You can do it but you will need a semi-rigid messenger which you can push through the mast. I used an old wire halyard. It was only just rigid enough & took a few tries to get it from top to bottom. Perhaps old standing rigging wire would be ok.
Just wanted to make sure it was indeed possible before I started pulling a new halyard through with the old one to realise it doesn't work half way through. I'm guessing its much easier to add halyards to where there isn't an old one or a mousing line in place?
Just tape the new one to the old and pull through. If that's not poss for some reason, pull a mouse through with the old one, then use the mouse to haul the new one. Piece of cake.
I normally take out the halyards leaving thinner rope tails through mast and sheaves. Then I can wash the green cr*p out, have sparkling 'new' halyards at start of season.
Personally I do not advise new halyard to old - as the joint (best is end butted to end and then sewn together with sail-twine) is usually too big to pass easily through the sheaves and may also get stuck on spreader bar in mast. The best IMHO is to get a reasonable light but strong line, 4 - 5mm should be fine. Sew this to halyard end and then with thin electrical tape cover the joint to make a smooth but tight joint. Now pull carefully through ....
I also take masking tape and wrap end of halyard and write on in permanent marker pen what halyard it is !! I also use same pen to mark mast sheaves which halyard !! You'd be surprised to know that I do this after forgetting which was which first time I did it !!
In my experience you run less risk of a jam if you use the old halyard to pull through a thin mousing line first and use that to draw down the new one. Ideally, pull down two mousers and leave one in place when you have finished, it will certainly come is useful for other jobs later, such as renewing cabling etc. I would not rely on taping to join old and new halyards for pulling through. It can make a bulky and insecure joint that is likely to catch and part company in the middle of the operation. Better to sew a proper loop on each end and join with twine.
My Halyards have a small loop whipped to the end. It's made from platted whipping twine & goes through the halyard so can't be pulled off without breaking. It's then very easy to ties a thin cord to the halyard. Pull the halyard out. Attach the new halyard to the cord & pull it back.
Tieing 2 halyards together is generally no use. The knot is too big. Taping together can be a big problem. Any stoppage & you end up pulling harder & the tape gives.
Sew & whip a small loop to the end & you'll have no problems.
If you do end up having to run a halyard or anything else for that matter without a mouse, then get yourself a cable-puller. These are availablel from electrical suppliers & are a thin metal strip on a drum with a small loop on the end. Fencing wire etc is generally too soft & will buckle. My cable puller get lent around the club frequently for pulling ropes & cables through masts as well as cables through boats. well worth a few £.
I am just going to do that.
I have just ordered 220 metres of 6mm white polyprop rope from ebay (£15 delivered).for this specific task, it should do the job fine.
It is important not to use too small a mousing line as it may fall down the side of a pulley wheel and jam.
All my halyards have twine loops in ends - but still prefer my method - as the halyard end is an abrupt face and often catches on the bar inside mast between spreaders. With my way - you have a smooth joint that generally passes over everything.
Cable puller - keep meaning to find and buy one !!
The last time I did this I used fishing line to sew the new halyard to the old, then taped over the joint. Fishing line has the advantage of being strong and thin, so poses less risk of snagging.
I have replaced several halyards by cutting off the eye & stiching the butt end to the butt end of the new halyard with whipping twine & winding some self amalgamating tape around the join.If you pull the tape tight it does not leave a bulky join.I also have a loop stiched to the end of the new Halyard to enable me to put a mouse in if I need to.
I have done the same ... and is usually OK - particularly if you have external halyards. But I have also lost halyards this way inside the mast - so now always use a smaller line as feeder ... IMHO better safe than sorry ...
I use the green tube braid thrown away by fishing boats . It slips over the halyard or wire and is then pulled tight. Tape the end for added security. Gives a nice tapered joint. If you can't find the green stuff, cut a bit of braid off an old bit of rigging.