Halyard sheave collapse
I had to take my mast down as the mainsail halyard sheave has collapsed. I cannot help wondering if it is due to an increased load of Dyneema halyard and higher tech sails – a point that has not been mentioned in any articles I have seen.
I have also had the pulley wheel collapse in the laminate self-tacking jib.
My temporary solution, to allow me to keep sailing until the end of the season, was to attach a pulley to the existing halyard with the old spare halyard rove through it. This I hoisted to the masthead – which I was able to do over the halyard axle as there was no load on the halyard. I then secured the original halyard as tight as possible to a spare cleat and hoisted the sail on the spare halyard. Sam Longley by email
MIKE COATES REPLIES: It is possible the use of Dyneema and therefore higher loads have contributed to the failure of the sheaves although I suspect UV degradation has also played a part in their collapse as they are often manufactured as a moulded product along with the sheave design itself.
Originally sheaves used in spars tended to be produced with a slight V-groove to help wire halyards run in the centre, preventing chafe on the sides of the sheave box. When synthetic materials like polyester and later Dyneema/ Spectra came along, sheaves were produced with a flatter, more rounded groove which tended to spread the load better on the sheave.
I don’t think it is particularly higher loads that are being used with high tech ropes – the problem is the reduction in their diameter producing higher point loading on the flatter bottomed grooves.
In some instances where wear has occurred on the sheave the halyard will settle into the grove and this will cause a splitting effect, fracturing the sheave.
Although difficult to find in every size, some spar and block manufacturers can supply sheaves that have been turned from aluminium and fitted with a low friction internal bush.
I’d suggest replacing any highly loaded sheaves with aluminium or stainless steel to increase sheave life.
The broken halyard sheave Sam Longley's temporary solution