|Topic Review (Newest First)|
|09-14-2011 12:19 PM|
|mitiempo||Harken used to have all these numbers on line for hundreds of designs - CompuSpec. But they have apparently removed it all because they say it was dated - now they say email or call for info.|
|09-14-2011 09:55 AM|
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.
|09-13-2011 06:18 PM|
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
|09-13-2011 05:25 PM|
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.
|09-13-2011 04:38 PM|
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.
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
|09-13-2011 02:07 PM|
|jimjazzdad||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...|
|09-12-2011 04:00 PM|
Originally Posted by PorFin View Post
+1 for PorFin!
|09-12-2011 02:54 PM|
Originally Posted by jimjazzdad View Post
Originally Posted by CapTim View Post
|09-12-2011 10:40 AM|
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.
|09-12-2011 09:21 AM|
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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