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

7261ab1f-6891-424f-a22f-14c946c08ba8

Gear: Fusion Panel-Stereo

Plug & Play StereoIt can be a real pain to install a marine stereo inside a boat, what with the tiresome business of running cables through cramped spaces and finding somewhere sensible to locate the speakers. The audiophiles at Fusion thought about this and came up with the ...read more

2019BestBoatsPromo-04

Best Boats 2019

Some years ago, the book Aak to Zumbra catalogued—and celebrated—the incredible diversity of watercraft that has evolved over the centuries, a diversity that remains evident to this day in the 11 winners comprising the “Class of 2019” in SAIL’s Best Boats contest. Indeed, it ...read more

TOTW_PromoSite

SAIL's Tip of the Week

Presented by Vetus-Maxwell.Got a tip? Send it to sailmail@sailmagazine.comGuaranteed result What you see on the end of this halyard isn’t a beautiful Flemish Eye worked by a rigger, but it will make a big difference when you have to “mouse” a line through the mast. If the ...read more

dometicadler-700x

How to: Upgrading Your Icebox

The time has come when the prospect of cold drinks and long-term food storage has you thinking about upgrading your icebox to DC-powered refrigeration. Duncan Kent has been there and done that, and has some adviceFresh food must be kept at a refrigerated temperature of 40 degrees ...read more

Jet-in-Belize

Cruising: Evolution of a Dream

There’s a time to go cruising and a time to stop. As Chris DiCroce found, you don’t always get to choose those timesAlbert Einstein said, “Imagination is more important than knowledge. For knowledge is limited, whereas imagination embraces the entire world, stimulating progress, ...read more

01a-rosemary-anchored-at-Qooqqut,-inland-from-Nuuk

Cruising: A Passage to Greenland

When a former winner of the Whitbread Round the World Race invites you to sail the Northwest Passage, there is only one sensible answer. No.More adventurous types might disagree, but they weren’t the ones facing frostbite of the lungs or the possibility of having the yacht’s hull ...read more