Skip to main content

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:
  • Updated:
    Original:

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

Rescue

Cruising: Safety Lessons Learned

It’s not often that sailors get a chance to put their rescue and MOB training to the test, rarer still that they do as quickly as newbie California sailor Khosrow “Koz” Khosravani did recently. If and when an emergency situation ever arises, though, it pays to be prepared. This ...read more

01-LEAD-'22.01.10_FALKEN-Maiden_Emma-Bow

At the Helm: Sailplans

The first thing you notice when you look at the sailplan for the Farr 65, Falken, which Mia and I recently added to the fleet here at 59-North, is the sheer number of headsails. Falken was built in 1999 as a racing boat to go around the world, and the crew would have carried the ...read more

01-PR-2-Throwing-it-Back-_©LaurensMorel

Racing Class Reunion

Where does an old VO70 go to retire? Right back to the racing circuit, apparently. This spring saw a remarkable contingent of Volvo Ocean Race one designs back on the water and duking it out on the Caribbean circuit. While it’s no surprise that some of the VO65 teams intending ...read more

05-Sailboats-moored-in-sheltered-waters-off-of-Kärrsön

Charter: Sweden

With 2,000 miles of coastline, 270,000 islands and seemingly countless bays and inlets, Sweden is truly a sailor’s paradise. One of the top sailing destinations here is the archipelago just outside the country’s second largest city Gothenburg (locally known as Göteborg), on the ...read more

fa70b13c-8eec-4c35-b30f-f89e497b469a

Crowdsourcing Age-of-Sail Weather Data

Although big, multi-million-dollar projects like the Large Hadron Collider and the human genome project with their legions of PHD’s tend to grab headlines, there’s still a part of play for the “citizen scientists” of the world. Amateur birders have long contributed to an ...read more

01-LEAD-Ultime-race-Yvan-Zedda,-OC-Sport-Pen-Duick

Ultims to Race Solo Around the World

For years now, maxi-trimarans, both solo-sailed and fully crewed, have been racing the clock on their own around the world in an effort to set ever faster records for the world’s fastest circumnavigation under sail. Back in 2000-01 there was also a no-holds-barred ...read more

P1-01-LEAD-018_CARYNBDAVIS_AMISTAD

Juneteenth on the Water

Discovering Amistad and Mystic Seaport Museum have partnered to organize their third annual Juneteenth festival, featuring concerts, speakers and a reflection on the lasting legacy of racial injustice in America. Declared a National Holiday in 2021, Juneteenth celebrates the end ...read more

Lead-2021-01-17-vue-03-34-av-tb-01

New Multihulls for 2022

Lagoon 51 In keeping with many of the more recently launched models created by French multihull builder Lagoon, the Lagoon 51 is all about comfort, “en plein air,” in particular, as the French might say. Topside, a whopping 80 percent of the boat’s flybridge is given over to ...read more