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Old 17-01-2020, 08:43   #1
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reefing points on ketch sails

My first post of the year, so Happy New Year to all!

When I got the new sails made for Nausikaa, I asked the sail maker to fit two reef points on each sail, main and mizzen, which he did. With hindsight, I now realise I could use a third reef on the main. Of itself, not a difficult thing to do. But this is not to the point.

Functionally, the existing reefs don't seem to be in the right place, though I have no reason to suspect the sailmaker didn't place them in the "correct" place, according to whatever reduction formula he used.

In summary, the first reef is quite low, and I hardly use it. The second reef is fairly deep, and with both sails on the second reef, I end up with equal (if not slightly more) canvass aloft on the mizzen as on the main, which intuitively seems incorrect on a ketch.

Admittedly, I have an unconventional rig. For those unfamiliar with Freedoms, I attach pics below. Essentially, I have two "mains", with the aft (mizzen) only slightly smaller than the forward (main). Both sails have similar aerodynamic profile, and both are tractor sails. Nausikaa will not sail happily with a full main only, on any point of sail apart from a dead run, and she is impossible to steer with a full mizzen alone, so balance is crucial. Like on any boat, I guess, but more so!

In setting the reefing points, it seems to me I must not only consider the geometry of each sail individually (as I am sure the sailmaker did), but also the balance of both main and mizzen on each practicable combination of reefs.

Reasoning follows fact that in sail aerodynamics, things seldom scale in straight proportions.

I wandered if anyone in the forum thought about this, and may know a ratio or formula that I can use to calculate where the reef points should go.

Any input much appreciated!
Rafa
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Old 18-01-2020, 04:15   #2
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Re: reefing points on ketch sails

The brochure photo seems to have the reefs symmetrical in main and mizzen--looks like you could take the same amount of reefs in each to keep the ratio the same. Did your sailmaker look at the brochure? Are there sail plans available from the manufacturer? That's where I'd go first.
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Old 18-01-2020, 06:14   #3
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Re: reefing points on ketch sails

Converting to roller reefing on one boom would allow you to continuously adjust balance between the two. I know it's a bit out of place on such a beautiful classic, but think just how beautiful the sailing would be with your boat perfectly balanced.
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Old 18-01-2020, 06:27   #4
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Re: reefing points on ketch sails

I have the same cat ketch rig on a 26 foot boat, so quite familiar with the sail plan.

I set the reef points to reduce sail area by equal percentages of total sail area in main and mizzen. This works well, keeping the boat in balance.

I would definitely quiz the sail maker on the logic for the reef settings. What you describe would not be ideal on my boat. I see no need for a smaller initial reef in the main.

I do however occasionally reef the main deeper than the mizzen, resulting in what I call schooner mode. When conditions are gusty and not quite requiring full and equal reefs this is a very satisfying sail configuration. However, I want to be able to choose when to use it. As you describe having schooner mode when both sails are in the second reef is not desirable in my mind. When double reefed I want both sails in proportion and well reduced.
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Old 18-01-2020, 10:56   #5
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Re: reefing points on ketch sails

I sail a Sea Pearl 28 cat ketch. Looking at your rig, and assuming that the pictures show your new sails, I would set up the mizzen for the second reef only, and the main with both reefs. Slab reefing works well with these full-battened sails, so I would run the mizzen reefing line through both reef points, grabbing in both slabs to the boom at the same time. IOW, always reef the mizzen to the second reef, ignoring the first. Then your worries with the mizzen are done. Then as the wind increases, I would reef the main to the first reef, then the second if necessary. As you know, balance is critical with this rig, and with either setup (mizzen to the second reef, main to first or second depending on wind), you would then use sheet trim to balance the boat for your course. FWIW, my Sea Pearl will balance perfectly for hands-free sailing on any point of sail except wing-and-wing (dead downwind) if you trim her properly. She does like to carry a bit of luff in the mizzen going to weather, but will follow the lifts and headers hands-off. She's almost as good as a schooner in that respect.
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Old 18-01-2020, 11:55   #6
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Re: reefing points on ketch sails

Eyeballing the drawings given here:

https://sailboatdata.com/sailboat/freedom-28-cat-ketch

it would appear that the CE (Centre of Effort) of each sail INDIVIDUALLY lies just about the same distance fore and aft of the CLP (Centre of Lateral Plane) of the hull regardless of whether the board is up or down. No doubt that was Gary Hoyt's intention (among other “experimental” aspects of doing so) when he modified the original conventional rig.

An obvious consequence of that is that when on a beam reach or above, the boat will only track satisfactorily when the area of sail worn each mast is the same. If you reef one sail but not the other the boat will go out of balance. Reef the “mizzen”, and you'll need to apply considerable LEE helm to make her hold her course. Reef the “main” and she'll require all kinds of WEATHER helm to hold her course. As you say: “she is impossible to steer with a full mizzen alone” That that must be so is so patently obvious that one suspects that Mr. Hoyt had his mind elsewhere the morning he designed this rig :-)!

However, reefing BOTH sails simultaneously, when it comes time to reef, will circumvent this particular problem, and reefing BOTH sails simultaneously, and in the correct amount, will keep 'er nicely balanced. So that's that out out of the way :-)

Now: What exactly is the right amount?

That is a little more difficult to determine, but to get a start: Assume that you never wish to heel more than 15 under any wind conditions. This is reasonable, because the vast majority of sailboats (due to fundamental physics and hydrodynamics having to do with hull form) sail best when heeled between 10 and 15. Let's now simplify this argument beyond reason, just for illustrative purposes:

Assume that under full sail a 12 knot wind will make you heel 15. You now go to the first reef on BOTH sails and the boat reduces heel to, say, 8. The wind rises to, say, 18 knots and the boat is again heeling 15. You again reduce sail by going to the second reef in BOTH sails, and the boat comes back to 8 heel.

The question then is this: How much area would you have to take out of each sail at each of these “steps” in order to balance the heeling moment generated by the remaining sail area against the righting moment generated by the combination of form stability and ballast stability?

Your sailmaker should be able to make that calculation for you, and therefore he should be able to position the reef points where required to obtain this objective of reducing heel to 8 when you take one more reef whenever the boat wishes to heel beyond 15.

As I said: This is an explanation simplified beyond reason, but it will set you thinking about what you have to say to your sailmaker :-)

All the best

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Old 18-01-2020, 21:51   #7
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Re: reefing points on ketch sails

While TrentePieds is correct in theory, in actual practice I can reef either my main or mizzen and still balance the Sea Pearl 28. The mizzen is the first to get shortened because the main is the more important driving sail. What I find interesting is that the position of the centerboard is critical to good balance because you can move the CLR to compensate for differing sail areas, reefing situations, and wind speeds. I can also adjust the kickup rudder angle to affect trim. Any split rig boat will talk to you, some more than others. You just have to be sensitive to what the boat wants. You have to balance sail area, centerboard position, sail trim, sheet trim, heel, course, sail shape, etc. I sail my cat ketch mostly without using the tiller at all, and have reached the point where the tiller is only needed to tack and gybe. It is true that trimming for certain conditions results in one sail or another not being optimum trim because its needed to balance the other in order self-steer. But i'n not racing, and I accept that small limitation so I can relax with an appropriate liquid refreshment, a good book, and a sandwich while the boat sails itself for hours.
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Old 19-01-2020, 08:58   #8
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Re: reefing points on ketch sails

Rhubstuff's post (#7) is a most welcome contribution to this discussion, proceeding from, and adding practical experience to, my utterly basic and entirely theoretical explanation, the only aim of which was to equip Seadago to have a more fruitful discussion with his sailmaker than the one he, apparently, had :-)

Seadago asked for "a ratio or formula that I can use to calculate where the reef points should go."

None of us have given him such a formula, and none of us can, because such a formula does not exist (as far as I know) in a cut and dried form that can be directly applied to all vessels. What DOES exist, however, is the procedure for determining where the reef points should go, and it is not a particularly onerous one. My hope was to set Seadago thinking along lines that will equip him to evaluate his sailmaker's advice before he omits to a not insignificant expenditure.

If Seadago wishes to pursue the question of where to put his reef points, I'm sure we'll all be glad help him through the necessary calculations :-)

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Old 22-01-2020, 06:40   #9
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Re: reefing points on ketch sails

Quote:
Originally Posted by Benz View Post
The brochure photo seems to have the reefs symmetrical in main and mizzen--looks like you could take the same amount of reefs in each to keep the ratio the same. Did your sailmaker look at the brochure? Are there sail plans available from the manufacturer? That's where I'd go first.
Hi Benz.

I had just posted, and looked at the brochure pic again, and realised the same thing! O could have the answer right there!!

My sails are slightly different though. For starters, I have wishbone booms (top pic), so the foot is not straight and parallel to the deck. Also, the wishbone boom has a choke fairly high up on the mast, which restricts how low I can set the reef without lowering the boom itself.

Alas! Freedom cat ketches are no longer built for many years, and my sails' geometry is different from the original production sails.

But you are quite correct. Allowing for perspective in the pic, it looks as if both sails have reefs at the same distance from the foot.

Maybe this is linearly scalable after all...

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Old 22-01-2020, 06:50   #10
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Re: reefing points on ketch sails

Quote:
Originally Posted by Sparx View Post
I have the same cat ketch rig on a 26 foot boat, so quite familiar with the sail plan.

I set the reef points to reduce sail area by equal percentages of total sail area in main and mizzen. This works well, keeping the boat in balance.

I would definitely quiz the sail maker on the logic for the reef settings. What you describe would not be ideal on my boat. I see no need for a smaller initial reef in the main.

I do however occasionally reef the main deeper than the mizzen, resulting in what I call schooner mode. When conditions are gusty and not quite requiring full and equal reefs this is a very satisfying sail configuration. However, I want to be able to choose when to use it. As you describe having schooner mode when both sails are in the second reef is not desirable in my mind. When double reefed I want both sails in proportion and well reduced.
Hi Brent. Thanks! that's my experience overall as well.

So... let me see if I got the maths right...

T (total sail area) = M (Main sail area) + m (Mizzen sail area)

reef 1 on either = X% of T on either sail; but the absolute value of X is the same on both sails. Correct?

Thanks again
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Old 22-01-2020, 08:20   #11
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Re: reefing points on ketch sails

Many thanks TrentePieds and Rhubstuff for interesting and informative posts.

I think it's 6 of one, and half-dozen of the other!

My search for "formulae" to do this goes beyond the academic domain.

My current sails look nothing like the triangular wrap-around sails in the original F30 CK brochure. Mi sails are single-ply, very roachy, and have full length battens.

When I decided to get new sails, I consulted Eric Sponberg on the right sail geometry for a small boat that is -by the size of the masts alone- underpowered by modern standards. Eric has done some brilliant work designing sails for large cat ketches and free-standing mast boats, and was kind enough to send me, free of charge, the sail specs for a F 35 ck. I then proceeded to shamelessly take Eric's design, scale it down -linearly- to my size (F 30), and then get the sails made to those specs. I should have been even more shameless, and asked Eric where to put the reefing points. Sadly, I didn't!

My general (current) reefing schedule is as follows with increasing wind speed:

1) Main full + Mizzen 1rst reef
2) Main 1rst reef + Mizzen 2nd reef
3) Main 1rst reef + Mizzen down
4) Main 2nd reef + Mizzen down
5) ... I should be somewhere else....

I don't have indicative wind speeds for the steps 1-5 above. The boat's behaviour varies considerably depending on the point of sail, with the same sail config, so it's a judgement of the moment all things considered (point of sail, wind speed, sea state, wave speed and direction, etc, etc) as to what configuration to have.

Problems with the above schedule:

The steps are not evenly spaced in terms of magnitude of overall change in conditions: (0) to (1), and (1) to (2) are short, then most of the sailing is done on (2) through a range of wind speeds and conditions. (3) I only use on a beam or shallow broad reach to the point where I may be close to or breaching the boat, going quickly to (4) thereafter, which is quite bad conditions already! IOW, this "schedule" allows me little practical flexibility.

Intuitively, a reefing step where both main and mizzen are on their second reef should be in the schedule between (2) and (3), and it isn't for the reasons I described earlier.

TrentePieds observation on the shift in the CE is bang on the money. There is no overlapping of the sails, so any change in config has a big relative effect, irrespective of the centreboard being up or down.

In asking for a "formula", if it exists, I wanted to include the CE as a factor in the overall equation. I would like to get to a point wrt balancing the sails similar to what Rhubstuff describes, if nothing else for comfort and safety, but also because my wind vane will not operate correctly -or at all- unless I
do!

Thanks again!
Rafa
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Old 23-01-2020, 01:25   #12
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Re: reefing points on ketch sails

I can't speak at all about the freedom or the wishbone rig as I've never sailed either but on my conventional ketch functionally my first reef is just striking the mizzen. then a first reef into the main, next thing to go is striking the staysail and after that the second reef goes into the main.
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