Mast bend and mainsail shape

Bending the mast is a fast and simple way of managing the power on your sail boat

As discussed here mast-rake and mast bend are two different aspects of mast “tune” that affect the boats balance and so feel and ultimately performance. This essay discusses mast BEND. There is an element of headsail shaping connected to mast bend but we will get to that in future posts.There are other tools in the mainsail shaping tool box, but this essay is about mast bend

The Basics:

More wind requires a flatter sail

Less wind prefers a fuller sail.

Mainsails have shaping both across the sail (broad seaming) and at the luff, known as Luff Curve

This “curve” is intimately connected with the characteristics of the mast, and so, of the boat. There are differences in the amount of luff curve required between masthead and fractional rigs, masts with spreaders in line–that is to say where the cap shrouds are perpendicular to the mast-Or with “swept back” spreaders, such as many newer boats have.

Some Definitions:

Chord: Is an imaginary line between the luff and the leech

Depth: (and draft, camber & shape-They are quite synonymous and generally used interchangeably by Sailmakers in their jargon) is that distance, perpendicular to the chord line, at which the sail’s surface is furthest away from the chord line.

In the sketch below:

The line ZZ is the chord line.

The portion “C” is the chord LENGTH on the sail.

The distance “a” is the chord DEPTH (draft camber shape…).

The distance “b” is the POSITION of the draft measured aft of the leading edge of the foil which is in this case to the right of the image. Thus a/c is the draft and b/c is it’s position.

Chord (length) draft and location of draft are all depicted in this image.

Chord (length) draft and location of draft are all depicted in this image.

Power: over and under (powered)

Power (force, energy …wind speed) is what gives the sensations you feel thru the wheel or tiller and via the heel angle of the boat. With headsails, managing the power is relatively easy: Too much power, then put up the small (er) one-Or more likely today either partially roll up the genoa or roll it up all the way and motor-sail.

Sail size and shape:

Having the right (sized) sails of the right shape for the conditions is, for those who prefer to sail for the sensations of how the boat feels thru the tiller/wheel, a key part of this sensation. Too much sail and/or sails that are too full makes the helm feel heavy and generates too much weather helm. In this condition the boat heels more than you want and always inclined to want to round up. This is of course unpleasant, occasionally quite noisy, slow and not infrequently intimidating for some on board.

The sensations of being underpowered are less dramatic, noisy, violent and so less intimidating.

Power on a sailing boat comes of course from the sails. Having the right shape in the sails is therefore key to getting the boat to “feel good” and sail well.

Managing the power

One tool in the box of adjustments available to sailors is adjusting the camber of the mainsail-What many folks refer to as the belly of the sail. Camber is the “depth” of a sail at a given point on the sail. (NB when we/you say the sail is blown out, usually this refers to the draft being further than 50% aft on the sail)

This picture shows me and a high school intern measuring the draft amount and location on a 420 mainsail strung up in the loft.

This picture shows me and a high school intern measuring the draft , the amount and its location on a 420 mainsail strung up in the loft.

The camber on any foil is expressed as an amount, at a location. So you might hear Sailmakers say something like…”it was 8% at 35% aft.” This means that what they were looking at was 8% of the fore and aft span (of the chord line) perpendicular from that line and that position was 35% aft, from the luff. Translated: the sail was 8% deep at 35% aft.

Another picture of working with Tristan discussing issues surrounding sail shape: Chord, draft position and so on.

Working with the intern and discussing issues surrounding sail shape: Chord, draft position and so on. This sail looks quite full because the mast is not bent.

So for example if the chord span (“c” above) was 10 feet and 8% of 10 feet is 9.6 inches so the sail surface was (“a” above) 9.6 inches away from the Chord line, and this point is (“b” above) 3 and a-half feet aft along the span from the leading edge.

So far so good?

Luff round

Now sails are not just triangular (Yes, mainsails have roach but many/most “cruising boat” sails have very little), the point is they are curved at the luff too. This is called, no surprise, “luff curve or luff round”. How much curve and at what position on the luff and how these points is determined are all components of the sail’s design.

On this J 30 main, the area of the sail to the left of the left hand tape is called  luff round. There is a lot of it on a J 30 because they have relatively bendy masts, especially above the hounds.

On this J-30 main, the area of the sail to the left of the left hand tape is called luff round. There is a lot of it on a J-30 because they have relatively bendy masts, especially above the hounds.

Luff round and mast bend are intimately connected. If the luff curve and the mast bend “match” is correct, you have a 6-speed stick shift gearbox. If the mainsail luff curve and the mast bend are not mated, pretty closely, then you have a 1953 shift on the column three speed with no fluid in the transmission, as it were. It is really critical and the more performance oriented the boat the more critical it becomes. AND this is why having a back stay adjuster (and knowing how and when to use it) is a great thing to know, if you like sailing your boat, as opposed to being on while it is sailing.

Sketch of a mainsail set on a straight mast

Sketch of a mainsail set on a straight mast

Briefly stated, when you bend the mast, you can flatten the mainsail. A flatter sail is to be preferred in more wind. Flatter sails reduce power in the sail, that is reducing load on the tiller/wheel. When you bend the mast, in windier conditions you will feel the helm go lighter, all other things being equal.

Mast bend flattens the mainsail

Mast bend flattens the mainsail. When the mast is straight …”straight mast” the sail is fuller. When the mast is bent, as at point “a”, the luff curve gets pulled forward and so flattens the sail.

 

Mast bend on the 12 meter Australlia ion 1980

Mast bend on the 12 meter Australlia in 1980. The defender, Freedom is to weather.

The picture above shows the 12 meters Australia and Freedom coming off the starting line in one of the 1980 races for the America’s Cup. The Australians had fabricated a fiberglass top to one of their spars that would bend much more dramatically than an aluminium one. The top of the mast was controllable, apart from the regular topmast backstay, by “jumpers” and “diamonds” all controlled by hydraulics in the mainsheet trimmers cockpit. It gave the boat the abiity to have more mainsail and the rounder shape to the top of the mast was more efficient. In under about 9 kts of wind we were faster than Freedom. We won one race and were in front by 20 minutes when the time limit expired.

The large roach not withstanding, study the luff on the let. IT has a degree of "luff curve". This will match the mast bend on the boat and so be used to de-power the sail.

The large roach not withstanding, study the luff on the left. It has a degree of “luff curve” about 6 inches to my eye. This will match the mast bend on the boat and so be used to de-power the sail.

If you do not have any mechanism to bend the mast, consider adding a way to do so. For almost all boats there are a variety of pretty simple solutions.  If you cannot find one or need help, contact me….It is not impossible that the money spent on such a set up will be good value with respect to making the boat more agreeable to sail in breeze and so less uncomfortable for all hands.

 

 

 

Headsails, 150, 130 what’s it mean?

Image

One of the most quoted yet least understood phrases used in discussions about headsails, their size, is the “LP”. Customers and prospects alike use phrases like 130, 150 or 100 regularly. But when I inquire as to their understanding of the number, it is rare for the average weekend sailor to get it defined correctly. It is actually very simple. The initials L.P. stand for Luff Perpendicular, usually written as LP. This is a dimension, but not an edge, of the triangle that is a headsail. Its definition is:

“A line drawn at right angles to the luff and that passes through the clew of the sail.”

The LP is drawn from the luff or the sail and passes through the clew. In this image, the line , parallels to the luff is indicated by the pencil. THe distance between the clew and the LP is "x" The percentage of LOP is then x/J

The LP is drawn from the luff of the sail and passes through the clew.  The distance between the clew and the LP is “x” and the percentage of LP is then x/J.

The “size” of a headsail is determined by taking the LP length  and dividing this number by the boat’s “J” The J dimension is, for the purposes of this discussion, is the distance from “the forward face of the mast at the deck to the intersection of the deck and the head stay pin at the stem head”. The J dimension and the LP are inseparably connected.

The LP number, this 150, 120 percent etc., is the length of the LP dimension divided by the boats “J”.  So for example if the boat’s “J” is 10 feet and the LP dimension on a sail is 13 feet, the sail has an LP of 130%.

Two sails, for the same boat can have the same LP yet have different clew heights

Two sails, for the same boat can have the same LP yet have different clew heights

In the image above, there are two sets of dashed liners. the red dash lines are the LP length.The green l one, long with short dashed between is the trim line. That is the axis along which the sail will want to sheet.  It is worthwhile noting too that the LP and the height of the clew of the sail are connected only by the fact that both a low clewed sail and a high clewed sail can both have the same LP.

I often am asked about all the missing area at the bottom of the sail, with respect to a high-clewed sail. There is no area missing if the sail has the desired LP. It is merely that the clew is high rather than low. The area of a genoa is arrived at by the formulae: LP x Luff length x 0.5. A moment’s reflection will indicate that the length of the foot, the most common answer to my question “what is the LP?” is incorrect.

The clew height is a function of where the sail is proposed to sheet AND the LP for the sail. And these are a  function of the purpose and use of the sail.

The clew height is a function of where the sail is proposed to sheet AND the LP for the sail. And these are a function of the purpose and use of the sail.

The length of the foot of the sail is a function of the LP and the clew height.

In the move image/sketch there are two LP lines the red dashed lines. They are the same length. You can see that there are however two clew positions. The clew positions are a function of the “trim” line, basically where on the deck the sail will sheet.

So, the height of the clew is governed by several variables but is driven by firstly what sort of sail it is, for what boat, for what use, where, and then so where the sail needs to sheet. This will be discussed in another post.

Safety at Sea seminar

On Saturday 07 Feb 2015 at the Safety at Sea seminar, produced by LandfallNavigation, I will be presenting a section on sails & sail fibers. This nearshore Safety at Sea seminar is being held at the Mystic Seaport in Ct. from 0800-1630

The following is a table of the basic properties, sailmakers are interested in, of the fibers currently used in making sails. In an earlier day, this information would be distributed at the event, but today, well it is here.

The information is from the North Sails website attributed to research by their cloth people

Polyester (Dacron)
Modulus (gr/denier) Tenacity (gr/denier) UV Resist. (mo. to 50% strength loss) Flex Loss (% in std. test) Cost ($/lb.) Uses
80 – 120 5 – 8 6 mo. 0% $2 Racing & Cruising Sailing
Pros: Tough, durable, inexpensive, many weights and finishes.
Cons: Relatively stretchy compared to aramids.
PEN (Pentex)
Modulus (gr/denier) Tenacity (gr/denier) UV Resist. (mo. to 50% strength loss) Flex Loss (% in std. test) Cost ($/lb.) Uses
250 10 5 mo. 5% $7 Club Racers & Cruisers
Pros: Fits between Polyester and Aramid in performance and cost.
Cons: Cannot be woven tightly, best used in laminates.
Kevlar 29 Twaron SM
Modulus (gr/denier) Tenacity (gr/denier) UV Resist. (mo. to 50% strength loss) Flex Loss (% in std. test) Cost ($/lb.) Uses
600 23 3 mo. 25% $15 Regatta Racing Sails
Pros: Light weight, low stretch.
Cons: Low flex and UV resistance.
Kevlar 49 Twaron HM
Modulus (gr/denier) Tenacity (gr/denier) UV Resist. (mo. to 50% strength loss) Flex Loss (% in std. test) Cost ($/lb.) Uses
940 24 3 mo. 28% $18+ Grand Prix Racing Sails
Pros: Light weight, very low stretch.
Cons: Low flex and UV resistance. Expensive.
Carbon
Modulus (gr/denier) Tenacity (gr/denier) UV Resist. (mo. to 50% strength loss) Flex Loss (% in std. test) Cost ($/lb.) Uses
1200 – 2500 20 – 40 No effect 30 – 100% $15 – $100 Grand Prix Racing Sails
Pros: Very light, extremely low stretch, good UV resistance.
Cons: Brittle, low flex resistance.
PBO (Zylon)
Modulus (gr/denier) Tenacity (gr/denier) UV Resist. (mo. to 50% strength loss) Flex Loss (% in std. test) Cost ($/lb.) Uses
1600 36 1.5 mo. 30% $60 Grand Prix Racing Sails
Pros: Extremely low stretch and light weight.
Cons: Low flex and very low UV resistance. Expensive.
Spectra / Dyneema
Modulus (gr/denier) Tenacity (gr/denier) UV Resist. (mo. to 50% strength loss) Flex Loss (% in std. test) Cost ($/lb.) Uses
1100 34 7 mo. No effect $25 – $35 Premium Cruisers
Pros: Very strong and durable.
Cons: Creep limits racing applications.
LCP (Vectran)
Modulus (gr/denier) Tenacity (gr/denier) UV Resist. (mo. to 50% strength loss) Flex Loss (% in std. test) Cost ($/lb.) Uses
580 28 1.5 mo. No effect without UV $25 Premium Cruisers
Pros: Good flex when protected for UV.
Cons: Requires complete UV protections.
Modulus: Stretch resistance per weight. Higher is better for upwind sails.
Tenacity: Breaking strength per weight. Higher is better for sails.
UV Resistance: Strength loss in a standardized exposure test.
Flex Loss: Percent breaking strength lost in an industry standard 50 fold test.

Safety @ Sea Seminar Mystic Seaport Museum

Save the DATE:

Saturday 7th February 2015

At the Mystic Seaport Museum, Mystic CT.

Cooper will be presenting on sails and sail cloth during this one day seminar.

For more information, read on, call or email me, joe@joecoopersailing.com

Cheers

Coop

LANDFALL TO HOST NEAR COASTAL SAFETY@SEA SEMINAR

US Sailing Certification Offered at Mystic Seaport on Saturday, February 7th

November 6, 2014 – Stamford, CT – Landfall, the nation’s leading marine outfitter and safety expert, will be offering the US Sailing certified, Near Coastal Safety@ Sea seminar at Mystic Seaport on Saturday, February 7th from 8:30 a.m. – 4:30 p.m. The hands-on seminar provides both novice and experienced mariners with the skills and information needed to safely navigate coastal waters from Long Island Sound to Cape Cod and the Gulf of Maine.

Topics covered during the seminar include:

  • Seamanship: Ralph Naranjo
  • Preventing and Managing Hypothermia: Sarah Hudson
  • Crew Overboard and Distress Signals: Mark Bologna
  • Modern Sails for Cruising and Racing: Joe Cooper
  • Damage Control: Will Keene, President
  • Communications: Eric Knott
  • Modern Sailing Clothing: Jerry Richards
  • Life Raft Demo: Capt’s Henry Marx & Mark Bologna

The event will be moderated by renowned sailor and instructor, Captain Henry Marx, the owner and President of Landfall Navigation. Captain Marx has more than 40 years of sailing experience on both coasts of the United States, the North Sea and the Caribbean. Captain Marx will be joined by a panel of experts including Ralph Naranjo, Instructor, Annapolis School of Seamanship and Editor at SAIL magazine; Sarah Hudson, Professor of Shipboard Medicine, Maine Maritime Academy; Capt. Mark Bolonga, Lead Marine Safety Equipment Specialist, Landfall; Joe Cooper, President and owner, Joe Cooper Sailing; Will Keane, President, Edson Marine; Captain Eric Knott, Safety Manager, Moran Towing; and Jerry Richards, National Sales Manager, Gill North America.

“95 percent of boating accidents happen less than 3 miles from shore,” stated Capt. Marx. “This seminar is designed for local boaters, sailors and cruisers and conducted by sailors and experts in their fields and upon completion, all attendees will earn an Official Near Coastal Safety at Sea Certification from US Sailing.”

“This was one of the best planned and executed seminars I have had the pleasure to attend,” said one 2014 participant. “The speakers were top notch, erudite as well as interesting.”

The Near Coastal Safety@Sea seminar is being held Saturday, February 7th from 8:30 a.m. – 4:30 p.m at The River Room at Latitude 41º Restaurant at the Mystic Seaport. For more information on the event visit: http://www.landfallnavigation.com/sasmystic.html. To register, visit: http://www.mysticseaport.org/event/safety-at-sea-seminar/

Offering unparalleled experience and in-depth product knowledge, Landfall has been the leader in marine safety since 1982. For more than 30 years, Landfall has been supplying sailors, boaters and fishers with the gear they need to arrive alive. Through the Marine Training Center (www.marinetrainingcenter.com), Landfall offers a comprehensive curriculum of additional classroom courses for recreational and professional mariners on topics of boating and seamanship. For more information, visit www.marinetrainingcenter.com, or call 1-203-487-0775 x21.

About Landfall

Landfall, formerly Landfall Navigation and The Dinghy Locker, is the nation’s leading specialist in offshore, inshore and sport-boat outfitting, navigation and marine safety. The company’s retail store, website and catalog offer a broad selection of gear from leading marine suppliers. Through the Marine Training Center (www.marinetrainingcenter.com), Landfall offers a comprehensive curriculum of classroom courses for recreational and professional mariners on topics of boating and seamanship. For more information, visit Landfallnav.com, or call 1-800- 941-2219.

About Mystic Seaport

Mystic Seaport is the nation’s leading maritime museum. Founded in 1929, the Museum is home to four National Historic Landmark vessels, including the Charles W. Morgan, America’s oldest commercial ship and the last wooden whaleship in the world. The museum is located one mile south of Exit 90 off I-95 in Mystic, CT. Admission is $24 for adults and $15 for children ages 6-17. Museum members and children 5 and under are admitted free. For more information, please visit www.mysticseaport.org.

Mast rake Versus Mast bend

Mast RAKE and mast BEND are two aspects of a boat’s sail plan and rigging used in making small adjustments to the mast, thus the performance, on a sailing boat. The two adjustments are quite different. These two words and attendent adjustments are frequently confused among weekend sailors. I will try to separate the two in this post.

Mast RAKE refers to the distance aft of perpendicular (to the waterline-Horizontal) the top of the mast is. It is used to fine-tune the boats balance and is generally not changed very often on cruising boats and perhaps never.

Mast BEND refers to applying tension to the backstay so that the mast bends to some degree. The amount of bend varies with the boat. BEND can be adjusted on the boat while sailing to change the sail shape and is adjusted depending on the wind speed.

I will address mast rake first.

MAST RAKE:

Some background:

Sail boat designers need to make sure the boat will ‘balance’ when sailing. This means in practice that there will be the ‘right’ amount of pressure, or ‘helm’, on the tiller or wheel when sailing up wind in about 10-15 knots of true wind. This balance is created by the relative positions of two features of the boat’s design. The sail plan’s Center of Effort (CE) and the boat’s (the hull, keel and rudder) Center of Lateral Resistance (CLR). The center of effort of the sail plan is quite easy to determine.

In the picture below you will see three short pencil lines at the pointy end of the crayon. This is the position of the sails C of E that I drew for this exercise. If you look carefully you will see the Designers hand written note indicating HIS C of E on the sail plan, (and a note on the sail’s size in square feet) immediately under where I found the C of E.

the intersections of the pencil lines is the Center of Effort of this sail

The intersections of the pencil lines is the Center of Effort of this sail

The designer calculates the C of E by drawing three lines on the sail plan. They are drawn from the center of one edge of the triangle (the sail) to the opposite angle as seen in the image below.

Big picture of the the center of effort

Big picture of the the center of effort

Lines are struck from the mid-point of the boom to the head, from the mid-point of the mast to clew and the mid-point of the leech to the tack. Where the lines intersect is the C of E. This point is an integral part of the sail and ‘moves’ with the sail. So if for instance the fore stay is lengthened, the mast HEAD moves aft (increasing the rake) the C of E moves aft (creating more weather helm). The reverse reduces weather helm and if excessive, creates neutral or sometimes lee helm.

Mid point of boom.

Mid point of boom.

The C of E is found by striking a line from each sides of the triangle to the opposite corner. Above is the mid-point of the boom, to the head of the sail.

A line from the mid point of the mast to the clew

A line from the mid point of the mast to the clew

The picture above shows a line from the mast mid-point to the clew.

This line is from the leech mid point to the tack

And this line is from the leech mid-point to the tack

Sailmakers need to know the mast rake for a few reasons all to do with making sure the sail(s) fit the boat.

RAKE is determined in most boats, not dinghies (they use another way) by measuring the distance the masthead is AFT of vertical, that is perpendicular to the waterline. It is measured at the boom and described as “xx inches of rake”, at least among Sailmakers & Riggers. Generally it looks like this:

Mast rake measurement on a J-105 rig.

Mast rake measurement on a J-105 rig.

The ruler shown above is set perpendicular to the water line, (on the dwg,) and just touching the back of the mast at the masthead (see picture of mast head below). Sailmakers do this, on a sail plan by using the right angle triangle on the right and laying the ruler alongside the triangle so it touches the mast head while perpendicular to the LWL. You can see here the rake on the J-105 on the sail plan is considerable.

Mast rake view at the top of the rig

Mast rake view at the top of the rig

To measure the rake on the boat you basically set up a plum bob, so you will need the following. The main halyard; a weight—a one gallon jug of water, fuel or rum, or the canvas tool bag is fine and a measuring tool: a 6-foot Stanley tape is fine. Ideally this measurement is done on a windless or light air day where the boat is not bouncing around. It should take 5 minutes or less. It is dead easy to do alone but a mate is always good to have.

Attach the weight to the main halyard and hoist the halyard only a couple of feet so the weight is half way between the bottom of the boom and the cabin top. Let the halyard/weight come to rest. Where the halyard crosses the boom, measure forward to the aft face of the mast and note the distance. This is the rake:-“xx inches”. So in the picture below, the amount of rake is that distance between the forward side of the ruler and the  aft side of the mast,

VIew of the plum bob/main halyard across the boom

View of the plum bob/main halyard across the boom

I used a J-105 as an example here because they have a lot of rake so it is clear in these pictures. Most ‘cruising’ boat’s do not have this much rake.

On the other side of the coin, If there is almost no rake, less than say 2 inches on a 35-foot boat, this bears looking into. If there is a lot, more than say 12-15 inches on the same boat, again this bears research. In the former, you may relate this to the light feel in the helm. In the latter, it will be a large factor in the boat’s weather helm.

Sailmakers need to know how much rake is on a boat when designing sails for several reasons. On a practical note, three key ones are: so the boom does not hit the dodger or bimini or on some boats the mast gallows. How much rake a boat has is also an indicator as to how the boat is set up; rigged or tuned up if you like. To use the J-105 example, if I were sent a measurement sheet that showed 3 inches of rake on a J-105 that would set of alarm bells because it is way off base.

Rake information is needed for designing headsails too. The C of E of headsails is determined the same way as for a mainsail. Thus the amount of rake has an impact on where the clew is going to land. If one was to merely duplicate the three edge dimensions for one boat of a particular class onto a sistership, without checking the sistership’s rake, AND the sistership had a lot of rake, then the clew would be a lot lower and perhaps the sail would not be able to be sheeted to the installed genoa track. The reverse is true if there is a lot less rake, the genoa may want to sheet where there is no track–Seen that happen too.

These two scenarios are less of a problem on cruising boats but a detail that consumes a lot of checking-time on race boats where the track locations are very specific and the tracks are usually pretty short, to save weight of course.

Next up Mast Bend