Appropriate sheave for dyneema?

[Jdege]

I recently(-ish) a ketch with its masts on tabernacles.

The standing rigging needs to be replaced, and I'm replacing the old wire with dyneema.

My question is with respect to my triatic stay. The existing stay was wire with a spliced thimble that was pinned to the mainmast head, then was run though a block at the top of the mizzen, then run down the mizzen. Toward the bottom the wire was spliced to double braid, which was then made fast to a clear at the base of the mast.

As I said, these masts are on tabernacles, and it's the triatic that is used to raise the mizzen. With the main up and the mizzen down, the triatic is run through the block at the top of the mizzen through a block temporarily tied to the base of the mainmast, and then to the main winch. With this you can raise the mizzen until it's lined up and pinned.

I'm thinking that with dyneema I'll do the same, splice the dyneema to a tail of something that's easier to handle and less likely to slip out of a cleat hitch.

But I'm wondering about that block at the top of the mizzen. The existing one looks like it was original to the boat, and if it was properly-sized for 3/16" wire rope it's unlikely to be properly-sized for 5mm dyneema.

And I vaguely remember reading that there proper shape of a sheave intended for dyneema is different than for wire rope.

Any guidance as to what kind of sheave I need for 5mm dyneema, and where I can find a block with such a sheave?

[Benz]

Any sheave intended for rope will do. You could even use a low friction ring (LFR), since you're not running a lot of rope through there at high speed/load. Any LFR that will comfortably fit 5mm line will have sufficient radius.

[GordMay]

Originally Posted by Benz

Any sheave [of appropriate diameter & profile*] intended for rope will do ...

* Sheave diameter:
Ropes, used round tight radii, can be adversely affected by compression, and flex fatigue. In order minimise these, it is important that the correct sheave diameter is chosen.
Rope type: Sleeve diameter
Braided Ropes: 8 x Rope Diameter
3 Strand Ropes: 10 x Rope Diameter
Multi-Plait Ropes: 10 x Rope Diameter
Aramid Ropes: 20 x Rope Diameter
Aramid ropes are very susceptible to bend fatigue, and therefore may require a much larger sheave diameter.

Sheave Profile:
The correct sheave profile is essential, to ensure the sheave rotes freely, and to eliminate any unnecessary abrasion and chafing. The profile of the groove in a sheave should support the entire rope.
Normally a semicircle of 10% greater diameter than that of the rope is appropriate.

‘V’ [or "U"] groove sheaves should be avoided, since they compress the rope, and have points of local friction, reducing the life of the rope.

I didn't find any specific recommendations for HPME / UHMWPE 12 strand braided ropes.