Clew block for self-tacking jib

[requiem]

I'm trying to spec a clew block for a self-tacking jib, and may be over-thinking it. The boat is an Oceanis 41.1, and if a picture helps this is the situation I have in mind:

One question I have is, between a soft shackle to attach the block to the clew board, the block itself, and the sheet, which should be spec'd to break first, or does it even matter? E.g. the soft shackle, so a flogging sail isn't also flinging the block about?

The second is what forces should I reasonably plan for? I don't have the exact sail area of the jib, but sailboatdata.com lists a ~41 m^2 foresail area which I can plug into Harken's genoa system loading calculator to get a sheet load of ~1000 kg at 34 knots wind, and ~2000 kg at 48 knots wind. (I assume that reefing would happen long before those points, of course.) A Harken 57 mm ESP swivel block has a working load of 1000 kg and a breaking load of 2000 kg, which I am thinking should be sufficient.

Is there something I might be missing?

[Benz]

Spec it so nothing breaks.

[requiem]

Originally Posted by Benz

Spec it so nothing breaks.

Ideally, yes. Looking back at my post, I'm thinking that the 100 mm size (breaking load 4400 kg) may be more appropriate. The boat itself shouldn't be out in such winds, but I am concerned about someone accidentally letting the entire sail unfurl in gusty conditions and want to insure a safe margin for error.

That, and at some point something will break, even it it's not until 80 knot winds, which is why I'm curious what people consider sufficiently improbably to use as their safety margin.

[wingssail]

Originally Posted by requiem

I'm trying to spec a clew block for a self-tacking jib, and may be over-thinking it. The boat is an Oceanis 41.1, and if a picture helps this is the situation I have in mind:

One question I have is, between a soft shackle to attach the block to the clew board, the block itself, and the sheet, which should be spec'd to break first, or does it even matter? E.g. the soft shackle, so a flogging sail isn't also flinging the block about?

The second is what forces should I reasonably plan for? I don't have the exact sail area of the jib, but sailboatdata.com lists a ~41 m^2 foresail area which I can plug into Harken's genoa system loading calculator to get a sheet load of ~1000 kg at 34 knots wind, and ~2000 kg at 48 knots wind. (I assume that reefing would happen long before those points, of course.) A Harken 57 mm ESP swivel block has a working load of 1000 kg and a breaking load of 2000 kg, which I am thinking should be sufficient.

Is there something I might be missing?

My feeling is that the 57mm block is far too small. You would be installing a block with a breaking strength which would be exceeded in 48 kts of wind. You cannot always plan about what wind strength you'll find yourself in, and flogging sails, which can happen, add shock loads to the clew equipment.

[Orion Jim]

You can call Harken and a tech will provide advice on which of their products best fit your need. Even if you don’t use their products you will now have a base of information from which to decide which other products fall within the same general working limits.

[requiem]

Thank you guys!

Originally Posted by wingssail

My feeling is that the 57mm block is far too small. You would be installing a block with a breaking strength which would be exceeded in 48 kts of wind.

That's what's bugging me. The funny thing is, someone with decent experience had suggested a Harken 55, and if I wander over to their Self-Tacking Jib and Staysail Systems page the suggested part for the 2:1 self-tacker at the "big boat" range is... a 57 mm block with 1100 kg MWL.

I did give them a brief call, and the guy suggested an 80 mm element block. We didn't get too much into the load limits though, so I'll keep digging. (I'm also going to go back and check the other existing components of the system.)

[donradcliffe]

Talk to someone who has spent time on a foredeck. You want absolute minimum weight of any hard pieces. When I think genoa attachments, I imagine being hit in the face by the flogging clew in 40 knots of breeze. I used to tie bowlines in 3/4 inch double braid through a single cringle. Plenty of reserve strength for chafe, and perhaps soft enough to save my teeth and skull.

[wingssail]

Originally Posted by requiem

...That's what's bugging me. The funny thing is, someone with decent experience had suggested a Harken 55, and if I wander over to their Self-Tacking Jib and Staysail Systems page the suggested part for the 2:1 self-tacker at the "big boat" range is... a 57 mm block with 1100 kg MWL...

Doesn't a 2:1 system mean that there are at least two blocks sharing the load?

[Rumrace]

Lovely Boat. When you want a floating Beneteau Key fob let me know. I 3D print Beneteau Jeanneau bits for a hobby.

I’m reminded just a few days ago an experienced German couple were killed by what appears to be a main running loose likely from a carriage brake failure.
So for what little I know overbuild so you’re not tempted to fix something in a storm. 3X better than what good enough

[waterman46]

Originally Posted by wingssail

Doesn't a 2:1 system mean that there are at least two blocks sharing the load?

The line running through the block gets half the load but block structure and attachment takes full load.

[Jammer]

I would at least consider a purpose-made clew block that is sewn to the sail, available from several manufacturers. It would eliminate a connection and allow you to sheet somewhat tighter to the traveler. This assumes you are using the block most or all of the time rather than using it only on rare occasions when you believe a self-tending jib is uniquely required. Ordinarily these are installed by a sailmaker who will include a flap of sailcloth covering most of the block.


IMO the running rigging should be sized so that the sail fails before any other component, but I am a fool and know little of such things.

[requiem]

Originally Posted by donradcliffe

Talk to someone who has spent time on a foredeck. You want absolute minimum weight of any hard pieces. When I think genoa attachments, I imagine being hit in the face by the flogging clew in 40 knots of breeze. I used to tie bowlines in 3/4 inch double braid through a single cringle. Plenty of reserve strength for chafe, and perhaps soft enough to save my teeth and skull.

Having had to re-tie the sheets once, although fortunately in far lighter winds on a far smaller boat, this is something I have no plans on doing here! Anything breaks free it's getting furled and lasso'd first!

Originally Posted by Jammer

I would at least consider a purpose-made clew block that is sewn to the sail, available from several manufacturers.
....
IMO the running rigging should be sized so that the sail fails before any other component, but I am a fool and know little of such things.

That would be tidier. Unfortunately with the time already taken to get the sail back from the loft, I don't think there's much appetite just yet. Yes, having the sail fail first would be preferred; one more strength number to look up. I'm thinking I should probably grab a book on the topic.

[Benz]

Tylaska makes (or made), a fuse for just such occasions. When loaded to x amount, it would blow, and the sail would sag off and be caught on a pennant, with the clew load far reduced. I never saw one blow, but have seen one in position.