Mainsail Reefing Techniques; Reefing by the numbers....

[Auspicious]

I strongly agree with Ol' Coot -- I just shove mine into the flakes of the main as I pull it down and back, tying down with gaskets as I go.

[CapnK]

OK, that's what I'd been doing. :) Thanks, all! Grog fer ya!

[rtbates]

What do you recommend for the reefing tie lines (nettles?)--the lines that gather up the bunt of the sail when the sail is reefed.

I don't have a loose footed sail, so the lines have to go under the boom. The sail could get torn if something goes wrong.

I've been using short pieces of line. Usually only need them when the sail is double reefed. The line has to pass through the small grommetts--maybe 1/4" wide



I've been thinking of using bungee cord, but its hard to get a knot to hold in bungee cord.

Please ignore the lack of wind in this photo

   

NOTHING. If you lose a reef line all the stress will be on these points and most likely tear your sail. If you feel you need these lines I'd suggest you go to a loose footed main. You'll get better sail shape anyway.  If not at least use a very small line that will hopefully break before the sail tears.

[Fortis]

On board Jester (A friends boat we are refitting) we have gone with light velcro straps. They hold beautifully, but if the sail were to let go, the straps wooudl release long before a danger of tearing the sail....and the straps would even be re-usable afterwards.

We sewed a little length of VB cord onto the end of the thin vlcro strap to make it really easy to thread through the eyelet of the sail and then pull through the strap proper.

I used to use bungee cords on my boat, and you can get a little plastic crimp-on clip that goes through the eyelet easily and then clips through and pulls tight into the mated end that is attached to the other end of the bungee cord. Our local chandlers stocked the tips.

Alex.

[Norm]

Hello all:
I have been pouring over two books:
The Science of yachts wind and water by HF Kay  Page 33, in particular.
Elements of Yacht Design by Norman Skene (original edition reprinted) Page 92 wind force graph.

Using AVERISERA as a model, I found the numbers to closely match our experience.  Validation.

Wind force is defined as 0.004 x wind speed squared.  We find results in pounds per square foot of:
10 kts = 0.75
15 kts = 1.25
20 kts = 2
25 kts = 4

Our full main and 100% jib have sail area of 468 sq ft
One reef (roughly 75% of full) and 100% jib have sail area of 351 sq ft
Two reef (roughly half full main) and 75% jib have sail area of 301 sq ft

I did all the arithmetic and saw that a full main and jib in 10 knots pretty much matches a small jib and double reef in 15 knots.  This also matches our gut reaction.

Other interesting aspects are to look at correlations to stiffness (static righting moment curves) and boat speed polars.  HF Kay has some terrific tables that provide guidance for further understanding the "numbers" associated with making the boat go.

Maybe cubemonkey will get free and make a table and graph that can be put in here?

What got this started is that my students keep asking for some metrics for reefing.  I come up with the usual reply:  "when you think it is time to reef, reef."  And, "if the rail is in the water, reef."

Not to mention, this weekend's Boston Harbor Islands Regatta looks to feature a nice 15 to 20 knots from the North East and driving rain.  We're thinking about heavy air.

So... I am going to work this up as an explanation.

Comments?  Critiques?

Best, Norman

[okawbow]

Your numbers make a lot of sense.

We also reef by the numbers. Number of degrees heeled, number of whitecaps, number of screams, etc...

[Norm]

yeah
we do pretty much the same thing.  the numbers are interesting as we can compare sail area with different combinations.

going to make a graph and have more detail in the weeks ahead.
norm

[Shawn T W.]

This is interesting . . . standing by to see the chart . . . Always learnin'

Shawn

[Bill NH]

Hi Norm,

Your analysis certainly points out why you need to reef in higher winds.  From an engineering/physics point of view you need to be careful though, as it's so simplified that it will not generate actual representative wind force values.  It can be used for COMPARING values, however, as long as you stay on the same point of sail and trim for the same shape... 

The reason the force predictions themselves aren't valid is that, as you certainly understand, the force on a sail does not just come from wind blowing against it, but also over it.  (It's both lift and drag)  Furthermore, the force generated is not just a function of the area of the sail but incorporates the angle off attack of the wind to the sail, sail shape, sail trim, twist, draft, etc.

The 0.004 x wind speed squared model is a drag model and only somewhat works to predict force if the sail is sheeted 90 degrees to the wind (all drag, no lift), which in reality is only happening dead downwind....  It's more useful for calculating anchor loads, etc.

However, the model does proportionately illustrate the relationship between changing wind speeds and sail area required.  You're on far safer ground stating that "when the wind speed doubles, the force multiplies by 4 and so your sail area needs to decrease proportionately if all else remains the same" than to state actual force values.  As a teacher I'd stick to a general explanation of more wind = less sail area and try to develop the instinct of when to reef, rather than trying to provide hard numbers that students will want to use as a hard & fast rule...

[...  At least for my boat, 15 kts seems a bit light to me for a double reefed main and a 75% jib to be equivalent to full working sail at 10 kts...    ]

[Norm]

Yes, Bill, it is a simple model.  The texts get into the complex model in some detail.  The thing we found is that our experience matches the simple model.  I am working on a way of understanding the complex model in order to explain it to new sailors.  I hope my study helps us manage our boat with more understanding and skill.

Who would have though study would be so much fun?  I am dragging up a lot of the physics and trig from my youth.  Gee that was a long time ago!

It is particularly interesting that the sail-change points occur at roughly the same force-levels.  We can sail with a full main and 100% jib (our largest) up to about 15 knots.  (Much less comfy than at 10 knots but still practical when racing.)  After that we have the choice of a reef and a small, 77%, jib or two reefs in the main and the 100%.  Pretty much the same force-level.  The boat does sail differently.  The small jib and larger main allow us to go to windward very quickly.  The other combination is preferred for reaching.  We observe that speed increases with the right sail plan.  A well known sailing fact, eh?

Averisera is a very narrow boat, the waterline is 25 feet and beam-waterline is 7 feet.  While we have a static righting moment curve showing positive stability to 130 degrees, the curve is shallow showing Averisera to be tender... empirically proven.  Hence, reefing early is necessary for us.  Reefing at the right place and with the right combination critical.

I also sail a very fat boat, 1987 C&C 38, heeling buries the beam and induces huge amounts of drag.  We see on the new wide-all-the-way-aft models that the opposite occurs.

The simple model is useful for getting some relative values for managing the sail plan on a single boat but not useful for comparing boats.   It will be interesting to see if anyone gets to do some empirical testing to compare numbers with experience.

This weekend's Boston Harbor Islands Regatta will be windy.  I expect to have a chance to look at things in a competitive environment.  Our choices will show up quickly as being right or wrong.

Study on...

Best regards, Norman

[TJim]

OK, I need help ..this is the way I did it and the numbers I got:

10 Kt.......004 X 10 X 10 =   .4
15 kt.......004 X 15 X 15 =   .9
20 KT......004 X 20 X 20 =   1.6
25 KT......004 X 25 X 25 =   2.5
30 KT......004 X 30 X 30 =   3.6

So, where did I screw up???  Also, the reefing by the numbers I use are not wind speed but percentage
of heel.  I usually reef at 30 degrees as that is where I usually start loosing speed and I must reef to
maintain speed.  The wind speed will vary with direction and relative direction of the wind.  Straighten me out on that formula.  I like the formula, I'd just like to come out with the right numbers. Tnx JZ

[AdriftAtSea]

I think Norm's OP is missing some numbers... cause according to Excel... I get the same numbers as you did.

Constant   0.004


Wind speed   Force
5   0.1
10   0.4
15   0.9
20   1.6
25   2.5
30   3.6
35   4.9
40   6.4
45   8.1

[TJim]

Kinda gives you a feel for why these hurricanes do so much damage !!!!! TJ

[liberty28]

We also reef by the numbers. Number of degrees heeled, number of whitecaps, number of screams, etc...

I totally understand that an activity such as when to reef a sail can be mathematically derived and that simple fact is probably one of the many reasons that sailing has such appeal because of it's many aspects from simple to complex. My personal preference for most things in life is the simpler the better. And IMHO there is no better guide to life's many aspects than pure unadulterated fear. Therefore, I too endorse the fear factor reefing system. Of course, I also would like to believe that there still be dragons out there, mate! ;)

[Bill NH]

Kinda gives you a feel for why these hurricanes do so much damage !!!!! TJ

Especially when you continue the table and see how rapidly the force grows as wind speed gets into hurricane strength...

Wind speed   Force

10      0.4
20      1.6
30      3.6
40      6.4
50    10.0
60    14.4
75    22.5
100  40.0
125  62.5
150  90.0

[sailorflo]

As a general rule I try to reef before the wind gets over the 20kt range, It is hard for me to shorten my canvas in stronger winds, the roller furling is nice but the main never wants to act right. so I opt to shorten sail early and save the rigging from being worked to hard,  I have been known to put the rail in the water from time to time, But that is not always a good thing as you lose sail area as you start to heel over. I would always recommened to always reef early so that things don't get out of hand.  Any one who has got them selfs in a stiff wind Knows that things seem to go wrong at the worst times.   Our boat is 37 ft so it takes a good blow to get her to go 7 kts, And if we get it up to 8.5 we are going to fast. We live on our boat and do not wish to push her to hard .

[Auspicious]

I tend to reef based on weather helm. If the rudder stays above 10 degrees to hold course I reef the main.

With my 100% jib, I get all the way to the third reef before touching the foresail. I'm still getting a handle on the new 135.

sail fast, dave

[Norm]

In the interest of throwing more fuel on the fire...

I figured out what the different sail area combinations were and applied wind force factors to the areas for a total measure of windforce.  Granted, this is pretty simple stuff.  But it is a starting point for study.

Rounding, we get:
full main and 100% jib = 470 sq ft
full main and 77% jib = 420 sq ft
one reef and 100% jib = 400 sq ft
one reef and 77% jib or two reefs and 100% jib = 350 sq ft
two reefs and 77% jib = 300 sq ft
two reefs and no headsails = 150 sq ft

In 15 knots, the wind force on a full main and 100% jib is about 600.  This is similar to a reef and the 77% (or two reefs and the 100%) force of 700 at 20 knots of wind.  No headsails and the second reef in 25 knots or more yields a wind force of 600.  We know from experience that these are comfortable levels.

This is a partial list of values.  If I can figure out how to put a table in here, I will.

Interesting to see the relationships in numbers.  Other things I want to study are the changes in drag with angles of heel and decline in windforce with angles of heel. 

For us, sailing is about having one's "head outside the boat."  It is not about sitting with a calculator sailing by numbers.  What's the fun in that?  The numbers help us make choices about sail combinations.

Note:  the Boston Harbor Islands Regatta went off as planned in the dense fog and sometimes driving rain.  Averisera pulled a third in Class B and a fleet position of 10th/29 finishers.  Of the 38 starters, nine did not finish.  Elizabeth/cubemonkey navigated and managed the tactics.  I worked the bow... jibs and kites. 

Some semi-amusing photos are on our blog, averisera dot com.

[maxiSwede]

I tend to reef based on weather helm. If the rudder stays above 10 degrees to hold course I reef the main.

With my 100% jib, I get all the way to the third reef before touching the foresail. I'm still getting a handle on the new 135.

sail fast, dave

Interesting thread with a scientific 'touch to it. For all of those who are not inclined to calculate when int's time reef, and has not yet gotten the experience needed to 'just know when...' I'll definetely second Auspicious comment.

It is not more complicated than that (on a cruisng keelboat) nd still one tends to wait too long with reefing way to often

[Norm]

"When you think it is time to reef, it is time to reef."
"Reef early and often."
Sage advice given years ago by Captain Andy Kurtis

One of the things that prompted he study is a recent beat home.  As we entered the outer harbor the wind picked up.  Sailing went form benign to frisky (?) pretty quickly.  E and I waited to reef, thinking things would settle down in a bit.  Caught reefing late and in a crowded channel, we discussed our late decision.  Everything worked out fine, no accidents or such and we certainly learned from the experience!

This may be an aside... or another thread... but I find that many boats are too hard to reef.  Of all the things to have set up to work well, reefing has to be pretty high on the list.  I find that is often not the case.

more to follow.