When sailing to weather, more mast rotation:

A. Depowers the main
B. Powers up the main
C. Doesn't matter

The only true answer is :

"dependents on mast design"

Wing masts depower with less rotation

Stiff masts (Hobie Tiger) power up with less rotation.

I think nacra mast are typically somewhere in between from comments I hear of Nacra F18 sailors at my club

Wouter

Normally rigged boat IE: Nacra 6.0 / Inter20 / Hobie 20 / Hobie 16...etc

Aluminium mast, tear-drop style mast, square or pin-head sail.... just standard stuff.

It depends if the mast is of a prebend setup (spreaders rakes aft and diamond wires tensioned to bend the mast in the long axis) or "soft" rigged setup (spreaders almost perpendicular to the mast, low tension on the diamonds).

A prebend setup will resist bend in the short axis and accept bend in the long axis. By under rotating a prebend mast, you are allowing more bend in the long axis, thus helping to flatten the sail, depower. Over rotating reduces flex in the long axis and increases sail body/power at the cost of more drag.

With a soft rig, less rotation reduces the mast bend along the more flexible short axis, thus the sail is fuller/powered up. More rotation places the short axis such that the mast flexs more, thus flattening the sail more to depower it.

Mike.

Great information guys. This particular question was raised by a couple of us at the bar last night. My boat has spreaders, but they are stationary thus you can not pre-bend the mast. Another guy had a H16. Another guy is an H20 driver.

We were talking about the cut of the newer Hobie 16 sails in relation to what Hobie did in the mid/late 90s to Hobie 20 sails. The old H20 sails oringinally were cut very generously thus a bunch of pre-bend was the main way to get those sails under control. Then the cut changed around 1996-1997. A lot of guys kept the same amount of pre-bend but rotated more.

I told them to change the spread angle and tension and screw rotating the mast on the H20.

There are two interconnected mechanisms to power and depower a sail using mast rotation alone:

1) Changing the section efficiency of the sail/mast airofoil by means of rotating the mast with regards to its most efficient angle for the conditions. Both under rotating and over rotating will reduce the efficiency of the airfoil, but over rotating costs more drag.

2) Changing the mast curvature using the different mast section flexibility in each axis. The mast's beaviour may be tunned to a certain extent using more or less pre-bend, as posted before.

Usually you want both mechanisms to work in synchrony, so that when you under rotate the mast, the airfoil loses power for both reasons simultaneously: due to the mast being more aligned to the sail and to the sail becoming flatter with the greater mast curvature.

There is one thing people neglect: it is necessary to know how much rotation is ideal for your mast in order to identify when it is under or over rotated. The ideal rotation is not a fixed figure. The ideal rotation of a 5% chord mast is approximately 53 deg, falling fast to 35 deg when the chord is 10%. Although possible, this is difficult to evaluate by just looking at the mast and sail. You need to divide the mast chord by the sail foot to find the chord % and figure the ideal angle. (take a look at the attached drawing/table copied from Tom Speer's text on the subject). Only after this is known and marked, can the pre-bend be tuned accordingly to make both mechanisms work in synchrony.

Best regards,

Luiz,

Thank you for that post. That's the single most informative thing I've seen on here in a while. It's obvious now that I look at the diagram but proper mast rotation has been a bit of a mystery to me. I know it's early, but that's my vote for post of 2005!

3rd factor is the top bending off to lee taking the leech with it and thus opening up the upper part of the sail and leech thus spilling air.

You want these 3 factor working in unison.

This is the reason why prebend wing masts have such a large range of (de) power levels.

Because you derotate a wing mast to loose power 3 things happen that reinforce another :

-1- You reduce the angle of the leading egde effectively reducing draft of the sail.

-2- You flatten the sail by use of prebend.

-3- You present to weak axis of the mast to the wind coming from the side thus pushing the (unsupported) top away to lee twisting off the top of the sail and flattening the top part even more.

Actually point 3 on skiff masts causes the top battens to invert (bending to windward) and this totally depowers the top.

End result is a rig that is totally depowered spilling air all over the place but with very little drag. In summary these rig both reduce lift (drive) AND drag by derotating the mast. These rigs are seen on A-cats, M20/M18, F18HT, Tornado's (I think) and Formula 16's. Ohh and the capricorn F18

Rigs that can not be set up with prebend (no-diamond wires) or that need the mast to bend over its weak axis to flatten the mainsail must rotate there mast further to depower. Making the top of the mast stand up. These rig reduce lift and drive but don't reduce drag. These rig are unsupported masts like Hobie 16's and Nacra 5.0's but also boats like Nacra 5.2 that have diamond wires but can't effectively produce prebend.

A 3rd way is the current F18 masts setup (excluding the capricorn F18) These are prebend rigs and they derotate first to depower but when it really starts honking they fully rotate to make use of the same principle as that described for Hobie 16's. So inititial these rig first depower by reducing lift and drag simulataniously but at a certain level they can only reduce power more when accepting to increase drag as well. It is my opinion that these rig are now seen with larger and larger square top sails in order to delay this switch over point. Hobie Tiger design is in my opinion as good example of this 3rd way.



A note on the side; I really took to wingmasts simply because the leech tension and mast rotation act like a pretensioned spring setup. By these two controls you can effectively set the point at which the top falls away WHILE sailing. It is a spring system that want two by at one of two extremes, roughly said. That is standing up when the sail force it to low to push it away OR falling away completely when the sail pressure is enough. Its reaction can be controlled very well. If the top falls away to quickly you just increase mast rotation a little bit and the leech tension will support the weak axis a little more thus setting the system to fall away at more sail pressure.

It is quite a neat system. If you really want to know invite yourself on a baot with a wingmast owned by a sailor who knows how to trim it. You'll be amazed how much control you have over such a rig.


I hope this helps

Wouter

Just a quick note regarding mast rotation and the diagram from Tom Speers site (it's worth checking out his site, lots of good information there. Tom works as an aerospace engineer, and is highly competent within his field).

The foil used as an example is a Clark Y foil, and methinks it is a bit to full forward for a sloop rig. So dont take that example as holy writ for all boats/sails.
The example is taken from an article about wingmasts and their design, so please take time to read his criteria on why he used the Clark Y foil before you go racing with your mast rotation set to 53degrees


Full Article

Quick side note... if anyone knows this guy, tell him his Wingmast directory on his web server is wide open. He needs to lock down that folder on his website or sooner or later a hacker will add some special sauce to his scientific dictation of airfoils.

Smart with aerodynamics... not so much with websites.

Additionally please note that REAL aerodynamics along sails can be very different from what theory predicts. Not many aerospace are great experts on beheviour of low velocity flow over rough (seams ?) and 2-dimensional curved planes. Yes I said 2 dimensional and not 3 dim. Because cloth is so thin that it does not always compare well with NACA 3 dimensional test wing sections. I've read up on Speers article and the optimal angle of attack analyses he presents may be scientic well done but does NOT square out well with pointing angles of soft sail boats. I will not go into detail that will take up much space, I only want to say that you should Speer article as a learning basis and understand that it diverges from real life experience in some important area's.

Wouter

Hey Wouter,just rotate the mast to the windward side of the boat and trim for the tell tales yes? Might try sailing on just the mast in light to mod air . 6-7 kts to see how my mast performs in stand alone mode.

Wouter,
Would you also consider a 4th factor? That factor being the righting moment (crew weight) vs stiffness of the mast?

Bill