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A-Cat Mast Question


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#1 bhyde

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Posted 23 March 2012 - 02:46 PM

Noobie A-Cat mast question.

We have a Firberfoam medium mast and we need to do a little repair where the main shrouds attaches to the mast. Our particular setup has three spectra loops exiting the front of the mast that are used to attach the main shrouds, forestays, and traps. Each loop is about 25mm long. The problem is the spectra loop for the main shrouds is a bit worn and we would like to replace it. It appears that the spectra loop is just glued into the mast with a knot on the inside to prevent it from pulling out of the (hopefully) backing plate.

Is this a typical setup for an A-Cat? I'm guessing that I just need to cut the loop and then gently drill the glue out of the hole to allow another loop to be slipped in (from inside the mast). Has anyone run into this before?

Looking at the fiberfoam website, it appears that the "normal" setup uses a custom SS T-ball fitting for attaching the shrouds/forestays/traps, but that isn't there. Got to say, I'm a little confused here.

#2 Remodel

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Posted 23 March 2012 - 04:11 PM

Noobie A-Cat mast question.

We have a Firberfoam medium mast and we need to do a little repair where the main shrouds attaches to the mast. Our particular setup has three spectra loops exiting the front of the mast that are used to attach the main shrouds, forestays, and traps. Each loop is about 25mm long. The problem is the spectra loop for the main shrouds is a bit worn and we would like to replace it. It appears that the spectra loop is just glued into the mast with a knot on the inside to prevent it from pulling out of the (hopefully) backing plate.

Is this a typical setup for an A-Cat? I'm guessing that I just need to cut the loop and then gently drill the glue out of the hole to allow another loop to be slipped in (from inside the mast). Has anyone run into this before?

Looking at the fiberfoam website, it appears that the "normal" setup uses a custom SS T-ball fitting for attaching the shrouds/forestays/traps, but that isn't there. Got to say, I'm a little confused here.

I've never seen the arrangement you describe, and it sounds fishy to me. However, I haven't been around A-Cats for several years.

I'd look at something like this:

Posted Image
Available at http://www.apsltd.co...tesj-hooks.aspx.

#3 TheFlash

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Posted 23 March 2012 - 06:58 PM

Hey Bob - I'd expect that there's something more than a simple know back there, and maybe a tight washer or something to keep that knot from undoing itself under creep loads and slipping right out of the mast. Colligo has a "soft shackle' diagram on their website - and you wouldn't need to create a shackle, but the knot is sort of Monkeyfist style and is meant to hold together. I suppose a little epoxy in the knot would help as well.

#4 Foghorn77

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Posted 23 March 2012 - 10:27 PM


Looking at the fiberfoam website, it appears that the "normal" setup uses a custom SS T-ball fitting for attaching the shrouds/forestays/traps, but that isn't there. Got to say, I'm a little confused here.


Bingo!!! All that I have owned or seen for that matter use the T-ball. Hall spars can supply everything you need.
I'd switch it over. You'll gain a little weight aloft but also gain great peace of mind.

#5 Lost in Translation

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Posted 24 March 2012 - 02:07 AM

I think I know the mast you got, and I bet it is set up in a really light and efficient way with just knots. You should ask the person you bought it from how best to maintain it. Most have hardware just as the others above have described.

#6 Trevor B

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Posted 24 March 2012 - 03:08 AM

I think I know the mast you got, and I bet it is set up in a really light and efficient way with just knots. You should ask the person you bought it from how best to maintain it. Most have hardware just as the others above have described.

Spectra loop with a "diamond" knots inside the mast works surprisingly well and it doesn't weigh anything.

#7 Wilpy

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Posted 05 April 2012 - 11:50 PM

Personally I would stay with the rope option rather than goping back to metal.
I sail on a Farrier F9R (approx 2.2t) and all the shrouds are rope and attached with soft lashings and loops into the rig.
If they are chaffed you should be able to slide them out with a mouse line attached and pull a new one in.

#8 Catnewbie

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Posted 06 April 2012 - 09:31 AM

BHYDE,

What you describe, is a smart, cheap and ultra-light device.

Could you post some pics, of the inside knots & arrangements when you will have put it appart

Thanks in advance

Cheers all

W

#9 bhyde

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Posted 06 April 2012 - 02:12 PM

BHYDE,

What you describe, is a smart, cheap and ultra-light device.

Could you post some pics, of the inside knots & arrangements when you will have put it appart

Thanks in advance

Cheers all

W

Yes I think your right. After looking at it a little more, we decided that the setup is pretty good. It's light, simply, and keeps chafing and windage to a minimum. Replacing the strops is no more inconvenient than replacing the metal parts normally found up there. I'll try to get some pix the next time I'll down at the boat.

Now, if someone would like to explain why most of the A-Cats I see have de-rotators attached to the trampoline instead of the boom, I'd appreciate it. On my F24 the de-rotator arm is connected to the boom with an adjustment line, and thus, the mast rotates at the same angle to the boom when the mainsheet is adjusted. This seems like the ideal arrangement, and considering mast rotation is so important on the A-Cat, I find it strange that this isn't used universally. Anyone have any thought on why "decoupling" the de-rotator from the boom is a better arrangement. Lot's to learn.

#10 Bang Zoom

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Posted 06 April 2012 - 05:54 PM


BHYDE,

What you describe, is a smart, cheap and ultra-light device.

Could you post some pics, of the inside knots & arrangements when you will have put it appart

Thanks in advance

Cheers all

W

Yes I think your right. After looking at it a little more, we decided that the setup is pretty good. It's light, simply, and keeps chafing and windage to a minimum. Replacing the strops is no more inconvenient than replacing the metal parts normally found up there. I'll try to get some pix the next time I'll down at the boat.

Now, if someone would like to explain why most of the A-Cats I see have de-rotators attached to the trampoline instead of the boom, I'd appreciate it. On my F24 the de-rotator arm is connected to the boom with an adjustment line, and thus, the mast rotates at the same angle to the boom when the mainsheet is adjusted. This seems like the ideal arrangement, and considering mast rotation is so important on the A-Cat, I find it strange that this isn't used universally. Anyone have any thought on why "decoupling" the de-rotator from the boom is a better arrangement. Lot's to learn.


personal prefference.

#11 TornadoCAN99

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Posted 06 April 2012 - 10:46 PM


BHYDE,

What you describe, is a smart, cheap and ultra-light device.

Could you post some pics, of the inside knots & arrangements when you will have put it appart

Thanks in advance

Cheers all

W

Yes I think your right. After looking at it a little more, we decided that the setup is pretty good. It's light, simply, and keeps chafing and windage to a minimum. Replacing the strops is no more inconvenient than replacing the metal parts normally found up there. I'll try to get some pix the next time I'll down at the boat.

Now, if someone would like to explain why most of the A-Cats I see have de-rotators attached to the trampoline instead of the boom, I'd appreciate it. On my F24 the de-rotator arm is connected to the boom with an adjustment line, and thus, the mast rotates at the same angle to the boom when the mainsheet is adjusted. This seems like the ideal arrangement, and considering mast rotation is so important on the A-Cat, I find it strange that this isn't used universally. Anyone have any thought on why "decoupling" the de-rotator from the boom is a better arrangement. Lot's to learn.


Modern cats typically use tramp connected rotators...the benefit is you have better options for controlling rotation while on the wire and for locking rotation in position...good for light wind in waves. I've sails Tornados with either setup and now use the low mount option full time.

#12 Wandering Geo

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Posted 06 April 2012 - 10:57 PM

Now, if someone would like to explain why most of the A-Cats I see have de-rotators attached to the trampoline instead of the boom, I'd appreciate it. Anyone have any thought on why "decoupling" the de-rotator from the boom is a better arrangement. Lot's to learn.
personal prefference.

Simple, light, low, less drag, easy, precise, repeatale adjustment, but BZ is right mostly personal pref.
Fiberfoam make both types of rotators. Check here Fiberfoam mast parts
Also have a look at the current removable hounds tee parts A12A and/or A12B

#13 SimonN

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Posted 06 April 2012 - 11:57 PM


Now, if someone would like to explain why most of the A-Cats I see have de-rotators attached to the trampoline instead of the boom, I'd appreciate it. Anyone have any thought on why "decoupling" the de-rotator from the boom is a better arrangement. Lot's to learn.
personal prefference.

Simple, light, low, less drag, easy, precise, repeatale adjustment, but BZ is right mostly personal pref.

I agree with this but I think there is more going on as well. The first priority is being able to adjust the rotation from out on the wire (as pointed out above) as this is a vital control in lighter conditions. I am only just beginning to understand how much this is adjusted. I know that Steve Brewin is constantly adjusting rotation as he is first sitting on teh side, then marginal trapezing, then flat trapping and back down through the sequence as the wind dies back. In light, shifty stuff he is also adjusting it before going into a tack and then when back up to speed.

The next thing is that on the A, I don't believe that the decoupling effect isn't actually that great. Going upwind, the traveller is on the centreline and easing the sheet simply twists the sail, rather than see the boom go out very much, so the relationship between boom and rotation stays very similar. It is similar offwind, where you fix the traveller in a psoition and play with leach tension. Yes, you also move the traveller position but it doesn't have a big effect on the relation between the mast and boom. More importantly, you are setting the rotation for the apparent wind while you are adjusting the sail for both apparent wind, optimum shape and power. In some ways, it seems to me to decouple the mast and sail makes sense.

#14 Catnewbie

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Posted 07 April 2012 - 12:10 PM

BHYDE,

With regards to the rotation set-up, I would make this comment;


The "indexation" of mast rotation with boom, is useful especially when sailing a leg @ 90° of the wind, just like when regattas were raced around a triangle instead of "banana" race like today.

In this case, as long as the traveller was fitted with a cleat in the middle of the rear cross-beam, and not on the traveller itself, it was very efficient to manage gusts only with the traveller. In this case the sail section remains constant and you manage only its AoA; not very academic, but much less tiring than managing the gust with the main pulley-block. Depend how old you are.

Windward, it si better to manage gust with main sail pulley-block, and if the mast is indexed with the boom, the rig's answer to easing is probably a bit less "clear" than with the mast rotation independent from the boom.

Tuning the mast windward has changed with the new masts, They have a longer chord (150 to 145 mm vs 135 mm and are a bit thinner 58/59 mm vs 64/63 mm for the old RIBA section.

These thinner section appreciate less angle with the sail, in doing so you have a little turbulence on both sides of the mast/sail luff, instead of a big dead zone on the winward side, when using a thicker mast section with a bigger angle. The overall section drag is lower with new section/lower angle.

Somewhere on this forum or on Boatdesign.net Mr Tom Speer provides a very clear & academic explanation of this issue.

With mast control independent from the boom, a little easing of the main, will flatten the top of the sail/mast section quicker than with the mast/boom connection.

Another difference, is simplicity: A control arm at the mast foot + a ringed hole in trampoline and one cleat on each hull.

Indexing mast rotation on the boom requires more stuff and the marginal efficiency of this complexity is probably very marginal, unless you are a bit old and if you want to sail inter-series races with a lot of leg @ 90° of the wind.

Hope it can help

Regards

W

#15 dacarls

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Posted 09 April 2012 - 01:35 AM

SimonN has it stated much better here. Steve Brewin's technique briefly described for mast rotation adjustments is generally known, and is critically important for speed in A-class cats.

#16 F-18 5150

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Posted 09 April 2012 - 04:41 AM

Most newer cats are going bottom rotation control. The newer F-18's have a double setting. One adjustable from the wire and one that is a blow all the way for downwind. So your upwind setting will be the same and you just blow the second cleat then when you round the bottom mark just pull the second cleat back in full and your set.

#17 bhyde

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Posted 09 April 2012 - 03:12 PM

Thanks guys. I'm thinking the de-rotator decoupled from the boom makes more sense per Simon's explanation. Exactly opposite of what I previously thought.

#18 Waynemarlow

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Posted 09 April 2012 - 09:36 PM

OK lower mast rotation it is, but I have a problem with the control line, has anybody got a diagram of the way it is rigged, at my club we do up to 4 laps per race and despite having a long length from each side of the rotator being pulled into the front beam by a 2:1 bungee, eventually you run out of line as you always go downwind the same way from the top buoy, feeding out the line from the same side.. What it needs is some sort of endless loop. Any clues.

#19 Lost in Translation

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Posted 09 April 2012 - 09:44 PM

the ends of my rotator lines run through the trampoline by the front beam and are tied together. It's not continuous but at least you don't need a take up system. I re-center the system every so often during the day.

#20 Tcatman

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Posted 09 April 2012 - 11:11 PM

OK lower mast rotation it is, but I have a problem with the control line, has anybody got a diagram of the way it is rigged, at my club we do up to 4 laps per race and despite having a long length from each side of the rotator being pulled into the front beam by a 2:1 bungee, eventually you run out of line as you always go downwind the same way from the top buoy, feeding out the line from the same side.. What it needs is some sort of endless loop. Any clues.


I mount a fairlead at the stern end of the rotator a cleat (midway ) and a turning block near the mast on the rotator arm centerline. The line with a stopper ball goes through the fairlead, cleat and block and ties off to the control lines coming through the tramp just underneath the rotator. when the line is pulled tight... you are at your upwind settings... when you go down wind.... (Mild especially) you reach in and blow the cleated line... allowing the rotator to go to you 90 setting... (stopper ball at the fairlead) When you go through the gate... you pull on the line and you are back to your previous upwind settings.... Eventually you have to recenter ...

The alternative of stichhing the line together...... works only if the mast never comes down (or you like to sew).

Stolen from someobody who stole it from somebody who probably stole it from an aussie!... its the way of the class...

#21 Bang Zoom

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Posted 10 April 2012 - 11:02 AM

OK lower mast rotation it is, but I have a problem with the control line, has anybody got a diagram of the way it is rigged, at my club we do up to 4 laps per race and despite having a long length from each side of the rotator being pulled into the front beam by a 2:1 bungee, eventually you run out of line as you always go downwind the same way from the top buoy, feeding out the line from the same side.. What it needs is some sort of endless loop. Any clues.


Sew the line in an endless loop. Line goes from cleat on deck under tramp thru hole in tramp to block on rotator (which is held on with a shackle for mast removal) back thru same hole to cleat on other side of boat. the tail on both sides goes thru a block at the shear and back under the boat to the other hull. this allows you to cleat from the other side.

#22 WetnWild

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Posted 10 April 2012 - 01:51 PM


OK lower mast rotation it is, but I have a problem with the control line, has anybody got a diagram of the way it is rigged, at my club we do up to 4 laps per race and despite having a long length from each side of the rotator being pulled into the front beam by a 2:1 bungee, eventually you run out of line as you always go downwind the same way from the top buoy, feeding out the line from the same side.. What it needs is some sort of endless loop. Any clues.


Sew the line in an endless loop. Line goes from cleat on deck under tramp thru hole in tramp to block on rotator (which is held on with a shackle for mast removal) back thru same hole to cleat on other side of boat. the tail on both sides goes thru a block at the shear and back under the boat to the other hull. this allows you to cleat from the other side.


I use that system with a slight enhancement. Where the line comes out of the cleat on each side it then runs through a small block tied by a 50mm loop to the sidestay tang. That makes it quick and easy to grab the tail and lift it to uncleat to ease rotation at the top mark or when needing to blow the rotation more when going downwind on the wire.

#23 Bang Zoom

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Posted 10 April 2012 - 02:00 PM



OK lower mast rotation it is, but I have a problem with the control line, has anybody got a diagram of the way it is rigged, at my club we do up to 4 laps per race and despite having a long length from each side of the rotator being pulled into the front beam by a 2:1 bungee, eventually you run out of line as you always go downwind the same way from the top buoy, feeding out the line from the same side.. What it needs is some sort of endless loop. Any clues.


Sew the line in an endless loop. Line goes from cleat on deck under tramp thru hole in tramp to block on rotator (which is held on with a shackle for mast removal) back thru same hole to cleat on other side of boat. the tail on both sides goes thru a block at the shear and back under the boat to the other hull. this allows you to cleat from the other side.


I use that system with a slight enhancement. Where the line comes out of the cleat on each side it then runs through a small block tied by a 50mm loop to the sidestay tang. That makes it quick and easy to grab the tail and lift it to uncleat to ease rotation at the top mark or when needing to blow the rotation more when going downwind on the wire.


Like this

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#24 erikM

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Posted 02 June 2012 - 06:51 AM

As an A cat beginner I'll be happy to read in this forum about the basic principles of mast rotation adjustments.
Some practical things such as:
Upwind in light, medium and heavy winds, the back of the mast should be pointing towards the centerboard, rudder pin, totally centered or ...
I have the same question for downwind (flat and flying a hull).

#25 erikM

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Posted 05 June 2012 - 05:30 AM

As an A cat beginner I'll be happy to read in this forum about the basic principles of mast rotation adjustments.
Some practical things such as:
Upwind in light, medium and heavy winds, the back of the mast should be pointing towards the centerboard, rudder pin, totally centered or ...
I have the same question for downwind (flat and flying a hull).



#26 erikM

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Posted 05 June 2012 - 05:35 AM

I have notreceived any answer to my questions about mast rotation.

I want to apologizefor asking such basic and possibly stupid questions. I fully understand that Aclass stars are more interested in esoteric issues such as “how stiff a stiffplatform should be” or "what is the optimum mast sideways flexibility, above shroud attachments".









#27 macca

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Posted 05 June 2012 - 06:16 AM

Thanks guys. I'm thinking the de-rotator decoupled from the boom makes more sense per Simon's explanation. Exactly opposite of what I previously thought.



If you have the gooseneck fitting on the mast made longer than the std fittings it is possible to have the rotation automatically adjust the outhaul settings for upwind and downwind. The more wind you get the less rotation you have upwind and the longer gooseneck fitting will push the boom end out more hence tightening the outhaul.

And when you go downwind you ease the rotation and you end up with a loose outhaul, If you get the length correct on the outhaul you will have a single setting for up and downwind.

#28 juniordave nz

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Posted 05 June 2012 - 08:46 AM

As an A cat beginner I'll be happy to read in this forum about the basic principles of mast rotation adjustments.
Some practical things such as:
Upwind in light, medium and heavy winds, the back of the mast should be pointing towards the centerboard, rudder pin, totally centered or ...
I have the same question for downwind (flat and flying a hull).


I'll answer with what I have found. But every boat/mast/sail combination is different.

Upwind light: Mast rotation at maybe 50degree off centre. Look to the luff, this should give you a nice full luff with a smooth transition from the mast to the sail.
Going up the wind range, bring the mast more towards centre and continue to look at the luff and try to keep that nice smooth shape.

Downwind light: Easy close to 90 degree, maybe 80. Again look to the luff for a good shape and telltales.
Again bring rotation back a little as wind strength increases and apparent wind goes forward.

I'm pretty new to the A-class as well and have a 30year old boat that I rebuilt will an alloy mast that's nice and bendy.
But the principles are still pretty much the same and I race with guys on DNA's and ASG3s that have been around in the class for years and have helped me out a lot.

But how I worked most of my trim out, is that I sailed along staring at the sail and not where I'm going (helps to have consistent wind for this). Then play with every sail control line you have.

#29 Lars Schrøder D13

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Posted 05 June 2012 - 10:02 AM

Put some tickles along the luff curve at the spreaders and at the forestays (as in all ashbisail). When they doesnt move on either side of the sail, the rotation is correct (rather move at little on the inside than outside).

Best

Lars

#30 erikM

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Posted 06 June 2012 - 06:11 AM

Thanks for your answer, but I think there is more to it than that.


Upwind, I believe most of us keep the telltale attached to the upwind frontstay (materializing the apparent wind) pointing more or less to the front face of the mast. The angle between apparent wind and boat centreline is then about 30°.
Keeping the mast also oriented at a 30° angle would certainly ensure the tickles on both sides of the sail, nicely floating as per your advice.
But this 30° angle and thus mast rotation would then remain constant whatever the wind speed .

On the other hand I thought that people would play with mast rotation to decrease the sail power in increasing wind speeds. I even heard that some people would center the mast (keeping then a very small angle say 10°) when overpowered.

I'd appreciate further insight.



#31 Lars Schrøder D13

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Posted 06 June 2012 - 06:55 AM

I havent thougth about any connection between the ticklers at the forestays, the mastrotation and the apperent wind - and Im not planning to do it in the future, either;-)

I do play a lot with the mastrotation and the ticklers at the luffcurve has worked fine for me.

If you come up with something smarter, please share with the rest of us (you should perhaps take a look at the dingenot Marstrom is making for getting the rigth mastroation). And if you are good at dutch (Im not) you can take a look at these comments by Chris Field - http://www.a-cat.nl/...-007-a-cat.html

Best

Lars

#32 erikM

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Posted 06 June 2012 - 09:36 AM

I havent thougth about any connection between the ticklers at the forestays, the mastrotation and the apperent wind - and Im not planning to do it in the future, either;-)

I do play a lot with the mastrotation and the ticklers at the luffcurve has worked fine for me.

If you come up with something smarter, please share with the rest of us (you should perhaps take a look at the dingenot Marstrom is making for getting the rigth mastroation). And if you are good at dutch (Im not) you can take a look at these comments by Chris Field - http://www.a-cat.nl/...-007-a-cat.html

Best

Lars



Lars, be assured that I have nothing to share, I am merely trying to understand how the system works. I would have guessed that mast rotation had everything to do with apparent wind and perhaps depth of the sail plan.
Considering the technical level of some of the threads I read, I am convinced that several of the top sailors in this community would be able to explain in simple but rational words this rotation stuff.
I am glad that your principle has worked for you, but I need to understand before applying (we are not all naturally talented).

#33 Lars Schrøder D13

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Posted 06 June 2012 - 10:48 AM

The third or fourth part of the equation is the speed of the apperent wind. Low windspeed = more depth in the sail and therefore the mast further out.

But hopefully some more advanced sailors will chime in as well......

Best

Lars

#34 Kshack

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Posted 06 June 2012 - 01:11 PM

Try this article by Landenberger. Should answer your questions about rotation, etc.

http://www.usaca.inf...nical&Itemid=31

#35 Lost in Translation

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Posted 06 June 2012 - 06:39 PM

Hi Erik,

I think there are some good resources posted here. Lars, I hadn't seen the article above. Pretty cool. If you want more info, it might be a good idea to post more about boat and experiences. so that people can have some context with which to comment. I am not an expert at all but can share some things I have learned / heard over time to get you started based on my sailing.


Rotation contributes a large amount to the overall camber of the sail as others have said. It is good to think about the sail and mast together as one unit. While the apparent wind when sailing upwind may be pointing as you describe, you don't necessarily want the same camber all of the time. In very light air you may not want much, then you power up, and then you depower again. As you note, when looking for power you can rotate out more. You can pull rotation back in as the breeze builds. In chop you will generally need more shape and in flat water less. In general people talk about "rotation in" or "back" as putting the mast more in line with the direction of the boat.

The boards and other items on these boats are placed in different positions depending on the design so it is a hard to give a rule of them on where the rotation should point as often done with other boats. You can play with it though to get a good feel. Generally too much rotation out will make the boat have a poor top end and be too draggy. Too much rotation in will leave you looking for power to get up to speed. You can actually not only feel this but even hear it if the mast is really eased in breeze as you sail upwind.

The Landy article Kshack posted above does a great job of explaining the trade offs in mast rotation relative to sail shape and breeze and is a classic. The new rigs really change everything too. With my old mast more rotation out really made the sail flatter when I sheeted hard just as Landy writes because the mast bent quite a bit side to side. With the new mast, I can feel the side to side bend is much stiffer. This gives a sense of more power in light air when rotation is out and the leech is still stiff. It is really pretty cool. In the breeze I generally under rotate the mast just as you say and with either the new mast or the old mast that works well. The new mast seems more sensitive to rotation and might be rotated a little less in general up wind.

Most people have trampolines with marks to track how far out the mast is rotated. I do not have the actual angles indicated by the marks but I vary between out to #3 and into #1 on these marks for upwind I think. Downwind in light air people will try to achieve more than 90 degrees of rotation and then go back to maybe 80 in breeze. In heavy breeze, you can pull on more rotation and try to depower the boat. There are some more sophisticated ways to measure mast rotation that I believe Marstrom has but I don't know this system.

Rotation is actually something that I think you will be able to feel fairly easily when sailing once you get the general principle and it is something with unique tuning to your mast, weight, sail, breeze, and water conditions. It is still stunning to me how much the boat's power changes in easing rotation in the bear away at the top mark (the boat feels like it gets a new puff) or when I pull rotation back on when setting up for the leeward mark (the boat feels like it just lost some breeze). I have heard it can be better to carry the rotation out when rounding the leeward mark and pulling it back in as you start upwind much like delaying a spinnaker drop until the end. I'm generally struggling with other stuff on the leeward rounding though!






#36 erikM

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Posted 06 June 2012 - 07:42 PM

Hi Erik,

I think there are some good resources posted here. Lars, I hadn't seen the article above. Pretty cool. If you want more info, it might be a good idea to post more about boat and experiences. so that people can have some context with which to comment. I am not an expert at all but can share some things I have learned / heard over time to get you started based on my sailing.


Rotation contributes a large amount to the overall camber of the sail as others have said. It is good to think about the sail and mast together as one unit. While the apparent wind when sailing upwind may be pointing as you describe, you don't necessarily want the same camber all of the time. In very light air you may not want much, then you power up, and then you depower again. As you note, when looking for power you can rotate out more. You can pull rotation back in as the breeze builds. In chop you will generally need more shape and in flat water less. In general people talk about "rotation in" or "back" as putting the mast more in line with the direction of the boat.

The boards and other items on these boats are placed in different positions depending on the design so it is a hard to give a rule of them on where the rotation should point as often done with other boats. You can play with it though to get a good feel. Generally too much rotation out will make the boat have a poor top end and be too draggy. Too much rotation in will leave you looking for power to get up to speed. You can actually not only feel this but even hear it if the mast is really eased in breeze as you sail upwind.

The Landy article Kshack posted above does a great job of explaining the trade offs in mast rotation relative to sail shape and breeze and is a classic. The new rigs really change everything too. With my old mast more rotation out really made the sail flatter when I sheeted hard just as Landy writes because the mast bent quite a bit side to side. With the new mast, I can feel the side to side bend is much stiffer. This gives a sense of more power in light air when rotation is out and the leech is still stiff. It is really pretty cool. In the breeze I generally under rotate the mast just as you say and with either the new mast or the old mast that works well. The new mast seems more sensitive to rotation and might be rotated a little less in general up wind.

Most people have trampolines with marks to track how far out the mast is rotated. I do not have the actual angles indicated by the marks but I vary between out to #3 and into #1 on these marks for upwind I think. Downwind in light air people will try to achieve more than 90 degrees of rotation and then go back to maybe 80 in breeze. In heavy breeze, you can pull on more rotation and try to depower the boat. There are some more sophisticated ways to measure mast rotation that I believe Marstrom has but I don't know this system.

Rotation is actually something that I think you will be able to feel fairly easily when sailing once you get the general principle and it is something with unique tuning to your mast, weight, sail, breeze, and water conditions. It is still stunning to me how much the boat's power changes in easing rotation in the bear away at the top mark (the boat feels like it gets a new puff) or when I pull rotation back on when setting up for the leeward mark (the boat feels like it just lost some breeze). I have heard it can be better to carry the rotation out when rounding the leeward mark and pulling it back in as you start upwind much like delaying a spinnaker drop until the end. I'm generally struggling with other stuff on the leeward rounding though!






Thanks for your answer "lost in translation". It is slowly getting clearer. However not fully clear yet. I'll give you a simple example. You (as most people do) say that downwind in light air the mast should be rotated at 90+ °. I don't know about you but if I sail with the tell tale (attached to the upwind front stay) or tickle (whatever the correct word is) at 90°, I go very slowly. I usually sail with this tickle at say 70° and maybe 60°. I hope I am not the only one to do so. In that case, why should the mast be rotated at 90+°? It would seem that to get a laminar flow on both sides of the sail the mast should be rotated at about the same angle as the apparent wind (so about 70°).




#37 Lost in Translation

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Posted 06 June 2012 - 08:19 PM

Erik, this relates to the point about considering the mast and sail together as one unit rather than just the mast. The idea is to get a lot of camber in the sail to have power. To do this does not mean the mast profile should be minimized to the wind but rather be used to help make a deeper and more powerful sail. Perhaps think of it like the leading edge of an airplane wing that extends a front flap. This flap points down somewhat to help the wing create more lift (and drag). It does not point straight into the apparent wind.

At any rate, try it because experience and real world results trump theory. You will see the boat performs a little better even though you are sailing a little higher than 90. It may also help you sail a little lower.

#38 erikM

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Posted 07 June 2012 - 09:53 AM

Erik, this relates to the point about considering the mast and sail together as one unit rather than just the mast. The idea is to get a lot of camber in the sail to have power. To do this does not mean the mast profile should be minimized to the wind but rather be used to help make a deeper and more powerful sail. Perhaps think of it like the leading edge of an airplane wing that extends a front flap. This flap points down somewhat to help the wing create more lift (and drag). It does not point straight into the apparent wind.

At any rate, try it because experience and real world results trump theory. You will see the boat performs a little better even though you are sailing a little higher than 90. It may also help you sail a little lower.


OK That makes sense. Thanks to all.

#39 AUS

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Posted 07 June 2012 - 01:58 PM

Lost,

Great posting! You have made every point very clear…. My only addition is to be careful when de-rotating upwind in a breeze at the top end. You will get to a point close to center (last couple of inches) where the mast sideways stiffness takes over and actually powers up the leach instead of allowing the top of the mast to work as it is suppose to. This is more evident on the older masts where the top is not as dynamic fore and aft as the new masts.
Also worth mentioning while on this topic, the day is gone of just dumping downhaul and rotation on the new rigs. As you mentioned, due to the extra dynamic bend of the new masts, the mast rotation has a lot more affect on the overall camber and power of the sail. There seems to be a point downwind where you need to keep a little control of the power depending on the wind and sea state. The power also needs to be matched to your mode of downwind sailing. Wire running doesn’t always mean you are looking for max power. The power control starts to take affect when it gets around 15kns and or when you start to get into chop. When you are on the wire downwind you actually create a lot more power than you may realize, having this balanced and controlled is the trick to affective wire running. Too much power and you will soak to low to control, thus loosing flow. To little power and you will be searching the high lane for power also reducing VMG. Get this balanced and it is a sweet feeling and makes the boat relatively easy to sail. The smoother you get the more power you can afford to carry. I am finding mast rotation as well as the traveler height is very critical to setting a balanced mode.
As a disclaimer the above notes are simply theories of mine that have evolved during my processes. I am looking forward to checking in with the good guys in a few weeks to asses that my progression is on the right path.
ericM Please don’t get discouraged when you don’t immediately get the answer you are looking for. There is no text book on this subject. The closest you will get is the class specific articles by guys like Landy that are worth reading from top to bottom a few times. like myself I think most people are reluctant to post any information as factual as there is many different theories out there even amongst the good guys. If you are after a specific answer on something it is worth starting a new thread so not to risk being over looked.
you have the right approach, keep asking questions but keep an open mind. Try and translate your thought processes into practice. There is no substitute to time on the water.
Happy sailing

#40 A-man

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Posted 07 June 2012 - 03:49 PM

Thanks for your answer "lost in translation". It is slowly getting clearer. However not fully clear yet. I'll give you a simple example. You (as most people do) say that downwind in light air the mast should be rotated at 90+ °. I don't know about you but if I sail with the tell tale (attached to the upwind front stay) or tickle (whatever the correct word is) at 90°, I go very slowly. I usually sail with this tickle at say 70° and maybe 60°. I hope I am not the only one to do so. In that case, why should the mast be rotated at 90+°? It would seem that to get a laminar flow on both sides of the sail the mast should be rotated at about the same angle as the apparent wind (so about 70°).


Really all of this is a valid way to sail. 90 degrees (tell tale for this discussion)is what you work towards, sailing 90 degrees can be fast (VMG) it can also be slow if you don't have the boat up to speed to sail that low. You can sail 60 to 70 degrees to build apparent wind and build boat speed and then "bleed" down to 90 degrees. But if the boat is moving too slow 90 degrees can feel (and be) slow. It takes time to get a feel for the boat and what works. Sailing around other boats (and racing) is the best way to figure it out. And lots and lots of time in the boat to generate the knowledge of what the boat wants to be fast and sail low.

Re read "Losts" and "Aus" and Landenbergs comments after sailing the next 3 times you go out.

A-Man

#41 Lost in Translation

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Posted 07 June 2012 - 09:16 PM

AUS, I hear your theories are working. ;) Thanks for the post. I want to start playing with this downwind myself.

#42 Bang Zoom

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Posted 07 June 2012 - 09:51 PM

All good stuff.

If you could find someone to train with I think you would find it a huge help.

My procedure for finding settings is to set up next to my wingman as on the start line, IE both in clear air and even. Make sure to set up with enough room that you don't fall down on him but aren't in a different breeze. The sailor to leeward gets going with the settings that he thinks are right for the conditions and just sails. No pulling strings unless he is way off or conditions change. Then the guy to weather can play with settings and match speed. At first you will make big gains. As you come to understand the rig you will start with settings that are pretty close and fall back as you try settings that don't work validating that what you started with were good settings. Switch back and forth as you tack and gybe, sharing static and learning spots. I think this is a fairly common technique.

FYI I mark my tramp at 22, 45 and 80 degrees of rotation and like my wingman (when I am lucky enough to have one) to do the same so we can compare easily. Also I prefer rotation control on the deck that I can reach from the wire running endlessly under the tramp with two to one on the rotator arm. It makes it easy to move it just an inch, or a half, which can make a difference.

Been thru pin heads, big heads, lots of luff curve, lots of broad seaming, stiff mast, soft masts, repaired masts and they all require different settings to get the most out of them. So I do the above.

Hope that helped and made sense.

#43 Jaffar

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Posted 25 June 2012 - 01:37 PM

As ErikM, I am a beginner in the A-Cat class with lots of questions on mast rotation and other things.The picture posted by Bang Zoom is tremendously helpful for me, as I am trying to properly setting up my control strings for mast rotation on a used Auscat (2001 Boyer Mk 4) with a new trampoline.

But for the time being I have first to properly take care of fixing a 12 cm crack at the bottom of one hull through which water is entering. I will post a question and a picture in the fix-it forum.

Jaffar

PS to ErikM: It's funny, I should be about one hour away from you...




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