# sail reef points

Discussion in 'Sailboats' started by Bod, Aug 21, 2012.

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### BodJunior Member

Hi guys,

I currently sail a 18' silouette in Cardigan Bay, Wales. i wish to add reefs to the mainsail and am using 'The sailmakers apprentice' as a guide. i am unsure where exactly to put the reefs. the book suggests a 30% reduction for the first reef (i am adding 2 lines).

here is the calculation i have used:
luff 593cm x foot 237cm = 140541/ 2 = 70270cm2 sail area
30% sail area = 21081/ foot 237 = 88.95cm

so first reef will be 88.95cm up the luff from boom. this seems a little small to me and i have been unable to find anything certain on internet so far. should i be confident with this calculation? any tips?

thanks

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### sharpii2Senior Member

I would seriously consider making the 2nd reef another 30% reduction of the original mainsail area. That leaves 40% of the original mainsail still standing to drive the boat. What you want is the minimal amount of sail area left that can drive the boat upwind reliably.

Keep in mind that the mainsail is really a triangle with a bit of curve added to the leach. So, when making the calculations, treat it as a triangle. With a triangle, the actual percentage of area reduced is is the square of the percentage of mainsail height that is removed. So, if you want 40% of the mainsail still up after your 2nd reef, you need to find the square root of 0.40 and multiply the height of your mainsail, then subtract that product from the height of your mainsail to get height of your 2nd row of reef points.

So, say your mainsail height (from boom to head) is 6.1 m. The square root of 0.4 is 0.63. 0.63 x 6.1m=3.8m. 6.1m-3.8m=2.3m. So, assuming you want 40% of your mainsail standing after the 2nd reef your 2nd row of reef points will be 2.3m above the boom for a 6.1m mainsail height. The math works the same for the first row, so the band of sail between the reef points and the one between the first row of reef points will not be the same height.

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### BodJunior Member

many thanks sharpii2 that really helped...

im only doing the 1st reef row for now and i will test it out before doing the second row. thank god for google as i have no idea how to calculate sq roots.

so my calculation was:
sq root of 70% = 0.836
x luff height 593 = 495.75
593 - 495.75 = 97.25cm from boom to reef row

so this was 10cm higher than my previous calculation so im feeling happier about this one! but is 30% reduction ok for 1st of 2 reef rows?? im gonna be cruising only and sometimes like to take on rough conditions....

4. Joined: May 2004
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### sharpii2Senior Member

Thanks for the compliment.

Another thing to consider when reefing is the Heeling Moment (HM) is often reduced more than the reduction of sail area (SA). This is because The Vertical Center of Area (VCA) of the sail is lowered, once the reef is put in. And reducing the HM is what you're really after.

This is more true on larger craft than it is on small one, as the Boom height is often a smaller fraction of the Beam.

But even on a smaller boat, the effect can be telling. I have come up with methods of calculating it.

I start with Boom height above the Water Line and multiply that by the SA that is up. Then I find the VCA of my sail area, above the Water Line and multiply that by the SA too. Then I add the two together to get the HM.

When calculating the reduced HM due to reefing, I use the same method over again, then subtract that from the original HM, to find how much HM has been reduced.

I can then divide the new HM by the old to see what proportion the HM has been reduced. I can then compare that with the proportion the SA has been reduced.

An easy method of finding the VCA of your sail is to do a scaled drawing of it on cardboard, cut the drawing out of the sheet, then balance on the edge of a table. You have to do this with reefed versions of your sail too.

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### BodJunior Member

wow you sound like you know your stuff. im impressed that i managed to understand it!

so i think it would be best to get the reef point height right first time, but what is the optimum HM reduction for an 18' boat that will be cruising? is it neccassary to be this precise?

thanks

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### sharpii2Senior Member

Precision is not as necessary as adequacy. There must be enough SA left, after the last reef, to get the boat to windward. Not as well as with full sail, but enough to keep you off the rocks. I imagine your boat is a twin keel. Am I right?

If it is, It can make do with less sail, as it has more lateral area, below the waterline, than your typical harbor yacht.

But really it is best that you contact other 'Silhouette' owners, or the designer, if you can, and get their advise.

I can only guess. That's how I came up with the 40% number. Th correct number might be 50%. Especially if this is a fractional rig and the jib is taken in first, leaving the main to stand alone.

At 50% of the main still standing, you will have approximately 1/3rd your original SA (assuming the jib has been struck). In my itty bitty boat designs, that is my cut off point. And that is just a guess, with no experience behind it.

Sorry I can't be more helpful.

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### BodJunior Member

correct it is a twin keel...

i havea masthead rig and i usually take the main down first as the main alone loves turning me to windward. but yes i agree it is worth considering how much SA to leave up.

the hurley owners club may be able to help. they state on their website that hurley masts are usually non standard heights but i shall conatct them directly and report back my findings.

no need to apologise you have given me a lot of food for thought, thanks

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### JetboySenior Member

I have a Venture 17 with a 23' hobie cat mast section and a fully battened roachy main sail from an isotope 16 catamaran.

I eyeballed my reef points. I was somewhat more limited in putting them in as I had battens to deal as far as placement. Anyway, my reef points are probably about 40% and then about 60%. Because the sail area is considerably larger than the original boat's sail, I've found that I have to put in a first reef around 10 knots if the wind is gusty at all. I've found that the first reef IMO is sometimes too much sail area reduction. There are times when I think I'd be happier with a 20% reduction.

Because, as sharpii explained, you are lowering the center of effort on the mast AND lowering the effort amount, the heeling forces are reduced considerably even by a relatively small amount of sail reduction. So it may not seem like the 30% reduction is a lot, but I think you'll find that it really makes a big difference.

I would hesitate to make a very large first reef reduction. I may end up with a 3rd reef point as a result of having too much sail area taken away with the first reef point at around 35-40%.

I also really like the ability to roller furl the main sail. Sure it's not true "reefing", I do roller reef it in heavier wind to balance the boat and it makes sailing in heavier wind much more enjoyable.

Good luck!

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### BodJunior Member

Jetboy sounds like your mast is too big for your boat, but that must be fun with the right winds!

my experience of reefing is nil but from what i have read 30% reduction seems like a good average for the first row. im finding my main is big and i have to let the sheet out so much the sail touches the spreaders to hold my course. this may be due to my small working jib, im looking to purchase a genoa asap and will fork out for a sailmaker if i cant find a second hand one soon...

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### JetboySenior Member

The mast added about 18" over the original one IIRC. Not a lot bigger. The biggest advantage is that it's a rotating wing mast so it's more efficient upwind, and the addition of the larger full roach main sail added a good amount of sail area.

I mostly sail in local lakes and reservoirs where it's pretty seldom for wind to exceed 15mph. I probably put a reef in 10% of the time I go out. It's really nice to have the extra sail area in light winds. The performance is much better. Especially due to the nature of the Venture 17 being a relatively light weight boat 700lbs with somewhat high freeboard to length ratio, going to wind in light winds was always a struggle with the original main sail and mast. I suspect the newer flatter sail compared to the original main sail from the 70's accounts for the vast majority of the improvement. I had the mast laying around and it fit the new sail better with more boom clearance for the cockpit area.

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### TynesiderJunior Member

I am not sure which model of silouette you have and if it is fitted with a roller reefing system.

Your plans for reefing the main are fine but if you have a roller reefing system you will find you can also achieve great sailing in poor weather by adjusting the amount of the head sail with the rolller reefing system.

Here up on the north sea east coast we very often reef once and then play tunes with the roller reefing system for a more comfortable trip without loosing much speed.

Mike

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### BodJunior Member

Tynesider, its a silouette mk111 but i dont have roller reefing as far as i know but thinking about it i should double check that!! actually i recall the mainsheet attacthment spins freely on the boom... would that indicate a possible roller reefing boom?

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### JetboySenior Member

One other thing I would suggest would be at least some very simple form of lazy jacks. I made a really basic set with a length of rope. I added a small hook on the front of the mast and two on the lower side of the boom.

Even though they are really simple, having a way to hold the mast up while you drop the halyard and a nice way to contain the sail as it comes down makes it a much easier process. And you can make your own for \$15.

Edit: Also, I am not a fan of boom roller reefing for the main sail. That's what my boat had originally. It was a lot harder to actually do in a stormy situation and rough water, and it gives a poor sail shape because you then basically have no outhaul so it's really hard to flatten your sail. Regular old reef points, especially with a jiffy reefing system is a lot easier IMO.

I'm not trying to convince you of anything. Just my experiences over 5 years with a similar sized boat - although certain my venture is more of an "economy" boat - ie cheap, I think some of my experiences in setting it up for the easy use could be worth while to consider.

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### BodJunior Member

agreed Jetboy iv heard many bad reports about roller reefing failing and damaging sails. would be nice to have the option tho so will check this weekend when i can finaly escape the dry midlands.
thanks i now understand what lazyjacks are! luckily i have small rope from the top of mast that attaches to the boom when dropping the sail so dont think i have the need...

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### TynesiderJunior Member

Sorry I was refering to a roller reefing system for a Genoa, I also agree with jetboy and I also am not a fan of boom roller reefing systems for the main sail.

Many of the Hurley Silouettes Mk III have a Genoa fitted, as I sail single handed I would not sail offshore without a Genoa as no way do I want to mess on the foredeck in a force 5/6.

Mike

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