four main sails placed on top of each other. One has a longer foot than the others, So I measure it. 2430 and on the sail is marked 2430 that's fine, now the other sails, are all same size, smaller, and are all marked as being 2503 (larger) so I measured them, all came in at 2410, which is great for Steppenwolf, she can have a larger Jib. 
 
Who else is sailing around with less sail area than is actually marked on their sails and Boom?
 
That's a loss of about half square metre of sail area. 

Cannot be shrinkage because the larger one has stayed the correct size and is not the youngest.
 
Any reason for this or were the sails simply incorrectly  measured. 
 

Have you checked the Half and Three Quarter widths.    Theses are the measurements that limit the area, the foot measurement is a maximum and actual is often less than that on the boom.

Hello Bob, just rechecked the rules: the Half and Three quarter heights are functions of the foot: (F/2)+500 and (F/4)+640 so these will decrease with the length of the foot. The Black Band on boom, has to match what the certificate says, which is based upon the foot of the mainsail as measured at time of certificate issue/update. A sail measured as 2503 should reach a band at 2503 not be 100mm short when tensioned.
 
 
I'm just wondering why, out of 4 sails, 75% are incorrectly measured?  

Sail makers tend to cram as much sail towards the top as they can. You'll find most sails 3/4 measurement is rather close, half height far less so.
 
The foot quoted on the certificate is a maximum, not a thou shalt. The leech is often cut hollow (And i'm not a sailmaker so this is a guess) so i would imagine that in order to get a leech that will stand properly and achieve maximum 3/4 height they are prepared to sacrifice a little on the foot. 
 
off the top of my head i can't remember the numbers but the three or four sails that i've measured that have failed (In 8 years of measuring) it's been on the 3/4 or the position of outboard edge of the top batten.
 
I've yet to see a foot that's been particularly close.
 
Its also worth remembering that with laminate sails, especially some of the older cloths, the shrinkage can be enormous. 

Hello Chris, thanks for the explanation, and  that it may well be shrinkage.
 
This obviously raises another question, if these sails did originally measure at 2505 and have 'shrunk' in the horizontal axis 10cm over 2.5metres.
 
How long before they start to shrink, 5 races, 1 season , no races just time spent in varying temperatures? 
 
Should a sail be remeasured and the loss in area compensated with a different sized jib, assuming of course that one might actually want to do this.
 

100mm is a huge amount of shrinkage over a sail that size?

I wouldn't expect a yacht sail to shrink that much let alone a Merlin one.

Miles - The foot may be undersize but what about the cross widths?
 
The fastest shape for a sail with 2503 associated cross widths will not necessarily have a foot right out to maximum size.
 
Therefore if you use a sail that has a 2410 foot length but 2503 cross widths on a rig/jib setup for a 2410 foot the sail will not measure and you'll be out of class. 
 

That's my point Chris. They have a measured foot length of 2503 written on them, but measure only 2410, at most 2430 depending upon how you interpret 'edge of sailcloth extended if necessary'.
 
Regardless of half and 3/4 measurements, the one written on the sail is supposed to be the actual foot measurement of the sail.
 
I know they are between 20 and 7 years old, (the 20year old one had only been used about 10 times) but same difference on all of them. My two year old one is 'as it says on the tin'.
 
Do sails actually shrink this much and when do they start shrinking?

I think you'll find that they've been marked up as per the certificate i.e. they comply with the measurements as laid down for the boat.
 
If you mark the actual foot length on the sail, use it on a boat set out for a short foot and the cross widths are for a larger sail you will be out of class.
 
100mm does seem a lot - why not measure the sails?
 

Chris, Bob, have measured sail, and see what you mean. Feel a right idiot now!
 
Half H=1750   gives foot 2500
3/4 =1265  gives foot 2503
 
Where foot is the black band measurement and NOT the actual foot length of the sail. So they haven't shrunk, it was me missing the small print and that it's the sail to fit the black bands and not the black bands to fit the sail sort of thing. 
  

I thought that would be the case. I label sails i measure according to the certificate regardless of foot length and i have in light of this discussion checked the rules. They are vague, the only clue is tat it says ACTUAL area of the jib. When i was taught how to measure my trainer went from the certificate so thats how I've been doing it, I never thought to second guess.
 
Hopefully ISAF will close some of these vagaries, but i'm confident the number on the certificate should be the number written on the sail. 

Sorry long post, but as we are close to hitting a 'go ' button on some expensive stuff i thought i would check.
As i read the rules there is an issue regarding the mainsail measurement; the foot length of the sail SHOULD be measured and this measurement is the one that is needed on the tack to certify a mainsail as per rule G2.2 b) ii. However this measurement is not used as part of the formula for measuring the mainsail. That measurement is 'F' and is the boom outer distance measurement (how that is measured i'm not 100% on!)
Now how does a sailmaker as part of IHC know what or where the boom outer distance mark is? Even if they have the boom bought to them, is it boom length + gooseneck?
If they are making a sail they need the boom 'F' measurement to get the 1/2 and 3/4 heights so they can manufacture a class legal sail. if they do not have the boom F measurement then how can they construct the 1/2 and 3/4 heights? 
Now if an IHC or RYA measurer is measuring the sails they need to mark on the foot length as per the class rules to certify the sail.(foot length as specified in the ERS - see below).
As far as i can tell like the OP i have sails which all vary but all carry the legend 2503mm on the tack, certainly wrong as defined by the ERS. the longest foot length i have is 2450mm. so as such this sail is illegal as the certification mark is incorrect, as another measurer cannot and will not be able to certify that sail with that foot length?? 
The only way to measure a mainsail is to have the boom F measurement, which means we need the boom to measure sails????

Merlin Rocket pre-amble:
"These Rules are Open Class Rules. Owners and manufacturers are strongly urged by the MROA to read the ERS definition of Open Class Rules to fully understand the implication and philosophy of the class."
 
ERS definitions:
Terminology
A term used in its defined sense is printed in “bold” if defined in the ERS and in “italic”
if defined in the RRS. Other words and terms are used in the sense ordinarily
understood in nautical or general use in English.
 
And then the Merlin Rocket sail rules:
G.2.2 CERTIFICATION
a) The official measurer shall certify mainsails and headsails in the tack and spinnakers on the head and shall sign and date the certification mark.
b) The following shall be written legibly and indelibly in metric units not less than 20mm in height:
i. The headsail area as defined in G.2.3(d) shall be marked adjacent to its tack 
ii. The foot length for a mainsail shall be marked adjacent to its tack. 
iii. The area of a spinnaker as defined in G.2.3(e ) shall be marked on its head.
iv. The maximum headsail area, and the maximum boom outer point distance shall be marked on the boom spar adjacent to the outer limit mark.
G.2.3 DEFINITIONS
a) The Total Sail Area in square metres is 13.80 – (0.6 x (L + 0.68)), but shall not exceed 10.2sqm.
b) Mainsail area shall be calculated as (L x F) /2, where L is the dimension between the lower limit mark and the upper limit mark on the mast, and F is the boom outer point distance. The mainsail area shall not exceed 80% of the Total Sail Area.
c) The maximum headsail area is the Total Sail Area less the mainsail area measured in G.2.3 (b).
d) The actual headsail area shall be calculated as (luff length x luff perpendicular) /2.
e) The spinnaker area shall be calculated as 0.25 x luff length x ((0.5 x Foot length) + quarter width + half width + three quarters width).

You dont need the boom, you need the certificate. 
 
That tells you:
 
1)Max foot length
2)Max 1/2
3) Max 3/4
 
As long as your measure measurements are equal to or less than those numbers it is OK to use on your boat.
 
You cannot get sails measured by IHC or just any RYA Measurer. You need an MROA approved measurer who will know this and where to look.

Not what the rules say about MROA approved measurer?? 
Rules regulating the use of equipment during a race are contained in Section C of these class rules, 
in ERS Part I and in the Racing Rules of Sailing.
The Merlin-Rocket class permits IHC (In House Certification) as approved by the RYA for sails in 
Section G only. (Subject to Class Ratification)
This introduction only provides an informal background and the National Merlin Rocket Class 
Rules begin on the next page.

So as stated the ERS AND CLASS RULES " L " and "F" are mast and boom measurements?

The class hasn't ratified IHC.

We have approved measures to ensure that the person signing the sail knows the class. MR sails are not difficult but there are quirks in our rules which can catch people out.

Well it has, it is clearly written in the rules. IHC for sails, ERS and the certification done by an official measurer as approved by the MNA. 
Nowhere in the rules does it state that sails can only be measured by MROA approved measurers? 
And still the rules are asking for rig measures and not measurements from the certificate?? 

Hi I suggest you speak to Dan Alsop, out technical man if you have a problem with the rules.
 
I can 100% assure you that the class has not ratified IHC and has no current intention to do so - it would be an AGM decision. Its possible that there are minor grammar conflicts between the certificate, measurement form and new format rules but noone else has raised this.
 
There are plenty of Merlin measurers scattered around the county if you have a look at the RYA technical page. 

 no problem with the rules, just want to clear up what has been giving us a headache as measuring a sail doesn't give the L and F measurement.
It just seems that what is written down in the rules isn't what is being measured! and that the rules clearly state:
 
 "The Merlin-Rocket class permits IHC (In House Certification) as approved by the RYA for sails in Section G only. (Subject to Class Ratification)"
 there in black and white on page 3 of the rules. This means any sail loft that is IHC certified can measure the sails (only the sails) but they can't as they need the L and F measurement which is taken from the mast and boom.......

I know some sailmakers are not keen on IHC and like the idea of an independent third party doing the measurement... So do I! 

Miles and others,
 
Miles and others,
 
Well what a thread!  Chris Martin has asked me to try to sort this one out, so here we go!
 
The class has definitely not ratified IHC for sails so measurement can only be done by an RYA Approved Class Maintenance Measurer, an RYA Approved Class Full Measurer, an RYA Approved Sail Measurer or a World Sailing International Measurer.  We strongly recommend using a Class Measurers because in spite of our move to ISAF format rules and their link to ERS, the Merlin rules necessarily still contain unusual features which our own measurers take in their stride.  I'm afraid this is a price we pay for sailing very interisting boxes of tricks!  The fact that IHC is mentioned at all was an integral part of the conversion to ISAF format.
 
Because the sail area is a function of the rig height and the apportionment of the area between the mainsail and jib is a matter of choice, these parameters must be selected before initial boat measurement and cannot thereafter be altered without full re-measurement.  The critical parameters are entered on the measurement form and certificate and are also displayed on the boom. 
 
Hence the 1/2 and 3/4 widths are functions of the boom outer point distance as shown on the certificate and boom (G.3.4).  (For SS how this is established is defined in ERS.)  If a mainsail is undersize this is irrelevant.
 
Miles - you are correct in observing that a measurer cannot determine whether the 1/2 and 3/4 widths are within limits without reference to the certificate (or boom).  
 
Similarly the max jib area must not exceed the figure entered on the certificate, however undersize the mainsail might be.  
 
The actual area of the jib is determined by the measurer and marked on the tack so that all can see if an oversize jib is being used.  
 
I agree that the wording of G.2.2.b.ii is incorrect.  It should refer to the boom outer point distance rather than the foot length, which is a hang over from our old rules which was not corrected when we did the ISAF conversion. Being the lead on this exercise, which was forced upon us in order to retain National Class status, I apologise for missing this.  In my defence I would explain that the exercise was very complicated, took several years and went through 24 drafts before completion!  Glitches like this rear up every now and then and we'll get this one sorted out asap.
 
 
Regarding the shrinking of laminate sails, I'm afraid this is a characteristic of the stuff - I had one mainsail which lost 100mm along its luff in 2 years.  
 
Hope this helps, but feel free to 'phone me if any of you still need help,
 
Regards,
 
Dan Alsop
Class Technical Officer
01 297 444 502
07 712 010 264 
 
 
 
 

Well explained, Dan,
 
It is perhaps worth adding for clarity, Scam, that when a measurer is carrying out certification of a Merlin Rocket mainsail they will never measure the luff or the foot of the sail itself.
 
Pretty much the only measurements that require checking in practise are the headwidth; the position of the top batten and lower battens and the half and three-quarter widths calculated from the certificate.
 
There are other checks for batten length, number spacing etc, but these are very rarely close to max.
 
The fundamental misunderstanding behind the recent posts is that the L & F for the sail come from the sail itself. They don't - they are from the certificate and are determined at the time of initial certification of the boat, probably before the sail is even made!
 
Well done all..
 
GGG 

Thanks for all the answers. 
I've done the RYA measurers course so was just trying to clarify what I can see needs changing (maybe because I'm reading the rules for the first time and not taking it for granted "how we measure") 
I bought this to the attention of the RYA tech dudes, this week when I saw them ..I think they will be in touch to clear up the error 
An ihc loft can measure their sails but they cannot certify them.. Complicated? You bet. 
Sorry if this has caused a headache!