A brief introduction on slamming & deckwetness.

1. SLAMMING
&
DECK WETNESS

2. SLAMMING
In rough seas, the vessel’s bow and stern may occasionally emerge from a wave and re-
enter the wave with a heavy impact or slam as the hull structure comes in contact with the
water.
The vessel experiences impulse loads with high-pressure peaks during the
impact between the vessel’s hull and water
The Conditions leading to slamming are:
High relative speed between ship and water
Small rise of floor
Shallow draft

4. Types of slamming
1.Bottom Slamming
The bottom slamming pressure is to be calculated for the bottom structure in way of the flat of bottom forward
of 0.25L measured from the forward perpendicular (FP).
2.Bowflare Slamming
The Bowflare slamming pressure is to be calculated for the side shell structure above the waterline in the area
forward of 0.25L from the FP.
3.Stern Slamming
The stern slamming pressure is to be calculated for the shell structure between 0.15L from the after
perpendicular (AP) and the aft end.

8. Effect of slamming
1.The area between 10 to 25 per cent of the length from the bow is the most likely to suffers
high pressure on bottom plating and sustain damage
2.Stress is developed along the main structure, called as whipping stress.
Due to that the deformation takes place in both the vertical and the
horizontal direction.

9. DECK WETNESS
The shipping of green water because of the Forward deck relative motion to the wave surface is defined as
deck wetness.
It is determined by the relative height of the deck above the ocean surface i.e. the height of the freeboard
The Probability of increasing deck wetness is determined by two methods.
STATIC SWELL-UP DYNAMIC SWELL-UP

10. STATIC SWELL-UP
Static swell-up of water at bow occurs due to two phenomenon
1.Bow wave generated by the vessel while moving in steel water.
2. Sinkage of the bow of the ship while running at a speed.
The effect of static swell-up is reduction of the freeboard height.
The effective freeboard is given by
Where
f’(l)=Effective Freeboard.
f(l)=Actual freeboard in still water.
hs(l)=Static swell-up.
ξ(e)=Elevation due to bow wave.
ξ(s)=Sinkage due to speed.
f’(l)=f(l)-hs(l)
hs(l)=ξ(e)-ξ(s)

11. DYNAMIC SWELL-UP
Dynamic swell-up occurs because of the bow , while immersing pushes water aside and ,while emerging
from below sucks in water.
The Dynamic swell-up is given by the following expression.
Where,
K=Constant depending up on the block-coefficient .
= 1/3(CB-0.45) 0.6<CB<0.8
L=Distance of CG from the forward Perpendicular.
S(l)=Relative bow speed.
h’(l )can be obtained by following equation
Where,
h’(l)=Amplitude of relative bow motion considering dynamic effect.
hd(l)=Amplitude of dynamic swell.
S(l)=Amplitude of relative bow speed.
hd (l)=KL
𝑆(𝑙)
𝑔𝑙
h’(l)S(l)
hd(l)
h’(l)=f’(l)-hd(l)+S(l)

12. Deck wetness can be reduced by
By increased Freeboard
By reducing speed
By changing the ship’s heading relative to the predominant waves