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IRSA 10R Measurement Forms


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As part of my attempts to upgrade and improve my aging Bentley (longer fin, lighter bulb, revised sail plan), I have been entering its new data into the IRSA 10R Measurement Forms in order to check its legality before having it officially measured.  The sail 1 / sail 2 pages of the spreadsheet produced many errors / warnings that I didn’t understand so I have spent a some time reviewing the underlying formulae in an attempt to understand why.  I have reached the conclusion that nearly all the errors & warnings generated by my sails are erroneous i.e. they are due to errors in the spreadsheet.  Also I have produced a private version of the spreadsheet that fixes all the issues bar the leech hollow problem, so I’m fairly confident that I’m correct in blaming the spreadsheet not my sail data.  Fixing the problems took far less time than finding them as the spreadsheet uses some very (over?) elaborate methods to do some fairly simple things.

I have listed the errors I have found to date below in case anyone else is having the same issues.  I have also included the responses to my observations that I received from the MYA Technical Officer.  I leave you to make up your own mind about the adequacy of the responses. 

If I’m right then this raises obvious questions about the process used to review and test the forms before they are released for use.  A more interesting question perhaps is “why has no one else found them yet?” I have now spoken to four 10R measurers and it is clear that none of them fully understand how the checks and warnings work and hence the possibility that the checks themselves could be wrong has not occurred to them.  Their attitude to the spreadsheet seems to fall into one of two camps. Either they just accept that the spreadsheet produces warnings for reasons that they don’t understand or they become frustrated and annoyed because they have to do additional checks that are clearly unnecessary.  Either attitude severely reduces their confidence that the spreadsheet is providing them with useful support when carrying out measurements.  In extreme cases it is seen as more of a hindrance than a help.

Please do not think that I am against the use of the spreadsheet.  In principle it is an excellent tool and the checking should help to catch many accidental errors in both data entry and calculation, however it is self-evident that it needs to be as error free as possible.  One might even suggest that there ought to be some sort of formal bug / query reporting mechanism so that errors, which will enevitably creep in, can be flagged up quickly and experience in its use shared.  Clearly a great deal of time (and cost?) has gone into producing this spreadsheet but if measurers don’t understand what it does and/or don’t trust the results it gives then much of the effort has arguably been wasted.

List of Anomalies found to Date.

John Millward report 18 February 2022 – GB replies

1          Number of Foot Heights

The spreadsheet attempts to count the number of foot heights entered and compare this with the length of the foot to ensure the right number of heights have been entered. Unfortunately the code tests for a valid entry by comparing it to zero, so if one of the entered foot height values is 0 (as would happen if the tack and clew heights were the same height or the length is an exact multiple of 50mm) then it doesn't count this as a valid foot height and gives a warning when the correct number of foot heights have been entered i.e. it ignores the zero height and thinks there should be one more height.

As replied to the Class Registrar some weeks ago - Any entry of zero in the legitimate spaces for measurements will give an ‘error?’ message. The calculations do not fail and there is no error in the calculations. The ‘error?’ message is to alert the measurer that something may be amiss and that he should check. Zero will be entered in ho in a very rare case i.e. the clew point lies on the perpendicular to the straight line between head point and tack point. Zero values in other legitimate spaces would normally indicate a hollow in the foot has not been bridged. The ITCA Technical Officer is aware of this and it is not regarded as something that is a priority to address.

2          Foot Hollows Check

There are 3 separate errors in the table.

1) The first two columns in the yellow table and the legend (S25 – S36 to U25 – U36) List the heights from clew to tack (hn -> h0) whereas the result column (V25 – 36), is based upon data in the “ Data for Foot Graphs “ table which lists the heights from tack to clew (i.e. in the reverse order). The result is that “Check / OK” warning appears against the incorrect h value.

The labelling of the heights hn to h0 in column S is reversed and will be corrected at some stage.

(JM Note – I have informed GB that the fault is not just column S.  Columns T & U are also reversed and the check / OK messages are therefore meaningless.)

2) The hollows check algorithm compares each height to the average of the two adjacent heights and issues a warning if it is less than the average. Obviously for this to work, there must be valid heights either side of the height being tested. Unfortunately the spreadsheet attempts to test the last h value (hn) and therefore this test is invalid and nearly always generates an error.

Not knowing how many heights will be measured and recorded it is not possible to know which will the last h value. Hence this will happen. The note to ‘check’ is an advisory message to alert the measurer that something may be amiss and that he should check.

(JM Note – The form already detects the last and the penultimate cross-widths in order to calculate A2 so the same method can be used for the heights.)

3) The foot hollows check is arguably too aggressive because it does not take account of rounding errors. The rules require that all measurements are rounded up (e.g. 35.1 becomes 36) but the check does not allow any tolerance in the check criteria. The result is that a perfectly straight line can produce hollows check warnings.

It is anticipated that there will be false negatives because of this. However setting a value below which the ‘check’ message does not appear would permit some real positives to be un-checked. Again, the note to ‘check’ is an advisory message to alert the measurer that something may be amiss and that he should check.

3          Leech Hollows Check

In contrast to the foot hollows check, the leech  hollows check is arguably not aggressive enough as the cross widths include the luff curve so any difference in cross widths due to the luff curve could mask a hollow.

Agreed. However, without measuring the offset of the luff itself from the vertical (or the offset of the leech) I do not see a simple way to address this. Please let me know if you do.

(JM Note – Shouldn’t we at least be consistent?  If the checks allow minor hollows in the leech to go undetected shouldn’t the same be true for the foot? False warnings will undermine confidence in the tool and potentially lead to real hollows being ignored)

4          Cross Widths Graph

Although this is only used for information it does contain a number of errors / anomalies

1) The leech profile graph draws a point which represents the head of the sail Unfortunately it makes the assumption that the cross width is zero at this point, so if your sail has a reasonable width at the head (as most do) the graph looks quite strange at this point. As the area calculation for A2 calculates a notional head cross width, it seems rather strange not to use this for the graph.

There were limits to the degree of realism that was sought in the diagrammatic representation of the sail. A notional head width could be estimated and used at some future stage.

2) The cross width differences have a similar problem to the foot widths problem 2) in that it calculates the last value as the difference between the last valid cross width (Cn) and zero. Thus the Cn cross width difference equals Cn, which is of course incorrect.

See comment above.

3) A minor issue is that the cross width differences are plotted at the cross widths and so it is not clear which cross width difference they refer to e.g. does the cross width plotted at C5 give the cross-width difference between C6 & C5 or C5 & C4. It would be clearer if the cross width differences were plotted between the cross width points i.e at 100, 300, 500 etc.

Most users are probably not aware of what the cross width differences demonstrate. The class registrars will be more aware of what they indicate. The adjustment you mention would be ‘more correct’ but I doubt it would make a great deal of difference in reality.

4) The foot height differences graph looks very odd at the clew and seems to have an extra point but I haven’t fully investigated this.

By all means let me know what you find.

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There are some oddities in the A class measuring spreadsheet which I posted about on here back in August last year but have not had any response.  They don't affect the fundamental measurement but confuse anyone like me trying to carry out a pre-measurement check themselves as there does not seem to be any supporting information in the documentation.

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The origins of the spreadsheet stem from attempts (worldwide) to reduce /remove the number of mathematical and measurement errors which were arriving on measurement forms checked by registrars.

The whole point of the graphical and check tables is Guidance and was never intended to be a definitive answer.  Competent (trained) Official Measurers know this is guidance.   They do not need to be IT specialists and understand the design of the macros behind the spreadsheet and  most would not wish to try to have such expertise.  Their expertise lies in other areas.  The whole point of guidance is to indicate possible problem areas which may need checking after all we are all human and can make mistakes.  Any official measurer who has checked and is satisfied there is no error in his measurements, has space on the measurement documents to indicate this or any issues he is concerned about. (and should do so).   Similarly if an error message persists such as could be caused by a correct zero measurement at ho. The official measurer will use the notes space to indicate the reason for this message and that he has checked the zero measurement and it is correct.  Such notes and messages on measurement forms  are helpful and necessary and not fatal to satisfying measurement requirements and do not hinder certification and registration.  (As the class administrator I have filed multiple Official Measurement documents over the years and I know of only one instance of a persistent error message on measurement forms.  Which was appropriately dealt with and the early spreadsheet version concerned corrected to deal with this.  I have not come across any challenges or official queries of the guidance by any measurers.)  

In all this it must be noted that the fact that the spreadsheet correctly calculates the rating based upon the data entered even with these guidance check anomalies and this has never been challenged anywhere in the world.

So where does that leave us.  Yes it would be a good idea to have these guidance anomalies reviewed by ITCA and appropriate corrections made to reduce the number of dubious checks indicated.  Are any of them fatal to the used of the spreadsheet in its current format? No.  How quickly should this happen? Well that is a matter of ITCA priorities and resources.  Certainly there are no current  financial  resources to pay someone for this task.  10R is a volunteer based organisation.  Even after adaptation substantial testing time would be required to ensure no inadvertent alteration to substantive parts of the spreadsheet had occurred.  

John, you seem to have expertise in hacking into "protected documents" i.e. this spreadsheet and  skills in writing revised macros.  Perhaps it is time to volunteer your  'private spreadsheet' to ITCA, via the Technical Officer, with necesscary supporting information on what you have done and the how's and why's.  Then this could be vetted/tested by the ITCA IT expert.  This could substantially reduce the time and effort of the 10R organisation in making any necesscary progress on these issues. 

 p.s The production of the "private spreadsheet" raises several questions. 

1. Copyright.

2. The level of 'protection' against unauthorised alteration in the current spreadsheet which may need upgrading.  

It might have been 'better' to offer to help solve the problems and or ask 'permission' to hack the spreadsheet first.


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A wise man once said that no information was of more use than wrong information.

The first image is the graph produced by the current spreadsheet from my mainsail data.  The second shows what I believe is a correct representation of the same data.





I have already offered any help I can give in confirming / correcting the anomalies I have found, but so far I have only met with explanations as to why they either don't matter or are not worth fixing.  Of course I accept that changing the spreadsheet comes with a cost but the publication of a bug / workaround list would seem to be no more effort than replying to my posting.  It was the seeming lack of enthusiasm to do even this that led me to post the list on this forum. (Ideally I would have posted it on the  IRSA or ITCA website so that measurers in other countries can comment but I have not found a suitable forum.)

P.S. I might be forgiven for expecting a little appreciation for the time I have put into trying to resolve the issues that several people seem to be having with the spreadsheet, instead I seem to be being accused of 'hacking' and copyright infringement (both of which I strongly refute BTW).  It doesn't encourage people to give their time in an effort to benefit the class does it?

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I apologise I was not aware from any posting or communication I have seen that you had offered your services to help ITCA resolve these issues.  

Maybe my use of the term 'hacking' is misplaced or a lack of understanding of the term but to me any unauthorised digging into protected electronic spreadsheets/documents the copyright of which is 'owned' by someone else, without their permission is infringement of intellectual property and 'therefore 'hacking'. (gain unauthorised access to data in a system or computer.)

I can see the differences in the two graphs but I cannot see that the differences will make a significant difference to a competent official measurer carrying out his duties.

I am not an IT expert and have no wish to learn that skill now.  However, I can say I have no idea what you mean by 'publishing a bug work around list' means or if one was published how to use it.  (I  feel most readers of this conversation and Official Measurers feel the same as no one else has commented).   It also seems to run dangers of multiple copies/variations of the official spreadsheet creeping in which is what a protected spreadsheet was intended to minimise/prevent.  I am still not convinced that to anyone other than an IT "expert" that in the short to medium term it makes any difference to the official measurement,  certification, and registration process.

If you want to give your time and effort to help the 10R class then please use official channels first.  (The COG, MYA ITCA systems). Stand for office and help run the class (the UK 10R COG have been looking for an IT expert to set up a new website and help run it, you appear to have relevant IT skills..... ).  A Forum It not always the best  medium.


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As a newcomer to this class I have now become quite familiar with the measurement spreadsheet and, ironically, now cannot see why I had problems initially. This forum has been so useful and through my thread as "potential 10 sailor" I am very grateful for the assistance and advice received from members.  I must single out John; whose efforts pointed me to the error I had been making. This is the true value of this forum. I not only see a great attraction in the 10R class but find myself impressed by the spreadsheet. The errors referred to earlier are, however, irritating and maybe some sort of enhanced guide to it's use would not go amis as I also spent time trying to eliminate them.  Only thing to sort out now would be a simpler keel depth measurement procedure, which would surely result in more measurers.  


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