Masthead sheave box - converting from cable

noexit

New member
39
3
Tacoma, WA
From the look of the pics that is  T-bird rig, likely built by SparTech (RIP) at their Redmond shop. While having a toggle would be a little better for articulation there are plenty of those rigs out there and the backstay is typically not a problem spot. 

A bare dyneema halyard will last at least 7-10 years in the PNW. If that is a T-bird you can have the halyard built such that the covered tail starts on the cabin top (forward of the clutch/winch) when the sail is hoisted. This will allow the sail to be fully lowered but you may need an extension pennant if you store the halyard on the boom end.

These boats also had wire/rope jib halyards where the splice would not pass out of the sheave at the top of the forestay and the splice lived just forward of the winches when hoisted.
Good eye. This is a T-bird with a SparTech mast. So If I just have my halyard made with the dyneema length the same as the wire length, and the covered tail the same length as the current rope tail, for both the main and jib, I'll be good?

One thing that hasn’t been discussed is -the old wire is somewhat shot. 
Does the wire travel through an older wire sheave?

Wire sheaves have a”V” profile as opposed to the “U” that is kinder and gentler to rope.

If it’s an old V profile there’s a good chance the old V sheave will be knarly and chafed and will abrade any new rope or dyneema running through it, in pretty short order and therefore it should be changed as well.

Just my 10 cents.
That was the very first post, and what this thread was originally about. It just got sidetracked by the discussion of the aircraft forks.

I didn't show the original sheave box, but it was very narrow, and broken. The one pictured below the pictured of my mast in the very first photo is a 30mm Ronstan, which is the closest that will fit with minor modification on the mast. I could do a 40mm that will take 10mm line, but I'd have to cut a lot more, and the mounting holes would cut into where the old ones were.

My thought is that I can get 10mm dyneema with the cover stripped, and it will be small enough going over the 30mm sheave that's rated for 8mm.

 

noexit

New member
39
3
Tacoma, WA
This is the original. Falling apart.

box3.jpg

box2.jpg box1.jpg

 

noexit

New member
39
3
Tacoma, WA
Good eye. This is a T-bird with a SparTech mast. So If I just have my halyard made with the dyneema length the same as the wire length, and the covered tail the same length as the current rope tail, for both the main and jib, I'll be good?

That was the very first post, and what this thread was originally about. It just got sidetracked by the discussion of the aircraft forks.

I didn't show the original sheave box, but it was very narrow, and broken. The one pictured below the pictured of my mast in the very first photo is a 30mm Ronstan, which is the closest that will fit with minor modification on the mast. I could do a 40mm that will take 10mm line, but I'd have to cut a lot more, and the mounting holes would cut into where the old ones were.

My thought is that I can get 10mm dyneema with the cover stripped, and it will be small enough going over the 30mm sheave that's rated for 8mm.
On the other hand, something like an 8mm Sampson XLS3 has a way higher average breaking load that I need for 201 sq ft main. That seems way easier and cheaper.

 

ctutmark

Super Anarchist
1,739
279
PNW
8mm XLS3 may be strong enough but I doubt you will be happy with the stretch, especially coming from the wire/rope halyard. Even with the larger sheave box you will probably need to go with a tapered halyard (dyneema core)  to account for the added bulk of the splice.  For your boat I would look at 8mm or 9mm for the tail section where it is not stripped. 

 




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