It is pretty well essential for a cruising dinghy to have provision for reefing, that is for reducing sail area as the wind strengthens. For a tryout dinghy cruise on an inland water or sheltered estuary you could manage without but you should have tested your arrangements for reefing before you go coastal sailing. Most dinghies designed for racing or general purpose sailing have a gooseneck which allows the mainsail to be partially rolled up onto the boom for reefing. That is better than nothing but it is not the ideal reefing method. A sail is not a flat sheet of cloth but is shaped three dimensionally. Hence when you roll it up from the lower edge you get creases and usually the outer end of the boom drops well below horizontal. Also the luff rope bunches up at the inboard end and it is awkward to use a kicking strap, although this can be done by attaching the kicking strap to a length of webbing rolled in with the sail. The preferred method for reefing a cruising dinghy is known as points or slab reefing and this has also now largely superceded roller reefing for big boat sailing. With slab reefing there are pairs of cringles (large eyelets) for each reef, one at the luff and the other at a similar level near the leech. Tying one of these pairs of cringles down to the boom takes a section out of the sail and also produces a bag of loose material. This loose material can then be rolled up and tied along the boom, usually by means of reef points which are short cords fixed through a row of eyelets running across the sail between each pair of main reefing cringles.
The reefing cringles at luff and leech are heavily loaded and the sail needs to be suitably reinforced at these points. Fitting reefing cringles to an existing sail is probably best left to a sail maker, it is less likely to be a DIY job than, for example, making a boat tent. Luff and leech cringles suitable for reefing are usually fitted with a small hydraulic press and special tools. Also, sailcloth is harder to sew than most tent materials. If you really want to fit your own reefing points to the sail I have heard a suggestion that one could use a loop of heavy polyester webbing to secure a stainless steel ring to the sail at the leech and luff. You would still need reinforcement patches on the sail where the webbing is sewn on. This method is sometimes used for attaching sheets to the clew of a foresail so I am sure it is strong enough for reefing lines provided that adequate stitching and reinforcement is used. I don't think many domestic sewing machines would stand a chance of sewing through two layers of webbing and several layers of sailcloth so hand stitching is required. A proper sail makers needle is needed, these are sold in chandlers shops. You may need to pre-make holes for the needle using a spike or even a large masonry nail used with a hammer and a block of scrap wood. I have also resorted to the use of pliers to pull the needle through.
When you take a reef you need to tension the sail between the luff and leech as well as just tying it down to the boom. The method I usually use with the HSC club boats is to start by tensioning the sail horizontally by taking a short cord through the luff cringle and tying it round the mast then taking a cord from the end of the boom through the leech cringle and tensioning this back to the end of the boom. The cord round the mast takes the considerable horizontal tension in the foot of the sail and importantly it keeps the luff of the sail close to the mast so that the luff rope is not trying to pull out of the mast groove. Having got the right sort of tension in the foot of the sail I then tie down down both leech and luff cringles to the boom and finally tie down the reefing points. The various short lengths of cord used can all be fitted with reefing pegs. Reefing pegs are an easy way to tie ropes tightly without using knots which could be hard to get undone later. They are described later in this script, click here to jump down to this section.
This link gives full details of an improved arrangement (perhaps better than as described above but needing a few extra bits of gear) for reefing a Wayfarer mainsail. This would be applicable to most other bermudian rigged dinghies. Perhaps we should set up a similar arrangement on our HSC club dinghies.
The main function of the reefing points between the luff and leech cringles is simply to tidy up the loose part of the sail, they do not carry the main loading on the sail. You don't need many reefing points, I have seen just two or three for each reef used successfully. Traditionally these reefing points consist of a short length of cord run through a small eyelet and kept in permanently on the sail with a couple of stopper knots. I just have the eyelets in the sail and thread the cords through them when needed, using reefing pegs (as above) to secure them.
I would advise against having your rows of reef points too closely spaced in the vertical direction. It is not necessary to adjust sail area in small steps and if you do you will spend a lot of time reefing since it is usually not really practical to keep a sailing dinghy fully on the move while the reefing operation is in progress. For most purposes two deep reefs are adequate, you might have three if you plan to do a lot of sea sailing. You can save taking out the lower batten by having the first row of reef points just below the lower batten. The link given above suggests that for a Wayfarer the first reef could be 69 cms from foot of sail which is just below first batten and the second reef 154 cms from foot which brings head of sail to top of shrouds. These dimensions could be scaled appropriately for other bermudian rigged dinghies.
You will probably also need to be able to reduce foresail to match reductions in mainsail area. The first thing I would say here is to check whether you actually need to use the foresail at all when the main is deep reefed. I discovered that my boat remained well balanced with a much reduced area of mainsail (actually a separate small mainsail rather than a reefed sail) and no foresail at all. Thus I usually remove the foresail altogether when the small size mainsail is in use and from then on I don't have to go on the foredeck to fiddle with foresails. However, the majority of boats do need a small foresail area to balance a deep reefed main. This means either changing to a small foresail or having a roller reefing gear for the foresail. Changing foresails at sea on a dinghy can be quite tricky. On most dinghies the safest way is to lean out across the foredeck rather than trying to climb right onto the foredeck. I don't have personnel experience of foresail reefing/furling systems but such systems should make foresail area reduction much easier and safer and are probably to be recommended for cruising.