Jump to content

10R Rules Quiz


Recommended Posts

To demonstrate how clear and simple the 10R Rules are; a little quiz to test your knowledge.

Suppose you have a 10R that has been measured with a full length fin and you have set the waterline marks precisely for this fin.  You now want to visit a 'shallow' venue and hence you have obtained a shorter fin.  Questions:

When (if ever) can you swop between the fins?

Does it make any difference if the short fin is lighter, heavier or exactly the same weight as the long fin. (Clue - you need to think about Archimedes Principle for the last case).

I will give my answers later.

Link to comment
Share on other sites

Essentially only one fin can be used for an event no changes permitted during an event.

Before deciding on the rest of this. Some  questions come to mind.

1. Are you intending to change  just fin or fin and lead?

2. Does change make boat more than 50g heavier? (If yes outside certificated event measurement tolerances)

2 Does change affect trim? (Even if displacement waterline length the same marks could be in the wrong place)

3. Does change affect waterline length? In particular make it longer?  (If longer outside certificated measurements and sails could be too big)

Simplest solution long term if differences affect certificated measurements is to have a multiple certificated boat.  One for each configuration and use whichever suits the conditions at the event.

Link to comment
Share on other sites

Been doing just this with the Pieces of Eight. Very aware that the full 700mm is only just sailable where I sail. Have reduced ballast to 2.3Kg from 3.6.

Reduced ballast has shortened the WL; will allow more Sail Area (SA) . Reduction in volume of ballast and Wetted surface area (WSA) is a plus.  WSA is reduced twofold; on the ballast itself and the reduced underwater hull volume / area.

Have not tried a shorter fin, but if I retain the same ballast then WL and hence SA remain the same but:  The small reduction in volume with the shorter fin will make a small difference to the boats performance.  Reduced wetted area and volume ; small I know.  A small reduction in weight will, microscopically, reduce the WL.  There will be dynamic differences also but these are so complicated to analyse.

Don't even start on block coefficient in relation to reducing volume and hence WL. Really important question on an extreme design like Pieces of Eight. Also the skin friction having far greater, relative, effect at our size than full size. Need a test tank ! So many questions.

If going down the two fins route, would multi certificate the boat, as Solent's comment. Allowing a SA tolerance to cover both options.


Link to comment
Share on other sites

I wasn't asking for advice. I have read the rules and formed my own opinion about them.  I am interested to know whether other people have interpreted them in the same way I have.   Given the response to my comments on the measurement forms, I felt there was little point in me raising a lone voice in comment on the rules.  On the other hand, if a significant number of people felt changes were needed then just maybe something would change.  I picked the multiple fin case as it is a fairly accessible example.  I actually think the clearest indication of the current state of rules are given by G.1.4 i) & j) and the accompanying figure L.2.0. but I suspect that few people have studied these rules enough to form an opinion as to what they are trying say and what they actually say.

If you want to know what I think the rules say about using multiple fins on a single certificate then:

Limitation C.4.4 b) states that you can use any appendage that complies with the limitations on the certificate USED FOR THE EVENT i.e. this limitation allows appendages to be changed during an event.

Limitation C.6.1 contradicts C.4.4.b) by stating that only one appendage is allowed to be used per event. 

Unfortunately 'event' is not defined in the rules, but we'll avoid that rabbit hole.

Limitation C 4.4. c) is also relevant.  This states that the boat must comply with the dimensions on the certificate used for the event.  Now any Physicist / Engineer will tell you that mass is a dimension.  (They will also tell you that when the certificate states the weight in Kilograms this is actually the mass not the weight) so I don't see how you can even switch to a lighter fin / bulb between events and still comply with this limitation (unless you add ballast to bring the 'weight' up to the value on the certificate - but see the following point).

Even if the replacement fin happens to 'weigh' the same as the original there is still a risk that it won't comply with the rules - this where Archimedes principle comes in.  If you have a short fin/bulb and a long fin/bulb that weigh the same, then the bulb on the long fin must weigh less than the bulb on the short fin (because the short fin must weigh less than the long fin).  As lead has a higher density than carbon, the volume displaced by the short fin/bulb will be less and hence the hull must displace more water to compensate.  The upshot of this is that the actual waterline length will be slightly longer with the short/fin bulb. and this could put you over the marks.  A similar effect occurs if you use a lighter fin/bulb and then add ballast inside the hull, but the increase in waterline length is bigger.

Limitation C.4.4. c) also affects the sails.  Since the certificate contains the measurements of the A rig, can someone explain how you can use a smaller rig and comply with this limitation?



Link to comment
Share on other sites

 I think the phrase is sometimes you cannot see the wood for the trees.

 First with regards to weight ( yes mass is component of weight but whereas mass remains the same weight can vary due to geographical  conditions so weight si relevant at an event location.   Remember matter in section C are not part of certification measurement but are the owners responsibility to ensure compliance and are subject to checking at event measurement. Not significant variation within UK). C.4.3 is relevant here.


When carrying out equipment inspection or measurement at an event the weight of the boat in sailing condition, dry and with its heaviest rig shall be found using calibrated equipment and rounded to the nearest 0.01 kg. The weight shall be not more than the weight recorded on the certificate plus a tolerance of 0.05 kg.

So essentially the boat cannot get more than 50g heavier than the weight recorded on the certificate but it can get lighter.  The reason for this limitation.  Essentially when setting the rule it was decided (by people who understand the science better than me)that  an increase of 50g or more will have a significant effect on displacement and therefore waterline length but a tolerance of 50 gave leeway for small permissible changes such as using different batteries whic might be heavier or minor repairs which might increase the weight.

From experience I would add that most owners declare a waterline length a few mm. longer than the length determined at tank flotation to allow for inevitable minor modifications. Calculating that the potential small loss in biggest sail area is less  significant than their overall sailing ability and construction and maintenance skills. (A pragmatic approach).

With regards to C.4.4 and C.6.1 These are both limitations at an event.   

C.4.4(b) The boat may use any hull appendages and ballast provided that the limitations relating to the certificate used for the event are complied with.

i.e the appendages used must comply with the Certificate. ( you could have a choice of appendages)  The plural  includes rudders.

C.6.1 Except when a hull appendage has been lost or damaged beyond repair the same hull appendages shall be used during an event. Such replacement may be made only with the approval of the race committee who shall then remove or cancel any event limitation mark attached to the replaced hull appendage.

i.e choose which of your appendages you are going to use for an event and stick with them for the whole event. (Even if it is a multiple day event.)

As a practical issue at event measurement for International, National and some ranking meetings the appendages declared at event measurement and are  marked with an event mark and spot checks take place during the event to ensure the declared appendages are the ones being used. (This course of action is supporteed by RRS and ERS and Sailing Instructions.

Re Sails.

In relation to C.4.4(c) a smaller sail can comply with the dimensions recorded on the certificate. Also see section K and  C.8 in particular (a) & (c) and G.1.2

All provide for alternative smaller sails provided they fall within the dimensions of the parent sail (i.e the jib or main  dimensions recorded on the certificate.)

As to Event - the dictionary definition "a planned public or social occasion" would seem to suffice.  In Radio Sailing Terms that is an occasion when Planned organised Racing takes place according to RRS, ERS & Class Rules.

Oh in relation to G.1.4 (i) & (j) I do not see a problem.  I Have seen them applied and they work.  Note the 900mm radius  template is also used  for a similar purpose in the M class rules.

Link to comment
Share on other sites

As a keen 10 sailor I spend my Sundays sailing.

Clearly some people have way too much time on their hands which would be better spent getting down to the club and sailing with the rest of us

It is actually a lot more fun than sitting indoors on a keyboard.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...