Differential control of jib and main

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cfwahl
Posts: 79
Joined: 23 Nov 2003, 23:01
Location: CAN 62

Differential control of jib and main

Post by cfwahl » 02 May 2004, 17:57

With an arm winch (or with a spectacular sort of drum on a drum winch) it is possible to intentionally make the jib behave differently from the main: sheet out at a somewhat faster or slower rate at close-hauled, for instance. Is this acceptable practice? I don't see what prohibits this, though there is nothing that permits it either (or permits perfectly simultaneous jib and main action, for that matter).

I notice that the 2003 Rules omit the requirement that was at C.7.7(a)(1) saying that sheets "shall be worked simultaneously and without any means of independent and/or automatic trim." (I speculate that perhaps this was done because no existing boat really had a perfect relationship of jib and main that did not involve differential in their movement, simply because the relationships of boom angle, and locations of hull-mounted sheet post or eye and the attachment point on the boom always vary from jib to main, and cannot be perfected; but that's not my main point.)

A couple of the questions posed for interpretations, issued as 3 and 4.15, dance around this issue, wiithout broaching it outright.

If you want to visualize the technical issues, I encourage you to look at the following articles by Lester Gilbert:
http://www.onemetre.net/Download/Sheeting/Sheeting.htm
http://www.onemetre.net/Download/Armwinch/armwinch.htm

However, it's possible to do the same thing using a crooked, two-ended winch arm, where the jib end is not opposed directly to the main end, but angled somewhat from the continuation of its axis.

Comments?
Charles Wahl

ralph kelley
Posts: 68
Joined: 23 Nov 2003, 17:57
Location: USA 41

Post by ralph kelley » 02 May 2004, 18:36

I do not see any problem with such an arrangement.

As you recognise, just about every boat has some degree of differential sheeting.

The geometry of the jib movement is based on the location of the jib pivit fitting and the sheet attachment fitting (which I will call jib-sheet length -- JSL) and the geometry of the main is based on the distance between the gooseneck and the sheet fitting on the boom which I will call the boom-sheet-length (BSL). If the two lengths are not the same, there will always be some differential swing even using a common feed from an arm or a drum.

In fact some folks purposely give the jib some differential arc just so that the jib can go further forward than 90 degrees so that, when wing and wing, the jib can be wing-and-wing when sailing on a broad reach.

And it is also possible to arrange the sheeting arrangements such that at the extreme end of the stroke, one sail comes in, and one goes out. For example, if you want to have a bit of control over weather helm in highly variable conditions, you could set up the controls such that in the extreme "in" position, the jib comes in to the tightest setting and the main goes out a small amount from the normal "in" position so that the weather helm is reduced. This can be accomplished via the trim control feature of the TX which would provide the final "in" setting.

As for an arm, you can achieve the same thing as the crock on the arm by having the jib sheet attached at a different point on the arm than the main sheet.

As you observed, the current C.7.7.(a) simply requires that the sheet is attached to the servo, states nothing more. And D.2.4.(a)(3) limits the sheet control to one servo.

So I don't see any problems with an arm winch configuration that you described


Ralph

Arvin S.

Post by Arvin S. » 28 Jun 2004, 13:38

ralph kelley wrote:I do not see any problem with such an arrangement.

As you recognise, just about every boat has some degree of differential sheeting.

As for an arm, you can achieve the same thing as the crock on the arm by having the jib sheet attached at a different point on the arm than the main sheet.

As you observed, the current C.7.7.(a) simply requires that the sheet is attached to the servo, states nothing more. And D.2.4.(a)(3) limits the sheet control to one servo.
As you both mentioned the 2003 rule has ommitted a lot of stuff about Remote control and thus freeing us with possible problems with interpretations in the future. But knowing the old rule and having this new rule will make us uneasy to use the above mentioned Crooked arm or different attachment point method. Also does this mean that we can actually use different points in our loop (for winch controlled models) for the main and for the jib?

I just came back from reading some rules interpretations and there are actually some interpretations relevant to this topic:

Interpretation 2003-IOM-4 item no. 13, 14 and 15
Interpretation 2003-IOM-3

The problem is that it is still vague.
The following have been cleared though:
1. Only one servo to control sails.
2. Arm servos are of course okay
3. Only one sheet control line is allowed
4. Main and Jib sheet MAY be attached directly to servo arm or drum.

Unfortunately there is no clarification if we can use the following:

1. Different attachment points for jib and main sheet on Sheet control line? (I think this is allowed as there is no prohibition or mention about it anyway) :?:

2. Different attachment points for jib and main SHEET directly to the Sail ARM? Say an offset hole? as suggested by ralf?

3. Is a double arm Sail servo system allowed? Much unlike those used for the Soling 1 meter class:?:

I hope someone can clarify this for us.

Or request for a more detailed interpretation with necessary drawings, as I fear maybe the interpretation requests that was submitted before that is relevant to the topic was not very specific :cry:

TTFN
Arvin

ralph kelley
Posts: 68
Joined: 23 Nov 2003, 17:57
Location: USA 41

Post by ralph kelley » 28 Jun 2004, 17:22

Let's not make our Rules any more complicated. The single servo interpetation seems quite adequate.

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