The design of offshore cruising yachts

33

-THE DESIGN OF OFFSHORE CRUISING YACHTS

D. Koopmans Naval A r c h i t e c t

More and more people are dreaming o f s a i l i n g away o r around. Because i t i s o f t e n d i f f i c u l t t o f i n d t h e r i g h t y a c h t i n t h e s t a n d a r d types some o f them decide t o l e t t h e b o a t b e d e s i g -ned and b u i l t t o t h e i r p e r s o n a l s p e c i f i c a t i o n s and i d e a s . T h i s l e c t u r e i s an a t t e m p t t o show t h e p o s s i b i l i t i e s and t h e r e s t r i c t i o n s which t h e d e s i g n e r i s c o n f r o n t e d w i t h when de-s i g n i n g a de-s p e c i f i c c r u i de-s e r .

Features l i k e how t o i n f o r m t h e c l i e n t w i t h t h e r i g h t a r g u -ments, t h e r e l a t i o n s between d e s i g n - f a c t o r s , t h e use o f d a t a from tank t e s t s , t h e i n f l u e n c e o f windage o f h u l l and r i g i n t h e h i g h e r windspeeds and t h e s t a b i l i t y i n extreme c i r c u m s t a n -ces a r e d i s c u s s e d .

1. I n t r o d u c t i o n 2. General : P h i l o s o p h y o f y a c h t d e s i g n The appearance o f t h e y a c h t The i n f l u e n c e o f t h e c u r r e n t r a t i n g r u l e s A c r u i s e r has t o be a l l - r o u n d 3. The h u l l 4. Balance 5. The r i g .

6. The i n f l u e n c e o f wind f o r c e s on r i g and h u l l 7. S t a b i l i t y i n extreme circumstances

1. INTRODUCTION

The a u t h o r i s an independent n a v a l a r c h i t e c t s i n c e 1963. He made t h e c h o i c e o f t h i s p r o f e s s i o n because he had an obsession f o r e v e r y t h i n g about b o a t i n g and t h e sea.

As a yachtsman he progressed from s a i l i n g on t h e i n l a n d l a k e s t o r a c i n g on t h e I J s s e l m e e r and l a t e r t h e i n t e r n a t i o n a l races i n Europe. Some years ago he stopped r a c i n g because he f e l t i t i m p o s s i b l e t o com-b i n e r a c i n g any l o n g e r w i t h t h e i n t e n s i v e c r u i s i n g w i t h h i s f a m i l y . I n h i s c a r e e r as a y a c h t d e s i g n e r he has f o l l o w e d more o r l e s s t h e same l i n e w i t h o u t s p e c i a l i s i n g t o o much a t any t i m e .

I n t h e d i s c u s s i o n I s h a l l r e s t r i c t m y s e l f as much as p o s s i b l e t o t h e d e s i g n o f s p e c i a l s , t h a t i s t o say, e s p e c i a l l y designed one o f f c r u i -s i n g y a c h t -s . N e v e r t h e l e -s -s an i m p o r t a n t p a r t o f my t h o u g h t -s upon t h e s u b j e c t a r e r e l a t e d t o those p u t i n t o p r a c t i s e when d e s i g n i n g y a c h t s f o r s e r i e - p r o d u c t i o n , m o s t l y so c a l l e d " c r u i s e r - r a c e r s " , n e a r l y always b u i l t i n f i b r e - g l a s s .

W h i l s t I am sure t h a t t h e use o f m u l t i h u l l s , p a r t i c u l a r l y i n t h e case o f t h e b i g g e r s i z e s can be o f i n t e r e s t i n t h i s theme, I am n o t g o i n g t o d i s c u s s m u l t i h u l l s s i m p l y because I d o n ' t have enough p e r s o n a l e x p e r i e n c e on t h a t p a r t i c u l a r s u b j e c t .

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-2. GENERAL

More and more people a r e l o o k i n g f o r a s a i l i n g y a c h t f o r extended c r u i s i n g . The means t h e y have t o l i v e on board f o r l o n g p e r i o d s o f time i n reasonable c o m f o r t .

O f t e n t h e y cannot f i n d j u s t t h e r i g h t boat w i t h i n t h e market o f e x i s t i n g s t a n d a r d boats because those boats do n o t e n t i r e l y s a t i s f y the needs o f t h e b l u e water s a i l o r i n g e n e r a l concept and c o n s t r u c -t i o n .

So people come t o t h e d e c i s i o n t o l e t t h a t v e r y p e r s o n a l "dreamboat" be designed and b u i l t as a " o n e - o f f " . T h i s means t h a t they m o s t l y s t a r t w i t h a v i s i t t o a p r o f e s s i o n a l y a c h t - d e s i g n e r .

I n g e n e r a l t h e commissioner w i l l s e l e c t a d e s i g n e r who a t t r a c t s him because o f h i s s t y l e , h i s p e r s o n a l approach and h i s e x p e r i e n c e . These p o i n t s have a v e r y i m p o r t a n t b e a r i n g , f i r s t l y because o f t h e préselec-t i o n which préselec-t a k e s p l a c e , and secondly because o f préselec-t h e need o f a good r e l a t i o n s h i p d u r i n g t h e d e s i g n i n g and e v e n t u a l l y b u i l d i n g s t a g e and t h e r e a f t e r .

The f i r s t c o n v e r s a t i o n between d e s i g n e r and commissioner i s v e r y i m -p o r t a n t . On t h e one hand t h e r e i s a man o r a cou-ple w i t h a f a i r l y good o u t l i n e d i d e a o f t h e new boat and on t h e o t h e r hand t h e d e s i g n e r who o f t e n has t o s t a r t by b r i n g i n g t h e ideas down t o r e a l i s t i c l e v e l s . Next t h e d e s i g n e r makes a pré-design and t h i s serves as a s t a r t i n g p o i n t f o r f u t h e r s t a g e s .

I n most cases t h e pré-design a l s o serves t o g e t an i n d i c a t i o n about the b u i l d i n g p r i c e .

P h i l o s o p h y o f y a c h t d e s i g n

For most people t h e s t a r t i n g p o i n t f o r t h e i r new boat i s based on t h e e x p e r i e n c e s w i t h t h e p r e v i o u s one. Shortcomings o f t h e o l d boat o f t e n become t h e g u i d e t o t h e new one.

I t i s up t o t h e d e s i g n e r t o make c l e a r t h a t t h e new boat has t o s u i t the r e q u i r e m e n t s and t h a t t h e r e q u i r e m e n t s have t o be f o r m u l a t e d w i t h the eye t o t h e expected use and s a i l i n g area.

I once r e a d t h e r e p o r t o f a man who designed f o r h i m s e l f a l i t t l e s t e e l y a c h t . H i s d e s i g n p h i l o s o p h y was t h a t a seagoing boat cannot be s t r o n g enough. The r e s u l t was n o t o n l y an immensely s t r o n g b u t a l s o an ex-t r e m e l y heavy b o a ex-t !

When t h e owner r e c o u n t e d t h e f i r s t s a i l i n g e x p e r i e n c e s he remarked: " F o r t u n a t e l y t h e boat was so s t r o n g l y c o n s t r u c t e d , t h e sea washed r i g h t over t h e c a b i n t o p w i t h o u t c a u s i n g any damage!"

Time a f t e r t i m e t h e d e s i g n e r has t o e x p l a i n t h a t d e s i g n i n g a b o a t i s , j u s t l i k e a c a r o r a p l a n e , always a compromise between s t r e n g t h and, i n our case, s a i l i n g q u a l i t i e s .

S a i l i n g people are o f t e n deeply i n t e r e s t e d i n t h e i r s p o r t .

The read books about s a i l i n g and d e s i g n i n g and t h e y t a l k a l o t about t h e o r i e s .

U n f o r t u n a t e l y those books were w r i t t e n some time ago r e f e r r i n g t o ex-p e r i e n c e s w i t h s t i l l o l d e r y a c h t s . Oex-pinions about s e a w o r t h i n e s s and

seakindness a r e o f t e n based on t h e r e s t r i c t e d c o n s t r u c t i v e p o s s i b i l i t i e s . The b e a u t i f u l and o f t e n admired curve between h u l l and l o n g k e e l was n o t chosen f o r b e t t e r b e h a v i o u r i n a seaway, on t h e c o n t r a r y , i t was a t one t i m e t h e o n l y way t o reach a low c e n t r e o f g r a v i t y w i t h o u t l o o s i n g t o o much s t r e n g t h and s t i f f n e s s .

Nowadays, i f someone chooses t o have t h a t shape, i t has t o be based on o t h e r , more r a t i o n a l c o n s i d e r a t i o n s .

The i d e a " r a t i o n a l " i s , when c o n s i d e r i n g y a c h t s , n o t so easy t o d e s c r i b e When a v e r y f u n c t i o n a l designed y a c h t d i s f i g u r e s some b e a u t i f u l bay where she i s anchored i t c o u l d be q u e s t i o n e d whether t h e owner i s happy w i t h h i s b o a t . For a y a c h t , meant f o r p l e a s u r e , t h e j o y o f t h e owner when l o o k i n g a t h i s boat i s one o f h e r f u n c t i o n s .

There a r e always a number o f unknown f a c t o r s i n a d e s i g n o f a s a i l i n g y a c h t . The r e l a t i o n between t h e f a c t o r s c o n c e r n i n g t h e b e h a v i o u r i n a seaway i s so complex t h a t i t cannot e n t i r e l y be understood o r c a l c u l a t e d . Experience and f e e l i n g a r e e s s e n t i a l f o r t h e y a c h t d e s i g n e r t o p r e v e n t f a i l u r e s .

Most r e l a t i o n s and shapes a r e , when an e x p e r i e n c e d d e s i g n e r i s d r a w i n g a c u r r e n t y a c h t , more o r l e s s a u t o m a t i c a l l y good. Slenderness, l o n g i t u -d i n a l c e n t e r o f buoyancy, p l a c e o f k e e l an-d mast can be f i x e -d by most d e s i g n e r s by eye.

Here we see a marked d i f f e r e n c e between t h e a t t i t u d e o f d e s i g n e r s . One s t a r t s d r a w i n g and c a l c u l a t e s t o c o n t r o l and c o r r e c t what he d i d , t h e o t h e r , who I f e a r w i l l w i n t h e game i n t h e l o n g t u r n , c a l c u l a t e s f i r s t b e f o r e d r a w i n g a s i n g l e l i n e , o r even r e s t r i c t s h i m s e l f t o l o o k i n g a t l i n e drawings b e i n g produced by t h e computer.

37

-The appearance o f t h e y a c h t

I a l r e a d y mentioned t h e importance o f t h e appearance o f a y a c h t . A t t h i s p o i n t , a p a r t from t h e p e r s o n a l s t y l e o f t h e d e s i g n e r , t h e f a s h i o n of t h e moment p l a y s an i m p o r t a n t r o l e .

I t i s an a r t t o f i n d t h e r i g h t c o m b i n a t i o n between t h e i n t e n d e d use and the shape o f t h e boat. For a c r u i s i n g y a c h t , which can s t i l l be con-s i d e r e d acon-s a l o n g l a con-s t i n g p r o d u c t , i t i con-s i m p o r t a n t t o con-s t r i v e f o r a more o r l e s s t i m e l e s s l o o k .

From t h e o v e r a l l i m p r e s s i o n o f t h e y a c h t i t has t o be c l e a r what t h e c h a r a c t e r o f t h e y a c h t i s . A slow c r u i s e r w i t h a s t r e a m l i n e d appearance soon l o o k s i i k e a h i g h l y u n s u c c e s s f u l r a c e r !

P e r s o n a l l y 1 l i k e t o see something o f t h e atmosphere o f t h e g r e a t s a i l i n g c l i p p e r s i n a modern c r u i s e r .

A c r u i s i n g y a c h t must have t h e appearance o f a l i t t l e s h i p more t h a n of a b i g b o a t .

The i n f l u e n c e o f c u r r e n t r a t i n g r u l e s

I n y a c h t d e s i g n , as i n most o t h e r t e c h n i q u e s and s p o r t s , t h e r e i s a tendency t o l o o k f o r t h e u l t i m a t e p r o p e r t i e s .

I n o u r case m o s t l y speed. I t i s q u i t e u n d e r s t a n d a b l e f o r r a c e r s and i t i s advocated by t h e p r e s e n t r a t i n g r u l e , t h e I.O.K.

I n f a c t t h i s r u l e i s n o t o n l y i m p o r t a n t f o r a c t u a l f e a t u r e s o f boat and r i g , b u t p r o b a b l y o f even more i n f l u e n c e f o r what i s n o t measured by t h e r u l e .

Very i m p o r t a n t r e s i s t a n c e making f e a t u r e s l i k e w e t t e d area and moment of i n e r t i a a r e n o t i n t h e r u l e .

I t i s a p i t y , i n comparation w i t h a l l t h e t r o u b l e , c o s t s and e s p e c i a l l y the r i s k s f o r t h e crew, t h a t improvement i n speed i s o n l y m a r g i n a l , and by no means u s e f u l . A b s o l u t e speed i s u s e l e s s i n races where o n l y speed d i f f e r e n c e s count.

Racing i n d i s p l a c e m e n t boats i s as r i d i c u l o u s as races between s n a i l s seen from t h e p o i n t o f a b s o l u t e speed!

N e v e r t h e l e s s t h e p r e s s u r e on t h e d e s i g n e r s o f r a c i n g y a c h t s l e a d s t o too low s a f e t y f a c t o r s , e s p e c i a l l y because t h e p o t e n t i a l winners t e n d to overdo t h i n g s i n t h i s r e s p e c t and s e t t h e s t a n d a r d s .

I n these developments t h e r a t i n g r u l e s however are n o t always r e s p o n -s i b l e f o r t h e n e g a t i v e e f f e c t -s . O f t e n o t h e r f a c t o r -s govern a c e r t a i n movement. An i n t e r r e s t i n g example i s t h e development o f b i g g e r and b i g g e r one t o n n e r s , t h e so c a l l e d "Jumbo's u n t i l t h e l i t t l e Ganbare demonstrated t h a t t h e I.O.R. r u l e i s n o t k i n d t o "Jumbo's" a t a l l .

For r e a l c r u i s e r s t h e mentioned c o n s i d e r a t i o n s might n o t be o f i n -t e r e s -t , on -t h e o -t h e r hand we see a s -t r o n g i n f l u e n c e o f -t h e r a c e r s i n the shape and c o n s t r u c t i o n o f c r u i s e r s as wel as a change o f mind when t h i n k i n g o f a c c e p t a b l e s a f e t y margins.

I t i s a m i s u n d e r s t a n d i n g t h a t c r u i s e r s a r e o f a l e s s d e s i g n q u a l i t y t h a n r a c e r s . A c r u i s e r s a i l s under t h e same law o f n a t u r e . I t i s up t o the y a c h t d e s i g n e r t o make t h e b e s t o f i t .

C r u i s e r s are i n p r a c t i c e m o s t l y slower t h a n r a c e r s because o f f a c t o r s such as h e a v i e r c o n s t r u c t e d h u l l and r i g , l i t t l e d r a u g h t , more devided s a i l a r e a , f i x e d p r o p e l l e r and so on.

From t a n k - t e s t s we can see t h a t t h e q u a l i t y o f h u l l f o r m s , between c r u i s i n g and r a c i n g h u l l s , does n o t d i f f e r ( f i g . 1 , 2)

A c r u i s e r has t o be a l l - r o u n d

A good c r u i s i n g y a c h t must have a r e a s o n a b l e "range" o f p r o p e r t i e s . By t h i s I do n o t o n l y mean t h e p r o p e r t i e s under s a i l and power, b u t a l s o t h e ease o f upkeep, t h e c o n s t r u c t i o n and l a y - o u t e t c .

The d e s i g n e r has t o be aware o f t h e f a c t t h a t t h e b o a t can be used i n o t h e r ways and c i r c u m s t a n c e s t h a n those which he i n t e n d e d . T h i s means f o r example t h a t t h e s h e l l must be s t r o n g enough t o s u p p o r t t h e b o a t when she i s d r i e d - o u t or t o w i t h s t a n d t h e t r e a t m e n t she w i l l g e t i n some busy commercial h a r b o u r s .

A w i n c h , c a r e f u l l y p l a c e d and a t t a c h e d t o serve a c e r t a i n s h e e t , can be used f o r o t h e r purposes i n a n o t h e r d i r e c t i o n and so on.

One o f t h e c o m p l i c a t i o n s f o r t h e d e s i g n e r o f c r u i s i n g y a c h t s i s l a c k o f c o n t r o l l a b l e feed-back.

The d e s i g n e r o f a f l a t o u t r a c e r i s as good as her p r i c e l i s t , f o r t h e c r u i s e r t h e jugdement i s more c o m p l i c a t e d .

Most owners l o v e t h e i r own b o a t s , a f a c t which can be v e r y p r o f i t a b l e f o r t h e d e s i g n e r from t h e commercial p o i n t o f view, b u t he seldom hears about t h e f a u l t s o f t h e b o a t b e f o r e she i s s o l d !

I n my own boat I have g o t an anchorwinch and c h a i n f o r t h e f i r s t t i m e i n my l i f e . I soon d i s c o v e r e d t h a t t h e c h a i n d i d n o t f i l l t h e c h a i n l o c k e r i n t h e way I assumed. C l i e n t s who I asked how t h e y s o l v e d t h e problem t o l d me t h a t t h e y had t o r e a r r a n g e t h e c h a i n t w i c e t o g e t i t i n t h e l o c k e r which t h e y c o n s i d e r e d q u i t e normal even where t h e r e was enough p l a c e f o r a b e t t e r arrangements

41

-I t w i l l be c l e a r t h a t -I c o n s i d e r t h e p e r s o n a l s a i l i n g e x p e r i e n c e o f a y a c h t d e s i g n e r as e s s e n t i a l , n e v e r t h e l e s s t h e r e are disadvantages as w e l l . The d e s i g n e r who n o r m a l l y s a i l s w i t h h i s own boat o r ovm d e s i g n w i l l tend t o , f i n d h i s own s o l u t i o n s t h e b e s t , s i m p l y because he i s accustomed t o them.

Another d i f f i c u l t y i s when t h e d e s i g n e r has g o t much more s a i l i n g e x p e r i e n c e t h a n h i s c l i e n t . For example: I t takes a l o t o f t i m e t o d e s i g n a good l o o k i n g g a l l e y , e s p e c i a l l y i n t h e b i g g e r b o a t s , t h a t can be used i n reasonable s a f e t y and c o m f o r t i n a seaway. Such a g a l l e y i s a l s o good f o r harbour use though n o t o p t i m a l . L o t s o f people however are unknown t o c o o k i n g i n a heavy sea and want a "Harbour g a l l e y " which i s much e a s i e r t o d e s i g n b u t more or l e s s use-l e s s as w e use-l use-l as dangerous i n a seaway.

I n t h e l a s t few years a l o t o f r e s e a r c h work has been c a r r i e d o u t es-p e c i a l l y i n H o l l a n d . Most o f t h e r e s e a r c h was done on y a c h t s o f t h e c r u i s e r - r a c e r t y p e , n e v e r t h e l e s s some t y p i c a l c r u i s e r s were a l s o towed and a n a l i s e d .

I do n o t t h i n k t h a t s c i e n t i f i c r e s e a r c h l e a d s t o super y a c h t s , b e t t e r t h a n b e s t i s d i f f i c u l t . However, t h e r e s e a r c h has l e d t o a marked im-provement i n t h e g e n e r a l d e s i g n q u a l i t y . The r i s k o f bad designs i s c o n s i d e r a b l y reduced by a b e t t e r u n d e r s t a n d i n g o f t h e t h e o r y and t h e a v a i l a b i l i t y o f b e t t e r s t a t i s t i c m a t e r i a l c o n c e r n i n g f a c t o r s l i k e : r e -s i -s t a n c e , -s t e e r i n g , b e h a v i o u r i n a -seaway, -shape o f k e e l and r u d d e r and so on.

For a l o n g d i s t a n c e c r u i s e r a h i g h topspeed i s u s e l e s s because n o r -m a l l y t h e s a i l s are r e e f e d or changed l o n g b e f o r e .

I t i s p o s s i b l e now t o chose f o r optimum h u l l f a c t o r s f o r a lower speed ( f i g . 1 and 2 ) .

Speed t o windward i s i m p o r t a n t f o r a c r u i s e r , b u t n o r m a l l y c r u i s i n g people p l a n t h e i r r o u t e s c a r e f u l l y t o p r e v e n t windward courses as much as p o s s i b l e , and when t h e y a r e on t h e wind t h e y o f t e n s a i l under r e e f e d canvas f o r c o m f o r t .

N e v e r t h e l e s s I c o n s i d e r good windward c a p a c i t i e s as e s s e n t i a l a l s o from t h e p o i n t o f s a f e t y u n l e s s one would depend on t h e a u x i l a r y .

THE HULL

The d i s c u s s i o n s about advantages and disadvantages o f t h e l o n g k e e l w i t h a t t a c h e d r u d d e r a r e over now.

I t i s c l e a r t o most s a i l o r s t h a t t h e concept o f t h e s h o r t deep k e e l w i t h separated r u d d e r i s s u p e r i o r i n terms o f speed and c o n t r o l . I t t o o k a l o n g t i m e f o r t h e d e s i g n e r s t o a d j u s t t h e h u l l l i n e s t o t h e new concept b e f o r e t h e advantages became c l e a r .

By way o f e x c e p t i o n t h i s b e t t e r system i s cheaper t o produce as w e l l , e s p e c i a l l y i n s e r i e - p r o d u c t i o n .

N e v e r t h e l e s s f o r t h e c r u i s i n g man who wants t o e x p l o r e d i f f e r e n t areas t h e r e a r e s t i l l good arguments t o c o n s i d e r t h e l o n g k e e l . The l o n g k e e l o f f e r s a l o t o f s t o r e f o r b a l l a s t , f r e s h w a t e r , d i e s e l and b i l g e w a t e r ( f i g . 3 ) . The b o a t can d r y - o u t e a s i l y and p r o p e l l e r and rudder a r e w e l l p r o t e c t e d .

I f t h e owner wants t o r e s t r i c t t h e d r a f t c o n s i d e r a b l y t h e c h o i c e becomes almost a p p a r e n t .

I n f i g . 4 a r e g i v e n t h e underwater p a r t s o f : (A) a l i g h t r a c e r w i t h max. d r a f t (B) a heavy c r u i s e r w i t h moderate d r a f t and (C) a heavy c r u i s e r w i t h s h a l l o w d r a f t .

I t can be seen t h a t t h e r e a l k e e l - h e i g h t o f t h e c r u i s e r (B) i s much l e s s t h a n c o u l d be expected from t h e d i f f e r e n c e i n d r a f t due t o t h e d i f f e r e n c e i n h u l l d r a f t (Here i s i n my o p i n i o n one o f t h e advantages o f t h e v e r y l i g h t b o a t s under t h e I.O.R. r u l e where t h e max. d r a f t i s a f u n c t i o n o f L ) .

Using t h e c a l c u l a t i n g model o f T.H. D e l f t (rap.260) t h e hatched a r e a o f (B) i s 2.4 t i m e s (A) f o r t h e same leeway angle I f e e l t h a t i n p r a c t i c e t h i s model i s t o o o p t i m i s t i c f o r k e e l s l i k e t h e example ( B ) , p r o -b a -b l y t h e r e l a t i o n t h i c k n e s s / h e i g h t a l s o p l a y s a r o l e .

When t h e space between t h e back o f t h e k e e l and t h e f r o n t o f skeg o r r u d d e r i s t o o s m a l l t h e r u d d e r i s w o r k i n g i n t h e downwash o f t h e k e e l . I n t h a t case i t i s b e t t e r t o chose t h e o l d concept o f t h e a t t a c h e d r u d d e r o f f i g . 3. Here t h e r u d d e r works t o g e t h e r w i t h t h e k e e l because t h e p r e s s u r e a l s o works on t h e a f t p a r t o f t h e k e e l , reason t o keep t h e p r o p e l l e r a p e r t u r e as s m a l l as p o s s i b l e .

L a t e r a l f o r c e s can a l s o be g e n e r a t e d by o t h e r means as i s d e m o n s t r a t e d by f i g . 4 t y p e (G). I n s p i t e o f t h e absence o f a r e a l k e e l t h i s b o a t s a i l s r e a s o n a b l y w e l l t o windward and, i n s p i t e o f h e r heavy d i s p l a c e -ment, she i s f a s t on a l l o t h e r courses.

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45

-BALANCE

The modern y a c h t s a r e c h a r a c t e r i s e d by b i g beam and narrow e n t r y - a n g l e o f t h e w a t e r l i n e s . The r e s u l t i s a s t r o n g l y c u r v e d m i d s h i p p a r t . T h i s shape has proved t o be a t t r a c t i v e from t h e p o i n t o f view o f accomoda-t i o n , s a i l i n g c o m f o r accomoda-t and speed accomoda-t o windward. I accomoda-t i s however n o accomoda-t accomoda-t h e b e s t way t o a t t a i n a good balance. H e e l i n g such a h u l l has n e a r l y always a l u f f i n g moment as a r e s u l t .

For a c r u i s e r a good balance i s e x t r e m e l y i m p o r t a n t e s p e c i a l l y when the r u d d e r , due t o t h e moderate d r a f t , i s l e s s e f f i c i e n t which i n t u r n means b i g g e r rudder f o r c e s and/or rudder a n g l e s .

Under water t h e c e n t r e o f a t t a c k o f l a t e r a l f o r c e s i s moving a l o n g a h o r i z o n t a l axes dependant o f speed and leewayangle.

The l o n g e r t h e k e e l t h e more i t moves.

'^his h o l d s t r u e f o r t h e s a i l s , t h e more t h e s a i l area i s u n d e r d i v i d e d the l e s s t h e c e n t r e o f a t t a c k o f t h e w i n d f o r c e moves.

The angle o f h e e l o f t h e y a c h t , and t h e r e b y t h e s t a b i l i t y , p l a y s an i m p o r t a n t r o l e . A h u l l n o r m a l l y generates her own l u f f i n g moment due t o h e e l i n g . An e x t r a l u f f i n g moment can be seen because t h e c e n t r e o f a t t a c k o f t h e s a i l f o r c e s moves t o leeward ( f i g . 5 ) .

I n p r a c t i c e t h e methode t o s t a r t t h e game w i t h a c e r t a i n l e a d between c e n t r e o f l a t e r a l p l a n e and t h e c e n t r e o f e f f o r d o f t h e s a i l s ( f i g . 5 and 6) i s w i d e l y used.

I t w i l l be c l e a r t h a t t h i s d i s t a n c e , expressed as a percentage o f l.w.1 i s a p r e t t y rough g u i d e .

F i g . 6 shows my boat i n two s i t u a t i o n s . S i t . 1 t h e l e a d i s 27%. I f t h e wind i n c r e a s e s t o o much f o r t h e s t a b i l i t y I s i m p l y lower t h e yankee r e d u c i n g the l e a d t o 1 8 % t o f i n d t h e boat p e r f e c t l y b a l a n c e d a g a i n !

( f i g . 6 s i t . 2)

Whereas i t i s n e a r l y always p o s s i b l e t o balance a s a i l i n g b o a t when on t h e wind t h i s does n o t h o l d so f o r courses from t h e w i n d .

Now t h e balance o f t h e h u l l i t s e l f and t h e e f f i c i e n c y o f t h e r u d d e r are i m p o r t a n t . U n f o r t u n a t e l y balanced h u l l s as w e l l as narrow h u l l s have a tendency t o r o l l , which can be t h e cause o f bad s t e e r i n g . . . ! as can be u n d e r s t o o d from f i g . 5.

I t w i l l be c l e a r t h a t i t i s i m p o s s i b l e t o c a l c u l a t e more o r l e s s ac-c u r a t e t h e r i g h t b a l a n ac-c e . T i l l now i t i s m a i n l y a q u e s t i o n o f f e e l i n g and e x p e r i e n c e .

The p r e f e r a n c e o f so much c r u i s i n g people f o r a l o n g k e e l from t h e p o i n t o f balance has more t o do w i t h t h e slow r a t e a t which d i v e r g e n c e s occur as w i t h t h e r e a l o f f - c o u r s e a n g l e s .

THE RIG

The q u e s t i o n "what i s t h e r i g h t r i g f o r a t y p i c a l c r u i s e r ? " i s o f t e n t h e s u b j e c t f o r a h o t d i s c u s s i o n .

Sometimes people seem t o f o r g e t t h a t s a i l are n o t h o i s t e d f o r a e s t e t i c reasons o r t o be e a s i l y h o i s t e d b u t t o d r i v e t h e b o a t .

For t h e c r u i s e r t h e r e are some i m p o r t a n t f a c t s . : The i n d i v i d u a l s a i l s must be easy enough t o be handled by t h e normal watch on deck. A l l systems must be such t h a t m i s t a k e s are p r e v e n t e d and t h a t , i f t h e y come t o pass, no major damage can o c c u r .

The p r e v e n t i o n o f wear and c h a f e and f a t i g u e s t r e s s e s i s a l s o h i g h on t h e l i s t .

W i t h modern developments l i k e r o l l e r r e e f i n g o f f o r e s a i l s and main-s a i l main-s watchemain-s can handle b i g g e r and b i g g e r main-s a i l main-s . People mumain-st r e a l i main-s e however t h a t t h e y make themselves more and more dependent upon expen-s i v e and complex gear f o r t h e p r i c e o f which t h e y c o u l d have expen-s a i l e d f o r a l o n g p e r i o d u s i n g j u s t t h a t s m a l l amount o f b o d i l y e f f o r t t o keep themselves i n good c o n d i t i o n !

THE INFLUENCE OF WINDFORGES ON RIGGING AND HULL

Tank t e s t i n g o f s a i l i n g b o a t s i n g e n e r a l s t o p s when t h e max. r e a l speed i s reached.

For s a i l i n g t o windward t h i s p o i n t i s reached a t a windspeed o f about 10 m t r . / s e c . (6 B f . )

The p o i n t when a s a i l i n g y a c h t s t o p s making p r o g r e s s t o windward i s reached sooner t h a n most people t h i n k .

When t h e autumn g a l e s blow over t h e y a c h t h a r b o u r you see t h a t most y a c h t s h e e l c o n s i d e r a b l y when t h e wind i s abeam, i n f a c t most y a c h t s a l r e a d y consume an i m p o r t a n t p a r t o f t h e i r s t a b i l i t y w i t h o u t any s a i l s e t .

ivlost c r u i s i n g y a c h t s are h e a v i l y r i g g e d . The s e a w o r t h i n e s s and t h e d u r a b i l i t y can be improved j u s t l i k e t h a t , b u t t h e p r i c e which has t o be p a i d i s windward a b i l i t y , a n o t h e r form o f s e a w o r t h i n e s s .

49

-To show t h e i n f l u e n c e o f t h e w i n d f o r c e s on r i g and h u l l I made some c a l c u l a t i o n s based on t h e f u l l s c a l e measurements o f " S t a n d f a s t " (Hiswa Symposium '75) and used t h e r e s u l t s f o r my own boat, " J a n t i n e I V " , a t y p i c a l c r u i s e r .

From the S t a n d f a s t r e s u l t s w i t h s a i l s nu. 1 and 6 ( f i g . 9a) I t o o k the t o t a l w i n d f o r c e s and d e t r a c t e d the c a l c u l a t e d w i n d f o r c e s o f r i g -g i n -g , h u l l and t h e p a r t o f the mast above and under t h e m a i n s a i l . I a l l o w e d f o r w i n d g r a d i e n t and o t h e r f a c t o r s g i v e n i n f i g . 7.

The so d e r i v e d r e a l s a i l f o r c e s were a p p l i e d t o t h e s i t u a t i o n s g i v e n i n f i g . 10 w i t h t h e a d d i t i o n o f the matching r i g and h u l l f o r c e s . From the known r i g h t i n g moment a t 30° a n g l e o f h e e l i t was p o s s i b l e t o c a l c u l a t e the adequate w i n d f o r c e s . The r e s u l t s are g i v e n i n f i g . 1 1 . A comparison between the r a c i n g r i g ( f r o m '75) and the h e a v i e r c r u i s i n g r i g f o r t h e same h u l l i s g i v e n i n f i g . 9b.

T h e r e a f t e r I c a l c u l a t e d t h e angle o f h e e l w i t h o u t any s a i l s e t and the speed when r u n n i n g under bare p o l e s . These r e s u l t s seem t o conform w i t h r e a l i t y . Angle o f h e e l l y i n g a h u l l : 5° 15,9 mtr/sec 7 B f . 10° 23,7 mtr/sec 9 B f . 15° 30,0 mtr/sec 11 B f . Running 4,2' 14,0 mtr/sec 7 B f . 5,9' 19,3 mtr/sec 8 B f . 6,7' 23,5 mtr/sec 9 Bf. 7,4' 26,7 mtr/sec 10 B f . STABILITY I N EXTREME CIRCUMSTANCES

A c r u i s e r i n t e n d e d t o c r o s s oceans w i l l meet winds o f g a l e f o r c e f r o m t i m e t o t i m e .

Normally a w e l l designed and b u i l t y a c h t w i l l cope w i t h those c o n d i t i o n s w i t h o u t problems, p r o v i d e d the crew r e a c t s i n t h e r i g h t way. The chosen heavy weather t e c h n i q u e w i l l be more dependant upon t h e c i r c u m s t a n c e s t h a n the s a f e t y o f t h e y a c h t .

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The p o s s i b i l i t y o f meeting r e a l extreme c i r c u m s t a n c e s i s n e v e r t h e l e s s always t h e r e . Now t h e chosen t e c h n i q u e w i l l be d i c t a t e d by t h e s t a t e o f t h e sea. I t i s i n those c o n d i t i o n s t h a t even b i g y a c h t s can be com-p l e t e l y r o l l e d o v e r .

I f t h e mast under those c i r c u m s t a n c e s w i l l s t a y i n p o s i t i o n , w h i c h i s p r o b a b l y n o t i n a l l cases d e s i r a b l e w i t h t h e eye on t u r n i n g back, t h e whole r i g must n o t o n l y be v e r y s t r o n g l y c o n s t r u c t e d b u t a l s o i t must be such t h a t no bending moments i s t h e mast can be g e n e r a t e d .

From t h e l i t e r a t u r e and r e p o r t s , from people who I know w e l l enough t o b e l i e v e , we see t h a t a y a c h t can c a p s i z e i n d i f f e r e n t ways.

N e v e r t h e l e s s i t i s d i f f i c u l t t o r e a l i s e what r e a l l y happens and t h e r e c o n s t r u c t i o n s a f t e r t h e event are n o t a c c u r a t e because o f t h e a c c e l e r a -t i o n s .

I t i s an e x p e r i e n c e which makes a v e r y deep i m p r e s s i o n and few people have g a t h e r e d enough e x p e r i e n c e t o be a b l e t o g i v e a c o o l r e p o r t . N e v e r t h e l e s s some c o n c l u s i o n s can be drawn:

The r i s k o f c a p s i z i n g i n l o n g i t u d i n a l d i r e c t i o n i s l e s s t h a n abeam, the known cases are always a c o m b i n a t i o n .

Boats b e i n g helmed a c t i v e l y seem t o be s a f e r t h a n boats which are l e f t t o themselves, p a r t i c u l a r l y t h e l i g h t r a c i n g boats can p r o -b a -b l y adopt t h e f a s t m o t o r -b o a t t e c h n i q u e s .

The b i g g e r t h e y a c h t t h e more energy i s necessary t o c a p s i z e , n e v e r -t h e l e s s a number o f r e a l l y b i g y a c h -t s are a c -t u a l l y c a p s i z e d .

I f e e l t h a t one o f t h e reasons o f c a p s i z i n g i s t h a t t h e y a c h t i s b e i n g t h r o w n away by an unusual steep wave. I n a s e r i e s o f p i c t u r e s I t r i e d t o i l l u s t r a t e how t h e phenomen o c c u r s . The camera moves w i t h t h e wave speed. Two y a c h t s o f t h e same s i z e are s t r u c k by a dangerous s t e e p wave. The f i r s t y a c h t i s an I.O.R. t y p e and t h e o t h e r a t y p i c a l c r u i s e r

(see f i g . 12)

d i s p . r i g beam f r e e b o a r d d r a f t v e r t . c . o f . g r .

I.O.R. l i g h t V . l i g h t b i g moderate deep c .w.1 C r u i s e r heavy heavy moderate I I _moderate I I

S i t . 1 Both y a c h t s are more o r l e s s s t a t i o n n a r y i n t r a n s v e r s e d i r e c -t i o n . Bo-th y a c h -t s " f e e l " -t h e n e a r i n g wave and s -t a r -t -t o h e e l .

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55

-S i t . 2 Both y a c h t s are b e i n g h i t by t h e s t e e p e s t p a r t o f t h e wave. The I.O.R. y a c h t r e a c t s Quicker because o f her r e l a t i v e l y s m a l l mass i n r e l a t i o n t o t h e p r o j e c t e d a r e a . The deep k e e l r e s i s t s t h e movement t h e r e -by a d d i n g t o t h e h e e l i n g f o r c e s .

The c r u i s e r cannot r e a c t as q u i c k , her k e e l , b e i n g s h a l l o w e r generates l e s s h e e l i n g moment and her heavy r i g g i n g has a c o n s i d e r a b l e s t e a d y i n g moment. Of course she takes more water on deck.

S i t . 3 The I.O.R. y a c h t i s moving w i t h n e a r l y t h e wave speed.

When t h e deck takes t h e water an e x t r a h e e l i n g moment i s g e n e r a t e d and the boat f l i p s over. The a n g l e o f h e e l o f t h e c r u i s e r i s s t i l l i n c r e a -s i n g .

S i t . 4 The I.O.R. y a c h t i s a c t u a l l y c a p s i z e d and whether she r o l l s 360° o r r o l l s back does n o t make any d i f f e r e n c e .

The c r u i s e r , b e i n g behind t h e wave t o p i s s l o w i n g down now.

Because o f t h e b i g mass i n r e l a t i o n t o her s t a b i l i t y she w i l l r o l l t o an a l a r m i n g a n g l e o f h e e l , a so c a l l e d knok-down, but she i s on the s a f e s i d e o f t h e wave and w i l l r e c o v e r .

I f e e l t h a t t h e d i s t r i b u t i o n o f w e i g h t i n h e i g h t i s p l a y i n g an i m p o r t a n t r o l e . I t i s s t r i k i n g i n t h i s c o n n e c t i o n t o note t h a t y a c h t s h a v i n g l o s t t h e i r mast seem t o c a p s i z e more e a s i l y .

Heavy c r u i s e r s w i t h v e r y heavy r i g s and h i g h c e n t r e o f g r a v i t y (Joshua, B y l g i a , S e n t i j n , Zeeuwse stromen) have a l l e x p e r i e n c e d s e v e r a l knock downs i n t h e Southern oceans b u t none o f them r o l l e d t h r o u g h .

I see c a p s i z i n g i n extreme c i r c u m s t a n c e s as a dynamic happening which i s beyond t h e normal s t a b i l i t y c a l c u l a t i o n s o f n a v a l a r c h i t e c t u r e . As a m a t t e r o f f a c t t h e normal s t a b i l i t y c a l c u l a t i o n s a r e i m p o r t a n t , p r o p u l s i o n t o windward i s based on t h e r i g h t i n g moment, a l s o i m p o r t a n t are t h e c a c u l a t i o n s o f r i g h t i n g arms a t angles o f h e e l o f more t h a n 90°, be i t o n l y t o l o o k a t which a n g l e t h e boat comes back t o h e r u p r i g h t p o s i t i o n . I t must be born i n mind t h a t incoming water has a b e n e f i c i a l i n f l u e n c e h e r e !

- 57

-Summing up:

P o s i t i v e f a c t o r s c o n c e r n i n g c a p s i z i n g a r e : s i z e - d i s t r i b u t i o n o f w e i g h t i n h e i g h t - d i s p l a c e m e n t - l o w c e n t r e o f g r a v i t y - h i g h f r e e b o a r d - s h a l l o w d r a f t - l i t t l e l a t e r a l area and i n d i r e c t l y : w e l l balanced h u l l - buoyancy i n stem - low gunwale - optimum p r i s m a t i c coëff and l o n g i t u d i n a l c e n t e r o f buoyancy

N e g a t i v e f a c t o r s c o n c e r n i n g c a p s i z i n g a r e : s m a l l s i z e - r e l a t i v e l y l i g h t d i s p l a c e m e n t c o n c e n t r a t i o n o f w e i g h t i n h e i g h t b i g beam on deck -deep d r a f t - low f r e e b o a r d - b i g l a t e r a l area and i n d i r e c t l y : bad balance - deep f o r e f o o t - b i g transom

F a c t o r s which a r e b e n e f i c i a l f o r speed a r e :

Length - r e l a t i v e l y l i g h t d i s p l a c e m e n t - low c e n t r e o f g r a v i t y - con-c e n t r a t i o n o f w e i g h t s - deep d r a f t - l i t t l e l a t e r a l area - a b i l i t y t o p l a n e low f r e e b o a r d and a l o t o f beam on d e c k h e i g h t .

REFERENCIES

1 Symposium y a c h t a r c h i t e c t u r e '75

" D e t e r m i n a t i o n o f s a i l f o r c e s based on f u l l s c a l e measurements and m o d e l t e s t s " ( J . G e r r i t s m a , G. Moeyes, J . K e r w i n )

2 Symposium y a c h t a r c h i t e c t u r e '77

"Developments i n t h e d e s i g n o f s a i l i n g y a c h t s " (C.W. van Tongeren and H.R.F. Körner)

3 L a b o r a t o r i u m voor scheepsbouwkunde T.H. D e l f t r a p p o r t No. 260;474;518.

4 1979 F a s t n e t Race i n q u i r y Report (R.O.R.C.) 5 Small b o a t s and b r e a k i n g waves " S a i l dec. '82"