Loads on my turning blocks? - SailNet Community

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  #1  
Old 09-11-2011
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Loads on my turning blocks?

I am putting a mast collar on my boat to lead lines aft to the cockpit.

Im looking at all manor of stand up blocks.
The harken folks sell a single line reefing kit that states for boats up to 27 foot.
My boat is an albin vega 27.

But upon looking in the box, i find 40mm blocks that seem light. THey are rated for less than 500 lbs max work load.

Then on the harken site, i did the end-of-boom calculator. at 60 knots wind, I will experience up to say, 2500-3000 lbs at the boom end.

So does that mean the halyards may also experience 2500-3000 lbs?
And the clew?

Most of the smaller blocks seem in the 800-1000 lbs working load range.

Confused on what to order for my turning blocks.

groundhog
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Old 09-11-2011
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So lets say you get loads at 3000 lbs on the halyard...

Does the sheve at the mast top divide that in two?
So the halyard comming down the mast is loaded at 1500 lbs?
And after the turning block even less?

groundhog
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Old 09-12-2011
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Quote:
Originally Posted by groundhog View Post
So lets say you get loads at 3000 lbs on the halyard...

Does the sheve at the mast top divide that in two?
So the halyard comming down the mast is loaded at 1500 lbs?
And after the turning block even less?

groundhog
No the sheaves are just changing de direction of pull, no load gain here.

There are som tables / formulas around that give the expected load for different lines on a boat - cant remember where at the moment
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Old 09-12-2011
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It's about 45 years since high school physics, but I am sure every time you turn around two pulleys, you halve the load...easy enough to research on the web. I used to have an Albin Vega - great boat. I would think a Series 40 block would be the right size for your boat. At 60 knots you will be severely reefed or under bare poles and the load on that block will be the least of your worries!
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Old 09-12-2011
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Quote:
Originally Posted by groundhog View Post
I am putting a mast collar on my boat to lead lines aft to the cockpit.

Im looking at all manor of stand up blocks.
The harken folks sell a single line reefing kit that states for boats up to 27 foot.
My boat is an albin vega 27.

But upon looking in the box, i find 40mm blocks that seem light. THey are rated for less than 500 lbs max work load.

Then on the harken site, i did the end-of-boom calculator. at 60 knots wind, I will experience up to say, 2500-3000 lbs at the boom end.

So does that mean the halyards may also experience 2500-3000 lbs?
And the clew?

Most of the smaller blocks seem in the 800-1000 lbs working load range.

Confused on what to order for my turning blocks.

groundhog
GH,

Whoa, there cowboy -- I think you're getting a little ahead of yourself.

If you're sailing in 60 kts apparent with a full mainsail set, those turning blocks coming loose are probably gonna be the least of your worries. Go back and recalculate your loads based on a reefing strategy -- remember to adjust your sail luff, boom and mainsheet attachment point dimensions as the sail area decreases.

I'm not an engineer or rigger, but my thinking is that your halyard numbers are a little over-the-top as well. The halyards tension the luff of the sail -- I'm thinking these numbers are going to remain fairly constant regradless of windspeed. Your sail slugs act to oppose the pressure on the luff generated by the camber in your main.
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Keep in mind that the forces on the line are not the same as the forces on the shiv. For instance.. lets say you hang a 100 lb weight on the halyard (where you would normally hang a sail, but for now, it's just a normal old weight).

Then, you pull on the other side of the halyard (the side you'd normally tie off to a cleat when you have the sail raised). So, at this point, you are pulling on one end of the halyard, and the 100 lb weight is attached to the other side.

Lets say you hoist the weight off the deck about 2 feet, and hold it steady at that height. With you doing that, you'll be pulling down with 100lbs of force. The weight, of course, would be pulling with the same.

At that moment, the rope will be supporting a total load of 100 lbs, but the shiv/pulley will be supporting 200 lbs.

In conclusion, whenever a rope turns 180 degrees around a turning block, the load on the block is double the load on the rope.

Calculating the load transfered from the sail to the halyard is less straightforward, and depends on a lot of factors. Those factors include: Sail weight, bolt rope vs. sail slugs, cunningham tension, halyard tension, and even, to a certain extent, wind speed. However, wind speed has less of an effect than you might think.. most of that force is spread out along the spars.

Depending on the level of accuracy you are looking for, you might consider buying/renting a gauge that measures linear loads (a fishing scale, if you want). Attach it to your halyard and go sailing. Many other methods/calculations will get you a ballpark range, but with so many variables, only measuring it gets you there.

It really is a lot lower than you might think. Consider that the rope you are using has never broken, or likely even come close (at least a result of load force). So determine the working load of that rope and figure that's the upper end of your loads, perhaps. Just another thought.
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Old 09-12-2011
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Two things. You are confusing the end of boom load (which the mainsheet would carry) with the reefing line load. Two different things. And as noted, you won't have a full main at 60 knots so that is way off - by a factor of 5 or more!

I would guess the max load the reefing lines will see is pretty close to the halyard load. That depends on how tight you have the halyard. Usually that's just enough to make the luff snug but not too much that you have vertical wrinkles in the sail. I suspect that is far less than the maximum end-of-boom mainsheet load.
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Old 09-12-2011
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GH,

Your question grabbed me -- I love problem solving.

Instead of dealing with generalities, I dug a little deeper and checked out the sail plan for an Albin Vega 27.

The luff of the main is 25.9 feet; the foot is 10.8 feet. I plugged these numbers into a mainsheet load calculator, using 15 kts as apparent wind speed. The result is 250 lbs.

For kicks, I came up with the numbers of what a reef might look like: I dropped the luff by 24" and the foot (with 88* angle at the tack) became 9.96'. This changed the mainsheet attachment point to -.84) I increased the apparent wind to 20 kts. The result is 349 lbs.

For a second reef, I cut the same distances off again (luff = 21.9; foot = 9.11; mainsheet attachment value -1.7), and increased wind to 30 kts -- the result is 602 lbs.

Just some hypothetical numbers here -- may be of some help as you think this through.
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Old 09-12-2011
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Quote:
Originally Posted by jimjazzdad View Post
It's about 45 years since high school physics, but I am sure every time you turn around two pulleys, you halve the load
This is wrong..
You must
Quote:
count the number of parts coming out of the moving block(s)
Source Brion Toss
All other sheaves or blocks are just altering the direction.

Quote:
Originally Posted by CapTim View Post

In conclusion, whenever a rope turns 180 degrees around a turning block, the load on the block is double the load on the rope.
And if you turn 90 degree around a turning block the load on the block will be 1.41 * the load on the rope
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Old 09-12-2011
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Quote:
Originally Posted by PorFin View Post
GH,

Your question grabbed me -- I love problem solving.

Instead of dealing with generalities, I dug a little deeper and checked out the sail plan for an Albin Vega 27.

The luff of the main is 25.9 feet; the foot is 10.8 feet. I plugged these numbers into a mainsheet load calculator, using 15 kts as apparent wind speed. The result is 250 lbs.

For kicks, I came up with the numbers of what a reef might look like: I dropped the luff by 24" and the foot (with 88* angle at the tack) became 9.96'. This changed the mainsheet attachment point to -.84) I increased the apparent wind to 20 kts. The result is 349 lbs.

For a second reef, I cut the same distances off again (luff = 21.9; foot = 9.11; mainsheet attachment value -1.7), and increased wind to 30 kts -- the result is 602 lbs.

Just some hypothetical numbers here -- may be of some help as you think this through.

+1 for PorFin!
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