No one likes to go up the mast, but when you do go up the mast, you should make it as safe as possible. This requires the proper gear and knowledge of how to use it.
There are three important parts to mast climbing
Coming Back Down
Going up may seem simple, but how do you raise all your weight to the top? You can have a trusted friend grind the main halyard winch to raise you up, but if you can't find a friend who is willing to hoist you up, you might have to go up alone.
They sell many different systems such as mast steps and mast climbers, but these are systems are bulky and complicated.
Metal mast steps add weight aloft, offer more places for halyards to foul, and are very expensive. They also entice sailors to climb the mast without a harness which can lead to deadly falls from the rigging.
Soft mast steps are attached to the main halyard and offer the same problem, you are tempted to climb them just like a ladder without any safety gear.
Mast climbers are interesting devices that attach to a line and allow you to shimmy your way up the halyard. They work well and offer a way to climb the mast in a hurry, but you are forced to stand up there while you are working aloft. Coming down is also rather tricky since you don't want to accidentally disengage the system and plummet to the deck.
I prefer the use of a gantline to climb the mast, it offers the safety and control of a pulley system and attaches to the bosun chair and safety harness. This system consists of three parts, two blocks and an ascender. These parts are not regular marine grade blocks, but rather blocks designed to hoist humans during rock climbing. They can not accidentally be opened and make the process very safe. The ascender is optional but it makes grabbing the gantline much easier.
The double blocks are made by C.A.M.P. and are relatively available at REI.
The ascender is made by Petzl Ascension and also relatively available at REI. I tie a small piece of dyneema to it and connect it to my harness. When I reach my desired location on the mast, I can let go of the gantline and the ascender will hold me in place while I work. The first time you let the ascender go may seem scary, but trust me, you will be fine (as long as it's tied to your harness!)
To go up, I feed the line through the double blocks and attach one block to the main halyard and run it up the mast. The main halyard is securely cleated to secure the upper block at the mast head and the lower block is securely attached to the bosun chair. I attach the ascender to the gantline and begin pulling myself up. This system offers a mechanical advantage of 4:1 and allows someone with weak arms like myself to hoist my heavy body to the top of the mast. If I were to loose my grip on the line and fell uncontrolled, I would be slowed somewhat by the friction imposed by the block system. I don't recommend testing out a free fall, but it will give you enough time to grab the gantline. If you were falling, the best way to stop your fall would be to grab the four lines involved in the pulley system which will easily bring you to a halt. Best yet is to avoid a free fall all together, which will be discussed in the safety section below.
Safety aloft is the most important thing to maintain while climbing a mast. With this in mind, we shall look at proper safety protocol.
The line for the gantline should be strong enough to support your body weight easily. I use 1/2" Three Lay from New England Ropes. I recommend the use of Three Lay because it offers more warning to chafe and if chafe occurs, it will usually only affect one of the three strands. Two of the three strands will be enough to hold your weight as you lower yourself down from the mast. If double braid were to get cut or chafed severely, your safety could be severely compromised, three lay is a little more insurance to a deadly fall.
The blocks should be inspected before each and every use to make sure they are free of cracks or corrosion. If there is any question, don't go up on them!
All attachments should be made with knots or threaded shackles. Never use a snap hook as a primary attachment. If the snap shackle opens, you will fall straight down.
The mast is both your friend and nemesis. It will keep you safe as long as you are close to it. If you move too far from the mast by wind or wave action, you will end up smashing back into the mast. This can be extremely dangerous and damaging! To avoid being swung away from the mast, you want to tie a safety line around the mast and to your safety harness. This will keep you close to the mast at all times. If you need to swing out to the end of the spreaders, simply tie it to the shroud and pull yourself out in a controlled manner. Never jump or swing because if you miss your target, you will smash back into the mast!
Attach a safety line from the bosun chair to your safety harness. This way, if you slip out of the chair, you will still hang by your harness.
Tie the rope ascender to your harness. This way if you let go of it by accident, you will not fall. The ascender will rise up until the safety line is tight, then it will hold you at that height.
Carry a messenger line with you so tools or parts can be passed from the deck up to you. This will allow you to send things back and forth without needing to climb or descend to retrieve them.
Tie a line that is secured to you to all tools you value. If a tool slips, it will reach incredible speeds as it plummets to the deck. This will damage your deck when it smashes into it and the tool will then bounce into the water to be lost forever. By tying a line to the tool, it will stay tethered to you and avoid all the damaging consequences.
Never use tools while someone is below you. Have any helpers relocate themselves to the bow or stern. If a tool or clevis pin slips out of your hand and hits them in the head, they could die from brain trauma.
If you are replacing your rigging, never disconnect all the shrouds or stays at the same time. Replace just one at a time and have that stay supported by a halyard. This way the mast head will always be supported. The lower shrouds are much less sensitive to this and can be replaced all at the same time.
Coming back down
After the job is finished, it is time to come back down to the deck. The safest way to do this is slowly. I take a firm grasp on the gantline and remove the ascender and let it hang by its tether line to the harness. I lower myself down hand under hand, never letting myself build up momentum.
Do not let the rope slide through your hands in a controlled manner. The friction will build heat leading to rope burn. When this happens, your instinct will be to let go! If you save yourself from the fall by grabbing the gantline bundle, you will still have rope burn on your hands, making any successive projects painful. This is why you avoid the whole situation by lowering yourself in a controlled manner until you reach the deck safely.
Lower yourself down slowly, and keep the safety line tied around the mast. When you get to the spreaders: wrap your legs around the mast, make sure there are no waves or wakes coming, untie it, pass it around under the spreaders, and tie it again. If conditions are not calm, use two safety lines. Keep the first one tied, tie the second on the other side of the spreaders, then untie the old safety line.
Following these steps you will be able to safely climb, work, and descend the mast without injury or casualty. Remember that anything you want to do will take longer to accomplish aloft and you need to be extremely careful to maintain perfect safety while aloft.