The ship used by the roll and pitch allowances
Squat the formulas are fixed; only the deep-water multiplier is yours
deep water when charted depth > multiplier × draft — raw figures, before tide or datum
base = Cb × (speed² ÷ 100)
squat = base in deep water, 2 × base in shallow water
Why the raw depth. The deep/shallow test is made on the charted depth against the
draft as they stand, not on the depth corrected for tide and datum. Squat is about the
water the hull is actually sitting in relative to its own draft, and correcting first would
move the boundary.
Minimum UKC and CATZOC how does your company’s policy use the zone of confidence?
The minimum UKC depends on the leg’s CATZOC: a percentage of its dynamic draught, or that band’s own equation. The charted depth is used as it stands.
The advanced column overrides the percentage. Leave it empty and the percentage is
used. Put an equation in it and that band uses the equation instead, for that band only.
The charted depth is reduced by the leg’s CATZOC depth allowance before anything else, and the minimum UKC is one figure for every leg, whatever its CATZOC.
CATZOC depth allowance metres taken off the charted depth
The defaults are the published depth accuracies (IHO S-57): A1 ±(0.5 m + 1 % of
the depth), A2 and B ±(1.0 m + 2 %), C ±(2.0 m + 5 %). D is only
“worse than C” and U is unassessed, so there is no figure to start from:
until your company’s is typed in, a leg on D or U has no allowance, and anything that
needs one shows no result rather than a pass.
With the minimum UKC by CATZOC, the charted depth is not
reduced: the allowance is there as
catzocAllowance for a band’s advanced
equation or a contour, if your policy uses it.
Contours and safety depth shown per waypoint, contours rounded up to the next whole metre, the safety depth to the next 0.1 m
The safety contour is the greater of the two formulas. Clear the second one and only
the first is used — that is a deliberate switch-off and it is remembered as one. If
either cannot be worked out on a waypoint, that waypoint has no safety contour (and so no
safety depth) rather than the other formula on its own, which may be the lower. It is
not the same as never having had a second formula at all, which is what an older file
without the field means; that gets the default back.
Keep this setup so the page opens with it next time
What is kept, and where. The ship (beam, LOA, numbering) and the whole policy —
squat multiplier, CATZOC, minimum UKC and contours — in this browser’s own storage
on this computer, nowhere else. Waypoints and the plan name are never kept here. A change
made after remembering is kept only when you press Remember again. For another
computer, or the DeckWatch app, save the plan as a file: it carries the setup too.
How each figure is worked out
The order, exactly as it runs.
- Squat, from the raw charted depth and draft.
- Roll allowance = ½ × beam × tan(roll).
- Pitch allowance = ½ × LOA × tan(pitch).
- Dynamic draught = draft + roll + pitch + squat + safety margin. Even keel — there is no trim term.
- Available water depth = charted depth − datum allowance + tide. When CATZOC comes off the charted depth, that band’s allowance is taken off here as well.
- Dynamic UKC = available water depth − dynamic draught.
- Minimum UKC: by CATZOC, from the band’s percentage or its own equation; or, when CATZOC comes off the depth instead, the policy’s one equation.
- Pass or fail, compared at full precision — not on the rounded figures you see.
- The three contours, from this leg’s own values.
- The safety depth, last: by default the safety contour as set on the ECDIS (the whole metre it rounds up to), or your own formula.
A blank is a zero here — deliberately, and only inside the arithmetic. A
waypoint with no tide entered is worked as no tide, not as unanswerable. The blank itself is
kept as a blank in the saved file, so it comes back a blank and not a nought.
Something typed that is not a number is not a nought — “0.3 m”,
“15 %”: the field turns red and the waypoint has no result until it is put
right. A minus printed as − or – is read as a minus.
Equations are read, not run. The contour and minimum-UKC fields are parsed by a small
reader that understands only numbers,
+ − × ÷, brackets and the names listed
beside each field. It cannot reach anything else on the page or in the browser, and a name it
does not recognise is refused when you type it rather than quietly treated as zero.
What can be used where. A contour may use the readings, the allowances, the dynamic
draught, the available water depth and the minimum UKC — but not the dynamic UKC, and
not another contour. The safety depth may use all a contour may, plus
safetyContour. A minimum-UKC equation may use all of those plus the dynamic UKC, but
not the minimum UKC itself, because that is the thing it is working out. A CATZOC depth
allowance comes first of all, so it may use only the charted depth, the draft, the tide and
the datum allowance; everything after it may use catzocAllowance. With the
minimum UKC by CATZOC it is worked out all the same but not taken off the depth, so a
band’s advanced equation or a contour can still add it where your policy does.
What this is. Arithmetic on figures you type, against a policy you set. It holds no
chart, no tide table and no squat model of your hull. The squat formula here is a rule of
thumb — it is not a manoeuvring-booklet figure and it does not know your block
coefficient at this trim, your channel, or the bank effect alongside it. Charted depths, ZOC,
tidal heights and the company’s UKC policy come from the chart, the publications and the
SMS. Nothing here is approved, certified or class-accepted, and the Master remains
responsible for the passage plan and for every depth on it.
A web version of a tool from DeckWatch, the work planner for ship’s deck officers.