The size of a vang/kicking strap ... discussion

[Bruce Andersen]

This is a complicated issue, particularly when it is being written to include the VISS vang.

An surface area measurement may be difficult for event measurers to calculate at pond side, and may not be accurate if measured while attached to the rig. Do we really need to make this more complicated?

Adding projected area is real - whether it adds to sail area or whether it closes the gap between the boom and the deck to add performance is a question about which everyone has an opinion.

My question is simple - does a plate type vang/fitting do more than simply control the upward motion of the boom? It must, in someone's opinion, or it would not have been developed as a replacement for the standard wire/turnbuckle device used for decades.

[Andrew Crocker]

Thanks Darin, Alberto, and Bruce for your comments.

If I may recapitulate somewhat.

I have no doubt that the larger “plate” style vangs are intended to add some advantage. I do not personally know what that advantage is intended to be but, provided they are compliant with the rules, there should be no issue as the IOM Class Rules do not define a “one design class”. Suggesting that an innovative design is somehow underhanded or a-priori non-compliant stretches the meaning of “closed class rules” beyond its intention. However, and this is how we find ourselves in this conversation, the rules do not specify any dimensional limits to the size of a vang, so how are we to determine whether these larger vangs are compliant with the rules?

Class rule F.2.3 (the only applicable rule when a vang as a single, un-combined fitting is being discussed) is frequently invoked as establishing a size limit for the vang. Class rule F.2.3, however, makes no mention of size/dimension:

• F.2.3 LIMITATIONS
  The function of items shall be limited to what is normally provided by items of their type.

However, because of statements in Interpretation 2015-IOM-1, there is an understanding that, beyond a certain size, a fitting that lies in the same plane as the sail can add material propulsive force and, therefore, now functions as a sail in addition to its normal function and therefore, can breach class rule F.2.3.

Unfortunately, Interpretation 2015-IOM-1 makes no specific statement as to how big a fitting needs to be for this additional function to manifest itself. It is unsatisfactory to say that any surface area adds additional propulsive force - in that case, all fittings are non-compliant under Rule F.2.3. Rule F.2.4(d), which was implemented as a result of Interpretation 2015-IOM-1, provides the only dimensional “clue” as to how big a fitting (such as vang) can be before it adds material propulsion (and so becomes a sail as well as its normal function) thus triggering F.2.3.

• F.2.4 CONSTRUCTION
  
  (d) Where the mast vang fitting and/or gooseneck:
  (1) are exposed,
  (2) are not of circular cross section, and
  (3) rotate,
  
  They shall not exceed 20mm in any cross section perpendicular to the axis of rotation.

GIven the above wording, the fact is that Rule F.2.4(d) currently restricts a “vang fitting” (not a vang) and “gooseneck” to, potentially, a surface area of 2000 sq mm as the diagram below clarifies.

It also needs to be noted that this surface area does not include that of the vang – which is not covered at all by F.2.4(d) – and could be at least another 300 – 400 sq mm for a total potential surface area of 2300 sq mm.

So, it is a reasonable interpretation of the rules to suggest that if a gooseneck/vang fitting with a surface area less than 2300 sq mm is allowable, a vang of similar size is also allowable and does not breach F.2.3.

My proposals, therefore, are not being written to “… include the Viss vang”. They are, rather, being constructed to ensure that we:

• have a better-defined and more comprehensive size limit (to avoid future problems)

• prevent combinations of fittings exceeding a defined limit.

• keep any existing fittings built to the current rules compliant

Under the proposed new rule, a measurer should only be required to satisfy themselves that the sum of the surface areas is less than the specified amount (say, 2300 sq mm). For fittings such as the “old faithful” vang/gooseneck/boom – which is known to have a surface area of between 300 and 400 sq mm and, therefore, significantly under 2300 sq mm, a quick glance at such a fitting by a measurer should be enough without the need for complex measurement. As Alberto correctly states, the onus is on the builder of a large vang/gooseneck/other fittings to satisfy a measurer that its surface area is compliant.

I would add that this is not an unusual approach to some of the class rules.

We do not, for example, seriously ask people to measure “… the difference between the largest and smallest value along the spar of any wall thickness dimension …” to determine that the variation is less than 0.1mm (F.3.4, Spar). Measurers understand that rule to mean that variable wall thicknesses along the spar, intended to stiffen the section at strategic locations, perhaps separated by mast joiners, are not permitted. However, if the mast is a continuous piece of aluminium section, it is taken as read that the wall thickness is constant without the need to get out the bore micrometer to check.

Apologies for the long post but it as well to ensure common understanding of where we current sit in the conversation.

Regards

Andrew

[Brad Gibson]

Hi Andrew,
Thank you for your outlay.
Regarding total allowable area, your reasoning is sound as per current 2.4 wording re area of an existing body as per the written rules.

One point that I feel should be taken into account for consistency is just what was seen as too large back in 2014/15.

The plate style gooseneck bodies of that time (Potter, AA and others), were deemed to be excessively large with free sail area gained. They were subsequently restricted to a width of 20mm as we know. What I believe needs remembering, is that these gooseneck bodies were no larger in height than 60mm with rigs set low in boats via some form of mast well. The efforts to restrict area, and subsequently require 100s of manufactured and sold items to be modified to comply, reached across both IOMICA and IRSA.
No variations of this style larger in body height were witnessed, nor in following years have they been seen by ways of using the fullest area allowed. In short, we don’t see 100mm high goosenecks.


While I agree that an allowable height of 100 mm under existing rules could give an area of 2000mms of gooseneck body +300mms for vang component, it is not consistent with that previous process. I feel the total area figure should reflect that of the area deemed allowed of the rulings of that time.

The 2014/2015 changes to existing fittings saw a 60mm gooseneck body height x 20mm body width that gave us 1200mms. If we then allow the vang area to your allowance we get 1500mms. This is far less than the suggested 2300mms.

I understand the desire to not overly restrict tolerances to prevent further manufacture processes not looking for undue advantage outside of function. In taking this into account I believe that any allowable combined vang component area above 1800-2000mms pushes well past consistency of previous rulings.

Regards
Brad Gibson
GBR 42

[Zvonko]

Hi All

Actually, the vang black plastic body shown in my previous post has a surface area of about 730 sq mm. When you add the rotating gooseneck, vang–boom fitting and the main boom fitting, the total exposed area comes to roughly 3000 sq mm.

Vang black plastic.png (2.96 KiB) Viewed 30 times

Regards Zvonko

[Alberto Spada]

Hi Andrew,
Thank you for your contribution.

I would like to thank Brad for reminding us of what happened ten years ago (which I did not mention out of delicacy, as I was involved at the time) with the gooseneck bodies that “created sail area” for about 25 square millimeters more than the current rule (and I believe that the others were more or less the same).

Now the rotating bodies of the goosenecks range from 50mm downwards (there is no need to go beyond 50mm in height), so the 100mm assumed is absolutely excessive if used in the calculation. To be generous, we can go up to 60mm (but I don't think any current manufacturer, AA-Parts, Potter, PG, or various self-builders, would go that far).

I also believe that there should be consistency with the process of ten years ago: back then, there was a scandal, now there is permissiveness: in my opinion, the truth lies somewhere in between. A surface area of 1800mm - maximum 2000mm can be considered reasonably consistent with the assessments made in 2014-2015 (using Brad's suggested method: maximum permitted area of the movable part of the gooseneck as per rule F.2.4 - example 60x20mm = 1200mm + standard vang area = approximately 300-350mm. The total is 1500-1550mm).

And in any case, let's remember that we are talking roughly about 0.6% of the surface area of the mainsail A. Let's not cause a scandal like we did ten years ago.