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-   -   Genoa Cut Flat for reefing???? (http://www.sailnet.com/forums/general-discussion-sailing-related/100681-genoa-cut-flat-reefing.html)

Fudzwollop 06-22-2013 02:58 AM

Genoa Cut Flat for reefing????
 
Hi Guys. I am about to make my Genoa, I have already made the roller furler with a 20mm aluminium tube (2mm wall) as the forestay which the sail will reef around, there is a swivel at the top. I was thinking I could just make the Genoa flat. That way it will reef flat as well. I am only cruising around and often have my kids on board so I don't need high level performance but I do want to be able to reef quickly without going forward if the wind picks up. Plus it is going to be damn convenient to reef the whole thing when approaching a nice little beach etc to explore. Can anyone tell me the difference in power one would expect form having a flat cut genoa over a Genoa with draft sewn in? Any other suggestions would also be welcome. (other then use a sail maker), I am a do-it-your-selfer, I build my own boats as well. I find that there is nothing more rewarding then using stuff you made your self.

thanks in advance

Andrew Stokes

celenoglu 06-22-2013 03:04 AM

Re: Genoa Cut Flat for reefing????
 
You can insert some batten like thick flat pieces parallel to the furler. This way the extra will be furled so the sail will always keep its shape. There should be more pieces inserted to the midlle of the sail and less towards the head and bottom.

smurphny 06-22-2013 05:37 AM

Re: Genoa Cut Flat for reefing????
 
Always great to hear of another DIY sailor. My boat came with a conventional jib furler system. One of the sails is a gigantic 160% Genoa which rolls up just fine. I don't think it is an issue to worry about. Having the right shape on your headsail is very important. Good luck with your project.

Jeff_H 06-22-2013 09:10 AM

Re: Genoa Cut Flat for reefing????
 
What you are proposing makes no sense at all. It will be too flat in light air, and too big to roll effectively in heavy going.

I have been working a paper studying the trade off between sail shape and sail size. This is intended to look at using more smaller and more powerfully shaped headsails vs bigger flatter sails. A part of that research has looked at how far you can roll a sail even with a tapered foam luff before its shape greatly diminishes its effectiveness requiring an excessive reduction in sail area and drive.

Even with a foam luff, the best you can expect is to roll the sail 10-15% of its area before the sail is too full to offset its reduced area. After that the creeping of the fabric on the rolled sail, with the head sliding towards the foot and vice versa, automatically powers up the sail.

If your goal is to optimize the wind range of the sail, this is best achieved with a smaller fuller cut headsail. Used in conjunction with a backstay adjuster, and cut for a lot of headstay sag in lighter winds, you can produce jibs with a very wide range. That would be a better approach to research.
Respectfully,
Jeff

smurphny 06-22-2013 09:39 AM

Re: Genoa Cut Flat for reefing????
 
Quote:

Originally Posted by Jeff_H (Post 1047835)
What you are proposing makes no sense at all. It will be too flat in light air, and too big to roll effectively in heavy going.

I have been working a paper studying the trade off between sail shape and sail size. This is intended to look at using more smaller and more powerfully shaped headsails vs bigger flatter sails. A part of that research has looked at how far you can roll a sail even with a tapered foam luff before its shape greatly diminishes its effectiveness requiring an excessive reduction in sail area and drive.

Even with a foam luff, the best you can expect is to roll the sail 10-15% of its area before the sail is too full to offset its reduced area. After that the creeping of the fabric on the rolled sail, with the head sliding towards the foot and vice versa, automatically powers up the sail.

If your goal is to optimize the wind range of the sail, this is best achieved with a smaller fuller cut headsail. Used in conjunction with a backstay adjuster, and cut for a lot of headstay sag in lighter winds, you can produce jibs with a very wide range. That would be a better approach to research.
Respectfully,
Jeff

That's why I rarely use the large genoa. It is really only useful for light wind and tends to cause too much weather helm in anything >10 knots. The OP is probably best off with a 100% "working jib" as far as size goes, cut to a standard profile/depth. There is a great little freeware program, Sailcut CAD Sailcut CAD | Sailcut, which I have used in building a couple of sails. It prints out nice drawings with detailed m/ms for the broadseaming.

Jeff_H 06-22-2013 10:23 AM

Re: Genoa Cut Flat for reefing????
 
100% is probably way too small for an AP working jibs on most boats in most venues. This is very boat design and geometry dependent but he would probably do better with something between 110% and 125%. The trouble with standard cutting patterns is that they do not account for headstay sag which varies very widely from boat to boat and with wind speed. If you are going to make a proper sail then sag needs to be measured and factored in.especially if he is concerned with maximizing the top and bottom of his wind range.

bobmcgov 06-22-2013 11:44 AM

Re: Genoa Cut Flat for reefing????
 
Problem with a flat sail is you get the negatives of a large headsail without the positives: heeling force, but no forward thrust. The attack angle will be wrong relative to apparent wind and mainsail sheeting angle, and it wil be very difficult to get the sail to 'set'.

I'm with Jeff on this one. A smaller sail with more powerful shape is the trend these days. Add a few means of shaping and depowering it -- magic box, jib cunningham, backstay adjuster, and moveable jib cars -- and that should give you decent performance from 5-20 kts. After that, you are on your own.:D Most tubular furlers, unless highly engineered for the purpose, will struggle to reef genoas and may not survive the torque loads. They do best as all-or-nothing deployment systems. The original tube furlers on the Chrysler Bucc and Mutt were that way. Not advised to reef the jib with them.

We designed our sails relatively flat and draft-forward because we have to survive high winds on a daily basis. But they suffer in light air; the boat is tricky to drive in less than 10 kts. Goes well in forty, mind you.;)

<a href="http://www.flickr.com/photos/63880797@N06/6180476850/" title="twist1 by Wyoming offgrid, on Flickr"><img src="http://farm7.staticflickr.com/6151/6180476850_19da0dd528.jpg" width="500" height="375" alt="twist1"></a>

It might help this discussion if we knew what boat you sail & what material you intend for the sails. Those data are kinda critical for thinking about sails. A Catalina 30 & 6.5oz Dacron is different than a Snipe & ripstop nylon.

smurphny 06-22-2013 02:10 PM

Re: Genoa Cut Flat for reefing????
 
Quote:

Originally Posted by Jeff_H (Post 1047854)
100% is probably way too small for an AP working jibs on most boats in most venues. This is very boat design and geometry dependent but he would probably do better with something between 110% and 125%. The trouble with standard cutting patterns is that they do not account for headstay sag which varies very widely from boat to boat and with wind speed. If you are going to make a proper sail then sag needs to be measured and factored in.especially if he is concerned with maximizing the top and bottom of his wind range.

Is there some rule of thumb or formula for calculating headstay sag? I just finished up a storm jib and really just estimated the amount of sag that would occur to my inner forestay but I agree that it's a m/m that often is ignored. Most of the headsails I've seen have little to no luff curvature sewn in to account for deflection. Calculating the direction of that deflection and in what wind direction and speed and the resultant curvature is also a difficult thing to figure.

Jeff_H 06-22-2013 06:43 PM

Re: Genoa Cut Flat for reefing????
 
Quote:

Originally Posted by smurphny (Post 1047936)
Is there some rule of thumb or formula for calculating headstay sag? I just finished up a storm jib and really just estimated the amount of sag that would occur to my inner forestay but I agree that it's a m/m that often is ignored. Most of the headsails I've seen have little to no luff curvature sewn in to account for deflection. Calculating the direction of that deflection and in what wind direction and speed and the resultant curvature is also a difficult thing to figure.

I don't know of any way to calculate headstay sag. On my boat the sailmaker put a piece of tape perpendicular to the luff on my jib half way up. We then raised the jib with a light line at its head and while sailing stretched the line tight to the tack. Pictures were taken from the tack with various backstay tensions. We could then use the pictures on a monitor to reasonably measure the range of headstay sag. This was a parameter in his computer design software. My AP jibs are designed for a wide range of sag options and I use that range to allow the same sail to function across a very wide range of windspeeds.

Fudzwollop 06-23-2013 05:31 AM

Re: Genoa Cut Flat for reefing????
 
Thanks heaps for your feedback guys. The boat is a 6 metre Jarcat Catamaran. Home built from ply. Not allowed to give you a link apparently as I haven't posted enough on this site. BUT if you google Jarcat Marine you'll find it :)

I was going to use 4oz Dacron, we don't get out a whole lot and I thought it would be easier to sew up. Also, I thought it might stretch a bit over time anyways creating draft. The alternative is to simply add some Luff curve and some Foot curve like they did in days past. I Have a copy of "Make your Own Sails", it is a bit dated but I am sure it would work fine still - like I said, I'm cruising not racing.

I am interested to understand how important adding draft is to a head-sail, and how much performance would be lost by not adding it. From what I am reading I should add a little at the very least, is this correct? And then I could add some foam etc to the middle of the luff to take up the bagging that would occur. Clearly I have little idea what I am doing here so please tell me straight if I am missing the point, I am sure I will appreciate it when I am out sailing using my DIY Genoa.

Thanks again in advance

Andrew Stokes:)


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