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Loads on my turning blocks?

6K views 15 replies 10 participants last post by  mitiempo 
#1 ·
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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#4 ·
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!
 
#9 ·
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
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.

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
 
#5 ·
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.
 
#6 ·
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.
 
#7 ·
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.
 
#8 ·
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.
 
#10 ·
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!
 
#11 ·
Thanks to knuterikt for correcting my mistake on counting mechanical advantage of a purchase (I SAID it was a long time ago...). However, gut feeling tells me that the load on the head of a mainsail is a lot less than mainsheet load? And does the boom act as a lever? Oh my, time to go back to the books...
 
#12 ·
There is another consideration here, in terms of shiv load on turning blocks. Regardless of sail size and wind speed, there is a functional limit to how much force can be imparted.

Consider this:

You are sailing with full rig in a hurricane. Good times! Wind speed, lets say, 3000 knots. Why not? In this situation, your turning blocks will be under no more stress than, say, 35 knots of wind.

The boat heels. So, in that hurricane, your boat isn't going to accept all that extra wind force.. it's just going to lay over and take a little rest in the water. So the maximum loads on your turning blocks is not just a function of sail size, but also how much force your boat can exert on the wind to keep itself upright. That force has been called "righting moment" by people much smarter than myself.

I only bring this up to suggest that one can do a lot of math - which we all agree is a ton of fun! - but in the end, it's more of an exercise in mental acrobatics more than sailboat mechanics.

Don't get me wrong.. I do love me some good mental acrobatics. But lets try this.. what type and size rope is your current halyard? Determine the max load on that line, assume it's the max load for your rig (safe assumption, since the halyard hasn't snapped in two on you (I hope)) and double that number. The result is the size of turning block you need.

I know it's not sexy.. but it'll get you on the water, if that's where you want to be :)
 
#13 ·
I see some folks did remember basic physics!

I guess not everyone hid in the back!

Another thing to consider is the maximum force the main sheet is likely to add. After all, often the maximum force boils down to the maximum the sailor can crank. Say, the purchase times 50 pounds. For design purposes, 80 pounds. See how that adds up. Again, I suspect the mast will be in the water before that. In general, the reefing tackle is matched to the mainsheet tackle. On my boat, at least, the loads are equal.
 
#14 ·
Despite what you think of SA there was a great thread with some good info in a civil form :) i cant find it right now BUT

There were some really good pictures of this posted on SA and the purchase depends on what is anchored wear and the load on the block does depend on the angles
 
#15 ·
Great points, folks.

The physics and mechanical engineering involved in arriving at a definitive answer are obviously beyond what any of us have covered here.

The bottom line is that I would tend to trust the recommendations of a reputable manufacturer like Harken when it comes to selcting block sizes. I'd bet that they've done the research, run the numbers , and added a safety factor to their calculations before making any such recommendation.
 
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