Sail Measurement

[Hartley18]

Well.. I did a silly thing last week. I put my hand up to scratch my back during a recent AGM and am now a "Class Measurer"! Yes, it's a horrible thought, I know!!..

What I'd like to know is what are the essential measurements to make on a mainsail, headsail and spinnaker to be able to compare two sails to determine whether or not one has much of a performance advantage over the other.

A few obvious ones are:
- Luff, Leech and Foot dimensions with the sail out flat (or should it be hoisted with a weight attached??)
- Sail area measurement, including roach (is there any easy formula for this?)

Has anyone out else out there tried to measure sails??

Thanks!

[blt2ski]

Here is a local rating groups, PHRF-NW protocals for joining etc, and what and how they measure.

not sure if this helps, but one way to measure etc.

Marty

[bubb2]

sail measurements it's a Zen thing that's all I know about the subject

[Hartley18]

Originally Posted by blt2ski

Here is a local rating groups, PHRF-NW protocals for joining etc, and what and how they measure.

not sure if this helps, but one way to measure etc.

Marty

Actually.. that helps a lot!! Thanks, Marty

They seem to be basing their sail measurments completely on Area - not dimensions as such. Is that right? Any idea how to calculate the "Area Factor" from the measurements??

[Jeff_H]

The methods for measuring sails was standardized during IOR days and has remained reasonably constant since. The sail is measured lying flat on the ground. The class or racing rules should determine maximum luff, leech and foot dimensions with most classes and rules also measuring girths at various points along the sail. Sailmakers are often called upon to measure sails and record the data on the sail so you might approach one of the name brand lofts and talk to them to see if they can show you how they measure a sail.

Jeff

[blt2ski]

Cam,

As Jeff mentioned, measuring a sail is not that hard. My local is using the girth measurements to make sure one is not trying to get a lot of SA thru large roach's, especially on the main sail.

I am sure there is a way to figure the area including the curved roach area's, but that is not something I have had to do. But as Jeff mentions, talk to a local loft, I am sure they can help you better figure and understand it better than Jeff or I via sailnet posts.

Marty

[Hartley18]

Thanks guys - I'll do that.

I've spent the past few nights trying to work out the formulas. It's been an interesting challenge for someone not much good at maths. I suppose I'd always taken it for granted, but measuring sail area seems to be a bit of a "black art" and a lot of guesstimation and different ways of doing the calcs produce different results.

The trick seems to be to minimise the number of measurements whilst keeping a handle on the roach - particularly in smaller classes like the Hartleys where excess roach can give you a decent advantage.

The current method in the Club is to measure the roach - separately from the main triangle - in three places (still not sure how that's done, but I'll find out soon enough!), but the PHRF method of meausuring the upper and mid-girth seems way easier and simpler even though the resulting sail area may not be as "accurate".

JeffH: It's interesting that you say "The sail is measured lying flat on the ground." - our current club measurement requires the sail to be hoisted and a 4.5kg weight hung off the bottom. Presumably to include stretch??

A final question for the designers out there: How important to performance comparison is the "sail area" anyway? Wouldn't a simple comparison of luff, foot and, say 2 girth measurements be sufficient??

[Jeff_H]

Here's the deal, for most measurement rules, sail area is measured as a simple triangle, typically the length of the luff x the length from the mast to the clew measurement band (depending on the rule sometimes measured along the boom, sometimes measured perpendicular to the luff) divided by two.

With regards to your question about whether the area contained in extra girth does really help?. Yes and typically not much.... upwind, roach gives very little advantage, but reaching and running the extra sail area can increase speed in light to moderate winds. If not measured and taken to extremes it could provide an unfair, unmeasured advantage.

To keep boats from taking unfair advantage of adding roach, under almost all rules there is a standard amount of roach that is permitted. This is usually defined as maximum permitted girths (measured perpendicular to the luff at specified points) and which are typically specified as percentages of the foot dimension. If the measured girths exceed those permitted percentages, then the boat gets a penalty but in some rating rules and classes, and in most one-design classes the sail cannot be used.

In many rules, these girths are found quite simply by folding the sail from head to foot, so that the sail is folded with the head laid over the tack. This produces a fold and the length of that fold is the Midpoint girth effectively the girth at the middle of the luff (MP/2).

With the midpoint girth held steady, the sail is then folded again so that the head now touches the mid-point fold and the new fold is measured at the new fold producing the quarter point girth (MP/4). Lastly it folded again so that the head now touches the quarter point girth and the new fold formed is measured producing the eighth length girth (MP/8). This protocol is illustrated in the Northwest Region PHRF site. NEW PHRF-NW PROTOCOL MEASUREMENT DATA

Some classes literally restrict the sail area and in those cases, I don't know if you have seen this: http://www.sailing.org/tools/documents/GuideSailArea-[941].pdf but it is from the International Sailing Federation and it descibes another set of methods for sail measuring, which may not be consistent with specific measurement rules but certainly would produce a very acurate sail area.


It should be noted that each one design class establishes its own measrement method and so if you are measuring boats of a one design class intending to race one design, you need to find the rules for that class.

In any case, there is no black magic or black art to it. Measuring sails is very straight forward, pretty quick, and quite simple.

Jeff

[rabbit238]

The Shields Class measures new sails each year and again before a regatta. Some of the procedures and can be found on the web site at shieldsclass.com and go to class rules and then Section V - Sails. As a current measurer I can discuss details further if needed. The first thing you need is the specifications.

[Hartley18]

Thanks guys - really helpful info. I appreciate it!

The Hartley TS18/21 Club is essentially a cruising club with a few people now interesting in racing, so they've decided to dust off the old Rules and have a look. Needless to say, a few people got a shock.

Most of it was okay (being loosely based on the TS16 rules which are a pretty tight one-design racing class over here), but the sail measurements were a bit of a mess and way more complicated than need be, but I think we can sort it out now..