Determination of Course-to-Steer
As we now know, the horizontal movement of water or current is always expressed in set (direction) and drift (speed). So, if we are trying to steer a vessel on a very specific course, say a through a narrow channel at night or in reduced visibility between two islands, a strong current taken on hard on our starboard bow would certainly make this job interesting unless we were able to compensate for this off setting current. In other words, we need be able to calculate a course heading which takes into account known set and drift (plus leeway) and thus making good the vessel’s ordered course.
Determination of course to steer and actual speed made good using intended vessel’s speed (i.e., you intend to run at or turn for an intended boat speed). This current triangle is a graphic vector diagram in which the first line drawn is the course or track line you intend to make good. It is drawn an indeterminate length from your beginning fix. Near its starting end make a tick mark and label it point A. Now, draw in, from point A, the current vector or Set in true degrees and its length in miles (or scale units) equal to that of the Drift speed in knots. Label this point B. Now, using your dividers or better yet, a drawing compass lay down an arc with its center at point B and its radius (in miles) equal to the intended speed of your vessel in knots. Where this arc crosses or intersects the track line make a mark and label it C. Draw a line from point B to C. The direction of this line is the course to steer at your intended boat speed. The length now established by the mark on your intended course line predicts the actual speed you will make good. It is this speed made good which should be used to calculate your estimated time en route.
Now review the plot on the following page. Learn it, understand it, and practice it.
“Engines Turning For” Course-To-Steer Study Problems:
Use the following deviation table along with 009º West Variation plus the compass deviation table from Lesson 10.
1. You are turning for 9 knots, westerly wind is causing 003º of leeway, and the current is 320º T at 1.2 knots. What TRUE course should you steer to remain southbound in the northern leg of York Spit Channel?
A. 203º T
B. 197º T
C. 194º T
D. 191º T
2. What is the TRUE heading to steer outbound in Thimble Shoal Channel if your engines are turning for 8.0 knots, the current 050ºT at 1.0 knots and a northerly wind is causing 003º of leeway?
A. 111º T
B. 104º T
C. 101º T
D. 098º T
3. What is the heading per magnetic compass (PMC) to steer inbound in the York River Entrance Channel if you engines are turning for 9.5 knots, the current is 076ºT at 1.2 knots, and a southwesterly wind causes 003º of leeway?
A. 313º PMC
B. 300º PMC
C. 305º PMC
D. 309º PMC
4. What is the TRUE heading to steer inbound in the York River Entrance Channel if you engines are turning for 9.8 knots, the current is 220ºT at 1.2 knots, and a northerly wind causes 003º of leeway?
A. 319º T
B. 315º T
C. 301º T
D. 298º T
5. What is the heading per magnetic compass (PMC) to steer eastbound in Thimble Shoal Channel if you engines are turning for 9.5 knots, the current is 110ºT at 1.2 knots, and a southerly wind causes 003º of leeway?
A. 119º PMC
B. 115º PMC
C. 108º PMC
D. 111º PMC