Tuning talk at Datchet radio sailing club

Preamble

This Sunday saw the Datchet Radio Sailing AGM and to add a bit of interest to encourage members in I was asked to give a talk on tuning. Well I would have but three of us were allowed to carry on sailing which we did for four and a half hours in the cold.

This Buildracetune web site covers most of the rig tuning basics. Setting up a Britpop, how you limit the variables in an IOM rig and pointers to the various videos on tuning different types of boats.

So I thought it might be interesting to talk to the club members about the theory of the new shroud set up I am now supporting designed by Sailboat RV. Their article on simplified rigging sets out what the are trying to achieve with no prebent rigs for club sailors.

Remember all this is theory for me at the moment but I will report on performance as I get the boat on the water.

How much prebend

My latest spar has a 7mm prebend with max draft Sailboat RC sails (mould 21) and shrouds that attach to the mast via sliding hounds. I start at the top of hound range in light weather and bring it down as the wind increases. I did have the shrouds set up on a 16mm prebent mast but that proved to be far too stiff. Never thought I would say that about an IOM mast.

I wanted to start with a bit of a teaser on prebend and set the mast up with max shroud tension with no jib luff or backstay tension and asked the audience to say how much prebend there was in the mast. (See first picture below). I then slackened off the shrouds and asked them again (See second picture below). There was supposed to be a look of surprise and would follow this up by saying you could create a prebend effect with a straight mast with the shrouds alone.

Mast with no backstay or forestay tension but with max shroud tension. Is this the equivalent of prebend.

Here is the mast with the shrouds completely slack. There is only 7mm of prebend and maybe less than that.

By varying the shroud tension one can control the bend of the mast. Mast ram for lower third, spreaders for middle third and shroud and hound position for the top third.

Set up for light winds

So I then set the boat up for light weather. The jib boom is set a cm off the deck with my light weight tie down and because I raised the boom I need to tighten the jib liff bowsie the same amount to maintain mast rake. The mast is set with the hounds set at the top of the setting range, a slight bend to take out some of the luff curve and check the mast rake so the boom is just above parallel to the deck. The mast ram is eased and the cunninghams are loose The jib luff will be relatively slack and with little tension will goosewing easily in very light breeze.

Once I am happy with the shape of the main, with its twist so the top batten is parallel to the boom I then check the leach tension on the run for both port and starboard gybes. If the leach is looser on say starboard gybe, one needs to tighten the starboard shroud or loosen the port shroud and visa versa.

Set up for medium winds

I drop the hounds a bit which tightens the shrouds slightly and lower the jib boom to the deck remembering to ease the jib luff tension bowsie by the same amount. I will increase the vang as needed and maybe add a mm or two of backstay.

Set up for top end A rig

I drop the hounds to the lower level and again this tightens the shrouds, adjust the vang and mast ram as needed and will apply the max backstay to flatten the sail slightly.

How does the movement of the hounds change the mast bend.

There are two types of bend to consider.

Fore and aft bend: The lowering of the hounds increases the length of stiffening wire running up the front of the mast creating a more even bend as the backstay is applied.

Sideways bend: The lowering of the shrouds increases the length of the mast unsupported by the hounds and allows the top of the mast to fall off sideways allowing the rig to depower in top end conditions. It enables the top of the sail to blade off in windy conditions.

Sheeting angles

I have a 10 and 20mm mark on the deck by the mainsheet post and a 12 degree mark on the deck under the end of the jib boom. According to various studies this is the optimum angle for a jib boom (unless you sail a TP52 or Americas cup boat) but we want to only have the mark as a baseline and move the jib inside and outside of this mark.

Jib leach twist

The amount of twist will vary from sail to sail. With my moulded sails, I use a lot of twist, up to 50mm. On my BG sail I would use approximatley 25mm. With my rig set up I would always check the jib leaches ability to open slightly in the puffs and also make sure it does not open to much as this will cause lack of acceleration and weather helm.

Those who have followed the blog know I am into mixing on the transmitter.

Use of mixing means in practice

When I sheet in, the side of the main boom goes to the 20mm mark on the deck and the jib boom sits outside the 12 degree mark. This gives me a great VMG mode. In flat water when the boat is up to speed I will move the sheet stick to left and this will tighten the sheets to the point when the main boom is near the centre and the jib is inside the 12 degree mark. This will be my high mode for use when I want to climb away from a leeward boat, hold a lane off a start line or get the most height out of a strong lift. If I move the stick to the right the sheet goes out the same amount which is great for tacking.

So much for the theory. I have used the rig with 16mm prebend and was really impressed with its performance upwind but I have a lot of work to do to understand the nuances. Today I sailed with the mast with the hounds in the heavy wind position and was able to sail a beautifully balanced boat in gusty conditions. (7mm prebend with Sailboat RC max draft sails). Some people may consider this rigging set up over complicated but I believe there are benefits.

If you want to try this out yourself, you could build your own rig, but be careful to get the measurements right. If you have a K2 or similar you can buy the rigs straight from Sailboat RC. I am working up this homebuild rig on the Proteus design.

With the above shroud system, I believe it is now possible to set up mast witout prebend and have full control in all weather conditions. Afterall there is little difference between 7mm prebend and no prebend.

One final point. This rig change will not benefit you if do not get out and practice. As my old 470 sailing partner Jim Saltenstall used to tell me, there is nothing better than time on the water for improving you ability. However if your try it and understand what it can do in each wind condition you will be the better for it.

Any questions, send me a message.