Sail Trim – Twist on a “People Boat” can be applied to a model yacht

INTRODUCTION

Add twist to your main sail for more power and speed in light air

NOTE – April 3, 2018

This Post is in development, needing fine editing and improvement of its images. It is published because I thought it was important for us all to have the opportunity to know about the subject of sail twist before the next outing at the pond.

Jerry Walker
Pima Micro Yacht Club
Website Administrator

model yacht sail trim descriiption
Add twist to your main sail for more power and speed in light air. Click the image for a graphic presentation about sail twist from Brian Chapman. Brian was, he passed in 2017, the UK Sailmakers Toronto owner and sail designer with 35 years of experience designing and building sails. Note; model yachts DO NOT USE A TRAVELER. Note: after looking at it twice I have decided its better than nothing but, overall, rather weak.)

 

This post contains three items found on the Internet about main sail twist. Also, clicking the image above takes you to a document developed by Brian Chapman, a sailmaker that does of good job of graphically describing sail trim.

ITEM 1 – UNDERSTANDING MAIN SAIL TWIST

Author: Quantum Sails, April 2017 – this article appeared on the Sailing World website: https://www.sailingworld.com/understanding-mainsail-twist

 

Mainsail twist can have a large impact on speed and performance. Quantum Sails explains what mainsail twist is and how to use it to get ahead on the racecourse.

Technically, twist is the change in the angle of attack (of the sail surface relative to the wind direction)  from the bottom of the sail to the top. Twist is necessitated by the wind speed changes, hence changing angle relative to the boat as you move away from the water. The drag induced by the water slows wind near the surface and shifts it relatively further forward, as opposed to the faster-flowing wind further aloft. This effect is exaggerated at lower wind speeds.

Understanding Twist

In the real world, twist means the leech of a sail must open up to some degree as we move from bottom to top.

Any time the distance between the clew and the head is shortened (easing the mainsheet or boom vang), twist increases. The same length of fabric is now strung between two points that are closer together, so the leech of the sail opens up. Conversely, pulling down on the clew reduces twist, closing off and rounding up the leech. A tight, round leech creates power and forces the boat to point, but it can also cause airflow to stall or overpower the boat (creating too much helm and heel). A twisted leech profile promotes airflow in light air when it’s hard to get air to stay attached. In heavy air, the flatter, more-open sections depower the sail and help keep the boat on its feet.

Having the right amount of mainsail twist is perhaps the single biggest key to upwind boat speed, especially on the new breed of fast sailboats that often relies on bigger mainsails and smaller foretriangles. A competent mainsail trimmer can get you in the ballpark, but the true boat-speed virtuoso understands, feels, and implements changes on a moment-to-moment basis, which makes the real difference.

In a very real sense, mainsail trimmers are driving the boat as much as the helmsperson. That’s why you often see trimmers hunched over, (usually directly in line with the view of the helmsperson!), as they ply their trade. They’re looking at the same inputs to guide their sense of feel to dictate the appropriate reaction: angle of heel, jib telltales, boat speed, waves, and wind angle.

In general terms, you can think of mainsail twist in three modes: light, medium, and heavy air.

Light Air

In light air, use extra twist and an open leech to promote attached flow and aid in acceleration. The top batten will be open, pointing 3-10 degrees to leeward from where the boom is pointing, and the top telltale should flow aft. Sail shape in light air will be full, so it’s important to keep the leech open and twisted to keep the sail from stalling. Once twist is set, position the boom on the centerline with the traveler for maximum power and pointing.

Medium Air

In medium conditions the boat should be moving well, so leech tension can be increased and twist reduced. This will force pointing. Overall sail shape will be flatter, so there is less danger of stall. If the boat is up to speed, it’s okay to reduce twist to the point at which the top telltale stalls (disappears behind the leech). The traveler will drop so the boom doesn’t get above centerline, and it will be lowered further to control heel as necessary. Using the traveler to control helm and heel in moderate conditions allows the trimmer to use twist to balance speed versus pointing.

Heavy Air

In heavy air, control of heel is paramount. More twist will help keep the boat upright. The boat will typically have to sail at wider angles (foot) to have the power necessary to blast through waves, and that will generate more heel. In smooth water, the helmsperson can “feather” more, or let the inside telltales lift in puffs. The overall sail shape will be as flat as possible, which will help induce twist and open the leech. The traveler, which is great for fine-tuning balance in moderate conditions, usually does not provide enough gross change to handle big puffs, so twisting the entire sail with the mainsheet works best. I typically pull the traveler up a couple of feet above the leeward coaming and play the sheet to control heel. In windy conditions, use the boom vang to help augment the mainsheet.

Playing the Game

On a moment-by-moment basis, the game is simple. The mainsail trimmer constantly tries to reduce twist (trimming harder) as long as the boat speed is up and heel is under control. The goal is to point as long as speed and helm allow. Keep in mind the golden rule: speed first, then try to point. Hypothetically, here’s how the thought process would work on board for a target boat speed of 7.2, true wind angle 38 degrees:

Out of the tack, ease the mainsheet at least until the top telltale flows, or until heel is under control. Pull the traveler up with one hand as you ease the mainsheet (if you need power).

Suppose speed turns at 5.8 and is building. Sheet harder as the speed comes back up to 7.2, lowering the traveler as necessary. Once you’re up to speed, full trim. If the tack is more into the waves than port was, don’t sheet as hard. If slowing, ease a half-inch of sheet for more twist. If the skipper presses for speed but there’s too much heel, ease some more. If the speed climbs too rapidly over target, sheet harder as the helmsperson feathers up.

Suppose a set of waves comes. Press and build speed over target and ease the mainsheet. Extra heel is okay, but not too much. Through the waves, back hard on the wind and sheet harder. When you reach a good angle, good speed, and no big waves, sheet harder still to make the driver work. If you get carried away, ease a fraction. For a big wave, ease a bunch over the top as the driver bears off to avoid the slam. Stay eased until speed comes back, then gradually sheet harder. And the game continues…

Getting the correct twist is a dynamic, ever-changing proposition, and it’s a little different on every boat. There is no one magic combination of twist versus traveler that works for every boat, so be prepared to re-educate yourself on each boat and don’t be afraid to ask for help!

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ITEM 2 – I KNOW SAIL TWIST IS IMPORTANT, BUT CAN YOU HELP ME UNDERSTAND WHY?

Author: Discussion on Quantum Sails website; question answered by David Flynn of Quantum Sails (quantumsails.com), November 2017.

 – Danielle P.

You are definitely not the only sailor who has asked this question! Here’s the lowdown on twist and why you need it. Twist is the change in the angle of attack from the bottom of the sail to the top of the sail and is caused by a change in wind speed, which changes angle relative to the boat the farther away you are from the surface of the water. The drag from the water slows the wind near the surface, shifting it further forward in comparison to the faster flowing wind further aloft. This effect is exaggerated at lower wind speeds. In practice, it means that the leech of a sail must open up to some degree as you look from bottom to top.

Any time the distance between the clew and the head is shortened (easing the mainsheet or boom vang), twist is increased. The same length of fabric is now strung between two points that are closer together, so the leech of the sail opens up. Conversely, pull down on the clew and twist is reduced, which closes off and rounds up the leech. A tight, round leech creates power and forces the boat to point, but it can also cause airflow to stall or overpower the boat (create too much helm and heel). In light air, when it is hard to get air to create lift, a twisted leech profile promotes airflow. In heavy air, flatter and more open sections depower the sail and help to keep the boat on its feet.

Mastering the boom vang is an important sail control when it comes to getting the right twist for the right conditions. Click here to read another sailor’s question about the vang.

 

THE DISCUSSION

MATT LECHNER

December 6, 2017. Twist on the mainsail also controlled by the traveller, and some jibs use a barber haul line to control twist I believe the term relates to the “twist” in the shape of the airfoil leading-edge of the sail(s), when there is “twist” in the sail the center of effort of the leading edge airfoil is farther aft than on the lower portions of the sail, hence the airfoil distribution is not uniform along a vertical type imaginary line – i.e. the airfoil is on a closer angle of attack to the wind at the bottom of the sail than at the top. Some twist is desirable in light to medium air as it can tend to give the sail a little belly to generate a more powerful airfoil. Too much belly and you lose windward ability however. Use of the cunningham hole on the mainsail can reduce the belly of the sail particularly in the sail’s lower half. Tightening up or loosening the outhaul also has an effect. The sea state also must be taken into consideration because if there is a sea or chop, the boat will need a little more power to push through and not hobby-horse so you arrange the sail adjustments to allow a slightly lesser angle of windward attack when the water is not flat. The boat’s tolerance for sailing with varying degrees of heel also enters into the equation, because some boats do best when sailed more or less flat, others do fine at greater angles of heel.

Also, if you vang down the main boom hard – be aware it will be lower to the deck as you tack, so try not to get clonked on the head if you do get knocked unconscious by a vanged-down boom, put your life jacket on immediately so that you do not float face down in the water when you slide off the deck.

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ITEM 3 – 3 REASONS TO TWIST YOUR SAIL OUT

April 30, 2014 Director of Education Bareboat Charter Crew Skipper

NauticEd is the World’s Most Advanced Sailing Education and Sailing Certification Program.

PERFECTING SAIL TRIM AND TWIST

Here are three reasons you should be twisting out the sail towards the top of the mast

  1. Get rid of excessive heeling
  2. Match the wind at the top of the sail to the wind gradient
  3. Drum Roll … The most important … Change heeling force into forward force.

(1)   Getting rid of excessive healing forces.

I’ve written on this topic a few times mostly because it is an important fundamental topic of understanding the forces on a sail.

In a right triangle, a force applied evenly over the surface can be considered to act in one place. This place is called the Center of Pressure and is the geometric center of the right triangle and is 1/3rd of the way up the triangle. It is found by crisscrossing the corners and midpoints.

The propensity to heel is called the Heeling Moment and it is derived  from a multiplication of the wind force magnitude and the height of the Center of Pressure.

Thus, the heeling can be reduced by lowering the Center of Pressure. You can do this obviously by reefing but also by twisting out the top of the sail which changes the triangle shape.

Twisted mainsail lowers and moves the center of pressure forward
Twisted mainsail lowers and moves the center of pressure forward

Twisted mainsail lowers and moves the center of pressure forward

This also has the added effect of moving the Center of Pressure forward which reduces your weather helm.

(2)   Match the wind at the top of the mast to the wind gradient

Wind velocity at the surface is less than wind velocity at the top of the mast due to friction of the surface on the wind. This is called Wind Velocity Gradient. In addition there is another effect called Wind Shear which is due to coriolus effect dependent on the distance from the equator and if northern or southern hemisphere. This wind shear creates a different direction of wind at the top of the mast than at the boom height because the wind is seen to twist as it slows down.

When you combine Wind Velocity Gradient and boat velocity you also get different apparent wind directions on the sail. This is best described in detail in our free basic sail trim course.

Because of this effect the wind at the top of the mast is more from an aft direction. Said to be “more aft”.

 

Wind Velocity is different in speed and direction between surface and top of mast.
Wind Velocity is different in speed and direction between surface and top of mast.

Wind Velocity is different in speed and direction between surface and top of mast.

When flying a sail then you already know to match the sail angle to the wind to make it most efficient. If the wind direction at the top of the mast is more aft the sail direction must change to be more let out moving up the mast. To achieve this you twist the sail out by allowing the aft of the boom to rise up. This loosens the leach of the sail and allows it to twist out at the top.

Adjust Sail angles up the mast to match apparent wind direction

Control your leach tension via the boomvang. Keep in mind also that your mainsheet will also control leach tension as well. If your mainsheet is in tight, loosening the boomvang will have no effect. When you let out the mainsheet this will deliver the leach control to the boomvang. You can then use the traveler to re-center the boom.

Adjust Sail angles up the mast to match apparent wind direction
Adjust Sail angles up the mast to match apparent wind direction

(3)   Changing heeling force into forward force

This is not talked about much but it is the most important when thinking about making your boat go fast.

The force acting on the sail from the wind can be thought of as being approximately in the direction perpendicular to the battens.  As the sail is twisted out in going up the mast the force then shifts from sideways unwanted heeling force to desired forward driving force by the nature of its direction.

Twisting the Sail Changes Heeling Force to Forward Force
Twisting the Sail Changes Heeling Force to Forward Force

This then is very important. As you know, not only should you always have your sails let out as much as possible just before luffing to fly the sails efficiently but you should additionally be looking up the sail and adjusting twist as much as possible to translate the resultant force to be forward acting rather that sideways acting. Increase twist until upper sail luffing occurs then tighten slightly.

As with point 2 above use the boomvang, the mainsheet and traveler as your controls.

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PMYC Club Racing Rules

The PMYC racing rules are a much simplified version of the International Racing Rules of Sailing. We established these simplified rules to make it easier for our skippers to participate in the racing.

Introduction

These rules are labeled the “PMYC Club Racing Rules” because they are a significantly modified version of the International Racing Rules of Sailing.
That is intentional as they accommodate what our members have clearly communicated during the past year (2016) — that learning the official rules is difficult and they just don’t want to be bothered. One Club member suggested that we do “Casual racing”. That essentially meant that following the official rules of racing was optional.

Using a set of rules that are not mandatory or having rules that are not obeyed because they are not known to skippers is a recipe for chaos. It can never be known for sure whether the boat that beats you did so by clean sailing or bending or breaking the rules and not acknowledging it. And, to make matters much worse, some of PMYC’s best skippers have been know to treat the official rules as optional.

But, we need some defined discipline for our racing. The remainder of this post is an attempt to provide some discipline in an easy-to-understand form. The PMYC rules take precedence over the international rules of racing. I believe they will work well for PMYC.  My sense is that they are easy to learn.

With the agreement of our members I suggest we have a focus on our racing discipline at every meeting — and perhaps every pond session. 

Note, the PMYC rules will not work for racing outside of PMYC as they are not the same as the International Rules of Racing. If you want to join the folks up in Phoenix you will have to study the official rules. They sail by them.

The PMYC Racing Rules have received one, very brief, review at the Feb. 3, Club meeting.  I think they were judged as reasonable; at least not unreasonable. They are subject to ongoing review and modification by the members so that we can have some pond discipline that our skippers are comfortable with and willing to sail by.

The biggest challenge of the PMYC Racing Rules is that they require voluntary compliance. And, compliance is easy because about the only behavior that is penalized is a collision.  And, a collision requires both/all parties involved to “take a turn”. The notion of penalizing all boats involved in a collision probably seems a bit unfair. There is validity in that notion. However, without imposing some greater set of rules, which you have all resoundingly rejected in the past these rules are just intended to allow everyone to have fun. If you are an innocent bystander, be patient. Take your turn and move on. The next time around your closest competitor will be taking the turn.

Here is something to think about. There are two secrets to not having to sail by any rules. One, stay clear of everyone. The down side of that approach is that you will be giving away the advantage to other boats. And, two, get the best start and be faster than everyone else.

PMYC racing rules

1. Starboard tack boat has right-of-way over a boat on port tack.

2. Windward boat stays clear of leeward boat.

3. There is no restriction on being on the course-side of the start line -- at the start signal. If you are on the course side of the line at the start signal, you have started early and must return to the start side of the line before sailing the race. Note, while returning to the start line you must stay out of the way of all boats that have started even if you are on starboard tack and all the other boats are on port. There is no rule indicating the course you sail to return to the start side of the start line.

4. General recalls - If so many boats start early that they cannot be individually identified, a general recall will be made - immediately ending the race. The race will be restarted. If you are over the start line early again, you will be disqualified from the race. The message is simple. Don't start early.

5. Hitting the boat to leeward of you while attempting to squeeze between another boat and the starboard mark on the start line; barging, is prohibited – take a turn.

6. Light contact between or among boats; anywhere on the course including at marks – allowed.

7. All other contact between two or more boats; anywhere on the course including at marks  – both/all involved boats take a turn - no discussion needed. The rationale is that one boat fouled and one boat failed to avoid a collision. We don’t know nor care which boat did what. If there is any doubt about whether it was a light hit , it is assumed to be in this category – take a turn.

8. Boats not racing are expected to stay clear of those that are racing. However, boats that are on the course and not racing are obstacles – stay clear of them. If you hit one you have fouled; take a turn.

9. When approaching a wall or other obstacle (debris in the water, a boat not racing) you will give boats around you the room to clear the obstacle. Failure to do so is a foul – take a turn.

10. Penalty turns are to be taken within a very short time (a minute or so) after a foul. Get clear of other boats to do it and understand that while executing a turn you do not have any right-of-way over other boats. Collisions during the penalty turns will not penalize the right-of way boats.

11. Failure to voluntarily take a turn when you don’t comply with the above behaviors can lead to broken tempers and a better boat might possibly loose through no fault of her own. You can’t be forced to take a turn but remember that, from the Racing Rules of Sailing: “A boat … shall compete in compliance with recognized principles of sportsmanship and fair play”.

12. PMYC Weed Rule - as debris in the water can be frequent and its attachment to a keel or rudder almost inevitable, it is permitted to get your boat to the pond side, clear the debris and relaunch it without penalty. The launching can be in the direction of the course leg but must not be at a speed higher than it came to the pond side.

PMYC Perennial Racing Trophy

Each year PMYC for the past three years PMYC has conducted a Winter Season Racing Series. This series is comprised of a monthly regatta, beginning in November and ending in May. During the 2015-2016 Winter Season a perennial trophy was designed and constructed by Bill Brown, one of our founding members.  The trophy is 7″ wide by 21″ high with plenty of room for nine years of winners. It is designed to be hung on a wall of each annual winner.

The winner of the 2015-2016 season was Bob Spraker, at the time a PMYC member.

As a result of a division of PMYC into two clubs, one sailing the Victoria model yacht (Tucson Model Yacht Club) and one sailing the Micro Magic (Pima Micro Yacht Club), there was no 2016-2017 series. Until PMYC has another winner, the trophy will hang on a wall in Jerry Walker’s den.  Jerry is PMYC’s most recent past Commodore.

 

Sail Day – February 10, 2017

It was a beautiful clear sunny day in the 80’s, good even for Tucson this time of year.

We had four Vic’s and the debut of the Micro Magic. The MM is on loan from John Albertson of Green Valley a long time friend of PMYC. The MM got an approved rating from three of the attendees. It sailed respectively, able to keep up with the Vics in very light air.

There was no racing today as I forgot to bring the marks. My mind is out of the groove after not sailing for six weeks. But, no one cared. We sailed and chatted. When two boats came together a little competition developed. Comments were made about one boat or the other sailing well but no one really cared. It was a pleasant day to be on the pond.

We are going to move ahead with the purchase five MM’s. Graupner is currently quoting 4-5 week lead time.

I am going to contact Graupner to clarify how we best order boats.

Read the MM building instructions provided by Graupner.

Where & When We Sail/Race

Reid Park Small Pond

June 1 – Oct. 30, Every Friday at 9:00 am

Nov 1 – May 30, Every friday at 12:00pm

The small pond is about a two minute walk north of the parking lot off Lakeside Lane (across the street from McDonald’s) on 22nd St.

Note: As the weather cools in October, we may change our sailing time to the winter schedule of 1:00pm. Watch the website for confirmation of the sailing time.

Note at the top of the right column the time we are sailing as it changes occasionally. When we have adverse weather, members will be notified by email if we have to reschedule the racing.

Parking and Sailing

The map below shows the big and small ponds and parking.

For the small pond enter Reid Park on Lakeshore Dr. at the McDonald’s on 22nd. St. just west of Alvernon and use the parking on the first left. Walk north to our sailing area.

Look for our boats.

For questions Contact Us.