Tight Strings - Sail Magazine

Tight Strings

Philip Donegan of Kemah, Texas asks: "My 35ft boat has a 379 ft2 mainsail. I’m using 10mm braid for my reefing lines, but I am thinking about replacing them with smaller Dyneema lines on the theory that the smaller diameter line will reduce the friction on the blocks. Am I correct, and if so, what is the minimum line strength I can use?" Win Fowler
Author:
Publish date:
ReefPhoto1

Philip Donegan of Kemah, Texas asks:

"My 35ft boat has a 379 ft2 mainsail. I’m using 10mm braid for my reefing lines, but I am thinking about replacing them with smaller Dyneema lines on the theory that the smaller diameter line will reduce the friction on the blocks. Am I correct, and if so, what is the minimum line strength I can use?"

Win Fowler replies:

Loading on a sail is a function of sail area and the apparent wind speed squared. The formula is L (load in lbs) = SA (sail area in ft2) x AWS2 (apparent wind speed in knots squared) x C (coefficient). The value for the coefficient is determined by a number of variables that I won’t go into here, but I like to use 0.00431. To compute the maximum load on your mainsail, I’m assuming the entire load is being taken either by the clew or the leech end of the reef. With that in mind, first determine the apparent wind speed when you will reef and then plug that number into the formula. For example, let’s assume your boat is very stiff and you don’t have to reef until the apparent wind is blowing 30 knots. In this case the computation would be as follows: Load = 379 x 302 x 0.00431=1,470lbs. The mainsail should never experience a higher load, because in winds over 30 knots you would either be taking in another reef or lowering the sail completely.

So on your boat, if your system is a single-part line, it must have a breaking strength of around 1,500lb. Remember, though, that you must also apply a safety factor of two, and preferably three times that amount to accommodate for things like shock loading, aging, chafe, knots, splices and small–radius sheaves in your reefing hardware. These factors all reduce a line’s strength.

This means that your 10mm line, which I assume is polyester double-braid with a breaking strength of about 4,000lb, is adequate. If you’ve got a two-part reefing system, the load can be half that amount.

Although you could replace your polyester line with 5mm Dyneema single braid—its breaking strength is over 5,000lb—I don’t think you’ll find it any easier to pull the 5mm Dyneema through the blocks than the 10mm double-braid unless the latter is rubbing against the cheeks of the blocks. If the sheaves are turning freely, the friction in the blocks is proportional to the load on the block, the radius of the sheave and the quality of the bearings and lubrication. Line diameter has no effect. Although the 5mm Dyneema line might be easier to pull through the sail’s reef cringle, it probably will be harder for you to winch in, and it certainly will be harder on your hands.

Related

TOTW_PromoSite

SAIL's Tip of the Week

Presented by Vetus-Maxwell.Got a tip? Send it to sailmail@sailmagazine.comLetting go the sheetTaking a loaded-up sheet off a winch when the boat tacks can be a just cause for nervousness. On a boat up to 40ft or so, the safest way is to first ease off a few inches, keeping the ...read more

USCGReadyForRescue_Identifier_FullColor

USCG Ready for Rescue Challenge

The U.S. Coast Guard is now collaborating with the U.S. Department of Homeland Security (DHS) on something it calls the “Ready for Rescue,” a $255,000 prize competition that is looking for ways that will make it easier to locate people, MOB victims in particular, in the water.The ...read more

04-CLR1718md1085-jpg

A Historic Win for Wendy Tuck

This past summer Australian sailor, Wendy Tuck (inset), became the first woman to win a round-the-world yacht race when she and her crew aboard Sanya Serenity Coast claimed the overall victory in the 2017-18 Clipper Race. “I am just so happy,” Tuck said at the finish in ...read more

daviscards

Davis Instruments: Quick Reference Cards

CHECK THESEIf you’re sailing with new crew this summer or your kids have suddenly and inexplicably started to look up from their phones and take an interest in the finer points of cruising, these Quick Reference Cards from Davis are a great way to further their boating education. ...read more

01-rbir18-596

Another Epic Round Britain Race

There are basically two kinds of offshore sailboat races out there: those that take place annually, like the Fastnet and Chicago-to-Mackinac races; and those that take place every other year, like the Transpac and Newport-Bermuda race, in part so the competitors have sufficient ...read more

01b_WALKING-KEDGE-OUT-cmykpromo

Getting More Use From Kedge Anchors

If you are cruising, you need at least two anchors on board for the simple reason that you must have a backup. Imagine having to slip your anchor on a stormy night with other boats dragging down on yours, or having your rope rode severed by some unseen underwater obstacle, ...read more

SailAwayCharter

How-to: Navigating on a Bareboat Charter

So you graduated from navigation class where you practiced dead reckoning, doubling the angle on the bow and maybe even celestial nav, and you now feel well prepared for your first charter trip. Well, you won’t be doing any of that on vacation—not past the first day, anyway.Most ...read more