Rule Interpretation RI-02-16

The Official Offsets are in essence excerpts from the design specifications and drawings of the J/105. For horizontal Sections of the deep keel above the bulb, of the shoal keel above Section 3 and between Sections “Bulb Axis” and 4, and of the rudder not included in the Official Offsets, the minimum thickness and maximum chord lengths are arrived at by linear interpolation between the published Sections.

Attached to this ruling are revised Official Offsets prepared by JBoats, which clarify and provide additional detail. These will replace the current Offsets located on the website and will form an appendix to our class rules which can be found on the official website https://j105.wpengine.com/j105rules.html#q8. See below for an explanation of the Official Offsets tables and drawings and some notes about chord length.

As contemplated by rule 5.4.3, surface fairing of the keel and rudder is permissible, provided that the minimum thickness, maximum chord length and foil shapes established by the Official Offsets are complied with. Subject to these limitations, permissible “surface fairing” includes smoothing out irregularities in the keel and rudder profile as well as filling in “low points” and removing “high points” on the side of a factory-supplied keel and rudder. However, any buildup of fairing material not required for surface fairing or for conforming the two sides of the keel, and any altering of the profile of the rudder or keel, are prohibited. For example, any fairing (by the buildup of faring material or otherwise) that would move the maximum thickness of the keel or rudder profile to a place other than that established by the Official Offsets, or would alter that profile in any way, is not permissible.

Under rules 1.2 and 1.3, it is not permissible to move the keel, redistribute lead in the keel or except as indicated below, add material to its leading edge or cut material off its trailing edge. Doing so, would fall under “altering rudder or keel profile” which is prohibited by rule 5.4.3. Likewise, changing the angle the leading or trailing edges form with the bottom of the hull is prohibited. On the other hand, it is permissible to add nominal amounts of fairing material to the leading edge as necessary to achieve a smooth nose radius or straight leading edge. In addition, for the purposes of bringing the trailing edge thickness close to the minimum, it is permissible to add a nominal amount of fairing material to the trailing edge of the keel or rudder, provided the maximum chord length is not exceeded. Also, reducing the cord length of the keel to the maximum permitted by the table is permissible.

Angle-cutting the trailing edge of a foil (keel or rudder) by up to 30 degrees to reduce vibration of these foils while the boat planes is also permitted. In other words, the standard 90 degree angle formed between the trailing edge of the foil and the centerline of the foil can be changed up to 30 degrees – so that the angle of the trailing edge is no greater than 120 degrees to a line parallel to the centerline, or 30 degrees to a line perpendicular to the center line. If angled, the trailing edge must be at a consistent angle, on a single plane. Thus, it is not permissible to cut two angles into the trailing edge to effectively create a v-shaped trailing edge. On the other hand, if the angled plane does not cut across the entire trailing edge of the foil, the angle may be larger than 30 degrees as long as the angle does not shorten one side of the foil by more than if a 30 degree angle were applied across the entire trailing edge of the foil (Max Depth of Cut). All or any part of the area bounded by the red triangle in the diagram may be removed by a single cut. For a minimum thickness trailing edge, the Max Depth works out to approximately 3 mm at Section A and approximately 2 mm at Sections B, C and D of the keel and at the rudder. For purposes of the Official Offsets, the thickness of any foil with an angled trailing edge will still be measured perpendicular to centerline.

Section A (Section 1 for a shoal keel) is located at the intersection of the molded keel sump of the hull and the lead keel casting. In the manufacturing process, the builder adds several layers of fiberglass wrap at this keel to hull joint. As a result, almost all J/105s built easily exceed the minimum offsets at this Section. While hollows and indentations on the molded keel sump as supplied by the builder may be filled in order to achieve a fair surface, removal of gelcoat (or laminate) on the boat is generally prohibited, except for light sanding of the gelcoat in preparation for the application of an overcoat material. Note that removal of gelcoat or glass from the sump area may also invalidate the builders structural warranty. Rules 1.2 and 1.3 also prohibit any modification of the trailing edge above Section A (or Section 1, in the case of a shoal keel), other than by adding a “nominal amount of fairing material” as referenced above, as this area is part of the molded keel sump.

Rules 1.2, 1.3 and 5.4.3 also govern modifications of the bulb. Again, surface fairing is permissible; however, any extension or shortening of the bulb, removing any of the lead (except as part of surface fairing, as discussed above), adding to the lead, any buildup of fairing material not required for fairing and any altering of the profile of the bulb are all prohibited.

The Technical Committee understands that severe grounding of a boat may lead to a dislocation of the keel or a need to replace the keel. In any such case, it is recommended that the owner contact his or her local fleet measurer or the Technical Committee prior to having repairs effected or a replacement keel installed.

<u>Explanation of the Offset Table:</u>

The J105 Class Deep Keel, Shoal Keel and Rudder Offset tables establish the shape and minimum thickness of each foil and the shape of the bulb on specific planes that are parallel to the waterline of the boat (referred to as “Sections”). Understanding of these tables is greatly enhanced by the accompanying drawings.

The upper table in the Offsets indicates the vertical location (“Location Below Hull” or “Location Below #1”), maximum chord length measured along the center line of the boat of the Section (“Maximum Chord Length”), and the minimum leading edge radius (“Leading Edge Radius”) for each Section of the keel and rudder and for the Section located on the tip of the bulb (“Bulb Axis”).

The left part of the lower table in the Offsets is populated with a series of minimum half-widths of each of the Sections (“Minimum Section Half Width”) that define one half of a symmetrical foil at each Section. Across to the left of the table is a listing of “Stations.” These are located aft from the leading edge of each Section at a percentage of each maximum chord length, measured along the centerline of the boat (for example, the “.10” Station of Section A of the deep keel with maximum chord length of 1,320 mm is located 10% or 132mm aft of the leading edge). All Stations are aligned perpendicular to centerline. The minimum Section half-width for each Station of a particular Section is shown across from the relevant Station in the column for the relevant Station. (For example, the minimum Section half-width for the “.10” Station of Section A of the deep keel is 49.9 mm.) To create the foil shape for each Section, a fair line is drawn through all plotted minimum Section half-width points and faired into a point tangent to the leading edge radius.

The right part of the lower table shows the designed vertical half-widths of the bulb below the Bulb Axis at the various Stations (“Vertical Tip Offset Below Axis”). The Notes below the tables spell out the shape of the bulb at the various Stations above the Bulb Axis (for the deep keel) and below the Bulb Axis (for both keels). For the shape of the bulb of the shoal keel above the Bulb Axis, see the drawing.

The Notes also indicate, among other things, how to properly locate and align each foil Section. The location of Sections at the trailing edge of the keel is established by measuring vertically down from a plane parallel to the lowest point of the hull to the trailing edge of the keel. Sections on the deep keel are located on a plane that forms an 82 degree (or 77.8 degrees, in the case of the shoal keel) angle with the trailing edge and intersects with the point at the trailing edge established by the trailing edge measurement. The location of Sections at the trailing edge of the rudder is established by measuring down from the top of the rudder along the trailing edge of the rudder. Sections on the rudder are located on a plane parallel to the bottom of the rudder intersecting with the measurement point at the trailing edge.

<u>Tolerances for Chord Length</u>

The Technical Committee is in the process of measuring the keels and rudders of a significant number of boats to determine the as-built variation in chord length. Once a sufficient cross section of boats has been measured, the Technical Committee will propose permissible tolerances (as originally contemplated by the rules) for the purpose of establishing minimum and maximum chord measurements. Those changes to the Official Offsets will be proposed as changes to our class rules.

<u>Other Developments</u>

The Technical Committee has developed and distributed to all fleets a set of jigs to more easily locate the keel Sections and to verify foil shape, minimum half-width and maximum chord length at each Section, and the shape of the bulb. This simplifies measurement in the field and, among other things, reduces the error factor of trying to measure around the keel filet and project an 82 degree angle (or 77.8 degree angle, in the case of the shoal keel), which was previously the process for location the top Section.