly will not profess to be an expert in the subject, but I do feel I’ve come a long ways from when I first learned how to sail on our Maine Cat 30. This topic appeals to my technical side, and I wanted to put together a guide that people joining us on Starry Horizons might find helpful, and hopefully others will as well.
Catamaran Sailing Basics
Parts of the Sail
Here are the main parts of the sail that you should know to understand the basics of sailing a catamaran:
Mainsail
The mainsail is the largest sail on the boat. It is attached to the mast on the luff (side) and the foot (bottom) attaches to the boom.
Our mainsail is a square-topped rig, which has a flat top with a diagonal batten in it to hold its shape. This style provides a bit more sail area. The standard offering is a pinhead mainsail, where the top comes to a point.
Headsail
There are a huge variety of possible headsails. Starry Horizons carries three, but so the simplest purposes, the headsail is the genoa.
Foot
The foot is the base of the sail. On a mainsail, the foot attaches to the boom. Our mainsail is loose-footed, which means it isn’t attached directly to the boom, but instead is attached by two lines at either corner of the foot. The
Luff
The luff is the forward edge of the sail. On the mainsail, it is attached to the mast. On the genoa, it is attached to the furler.
Leech
The leech is the third side of the sail, and on both the mainsail and headsails, it is loose.
Clew
The clew is the aft corner of the foot of the sail.
On the mainsail, a line called an outhaul runs from the clew into the boom.
On out headsails, the clew is attached to two lines called sheets.
Tack
The tack is the forward corner of the sail. In our mainsail, we have a line called a cunningham which pulls the tack forward and attaches it.
On our genoa, the tack is attached to the base of our furler.
On our bigger headsails, the furlers attach to the boat via what we call an adjustable tackline.
Head
The head is the very top of the sail. Our mainsail is attached via a line called the main halyard. The headsails are attached via a line called the head halyard.
Mainsheet
The mainsheet is the line that runs from the boom to the deck. This is the line that controls the mainsail.
What is a Traveler?
The traveler is a track that runs along the top of the cabin across the beam of the boat. It has track cars, which is how the mainsheet is attached to the traveler.
The traveler can be moved from side to side using a continuous line that is led to the helm.
What are Tell-Tales?
A tell-tale is a small piece of yarn attached to the sail on the body of the sail. We have several on each sail, and we use these tell-tales as indicators to what the wind is doing on the sail.
Catamaran Sail Trim
Angle of Attack and Sail Twist
These are the two primary concerns when considering catamaran sail trim. Here’s how I break them down:
Angle of Attack (AOA)
Definition: The angle between the sail’s chord and the direction of the wind.
Key Question: Is the sail trimmed to maximize power and attached airflow?
How to Answer: Look at your telltales. Along the leech of the mainsail, the telltales should be streaming straight aft. Your genoa will likely have tell-tales on each side of the sail, and these should be streaming aft in parallel.
How to Control: You can use both your traveller and sheets to adjust your angle of attack, or you can adjust your point of sail (change from close-hauled to a close reach).
Tips and Tricks: “When in doubt, let it out” is a good phrase. On a catamaran, I would suggest that you play with the traveller before adjusting the sheet. I’ve found it’s far too easy to over trim the sail, in essence stalling the boat. You may be sailing, but it won’t be as efficient as possible. The exception to this rule is sailing close-hauled, where having the traveller slightly to windward seems to do the trick.
For the genoa, whatever side of the sail has the telltale that isn’t flowing straight, adjust the sail (modifying the angle of attack) towards that side. For example, if you are on a starboard tack and the leeward (port side) telltales are not streaming aft, let out the sheet or try moving the genoa cars forward.
Sail Twist
Definition: The difference in AOA from the head to the foot of a sail.
Key Question: How much twist should I have in the sails for the current conditions?
How to Answer: This one you’re just going to have to know. The general rule of thumb is you want more twist in light winds, less twist in moderate winds and back to more twist in heavy wind.
How to Control: You can use both the sheet and traveler to adjust sail twist. Moving the traveler to windward and letting out the main sheet to keep the boom in place will increase the twist of the mainsail. For the Genoa, rigging a barber hauler can provide a significantly increased amount of control over twist, as well as opening up the AOA on deeper sail angles.
Tips and Tricks: Increasing twist in a sail in heavy air can be a quick way to slightly depower if necessary, just let out the sheet. If you get to this point though, you should probably be thinking about putting in a reef!
Traveller Out, Sheet On
If you remember nothing else, this is the thing to remember. As a general rule, to trim the main on a cruising catamaran, you should let out the traveler first, while keeping the main sheet pretty tight. This seems to work fairly well for the length of the traveler, and then you can start letting out the main sheet. Don’t be afraid to experiment, especially with adding some twist to the main if the telltales aren’t flying just right.
Move Your Jib/Genoa Cars
This can be an easy thing to forget, but the closer to the wind your sailing, the further aft your genoa cars should be. This helps to reduce twist and get the most power out of the sail. As you sail deeper angles, you need to move those cars forward. Eventually, a barber hauler will prove incredibly useful on everything from a close reach and deeper.
Diagrams
I’ve put this big diagram together to be a handy guide aboard our boat and a “cheat sheet” to basic sail trim. I welcome any feedback and will constantly try to be improving it. Also included are diagrams for our Doyle Screecher and Asymmetric Spinnaker. These diagrams are a bit more general and are included because I wanted a handy place to keep track of the apparent wind angles (AWA) and apparent wind speed (AWS) that Doyle has suggested for these sails. If you have these type of sails on board, I highly recommend that you listen to the manufacturer’s recommendations, rather than using these numbers.
You can also download this file in pdf format here: Sail Trim for a Cruising Catamaran.
Hopefully, this is helpful and please don’t hesitate to provide feedback. I’d love to hear your suggestions on ways to improve and make the boat go faster!
Article and diagrams very helpful. Wondering if you could post more about the screecher. Curious to see the other points of sail for it.
Thank you!
Ed
34′ Gemini 105mc
Thanks this has been very helpful! Just bought a car this year, and this was still the most clear and easy guideline to sail trim I could find, even though you wrote this article years ago!
Aw thank you!
Amy and David,
Thank you for sharing your experience. I will be printing this out and keeping it handy on our Helia.
Steven
Salty Britches
This is a very nice article — thanks for putting it together.
Thanks for reading!
I have designed a cat-rig which drives the yacht directly to the destination without tacking, without healing, and can sail astern. I’ll send you a sketch if you request one without charge but I’m not a draftsman or model maker
Just came across this article. Excellent stuff David. Thank you. As newbie sailors, we really struggle with Sail trim and I found this super helpful
Cheers
Jo
SV Double Trouble
Hi David, awesome stuff. Really appreciate it. Sailing from Cascais to Lagos today and looking for tips. We’re Auckland based usually but unfortunately missed you when you were there. Sounds like you had a nice day trip with our friends Tim and Adrienne Chrisp.
Cheers
Connan
Old post but…
“but the closer to the wind your sailing, the further aft your genoa cars should be. This helps to reduce twist and get the most power out of the sail. As you sail deeper angles, you need to move those cars forward.”
Just getting into cats, but on a monohull moving cars aft increases twist. What’s up?
Thanks, I enjoy your site
Hmm… well, I don’t have really any experience on monohulls so I can’t really speak to that. All I know is that if I have the genoa cars forward due to reefing (and we’re sailing upwind), and then we unfurl the whole genoa without moving the cars aft, the sail shape clearly isn’t right and the telltales don’t stream properly. Move the cars aft and all is right with the world again.
Sail trim is sail trim dosent matter if on a cat, mono hull, trimaran, or city bus. Moving cars aft does increase twist as you sheet in that lead will need to come aft for proper twist. As you ease out it will need to go forward.
This is such a well written and explanatory article. Thank you. I find myself returning to reread this quite often.
I do have a question regarding mainsail trimming. When the mainsail leech telltales are curling, in general, which way should I move my traveller? Should I move the sail towards the curl or away from the curl? I have tried to approach this in a practical manner while sailing but often times swell and waves makes it difficult to tell if I am making a difference.
I am basically looking for a cheat like I use with our genoa. If a telltale is lifting I know to move the sail in the direction of the lift. Is it the same with the mainsail leech telltales?
I’m glad you’ve found it helpful! Unfortunately, I don’t really have much help to offer with mainsail trimming. I don’t really have a racing background, and while cruising I generally go for the set it and forget it mentality. Basically, I try to do 2 things: 1) Go by the “when it doubt, let it out” theory and 2) try to generally line up the leech of the mainsail with the leech of the genoa. I have a bit of a hard time sometimes telling exactly when the mainsail is luffing, so that’s when I try to use the 2nd part to help as a guide.
Also, based on some of the articles I’ve read, I think (and this is key, this is me thinking, so not very scientific or official) that if the telltale is curling to the leeward side of the sail, then the sail is overtrimmed and needs to be let out. Sorry I can’t offer more help, but I’d check out some of the racing sailing forums and I’m sure they’d have a lot more advice to offer!
Depends on which telltale you are looking at. Leech telltales are for accessing the twist in the sail, telltale in the body of the sail are for angle of attack. The idea is to have attached flow over the sail front to back, angle of attack is for the front of the sail, twist is for the back. If the lower leech telltales are flying up and are not streaming streaming straight aft tighten the sheet, if the upper tails are flying up ease the sheet. A telltale in the body of the main is used just like the ones on the the jib, so adjust the angle of attack with the traveler for those first and then adjust the sheet for the ones on the leech. If you have a vang you can use it to adjust the twist and the sheet to adjust the angle of attack.
On the jib you can actually use the luff telltales to access twist because of its different planform design and clean luff compared to the main. That is accomplished by moving the lead forward to reduce twist and aft to increase it, ideally windward and leeward telltales will be streaming aft top to bottom. Keep in mind however inducing extra twist can be used to quickly depower both the main and the jib prior to reefing as a tactic.
Hi@ I’m really glad to find your diagrams and test, but I waant to ask you about the length of your traveler, and the fact that its straight; do you think your (estimate 6 to 8′ traveler is long enuf for cruising purposes? and; would a curved track have been better? There are several contributors on this topic who seem to believe otherwise.. Now that you have another 18 months experience… thanks
Hi Dennis,
I would estimate our traveller is closer to 12 feet long as it stretches almost the full length of our coachroof. I don’t know enough about a curved track to offer a comment, but for us, when we let the mainsheet out much past the ends of the traveller, we use a preventer which can also act as a boom vang. This set up is fairly easy and works for us, but I’m sure there are better ways to get more performance. We just care about getting there eventually!
Hope that helps,
David
Thanks for posting that; much appreciated. I really struggle to find anything online relating to sailing instruction for cruising cats.
Hey David! (Great name by the way!) These guidelines may not get you to the finish line first, but they’ll definitely get you from Point A to Point B, which is pretty much the whole purpose of cruising. 🙂
Ditto – many thanks for the diagram
You’re very welcome! Hope it helps!
This is by far the best explanation of traveller use I have come across. You have cleared up some nagging doubts I have had since buying our cat last year. Thanks for not getting wrapped up in technical jargon. Amazing how hard it is to find this info online.
Thanks Ben! I’m glad you found this helpful. Like I said, I’m certainly no expert, but this guide seemed to work fairly well for us as we came across the Atlantic.