Loads & Forces on a Sailboat - SailNet Community
 
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post #1 of 7 Old 05-01-2009 Thread Starter
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Loads & Forces on a Sailboat

As I gradually replace all the gear on my sailboat, I've come across a consistent problem. Rope, winches, blocks, anchor chain, cleats, etc. are often rated with a "safe working load." Great to know, but this information is useless unless I know what kind of load these components will be under once installed on my boat. Right now I'm clueless as to how strong something like my headsail halyard needs to be, or what kind of load my winches will encounter. I just take peoples advice on what I need, but this is inconsistent and I'd like to know more. Any thoughts? Anybody know a website or book that gives a way to figure out what kind of loads & forces are happening on my boat so I can more accurately buy gear?

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post #2 of 7 Old 05-01-2009
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The tensile strength of ropes used on a boat are much more h'gher than the loads. 12.5 mm rope has a tensile stregth of at least 10.000 lbs. This value can decrease as much as 50% depend'ng on the knot used. The expected load on any corner of a sail is not more than 2000 lbs. This means you have at least a safety factor of two including knotting.

The main factor for rope diameter is the handling properties. A thin ropewill be difficult to handle.
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post #3 of 7 Old 05-01-2009
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Originally Posted by celenoglu View Post
The expected load on any corner of a sail is not more than 2000 lbs.
That's not true. It depends on the size of the sail. Open 70's have sheet loads that are over 15k lbs. There are ways to calculate the loads on various sheets and halyards. IIRC Harken has some calculation formulas in their catalog; and they were posted on the mauriprosailing.com website. Harken had them as calculators on their website where you could just enter in the numbers but for some reason they took them down (possibly for liability reasons if loads were exceeded).

Here you go:

Harken Loading Formulas
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post #4 of 7 Old 05-01-2009
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Mo' betta
Harken Compu Spec

They list two versions of your boat model. Click on "View Loading Report" to get all the load info you likely need.

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post #5 of 7 Old 05-01-2009 Thread Starter
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Good stuff thanks guys that helps a lot. What about info on anchor line/chain loads and rigging loads?

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post #6 of 7 Old 05-01-2009
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Originally Posted by FishFinder View Post
Good stuff thanks guys that helps a lot. What about info on anchor line/chain loads and rigging loads?
I highly recommend The Complete Book of Anchoring and Mooring by Earl R. Hinz. Talks specifically about working loads on various shapes and sizes of boats, and how the loads scale based on wind speed.

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post #7 of 7 Old 05-01-2009
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Wrote this for another forum:

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To give you an idea of what the forces are like, this table is based on one found in The Complete Anchoring Handbook, by Alain Poiraud, et al.



Of course, it depends on the windage of your boat. A bimini, dodger, roller furling headsail, etc., will all increase the forces on the anchor.

The numbers above assume that the boat is solidly anchored... and those numbers will change, depending on what kind of bottom the anchorage has, what kind of anchor you're using, and things like that.

To give you an idea of what forces an next generation anchor like the Rocna can generate, the 15 kg version was found to repeatedly hold in excess of 4500 lbs. of force. The 10 kg Rocna will obviously not generate forces quite that high, as it has a much smaller surface, 795 sq. cm. vs. 1030 sq. cm. Assuming the surface area is proportional to the force it can generate, the Rocna 10 would be able to generate 3500 lbs. or so repeatably.

Based on that, the rope portion of the rode would be the weak link, provided you're using a load-rated shackle for the anchor-chain connection. This is why I was recommending upsizing to at least 9/16" line.

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Good stuff thanks guys that helps a lot. What about info on anchor line/chain loads and rigging loads?

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