I will be up for a new mainsail halyard soon.
I have 3 mm wire spliced to 10mm braid.
I was looking at the options and because I have a masthead sheave suitable for the thin line it is either wire to braid (bought) or dyneema to braid which I (think) I can splice myself.
I checked various spicing/ testing youtubes for dyneema and it seems that it doesn't like to bend in a sharp radius.
It loses a lot of strength in that situation.
My plan was to do an eye splice and fit a halyard shackle (which is what I use now) but am concerned about the tight radius on the eye splice once the halyard is hauled tight.
Are you with me?
Any ideas?
Is the dyneema strong enough for that task even under that stressed condition?
gary
The masthead pulley is probably 40mm. The eye splice around the thimble would be similar. 3 or 4mm dynema would be plenty strong enough. My boat trailer has 6mm dynema and my race car trailer has 6mm dynema around a thimble and it has no problem pulling the car up.
Thanks Ramona.
I will give it a go.
Not a lot of cash at stake.
What diameter?
Presently 4mm wire, I was thinking of 5mm Dyneema as smaller than that would be fiddly to splice.
Also thinking of putting a simple sleeve over the eye to alleviate wear. Plastic fuel tube?
gary
I think it's more a case of what's the smallest diameter rope the clutch/cleat will take.
Also, the smallest diameter you're willing to pull up by hand... 
Hi Jethrow.
I plane to try a dyneema to braid splice.
The line that I actually handle will be 10 or maybe 12 mm.
gary
The easiest way is to use 6mm Spectra. Strip the outer polyester cover off for 2 meters or so. I take the halyard back to the cabin top to a winch with a 6 inch cleat behind it. I avoid jammers and camcleats and use 6 inch plastic cleats for all the halyards and reefing lines. Some of these cleats are 45 years old and very worn but they never fail!
In your case it will all depend on what size suits the top sheave. You have 10mm over it now so down sizing wont be a problem. 6mm is fine for a 34 footer and is no problem on the hands.
Nobody has addressed the issue of what's likely to be a steel or aluminium sheave that's had wire running over it for however many years, i.e. could have sharp edges that will tear Dyneema to shreds. If a visual inspection confirms the sheave does not have razor edges, fitting a Dyneema cover at the working tip will be essential, and has the added benefit of keeping the strength core out of the sun. If it does have sharp edges, replace it.
Don't worry about the bend radius around your halyard shackle, all you're losing is a portion of the eye strength, which is effectively double line strength. If you lost 30% due to bend radius, the eye is still stronger than the base line.
10mm braid into 4mm Dyneema is doable, 3mm will be too big a transition. To give similar performance to existing wire, select a high spec single braid, we would use Marlow D12 MAX 99.
...as for the advice of replacing your 4mm wire/10mm rope with stripped 6mm Spectra, take that with a grain of salt. Not apples and apples in terms of strength, robustness or stretch characteristics, and we certainly wouldn't be recommending 6mm Spectra for a halyard on a 34 footer.
If in doubt, discuss with a professional.
Select to expand quoteRE; If in doubt, discuss with a professional.
First of all, Thank you Wet Tech Rigging for taking the time for the very informative input.
I am in Japan and do not have access to rigging experts so your advice is greatly appreciated.
RE likely to be a steel or aluminium sheave that's had wire running over it for however many years, i.e. could have sharp edges that will tear Dyneema to shreds.
I was aware of this and was thinking (after masthead inspection) of whether to go braid to small diameter dyneema or replace the sheave and go simply for 10 mm double braid for the whole halyard. That would make the job much easier and I am not worried about stretch and a bit of re-tensioning.
Then I looked up at the mast head (this is not my boats but a friend's Yamaha 25) and noticed that she is 7/8th rig.
I have actually sailed her 3 times!!!
So, How do I get to the top of a 7/8th rig mast ?
The foresail or spinnaker halyard will take me almost 2 meters short of the masthead.
If I use the mainsail halyard , I will get to the top but not be able to remove the sheave or even rotate it to inspect properly
I have gotten hold of some 5mm Dyneema and practiced this splice and am pretty happy with the result, but am still up in the air (well not actually) about what to do at the mast head.
Cause I don't see how to get there and work.
gary.
