The relative motion (relative velocity) plot (relvel) gives the deck officer an understanding of the combined movement of vessels when under way and making way. The relative motion plot takes the following factors into account – when given any four factors, the remaining two can be calculated:<\/p>\n
- \n
- The true course of the observing (own) vessel.<\/li>\n
- The actual speed of the observing (own) vessel.<\/li>\n
- The true course of another ship (the target observed).<\/li>\n
- The actual speed of another vessel (the target observed).<\/li>\n
- The combined movement of both vessels.<\/li>\n
- The combined speed of both vessels.<\/li>\n<\/ol>\n
Plotting procedure.<\/strong> The relative velocity problem is solved by the use of vectors on a plotting diagram as follows:<\/p>\n
- \n
- Lay off the observing (own) ship\u2019s true course.<\/li>\n
- Plot the first bearing and distance of the target observed and next to it insert the time
\nand the letter \u201cO\u201d.<\/li>\n - Plot the second bearing and distance and insert the time next to it (six minutes later).<\/li>\n
- Plot the third bearing and distance and next to it insert the time and the letter \u201cA\u201d (a
\nfurther six minutes later).<\/li>\n - Join \u201cO\u201d and \u201cA\u201d and extend this line past the centre of the plot. This is the projected
\nmovement of the target and this extension will indicate whether a risk of collision will result. Insert an arrow in the direction of movement.<\/li>\n - Insert the observing (own) ship\u2019s course in reverse from point \u201cO\u201d. The length of this vector is the distance travelled by own ship at it\u2019s present speed for 12 minutes. Mark the end of this vector with the letter \u201cW\u201d. Insert an arrow in the direction of own ship\u2019s course.<\/li>\n
- Complete the triangle by joining \u201cW\u201d to \u201cA\u201d. Insert an arrow in the direction of \u201cA\u201d.<\/li>\n
- Determine the direction of \u201cW-A\u201d which is the other ship\u2019s true course.<\/li>\n
- Measure the length of \u201cW-A\u201d which is the distance travelled by the other ship in 12 minutes. From this calculate the other ship\u2019s true speed (distance of \u201cW-A\u201d multiplied by five).<\/li>\n<\/ol>\n
![\"\"](\"https:\/\/maritimesa.org\/nautical-science-grade-12\/wp-content\/uploads\/sites\/7\/2020\/09\/12-1-3-7-fig5.jpg\")
A graphical representation of the procedure described above.<\/strong><\/p><\/div>\n
Summary of Vector components.<\/strong><\/p>\n
- \n
- OA<\/strong> Relative movement of vessels which includes relative direction and the relative movement over the period between the first and last plots.<\/li>\n
- WO<\/strong> Own vessel\u2019s true course and the distance covered over the same time period as OA.<\/li>\n
- WA<\/strong> Other vessel\u2019s true course and distance covered over the same time period as OA.<\/li>\n<\/ul>\n
Target report.<\/strong> The results of the plotting procedure are used to compile a Target Report which has the following elements:<\/p>\n
- \n
- Target identity\/name.<\/li>\n
- Time of first plot.<\/li>\n
- Bearing and range of first plot.<\/li>\n
- Closest Point of Approach (range and bearing).<\/li>\n
- Time to Closest Point of Approach.<\/li>\n
- Target\u2019s true course.<\/li>\n
- Target\u2019s true speed.<\/li>\n<\/ol>\n","protected":false},"excerpt":{"rendered":"
Why do we need to do radar plotting? The International Regulations for the Prevention of Collisions at Sea (1972) as amended (Colregs) requires the systematic observations of targets. Extracts from Rule 7 b. and c. state: Rule 7b. \u201cProper use shall be made of radar equipment if fitted and operational, […]<\/p>\n","protected":false},"author":6,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[14,23],"tags":[],"class_list":["post-267","post","type-post","status-publish","format-standard","hentry","category-12-1-3-radar","category-12-1-3-8-radar-plotting"],"_links":{"self":[{"href":"https:\/\/maritimesa.org\/nautical-science-grade-12\/wp-json\/wp\/v2\/posts\/267"}],"collection":[{"href":"https:\/\/maritimesa.org\/nautical-science-grade-12\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/maritimesa.org\/nautical-science-grade-12\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/maritimesa.org\/nautical-science-grade-12\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/maritimesa.org\/nautical-science-grade-12\/wp-json\/wp\/v2\/comments?post=267"}],"version-history":[{"count":9,"href":"https:\/\/maritimesa.org\/nautical-science-grade-12\/wp-json\/wp\/v2\/posts\/267\/revisions"}],"predecessor-version":[{"id":435,"href":"https:\/\/maritimesa.org\/nautical-science-grade-12\/wp-json\/wp\/v2\/posts\/267\/revisions\/435"}],"wp:attachment":[{"href":"https:\/\/maritimesa.org\/nautical-science-grade-12\/wp-json\/wp\/v2\/media?parent=267"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/maritimesa.org\/nautical-science-grade-12\/wp-json\/wp\/v2\/categories?post=267"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/maritimesa.org\/nautical-science-grade-12\/wp-json\/wp\/v2\/tags?post=267"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}
- WO<\/strong> Own vessel\u2019s true course and the distance covered over the same time period as OA.<\/li>\n
- OA<\/strong> Relative movement of vessels which includes relative direction and the relative movement over the period between the first and last plots.<\/li>\n