Seat Back measurement Interpretation needed?

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    Avatar photoJeff Kent

      The discussion on how to define A. fueselage,13 continues..

      13. Seat back shall be flat, measuring 11’’ (280. mm) in length at the center line, no
      maximum height, the crown on top of seat must be a minimum of 2’’
      (50.8 mm) radius with minimum seat width 4’’.

      It appears there may be a consensus of the TC on this . Discussions have taken place offline and prior to the existence of this TC Home page

      how to measure radius and where it applies according to specification

      There is some variation of opinions but I believe the following most of board agrees to :

      Seat back shall be flat
      2. Seat length must be 11″(280mm) min measured from base of seat along flat on centerline, to crown, No maximum dimension specified

      3. Radius specification is at top extent of seatback flat area which is specified by the word “Crown”, min dimension is 2″ (50.8mm)

      4. Min width is 4″(101.6mm). Which in turn is 2x min radius when the top of seat forms a complete semi- circle profile (popular). In reality this applies is in the area located between surface of rear deck level and at a point 2″ (50.8mm) down from “crown”

      Fellow TC members, please officially state opinion on these findings ..If the majority agree I can combine this wording into an interpretation, or just a better definition used by the TC. If diagrams would help they can be added, several have been created

      I am of the point of view that this is what has always been considered the intent of this specification. Nearly all of the fuselages out there would comply short of several new builds

      thank you




      Dear All,

      In this question first of all I need to point out the insignificance of this rule. It does not influence any of the boat’s performances, it is only an aesthetic option.

      If I were to decide, I would delete A/13 completely from the specifications and let everybody use their imagination and skills to fulfill their creative ambitions and as it fits to serve their artistic thinking.

      It is the only part of the DN that can give any personality to it and distinguish the different design styles.

      In my interpretation, the current rule A/13 is a minimum area that can be placed anywhere on the backseat’s centerline. If the backseat is larger than the minimum, the above the deck area can be considered the backseat support that has no dimensions specified.

      My interpretation of the current rule is:

      If the backseat’s flat area at any point on the centerline is larger than 11″ in length 4″ in width and covers radius 2″ on the top side until the minimum length, it fulfills the rule A/13.

      The intention of the rule is not very clear, some say it was a safety issue, some say it has something to do with hiking at the back when very hard masts were common.

      I hope in reality we are not overthinking this issue.

      In Jeff’s list, point 4 has a problem by overruling the specifications, where there is no mention of the rear deck level. It would require a ballot to change the specifications.


      Avatar photoSteve Orlebeke

        I agree with Jeff on all points. 2″ minimum radius applies to the top of the seat back no matter how long it is. That’s how I have interpreted this rule for all of the boats I have built over the years.


        Tomek Zakrzewski

          I think the real question mark is related to 2” radius and where is has to be met. My opinion is that the rule makes it clear: “the crown on top of seat must be a minimum of 2’’(50.8 mm) radius”. There is only one top of the crown and it doesn’t matter if seat back is 11” or more. Top is top. There is no word about area.
          Therefore I believe below (attached) drawing represents my take on A.13.
          Green = compliant
          Red = illegal

          the discussion about how it influences boat performance and is non relevant for this case.

          I agree to 1-4 mentioned by Jeff.

          Avatar photoRichard Larsson

            Tomek, i think you are right that the top of seat is the most upper point. Your interpretation could be considered right according to the rule. Anyway it is hard to me to accept the two green “corners” with smaller radius. They could in your interpretaton be sharp corners as well. I think we should include them in the top of the seat back.

            In words Tomeks interpretation could be something like this: “It has to be possible to draw half a circle with 2″ radius on the seat back that in the top of the circle intersect with the top of the seat back” (if i interpret it right).

            I agree with Jeff.


            There are a couple of questions in my mind:

            1. How does any attached extension to the backseat influence the flatness and size of the backseat in measuring? Like the commonly used sponge kind of plastic attached to the headrest part to hold the helmet during sailing? Is that allowed?

            2. How would you distinguish between the backseat and the headrest support?



            Avatar photoPaul Goodwin

              I agree with Jeff’s summary of the intent of the Seat Back Specification A. fueselage,13:

              Seat back shall be flat

              2. Seat length must be 11″(280mm) min measured from base of seat along flat on centerline, to crown, No maximum dimension specified
              3. Radius specification is at top extent of seatback flat area which is specified by the word “Crown”, min dimension is 2″ (50.8mm)
              4. Min width is 4″(101.6mm). Which in turn is 2x min radius when the top of seat forms a complete semi- circle profile (popular). In reality this applies is in the area located between surface of rear deck level and at a point 2″ (50.8mm) down from “crown”
              I do not believe an official Interpretation is required, since I have always thought these clarifications were clear in the wording of the Specification. However if the TC Chairman feels an Interpretation would add additional clarity to the Specification I would support that.
              I think Richard makes a good point that the corner radius in the green outline in Tomek’s drawing is less than 2″. I think that all radii along the top of the seat back should be 2″ minimum. Otherwise the corners on Tomek’s Green outline could be reduced to sharp corners and create a safety issue. The seat back is directly behind your head in a collision and can cause serious injury. But this could require an Interpretation, or possibly even a Specification change.
              If a sailor decides to add some sort of padding to the seat back I believe that should be allowed since it could provide additional safety, or at least comfort. But this additional padding would need to be added to a seat back that already meets the Specification.
              Paul Goodwin


              Hi All,

              I agree with Richard and Paul. If the top needs to be a 2″ radius, then any lesser value shouldn’t be acceptable on any part of the edge line of the headrest.

              I am still saying that it is not a crucial issue. And I would allow any form or shape.

              I dont see any safety problems with the headrest regarding the top radius, as there are many much sharper edges around your head, like the boom, the side panels top, the tiller, and so on. Why they do not make any concerns about safety? Why the thickness of the backseat (or the lack of it) is not an issue here if we talk about safety?

              The padding on the headrest provides hold for the skipper’s helmet and head and helps the neck not to be tired too soon. That is a functionality that gives an advantage in sailing. Noone uses it for additional safety.

              We are talking about some aesthetic issues that in reality not worth the e-paper, but not questioning something that has an absolute advantage over the others, besides overwrites the allowed parameters in the rules…

              I still have 2 questions, which could be important not to be overlooked and to define in this discussion:

              1. I still dont know what separates the headrest from the support? This could be answered.

              2. If I glue anything to the hull or any part of the DN, is it outside of the measurement dimensions?





              I find another issue here. How big the fairing or rounding can be on the edge of the headrest part on the seat back?

              Also, I find myself asking again where the headrest support begins here and where is the top of the headrest where the 2 radii arc should touch the top?

              I guess at the end of the flat part? Is that easy to find that part at the measuring? Does that issue concern many hulls not covering the minimum radius?

              See pic.


              Avatar photoJeff Kent

                Peter, based upon the majority opinion of the TC members the rendering you put up the blue area part of the seatback would not pass

                The area being measured is the “Flat” portion. The top area of seat specification you show is correctly displayed by the top of green line on centerline.( assuming green line having the min 2″ radius and being located at top of flat area “Crown”)

                The issue here is that flat area does not extend out to the tangent to the curved green line everywhere . Therefore seat as shown is too narrow

                Edge radi along top of seat to support is not considered part of the measured seat .

                currently there is no constraints to a support structure

                The seat back specification in the past and currently in effect essentially is the green line . Any flat area to either side of center extending beyond the green line is open The green line area is the minimum requirement

                As far as Paul’s comment about “padding ” applied to seats I agree, the seat back must conform to specification prior .

                At this time I believe we can consider this discussion concluded and deemed “clarification” As the majority are in Agreement

                Tomek Zakrzewski

                  I’ve received an enquiry from Vaiko C-6 who has been producing hulls for decades with headrest as shown on the drawing asking to confirm that this design fulfills A.13.

                  Vaiko is confused with various opinions presented in this topic, however his interpretation of A.13 is that top crown has to be 2” radius minimum. Area outside is not limited by the rule.

                  I agree with Vaiko. In his drawing corners are rounded to 10mm radius but this is just individual preference.
                  Drawing here:

                  Avatar photoDeb Sec


                    It has been my continuing opinion to A.13, that it applies to portion at centerline “crown” what happens outside the 2″ radius is beyond the controlled area

                    therefore I would believe there is no issue with Vaiko’s approach.


                    Avatar photoPaul Goodwin

                      Personally I don’t have any issue with Vaiko’s approach, but I don’t think it meets the Specification. To be allowed I think we should introduce a change to the specification with a minimum radius for construction like Vaiko uses. Otherwise the outside corners could be less than 90 degrees and 0 radius (pointy).


                      Avatar photoRichard Larsson

                        at 6:35 pm
                        Richard LarssonRichard Larsson
                        I agree with Paul. We should decide if the green line in Peters picture is a minimum surface that could be bigger outside the line and be shaped free, or if the upper edge of the flat surface should be at least a 2″ curved radius.
                        As Jeff pointed out above we only talk about the flat surface. That makes some complicated effects. If Vaiko would provide a sticker 0.1mm thick shaped as the green line all his hulls would be clear. The surface behind the sticker would be a support for the sticker and the sticker would be defined as back seat. This is not cool, or maybe it is?
                        Peter, now i realize the problem with this. It is way harder to handle than I thought.
                        My adviser board helped me to clear up this problem. We discussed how a back seat should be defined. We think it could be ¼” thick from the surface that has to be flat. The back seat is allowed to be faired with a ¼” radius as the corners of the hull are. We measure the sides of the hull as if they were not faired. Let’s measure the back seat in the same way. The “green line” (the measurement of the rule) then will be measured in the widest spot of the upper part and sides of the back seat not further back/down than ¼” perpendicular to the flat surface. The measured edge of the back seat shall, all the way, have a convex curve of minimum 2” radius. Less radius along the upper edge of the back seat are not allowed. (Sorry Vaiko, I gladly interpret this somehow so the 10mm radius is ok. Please make another interpretation, maybe I will support it. Otherwise it is very easy to correct the back seat).

                        We also want to interpret the support of the back seat. This is our suggestion. The support of the back seat is defined as the material above the deck and behind the back seat of ¼” thickness. The support cannot be higher from the deck, or wider, than the back seat. The support cannot stretch further back than 1/3 of the length of the deck.


                        Hi Rich,

                        I agree with your current interpretation. A 0.1 mm tape would be sufficient to represent the backseat and support is not defined.

                        The rest of your suggestions are not interpretations, but proposals.

                        As of these, it would need to be supported first by any of the CGG’s.

                        I am not recommending calling these interpretations, but proposals.

                        I am not supporting it anyway, as I pointed out earlier. I think it makes unnecessarily complicated rules and against the EPIC agreement as DN should be simple to build. Specification changes are to make building more simple and easier. Bringing rules that making unnecessary complications does not make it easier to build.



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