Determining the leeway drift characteristics of tropical Pacific island craft

Applied Ocean Researcli 44 (2014) 92-101

ELSEVIER

C o n t e n t s l i s t s a v a i l a b l e a t S c i e n c e D i r e c t

Applied Ocean Research

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p o r

O C E A N

R E S E A R C H

Determining the leeway drift characteristics of tropical Pacific

island craft

BenA. Brushett^''''*, Arthur A. AllenS Victoria C. Futch'', Brian A. King'', Cliarles J. Lemclcert^

^Asia Pacific ASA, PO Box 1679, Surfers Paradise, QLD 4217, Australia

"U.S. Coast Guard, Office of Search and Rescue (CG-SAR-t), U.S. Coast Guard International Ice Patrol, 1 Chelsea Street, New London, CT 06320, United States '' U.S. Coast Guard, U.S. Coast Guard Academy, Science Department (dsm), 27 Mohegan Avenue, New London, CT 06320, United States

A R T I C L E I N F O A B S T R A C T

Article history: Received 15 August 2013

Received in revised form 9 November 2013 Accepted 20 November 2013

Ke^'words; Panga Outrigger canoe Personal water craft Search and rescue Tropical Western Pacific

Federated States of Micronesia (FSIVl) Guam

A n a c c u r a t e u n d e r s t a n d i n g o f t h e l e e w a y d r i f t c h a r a c t e r i s t i c s o f d r i f d n g o b j e c t s is r e q u i r e d t o e f f e c t i v e l y forecast ttie d r i f t o f persons, vessels o r o b j e c t s l o s t a t sea, a n d to g e n e r a t e e f f i c i e n t search areas t o m a x i m i s e t h e p r o b a b i l i t y o f successfully l o c a t i n g t h o s e m i s s i n g . Presently, t h e m o s t e f f e c d v e m e t h o d f o r c a l c u l a t i n g t h e l e e w a y d r i f t c h a r a c t e r i s ü c s o f a n o b j e c t or vessel is t o e m p i r i c a l l y d e r i v e t h e l e e w a y c o e f f i c i e n t s o f t h a t o b j e c t t h r o u g h f i e l d studies. T h e m a i n goal o f t h e studies is t o m e a s u r e h o w t h e o b j e c t d r i f t s i n r e l a t i o n to t h e surface c u r r e n t s , d u e t o t h e w i n d a n d w a v e a c t i o n u p o n i t . T h i s p a p e r o u t l i n e s t h e d e t e r m i n a t i o n o f d o w n w i n d a n d c r o s s w i n d l e e w a y c o e f f i c i e n t s f o r t h r e e s m a l l c r a f t c o m m o n t o Pacific i s l a n d c o m m u n i d e s for w h i c h n o a c c u r a t e l e e w a y c o e f f i c i e n t s e x i s t . These c r a f t w e r e : a 19 f o o t (5.8 m ) fibreglass s k i f f ( k n o w n l o c a l l y as pangas, fibres, o r b a n a n a b o a t s ) ; a 2 0 f o o t (5.97 m ) fibreglass o u t r i g g e r c a n o e ; a n d a 2 - p e r s o n s i t d o w n p e r s o n a l w a t e r c r a f t ( P W C ) . D u e t o t h e v a s t distances b e t w e e n pacific islands a n d t h e r e m o t e n e s s o f these l o c a d o n s i t can b e several d a y s u n t i l a search can be m o u n t e d t o rescue those l o s t a t sea, h e n c e i t i s p a r a m o u n t t h a t an a c c u r a t e d e s c r i p t i o n o f t h e d r i f t o f these t r o p i c a l pacific c r a f t is a v a i l a b l e f o r use i n search a n d rescue (SAR) d r i f t m o d e l s , t o d e f i n e a p p r o p r i a t e search areas. T h i s s t u d y successfully d e r i v e d t h e l e e w a y c o e f f i c i e n t s r e q u i r e d f o r each o f t h e s e t h r e e c r a f t . The l e e w a y speed o f t h e o u t r i g g e r c a n o e a n d PWC, b o t h w i t h o n e p e r s o n o n b o a r d (FOB) e q u i v a l e n t l o a d i n g , w e r e calculated t o be 2.40% a n d 4.24% o f t h e w i n d speed r e s p e c d v e l y . The l e e w a y speed o f t h e s k i f f w a s f o u n d t o range b e t w e e n 7 . 7 1 % a n d 4.40% o f t h e w i n d speed for e q u i v a l e n t l o a d i n g b e t w e e n 1 POB a n d 13 FOB.

The r e s u l t s o f these field tests have s u b s e q u e n t l y been i m p l e m e n t e d i n t o search a n d rescue m o d e l s b y several SAR o r g a n i s a t i o n s w o r l d w i d e . These r e s u l t s s h o w t h a t t h e findings h e r e i n have t h e p o t e n t i a l t o b o t h increase t h e l i k e l i h o o d o f finding p e r s o n s a d r i f t a t sea a l i v e , as w e l l as r e d u c i n g search costs t h r o u g h m o r e e f f e c t i v e d r i f t p r e d i c d o n a n d e f f i c i e n t search area f o r m u l a d o n .

® 2 0 1 4 Elsevier L t d . A l l r i g h t s r e s e r v e d .

1. Introduction

Several k e y elements are r e q u i r e d to successfully p r e d i c t the d r i f t o f a person o r o b j e c t at sea; these i n c l u d e search a n d rescue (SAR) d r i f t forecast models, i n p u t w i n d and c u r r e n t forecast data and the d r i f t object's l e e w a y d r i f t coefficients. M a r i t i m e search a n d rescue (SAR) d r i f t forecast m o d e l s are used to n u m e r i c a l l y m o d e l t h e d r i f t o f an object a t sea; h o w e v e r these m o d e l s are o n l y as effective as the i n p u t data p r o v i d e d . Both accurate e x t e r n a l f o r c i n g data ( w i n d s and c u r r e n t s ) a n d a w e l l - d e f i n e d r e p r e s e n t a t i o n o f h o w the o b j e c t m a y d r i f t due to the e x t e r n a l forces u p o n i t are essential m o d e l i n p u t s . The forces a c t i n g u p o n a d r i f t o b j e c t i n c l u d e those f r o m w i n d , w a v e s

* Corresponding author at: Griffith School of Engineering, Gnffith University, Gold Coast, QLD 4222, Australia. Tel.: +61 403562553.

E-mail addresses: ben.brushettOstudent.griffith.edu.au, bbrushett@apasa.com.au (B.A. Brushett).

0141-1187/$ - see front matter ® 2014 Elsevier Ltd. All rights reserved. http://dx.d0i.0rg/l 0.1016/j.apor.2013.11.004

a n d c u r r e n t s . Prior studies have s h o w n t h a t the d r i f t o f a n object due t o w a v e a c t i o n ( f o r c i n g ) o n l y becomes s i g n i f i c a n t once t h e d r i f t objects have a l e n g t h scale greater t h a n t h a t o f t h e w a v e l e n g t h [ 1 ] , and as the d r i f t objects i n v e s t i g a t e d h e r e i n have a l e n g t h less t h a n the w a v e l e n g t h , effects due to w a v e f o r c i n g m a y be d i s r e g a r d e d . W i n d a n d c u r r e n t f o r c i n g m a y be p r o v i d e d t h r o u g h a n u m b e r o f means, i n c l u d i n g near real t i m e observations and m o r e c o m m o n l y , n u m e r i c a l forecast m o d e l s . As t l i e object d r i f t s w i t h the c u r r e n t s , i t is exposed to t h e effects o f the w i n d a n d w a v e s . The c o m b i n e d effect u p o n the d r i f t o f an o b j e c t due t o w i n d a n d waves is d e s c r i b e d as t h e " l e e w a y " o f t h e object.

The l e e w a y o f an o b j e c t varies f r o m o b j e c t t o o b j e c t and t h e r e -fore a n e w set o f l e e w a y coefficients is r e q u i r e d f o r each d r i f t object to a c c u r a t e l y d e t e r m i n e t h e i r l e e w a y d r i f t characteristics. W i t h o u t the c o r r e c t l e e w a y coefficients, i t is i m p o s s i b l e to a c c u r a t e l y forecast h o w t h a t o b j e c t m a y d r i f t Leeway f i e l d tests are c u r r e n t l y t h e m o s t

BA. Brushett etal./Apphed Ocean Research 44(2014)92-101 ₉₃

A ^'

Pacific Ocean Legetid ;? Düpioyniefil Lccalion • Drilt Tfack Chunk FSM 100 20E) 5K[l0JTii>lrL'; Puluwat FSM

Fig. 1. Location map showing the tracl<s of the leeway drift objects at the three loca-tions studied; Chuuk (FSM). Puluwat (FSM) and Guam.

c o m m o n and m o s t accurate m e t h o d for d e t e r m i n i n g the leeway co-efficients o f a d r i f t object. A standard approach t o the leeway field tests is o u t l i n e d by Breivik et al. [2]. The leeway study was carried out at three locations w i t h i n the t r o p i c a l N o r t h Pacific Ocean d u r i n g the m o n t h s o f M a y and June 2012 (refer to Fig. 1). The i n i t i a l 5-day d r i f t of the skiffs (and m u l t i p l e single-day d r i f t s of the PWC and outrigger ca-noe) c o m m e n c e d a p p r o x i m a t e l y 15 k m o f f the w e s t e r n coast of Chuuk Lagoon i n the Federated States o f Micronesia (FSM). The next study location (one single-day d r i f t of all craft) took place a p p r o x i m a t e l y 20 k m to the n o r t h of Puluwat A t o l l ( w e s t e r n m o s t land features o f Chuuk State, FSM). The final study (single-day, all craft) took place a p p r o x i m a t e l y 10 k m to the w e s t o f Apra Harbour, Guam. A l l drifts w e r e u n d e r t a k e n in deep w a t e r w h e r e the d o m i n a n t c u r r e n t forcing was a t t r i b u t a b l e to the w e s t w a r d flowing N o r t h Equatorial Current (NEC).

The t o t a l d r i f t of an object at sea can be s u m m a r i s e d by the three equations b e l o w (adapted f r o m Hackett et al. [3]). Eq. (1) shows that the total d r i f t is a s u m m a t i o n of the d r i f t due to currents (relative to the earth) plus the d r i f t due to l e e w a y (slip relative to the a m b i e n t currents). The d r i f t due to currents is a result o f the c o m b i n a t i o n o f surface currents (derived f r o m Ekman d r i f t , baroclinic m o t i o n , tidal currents and inertial currents), as w e l l as d r i f t due to w a v e induced currents o r Stokes d r i f t (Eq. (2)). Leeway d r i f t is the s u m of the d r i f t due to t h e w i n d s acring o n the object plus the d r i f t due to the w a v e forces acting on the object (Eq. (3)).

DT = DC+DL ( 1 )

w h e r e Dj- = t o t a l object d r i f t ; Dc = d r i f t due to c u r r e n t forces (relarive to the e a r t h ) ; Di = d r i f t due to leeway (relative to the currents).

A n d

Dc = Dsc + Dsa

w h e r e Dc = d r i f t due to currents; Dsc

-Dsa = d r i f t due to Stokes d r i f t

A n d

DL = Dwi + Dwa

(2) : d r i f t due to surface currents;

(3) w h e r e Di = d r i f t due to leeway; Dwi = d r i f t due to w i n d forces; Dwa = d r i f t due to w a v e forces.

The effect o f Stokes d r i f t m a y be present for the d r i f t of an o b -j e c t o n the w a t e r surface, i n t w o f o r m s . The first is Stokes d r i f t due to w i n d generated waves, and the second is the Stokes d r i f t due to s w e l l . The w i n d generated w a v e - i n d u c e d Stol<es d r i f t p r e d o m i n a t e l y acts i n a d o w n w i n d d i r e c t i o n (the same direcrion as the w i n d ) ; h o w -ever the s w e l l - i n d u c e d Stokes d r i f t acts in the d i r e c t i o n of the s w e l l , w h i c h is n o t necessarily the same d i r e c d o n as t h e w i n d generated waves, and hence may n o t be in the d o w n w i n d d i r e c r i o n . As i t was

n o t possible to determine the swell direcrion in this s t u d y and due to the m i n i m a l swell encountered, any Stokes d r i f t w a s assumed to be a result of w i n d generated waves only, and act i n t h e d o w n w i n d direcrion. The s w e l l - i n d u c e d Stokes d r i f t may become a n i m p o r t a n t factor in higher energetic areas w i t h larger swell sizes. Once the d r i f t due to surface currents has been subtracted f r o m the t o t a l d r i f t , the e m p i r i c a l l y derived leeway d r i f t of the object cannot d i s t i n g u i s h be-t w e e n be-the d o w n w i n d leeway d r i f be-t effecbe-ts and be-the d o w n w i n d Sbe-tokes d r i f t effects on the d r i f t of the object, and therefore t h e effects that Stokes d r i f t m a y have on the d r i f t o f the object are i n c l u d e d in the regression of the leeway of the o b j e c t As a result of t h i s , Breivik et al. [2] r e c o m m e n d that for small craft i t is m o s t pracrical to express leeway as a f u n c t i o n o f the w i n d only.

Breivik and A l l e n [4] suggest that the d r i f t due to w a v e forces m a y be ignored for small craft whose length is less t h a n t h a t o f the w a v e l e n g t h , as the d r i f t due to w a v e f o r c i n g may o n l y become signif-icant once the object's length is similar to the w a v e l e n g t h (e.g. large vessels).

In s u m m a r y , as the lengths of the craft used in this study w e r e significantly less than the w a v e l e n g t h , the effects o f w a v e forces w e r e assumed to be negligible, and as the w i n d generated w a v e - i n d u c e d Stokes d r i f t was accounted for in the leeway coefficients derived for the objects, the total d r i f t o f the objects was calculated as a s u m of the d r i f t due to the surface currents and the d r i f t due to the w i n d .

The d e f i n i t i o n o f leeway has evolved over t i m e , w i t h each itera-t i o n b e c o m i n g m o r e rigorous and less ambiguous. The m o s itera-t recenitera-t d e f i n i t i o n o f leeway is listed by Breivik e t a l . [1,2] w h e r e i t is defined as;

"Leeway is the motion of the object induced by wind (10 m refer-ence height) and waves relative to the ambient current (between 0.3 and 1.0 m depth)".

This d e f i n i t i o n allows the SAR responder to use standard 10 m reference height model forecast w i n d s and the surface layer of c u r r e n t forecast models or currents measured by HF r a d a r

There are t w o methods o f describing the leeway o f a d r i f r i n g o b -j e c t Both m e t h o d s refer to the speed of the d r i f t of the ob-ject w h e n c o m p a r e d to the 10 m reference height w i n d speed. The first m e t h o d refers to the object's leeway speed and divergence angle referenced to the d o w n w i n d d i r e c t i o n and speed. The second m e t h o d d e c o m -poses the leeway speed and divergence angle i n t o d o w n w i n d leeway ( D W L ) and crosswind leeway (CWL) vectors. The f o r m e r m e t h o d , u r i l -ising leeway speed and divergence angle, has h i s t o r i c a l l y been used for m a n u a l d r i f t planning, h o w e v e r Allen [5] noted that w h e n using n u m e r i c a l m o d e l soludons for d r i f t p l a n n i n g , the l e e w a y divergence angle can cause the solution to become unstable at l o w w i n d speeds w h e n w i n d d i r e c d o n fluctuates. As a result, the latter m e t h o d using D W L and CWL is the preferred m e t h o d for n u m e r i c a l SAR models as i t does n o t suffer the same shortfall and remains n u m e r i c a l l y stable, even at l o w w i n d speeds.

The leeway coefficients can be calculated t h r o u g h e i t h e r a c o n -strained or non-con-strained linear regression w i t h the 10 m w i n d speed. The constrained t h r o u g h zero regression i m p l i e s t h a t the lee-w a y lee-w i l l be zero lee-w h e n there is no lee-w i n d , lee-w h i l s t the n o n - c o n s t r a i n e d linear regression implies t h a t there may srill be some residual lee-w a y d r i f t o f the object by f o r c i n g other t h a n lee-w i n d s lee-w h e n lee-w i n d s are zero. U t i l i s i n g the constrained t h r o u g h zero regression provides the m o s t stable n u m e r i c a l s o l u t i o n for m o d e l l i n g search o b j e c t trajecto-ries, whereas n u m e r i c a l models u t i l i s i n g the u n c o n s t r a i n e d regres-sion m a y incur difficulties i f zero w i n d speeds are e n c o u n t e r e d (due to h a v i n g no w i n d d i r e c t i o n in w h i c h to a p p l y the leeway c o m p o n e n t ) . This is generally n o t a p r o b l e m as zero w i n d speeds rarely occur; h o w -ever there are s-everal approaches w h i c h can be utilised b y n u m e r i c a l models to c i r c u m v e n t this p o t e n t i a l issue, w h i c h i n c l u d e ; (a) carry-ing f o r w a r d the w i n d d i r e c t i o n f r o m the previous m o d e l rime step to calculate the residual trajectory w h e n t h e r e is zero w i n d speed; ( b )

94 B A Brushett et al. / Applied Ocean Research 44 (2014) 92-101

r e m o v i n g tlie residua! trajectory f o r cases w t i e r e tliere is zero w i n d speed; o r (c) no m o d i f i c a t i o n to t h e model code, as conditions w i t h zero w i n d speeds are infrequent. Each approach has their m e r i t s and drawbacks, so i t is i m p o r t a n t to i m p l e m e n t the approach w h i c h best suits t h e particular a p p l i c a t i o n .

Leeway field tests have been carried o u t i n one f o r m or another since t h e first recorded results by Pingree [6] w h o carried o u t studies o n the d r i f t o f Navy life rafts i n W o r i d W a r II. A t h o r o u g h r e v i e w of the various leeway e x p e r i m e n t s / f i e l d tests conducted up u n t i l 1999, as w e l l as a s u m m a r y o f the leeway speed a n d divergence angle of 63 d r i f t objects is c o n t a i n e d w i t h i n Allen and Plourde [ 7 ] . A f u r t h e r r e v i e w o f leeway divergence was published by A l l e n [5] w h o p r o v i d e d the CWL a n d D W L coefficients for t h e 63 objects defined i n Allen and Plourde [ 7 ] . Since then, there have been f u r t h e r l e e w a y field tests undertake'n by various organisations and countries w o r i d w i d e i n c l u d i n g the U n i t e d States o f America, Canada, N o r w a y and France.

A semi analytical n u m e r i c a l approach to calculating the leeway of s h i p p i n g containers at various i m m e r s i o n levels w a s presented b y Daniel et al. [ 8 ] . This w a s a unique approach as t h u s far, the m a j o r i t y of o t h e r leeway studies utilised an empirical approach to d e t e r m i n i n g the leeway coefficients. Daniel et al. [8] presented three case studies o f previous s h i p p i n g incidents w h i c h occurred i n 1 9 9 3 , 1 9 9 6 a n d 1997 w h e r e s h i p p i n g containers w e r e a d r i f t at sea, a n d a case s t u d y c o m -p a r i n g t h e results f r o m a -previous leeway field test carried o u t o n 2 0 foot s h i p p i n g containers o f f B r i t t a n y (France) i n 1 9 9 1 - 1 9 9 2 . Leeway field tests w e r e carried o u t i n 2008 as a j o i n t v e n t u r e b e t w e e n Nor-w a y , France and the US. The studies Nor-w e r e u n d e r t a k e n i n N o r Nor-w a y , and investigated the leeway of; a 1:1 / 3 sized model o f a 4 0 foot s h i p p i n g container, a W W I I m i n e , a n d a 2 2 0 L (550-gallon) o i l d r u m . A n o t h e r j o i n t v e n t u r e b e t w e e n N o r w a y , France and t h e US saw f u r t h e r lee-w a y field tests lee-w e r e carried o u t i n 2009 in N o r lee-w a y [ 9 ] . This study successfully collected leeway d r i f t data for; a f u l l sized 2 0 f o o t ship-p i n g container, a 4.5 m oship-pen a l u m i n i u m skiff, Sunfish sailing dinghy, a person i n w a t e r (PIW) i n the deceased p o s i t i o n and a d d i t i o n a l data w e r e collected for t h e V W I I m i n e . A l l objects w e r e a p p r o p r i a t e l y i n -s t r u m e n t e d to f o l l o w t h e direct m e t h o d of c o l l e c t i n g l e e w a y data. Breivik et al. [10] c o m b i n e d the s h i p p i n g container results f r o m the 2008 a n d 2009 l e e w a y field tests i n N o r w a y a n d compared t h e m to the semi analytical leeway m o d e l of s h i p p i n g containers presented by Daniel et al. [8). Further, results w e r e extrapolated to account f o r d i f f e r e n t i m m e r s i o n levels of s h i p p i n g containers ( i m m e r s i o n level is one u n c e r t a i n t y faced by SAR responders w h e n m o d e l l i n g d r i f t i n g objects).

The frequency i n w h i c h the d r i f t object changes d i r e c r i o n f r o m left o f d o w n w i n d (positive cross w i n d ) to r i g h t of d o w n w i n d (nega-tive cross w i n d ) is k n o w n as the j i b i n g frequency, j i b i n g frequency is measured as a percentage per h o u r over the d u r a r i o n t h a t an object m a y be adrift. A l l e n [ 5 ] i n t r o d u c e d the j i b i n g frequency concept i n terms o f d e f i n i n g t h e search areas for d r i f t objects. A d r i f t i n g object may j i b e suddenly, w i t h an instantaneous change i n CWL sign, or i t may occur gradually over time w h i c h is m o r e d i f f i c u l t to d e t e r m i n e . It may be possible to n u m e r i c a l l y i d e n t i f y a gradual j i b e f r o m t h e d r i f t track a n d w i n d data h o w e v e r i f there is a l i m i t e d a m o u n t o f d r i f t data i t m a y be deemed sufficient to visually i n t e r p r e t a progressive vector d i a g r a m of the d r i f t r u n to i d e n t i f y the j i b i n g events.

The purpose o f this study w a s to undertalce a series o f leeway field studies to d e t e r m i n e the leeway coefficients o f three w a t e r craft c o m m o n to tropical Pacific islands, whose leeway coefficients w e r e previously u n k n o w n . These three craft i n c l u d e d a 5.8 m (19 foot) fi-breglass skiff, a 5.97 m (19.6 f o o t ) fifi-breglass o u t i i g g e r canoe, a n d a 2-person sit d o w n personal w a t e r craft (PWC). The outcomes o f this study a l l o w SAR planners to more accurately forecast the d r i f t o f the three objects, and plan search efforts more effectively. I m p r o v e d def-i n def-i t def-i o n o f search areas def-increases t h e Idef-il<eIdef-ihood o f fdef-inddef-ing the mdef-issdef-ing persons or craft quiclcer, and hence reduced search times increase the p r o b a b i l i t y o f finding t h e missing persons and increase their chances

Table 1

Leeway object categor ies.

Categoi-y Object size Leeway method Instrumentation

1 Small (e.g. Indirect Location device only

EPIRB, floating debris)

2 Small to Direct Location device plus

medium (e.g. current meter

PIW, PWC)

3 Medium (e.g. Direct Location device,

life raft, skiff) current meter and

weatherstation

4 Large (e.g. large Direct Location device.

boat, shipping current meter and

container) weatherstation

o f survival [11 ]. In a d d i t i o n , search efforts a n d the related h i g h costs involved w i t h m a r i t i m e searches m a y be reduced.

2. Methodology

A standard m e t h o d o l o g y for d e t e r m i n i n g the leeway o f floating ob¬ jects was set o u t by Breivik e t al. [2] to ensure subsequent field tests to gather leeway data o n o t h e r objects of interest could be conducted i n a consistent m a n n e r This allows for interchangeable data w h i c h are able to be i m p l e m e n t e d i n t o the various numerical SAR models cur-r e n t l y i n use by vacur-rious SAR ocur-rganisations w o cur-r l d w i d e . A n o v e cur-r v i e w o f some m a r i t i m e SAR models and their use w i t h ocean c u r r e n t forecast data for p r e d i c t i n g the d r i f t o f objects or craft is included i n Davidson et al. [ 12 j . Further i n f o r m a t i o n i n regards t o these SAR models i n c l u d e ; the U n i t e d States Coast Guard SAROPS (Search and Rescue O p t i m a l Planning System) [ 1 3 ] ; t h e Leeway m o d e l used by t h e N o r w e g i a n Coast Guard [ 4 | ; the SARMAP m o d e l used by M a r i t i m e N e w Zealand and the Australian M a r i t i m e Safety A u t h o r i t y [14,15|; the Canadian Search and Rescue Planning p r o g r a m (CANSARP) utilised by the Cana-dian Coast Guard [ 1 6 ] ; and t h e French MOTHY (Modèle Océanique de Transport d'Hydrocarbures) system r u n b y Météo-France [ 1 7 | .

There are t w o methods for d e t e r m i n i n g the leeway of an object, the direct m e t h o d and t h e i n d i r e c t m e t h o d [2[. The direct m e t h o d uses a c u r r e n t meter d i r e c t l y attached o r tethered to t h e object t h a t is being studied, as opposed to the i n d i r e c t m e t h o d w h i c h estimates the currents f r o m a nearby vessel or object to infer t h e leeway slip o f the study o b j e c t The i n d i r e c t m e t h o d is n o t as accurate as t h e d i r e c t m e t h o d ; h o w e v e r i t m a y be necessary w h e n tlie study objects are t o o small to either fit or tether a c u r r e n t m e t e r to (for example, m e d i c a l waste). The preferred m e t h o d for d e t e r m i n i n g the leeway o f an o b j e c t is the direct m e t h o d .

In this standardised m e t h o d o l o g y , Breivik et al. [ 2 | i d e n t i f i e d f o u r categories o f leeway objects w h i c h are categorised based o n t h e i r size a n d a b i l i t y to carry various i n s t r u m e n t a t i o n , as o u t i i n e d i n Table 1.

The three objects studied i n this field test all fell w i t h i n categories 2 and 3. The PWC and the O u t i i g g e r canoe w e r e b o t h deemed t o o small to adequately a c c o m m o d a t e w e a t h e r starions, a n d therefore fell w i t h i n category 2, w h i l s t the skiffs w e r e o u t f i t t e d w i t h w e a t h e r stations, and hence fell w i t h i n category 3. The direct m e t h o d was used for calculating the leeway for all three craft in this test as each w a s able to carry a cuiTent m e t e r for direct measurements o f the surface currents.

A f u l l descriprion o f the bacl<ground, model setup and results f o r this study is contained w i t h i n the technical d o c u m e n t of A l l e n et a l . [18].

2 . J . Drift objects

Several craft are c o m m o n to the tropical Pacific island i n h a b i t a n t s , w h i c h include the 19 or 23 f o o t fibreglass skiff, also c o m m o n l y k n o w n

B.A. Brushett et al. / Applied Ocean Research 44 (2014) 92-101 95

to locals as pangas, fibres (due to their fibreglass c o n s t r u c d o n ) , or banana boats (due to their c u r v e d appearance). Outrigger canoes and PWC are also c o m m o n .

2.1.1. Skiff

The Search and Rescue Exercise (SAREX) conducted by the USCG d u r i n g the leeway field tests r e q u i r e d a continuous 5-day d r i f t of a 19 foot (5.8 m ) fibreglass s k i f f T w o identical skiffs w e r e used in a leapfrog d e p l o y m e n t to safeguard against unforseen technical issues and a p o t e n d a l loss o f data. The t w o identical 19 foot fibreglass skiffs (Skiff-One and Skiff-Two, refer to Fig. 2a) w e r e o u t f i t t e d w i t h the nec-essary i n s t r u m e n t a t i o n i n c l u d i n g c u r r e n t meters, w e a t h e r stations, GPS (global p o s i d o n i n g system) I r i d i u m t r a n s m i t t e r s and flashing lights. Skiff-One was deployed first, for a p p r o x i m a t e l y 24 h before Skiff-Two was deployed nearby t o the locarion o f Skiff-One at 24 h. Once Sldff-Two was d r i f t i n g i n clear water, Skiff-One was t h e n recov-ered and once o n board the s u p p o r t vessel, the data f r o m the c u r r e n t meter and weather starion w e r e d o w n l o a d e d (to ensure the data had recorded correctly and all sensors w e r e w o r k i n g correctly). This d e -p l o y m e n t system also had the added benefit o f being able to check the c o n d i r i o n of the skiffs (e.g. i f t h e y had filled w i t h rain w a t e r f r o m o v e r n i g h t storms) and to ensure all batteries for the i n s t r u m e n t a r i o n w e r e f u l l y charged b e t w e e n d e p l o y m e n t s . This 2 4 h leap frog d e -p l o y m e n t schedule ensured t h a t any faults w i t h the i n s t r u m e n t a t i o n , w o u l d o n l y result in a m a x i m u m o f 24 h of data lost.

2.1.2. Outrigger canoe

A v a r i e t y of d i f f e r e n t outrigger canoes are c o m m o n to the tropical Pacific islands, v a r y i n g f r o m small 1-person craft, up to larger 2 0 ¬ 30 foot versions, w h i c h can be fitted w i t h a sail for longer distance j o u r n e y s . Some outrigger canoes are constructed in the tradirional ways f r o m rimber, w h i l s t others are constructed o f fibreglass. The outrigger canoe selected for this study was a 5.97 m fibreglass design w h i c h was designed to carry 1-2 persons (Fig. 2b).

2.1.3. Personal water craft

The PWC used in these leeway d r i f t tests was an older style 2.7 m Yamaha 2-person sit d o w n type (Fig. 2c). PWCs are c o m m o n l y used for recrearional use i n coastal a n d near shore w a t e r w a y s . Other sizes and style PWCs also include one person stand up style, as w e l l as 3 and 4 person sit d o w n styles. The PWC used i n this study had the e n -gine r e m o v e d and a d o w n w a r d facing ADCP (acoustic doppler c u r r e n t profiler) m o u n t e d t h r o u g h the centre of the h u l l .

2.2. Instrumentation

Each o f the d r i f t objects w e r e o u t f i t t e d w i t h various i n s t r u m e n t a -rion; i n c l u d i n g GPS beacons w i t h I r i d i u m satellite transmitters, ADCP c u r r e n t meters, and w e a t h e r starions, as w e l l as RDF (radio d i r e c t i o n finding) beacons and strobe lights. The f o l l o w i n g secrion outlines the i n s t r u m e n t a r i o n specifics and data s a m p l i n g periods urilised on each of the d r i f t objects studied. Table 2 gives a b r i e f o v e r v i e w of the i n -s t r u m e n t -s fitted to the d r i f t object-s.

2.2.1. Location devices

Each o f the craft was fitted w i t h Clearwater I n s t r u m e n t a r i o n GPS/ I r i d i u m t r a n s m i t t e r beacons, each w i t h a u n i q u e j M E I ( I n t e r n a t i o n a l M o b i l e starion E q u i p m e n t I d e n d f i c a d o n ) n u m b e r , w h i c h t r a n s m i t t e d the location of each o f the d r i f t objects back to the I r i d i u m receiver on board the USCGC Sequoia every 10 m i n ( o n the 10 m i n m a r k ) i n near real rime ( a p p r o x i m a t e l y 3 0 - 4 5 s delay). This enabled the posirion and speed over g r o u n d to be recorded for each o f the craft, as w e l l as to facilitate the l o c a t i o n and recovery o f the craft at the end o f each d r i f t r u n . The skiffs w e r e fitted w i t h Carmanah Marine Lanterns ( M 7 0 4 - 5 ) to enable the skiffs to be seen at n i g h t or under l o w l i g h t and to aid i n their recovery. These lanterns w e r e essenrially

l l 5.E0 •' 5.E0 •' (b) n ^

i '

(C) v ^ i ^ ^ r '

_L

Fig. 2. Craft tested: (a) 5.8 m sl<iff, (b) 5.97 m outrigger canoe, and (c) personal water craft. Line drawings on the nght side show the dimensions (in metres) of the corre-sponding d r i f t objects tested.

a solar p o w e r e d flashing light, w i t h a l i g h t sensor that s w i t c h e d t h e m o f f d u r i n g the day a n d on again at n i g h t The outrigger canoe and the PWC w e r e b o t h fitted w i t h NovaTech c o m b i n a r i o n RDF flashing beacons. These devices w e r e fitted w i t h an RDF t r a n s m i t t e r t h a t also c o n t a i n e d a flashing strobe l i g h t w h i c h a u t o m a r i c a l l y s w i t c h e d o n w h e n i t w a s dark.

2.2.2. Current meters

T w o d i f f e r e n t types o f c u r r e n t meters w e r e used i n this study to measure the sea surface currents relative to t h e d r i f t objects. The first was the N o r t e k AquaDopp 2 M H z ADCP, w h i c h was fitted to each o f the skiffs and the outrigger canoe. The second type of c u r r e n t m e t e r used was an RDl W o r k h o r s e M o n i t o r 1228.8 kHz ADCP, fitted to the PWC i n a special g i m b a l setup to m i n i m i s e rilt One and Skiff-T w o each had their respective ADCP fitted to the t r a n s o m , w h e r e the o u t b o a r d engine w o u l d have been fixed. The outrigger canoe had the ADCP fitted to the side o f the h u l l on the same side as the outrigger, s l i g h t l y offset f r o m amidships. It was positioned on the o u t r i g g e r side o f the h u l l to ensure i t was n o t damaged d u r i n g d e p l o y m e n t and recovery.

The s a m p l i n g frequency, s a m p l i n g average, b l a n k i n g distance, b i n size, n u m b e r o f bins and head d e p t h for the c u r r e n t m e t e r s are all listed for each o f the f o u r d r i f t objects i n Table 3. Data w e r e averaged over the surface 6 - 8 bins ( d e p e n d i n g o n the c u r r e n t m e t e r ) , and 1 m i n averages w e r e adjusted to account for magnetic v a r i a t i o n , and t h e n rotated a f u r t h e r 180= to account for leeway f r a m e of reference. The 1 m i n samples w e r e t h e n averaged to 10 m i n samples as B r e i v i k and A l l e n [4] established the m a x i m u m correlarion of l e e w a y occurred w i t h zero lag at 10 m i n samples.

2.2.3. Weather stations

A w e a t h e r starion w a s m o u n t e d to each o f t h e skiffs, a n d was d e -ployed i n close p r o x i m i t y to the outrigger canoe and the PWC, as they w e r e too small to d i r e c d y m o u n t a weather station to. W e a t h e r sta-t i o n measuremensta-ts f r o m sta-the skiffs could sta-t h e n also be m a d e available to the leeway regression calculations for the PWC and o u t r i g g e r canoe as w i n d s o n the ocean are relatively consistent and do n o t fluctuate considerably over small distances, i t is acceptable to use t h e w i n d s measured o n a nearby object [ 2 | . The w e a t h e r stations fitted to the skiffs w e r e Coastal E n v i r o n m e n t a l System WeatherPak 2 0 0 0 units, each fitted w i t h a Gill ultrasonic anemometer, w h i c h is an i m p r o v e -m e n t over the older -mechanical style a n e -m o -m e t e r as t h e ultrasonic

96 BA. Brushett et al./AppUed Ocean Research 44 (2014) 92-101

Table 2

Instrumentation installed on drift objects.

Drift object Current meter Weather station GPS o t h e r

Skiff-One Skiff-Two Outrigger canoe PWC

Nortek ADCP - AquaDopp 2 MHz

Nortek ADCP - AquaDopp 2 MHz

Nortek ADCP - AquaDopp 2 MHz

RDI ADCP - Workhorse monitor 1228.8 kHz

Coastal Environmental System - WeatherPak 2000 Coastal Environmental System - WeatherPak 2000

Clearwater iridium Beacon Clearwater Iridium Beacon Clearwater Iridium Beacon Clearwater Iridium Beacon

Carmanah Marine Lantern -M704-5

Carmanah Marine Lantern -M704-5

NovaTech RDF Flasher Beacon -RF-700C1

NovaTech RDF Flasher Beacon -RF-700C1

Table 3

Current meter specifics.

Drift object Current meter

Sampling Sampling frequency (Hz) average (min)

Blanking

distance (cm) Bin size (cm)

Number of bins

used Head depth (cm)

Skiff-One and Skiff-Two Outrigger canoe PWC AquaDopp 2 MHz ADCP AquaDopp 2 MHz ADCP RDI Workhorse Monitor 1228.8 kHz 1.0 l a n d 10 1.0 l a n d 10 1.0 l a n d 10 10 10 50 and 25» 10 10 Sand 10» 8 6 6 and 5» 2 0 - 2 5 5 - 1 0 10

• Initial PWC run had a blanking distance of 50 cm and 5 cm bin size ( x 6 bins), subsequent runs had a blanking distance of 25 cm and 10 cm bin size ( x 5 bins).

versions do n o t have a dead band. Resolution of w i n d d i r e c t i o n was 1° w i t h an accuracy o f ± 3 ° . The m i n i m u m w i n d speed threshold for the a n e m o m e t e r s was 0.01 m / s . The WeatherPaks also measured: w i n d gust, air temperature, GPS position, internal t e m p e r a t u r e and battery voltage. The u n i t f i t t e d to Sldff-Two also contained a h u m i d i t y sensor and a barometer. The barometer w a s used to correct the offset o f the pressure sensor o n the ADCP, w h i c h measured the d e p t h o f the ADCP in the w a t e r . A l l samples f r o m the V^eatherPaks w e r e taken at a frequency o f 1 Hz, and then averaged over 10 m i n to align w i t h the 10 m i n averages o f the c u r r e n t meters. The a n e m o m e t e r height was 1.79 m and 1.83 m above the w a t e r l i n e for Skiff-One a n d Skiff-Two respectively.

2.3. Object loading

One o f the m a n y uncertainties faced w i t h p r e d i c t i n g the d r i f t o f an object at sea, is the state i n w h i c h the object or craft is i n . Objects w i l l e x h i b i t d i f f e r e n t d r i f t characteristics d e p e n d i n g o n the loading to w h i c h t h e y are subject. D r i f t o b j e c t s / c r a f t w h i c h are heavily loaded w i l l sit l o w e r i n t h e w a t e r , thus increasing t h e i r cross sectional area exposed to currents, as w e l l as decreasing the cross sectional area exposed to w i n d . This has the combined o u t c o m e o f increasing the effects of currents w h i l s t decreasing the effects o f w i n d s u p o n the d r i f t o f the object, hence r e d u c i n g the m a g n i t u d e o f the leeway o f the object. The reverse is also true, w h e r e b y a decrease i n the load-i n g o f a d r load-i f t object w load-i l l load-increase the object's leeway, thus a l l o w load-i n g load-i t to f o l l o w the w i n d s more and the currents less. To understand h o w the d r i f t objects w o u l d d r i f t under these d i f f e r i n g l o a d i n g c i r c u m -stances i t is i m p o r t a n t to test the object's leeway d r i f t u n d e r v a r y i n g loadings. Previous studies by Breivik et al. [10] and Daniel et al. [8] investigated h o w s l i i p p i n g containers d r i f t e d under d i f f e r i n g i m m e r -sion levels ( w h i c h has the s i m i l a r effect to d i f f e r i n g loadings o f the craft).

The skiffs w e r e tested under several d i f f e r e n t loadings, using sand bags as extra w e i g h t The loadings w e r e tested i n terms o f persons o n board (POB) and i n c l u d e d ; 1 POB, 2 POB, 4 POB, and 13 POB equivalent loadings. In a d d i t i o n , sand bags w e r e placed at the stern o f the skiffs to s i m u l a t e the w e i g h t of the standard 4 0 h p o u t b o a r d m o t o r As the PWC a n d Outrigger Canoe are smaller objects w i t h l i m i t e d c a r r y i n g capaciries, they w e r e tested i n one c o n f i g u r a t i o n - w i t h 1 POB, w h i c h w o u l d be t h e i r m o s t lilcely l o a d i n g configuration.

A

Fig. 3. Chuuk drift runs: 28th of May 2012 to 2nd of June 2012.

2.4. Data processing

The w i n d speeds measured o n the skiffs w e r e a d j u s t e d f r o m t h e i r m e a s u r e m e n t h e i g h t up to the standard 10 m reference height f o l -l o w i n g S m i t h [ 1 9 ] . These w i n d speeds w e r e t h e n corrected by using the GPS positions to a l l o w for the m o v e m e n t o f t h e skiffs. The 10 m i n samples o f the w i n d s and currents w e r e m a t c h e d i n time, and the measured leeway was decomposed i n t o the D W L a n d CWL c o m -ponents. The CWL was split i n t o positive and negative d e p e n d i n g o n w h e t h e r d r i f t w a s to the left (negative) or r i g h t ( p o s i t i v e ) of the d o w n w i n d d i r e c t i o n . A d d i t i o n a l l y , a ( - 1 ) times the negative CWL co-efficient was also calculated to enable b o t h the p o s i t i v e and negarive CWL values to be p l o t t e d on the same positive axis. A linear regres-sion using a least squares best fit was carried o u t for t h e l e e w a y speed, DWL and CWL, w i t h each regressed against the w i n d speed (adjusted to 10 m height). This was repeated for b o t h u n c o n s t r a i n e d and c o n -strained t h r o u g h zero linear regressions. F r o m the l i n e a r regression, the slope, y i n t e r c e p t and r^ values w e r e calculated, as w e l l as the standard error t e r m (Syx).

The nine leeway coefficients i d e n t i f i e d by B r e i v i k and A l l e n [ 4 | a n d Breivik et al. [2] are s h o w n in Eqs. ( 4 ) - ( 6 ) . N o t e these can be compressed to six coefficients for the c o n s t r a i n e d t h r o u g h zero

BA. Brushett et al. / Applied Ocean Research 44 (2014) 92-101 97

Table 4

Summary of the drift runs.

Deployment date Retrieval date Duration 10 m wind speed

Drift object R u n * P 0 B # time UTC time UTC (hh:mm) range ( m / s ) Location

SI(iff-One 1 2 2 8 / 5 / 2 0 1 2 0:48 2 8 / 5 / 2 0 1 2 23:32 22:44 2.0-9.1 Chuuk PWC 1 0 2 8 / 5 / 2 0 1 2 1 : 0 0 2 8 / 5 / 2 0 1 2 6:02 05:02 4.8-9.1 Chuuk Canoe 1 1 2 8 / 5 / 2 0 1 2 23:06 2 9 / 5 / 2 0 1 2 6:20 07:14 2 . 7 - 4 7 Chuuk PWC» 2 0 2 8 / 5 / 2 0 1 2 23:19 2 9 / 5 / 2 0 1 2 6:00 06:41 n/a Chuuk Sl<iff-Two 1 2 2 8 / 5 / 2 0 1 2 22:45 2 9 / 5 / 2 0 1 2 22:59 24:14 2 7 - 9 . 3 Chuuk Sl<iff-One 2 2 2 9 / 5 / 2 0 1 2 23:30 3 0 / 5 / 2 0 1 2 22:22 22:52 2.8-9.0 Chuuk Canoe 2 1 3 0 / 5 / 2 0 1 2 0:28 3 0 / 5 / 2 0 1 2 5:52 05:24 2.8-6.2 Chuuk Sltiff-Two 2 2 3 0 / 5 / 2 0 1 2 22:14 3 1 / 5 / 2 0 1 2 2 3 : 1 5 25:01 2.9-11.9 Chuuk Canoe 3 1 3 1 / 5 / 2 0 1 2 22:52 1/5/2012 5:46 06:54 5.3-9.0 Chuuk SI(iff-One 3 2 3 1 / 5 / 2 0 1 2 22:32 2 / 5 / 2 0 1 2 2:26 27:54 5.3-9.1 Chuuk PWC 3 1 1 / 6 / 2 0 1 2 2 2 : 1 6 2 / 6 / 2 0 1 2 2:05 03:49 6.9-8.4 Chuuk SI(iff-One 4 1 8 / 6 / 2 0 1 2 2 1 : 1 7 9 / 6 / 2 0 1 2 8:02 10:45 1.4-4.7 Puluwat SIciff-Two 3 4 8 / 6 / 2 0 1 2 2 1 : 1 1 9 / 6 / 2 0 1 2 8:23 11:12 1.6-4.6 Puluwat Canoe 4 1 8 / 6 / 2 0 1 2 21:25 9 / 6 / 2 0 1 2 8:55 11:30 1.6-4.6 Puluwat PWC" 4 1 8 / 6 / 2 0 1 2 21:28 9 / 6 / 2 0 1 2 8:36 11:08 n/a Puluwat

Sl<iff-One' 5 6 1 2 / 6 / 2 0 1 2 0:24 1 2 / 6 / 2 0 1 2 0:41 0:17 n/a Guam

Sl<iff-Two 4 4 1 2 / 6 / 2 0 1 2 0:19 1 2 / 6 / 2 0 1 2 22:12 21:53 2.2-9.6 Guam

Canoe 5 1 1 2 / 6 / 2 0 1 2 0:29 1 2 / 6 / 2 0 1 2 23:21 22:52 2.2-9.6 Guam

PWC 5 1 1 2 / 6 / 2 0 1 2 0:33 1 2 / 6 / 2 0 1 2 22:37 22:04 2.2-9.6 Guam

° Battery failed.

' ADCP hung up o n ; gimbals during deployment, tilt exceeded tolerances. ' Skiff was overloaded (much greater than 6 POB), unsuccessful recovery.

Table 5

Unconstrained linear regression of leeway speed and d o w n w i n d leeway parameters.

Drift object Leeway speed DWL

Slope (%) y ( c m / s ) Sy, (cm/s) r^ Slope (%) y ( c m / s ) S,, ( c m / s )

Panga w / 1 3.28 1 5 J 2 1.59 0.71 3.89 11.90 1.69 0.75 POB Panga w / 2 2.87 15.39 4.19 0.53 2.79 14.17 3 7 5 0.57 POB Panga w / 4 3.80 6.51 1.97 0.92 2.92 8.25 2.15 0.86 POB Panga w / 1 3 3.98 1.61 2.16 0.80 4.03 1.19 2.12 0.82 POB Outrigger 1.30 6.13 3.98 0.24 1.42 4.92 3.90 0.28 canoe PWC 3.46 4.99 3.31 0.67 3.59 3.41 3.20 0.72

regression, as ttie y intercept is zero. Tlie subscripts refer to t l i e vector c o m p o n e n t s of leeway; DWL (d), positive CWL (c + ) and negative CWL

Ld = OdW|o + bd + £d (4)

(5)

Lc- = ac-W^o + bc- + Ec- (6)

w h e r e L = predicted leeway ( c m / s ) at a given value of W i o w i n d speed.

a = Slope o f the regression line t h r o u g h the data, to give l e e w a y as (%)

o f IVio w i n d speed; IVio = w i n d speed at 10 m reference h e i g h t ; b = Y intercept or offset t e r m for unconstrained regression; E = a d d i t i o n a l error t e r m .

Progressive vector diagrams (PVD) w e r e generated by p l o t t i n g the leeway d r i f t w i t h respect to the d o w n w i n d d i r e c t i o n for each d r i f t r u n . This a l l o w e d the j i b i n g analysis to be undertalcen, as the PVD allows the switches between positive and negative leeway over prolonged periods (several 10 m i n time samples) to be readily i d e n t i f i e d (thus i n d i c a t i n g a j i b i n g event).

3. Results

The d r i f t tracl<s f r o m the leeway field tests undertaken in the t r o p -ical Pacific ocean d u r i n g M a y / J u n e 2012 are s h o w n i n Fig. 3 (Chuuk, FSM), Fig. 4 (Puluwat atoll, FSM), and Fig. 5 (Guam). A s u m m a r y of

A

Fig. 4. Puluwat drift runs: 8th of June 2012 to 9th of June 2012.

each of the d r i f t runs is p r o v i d e d in Table 4. The results f r o m the anal-ysis and linear regression o f the leeway coefficients f o r each of the i n d i v i d u a l d r i f t objects (skiff, outrigger canoe, and PWC) are s h o w n i n Tables 5-8. For b r e v i t y only the linear regression plots for the l e e w a y speed, DWL and CWL of the 2 POB s k i f f are s h o w n ( r e f e r to Fig. 6), w h i l s t the full analysis of each of the d r i f t objects ( u n d e r all loadings

98 BA Brushett et al./Applied Ocean Research 44 (2014) 92-101

A

Skiff (2-POB) - Leeway Speed

« 5 - I Fig. 5. Guam drift runs: 12tli of June 2012 to 12th of June 2012.

tested) i n c l u d i n g the linear regression plots can be f o u n d i n the lee-w a y field test technical r e p o r t [18). The j i b i n g frequency analysis for all d r i f t objects follows and is s h o w n i n Table 9.

3.1. Summary of the drift runs

Table 4 b e l o w provides a s u m m a r y o f the d r i f t runs and outlines the d e p l o y m e n t Hmes, retrieval dmes, d u r a t i o n o f the r u n as w e l l as the 10 m reference height w i n d speed range, and the locations for each o f the i n d i v i d u a l leeway runs. It should be n o t e d there w e r e three runs t h a t d i d n o t r e t u r n usable data, these i n c l u d e d PWC Run-2 (ADCP battery failed), PWC Run-4 (ADCP was tilted beyond toler-ances i n gimbals d u r i n g d e p l o y m e n t ) , and Skiff-One Run-5 (skiff was overioaded beyond the 6 POB loading w h i c h resulted i n unsuccessful recovery).

3.2. Regression of leeway components

A graphical representation o f the linear regression for the 2-POB l o a d i n g o f the skiffs is s h o w n i n Fig. 6 (constrained i n d i c a t e d by thick red lines, unconstrained indicated by t h i n blue lines) for the leeway speed (a), DWL (b) and CWL (c). The complete linear regression results for t h e leeway coefficients (leeway speed, CWL and DWL), and 95% confidence level statistics of the skiff (under the f o u r loadings tested), the outrigger canoe and PWC are s u m m a r i s e d i n Tables 5 - 8 .

w,n„ (m/s)

-Linear Regression (Unconstrained) Linear Regression (Coitstramed)

Sl<iff (2-POB) - CWL

0 2 4 6 8 10 IZ —Linear Regression (Uncoiisliained) — Linear Regression (Constrained)

4 6 w , „ „ (m/s)

-Linear Regression (Unconstrained) —Linear Regression (Constrained)

3.3. Jibing frequency

The PVD of the five d r i f t runs o f the s k i f f w i t h 2-POB loading is s h o w n i n Fig. 7. Jibing events are m o r e cleariy visible i n the zoomed figure (b), and are indicated by the four black arrows. A s u m m a r y o f the j i b i n g frequency o f all o f the d r i f t objects for all of the d r i f t runs is presented i n Table 9.

Fig. 6. Leeway plots showing the linear regression of leeway components ( c m / s ) against the wind speed ( m / s ) adjusted to standard 10m reference h e i g h t (a) Leeway Speed, (b) Down W i n d Leeway, and (c) Cross Wind Leeway - including both positive (left of d o w n w i n d ) and -1 times negative (right of down w i n d ) components. The thick red solid line shows the constrained through zero linear regression, w h i l s t the thin blue solid line shows the unconstrained linear regression. Dashed lines show the respective 95% confidence limits of the regressions.

4. Discussion

It w a s f o u n d that depending on the loading o f the Panga skiffs, the constrained DWL m a y be up to 7.23% o f the 10 m reference height w i n d speed, w h i c h is significantly higher ( a l m o s t d o u b l e ) t h a n the DWL previously recorded for s i m i l a r s i z e d / c o n f i g u r e d craft such as the 4.15 m a l u m i n i u m skiff or the Cathedral h u l l - Boston Whaler, w h i c h have DWL coefficients o f 3.95% and 3.15% respecrively. Should the l e e w a y coefficients o f these s i m i l a r craft be used as a p r o x y for the l e e w a y coefficients of the Panga skiff, instead o f the leeway coef-ficients calculated herein, the search areas w o u l d fall q u i t e short o f the actual location o f the Panga s k i f f The search object w o u l d t h e n be outside o f the search area, and conrinue d r i f t i n g f u r t h e r outside the

search area as time goes on, thus g i v i n g a v e r y l o w p r o b a b i l i t y o f a successful search o u t c o m e .

The outrigger canoe e x h i b i t e d m u c h l o w e r l e e w a y speed and D W L coefficients c o m p a r e d to the other craft tested (skiff a n d PWC), w h i c h was n o t s u r p r i s i n g as the outrigger canoe floated deeper i n the w a t e r compared to the s k i f f and PWC ( a p p r o x i m a t e l y double the d r a f t o f the s k i f f and PWC). The c o m b i n e d effect o f deeper d r a f t as w e l l as t h e addirional drag o f the outrigger w o u l d have c o n t r i b u t e d to r e t a r d i n g the l e e w a y speed.

W h i l s t the d o w n w i n d leeway c o m p o n e n t tends to have a d i r e c t l y linear r e l a t i o n s h i p w i t h the w i n d speed, the c o r r e l a t i o n b e t w e e n the c r o s s w i n d l e e w a y c o m p o n e n t and w i n d speed m a y n o t necessarily be l i n e a r l y p r o p o r t i o n a l to w i n d speed [5,7], This is e v i d e n t i n the

B.A.Bnishett et al. / Applied Ocean Research 44 (2014) 92-Wl 99

Table 6

Constrained through zero, linear regression of leeway speed and downwind leeway parameters. Range of 10 m

windspeed # o f l O-min

Drift object ( m / s ) samples Leeway speed DWL Slope (%) S,,, ( c m / s ) r^ Slope (%) S,,, ( c m / s ) Panga w / 1 1.4-4.7 64 7.71 3 7 3 - 0 . 6 4 7.23 3.06 0.17 POB Panga w / 2 2.6-10.6 655 5.32 5.75 012 5.04 5.22 0.18 POB Panga w / 4 1.6-11.9 227 4.92 2.91 0.84 4.33 3.46 0.63 POB Panga w / 1 3 2.0-5.2 17 4.40 2.15 0.80 4.342 2.08 0.81 POB Outrigger 1.6-9.6 307 2.40 4.44 0.05 2.30 4.21 0.16 canoe PWC 2.2-9.6 181 4.24 3.49 0.65 4.12 3.28 0.70 Table 7

Unconstrained linear regression of crosswind leeway parameters.

Drift object + CWL - C W L + C W L - ( - CWL)

Slope (%) ^ ( c m / s ) Syx (cm/s) Slope (%) y ( c m / s ) S,x (cm/s) Slope(%) y ( c m / s ) Syx ( c m / s )

Panga w / 1 - 0 . 6 6 11.29 2.73 n/a n/a n/a n/a n/a n/a

POB

Panga w / 2 - 0 . 2 0 11.27 5.60 -2,28 3.36 2 7 1 0.73 4.97 5.05

POB

Panga w / 4 2.86 - 3 . 8 1 3.60 n/a n/a n/a n/a n/a n/a

POB

Panga w / 1 3 n/a n/a n/a 0.0009 - 2 . 4 9 1.41 n/a n/a n/a

POB

Outrigger n/a n/a n/a n/a n/a n/a n/a n/a n/a

canoe

PWC n/a n/a n/a n/a n/a n/a n/a n/a n/a

Table 8

Constrained through zero, linear regression of crosswind leeway parameters.

Drift object + CWL - C W L + CWL - ; - C W L )

Slope (%) Syx(cm/s) Slope (%) S y x ( c m / s ) Slope (%) S y x { c m / s )

Panga w / 1 POB 2.52 3.63 n/a n/a n/a n/a

Panga w / 2 POB 1.47 6.12 - 1 . 6 4 2.83 1.51 5.21

Panga w / 4 POB 2.21 3.80 n/a n/a n/a n/a

Panga w / 1 3 POB n/a n/a - 0 . 6 3 1.54 n/a n/a

Outrigger canoe 0.61 2 7 1 - 0 . 3 4 1.85 0.54 2.58

PWC 0.93 3.06 - 0 . 3 7 1.42 0 8 6 3.41

linear regression (using least squares line o f best f i t ) of CWL to W^Q w i n d speed for the results herein, and has often been the case w i t h other leeway studies [2,9,10]. The r^ values are n o t s h o w n i n the CWL results tables as they are very l o w , and have been o m i t t e d for b r e v i t y . W h e n there is insufficient CWL data to effectively regress against the w i n d speed, those results m a y have to be o m i t t e d , w h i c h was the case w i t h various loadings o f the Skiff, the PWC and the Outrigger canoe. The 2 POB loading of the s k i f f d i d r e t u r n adequate data to p e r f o r m the linear regression o f CWL coefficients, due to the extended r u n rimes the s k i f f was loaded in this c o n f i g u r a d o n (over 109 h in total).

The leeway speed refers to the total leeway speed, or the c o m b i n e d DWL and CWL leeway vectors. This value is greater than the DWL, b u t n o t usually significantly greater, as t h e p r e d o m i n a n t d i r e c d o n o f leeway speed is towards the d o w n w i n d direction.

The r^ value indicates h o w w e l l the regression line fits the data. Values close to one indicate a perfect fit, w h i l s t values close to zero indicate a poor fit. As the regression can be either unconstrained, or constrained t h r o u g h zero, an value can be given for b o t h regressions for the same dataset. Typically, unconstrained r^ values w i l l be higher than those constrained, as the constraint t h r o u g h zero can artificially skew the data w h e n i t is forced to pass t h r o u g h zero.

The rate o f expansion of the search area is related to the uncer-t a i n uncer-t y o f uncer-the d r i f uncer-t characuncer-terisuncer-tics of uncer-the objecuncer-t in q u e s uncer-t i o n . The Syx error t e r m used i n m a n y o f the stochastic search and rescue models avail-able controls this l e v e l o f uncertainty i n the object's d r i f t ; thus a larger

Syx t e r m w i l l result i n a more r a p i d l y e x p a n d i n g search area. W h i l s t

a large search area has a higher p r o b a b i l i t y o f c o n t a i n m e n t (POC), w h e r e b y i t is m o r e lilcely that the search object w i l l r e m a i n w i t h i n t h e search area; this is balanced by the availability o f resources to adequately search that area. The 95% p r e d i c t i o n l i m i t s (indicated by the dashed lines i n Fig. 6) may vary b e t w e e n the u n c o n s t r a i n e d and constrained analysis o f the leeway coefficients. A n u n c o n s t r a i n e d re-gression w i l l generally give tighter p r e d i c t i o n l i m i t s c o m p a r e d to the constrained t h r o u g h zero regression o f the same data, as indicated by the data i n Tables 5 - 8 . The 95% prediction l i m i t s are d i r e c d y related to the standard error (Syx), and larger Syx values indicate larger or w i d e r 95% p r e d i c t i o n l i m i t s .

The j i b i n g frequency indicates h o w often the o b j e c t changes its CWL sign as a percentage per hour. Jibing is a nautical t e r m w h i c h refers to w h e n a yacht changes tack (course) and passes its stern t h r o u g h the eye o f the w i n d . Allen [ 5 | i n t r o d u c e d t h e use of j i b i n g w h e n d e f i n i n g the search area for objects a d r i f t at sea, a n d its use in

100 B A Bmshett et al. / Applied Ocean Research 44 (2014)92-101

( 9 ) Fibreglass Panga Skiff 5.8 m, 2 POB loading, PVD of Down and Croswind Components of Leeway

7 5 ) 1 ! ! 1 i : 1 1 U I

-4.5 -4 -3.5 -3 -2.5 -2 -1.5 -1 •O.S 0

Crosswind Displacement (km)

Fig. 7. Progressive vector diagram (PVD) showing the d o w n wind and cross wind leeway vector components of each drift run for the fibreglass skiff w i t h 2-POB loading (a). Zoomed in view (b) shown for the left t w o tracks. The far left track indicates four of the six j i b i n g events that occurred d u n n g that drift run (indicated by four black arrows). Down w i n d is shown as the black vertical line, w i t h the w i n d blowing from the bottom of the figure towards the top.

SAR m o d e l l i n g . M o d e l l i n g the frequency o f j i b i n g can be d i f f i c u l t due to the c o m p l e x dynamics i n v o l v e d w i t h m o d e l l i n g the f l o w of fluids a r o u n d a d r i f t i n g object, especially w h e n t h a t object is at the interface b e t w e e n t w o fluids of s i g n i f i c a n d y d i f f e r e n t densities ( w a t e r / a i r i n -terface). It is proposed t h a t rapid shifts i n e n v i r o n m e n t a l forcing, such as a change i n w i n d strength and d i r e c t i o n m a y cause j i b i n g to occur. There are c u r r e n t l y no statistical models available to d e t e r m i n e the j i b i n g frequency of an object; h o w e v e r i t was indicated by A l l e n [5] that past observations have y i e l d e d a j i b i n g frequency i n the range of 3-7% per hour, w h i c h m a y be used as a guideline for c u r r e n t and f u t u r e studies. The s k i f f o n l y e x h i b i t e d j i b i n g o n the runs w h e n i t was u n d e r the 2-POB loading, w i t h a j i b i n g frequency o f 5.5% w h i c h is w i t h i n the range suggested by A l l e n [ 5 ] . The outrigger canoe j i b e d o n several o f its runs, r e t u r n i n g a higher j i b i n g frequency of 7.8% w h i c h is s l i g h t l y larger than the a f o r e m e n t i o n e d suggested upper l i m i t of 7%. The PWC did not e x h i b i t any j i b i n g events. Longer r u n times w h e r e

Table 9 Jibing frequency

Range of 10 CWL

m wind Hours:minutes switches Frequency per Drift object speed ( m / s ) of samples Üibes) hour(%)

Panga w / 1 1.4-4.7 10:40 0 0 POB Panga w / 2 2.6-10.6 109:10 6 5.5 POB Panga w / 4 1.6-11.9 37:50 0 0 POB Panga w / 1 3 2.0-5.2 02:50 0 0 POB Panga total 1.4-11.9 160:30 6 3.7 Outrigger 1.6-9.6 51:10 4 7.8 canoe PWC 2.2-9.6 30:10 0 0

the d r i f t object is a l l o w e d to d r i f t w i t h o u t being i n t e r f e r e d w i t h are r e q u i r e d to effectively ascertain the j i b i n g frequency o f a d r i f t o b -ject, a n d as the j i b i n g events are relarively rare - a larger n u m b e r of samples are r e q u i r e d to encapsulate these i n f r e q u e n t events.

D u r i n g the first d e p l o y m e n t , to the west o f Chuuk (FSM), all of the d r i f t objects traclced in a w e s t to n o r t h w e s t e r i y d i r e c t i o n t h r o u g h -o u t the 5-day d e p l -o y m e n t d u r a t i -o n (Fig. 3). The skiffs w e r e d r i f t e d for a p p r o x i m a t e l y 24 h intervals, a l t e r n a t i n g b e t w e e n Skiff- 1 and Skiff-2 over t h e 5-days. The other craft (PWC and outrigger canoe) w e r e d r i f t e d d u r i n g d a y l i g h t hours o n l y over the 5-days. The w e s t e r i y d r i f t e x h i b i t e d by all craft d u r i n g this d e p l o y m e n t was a t t r i b u t a b l e to the w e s t e r l y surface currents w h i c h are p r e d o m i n a t e l y d r i v e n by the easteriy trade w i n d s .

The short d r i f t tracks s h o w n i n Fig. 4 depict the shorter d u r a t i o n d e p l o y m e n t ( - 1 1 h) nearby to Puluwat, FSM. These tracks all take a south w e s t e r l y d i r e c t i o n , w i t h the skiff trades b e i n g the longest (as t h e y d r i f t e d the fastest) w h i l s t the outrigger canoe track was the shortest (due to its slower d r i f t ) . The w i n d s and w e a t h e r w e r e calm d u r i n g this r u n , w i t h m a x i m u m w i n d speed o f 4.7 m / s (9.1 kts). This was the o n l y r u n w h i c h took a southerly d i r e c t i o n ( a l b e i t s t i l l w i t h a w e s t e r i y c o m p o n e n t ) w h i l s t the other d e p l o y m e n t s near C h u u k and G u a m b o t h tracked t o w a r d s the n o r t h w e s t

Fig, 5 shows the d r i f t p a t h o f the objects w h e n d e p l o y e d o f f the w e s t e r n coast o f Guam, a p p r o x i m a t e l y 9.2 k m w e s t o f A p r a H a r b o u r The d r i f t objects took a n o r t h w e s t w a r d d r i f t t r a j e c t o r y f o r the first - 9 h of the r u n , before changing d i r e c t i o n to d r i f t t o w a r d s the n o r t h for a p p r o x i m a t e l y 5 h, and t h e n a l t e r i n g course again back t o w a r d s the n o r t h w e s t f o r the remainder of the d r i f t r u n . A l l t h r e e d r i f t o b -jects took s i m i l a r trajectories, w h i c h indicate t h a t a w i n d d i r e c r i o n

change came t h r o u g h at the times that the objects each changed t h e i r courses, and t h a t w i n d change was the d r i v i n g force for t h e objects to change course. This was c o n f i r m e d in the w i n d records f r o m the w e a t h e r station aboard the skiff, w h i c h showed t h a t the w i n d chang-i n g d chang-i r e c t chang-i o n , b l o w chang-i n g p r e d o m chang-i n a t e l y f r o m the east, before s w chang-i n g chang-i n g t o w a r d s the s o u t h east, and t h e n back t o w a r d s the east s o u t h e a s t The s k i f f and PWC d i d not record any j i b i n g events d u r i n g this r u n ; h o w e v e r the outrigger canoe j i b e d twice, and b o t h rimes w h e n i t was observed to j i b e there was a change in b o t h w i n d i n t e n s i t y and d i r e c r i o n (recorded by the w e a t h e r station aboard t h e sldff), i n d i c a t -i n g the w -i n d s h -i f t caused the outr-igger canoe t o j -i b e . B o t h the sk-iff and t h e PWC d r i f t e d in a v e r y s i m i l a r fashion, b o t h finishing w i t h i n ~ 2 k m o f each other, w h i l s t there was a greater separarion b e t w e e n these t w o d r i f t objects and the outrigger canoe, w h i c h d r i f t e d m u c h slower, and finished the d r i f t r u n a p p r o x i m a t e l y - 7 k m b e h i n d the other t w o c r a f t W h i l s t i t appears that the changes i n d i r e c t i o n occur eariier for the outrigger canoe (closer to the start o f t h e d r i f t path) c o m p a r e d to the other t w o d r i f t objects, i t is in fact d u e to the o u t -rigger canoe m o v i n g slower, and hence lagging b e h i n d the o t h e r t w o

fi.A Bnishett et al./Applied Ocean Research 44 (2014) 92-Wl 101

d r i f t objects w l i i c l i w e r e able to d r i f t f u r t l i e r before t h e w i n d change came t h r o u g h , and hence all three objects w e r e subjected to the w i n d change event at essentially the same d m e ; h o w e v e r their positions s l i g h d y varied spatially.

Sector Guam. W e w o u l d also like to thank the officers and c r e w o f the USCGC SEQUOIA for t h e i r outstanding professionalism and g o o d h u -m o u r t h r o u g h o u t t h e cruise. W e express o u r sincere a p p r e c i a t i o n to the reviewers of this paper for their constructive r e c o m m e n d a t i o n s .

5. Conclusions/recommendations

The m e t h o d o l o g y undertaken d u r i n g the leeway field tests to d e -t e r m i n e -the s-tandard leeway coefficien-ts o f -three c o m m o n -tropical pacific island craft; a 5.8 m fibreglass skiff (Panga), a 5.9 m outrigger canoe, and a t w o person sit d o w n PWC, has been described herein. Data w e r e successfully recovered for each o f t h e three d r i f t objects, and their leeway coefficients w e r e effectively calculated u t i l i s i n g cur-r e n t l y cur-recognised methods, i n line w i t h othecur-r leeway studies undecur-r- under-taken.

I t is r e c o m m e n d e d that the three d r i f t objects (and their associated leeway coefficients) are added to t h e leeway databases for search and rescue d r i f t objects, and i m p l e m e n t e d i n t o t h e various search and rescue models used i n t e r n a r i o n a l l y . Already the leeway data f r o m these three d r i f t objects have been integrated i n t o the USCG SAROPS search and rescue d r i f t forecast system and have been used d u r i n g several SAR incidents w i t h i n the Tropical Pacific region w i t h success-ful results. The leeway coefficients calculated herein are c u r r e n t l y being i m p l e m e n t e d i n t o the A u s t r a l i a n and N e w Zealand m a r i t i m e SAR systems.

It is imperative that c o n t i n u e d research i n t o the leeway of c o m m o n search objects is undertalcen to ensure that the databases o f leeway d r i f t objects is as up to date and complete as possible. Revisiting c o m -m o n search objects (e.g. PIW) that have been studied i n the past, w i t h n e w methods and techniques ( d i r e c t m e t h o d ) and m o r e advanced i n s t r u m e n t a t i o n w i l l lead to a better descriprion o f t h e i r d r i f t charac-teristics and m i n i m i s a t i o n of the d r i f t error, and hence r e d u c t i o n i n the search area sizes r e q u i r e d to adequately c o n t a i n these objects. I t is i m p o r t a n t that there are regional leeway databases o f c o m m o n craft w h i c h are specific to an area, and t h a t these are updated accordingly.

Acknowledgements

This w o r k was financially s u p p o r t e d by t h e US Coast Guard Office o f Search and Rescue, US Coast Guard Academy, US Coast Guard Dis-t r i c Dis-t FourDis-teen, GriffiDis-th UniversiDis-ty and Dis-the AusDis-tralian Research Coun-cil's Linl<age Projects f u n d i n g scheme LP0991159. The authors w o u l d like to acknowledge t h e assistance received f r o m t h e Coast Guard Academy w a t e r f r o n t personnel, RJ Burns; Cadets A r n o l d , Kennedy, and Byrd in preparation o f the PWC and the personnel at Coast Guard

References

Breivik 0 , Allen AA, Maisondieu C. Olagnon IVl. Advances in search and rescue at sea. Ocean Dynamics 2013;63:83-8.

Breivik 0 , Allen AA, Ivlaisondieu C, Roth J-C. Wind-induced drift of objects at sea: the Leeway field method. Applied Ocean Research 2011 ;33:100-9. Hackett B, Breivik 0 , Wettre C. Forecasting the drift of objects and substances in the ocean. In: EP ChassigneL J Verron (Eds.), Ocean weather forecasting - an integrated view of oceanography. Dordrecht: Springer; 2006, pp. 507-23. Breivik 0 . Allen AA. An operational search and rescue model for the Norwegian Sea and the North Sea. Journal of Manne Systems 2008:69:99-113.

Allen AA. Leeway divergence. Washington DC: U.S. Coast Guard; 2005. Pingree F Forethoughts on rubber rafts, Woods Hole Oceanographic Institution. 1944.

Allen AA, Plourde JV. Review of Leeway: field experiments and implementation. Washington DC: U.S. Coast Guard; 1999.

Daniel P.Jan G. Cabioc'h F, Landau Y, Loiseau E. Drift modelling of cargo con-tainers. Spill Science & Technology Bulletin 2002;7:279-88.

Allen AA, Roth J-C, Maisondieu C, Breivik 0. Forest B. Field determination of the Leeway of drifting objects. Oslo; Norwegian Meteorological Institute: 2010. Breivik 0, Allen AA, Maisondieu C, Roth JC, Forest B. The Leeway o f shipping containers at different immersion levels. Ocean Dynamics 2012:62:741-52. Australian Maritime Safety Authority. National search and rescue man-ual 2011 version 11; 2011 [online]. Available: http://natsar.amsa.gov.au/ Manuals/Search_and.Resciie-Man™l/documents/NATSAKMAN2011.pd/ [ac-cessed 19.07.13].

Davidson FJ, Allen AA, Brassington GB. Breivik 0. Daniel P. Kamachi M. Applica-tions of GODAE ocean current forecasts to search and rescue and ship routing. Oceanography 2009;22(3):176-81.

Kratzke T M , Stone LD, Frost JR. Search and rescue optimal planning system. In: IEEE Proceedings of the 13 international conference on information fusion, Edinburgh. 2010.

Spaulding ML, Howlett E. Application of SARMAP to esdmate probable search area for objects lost at sea. Marine Technology Societyjournal 1996:30(2):17-25.

Spaulding ML, Isaji T. Hall P, Allen AA. A hierarchy of stochastic particle models for search and rescue (SAR): application to predict surface drifter trajectories using HF radar current forcing. Journal of Manne Environmental Engineering 2006;8(3):181-214.

Canadian Coast Guard, Canadian Coast Guard College CANSARP Development Group Web Site. CANSARP user manual; 2009. [onlinel. Available: http://loki. cgcgc.ca/cansarp/cansarpmanualseptl609.pdf [accessed 6.11.13].

Daniel P. Marty F. Josse P. Skandrani C, Benshila R. Improvement of drift calcu-lation in MOTHY operational oil spill prediction system. I n : 2003 international oil spill conference, Vancouver. 2003.

Allen AA, Brushett BA, Futch VC. Results from the Guam/Chuuk/Puluwat 2012 Leeway field tests. Gold Coast: Griffith University; 2013.

Smith SD. Coefficients for sea surface wind stress, heat flux, and w i n d profiles. Journal of Geophysical Research 1988;93:15467-72.