Performance Sailing - When to change sails on a reach

7

PERFORMANCE SAILING

When to change

sails on a reach

John J. Collins uses a computer to resolve

the spinnakcr-vs.-genoa question

Two years ago I wrote about the PC version of the Interna-tional Measurement System (IMS) Velocity-Prediction Pro-gram (VPP) ("Plotting Peak Performance," November 1986). In that article I described a number of useful things you could do to improve boatspnod, providod you had an IMS certificate for your boat or for a sistersiiip. The program, which runs on IBM PCs aird compatibles, is available for $150 from the United States Yacht Racing Union (USYRU, P.O. Box 209, Newport, RI 02840). In this article I discuss yet another way to improve your sailing via the VPP

On a plot of boatspeed estimates derived from that Velocity-Prediction Program (VPP), the optimum spinna¬ ker/genoa change point—the point at which you should change headsails—is where the genoa and spinnaker speed curves cross. The figure shows the speed curves for a per-formance racer/cruiser. These curves are commonly called polars when presented in a circular format. I think that the rectangular format used here, however, is easier to read.

In real life finding the change point is a bit more complex than these speed curves show, for the actual optimum spin¬ naker/genoa change point varies from boat to boat depend-ing on the boat's characteristics. High-performance boats tend to have higher wind-angle change points because the apparent wind gets drawn forward more quickly than it does for lower-performance boats. To give you an idea of what I mean, some typical optimum spinnaker/genoa • change points for a number of different boat types are

shown in Table 1.

Now look at the shapes of the speed curves shown in the When changing to a headsail or a spinnaker, you should know whether to sail high or low of the course

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figure. You'll see that boatspeed is usually a little higher a few degrees c7oser to the wind than at the change point with a genoa. It's also a little faster a few degrees farther horn the wind than at the change point with a spinnaker, This situation applies to .almost all boats whose speed curves 1 have examined, with the exception of some low-performance cruising boats sailing at low wind speeds.

T h i s double-hump shape of the speed curves is called an E.A. (ele-phant's a * * ]. It means that you should

never sail your boat right at the

opti-mum spinnaker/genoa change point. If the wind is blowing at this angle and you are on the rhumb-line course, you will be better off sailing half the distance a little high of the rhumb line with your genoa and the rest of the distance a little low of the rhumb line with your spinnaker. Never sail right on, or close to, the wind angle that marks the optimum change point.

Obviously, you have to look at the shapes ofthe curves for yom- own boat to see how far from the rhumb line to deviate for the best performance. Since any course deviation from the rhumb line means you are sailing a slightly longer distance, you have to make sure that the increase in boat-speed more than offsets the longer dis-tance sailed. Table 2 shows how much you must increase your boat's speed to equal the added distance sailed. V The boatspeed increase you need to offset a 5-degree course deviation is • so low that you should never sail

within 5 degrees, high or low, of the

optimum change point, just how far beyond you can deviate depends on how deep your E.A. is in the speed curve at the change point. In most cases the best course deviation will be somewhere between 5 and 10 degrees. For example, at 6 knots of boatspeed you can pick up an average 0.1-knot boatspeed increase with a 5-degree course alteration. You can get a 0.2-knot average increase w i t h a 10-degree course change. As Table 2 shows, you are better off making a 10-degree course change because the dif-ference between the actual speed increase and the required speed in-crease is greater at 10 degrees (0.2

Optimum spinna!<©r/genoa cliangs points

Boat type W i n d speed (kts) , U l t r a l i g h t L i g h t racer L i g h t R/C H e a v y R/C S l o w c r u i s e r 6 . , . 9 4 ° U 5 ° U 4 ° U3" 7 7 ° ;:;8 ' 9 3 ° • 8 4 ° a r 8 0 ° 75° : ; i O 9 7 ° 9 1 ° 8 3 ° 8 0 ° 7 7 ° . , • • , 1 4 : , ; 1 0 3 ° " 1 0 1 ° 9 2 ° 8 8 ° 8 5 ° .,,;:',20, 1 1 0 ° 1 0 7 ° 1 0 1 ° 9 6 ° 9 6 °

Table 1: This general guideline suggests when to change from a genoa to a spinnaker in reaching conditions. Wind angles are expressed in true wind

Boatspeed increase offset

Boatspeed (kts) ' • " ' ^ "''-^' '^^ 1 0 Course d e v i a t i o n : 5 ° ( increase, k t s ) 0 . 0 1 5 0 . 0 2 3 0 , 0 3 0 0 , 0 3 8 10° (increase, k t s ) ', 0 , 0 6 2 . 0 , 0 9 3 0 , 1 2 4 0 . 1 5 4 15° (increase, k t s ) 0 . 1 4 1 • • ' 0 , 2 1 2 , - ' .• 0 , 2 8 2 0.35'3 ' .Table 2; When changing sails on a reach, you will need incremental speed over /the base speed to compensate for the additional distance sailed

Speed curves vary, depending on t h e , ' characteristics of the i n d i v i d u a l boat;', the ones shown here arc typical for a performance racer/cruiser '. . i-'O'Vv k n o t actual i n c r e a s e ' m i n u s the 0.093 k n o t r e q u i r e d i n c r e a s e gives y o u a g a i n o f 0.107 k n o t ) t h a n i t is at 5 de-grees (0.1 k n o t m i n u s 0.023 k n o t f o r a g a i n o f 0.077 k n o t ) . - ' - ' W i t h a 15-degree c o u r s e c h a n g e y o u w o u l d need a speed increase o f 0.319 k n o t (0.212 k n o t is t h e increase necessary to offset the added distance at 15-degrees p l u s t h e 0.107 k n o t , w h i c h is t h e a c t u a l speed i n c r e a s e y o u get s a i l i n g at 10 degrees). Y o u ' r e u n l i k e l y t o get a n i n c r e a s e g r e a t e r t h a n 0.319 k n o t , w h i c h is w h a t y o u ' d need to better the 10-degree case.

W h e n y o u m a k e a course d e v i a t i o n f i ' o m the change p o i n t , y o u are b r i d g -ing the h o l l o w i n the speed c u r v e (the E . A . ) a n d t h o r o b y i n c r o a s i n g y o u r average boatspeed over w h a t i t w o u l d be i f y o u sailed at, or close t o , t h e op-: t i m u m spinnaker/genoa c h a n g e angle. I f y o u can average 8.1 k n o t s i n s t e a d o f 8.0 knots over a n 8-mile leg, t h e r e f o r e , y o u w i l l save a b o u t 4 5 s e c o n d s o f elapsed t i m e . That's w o r t h t h i n k i n g h a r d about. p ; ; • : ^ Remember f h e basic c o n c e p t a n d never sail at, or w i t h i n , 5 degrees o f the o p t i m u r r i spinnaker/genoa c h a n g e p o i n t ; y o u w o n ' t be g o i n g as fast as y o u s h o u l d be. ^ " ^ ^ ^ ^ ^

John Colhns has been a student of handicap rules for many years and is chairman of USYRU's PHRF : Committee. ,, ' , , • ,