Turning Moment due to Wind
Can Wind turn the Vessel? Yes
Can we predict which way vessel with headway will turn?
Towards the wind, that was easy!
Can we predict which way vessel with sternway will turn?
I am wondering……..!
Well we can figure out above with little bit of science! So we will just explore the turning effect due to wind.
Wind will cause below 2 effects on vessel:-
Vessel will drift downwind due to force of wind as discussed in last chapter.
Vessel will turn due to wind which we will cover as below.
Vessel at even keel stopped in beam wind from Port side
In above figure, P is the pivot point of vessel almost at midship since vessel is stopped. Now since windage area aft of pivot point is more as compared to forward due to Appendage (accommodation) situated aft, so the point W (center of action of wind force) will be slightly aft of midship. Hence there is a short turning lever thus a relatively weak turning moment, nevertheless vessel will have weak tendency to turn to port.
Vessel at even keel moving with headway in beam wind from Port side
In above figure P(Pivot point) is situated at 1/4th ship length from forward since vessel has headway. Windage point (W) remain at same position. As we can see now there is much larger turning lever hence vessel will have a much larger turning moment to port hence strong tendency to turn to port.
Clearing some doubts:
What will happen if vl had midship accommodation, of in case of a car carrier, will it turn to port or stbd or no turn at all?
Normally people have understanding that due to large accommodation block situated aft, wind will tend to blow accommodation block away, that’s the reason vessel tend to turn to port.
Now let’s imagine a vessel where there is no accommodation block.
When there is no accommodation block, windage point W can be expected to be located right at midship (geometrical center of freeboard). Since pivot point is still located in fwd part. There is still a separation of P and W, this separation will act as turning lever and vessel will still experience turning lever to port.
Vessel at even keel moving with sternway in beam wind from Port side
As we can see in above figure, Windage point is still located slightly aft of midship but due to sternway of vessel now Pivot point has moved to 1/4th ship length from stern. In this new configuration wind is acting forward of pivot point and there is a reasonably large turning lever and moment which will tend to turn vessel to starboard.
In few cases when vessel is fully loaded and main deck freeboard is very less and accommodation freeboard is comparatively larger, it can happen that windage point(W) is situated aft of pivot point(P) and just forward of accommodation. In this case there will be small very small turning lever and turning moment and vessel will have tendency to turn to port.
“ Hence one should not have this doubt that due to wind acting on accommodation stern will always move away from wind and vessel will turn to port. This will depend upon vessel movement, freeboard and lateral area of accommodation block.”
Vessel trim by stern moving with headway in beam wind from Port side
-Weak turning moment bow towards wind.
Now due to trim a larger section of hull in forward part is exposed to wind as compared to aft part hence windage W point is located slightly forward of midship. Pivot point is located at 1/4th of ship length from bow due to head way. So there is a rather small turning lever thus smaller turning moment but vessel will still have tendency to turn to port.
In few cases of small vessels like barges, small tanker or bulk carriers with small accommodations in empty condition with large trim the windage area of forward part may be so large that windage point W may be located well forward of misdhip. If such vessels are stopped (Like just casting of from berth and got underway) the pivot point will be located at midship and bow will violently move downwind and stern may rapidly swing to port and come in contact with berth (if casting off from port side berth). So pilots of Masters of such vessel must expect and be ready to take bold corrective action as soon as all lines are casted off.
Vessel trim by stern moving with sternway in beam wind from Port side
-Strong turning moment bow away from wind and stern into the wind.
Due to larger turning lever vessel will have strong tendency to turn to stbd (i.e. Stern will seek the wind while falling back).
Above were some generic scenarios which will hold good for most of the cases however similar line of thought may be used to predict behavior of different types of vessels like with forward accommodation, very small accommodation, vessel with very large freeboard or trim etc. There is no need to exactly calculate location of Windage point W or pivot point P, Just an intelligent estimate should be fine.
As long as one expects such behavior of vessel in wind and ready to apply corrective action, he can handle vessels safely in strong winds.
So with above understanding we have covered one of the most important and often misunderstood concept which we will be using in day to day pilotage/ship handling.
Observing Wind Direction
Usually all the pilots develop a habit of observing various items to get an idea of wind direction and strength.
1. Flags on your vessel or near by vessels.
2. Wind socks on jetty or other structure.
3. Smoke coming out from own vessel.
4. Cross check your visual observation with anemometer for wind direction and strength.