Hey guys,
I`ve been looking at the pic Wouter posted a while back somewhere of Matt & Jennifer going upwind (Think it was the Squaretop trimming post ?). Now I have me confused, I know it`s a good thing to twist off the main when overpowered to get the top to fall away & depower the rig, but how much is right / too much / not enough ? Looking at the pic, they hardly look overpowered (wave state etc says about 8knots, maybe 10.), but Matt has much more twist & fullness than Jennifer, who looks to be pointing higher than Matt., which would confirm my thought, ie that too much twist & you struggle to point. In my opinion even Jennifer has much more twist than I`d be happy to sail with, I`ve been coached to believe that the leech should be a straight line, in line with the mast, when viewed from directly behind the boat, for optimum pointing ability. Any tighter and you hook the leech, but any looser and you are spilling power higher up and negatively affecting your pointing ability. I know some twist is good, something to do with laminar flow & wind higher up going faster, but inducing that much twist leads me to believe the wind higher up would be travelling at a much different direction to that lower down, is that really the case ?
Any answers that don`t add confusion would be much appreciated !

I think they are both closing on the A-mark and matt appears to be steering towards it and dumping some main in order to not let the luff hull fly to high. Jennifer appears to be pointing higher indeed.

Glenn Ashby says in the now famous catsailing DVD that he sails with 250 to 300 mm twist in pretty much all conditions. I notice that while is looks strange that the boat speeds up when adding about that much twist. I'm probably still oversheeting my main but I'm getting there.

And both Jennifer and Matt are lightweights (65 kg and about 70 kg). They can probably trap when I'm (at 85 kg) are still in.

They large amount of twist in the squaretops sails has to do with tip cortex drag. If the angle of attack of the top part of the sail is to great than you create alot of sucking and pressure across the sail this feels like power and keeps you out on the trap. HOWEVER the vicinity of the top of the sail allows air from the overpressure zone to circle over the squaretop to the underpressure zone, creating what is called a tip vortex. This is felt like drag. So by sheeting in can can lift the boat but only at the expense of disproportionally amount of extra drag. The boat feels bound up then. Now by either sheeting up or using your downhaul you can open the upper leech and have the top twist away. This results in less heeling force but also in alot less drag. The net result is more drive to propel the boat.

The Aussie Taipan sailors webpage mentions that the 4.9 needs to be sailed for speed all the time and this addagio hold true for the F16's. Don't sheet the main to keep you out but rather sheet is ONLY to maximize speed. It may well be that you'll be sitting while all others are trapping on other boats. But despite that your speed will be better.

I found this again last sunday. Like I said I'm still an oversheeter

Wouter

Ok, so I think what you`re saying is the following :
The main advantage of squaretop over pinhead sails is that the airflow over a squaretop sail is more laminar (a pinhead will always have wind going up the leech, trying to find shortest distance.)
HOWEVER : If you are slightly oversheeted on a squaretop sail, the drag is higher than on a pinhead, since the pressurised air "jumps over" the head, causing a lot more drag. Am I right in assuming that a pinhead sail would be a bit more forgiving in this regard, since the air has no head to hook over, or would it be even worse as the air is already travelling up the leech to a certain extent ?
If my assumption is correct then correct settings while sailing with a squaretop is more critical than a pinhead sail.
??
Steve

I dont think that is is correct to say that a square top have more laminar flow. I always believed that you had either laminar or turbulent flow over the sail? But what you say about pressure leaks is the same explanation I have heard earlier explaining why square tops are more efficient.

Now, I believed the extra drag caused by an oversheated sail was mainly due to early separation and an increased tip vortex. I did not believe that pressure leaking across from windward was a major contributor to this even if it's part of the "big picture" when taking into account circulation?

If Glenn says 250-300mm of twist, on an A cat. Assuming that he designs the sail with about 8 degrees twist and that it have a 8000mm leech, the buildt in twist is 640mm (I dont know how much buildt in twist he uses, but I think 8% should be about right for a sloop rigged cat). That must mean that he sheets the main quite hard to achieve 250-300mm.

A tight leech is paramount to achieve height while beating. The pictures of Jennifer and Matt must have been taken while rounding the windward mark, or else they would not have been in front.


I would say that a square top is easier to trim and sail well with compared to a pinhead, as the squaretop reacts better to mainsheet and downhaul (just an opinion).
Without knowing how to read the sail (telltales+boat's course+what gear you are in+knowing what you want to do+knowing what shape you want) you are equally slow whatever kind of mainsail you have

An oversheated main is slow, but can give you some extra height if you need it. Take care not to hook the leech..

Extra advantages : better gust response, better efficiency of area in sail that is somewhat away from edges (=middle section); more efficient use of the area that is in the top (the upper section of a pinhead does nothing but create drag); More area up high, lifting hull early and possible milking more energy from higher air layers. Better distribution of total sail area. (A sq mtr on the bottom near the foot doesn't do much in the top it does even when its efficiency is less than the middle part).




Drag is related to size of the pressure zones. a square top can just create larger pressure zones there and so larger drag.




It is difficult to say in one simple phrase. It is not that easy. Pin head suffer from the mast section being very big in relation to the pin head area as well for example. On the race course however, pinheads are uncompetitive and that should say enough.




Yes.

But the rewards when trimmed right are just a significant.

Wouter

Steve,

What you can not see is that the A mark in this picture is right behind the gals left elbow, and both of us were driving into the mark. The twist evident in this picture is from the fact that the main is not sheeted in anywhere near where it would be to go to weather efficiently and be able to judge against the numers being quoted in the other posts here.

My opinion of the square top sail to date is that it is a dream to sail. When trimmed, it does everything just as advertized. The problem I found in racing uni especially at this regatta where it was very gusty, was that without a crew, the sail was not often properly trimmed. With very small adjustments in the downhaul, one can keep the main flowing perfectly. When you are alone you pick your best guess at it and go.

With my current set up I have the ability to trim my sail out to be board flat if needed. I usually sail relatively light, so flatter is better for me. As you can see in the pic, making it very full is also possible. For going to weather, I set my rotation to get good transition around the leward side, which is slightly under rotated relative to traditional tear drop shapped masts. I then have my crew run the down haul so that my leach breaks equally up the sail as I sheet in. This becomes my reference point before we start, and during the leg they will downhaul more or less with the puffs. This way the sheeting is minimized, because when the sheet is eased a viscious circle is started, you get big twist to depower, but the sail also becomes very much fuller, which adds power and momentarily hurts your pointing ability, so you have to counter by letting out more sail than necessary.

Anyway, hope this did not add to your confusion.

Matt

Just to give you an idea of the amount of twist that is sometimes needed...



This pic was taken last Saturday showing me going upwind in 22mph of breeze with a combined 170Kg+ on the wire. The top two leech streamers are flowing nicely and I'm nearly block-to-block on the main but the top of the sail has flicked off and given a twist in the region of 5-600mm by my guesstimate. Gotta love those square tops!

Man, what have I been writing?? This is pure bunk of course! Twist is not expressed as a function of the leech length but the angle of the chords. Woke up in bed and realised what I had written earlier, can only blame it on the cold I caught on saturday

Praise yourself lucky that Sam Evans didn't got around to given you a reality check on this yet. !

Wouter

Sorry Rolf, I don't quite understand your concern...

When discussing mainsail twist with fellow catsailors, we've always talked about the sail twisting off so many inches (or feet). My understanding of this measure has been that if an imaginary line were extended through the head of the sail and out to the arc described by the boom, the difference between the intersect and the boom position gives the measure of twist in feet, inches, mm, whatever?

Have I been misunderstanding, misdescribing for the last 30 years?

Jalani, your description of twist is a correct one.

It was I who messed it up wanting to calculate how much twist 250mm would be on an A-cat expressed as an percentage. The reason I wanted to do this earlier, was to get an idea about how hard he sheeted his sail as compared to the amount of twist designed into the sail..


While I have your attention. In the picture you posted earlier on, of you going upwind with bit-time twist. Is your boom bending noticabely, causing the foot of your main to be very deep?
It's hard to tell from the picture, but how much is your mast bending? It looks fairly straight, even with what I guess is quite high sheet tension.
You also seem to have a carbon mast, how sensite is it to downhaul compared to an alu section?

Would be nice (and relevant to the thread) to hear your opinions on this.

Thanks for the response Rolf,

In answer to your questions:


Yes, it is bending more than I would've liked. I hadn't realised until I saw the pictures. The outhaul was set up quite tight with about 30mm of draft when unloaded. From the photos it looks like under load the draft has become excessive and could be more than double the static setting. In future I'll have to bear this in mind and/or get a stiffer boom.



Because I knew that I was going to be sailing with a fat b*****d for the day, I set the mast up very straight (minimal prebend).


This sort of comes into your second question as well. I have found the carbon mast to be all round more responsive. Bend can be induced very quickly and easily with downhaul and/or rotation. In the photo the spanner is pointed just forward of the shroud so we are in effect slightly over-rotated - this keeps the slot clear as the mast doesn't sag in its fore and aft axis but allows some sideways bend and the leech to open. Additional downhaul would cause even more sideways bend and more sail twist. De-rotation would (without altering the downhaul) flatten the sail more but reduce sideways mast bend, give less 'drive' and straighten the leech. Alternatively we could just ease the downhaul and this would allow the leech to stand up again. However, from the photo you can see that, apart from the deep foot, the sail is set really well and the power available to us is being used very effectively - we're not overpowered.

The short answer I guess, is that the carbon mast IS more responsive than an aluminium one....



To illustrate the point about mast bend, this picture was taken just before the one posted earlier. Although in this one I have eased the sheet slightly compared to the later one, you can still see the sideways mast bend above the spreaders. It also illustrates how easing the sheet just an inch or so results in dramatically increased twist. A powerful mainsheet on these boats is an ABSOLUTE MUST!

Matt, thanks for the reply, since you were there ! Without knowing where the weather mark was it`s hard to tell why you had so much fullness and twist. At first I thought you might be easing the mainsheet to get the hull back down in a gust, which Wouter also presumed.
Your following statement is interesting :
"With my current set up I have the ability to trim my sail out to be board flat if needed. I usually sail relatively light, so flatter is better for me. As you can see in the pic, making it very full is also possible. For going to weather, I set my rotation to get good transition around the leward side, which is slightly under rotated relative to traditional tear drop shapped masts. I then have my crew run the down haul so that my leach breaks equally up the sail as I sheet in. This becomes my reference point before we start, and during the leg they will downhaul more or less with the puffs. This way the sheeting is minimized, because when the sheet is eased a viscious circle is started, you get big twist to depower, but the sail also becomes very much fuller, which adds power and momentarily hurts your pointing ability, so you have to counter by letting out more sail than necessary."
I agree fully here, I`m also on the light side. In our Solo (Uni) champs a few weeks back I started playing with a few settings upwind and changed the way I play the main quite a lot (we had 12-18knots most races, 18 boats):
First get settled in on the upwind leg, main & traveller centred, downhaul on minimum. If I`m overpowered, first get the downhaul on until it`s at it`s max - If I`m still overpowered, bring mast rotation back (de-rotate), then I start playing the traveller, I will almost never ease main, since as you said it puts so much fullness into the sail initially that you first poewr up before you depower and I find the boat becomes "flighty" if you`re constantly trimming the main. Playing traveller keeps the mainsail shape & profile the same.
I found that I don`t point as high as my competition, but I have a LOT more speed than some, to the extent that I don`t worry about my heading, just foot like mad for speed, and found myself at the weather mark in the top 4 in all races. I don`t know if the way I`m adjusting the main hurts my pointing or not, but it settles the boat down and I can concentrate on my speed.
If the gust drops off I`ll ease downhaul first, then mast rotator, before coming in off the wire.

Steve

Thanks for the clarifications.

I asked about the downhaul because your mast seemed very straight (little pre-bend and little downhaul applied) while your sail-trim looked like you was dropping power.

It might be fun to hear that I had the pleasure of following a two time Tornado olympian on a coach boat a week ago while he sailed his T. He used a similar trim to go fast upwind in a stiff breeze, albeith not with as extreme a twist (hard to say from the angles of your pictures). He also had a lot more mast-bend..

In the last picture, it looks like your boom bends less, so there must be less pressure on the leech.

Agree on the need for powerful mainsheets (and downhauls!). We have a 9:1 mainsheet on our T, and pull with something like 50-60kgs in a breeze.

John,
Yeah I thought you had a loose foot to keep the power up, 170kg needs power !
Looking at your mast rotator position leads me to one of two possible conclusions based on your comments :
-If you eased off on your diamond wires to get less pre-bend, your mast would bend in it`s min. axis, so you`d get an even bend in the mast when rotated as you have, leech would open fairly evenly.
-If you raked spreaders forward to get less pre-bend, but have tight diamonds, your leech would only twist off above the hounds, where the mast can bend in minor axis.
So in effect you would be getting the top of the mast to bend to weather in it`s minor axis, but with lots of rotation allowing that to bend toward the leech, letting the top twist off ? (I`m assuming you eased off on the diamond wires only without changing the spreader rake, or you wouldn`t be worried about the mast sagging into the slot when derotating.)
With this much rotation you are not letting the top of the mast "fall away" to leeward as with that much weight on board you probably don`t need to.
Am I on the right track here ? Just a thought - with that much rotation are you not getting a disturbed airflow over the luff of the main ?

Steve

Steve,

Yes you are right in your assumption - I took two full turns out of the diamonds to reduce the pre-bend, as a result the bend in the mast is fairly even, but also remember that unlike an alu mast, a carbon mast is built with specific bend characteristics. A tapered carbon mast (as I have) even more so.

Without diamonds the bare mast is softer twothirds of the way between the spreaders and hounds. It appears to be slightly more flexible in the topmast as well. This seems to give a straightish section from base to just above the spreaders, then a gentle curve through the hounds to a top that flexes and flicks off in the gusts.

The unloaded prebend though (major axis) is an even curve along the length of the luff groove. When the mast is loaded and rotated the tapered section is able to bend to weather and back freeing the leech 'automatically'.

As to disturbed airflow - the flow at the leech is fine as demonstrated by the streamers. From recollection my centre (in terms of fore/aft) tell tales were flying correctly but I believe that the front tell tales in the upper third were lifting occasionally to signify an updraft every so often. A lot of the time though they were flying correctly.

As we were the fastest thing out there (and that includes the rescue boats) and two Dart 18's capsized and retired and a Nacra F18 lost its mast (too little diamond tension), I wasn't going to worry too much about a little turbulence at times!

Thanks for reply, John.
Understand the carbon mast stiffness, one thing you can`t do easily with aluminium unless it`s tapered. Maybe one day that technology will be possible way down south in Africa, but for now carbon masts are unaffordable for us.
We used to have stiffer alu masts than we now have, and also used to over-rotate with very loose diamonds, but now have softer alu masts and do the exact opposite.
Forgot to mention in my last posts - the Stealth looks really sweet at speed ! The boat looks larger than the 16ft that she is, must be the width & those tall bows.

Cheers
Steve

Hi all,

great post.

Steves comments re Pinheads V's Square tops, reminds me of something I noticed at recent F16 challenge. Front Mossies (all mossies have pinhead sails) in the light winds showed very tight leaches, in line with mast but still moved very well although no twist was evident. At the same time I felt I had some twist in my square top, but was hardly able to catch them if at all.
Don't know if it means anything but I wouldn't imediatly say that pinheads don't perform as well. Just differently?

Regards Gary.

Steve,

Take a close look at the relative angle your competition is pointing into the wind and your own. Then look at who got to the weather mark first. Most often the boats looking like they are out pointing you are in reality just pinching. Cat sailing is about boat speed. Within reason if you are making better speed, chances are you are going to get there faster.

That said sailing under control with the crew controling the sail shape as opposed to the skipper running the main sheet is somewhat difficult to get used to and takes a lot of coordination and communication. When the moves are all anticipated, the contol line adjustments needed are ususally very slight, and coordinating them with the tiller to maximize up wind gains takes practise. Tiller motion needs to kept to a minimum to keep drag down. Yet you want to put that foot or 2 in the bank every chance you can get without moving too much or sacrificing your boat speed.

To me that is the thrill of sailing. Trying to run that tight edge to get everything out of the boat possible, all while going very fast and having fun.

...oh ya, and the rum.

Matt

Hi Matt,
Agree on all points (especially the rum ).
I sail 90% sloop & 10% solo, so my learnings at the solo champs showed me that to sail well solo you have to change your sloop-sailing mindset (that, and the crew is toooo slow getting the kite up when sailing solo !).

Ok, except the bit about who gets to the weather mark first. He points like mad, goes like hell, and won 8/8 races. Not fair. The rest of us have a bit to learn.

When sloop I sail with stick in one hand, main in other. Downhaul is on deck at my feet when trapezing, I play it rather than crew. Crew has jibsheet & traveller, and plays traveller at my request.
When solo, I cleat the main, play traveller, downhaul is at my feet for easy reach. I`m going to set up mast rotation accessible from the wire as well, as I lost a lot of ground to a competitor on an upwind leg because I forgot to get the mast rotator back in for the upwind leg and could only change it when I tacked, felt too powered up for that leg and started sailing horribly.

Rolf : the 8deg of twist built into mainsails, is that pretty much a standard amount built into most mains, or can it vary depending on the boat / crew weight / mast prebend etc ? Or is that a sort of "benchmark" figure that can be adjusted by sailmakers to suit certain conditions / requirements ?

Steve