Main sheet material

Moderator: GreenLake

Postby GreenLake » Mon Dec 07, 2009 3:32 pm

The 8mm would be 5/16. Sounds minimal to me. But, as I wrote, I'm running 7/16, which comes to about 11mm more or less.

With better mechanical advantage and ratchet blocks, as in your planned setup, I might do with a bit less, but in that case I'd probably aim for 10mm (roughly 3/8").
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Postby K.C. Walker » Mon Dec 07, 2009 9:05 pm

The reason I was asking about the 8 mm Bzzz line in particular is that it is supposed to be easy on the hands but runs very smoothly through the blocks, even as small as 40 mm. Here is a paragraph also from Jim Young sailing that talks about sizing line and blocks.

Some words about diameter; some sailors are in the habit of stuffing the largest line that they can between the faces of a block in the mistaken notion that the fat line will be easier to handle. Unfortunately, they are working against themselves, fighting the internal resistance of the larger line to turn over a small block. The end result is a system that is harder to pull in, and does not play out as easily, as one that is properly sized. A safe working formula for most double-braided lines (a double-braided line is one that has a braided core with a braided cover over it) is that the block diameter should be about 7 times the diameter of the line, or 8 to 1 if the line is very 'stiff' with a tight weave on the outer cover. So the over-sized half-inch mainsheet seen on some catamarans would need 3 1/2-inch blocks (90mm!) to run smoothly without additional friction. On single-braided lines, the safe ratio is closer to 5 to 1 for a medium braid or even down to 3 to 1 if the line is a very loose braid, so something like 3mm Endura can be used on Harken 16mm blocks, or 8mm Bzzz with 40mm Carbos without adding internal friction.

KC
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Postby GreenLake » Mon Dec 07, 2009 9:30 pm

I hadn't heard of those specific rules of thumb, but, yes, the principle is sound. The 7/16" sheet I have currently is most likely oversized compared to not only the diameter of the sheaves but also compared to the opening between the faces. (As I've written before, the setup was largely inherited - but I haven't felt any real urge to do something about it yet, either - I think the block at the mainsheet fitting on the CB - original hardware, isn't even 40mm, so I should see a lot of this effect).

For the friction, there are two kinds that matter. On is the free-running friction, against which a sail needs to pull in light airs - on a jibe, for example. The other is the friction under full load.

The former is substantial in my setup, and would be the most likely reason for me to contemplate a change.

The story on the latter isn't as clear cut. When hauling in on a mainsheet, I would have to work against that friction. When holding the mainsheet in hand, it either doesn't contribute or works in my favor. When paying out a bit, the friction would act to reduce the force I need to slow the sheet from running through the block.

The next question I would have is to go from simply stating "there is friction" to finding out "how much", and especially, how much in relation to the force with which I have to haul on the mainsheet anyway? If I use a 40mm block where the rule of thumb says 90mm, do I need to pull twice as hard, or 10% hader, or even 5%?
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Postby K.C. Walker » Mon Dec 07, 2009 10:27 pm

GreenLake wrote:
The next question I would have is to go from simply stating "there is friction" to finding out "how much", and especially, how much in relation to the force with which I have to haul on the mainsheet anyway? If I use a 40mm block where the rule of thumb says 90mm, do I need to pull twice as hard, or 10% hader, or even 5%?


Yeah, good question. I inherited 3/8 line on my boat.

KC
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Postby GreenLake » Tue Dec 08, 2009 3:48 am

Until I hear something convincing or until I can run a comparison, my thingking would be to ignore the friction in the loaded case. But improving the friction in the light wind jibe scenario would definitely improve boat handling.

Reducing the weight of the main sheet supported by the boom would matter in that scenario as well - at least that's what my own, not too scientific, observations seem to hint at. The mainsheet seems to pull the boom in by its weight in some very light airs.
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Rope Tricks

Postby K.C. Walker » Tue Dec 08, 2009 10:55 pm

GreenLake wrote:Until I hear something convincing or until I can run a comparison, my thingking would be to ignore the friction in the loaded case.

GreenLake,

Okay, I decided to try some rope tricks. I put couple of 5 pound weights in the bucket and hung it from my old 5:1 Lewmar vang. It's set up as two fiddle blocks with 40 mm and 25 mm sheaves. It's got 3/8 inch double braided which I'm pretty sure is original. The line does not touch the cheeks. There is a noticeable amount of friction in this set up. The sheaves are not ball bearing so obviously there's friction there. It is obvious in playing out the line that there is a fair amount of drag.

Next, I hung the 10 pound bucket from two new Harken Carbo blocks in a 3:1 set up, one of them 40 mm the other one 75 mm. I also used 3/8 line. Interestingly, the 3:1 set up is almost as easy to haul in as the 5:1. I flipped over the Lewmar to make it 4:1 with two 25 mm and one 40 mm. In this set up the effort to haul in the load was pretty close but was more difficult with the Lewmar. Now, with the Harken blocks freewheeling the effort to hold the weight up was definitely less with the friction in the Lewmar, especially at 5:1. Switching on either one of the ratchets on the Harken's makes them easier to hold even at 3:1 with both set the weight of the line was just about enough to hold the 10 pound bucket.

Then, I grabbed a cheap old Ronstan 3:1 vang, this was set up with 3/8 line as well and was pretty much stuffed into the blocks. It drags on the cheeks. The friction in that one is overwhelming.

So, I believe that sizing the line and blocks correctly, not to mention using quality blocks, does make a huge difference in the friction.

KC
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Postby GreenLake » Wed Dec 09, 2009 3:43 pm

Very interesting. The sheave friction (both from the rope bending and the bearing) is of course more of a factor with higher purchases - which you were contemplating.
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