380 S2: Block on top of 380 mainsail or not.

[meirriba]

Originally Posted by Bean Counter

This topic was covered in http://www.cruisersforum.com/forums/...tml#post823720

Also to test the physical theory I got a boat hook and placed vertically it on a set of bathroom scales. I hung a bucket of water weighing 20 pounds on boat hook =20 lbs. This equals the halyard lock scenario.

I then hoisted the bucket with a 1:1 pull over the top of the boat hook - reading on the scales 40 lbs.

Hoisted the bucket with a 2:1 and reading was close enough to 30 lbs so a 10 lb reduction in load on the scales = 25% reduction on compression from single line pull causing a 40 lb load.

So we are talking about the load on the mast not the boat. Anyway I am now satisfied that 2:1 decreases compression load on the mast itself.

So your scientific test required 40lb to raise 20lbs bucket?
And then lessened the total tension to 30 when you have added double purchase?
Perhaps you should add double purchase also on you anchor rode to reduce the total tension in high wind.
Frankly, assuming you are right and I am mistaken (happened to me before...) I do not understand the physics of these arrangements you propose.

[jbinbi]

Originally Posted by Bean Counter

This topic was covered in http://www.cruisersforum.com/forums/...tml#post823720

Also to test the physical theory I got a boat hook and placed vertically it on a set of bathroom scales. I hung a bucket of water weighing 20 pounds on boat hook =20 lbs. This equals the halyard lock scenario.

I then hoisted the bucket with a 1:1 pull over the top of the boat hook - reading on the scales 40 lbs.

Hoisted the bucket with a 2:1 and reading was close enough to 30 lbs so a 10 lb reduction in load on the scales = 25% reduction on compression from single line pull causing a 40 lb load.

So we are talking about the load on the mast not the boat. Anyway I am now satisfied that 2:1 decreases compression load on the mast itself.

1. Thanks for the other link, i will read that.
2. Sorry to say, but basic physics lab would tell you that if you hung a 40lb wt from the ceiling with a spring scale it would show 40lbs with 1 wire, 20lbs each with 2 wires, 10lbs each with 4 wires.
3. There is less force on the masthead sheeve with a 2:1, but for my boat, I don't believe this should be an issue. For the first 8 years of 380 production, it didn't have a 2:1.

My mainsail is not even close to the size of say a Beneteau/Hunter/Catalina/Juneau 50' mainsail and none of them have 2:1 purchase.

The mainsail on a 450 is much larger than those, so I can understand the need for it.

[Bean Counter]

Originally Posted by meirriba

So your scientific test required 40lb to raise 20lbs bucket?
And then lessened the total tension to 30 when you have added double purchase?
Perhaps you should add double purchase also on you anchor rode to reduce the total tension in high wind.
Frankly, assuming you are right and I am mistaken (happened to me before...) I do not understand the physics of these arrangements you propose.

Remember we are talking compression load on the mast. If you think about it, the load from the bucket is 20 lbs sitting there by itself. But to hoist the bucket up will require 20 lbs of weight to lift it to the top of the boat hook (mast) So when I stand there I am taking 20 lbs of force in my hand from the pulley added to this is the 20 lbs of the bucket itself. So the force from the bucket is 20 pound and the force holding it in place via the halyard is 20 pounds = total force being experienced by the boathook/mast = 40 lbs. Try it for your self. Also read post number 37 on the Leopard 42 I referenced earlier. Also see attached diagram.

[jbinbi]

Really not worried about compression on the mast. The compression is taken by forestay , shrouds, halyard, luff.

The only issue is the sheave having 2x the force. And possibly my double braid halyard stretching.

[Bean Counter]

Originally Posted by jbinbi

1. Thanks for the other link, i will read that.
2. Sorry to say, but basic physics lab would tell you that if you hung a 40lb wt from the ceiling with a spring scale it would show 40lbs with 1 wire, 20lbs each with 2 wires, 10lbs each with 4 wires.
3. There is less force on the masthead sheeve with a 2:1, but for my boat, I don't believe this should be an issue. For the first 8 years of 380 production, it didn't have a 2:1.

My mainsail is not even close to the size of say a Beneteau/Hunter/Catalina/Juneau 50' mainsail and none of them have 2:1 purchase.

The mainsail on a 450 is much larger than those, so I can understand the need for it.

If you put a spring scale on the ceiling and then a pulley at the bottom of it, then hoist up 5 pounds, what do you think the spring scale will read while you hold the line at ground level.?
Try it and see.

You will have noted that Lagoon changes rigging and mast styles from year to year for many models. The latest trend it seems is to build a lighter (probably more economical mast). A lighter mast may compress more easily, also they could cut down on the size of the sheaves and pins and so on with aim to cut costs with a lighter rig. I have no idea if they have done this with the 380 over time, but you would have to ask the mast builder, and from my experience with Z Spars for example, they are not great communicators on mast rigging (ie. - no response at all). In my view a lot of the systems have come from someone's professional drawing board, and there has been some thought behind it. We can all guess at Lagoons motives for the types of rigs they put together or have designed for them,, but until you get a technical response from someone at Lagoon, for myself I would not be game to try it. Any boat that has a single halyard should have had the halyard loads (and all the other loads) specified for those loads. If we as amateurs in the design game think we know better, or run off anecdotal evidence that because it works for someone else's boat it must be OK- well that is chancing your arm with some costs associated with repair if it fails.

We all might assume Lagoon put a 2:1 to make it easier to lift the sail only and that the rig and components were actually designed to handle 1:1. But then again they may have changed to 2:1 because they had too many complaints about clutch slip, sheave failure etc. I don't know. There are too many unknowns to make a call, unless Lagoon themselves make a comment.

Anyway, I was not attempting to create a storm in a tea cup, and say dire things will happen if you use 1:1, probably won't make much difference, the main purpose was to get to the bottom of the 2:1 v 1:1 compression effects and I myself have learned something as a result of your original post, because I was considering going to 1:1 on the 450 to get the sail up quicker, but now I have studied this all in more detail that is not going to happen. I am even now considering changing my Gennaker Halyard to 2:1 like the newer 450's have.
Sorry for the long winded reply.

[jbinbi]

appreciate your comments. on a 450, i wouldn't do it. on the 380, with a main about half your size, different story. Had a comment from the lagoon forums on yahoo where i have the same post topic of a 410 owner who had a professional rigger say he could take the 2:1 off. Since I have the original halyard which is 2 years old and is just double braid, stretch is my concern.

[PaulinOz]

Quote
"Also to test the physical theory I got a boat hook and placed vertically it on a set of bathroom scales. I hung a bucket of water weighing 20 pounds on boat hook =20 lbs. This equals the halyard lock scenario.

I then hoisted the bucket with a 1:1 pull over the top of the boat hook - reading on the scales 40 lbs."

I am also struggling with this,if in the above example you tied the halyard off to a cleat attached at the bottom of the boat hook above the scales, what would the reading be on the scales placed beneath the boat hook?

[Bean Counter]

Originally Posted by PaulinOz

Quote
"Also to test the physical theory I got a boat hook and placed vertically it on a set of bathroom scales. I hung a bucket of water weighing 20 pounds on boat hook =20 lbs. This equals the halyard lock scenario.

I then hoisted the bucket with a 1:1 pull over the top of the boat hook - reading on the scales 40 lbs."

I am also struggling with this,if in the above example you tied the halyard off to a cleat attached at the bottom of the boat hook above the scales, what would the reading be on the scales placed beneath the boat hook?

It would read 20 pounds, but then the scales are not measuring the compression force along the length of the mast, just the weight of the bucket.

[team karst]

Old thread yes. but a related question. My L380 manual calls for a 12mm main uphaul rope with no block and a 14mm halyard with the block. Is this likely a mistake? I think those diameters should be reversed.

[meirriba]

Not that I am familiar with L380, but common sense physics suggests that your thinking is correct.