Catsailor.com pros and cons of square top mainsail
Posted By: Brian P pros and cons of square top mainsail - 01/13/10 12:01 PM
hi guys, am just graduating form mosquito catamaran to a stingray caamaran. much the same shape (hulls like a tornado).
and some of the boats carry a standard shaped mainsail, and some newer rigs are running the big head or square top mainsails. my question is: what are the pros and cons of the squar top???
are they faster in heavy wind or lighter wind???
cheers
Posted By: Codblow Re: pros and cons of square top mainsail - 01/13/10 12:59 PM
beg to differ on the turd polishing , myth busters busted that myth
square head on my boat seems much better all round for me
Posted By: Anonymous
Re: pros and cons of square top mainsail - 01/13/10 01:55 PM
i look forward to hear the response of the guys who can really explain the technical aspects of your question, but from my experience i find the extra sail area on top leads to more power (not necessarily speed.. it depends how you utilize this power).
This is a great advantage in light air.. and i can handle it without to much problems up to 20knots.. above that i am rough handling my boat to keep it from capsizing and need to do everything i can to depower it (upwind) down/outhaul.. traveled out, jib furled... etc.
PS I usually solo my 5.5
I have recently acquired a pin top tornado sail that was cut for a prindle 18 (its a bit short for my rig) and i haven't had a chance to try it out.. but if all goes well... it will be my sail for heavy air (above 20)
Posted By: RickWhite
Re: pros and cons of square top mainsail - 01/13/10 02:36 PM
I believe it was Dave Calvert that started the concept for windsurfers, before he started designing multihull sails.
The theory is you have a fuller, more powerful area of the sail up high for lighter winds.
Now, when a puff hits, because of the length of the batten sticking out from the upper mast, the wind uses that leverage to allow the top of the sail to blow off to leeward, thus depowering in the puff, lowering the center of effort, and also reducing the heeling moment.
Sort of an automatic transmission, it shifts to a high gear in the puffs, and when the puff subside, it shift down again for more power.
Add to that the ability to downhaul and it becomes even more useful.
And they definitely work much better than pinheads
Rick
Posted By: rexdenton Re: pros and cons of square top mainsail - 01/13/10 02:43 PM
It's more nuanced than the question asks. In terms of power, it is basically as simple as 'its windier up there' as there can be as much as 2-5 mph difference 25 feet above the water surface. Placing more sail area up top takes advantage of more power.
Now we get into nuance...Square top sails are usually designed to match the mast, which has a very dynamic interaction with the sail plan. The front of a performance cat sail is cut with a curve in the luff (the part that goes into the mast track) which, when the mast bends, allows the sail to flatten. Without going into a lot of detail (you can spend hours trying and still not be able to visualize the process until seeing it with your own eyes on a boat) the square top works in tandem with the cunningham/downhaul/rotator to impose a shape on the sail that is most conducive to sailing conditions. The square-top provides leverage on the top of the sail down from the mainsheet (think square top 'pulls more downward' vs than does a pin-head sail (think 'pulls more backward'), and is therefore able to bend the top of the mast more efficiently. The result is greater ability to tune the rig according to sailing conditions, and an edge on speed overall.
That said, I haven't figured it all out...
Hope that helps
Posted By: Rolf_Nilsen
Re: pros and cons of square top mainsail - 01/13/10 02:57 PM
Less drag for more area, higher up => better lift/drag ratio. A sail that is in my opinion easier to handle and sail well with.
The "automatic gust response" is fiddly to get right and depends on a lot of variables. That is not the main point. A more efficient and userfriendly sail is.
am just graduating form mosquito catamaran to a stingray caamaran.
I dont knof if there is such a thing as "graduating" from mozzies. Seems like there is a lot of skill and experience in those fleets.
Posted By: John Williams Re: pros and cons of square top mainsail - 01/13/10 04:08 PM
There are cons?
Posted By: Dan_Delave Re: pros and cons of square top mainsail - 01/13/10 04:24 PM
This is not the engineering of the sail, just my experience with it.
When I put the squaretop on the Tiger I expected to get better performance out of it in light winds. The first day racing it was miserable. I was sailing in bumpy conditions in light winds and finished at the back of the fleet. The next morning I looked at the leech, while on the beach. I noticed, sheeting and loosening the main, that there was a huge hook at the top while sheeted in. I marked the mainsheet where the hook stopped and raced that day with that slack in the sheet. I finish in the top three with the same conditions as the day before...much better.
So where do I think the squaretop shines? In windy conditions where you do not want to do too much sawing if you hold your own mainsheet. As Rick pointed out the top of the sail will depower on its own as a puff hits it. I usually play my own mainsheet and do not like to pump it in and out. Doing that too much, one handed, hurts the shoulder. The squaretop is perfect for this application.
Later,
Dan
Posted By: Rolf_Nilsen Re: pros and cons of square top mainsail - 01/13/10 04:25 PM
Good post!
Higher sheeting loads. More area at the top to flip you forwards if running dead downwind in extreme conditions, or when transiting from beating to reaching to downwind.
There are some cons, but the pros outweight them massively in my opinion.
Posted By: RickWhite Re: pros and cons of square top mainsail - 01/13/10 04:30 PM
Dan,
That is what I teach for sail trim. Line up the points of mast base and sheeting center, trim so the sail is slightly off center of mast and mark the sheet. That way you know exactly where to sheet in after any maneuver, i.e., tack, start, mark rounding.
At least you are in the ballpark. In a puff, sheet in a bit more, in a lull, ease off a bit.
You certainly do not want to oversheet a square head, or any other sail for that matter.
Rick
Posted By: Will_R Re: pros and cons of square top mainsail - 01/13/10 07:59 PM
Marchaj's book "Sail Performance" does a good job discussing head shape. The square head does have more area up higher, but b/c of the shape it's much more efficient (more equal loading along the chord length). I don't remember the percentage, but if you look at flow and usable area of a pin head, it looses something like 30% at the top.
I'll pull the book back out if I have time and find the section to reference.
Posted By: Jeff_Bowers Re: pros and cons of square top mainsail - 01/13/10 09:33 PM
Rick,
Does that spot always hold true or do you have to sheat more if you apply more downhaul creating more mast bend?
Posted By: RickWhite Re: pros and cons of square top mainsail - 01/13/10 09:37 PM
If the mast bend makes the blocks closer together (lowering the clew), then it will change. But, for most boats the downhaul does not significantly change the relativity of the sheeting blocks.
Rick
Posted By: Anonymous
Re: pros and cons of square top mainsail - 01/13/10 09:46 PM
does this apply downwind too?
Posted By: Brian P Re: pros and cons of square top mainsail - 01/13/10 11:33 PM
thanks guys for all the input. sounds like the squaretop is the winner, especially if the rig can become that little bit more "automatic". doe sthe extra sail up top not cause more healing moment due to the extra sail power "higher up the lever", i guess if it automatically twisits off and depowers itself it shouldnt cause too much drama, in 2o knots plus i guess i would be more inclined to run an older mainsail anyway.
thanks guys!!!!!!
p.s. dont stop with the info!!!!!!
its great to keep learning, even after racing for 30 years. you can always learn more
Posted By: TheManShed Re: pros and cons of square top mainsail - 01/14/10 02:51 AM
There was a huge string last year on this in SA with allsorts of technical stuff.
I remember asking Bill Roberts about it a very long time ago when the came back after all he was a rocket scientist at Pratt Whitney. It is not new but old technology that was re-born again in the late 70's early 80's or there about for beach cats.
He said some simple things to me:
A sail is a wing.
Ever see pointy wings on modern airplanes?
Also a bunch of scientific stuff about the way the wind leaves the sail, and the vortex of wind not going down the face of the sail.
Bill made fast boats and knows more about airplanes and rockets then me I took his word for it.
When I went flat head my boat sailed faster.
Posted By: JeffS Re: pros and cons of square top mainsail - 01/14/10 05:45 AM
G'day Brian Big head sail is the way to go, Mitch Bayliss was here last weekend and he went like stink. I had a square top on my big rig Stingray though a lot smaller than the squares now. I found that to make the main hold it's shape I had to have tight main sheet but if I had the mast rotated properly it pulled the top of the mast over backwards and depowered the boat so I used less mast rotation. The 5.7's have gone from my square top to a F18 style, in light wind from what I've seen they seem to lay off quite a bit if you let any main tension off. Maybe super stiff top batten needed? Whats your new boat?
regards
Posted By: scooby_simon Re: pros and cons of square top mainsail - 01/14/10 12:53 PM
If the mast bend makes the blocks closer together (lowering the clew), then it will change. But, for most boats the downhaul does not significantly change the relativity of the sheeting blocks.
Rick
Disagree.
When applying DH you are streaching the sail and bending the mast; this will change the sheet position.
THink about what is ahppening; mast bends and sail flattens (top of the sxail falls away); to get the sail back into a good foil, you have to pull more leach tension; this means more mainsheet.
When sailing without any extra DH, I have about 20cm between the blocks on my F16; when sailing with full DH (but needing to sheet in) my blocks will be almost block to block.
Posted By: MitchB Re: pros and cons of square top mainsail - 01/14/10 10:08 PM
Brian - From a completely non technical point of view I'll say this:
In light airs/choppy water the square top seems slower, in flat seas it's about the same.
As the wind started to fill in a Victor (13kts) and we were able to stretch out on the wire the big head sail seemed to be faster, I sailed over the top of Brett and gained about 100m on Todd in one upwind work.
However - Todd did win the series with a pin head sail - it should be mentioned he is an excellent sailor with 15+ years experience on the boat (I hope he doesn't get a big head sail!)
Posted By: Will_R Re: pros and cons of square top mainsail - 01/14/10 10:14 PM
Brian - From a completely non technical point of view I'll say this:
In light airs/choppy water the square top seems slower, in flat seas it's about the same.
As the wind started to fill in a Victor (13kts) and we were able to stretch out on the wire the big head sail seemed to be faster, I sailed over the top of Brett and gained about 100m on Todd in one upwind work.
I'm going to go out on a limb here and say that this is probably b/c it's much easier to oversheet a squarehead in light/med conditions than it is a pin head. Oversheeting is a painful death in light air.
Posted By: Brian P Re: pros and cons of square top mainsail - 01/14/10 11:43 PM
THANKS GUYS!! , mitch, but isnt your crew weight about 35kg more than todd, in the light wind he is bound to be on the wire earlier than most and up and going?? how much do you think that is making a difference in that light to moderate wind range rather than just the diff in the sail?
Posted By: scooby_simon Re: pros and cons of square top mainsail - 01/15/10 12:23 AM
Brian - From a completely non technical point of view I'll say this:
In light airs/choppy water the square top seems slower, in flat seas it's about the same.
As the wind started to fill in a Victor (13kts) and we were able to stretch out on the wire the big head sail seemed to be faster, I sailed over the top of Brett and gained about 100m on Todd in one upwind work.
I'm going to go out on a limb here and say that this is probably b/c it's much easier to oversheet a squarehead in light/med conditions than it is a pin head. Oversheeting is a painful death in light air.
Agreed; you needd to watch what is happening with the leach and head on a fat-top sails
Posted By: MitchB Re: pros and cons of square top mainsail - 01/15/10 06:21 AM
Our crew weight was actually more like 45kg more than Todd! Yes that helps I suppose haha.
Thanks for the sailing lessons guys, I am faily aware of everything mentioned, BUT, I still feel on the stingray that the older style sails hold their shape better and drive more easily in light winds+chop.
I guess the other thing is Brian that its probably not that bigger issue in the stingray class - When time comes to buy a new sail I would get the Big Head everytime, but the older sails still go pretty damn fast!
Posted By: Brian P Re: pros and cons of square top mainsail - 01/15/10 06:30 AM
i think your right bro!!! i would need some pretty hard convincing not to buy one. i was hugely impressed with the shape of rex's mainsail from goodall's
now, to find someone that wants to sell.......
on a different point mitch, did you send your CV in??
sorry my email been dead for about a week now!!!
Posted By: Will_R Re: pros and cons of square top mainsail - 01/15/10 04:38 PM
BUT, I still feel on the stingray that the older style sails hold their shape better and drive more easily in light winds+chop.
Back in the days of the failed F18HT, Randy built a main out of PBO that was clewless... sort of. He took the material that would be in that area and replaced it with spectra. That sail area was then added to the head of the sail. It really was a cool sail to look at and touch. We all called it "The Fly Swatter" b/c we were just flies on the wall when he had that sail in light-med conditions. The extra sail area up top REALLY paid off and this was square head vs. square head.
Posted By: Smiths_Cat Re: pros and cons of square top mainsail - 01/15/10 08:56 PM
from a practical point of view:
a good pin head is still better than a bad square head imo, but a good square head is better than a good pin head.
from a theoritical point of view, it is about higher Reynolds number and lower local lift coefficients, which leads to a less early stalling and hence more lift and less drag. Also the mast-to-sail-size ratio is more favorable if there is more chord. The gust unloading theory, which Rick explained so well, does probably the most good for the sail, especially for uni-riggs
Cheers,
Klaus
Posted By: Luiz Re: pros and cons of square top mainsail - 01/16/10 04:17 PM
FWIW, Bill Roberts once wrote the following regarding square top mainsails:
...What the square-top sail/wingtip does is prevent spanwise flow down the low pressure side of the sail that originates from the sail top where the sail is narrow, where the leech is forward relative to the sail below. On a pin head sail or pointed wing tip, the air exiting the sail/wing on the pressure side near the tip turns 90 degrees at the trailing edge and flows or is pulled spanwise down the sail, to an area immediately below where the pressure is lower. Air wants to run downhill just like water. The air on the pressure side of the sail near the tip is at a higher pressure level than the air on the leeward side of the sail at, say, 50% chord a foot or so, down from the narrow leech above. The pressure side air at the leech sail dives vertically down the sail, leeward side into an area of low pressure on the leeward side. This spoils the delta P across the sail at the top and reduces the span effectiveness of the sail or wing. The square top moves the leech aft and this delays the opportunity for the air exiting the windward side leech to flow spanwise down the sail to a low pressure area immediately below. With this spanwise flow diminished/eliminated, the span of the sail/wing is more EFFECTIVE, it more closely approaches the ideal lift for a given span. You could say the square top sail or square wing tip is an aerodynamic dam that prevents a parasitic spanwise flow which reduces the effectiveness of a sail or wing...
I'm still trying to understand it all...
Luiz
Posted By: Smiths_Cat Re: pros and cons of square top mainsail - 01/16/10 05:48 PM
I'm still trying to understand it all...
Don`t worry. The explaination from said Bill Roberts is wrong (or so badly formulated, that I miss the message).
Put tell-tales on your sail, and you will see in which direction the air flows. I have never spotted a spanwise downward flow on a well trimmed pinhead sail.
Effect of taper ratio (and sweep) on wings is well examined, documented and understood in aeronautics since long (80 years or so). Google for "wing design" and/or "wing taper ratio".
Even without the theoretical background, naval architects found by evolution the positive effect of taper, e.g. in a gaff rig, which was for long the standart fore and aft rigg, several centuries ago.
And by the way, pinheads like aft swept masts. A well known fact for H16s and also discovered on many Bermuda rigged sloops (with pinheads) years ago.
If we would only listen more to our predecessors, technical advance would be twice as fast...
Cheers,
Klaus
Posted By: Wouter Re: pros and cons of square top mainsail - 01/16/10 10:26 PM
The explaination from said Bill Roberts is wrong
I remember going down that road many years ago.
Long time catsailor forums members will remember what I mean.
Wouter
Posted By: Leo Ambtman Re: pros and cons of square top mainsail - 02/11/10 09:03 PM
I sailed a classic Tornado (pinhead sail). Now I sail the new Tornado (square top). I doubt the new one is faster when reaching in heavy wind (both double trapeze). At the classic T. center of effort is lower and probably less drag. In heavy wind you try to eliminate the top of the square sail by a lot of downhaul so this makes it more or less a pinhead sail. However this is in contradiction to speedwindsurfing where you see a square sail with a loose leech. I think the topspeed of a catamaran (no spi) is for 99 pecent fixed by maximum heeling moment so in the end it makes no difference for top speed what kind of sail is used as long as the trim is correct (flat!).