T E C H N I S C H E H O G E S C H O O L DELFT
AFDELING DER MARITIEME TECHNIEK
L A B O R A T O R I U M V O O R S C H E E P S H Y D R O M E C H A N I C A
THE PURPOSE OF THE SEAKEEPING EXPERIMENTS.
J . GERRITSMA REPORTNR. 139-11
l l t h I T T C , TOKYO 1966
Delft University of Technology
Ship Hydromechanics Laboratory Mekelweg 2
2628 CD D E L F T The Netherlands Phone 0 1 5 - 7 8 6 8 8 2
THE PURPOSE OF THE SEAKEEPINQ EXPERIMENTS.
Ths r a p i d davalopmect o f eealceeping r e s e a r c h i n t h e l a s t f i f t e e n y e a r s r e s u l t e d i n some oases i n t h e c o n s t r u c t i o n o f s p e c i a l seakeepingr l a b o r a t o r i e s . I n a d d i t i o n many c o n v e n t i o n a l t o w i n g t a n k s e x t e n d e d t h e i r equipment w i t h a wave maiker and i n s t r u m e n t a t i o n t o measure s h i p model m o t i o n s i n waves, and wave b e n d i n g moments.
I n many oases t h e purpose o f t h e s e e x p e r i m e n t a l f a c i l i t i e s i s t o a s -s i -s t t h e -s e a k e e p i n g r e -s e a r c h i n t h e development o f -s h i p form-s f o r optimum b e h a v i o u r i n waves. A l s o o t h e r f l o a t i n g s t r u c t u r e s , f o r i n s t a n c e d r i l l i n g p l a t f o r m s , a r e o f i n t e r e s t i n t h i s r e s p e c t .
The d e f i n i t i o n o f optimum b e h a v i o u r i s a problem i n i t s e l f . I t may be governed by m o t i o n a m p l i t u d e s o r a c c e l e r a t i o n s , w e t n e s s , slamming o r s p t e d -I s s B t o m e n t i o n a few i t e m s , b u t v e r y l i t t l e i s known about c r i t e r i a w h i c h c o u l d be used i n t h i s r e s p e c t . A n o t h e r purpose o f s e a k e e p i n g e x p e r i m e n t s i s f o r i n s t a n c e t h e d e t e r m i n a t i o n o f wave l o a d s on s h i p s f o r t h a purpose o f ^ ^ s t r u o t u r a l d e s i g n . I n t h i s case t h e e x p e r i m e n t may be used f o r t h e d i r e c t e v a l u a t i o n o f t h e wave l o a d on a p a r t i c u l a r s h i p f o r m o r s y s t e m a t i c t e s t s a r e used t o e x t r a p o l a t e e x i s t i n g knowledge t o g r e a t e r s h i p l e n g t h s o r f u l -l e r s h i p f o r m s .
We can d i s t i n g u i s h two d i f f e r e n t approaches i n t h e use o f s h i p model e x p e r i m e n t a l t h e pure e m p i r i c a l e x p e r i m e n t and what I l i k e t o c a l l t h e f u n d a m e n t a l e x p e r i m e n t . A c l a s s i c example o f t h e e m p i r i c a l e x p e r i m e n t i s t h e commercial s h i p model t e s t i n s t i l l w a t e r w h i c h must p r o v i d e t h e d a t a f o r t h e d e s i g n o f t h e power p l a n t . I n t h i s case t h e s t a t i s t i c s o f t h e t e s t r e s u l t s p l a y a dominant r o l e and knowledge o f t h e s u b j e c t depends f o r a l a r g e p a r t on t h e number o f s h i p models t a s t e d . I t i s n o t e d t h a t t h e c r i t e -r i u m f o -r o p t i m i u a t i o n l o o k s -r a t h e -r s i m p l e i n t h i s c a a e i t h e powe-r must be a s s m a l l aa p o s s i b l e f o r a g i v e n speed and d i s p l a c e m e n t .
The o t h e r extreme i s t h e f u n d a m e n t a l e x p e r i m e n t w h i c h i s used t o
check a t h e o r y o r w h i c h i s used t o a s s i s t t h e o r e t i c a l r e s e a r c h . The m u t u a l i n f l u e n c t o f t h e o r y and e x p e r i m e n t may r e s u l t i n s o l u t i o n s o f t h e p r o b l e m w h i c h r e p l a c e e n t i r e l y t h e model t e s t r o u t i n e by computer work. C l e a r l y t h i s method i s v a l u a b l e i n cases where a p u r e e m p i r i c a l a p p r o a c h w o u l d r e -s u l t i n an e x c e -s -s i v e number o f model t e -s t -s .
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The e a r l y e x p e r i m e n t a l s e a k e e p i n g r e s e a r c h s t a r t e d w i t h s y s t e m a t i c m o t i o n and wave l o a d t e s t s u s i n g s h i p models t n r e g u l a r waves. These ex-p e r i m e n t s were u s e f u l f o r a f i r s t o r i e n t a t i o n b u t t h e r e s u l t s do n o t g i v s s u f f i c i e n t i n f o r m a t i o n f o r a d e t a i l e d a n a l y s i s , w h i c h c o u l d be used f o r o p t i m i z a t i o n p u r p o s e s . T h i s i s due t o t h e f a c t t h a t a l a r g e number o f p a r a -m e t e r s i s i n v o l v e d i n s e a k e e p i n g p r o b l e -m s . I'or i n s t a n c e i n t h e a p p a r e n t l y s i m p l e case o f s h i p m o t i o n s i n l o n g i t u d i n a l waves we have t o d e a l w i t h 12 c o e f f i c i e n t s i n t h e l e f t hand s i d e s o f t h e c o u p l e d e q u a t i o n s o f m o t i o n , and most o f them have a s i g n i f i c a n t i n f l u e n c e on t h e s h i p r e s p o n s e i n waves.
A f u r t h e r c o m p l i c a t i o n i s t h e r e l a t i v e s m a l l i n f l u e n c e o f t h e s h i p form on t h e s h i p m o t i o n ; t h e s h i p l e n g t h and t h e w e i g h t d i s t r i b u t i o n have much more i n f l u e n c e . A c o n c l u s i o n c o u l d be t h a t f a i r l y d r a s t i c changes o f t h e s h i p f o r m a r e n e c e s s a r y t o a l t e r t h e b e h a v i o u r i n waves when t h e s h i p l e n g t h i s f i x e d .
To a v o i d an u n r e a l i s t i c l a r g e number o f s h i p model t e s t s i n waves, t h e development o f methods f o r t h e c o m p u t a t i o n o f s h i p m o t i o n s and b e n d i n g mo-ments i n waves was c l e a r l y n e c e s s a r y and i n an e a r l y s t a g e t h e s t r i p t h e o r y was i n t r o d u c e d . The a p p a r e n t l y crude a s s u m p t i o n s , as used i n t h i s method, needed e x p e r i m e n t a l v e r i f i c a t i o n and a number o f f u n d a m e n t a l e x p e r i m e n t s were c a r r i e d o u t f o r t h i s p u r p o s e . I n t h i s case s u c c e s s f u l s o l u t i o n s f o r
t h e head sea c o n d i t i o n were o b t a i n e d i n a r e l a t i v e l y s h o r t t i m e . Exemples o f such work a r e g i v e n i n t h e Seakeeping Committee R e p o r t o f t h i s C o n f e r e n c e ,
E x t e n s i v e s t u d i e s o f s h i p m o t i o n s i n head seas can now be c a r r i e d o u t on a computer w h i c h saves a c o n s i d e r a b l e amount o f t i m e and money.
There i s no doubt t h a t f u n d a m e n t a l e x p e r i m e n t s w i l l r e m a i n a v e r y ne-c e s s a r y t o o l i n t h e f u t u r e s e a k e e p i n g r e s e a r ne-c h . I n t h i s r e s p e ne-c t I m e n t i o n o n l y one example. For o p t i m i z i n g t h e s h i p f o r m we have t o s t u d y i n more de-t a i l de-t h e c o n d i de-t i o n s o f s h i p speed and c o u r s e i n a g i v e n seaway w h i c h l e a d t o w e t n e s s , slamming and p r o p e l l e r emergence. I n f o r m a t i o n c o n c e r n i n g t h e s e phenomenae c o u l d a l l o w ua t o p r e d i c t w i t h s u f f i c i e n t a c c u r a c y t h e p r o b a b i l i -t y o f -t h e i r o c c u r r e n c e i n a g i v e n se wjiy. These p r e d i c -t i o n s may be used -t o compare d i f f e r e n t h u l l forms w i t h r e s p e c t t o t h e i r s e a k e e p i n g q u a l i t i e s . A l s o i n t h i s case d e t a i l e d and s i m p l i f i e d e x p e r i m e n t s w i l l be used t o check and e x t e n d e x i s t i n g methods.
There w i l l always r e m a i n a group o f problems i n s e a k e e p i n g r e s e a r c h where t h a pure e m p i r i c a l a p p r o a c h i s t h e b e s t or t h e q u i c k e s t way t o s o l v e t h e p r o b l e m . One example i s f o r i n s t a n c e t h e d e t e r m i n a t i o n o f t h e i n t e r -a c t i o n e f f e c t s between two s h i p s on p -a r -a l l e l c o u r s e s i n -a se-aw-ay -as o c c u r s
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d u r i n g fu«llings o p e r a t i o n s o f n a v a l v e a e e l e . I &m i n t e r e s t e d t o know whether t h e s e and s i m i l a r e x p e r i m e n t s a r e a c t u a l l y s u c c e s s f u l i n s o l v i n g p r a c t i c a l s h i p m o t i o n p r o b l e m s . Because I have no e x p e r i e n c e w i t h such e x p e r i m e n t s and l i t e r a t u r e on t h e s u b j e c t i s r a t h e r s c a r c e .
I may i n v i t e t h e d i s c u s s e r s t o g i v e t h e i r o p i n i o n about t h e use o f f u n d a m e n t a l and e m p i r i c a l e x p e r i m e n t s i n t h e t o w i n g t a n k p r a c t i c e .
