Mast theory

I'd like to understand the basic theory behind mast shape and settings - in particular, the effect of spreaders. For instance, if I have 1.5" of pre-bend, does it matter whether the spreaders are in their forward or backward position? As far as I can see, I'd need more diamond tension to achieve a given amount of pre-bend if the spreaders are in the forwards position rather than back, but does that affect sailing performance?

Let the debate ensue!

from what i gather if the spreaders are not swept back than you would not get any prebend at all... ie on my Supercat 19 i have spreaders that are inline with the mast and only help the mast from bending sideways when your mast is rotated

Sail makers cut a constant luff curve. In order to get your sail to flow well at top, middle, bottom, the mast must generally follow the luff curve of the sail. Lets say there is a amount of luff curve your sail maker has placed in your sail (X). In order to match X in your mast you can play with your spreaders and/or your diamond tension. You can place your spreaders forward and put huge amount of diamond tension and get to that magic number, X. The result of this would be a mast that would be very stiff down low and bend a lot up higher ( NOT a constant curve). Now lets put a lot of sweep in the spreaders and less diamond tension to get to X. The result will be a mast that bends more down low than up top (NOT a constant curve). If you play with both in relation with other you can come up with a curve that is more constant (even) and therefore better matches your sail. This will allow you to get your sail to flow at top, middle, bottom. That is a good thing.

Ken

I think what he is asking is if a given amount of pre-bend can be obtained with 1.5 inches of spreader rake and 500lbs of diamond wire tension and the same amount of rake can be obtained with 1 inch of spreader rake and 750lbs of diamond wire tension (for example!!!), is one setting preferable over the other?

The amount of spreader rake will affect some of the mast bend difference between fully loaded and lightly loaded. As the rig is loaded through wind pressure and righting moment (trapeze), the diamond wire(s) are in additional tension as the mast tries to bend away in the middle.

The amount of rake in the spreaders affects how much additional forward bending pressure is applied to the mast as it loads sideways from these dynamic loads. More spreader rake means that the load on the diamonds will translate more into a forward bending moment on the mast as the load increases and you will experience a greater difference in the flexing of the mast from 8 knots and sitting on the trampoline to full trapezing in 20. Less spreader rake translates into less forward bending moment on the mast yielding a smaller range in the flexing of the mast in the same situation.

Perhaps it makes sense for the dynamic mast bending characterics to be very similar between two teams of different weights. The lighter team would want more spreader rake because they dynamically load the diamonds a little less than the heavier team. To see the same dynamics, the heavier team would want less rake because their diamond wires are actually going to carry a little more load.

Lighter teams will probably want to see the mast flex through a wider range as the wind increases. Heavier teams typically want to see it flex less to accommodate the extra weight.

The preferable setting is the one that best matches the shape of the sail.

Thanks for the feedback. I can see the case for matching the shape of the sail's luff - this must be the 'best' position, but that would imply everyone would set their mast up in the same way. I have had advice from the designer / manufacturer to go to the "fat boy" setting, with minimum spreader rake, and maximum mast pre-bend. I am trying to understand the theoretical advantage this offers over the middle setting for each. I gather that with the spreaders raked less (putting them further forward), I will get a stiffer mast, at least in the lower section, and this will generate more power. And if I want even more power, I can ease off the diamond tension and get less mast pre-bend.

Just a small thing. Luff curve is not constant, it is supposed to change according to how the mast bends. Above the hounds it bends very differently compared to around the spreaders.

I am no expert, and could be VERY wrong,... but i thought pre-bend was mostly a result in spreader rake.. not diamond wire tension .

Everyone, please correct me if i am wrong

i believe its a combination of both... if you have alot of spreader rake you can adjust the mast prebend by loosening/tighting the diamond wires.. so the looser the wires the less the prebend...

Pre-bend is a function of spreader rake and diamond tension. Less spreader rake and tension and the mast will be slower to react to gusts. More rake and tension and the mast will depower faster when you pull on downhaul or even when you go out on trapeeze. As always it is a compromise, but if you are racing it is critical to get a luff curve matching the bending characteristica of your mast. You can adjust it a bit with pre-bend, but getting the sweet spot where the mast do the work for you and depower in gusts is harder then.

I always have to remind myself that more power comes at the cost of more drag, it will not always be faster to have more power!

So to add to the confusion...when setting spreader rake, we typically measure by placing a straight edge (ie batten) across the two diamond cables at the spreader tips, then measure the distance from the straight edge to the mast track. When we're at the rake distance we want, then we crank on diamond tension (using a Loos guage). Typically, if we re-measure the rake setting as above, it does not change after diamond tensioning. So, to check pre-bend, you're suppose to stretch a line from mast top to mast foot and measure from the line to the mast track at the spreaders. This should change with differing tensions (the ends of the mast are flexing in the fore/aft direction with less/more tension, respectively).

I have never done the pre-bend measurement...since it can't be measured easily without tipping the boat on it's side and I'm too lazy :-)

Ahh, so how do you know that you have a good setup <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

Well, I will repeat my question from the F16 forum about sail stalling here:
So far, I understood that I can adjust the twist of the main with pre-bend. But what is the best light wind twist? Should my main stall at the top of the mast/sail, in the middle or just above the boom? I am used that pin head sails always stall at the top first, however the square top sail which I have now stalls above the boom first. Do I still have to much twist?

Cheers,

Klaus

Set up technique is here:
http://www.hurricane59.com/rigging_sailing.html

Cheshirecatman

Klaus, you are the aeorodynaimist, you should be telling us <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

In my opinion the sail should stall over the whole at the same time when you have perfect trim. If your uni-rig stalls over the boom first, I would say you should try less twist. But, that would depend on conditions. In chop you probably want more twist. That is my opinion..

Hi Rolf,


Well, a theory without practical back up might be wrong, hence I ask.


That's what I think as well. But I see that you use subjunctive. Did you ever achieve this 'perfect trim' (I assume you use tell tales at the leech)?
Actually I have already a quite relaxed dimaond wire tension, maybe the leech is cut to long <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />...

Apart from confusing sail trim, my wife and I have had great time on the boat already. My wife did helm under spi for the first time and really pushed it to the limit and now she is really thrilled (she dived the bow up to beam, nothing happend, but a big shower for us).
You will be surprised, how sensitive these boats react on the smallest changes. I hope you get your boat on the water soon, so you can share this.

Cheers,

Klaus

Klaus,

Dependent on what you call light winds.

Stalling of the sail in light winds can also because by having too much draft in the sail; as in contrast to having a too large angle of attack. I think this could be the case here as the bottom part of the sail is widest and therefor the flow needs to have relatively alot of "internal energy" to overcome the "long" negative pressure gradient at the back of the sail.

In really light winds you'll want a flat sail with a considerable amount of twist. The flatness assures the sail won't stall due to negative pressure gradients that are too large and the (relatively) large twist on the leech compensates for the windshear.

However at some point in windspeed the wind itself transitions abruptly from laminair flow (with a significant windshear gradient) to a turbulant flow (with much less windshear) and then you'll need to bring the leech back in. When the wind increases more then the increase the draft; as the airflow is becoming more energized. You are now in medium wind conditions typically 6- 12 knots.

And only in high winds due to increase twist and decrease draft again.

In simple terms ; very light wind sailing has an almost identical sail profile as high wind sailing where the sail trim in the medium conditions is with a relatively full draft and tight leech.

Flat and rough water are influence factors as well. In flat water you can typically sail with a leech that is a little bit more tight and with a relatively unform draft profile. In rought water you sail with more twist and a draft that is located more in the lower part of the sail.

You use mainsheet, downhaul, diamond wire tension, spreader rake simultaniously to adjust the mainsail to these different conditions. Sadly it is not like one control only affects one adjustment. For example when you sheet the mainsheet tighter then you reduce twist and reduce draft in the top of the sail. Only be also using the downhaul at the same time can you increase twist again while flattening out the top even more. So if you only want to reduce draft in the top of the sail but keep the same twist profile then you have to adjust both settings simultaniously. This is sadly one of the more easy ones. Setting the draft profile properly also requires resetting the spreader rake and or the diamond wire tension together with the mainsheet, downhaul AND mast rotation.

As an aerodynamists you have more then enough learning and experimenting to do on your F16 !

But that is all part of the fun right.

Wouter

Hi Wouter,



That's one of the reasons way I changed from Dart 18.

Thanks for the details, so far they are clear for me. We have noticeable wind shear only a couple of days per year, hence 'my light wind', is what you call medium wind. My concern is, that (maybe) I have used already all trim to reduce twist and still the lower part stalls first, even with maximum outhaul (e. g. not to much draft). Or is it maybe not possible to close the top of a square head sail? So what is with your sail with maximum closed leech, stalls at the top or bottom?
Next week-end I will try to relax the dimaond wires even more (spreader rake is not adjustable on my boat <img src="http://www.catsailor.com/forums/images/graemlins/frown.gif" alt="" /> ).

BTW: Are you a soccer fan? Excellent play of the orange team, maybe we will see another great match between Geramy and the Netherlands <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" />

Cheers,

Klaus

Ok, to be more accurate: On a reach or downwind in light winds (not yet heeling up), I would like to stall all over the sail at once, so I maximise lift. Upwind I would like to stall in the middle first. That is because I would like to have a good lift distribution over the mast height just before the stall to maximise drag/lift. That's my theory, not necessarily the truth.

Cheers,

Klaus

Large squaretop sails like a bit of twist along the leech. Pulling the leech tight on these sails with slow the boat down. The explanation given for this is often the unacceptable increase in drag because of having a large tip vortex.

On the French Sailing DVD Glenn Ashby says in an interview that his top is often around 300 mm twisting off, as taken from what would be a perfectly straight leech.

But you can feel it too. If the leech is to tight up top then the boats hangs up, it feels powerful but not fast. You heel easily but don't accellerate quickly in the gusts You don't get that nice "streaming" feeling that you would get with the right amount of twist.

Try it a few times, you will learn to feel it.

Wouter

Klaus,

if you are unable to get your top to stall first, it might be that you could benefit from having your sail recut. Usually I oversheet the squaretop by error in light winds and stall the top, not the other way around.
Do you know how much development have gone into that sail? Is it an old sail or a new one? Can the sail have stretched badly?

Oh, and pictures please.. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Put the boat over on the side, sheet in and put on downhaul, then do a picture upwards from the boom and one from behind the boat.

Thanks Rolf, that's exact the information I needed.

I can only guess, that they used an A-cat cut, but the mast is stiffer due to the reinforcement for the second trapez. Sometimes the leech between the top and second batten flutters a bit.

2004, pentex mylar from Petrucci, but not sailed much.

Could be, but I don't think so (6:1 mains sheet and sailed on lakes in the Alpes).

I will try to take some pictures next week end. (try = have the patience, not to be all the time on the water <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />).

Cheers,

Klaus

Wouter,

Do you have source for this DVD? Is it the one on Multihulls-world website? I can't seem to create an order there... <img src="http://www.catsailor.com/forums/images/graemlins/confused.gif" alt="" />

Mike.

Hmmm, what kind of battens do you have up there? Tapered and well tested? If the batten leech ends are too soft, you can get some flutter in the leech and more twist than you wanted in the top. Have the top inverted with full downhaul and strong wind?

If your mainsail goes slack in between battens in the leech in light winds, I would be tempted to speculate that there is too much luff round for your mast setup. Easiest and fastest way to test it is probably by putting the boat over on the hard and set mainsheet and downhaul, then adjust diamond tension. You could do the same on the water, but I like to get in the ballpark with the boat on its side and on the hard. Of course, that theory goes against your twist problem.. I would have put the boat over on its side and spent the time to find out what is really going on.

I think that is the same one, but give me a link to the multihulls-world order page and I'll check for you.

I seem to remember that Ricks online store featured these DVD's too !

Here is the link to the Rick White online store offer :

https://store.catsailor.com/ProductCart/pc/viewPrd.asp?idcategory=178&idproduct=13792

Wouter

Actually the battens are what I found in the sail, when I bought it.


No, never, but I think I never applied full downhaul, because I had already enough twist. Actually in strong winds, I am happy with the sail. However having some thoughts about the battens is never wrong.


Maybe I used the wrong wording in my previous posting: It does not go slack, it is tight, but the leech flutters. I think that the distance between the battens is just a bit to long. I do not really worry about that.


I lack the expirience to judge from the shape of the sail only. I need to go on the water and see the tell tales to understand what happens.

Rolf, thank you very much for the advices. Now, I know, that I should be able to stall the top. And that means that I have still to tweak on the settings. I will have a look at the batten(s) and try if I still can reduce the diamond tension before making them slack.

Cheers,

Klaus

Bingo!

I've placed my order.

Cheers!

A couple of questions to help my education.

1. Rolf: Can you explain too much power/drag. I dont understand.

2. Wouter: I was led to believe the only factor that stalls a wing is angle of attack. I think I understand what you are saying (i.e. tweaking changes the foil shape and therefore changes the stall angle of attack). Or am I misunderstanding you?

David

David,

when you increase the power by making the foil deeper (more draft) the drag will also increase. If we say the windspeed is relatively constant there will be a given draft which have the best lift to drag ratio for that windspeed. If we sail with more draft than optimum, we will be slower than somebody with optimum draft or even less than optimum as they have less power but also far less drag.

Perhaps I should do a graph..

This is all very theorethical and in real life it is most important before you get out on the trapeze. It is especially important in light winds in my opinion!

I'll dare to say something about the second question as well. Draft and airspeed are also part of deciding the point of stall to a greater or lesser degree depending largely on the speeds and angles you work with. That is my understanding at least.

There is a good trick you can use this full camber for. In light winds if the leg is long enough and you are having trouble catching/overtaking a nearby boat ahead when sailing to windward, try easing the outhaul and downhaul to increase camber and start to pinch slightly. If done properly you fall back a little but you lift above the other boat. You can then bear off a little and trim accordingly for greater boatspeed. This helps get you past the other boat. If they try luffing they slow down, plus you have more lift from your boards with the increased boatspeed.

Cheshirecatman

sounds fair....

Hi David,



Actually, there are many factors which let the sail or parts of the sail stall. Some are linked to the local angle of attack, the draft (camber), roughness, mast shape and perturbations and to a small amount to the speed (actually Reynolds number). Others are linked to the plan form of the sail and the its aspect ratio among many others.

The good thing of all these parameters is, that you don't have to care about them as long as you know if and where your sails stalls and how to stop it. Put tell tales in your sail and at the leech. They cause very very little extra drag, so don't be thrifty. I have them at three different heights each with a sequence of four or five tell tales from about 10cm behind the luff to the leech. With this (and a minimum transparancy of the sail of course) you will easly see the effect of sheet, downhaul and outhaul tension and mastrotation and -prebend.

Avoid stalling on the leeward side of the sail at upwind courses, if possible. But you may accept some stalling at the leeward leech if you are on a reach or going downwind. Stalling on the windward side is harmless and an indicator that you can tighten the sheet (if you are not over powered of course).

Hope that answers your question.

Cheers,

Klaus

Hi Rolf and all,



Finally I made some pictures of my sail. I was in a hurry because of an approaching thunder storm, so I didn't have the time to play around and I couldn't test the new diamond setting on the water.
The main sheet was tight, downhaul open, rotation open (at least should be open), outhaul tight and the diamonds close to slack. Analysed with your eyes, should I be able to hook the top leech more than that? Thanks.

Cheers,

Klaus

and #2

Sail doesn't have a good shape.

Using a tape measure with the sail laying flat on the ground I mark the sail at my max draft position. I use sticky back sailcloth, 3 on each side, contrasting colour to sail colour. I would relocate your tell tales to Max draft, one 15cm closer to the mast and a couple on the leech side of Max draft, same spacing. Should be easy to tune your sail with that setup and a couple of leech ribbons.

I read a quote from Dennis Conner once to the effect of "only an idiot uses the battens that come with a sail". Typically blunt but it is a good point. Battens need work too.

Darryn
Mozzie
1782

Hi Darryn,

thanks for the hints. Please could you specify what is wrong with sail shape and battens (e.g. too much camber, too flat, too hard, too soft, etc.)? Maybe you have a picture of a well shaped sail, which you want to post.

There are tell tales in the sail, but the poor quality of the digital camera makes them hard to see. However I know where and when the sail stalls (just above the boom). But I am not (yet) able to stall the sail at the top as well (e.g. cannot close the leech enough?).
Thanks.

Cheers,

Klaus

Looks to me like max draft is too far back at the bottom of the sail and too far forward towards the top.

At the second from top batten the leach seems to hook inwards. I would try a stiffer batten.

Try sanding the bottom batten to encourage some bend at max draft. Not much sanding. The batten also doesn't exit the batten holder on the luff squarely. Loose?

Looks like it needs more downhaul. Some more pics with increasing amounts of downhaul and mainsheet tension would be interesting. Force your rotation to your normal upwind setting. Might be all your sail needs as there is not much mast bend. Position your boat on a beat when you take the pics, need some wind too, hold the boat down. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Marking your sail and moving your telltales takes the guesswork out of setting your sail up.

Strangely I have some pictures of mainsails from the same angle <img src="http://www.catsailor.com/forums/images/graemlins/smirk.gif" alt="" /> I will find a real ugly one and we can pick it to bits.

Darryn
Mozzie
1782

Good sail, but not setup right.

Whats wrong?

Darryn
Mozzie
1782

to much pre-bend causing the draft to move forward?

Looks like the top is closed. Maybe a lot of prebend and no rotation?

Regardless of what I think, I believe the direction we're going in on this thread (photos with answers) is GENIOUS.

Keep it up!

Lot of cunningham tension and slack diamond wires (or is this an optical effect?). Very little rotation.

Klaus, does the sail stall first above the boom on the windward side when pointing or on the leeward side when footing?

On the leeward side when footing. For light wind sailing I woule like to stall the whole sail, not only the lower part.

Cheers,

Klaus

So if we are talking about 5knts+ (no laminar boundary layer) you want a fuller sail downstairs and a flatter sail upstairs. I would begin loosening a bit of outhaul (which gives you the lower part) and maybe try with a little more of rotation (which combined with sheet tension gives you the upper part). If the diamond wires are almost slack, maybe you want to try a little bit more of prebend by applying diamond tension, so the mast follows the luff curve better (I think a little of prebend is necessary in all winds). Cheers, Claus <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

By the way, a nice discussion about under/over rotation and wingmasts can be found here: http://www.tspeer.com/Wingmasts/teardropPaper.htm

that's a good link. Actually my problem is not how the best trim would look like in theory, but how can I close my leech in reality.
For strong wind my sail looks like I wish. In light winds I couldn't close my leech as much as I wished (with tight main sheet and open downhaul). Maybe my wish of a closed leech is not realistic at all, but with your suggestions it could be that the battens in the top are too stiff or I still have too much pre-bend or the sail cut is not ok. Hopefully I will have better wind next week end, so I can figure it out and make some more pictures. So far thanks for your comments.

Cheers,

Klaus

I spent 1 minute with accumeasure (download on the Uk Halsey site), result attached. No significant twist in that mainsail, but you really should flip the boat over when taking photos like this, so you can get some force pulling on the leech.

Lots of gotchas to look out for in that analyzis, especially the top one where I dont know the angle of the batten! I would say the sail is a bit deep for my taste all over, but the twist profile is where I would expect it to be with no kind of force pulling on the leech.

I think your problem is not the leech but the sail shape in the lower part, i.e. chamber too much aft and too flat close to the boom.

In this photo downhaul and mainsheet are cranked on as hard as possible, much more then sailing setting, still cannot pull the depth out of the lower part of the sail. We decided the mast wasn't bending enough below hound fitting to suit the luff curve so Gordon (former owner in pictured) recut the luff curve in that area. The sail is now competitive and has won races against other Mozzies, last by a long way before recut.


Darryn
Mozzie
1782

Darryn,

Do you by chance have a photo before the recut?

The mozzie mast is quiet a different animal to the F16 mast.!

Marcus

Thanks Rolf,

as I said, I don't how to use this numbers. But I have found a pictures of Gilos Blade Sail with open downhaul and it looks pretty much like my sail, at least in my eyes.
I agree with the excessive camber, but with the downhaul I can reduce it quite efficient. No pics in the moment, sorry.
I will continue with on the water testing, because I can understand what tell-tales tell to me.



Claus,

I tried various outhaul tension (on the water) and I am pretty sure that neither the sail is to flat there nor is the camber to much aft. Both more camber and more forward position of camber let the bottom stall earlier.

Thanks again for your input.

Cheers,

Klaus

From the pics I think your leech is too tight.

The larger the squaretop is the more leech twist you want "see".

I write "see" as in this case it is different from actually having more leech twist. Basically the twist angle is the same but the large head shows more falling away to leeward at its tip.

Wouter

Klaus, where are you sailing? I'd like to have a look at your sail in flesh since I believe my A-Kat sail has the same problem.

Baltic,

Very good idea. I will give you a call later.

Wouter,

I agree with you for upwind courses. But if I am on a reach, drag doesn't matter, only lift until you can't hold the heeling moment any more. And you get the maximum lift out of a sail or wing, if you bring all parts of it close to stall at the same time. And exactly for this condition I want (or I hope at least) to set up the sail as well. I can always open my leech with the downhaul for the upwind leg.
Maybe it is not possible to stall the top with a square head sail + wing mast and without jib (which protects the lower and middle part of th emain from stalling) <img src="http://www.catsailor.com/forums/images/graemlins/frown.gif" alt="" />, but before I make early assumptions I will listen if you guys can stall the top of a cat rigged square headed and wing masted boat.

Cheers,

Klaus

Hi Marcus, that is the photo before the recut. Looking for the ability to flatten the sail completely, a small pocket down low is Ok but the depth extending all the way up to the hound fitting with all adjustments maxed means that when downhaul and mainsheet are eased to normal sailing positions you cannot point. Fast downwind but low VMG upwind. I'm sure you know this already with your sailmaking skills, I'm putting this on here for those who might not.

Mozzie pear shape mast and pin head sail is very different to F16 wing mast and square top which this topic is refocusing on, a cue for a timely exit I think <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Darryn
Mozzie
1782