Cunningham / Downhual Primary - Suggest changing Class Rule to allow doubling of thinner rope
Dear Jonathon,
We are sorry that you feel the only way to be heard is to post this in our forum.
If you would like a response, please can you submit your request directly to the International Class measurer via the contact page on this site, who will reply to you in a timely manner and help with your queury.
Speak again,
Sammy Isaacs-Johnson
RS Aero UK Class Manager and International Class Manager
Reply 19/01/2026 13:04:00
Sammyblueij
Posts: 132
In other posts on this Forum I have mentioned I use a 3 mm line doubled:-
Technical > Replace Downhaul Line
&
Class Rules - Querioes & Suggestions > Securing Downhaul Tail in Gooseneck Notch
In the 2nd posting David Rickard has stated that doing so is outside class rules: it is not explicitly allowed.
The Class Rules give a lower limit for the Downhaul Primary of 2.5 mm diamater; whereas the rope supplied with the two new boats I have purchased in the UK has been 4 mm; tapered to pass easily through the upper / floating two sheave block.
I have used 3 mm rope doubled on both Aeros I have owned and together with: the lower double sheave block replaced by two singles, and more flexible 4 mm seccondary rope have a system that I find very easy to use, and that does not need added bungee to ease.
I have discussed this application with the rope manufacturer, Marlow's, technical support. They were unequivocal that doubling a 3 mm rope is technically better than using a single 4 mm rope, whether Vectran cored or SK78 Dyneema.
With either Dyneema or Vectran the breaking strength of the 3 mm is just > 1/2 the breaking strength of the 4 mm.
So doubling the 3 mm restores the breaking strength to the original supplied.
3 mm line does not need to be tapered to pass through the floating double sheave.
BUT:
ALL ropes loose strength as they: pass through a block, arond a cringle, or around itself as they are tied in a knot.
The ratio used is the D/d - where
D - the diamter being bent around: sheave, cringle, or itself
d - the diamter of the rope
In knots the strength reduction can reach 70%, depending upn the knot.
The Tack Cringle is 1 cm, 10 mm, width and 1/2 round section so 10 mm diamater.
The strength reduction of 2 X 3 mm rope passing round the tack cringle is < than the strengthn reduction of a single 4 mm.
The two strand figure of eight knot I use will give < strength reduction than 4 mm overhand or 1/2 hitch knot.
The two 3 mm strands slip,easily into the Gooseneck Notch, and in many years of sailing have never slipped out.
For all these reasons I propose that the rules be ammended to allow doubling of rope < 4 mm, or 3 mm or less (2.5 mm) diameter.
I do not know whether 3.5 mm diameter rope can thread through the double sheave without tapering.
In my opinion this change is more in keeping with the Aero design philosophy of simplicity, i.e. KIS.
Before giving a ruling I would urge David and others to try this out for themselves.
And please can I have a ruling before February 8th 2026 if possible?
As that is the date of the re-scheduled Grafham Grand Prix. I plan to enter & would not want to be protested as my boat is outside class rules.
Reply 19/01/2026 12:54:00
Jonathan Rickels
Posts: 120
