Getting ready to order a new set of sails for my 5.2 and I like the idea of going to the square top sail. This adds about 11 sq. ft. to the main moving the center of effort aft somewhat. Since I'm also planning to change the standing rigging now would be the time to tweek the dimensions so that moving the mast forward to get the center of effort back to where it belongs would be easy to do.
Has anyone sorted this out, or is there enough adjustment in the standard rig dimensions to accommodate this change?
dg
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dg
NACRA 5.2 #400
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Going Square Top
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Posted: Aug 21, 2016 - 09:39 PM
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Posted: Aug 21, 2016 - 10:43 PM
That's interesting. I found that after I got my square top I wanted more rake to balance the boat and keep the bows from burying. I just bought murrays stock rigging and with a shortened forestay to accommodate a roller furler. I also run a second adjuster on my forestay to get the rake I want (about halfway between the beams). I also replaced the stock straight spreaders with ones that could be raked back so I could bend my mast a bit backwards with the downhaul and depower a bit. You can't get as much bend as with a bendy carbon mast but it works.
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Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
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Posted: Aug 21, 2016 - 11:02 PM
Draw yourself a simple triangle, then do the math.
You can only use Pythagoras, A sqrd + B sqrd = C sqrd), with the mast vertical. Law of sines, & co-sines will give you the numbers with mast less than vertical, (ie not a "right" trianlge).
If you measure the chord of your sail, over several spots, you can average what we term in transport category aircraft as the Mean Aerodynamic Chord, (MAC).
I think you will find that the center of effort over the MAC with the new sail is very small. It will take very little mast rake adjustment.
Order the new forestay 1" shorter than factory.
Moving the forestay pin a hole or two should be plenty. I think with 10 hole adjusters, you will have adequate adjustment.
Time on the water is your friend. Take the boat out & sail, then move the mast 1 hole & see if you like it better.
Take some notes, match wind, then rake 1 hole. A GPS & a bingo chart & grease pencil is better than memory.
In the end, simply sailing, in light to big wind will show you where you want the mast rake.
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Posted: Aug 22, 2016 - 01:51 AM
Dave, I think the reason you would be burying your bows more now is the addition of 11 square feet of sail in the upper portion of the main. So not only did the center of effort move back, but it is moving up and that is something I hadn't really thought about before.
The reason I would want to move the center of effort back forward (when using a square top) is that if you didn't I would expect an increase in weather helm and if you don't correct that you are essentially slowing the boat down by fighting the weather helm. I have always understood that "fast" is when you have only a very slight amount of weather helm, but I will defer to the racers on this issue.
I also think there is a good reason to bring the pin head sail along for those windy days.
dg
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dg
NACRA 5.2 #400
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Posted: Aug 22, 2016 - 05:08 PM
I have a question regarding this train of thought "moving the center of effort aft" . Where did this information come from? Is this what sail makers are saying about adding a square top sail?
I will explain the reason form my question and then the experts can set me straight. Sail design has been evolving towards higher aspect designs which is a faster sail. What this high aspect means is that the sail is taller, but also narrower front to back. It is said that the aft part of the sail (back half) of the main is a good deal of drag as well as a lot of surface that contributes to pressure to turn the boat over (simply blowing it sideways). The high aspect design result in more forward speed by eliminating a lot of the aft of the main sail sail area as well as adding sail area to the forward part of sail resulting in more force on the boat in the forward direction rather than just pushing it sideways, the go fast equation (the lifting wing part). I would think this would move the center of effort forward rather than back if properly designed. No?
In regard to a square top sail versus the pin head, the assumption I had, was you are taking sail away from the back half of the sail and moving sail area up to the top to get a more uniform wing from top to bottom with the result being a higher aspect shape than what you get from a pin head main. This result is more pressure forward rather than sideway for a faster sail.
The other benefit of the higher aspect sail is that twisting off the square top is very easy thereby reducing sail area very fast in puffy conditions. This makes controlling the sail much easier than conventional pin heads.
In summary my impression of a square top was not just piling on sail at the top of the mast. Am I just making this stuff up?
Thanks in advance
Edited by bruiser on Aug 22, 2016 - 06:11 PM. -
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Posted: Aug 22, 2016 - 06:34 PM
I think ideally the daggerboards represent where the center of effort is applied at the water when going to weather. With a jib and main configuration the combined lifting effort of the two sails should work out to be at about the vertical center line of the daggerboards. If the center of effort is in front of the boards the boat will want to fall off the wind and when behind the boards the boat wants to round up. Adding 11 square feet of sail to the main changes this balancing act. In the case of my 5.2 neither the height of the mast or the clew position is changing, but there will be a little more sail working higher up, so the aspect ratio is only changing very slightly.
I asked Andrew at SLO this same question and here is his response:
There are a number of mitigating factors with the square top. As the breeze builds or in puffs, the top of the sail will twist off essentially depowering a little by reducing the effective sail area. This makes the sail appear closer to a pinhead with a similar center of effort. Additionally, the added area of the square top doesn't move the center of effort considerably (there is still plenty of area down low to keep the center from moving too much) though it will be a little further aft than the standard pinhead. In lighter air, you'll likely want to keep the mast a little further forward. We have many customers racing with these square tops and we haven't received any feedback regarding negative weather helm. I don't know if they have adjusted their rigs or if they are just using the sails with their stock setups. You should have enough adjustment with the rig to try the new sail with the stock tune and then adjust from there. I know this is not an exact answer but I hope it leads you in the right directions.
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So for now I will stay with the current lengths for the standing rigging. If I ever finish this project and go sailing I will report on how the helm works out. When I last sailed the boat several years ago I know I was happy with the balance of the helm when going to weather.
dg
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dg
NACRA 5.2 #400
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Posted: Aug 22, 2016 - 07:59 PM
My theory on the square top goes the same with adding a spinnaker to a non-spinnaker boat and that is don't do it unless the boat was set up that way from the start... there may be a problem setting the pre-bend as you spreader bars and set... Again my opinion...
I have not been happy with making the change on both a NACRA 5.8 nor a Prindle 18 which stupid me just did as part of a group purchase where the price was to ridiculous to pass up..
When I did it on the 5.8 years ago there were a lot of adjustment I had to play with, one being the mast pre-bend which got in the ball park on the other the mast rake... there was no issue with the the helm but more so a dragging feel on the boat with a close haul and a beam reach... with the narro beam these boat could point pretty high.... on a beam reach it was OK, but on a broad reach it screamed.. After countless adjustments and recommendation from EP sails I gave up on it... One day I said enough and bought a left over stock pin head sail from the factory set everything back to the known variables and was happy again... the boat preformed perfect on all points of sail... plus there were no DP-N modification factors to deal with..
On the Prindle 18 the jury is still out as I have not had them in heavy wind as of yet but the few times I have had them out there are some pluses and minuses... so far mostly minuses. It does accelerate a little faster and points a hair better, but that is it.. I feel I have lost some energy on the beam and broad reaches. I am planning to have it out this weekend as I'll have a good crew and we're suppose to have some heavy wind.. this will determine if these sails are up for the task or if I wasted money..
I know sounds odd and can open up a whole new discussion..
Edited by JohnES on Aug 22, 2016 - 08:09 PM.
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John Schwartz
Ventura, CA
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Posted: Aug 22, 2016 - 09:00 PM
That is how the Squaretop main I have from EP is done, the total size (SqFt) of the main is virtually the same as stock (less than 5% difference) even though it has a pretty large 53 inch top.
Most stock beachcat sails (on the older design boats) have a large roach (curve on the back) of the main so there is plenty of material that can be relocated to the top.
If you entering races using the Portsmouth ratings the penalty for a Non-Standard (within 5% area) main is only .995 but an oversize main is much more (can't locate the ratings on http://www.ussailing.org/ right now)
So if you have one custom made be sure and specify to the sailmaker to keep the sq.ft. close.
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Damon Linkous
1992 Hobie 18
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Posted: Aug 23, 2016 - 12:58 AM
The reason you cant find the ratings(I emailed them) they voted recently to make the portsmouth ratings only available to US sailing members including the penalty information. The second I get a hard copy I'll post it..
In the meantime you can find the current ratings without being a member by going to regatta network and try registering for an event on the calendar. On most of them, not all, there is a "lookup code" down at the bottom if they are asking for your boats rating. You can find all the ratings there searching by make. -
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Posted: Aug 23, 2016 - 04:29 AM
Wow, just wow.
Sailing must be getting too popular, they have to keep it down somehow.
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Damon Linkous
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Posted: Aug 23, 2016 - 09:48 AM
All the more reason to move away from the DP-N and go to a system like SCHRS that is dedicated to catamarans...
Also, I have the ratings and modification factors that I downloaded in June 2016 in a spreadsheet if anyone wants them... they haven't changed much over the years...
Edited by JohnES on Aug 23, 2016 - 09:51 AM.
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John Schwartz
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Posted: Aug 23, 2016 - 02:10 PM
Damon: I sent you an e-mail with regard to the Portsmouth Tables.
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Jerome Vaughan
Hobie 16
Clinton, Mississippi
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Posted: Aug 23, 2016 - 06:32 PM
Ok, so we are all right on this topic. I think there are some risks to making changes to the original designer's sail plan, but there are also some advantages. I have both spoken with and corresponded with Andrew at SLO and here is his response after reading the forum string:
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If possible, I'd like you to give me a call so I can explain how the area is calculated which will give you a better reference for the area values. Our standard pinhead main is 154 square feet while the square top is 165 square feet. Again, if you call, I can better explain what this means as the two sails are actually measured a little differently.
I read through the forum and "bruiser" brought up an interesting point regarding the aspect ratio of wings or sails. While he is correct, the aspect ratio of a square top mainsail is lower than a pinhead for the same rig. This is because aspect ratio is span (or luff length) squared divided by area. Since the luff length is fixed with the stock N5.2 rig, when you divide span squared by a larger area, you end up with a smaller aspect ratio. He is correct that modern sail plans are higher aspect than traditional rigs but that is because they are building taller rigs with shorter booms and keeping either the same or slightly larger areas.
Additionally, I think "Edchris177" summed it up perfectly in that the movement of the aerodynamic center will be small and that time on the water will be the best determination for how to set the rig up for the square top. Regarding your post about the daggerboards, you mention wanting the center of effort of the sails located at the center of the daggerboards. This is somewhat incorrect in that the rudders contribute to the center of lateral resistance in addition to the daggerboards. This means that the center of lateral resistance is somewhat behind the daggerboards. You don't want the center of effort of the sails directly in line with the center of lateral resistance, but they will likely end up being close. I look forward to hearing from you and let us know if you have any other questions.
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In my discussion with Andrew he said that the older software package that they used to design their Nacra 5.2 sails for both the square top sail and the pin head didn't take into consideration the overlap of the seams when calculating sail area. With so many more seams and overlaps and overlap width in the paneled sail the old software overstates the sail area, so it is not actually 11 sq. ft. it is something in the 7 to 8 ft. range. To get an accurate measurement they would have to design both sails in their newer software that would better sort this overlap difference out and probably not worth anyone's time.
We also talked about the lateral loads on the rudders a little more and there we got into a discussion of rudder types, angles etc. and that goes beyond this discussion thread.
I really liked corresponding and talking with Andrew who is clearly very knowledgeable, even for an aeronautical engineer. I also appreciate that SLO supports this website, so I am going to order a square top sail and will report my results.
dg
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dg
NACRA 5.2 #400
This End Up
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Posted: Aug 23, 2016 - 06:54 PM
Thanks Jerome -
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Posted: Aug 24, 2016 - 01:13 PM
Why the hell would us sailing make it hard to get the ratings(i had the rating page bookmarked) then it disappeared.You think they would be happy to have the info available for all in the interest of making sailing more popular!
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Carl
Dart 18x2
Nacra 5.8
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Posted: Aug 24, 2016 - 02:07 PM
It is not like you couldn't ask a US Sailing Member to get it for you either... All the more reason we should adopt the SCHRS handicap for our little Ventura races...
I downloaded the tables to a spreadsheet in June before my membership laps.. shoot me PM if you want it..
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John Schwartz
Ventura, CA
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Posted: Aug 24, 2016 - 02:27 PM
sent you a pm john.
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Carl
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Posted: Aug 24, 2016 - 02:46 PM
they make it very hard to get info unless you pay for a membership - aka they have their heads up their ass's -
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Posted: Aug 24, 2016 - 08:29 PM
That was some good info, thanks for taking the time to share. I've always thought sail design is a mix of science, art, voodoo, and BS.
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Damon Linkous
1992 Hobie 18
Memphis, TN
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Posted: Aug 24, 2016 - 08:51 PM
Not for the boys that are spending more on one sail that I had for the down-payment on my house...
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John Schwartz
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