hi gents, I need to climb up my mast to replace the nav lights and wind vane I haven't done this before and I don't know anyone who has.My question is can I use the jib uphaul to raise the mainsheet block to the top of the mast and use the bottom block attached to the bosuns chair to raise myself up the mast. This will give me a 4/1 ratio the rope will pass through a single sheave and back to my winch. I can use the spinnacker uphaul attached to the bosuns chair as an extra saqfety line. OK or foolhardy.

Falling from more than a couple of meters will really spoil your day. I'm probably more cautious than most when doing this. I got advice from a friend who's an experienced rock climber before setting up my system.

I use a safety system that's completely independant from the chair and whatever is pulling it up. The safety protects you not only against the main support failing (line breakage, block failure, knot failure etc), but also a chair failure or even falling out of the chair. A bit hard to describe without pictures, but very simple. The essentials are a harness, about 1.5m of 8mm braid, a short length of ~6mm braid tied securely in a loop, and a spare rope halyard tied at the base of the mast, pulled taut, and cleated off securely. You tie the braid loop to the halyard with a prusik knot, and use the 1.5m braid to connect your harness to the loop (using a bowline at each end). The prusik knot is the key (google it). While you're moving up or down, just grab the prusik knot and slide it with you. But if there is a failure, tension will cause it to lock. From time to time you might need to stop to untie and retie your 1.5m braid at one end, to navigate past spreaders etc.

Mind you, your problems wouldn't be entirely over if you did have a breakage and ended up dangling from your harness and safety line near the masthead. But you'd be in a much better situation than the alternative.

Always tie the halyard to your chair with a bowline, don't use a snap shackle.

Theace. That's the way I do it. Simply because, my mate couldn't winch me up.
We need it 1 to 4 , very convenient . I used extra long brand new rope through the blocks.
Alway use the way Morning Bird suggest .
Main halyard winch the block. Spin. halyard as safety back up.
Make sure your tool bag and tools are secure, if you drop anything,
Could be potential injury or damage.
To stop me going up and down, I take spare light rope with me
In case I need more tools, drop the rope and winch the bucket with more stuff.

Using a second person to winch you up is obviously best, but if you need to ascend your mast solo, you just need a couple of high-quality mountain climbing ascenders and a good climbing harness, in addition to your bosun's chair. Paddy Pallin or similar can set you up.

I use the system that Allen Edwards describes in his article (l-36.com/mast_climbing.php). I've done it many times and I thoroughly recommend this setup. The key thing is to use TWO halyards, one of which you actually ascend, and the other as a safety line that you tie a Prusik knot to and attach to your climbing harness. Very safe and very easy.

Don't tell anyone but I can get to my 2nd set of spreaders while sailing with out a chair or safety line!