Replacing Sheaves

I need to replace the sheaves in my boom as well as those at the masthead. 

Also, according to our local rigger, our mast head unit was totally under-spec (Sparcraft) and so we followed his advice and let him cut out that unit and totally rebuild it to spec. Our problem that led to this was the main halyard would jump off the sheave and get irretrievably jammed between the separating plate and the sheave which was frustrating and potentially very dangerous. Otherwise, replacing the sheaves would have been the same as the above

Peter the rigger told us the main problem which he has fixed on a number of production boats, was the plate that separates the port and starboard sheaves is too thin and not welded in place, so it tends to move/bend under load, particularly if you say were trying to reef or raise the main while running down wind. On the Sparcraft masts all that holds the plate in place is a slide too and bottom and the sheave axles running through it. The rigger built one with a sturdier plate that was welded in place. Sheaves could still be removed as per normal for inspection/replacement 

I turn my halyards round end to end every few years and I have even switched the jib and main halyards around to even out the wear.  Replacing Dyneema halyards is not cheap, so best avoided where possible.

If it’s a Sparcraft mast you can climb on the main halyard but still replace the sheaves. The pins are fairly easy to extract. Depends on your climbing skills and comfort operating at masthead though. I’ve done it using climbing strops looped over the masthead to take the my load temporarily.  I’ll leave you to think through safety lines and alternate descent solutions ‘just in case’ you can’t replace the sheaves. 

Edited by Mark_J1 - 07 February 2026 at 22:22