Why are these Masthead blocks breaking?

[Impetuous too]

Dear ladies and gents,

Just wondered if your esteemed selves may have any thoughts on what is going wrong at our masthead. We built our wooden mast ourselves (it's good and straight) and had our masthead unit built custom from a good marine fabricators in the UK.

We've been sailing for two years now. After around 10,000 miles on a routine trip aloft we noticed our forward main halyard block was broken. No problem we thought, it's seen some use, so changed it. We had two spares. Then after a cruise around the South Island of New Zealand we'd seen some fun weather so around 3,000 miles later I went up to check things; it was broken again. Hmmm.... Used our second spare and wondered a bit. Now only 200 miles later I go up to varnish the mast and it's already broken. Makes me wonder about those first 10,000 miles when we'd been up but not necessarily checked the block itself...

The main Halyard goes up from the sail, through the masthead fabrication through a Tufnol block at the aft, then the forward block then down. We are using 11mm Spectra on 11mm blocks. The block has split in the same way each time. It runs on a Brass bushing over a machine bolt. The aft one has never broken and remains in good condition. I went up today with a set square and ruler. As you can imagine in such circumstances it is difficult to be precise but as far as I could make out measuring around the various hardware, the bolt holding the block appears square and true within what I could ascertain.

Our suspicions are that perhaps Tufnol might be the problem. Is the Spectra spreading out and forcing the fibres appart? Any other ideas?

I've attached three photos. The first of the broken block, second the new block broken in place, the third of the masthead unit.

Many thanks in advance,

[svHyLyte]

The sheaves are failing due to "wedging action" caused by the line being pulled into the sheave channel and forcing the shoulders of the sheave channel apart (the sheave channel is the U-shape the line fits into about the sheave's circumference). That action is applying transverse tension to the sheave body which, eventually, splits along the center about the axle/pin as shown. The material is good in compression but weak in tension hence it splits (in the same fashion that a plank will split with wedges driven into it.) Note that Spectra, particularly in tension, is/becomes very "hard" (i.e. become incompressible) hence, under the applied load it is the sheave that must "give". If it is too difficult to change to larger--i.e. wider--sheaves, or to machined aluminum sheaves that fully cradle the line, change down to 10mm line.

FWIW...

[Delancey]

Wow, haven't seen Tufnol sheaves in like, since forever. That's your problem. Where are you even getting replacements? Seems like it was abandoned as a material for sheaves in the late seventies early eighties for precisely that reason. Get sheaves made from some other material and your problem will go away.

[Impetuous too]

Thanks both, will do.

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