Hal­yard sheave col­lapse

Practical Boat Owner - - Ask The Experts -

Q

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

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