Com­puter anal­y­sis helps us un­der­stand that when it’s fast, ease the spinnaker sheet un­til the luff curls.

Sailing World - - Contents -

Ap­ply­ing a sci­en­tific ap­proach to un­der­stand­ing the tenets of spinnaker trim con­firms that the curl is fast.

O The pri­mary ad­vice for spinnaker trim­mers to­day is the same as it was 50 years ago: “Ease the sheet un­til the lead­ing edge be­gins to curl, then trim in slightly. And re­peat.” As a sail de­signer im­mersed in the science of how to make and trim fast sails for most of my life, I find the tech­nique more than a lit­tle dis­ap­point­ing. Doesn’t mod­ern tech­nol­ogy have a lit­tle more to of­fer when it comes to the art of mak­ing a boat sail fast down­wind?

Yes, it does. The Sail­ing Yacht Re­search Foun­da­tion part­nered with North Sails in a two- part study to help pre­dict the force and mo­ments gen­er­ated by gen­nakers and spin­nakers. The study, known as Down­wind Aero Mo­ments and Forces Pro­ject, al­lows hand­i­cap rule- makers to bet­ter un­der­stand per­for­mance trade-offs as­so­ci­ated with dif­fer­ent down­wind sails. The re­search al­lowed the opportunity to bring the specifics of down­wind sail trim into sharper fo­cus.

Com­puter anal­y­sis of up­wind sail trim is noth­ing new. Be­cause sail ma­te­ri­als are stiffer and the shapes flat­ter and rel­a­tively sta­ble, up­wind sail per­for­mance can be ac­cu­rately mod­eled with straight­for­ward com­pu­ta­tional fluid dy­nam­ics. Down­wind sails, how­ever, are rel­a­tively stretchy and op­er­ate at much lower wind­speeds, so the propen­sity for the air­flow to stall, or sep­a­rate from the sail, is much higher. A spinnaker is an­chored only at the cor­ners, so the sail moves around more, and as a re­sult, trim­ming must be more dy­namic. While it’s stan­dard to cleat off a jib sheet, a good spinnaker trim­mer is con­stantly work­ing the sheet. Ac­count­ing for how and why a spinnaker moves, and how that im­pacts shape, air­flow and many other fac­tors, is a chal­lenge with which we’ve only re­cently started to come to grips.

Be­fore we get too deep into the topic, let’s re­view a few fun­da­men­tals of aero­dy­namic ter­mi­nol­ogy. The two ma­jor forces act­ing on any wing are lift and drag. Drag al­ways acts in the di­rec­tion of the wind, while lift is the force com­po­nent per­pen­dic­u­lar to the drag.

The forces on a sail are the

same, only the wing is ver­ti­cal. Flow over the sail is gen­er­ated by ap­par­ent wind, which is a com­bi­na­tion of the true wind and the air­flow cre­ated by for­ward move­ment of the boat.

When sail­ing up­wind, the re­la­tion­ship be­tween lift and drag is sim­ple. Lift from the sails, bal­anced by lift from the keel, helps to drive the boat for­ward. Drag, pointed in the di­rec­tion of the ap­par­ent wind (25 de­grees off cen­ter­line for an av­er­age keel­boat), is work­ing al­most di­rectly counter to for­ward progress. The trim­mer’s goal is to max­i­mize lift and minimize drag.

As the boat bears away and ap­par­ent wind swings aft, the re­la­tion­ship changes. With the ap­par­ent wind 90 de­grees to the boat’s cen­ter­line, lift is pro­vid­ing the driv­ing force that pushes the boat for­ward, whereas drag is work­ing to heel it to lee­ward. As the boat turns far­ther down­wind, the di­rec­tion of lift shifts to weather, while drag grad­u­ally comes more in line with the in­tended di­rec­tion of the boat. When a boat is dead down­wind, drag pro­vides all the drive force, and lift be­comes the heel­ing force to weather.

With this in mind, let’s take a look at two dif­fer­ent points of down­wind sail to see if mod­ern science can get a lit­tle deeper than the time-hon­ored ad­vice men­tioned at the start of this story. To iso­late the spinnaker, we locked in the on­set flow (ap­par­ent wind­speed and an­gle) and the main­sail trim, and var­ied only spinnaker trim. As one might ex­pect, for any given ap­par­ent wind an­gle and wind strength, there’s an op­ti­mum trim for the spinnaker. Ease too much, and the lead­ing edge be­gins to fold and drive force is re­duced. Trim too much, and the sail stalls, and flow and pres­sure drop, which also re­duces drive force. Any­one who’s spent more than a few min­utes work­ing a spinnaker sheet knows these ba­sics, re­gard­less of whether they know the ter­mi­nol­ogy or un­der­stand the science.

How fine is that groove of op­ti­mum trim? How much do you ac­tu­ally lose by not hav­ing op­ti­mum trim? And how much does this vary de­pend­ing on the point of sail? For these an­swers, we turn to the com­puter.


In mod­er­ate wind speeds, a mod­ern sprit boat of­ten finds the best VMG to­ward a lee­ward mark at an ap­par­ent wind an­gle of ap­prox­i­mately 85 de­grees, or just for­ward of abeam.

The di­a­grams al­low us to look at lift and drag some­what in­de­pen­dently. Pres­sure on the spinnaker is in­di­cated by the color of the sail: Yel­low in­di­cates neu­tral pres­sure, red in­di­cates pos­i­tive pres­sure, and green in­di­cates neg­a­tive pres­sure. These col­ors are spe­cific to the side of the sail shown in the di­a­gram. If you’re look­ing at the in­side of the sail, pos­i­tive pres­sure — in­di­cated by red and or­ange hues — keeps the sail full and pulling the boat for­ward.

The three hor­i­zon­tal slices in each im­age are col­ored to in­di­cate wind ve­loc­ity, with blue rep­re­sent­ing stalled air, and green to yel­low to red rep­re­sent­ing air­flow of in­creas­ing ve­loc­ity. The im­ages are ar­ranged clock­wise from the top left from tight­est trim to most eased.

As ex­pected, the tight­est trim has the slow­est-mov­ing air around the sail sur­face. The large ar­eas of blue and green in­di­cate stalled air across most of the sail’s cross sec­tion be­cause the wind is not flow­ing across the full width of the sail. As with an air­plane wing, stalling a sail re­sults in a sud­den de­crease in lift and an in­crease in drag. The most eased trim set­ting has the fastest- mov­ing air around the sail sur­face, but it also has the

A sim­u­lated look at a spinnaker shows pres­sure dis­tri­bu­tion and air­flow ve­loc­ity. Yel­low in­di­cates neu­tral pres­sure, red is pos­i­tive pres­sure, and green is neg­a­tive pres­sure on the sail. Of the hor­i­zon­tal slices, blue rep­re­sents stalled air and green rep­re­sents pos­i­tive air­flow.

At an ap­par­ent wind an­gle of 85 de­grees, the dan­ger is in trim­ming too ag­gres­sively. The trio of im­ages at right is a look at the pres­sure and air­flow ve­loc­ity for op­ti­mal trim (mid­dle) as well as slightly over- (top) and un­der­trimmed. The sig­nif­i­cant amount of stalled air in the top im­age (shown in blue on the hor­i­zon­tal slices) cre­ates drag, which pulls the rig side­ways, since drag is al­ways in line with the ap­par­ent wind. The bottom two im­ages have less drag and more at­tached flow — note the thin hor­i­zon­tal slices of yel­low and red along the outer sur­face of the spinnaker — which gen­er­ates max­i­mum lift and pulls the boat for­ward. The red strip along the luff of the spinnaker rep­re­sents the curl.

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