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Theory behind Turning dynamics of ships


published on 6 May 2020 -  2304 -

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Found on YouTube. Created by
"ISSIMS GmbH - Marine Prediction Technology"

SAMMON Lecturing Video describing

"Theory behind turning dynamics of ships"

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SAMMON - the IDEAL tool to identify manoeuvring capabilities of a ship - SAMMON - learning the EFFECTIVE way

Comment by Gunter Schütze via LinkedIn:
"Prof. Benedict a very interesting video by HS Wismar / Maritime Simulation Center Warnemünde ( MSCW) & Innovative Ship Simulation and Maritime Systems GmbH (ISSIMS). Excellent and understandable the dynamic effects of the forces and moments in turning circle explained. A really good illustration for students, nautical officer and Captains to lead them to the dynamic basics and to refresh the knowldege. I can only recommend this video. Thanks for sharing."
Germany, Wismar

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Knud Benedict Germany
on 21 December 2020, 11:49 UTC

Thanks to Gunter for the recommendations and Michael for bringing in ideas and questions. To answer your questions, I like to explain the following:
1) The transverse force Y(ß) can be explained by two models:
a. Either it can be seen from its nature as a Lift force, i.e. the same force who lifts an airplane wing up in the air, works now horizontally on the ship hull due to the drift angle ß. It is generated by the uneven force distribution caused by an additional circular flow around the body (wing, or here the ship’s hull). This lift force has it maximum close to the wing nose – this means for the ship it is working more to the bow, so it turns the ship: I made an additional movie on that effect at out ISSIMS website on YouTube (“Effect of Drift Force and unstable moment”, https://youtu.be/agQPMQNwhxU.)
b. A second way is to look at the effect of the transverse speed along the ship length (This as named as “Slender body theory”). You are right that the transverse speed distribution is constant. However, the force due this inflow is not constant: because of the friction in the flow there is a boundary layer around the ship which is getting thicker to the stern and therefore the transverse forces are smaller at the stern than on the bow – this model also explains why the bow turns to increase the drift angle, which is called as unstable moment.
2) The "additional force at the stern" Y(r) is not just the aft component of the above evenly distributed drift force. The rotation /Rate of Turn creates a linear transverse speed distribution with maximum at bow / stern and minimum / zero at the ships center. This has an effect which can be explained by the wing/lift model under a) as curved centerline of the wind profile
3) You are right: A centripetal force is a force that makes a body follow a curved path. Its direction is always orthogonal to the motion of the body and towards the fixed point of the instantaneous center of curvature of the path - so in case of the ship motion on a circle these are the forces due to drift Y(ß) and turning Y(r). These forces are balanced by the centrifugal forces (which is Ycentrifugal in the movie). The ship is able to move in a steady state motion - This is also an equilibrium condition at constant speed and also constant rate of turn.
I do hope my explanations are of some help for professional as you are. And I should underline that these are attempts of us as humans to understand the nature – that’s why I mentioned two models (there are even more…). And they are not “Laws” - even the equations of Newton and Einstein are not Laws, but they represent our current views and approaches to describe the nature with our brain in order to think and plan ahead (– and have to be checked and approved continoulsy in daily life). In this way I made some comments in my movie about “digital twins” as key maritime innovations (https://youtu.be/jYRq6xRrVUw) you can have I look if you like.
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Mr Michael Nicholson Shipmove Ltd., United Kingdom
on 7 May 2020, 09:43 UTC

I either; don't fully understand, or disagree. 1) Why must the working force (transverse force) Y(B) be acting close to the bow. The rudder is providing a moment, I don't see why/how the ship also provides a turning moment. The drift angle of the ship to its bodily movement is uniform. Surely the forces would be distributed evenly?
2) Is the "additional force at the stern" Y(r) not just the the aft component of the above evenly distributed drift force.
3) The use of the term Equilibrium, is this correct? For an object to describe an arc or a circle requires an acceleration towards the centre, caused by centripetal force. This is not in equilibrium. Equilibrium would be a straight line at uniform speed.
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