What is a suitable ratio for the kicker depends heavily on the age and type of your spars.

At the time of the Mk 1 GRP boats ,  and probably through to Mk 2 at least,  it was normal to have comparatively modest purchase,  typically in the range 2:1 to 4:1,  with 3:1 perhaps the most popular.  The kicker was used primarily to stop the boom lifting when off the wind,  not to depower the rig by bending the spars in order to flatten the sail.  Sail twist when well off the wind was controlled by the mainsheet,  not the kicker,  which is why the full width sheet horse was developed,  and this was a feature of the Mk 1 and Mk 2 GRP boats;   however it had fallen out of favour by the time of the Mk 3 boats,  and that may possibly indicate that more powerful kickers were then in use by then.

With modern spars,  the kicker is used to actually bend the spars,  in order to flatten the sail in stronger winds,  and to control twist.  Typical kicker purchase on the modern rig is 16:1 or 18:1.

For what it is worth,  15 years ago I had a modern powerful kicker on a Series 1 (wooden) boat from 1979 (sail no. 11930),  and both boat and rig were well able to handle the loads.   However I would counsel against using such a powerful kicker on a boat with early spars,  e.g. the IYE ones,  contemporaneous with the early GRP boats;   such spars were not designed for those loads in the first place,  and many of them are no longer anything like as strong as they were when new,  because of deterioration (cracks,  corrosion,  and extra holes drilled,  just for starters) in the several decades since they were made.

Hope this helps,

Oliver

• This reply was modified 4 years, 2 months ago by Oliver Shaw. Reason: Highlighting

Rob

Oliver has given you some information regarding efficiency ratios used over the years.  Early boats I sailed had simple multi purchase block and tackle systems with an integral jamb cleat on the lower multi block for the crew to make the adjustments.  Problem with the multi block systems was friction with the ropes running though the blocks – the next step was to use a leaver system with a two part adjustment rope which was either cleated on the mast or lead through block along the side of the plate case or even better to the side benches.

The modern systems use cascade blocks lead back to the side benches or to the aft end of the plate case.  can be powerful but can be set up to limit loads.

For a Cunningham system a simple single run of 6mm rope from an eye on the mast though the eye or block on the sail down the other side of the mast to a jamb cleat on the mast.  You can of course double up the purchase and also lead the ropes back to cleats on the side benches or on the plate case.

There are pictures of rigging systems on this web site but most show newer series boats – I have a set of sketches and some photos of older systems – send me an email if you would like a copy scorbet@btinternet.com

Steve Corbet