Dumb Sheaves

[Wrinklybear]

I am thinking of replacing the dumb sheave system of rigging a 14' catboat w small standard rotating cheeked sheaves. These will be suspended from the original dumb sheave throughmast holes by cordage and will lie outside a glued-up hollow 3.5" dia mast.

Will this weaken the system by moving the centre of down force away from the mast centreline? Will it make enough difference to be critical?

Any theories (or better yet - facts) wld be welcome.

Bear

[osteoderm]

What sort of sail and rig do you have? What is the trouble with the dumbsheave? Properly drilled and faired, with correctly-sized line fairly led, dumbsheaves are great on small craft rigs; cheap, light, strong, and effective.
Many small boat rigs with freestanding masts and dumbsheaves lead the tail of the halyard to the stemhead as a sort of forestay, particularly on small gunter and lug rigs. Another alternative for a lug rig would be a cheekblock at the masthead, on the same side as the yard.
If you have a small marconi sail, you may find that the fall of the halyard aft of the mast (given your planned-for block-on-a-strop aloft) may well interfere with the luff of the sail, whether laced on or otherwise. Leading the halyard tail to a belay point other than to the mast step or partners will either cause a foul lead or an undue lateral strain.
A small sheave box and sheave at the masthead is what i suspect you'd really prefer. Depending on the size/shape of the plug at the head of your hollow mast, this isn't really such a hard thing to do; just be sure you have enough wood to work with up there. It's just a matter of opening up the dumbsheave hole a bit into a rectangular slot, and pinning a sheave in the opening. Since the sheave only needs clearance for line at the top, and given how small such sheaves can be easily had, the hole need not be very large at all.
i've made this same conversion on other hollow masts for somewhat larger boats, using two small sheaves instead of a single larger one; this leads the line as far fore and aft as possible (for that fair lead), while keeping the slot as small as possible in the vertical dimension.

[Wrinklybear]

Osteo -

Thx for yr note...y're quite right, I wld prefer built-in sheaves, but the mast is 10ft6" tall and is less than 2 1/2 inches diameter at the top. Basically, I'm afraid to cut into the damn thing any more than I already have!

This question applies to a single gaff-rigged mainsail on a small catboat, and while I agree with yr comments in theory, I worry about abrasion of the 1/4" halyards, especially since I have been unable to find any thin copper flashing as a dumb-sheave hole lining. I am considering the use of tin flashing - will that last, do y think?

Cheers, Bear.

[Brion Toss]

Hello,
I understand your reluctance to carve away more wood. Is this a 1-part halyard? Is the problem that it's too hard to hoist? If so, consider deadending the line aloft, then go down to a small block or lizard on the sail, then back up to the dumbsheave. You'll double the purchase, overcoming much of the friction.
Mounting a cheek block would probably not make for too much side moment, but it would be foul on one tack, yes?
Fair leads,
Brion Toss

[Wrinklybear]

Brion :

The lizard sounds like an excellent idea - don't know why it didn't occur to me. And y're right abt a fouled lead as well.

Thx for the feedback guys.

Bear.

[NickfromWI]

What's a dumb-sheave?

[Mike Maddin]

Nick,

I'm glad you asked, because I had been thinking "deadeye". When I looked it up though, I finally found this:
www.simplicityboats.com/dumbsheave.html

Best Wishes,

Mike

[NickfromWI]

So basically you just drill a hole in the top of the mast and run the line through that?

Sounds affordable!

[Brion Toss]

Hi Nick,
Yup, cheap as drill bits and chisels, but of course high-friction. So it is pretty much relegated to relatively low loads, like this one, and notably at the bottom of topmasts, for the heelrope.
Fair leads,
Brion Toss