A lot of my students lately asked me how to calculate or determine their wheel-over point. Since most of them are onboard their ships, they usually communicate with me through Facebook Messenger. Of course, it is so difficult for me to give a lecture via FB messenger every time somebody will ask me about it. Thus, I decided to write this blog post so I can just direct them here so as to save time.
Let’s begin! So what is a wheel-over point and why do we have to determine its position?
A big vessel cannot make a turn at one point. Thus, it is very important that her turning ability within a restricted area is considered. For a safe turn, it is necessary for a vessel to determine its wheel-over position or point. The wheel-over point is a point where the rudder is put over while considering the distance to the new course and the ship’s turning characteristics.
With the given information below, let’s start to determine the wheel-over position for the vessel.
- Current course: 090 deg T
- Next course: 045 deg T
- Speed Over the Ground (SOG) : 12 knots
- Turning Radius ( TR = SOG/ROT) = 1.0 nm
- Length Over-all (LOA): 235 meters
- Point-of-overcoming the inertia (POI): 1.5 x LOA = 352.5 meters or 0.2 meters . Also known as the execution point wherein the ship starts to turn ( Wheelhouse Poster)
Provided that your course lines are already constructed on the chart, draw two (2) parallel indexes at turning radius distance (1.0 nm) from your course lines inside the turn. Please see the below illustration.
2. Identify and mark your “datum”. It is where the two parallel indexes meet or intersect.
3. Take a pencil compass and spread it to the distance of turning radius which is 1.0 nm. Point the steel tip of the pencil compass on the datum and then draw a turn onto your course such as illustrated below.
4. Apply your point of overcoming the inertia backward. Your wheel-over position or point is approximately 0.2 nm from where the turn tangent with your initial course.
5. Drag your next course in the position of your wheel-over point to create your wheel-over line.
The wheel-over line is used to indicate the points of wheel-over when the vessel is not exactly at her course line during the turn or alteration of course.
That’s it! I hope that this post will add value to you and I hope you enjoy reading my post. If you have any question or comment regarding this post, please don’t forget to leave a note in the comment box below. I would love to hear from you.
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