예술
뷰티 및 웰니스
공예
문화 및 역사
엔터테인먼트
환경
음식 및 음료
역공학
과학
스포츠
기술
웨어러블
Speed Through the Water: Using and Calibrating the Log
Dead reckoning needs a speed, and the word 'knot' is a fossil of how it was first measured: a wedge of wood on a line knotted at even intervals, thrown over the stern, with the knots counted as they ran out during a measured interval of a sand glass.
The chip log, already published here, is that instrument. This rung is about USING it honestly — because every log measures speed through the WATER, and the water itself is usually moving. A log reading five knots in a two-knot foul tide is a boat making three over the ground, and no log will ever tell you that.
You will build a simple log, calibrate it over a measured distance, and learn the difference between speed, distance run and distance made good — three quantities that are routinely confused and are never the same number.
초급
1 day
안내
1
1
Understand why the knot spacing is what it is
Understand why the knot spacing is what it is
A nautical mile is 1852 metres. If knots are spaced at 1/120th of that — about 15.4 metres — then the number running out in 30 seconds is the speed in nautical miles per hour directly.
Thirty seconds is 1/120th of an hour, so the two ratios cancel and the count IS the speed. That is the whole elegance of the device: no calculation at sea, in the dark, by a tired watch. Choose a different glass and you must re-space the knots to match.
이 단계의 재료:
먹줄(수평 실)60 metre필요한 도구:
줄자
유성 매직2
2
Make a chip that stays put in the water
Make a chip that stays put in the water
Cut a quadrant of thin board about 200 mm on its straight edges, weight the curved edge with lead so it floats upright, and bridle it from three points so it presents its face to the pull.
The chip must stay still in the water while the line runs off the reel — it is a sea anchor, not a float. If it skids or planes, every reading is short. The traditional bridle has one leg on a tapered peg that pulls free when the line is jerked, so the chip turns edge-on and can be hauled in easily.
이 단계의 재료:
합판1 장먹줄(수평 실)10 metre필요한 도구:
손톱(핸드소)
사포3
3
Leave stray line before the first knot
Leave stray line before the first knot
Mark a length of 'stray line' — typically 10 to 20 metres — before the first knot, and only start the glass when the stray-line marker passes your hand.
Close astern the water is disturbed by the boat's wake and is travelling with her, so a chip in that zone is dragged along and reads slow. The stray line gets the chip out into undisturbed water before timing begins. Skipping it is the single commonest reason a hand log under-reads.
필요한 도구:
유성 매직줄자4
4
Calibrate against a measured distance, both ways
Calibrate against a measured distance, both ways
Run a known distance between two fixed marks at steady throttle, timing it, then run the SAME distance back at the same setting.
Averaging the two runs cancels the current, which otherwise adds to one and subtracts from the other. One run alone measures your speed over the ground on that day's tide and tells you nothing reliable about the log. Do it in calm conditions, and repeat at two or three speeds, because log errors are rarely a constant percentage.
필요한 도구:
스톱워치5
5
Knot spacing, calibration and the three distances
Knot spacing, calibration and the three distances
Jupyter 노트북 불러오는 중…
필요한 도구:
스톱워치6
6
Record the correction and use it every time
Record the correction and use it every time
Write the correction factor for each speed on a card by the chart table, and apply it to every log reading before it goes into the plot.
A calibration in a notebook you do not consult is worth nothing. Re-check it after anything that changes the hull — a season's weed growth easily costs five per cent, which over a night passage is several miles of error accumulating steadily in one direction.
이 단계의 재료:
카드지4 개관련 블루프린트
이 블루프린트들은 지식을 공유합니다 — 기술, 재료 또는 원리
CC0 퍼블릭 도메인
이 블루프린트는 CC0로 공개되었습니다. 어떤 목적으로든 자유롭게 복사, 수정, 배포 및 사용할 수 있습니다.
제품 구매를 통해 메이커를 지원하세요. 판매자가 설정한 메이커 커미션 을 받거나, 이 블루프린트의 새로운 반복을 만들어 연결로 포함시킬 수 있습니다.