If you really cannot wait for calmer weather or even until moored safely alongside, this is how to climb the mast while sailing.
Of all the things I have encountered over my sailing career, climbing a mast at sea is one of the top five to get my heart racing. Any motion felt on deck is amplified the higher you go, leaving the climber not only to contend with the height but being flung around like a rag doll with every roll of the boat.
If there is any option not to climb the mast while at sea then take it – get to a port of refuge or at least some sheltered waters to anchor before going aloft. If the problem must be solved while at sea then ask yourself whether you could wait until conditions are light. The rougher the seas the greater the risk when going aloft.
When planning for long offshore or ocean passages, put together a dedicated mast climbing kit, then practise your procedure on the dock and in sheltered waters to make the live exercise a lot less daunting. Invest in a good quality climbing harness, big enough to fit over layers of foul weather gear, and a lightweight helmet – it is a good idea to remove helmet ear flaps for better communication.
Wear plenty of clothing to avoid bruising from harness straps and knocks against the mast and rigging.
Before going aloft think about the job you need to perform and gather together the tools you will need in a bag that can be closed while you are climbing but opened with one hand while aloft. I use a small drybag with a Velcro mouth but there are alternatives.
Having duplicate halyards is wise for any offshore yacht, and ideally use a spare main halyard to make this climb. If your boat uses a topping lift, then check that the breaking load is suitable for it to be used in this way.
If you have a double sheave box at the top of the mast but do not want a permanently rigged second halyard then rig a mousing line instead and carry a spare halyard. If this is not possible, then masthead spinnaker or jib halyards can be flipped over the shrouds and used in the same way.
Fractional halyards are less suitable for this type of climb.
Using a safety halyard is advisable but not always practical or possible. If it is vital you climb and a safety is not available, then it is essential the person winching is experienced and has practiced with you a controlled decent from the mast under sail.
The primary danger is losing hold of the rig and swinging freely, which can lead to injury from high speed collisions with the mast.
My preferred method for preventing that is to climb abaft the mast with the boat heeled moderately. This reduces rolling and the mainsail offers a solid surface for a climber to lean against. I set the boat up with mainsail alone sailing at around 60° TWA, adjusting the traveller and sheet to ensure the boat does not round up.
Even for those adept at mast climbing in port, it can take a great deal of strength at sea just to stay connected to the mast. Generally the crew will resort to winching while the climber uses their arms and legs to keep aft of the mast, leaning into the mainsail, pushing up off the top of windward mast steps or even batten cars.
Swap grinders regularly if you are able and always ensure the person tailing the winch is looking up. Once in position to work the climber should use short strops to tether themselves next to the mast to reduce the risk of swinging around while working.
The descent can actually be a lot scarier for the climber than the ascent: handholds are beneath your line of sight and as the halyard lengthens the potential to swing is greater.
A good descent is smooth and at just the right speed. The cockpit crew should stand as far back from the winch as possible looking up; spare crew should ensure the halyard is free to run out and also stand by the jammer in case of problems.
Beware of spreaders, running backstays or radar brackets where the climber may need a bit more time to manoeuvre their legs and arms over an obstacle, and communicate constantly about the speed of the decent.