Instructor self-study

Tides & Currents

Eastern Lake Ontario is essentially non-tidal — so this is both a knowledge gap for the IYT exam and a delivery challenge Jon will probe: how do you teach tidal competency in fresh water? This module covers the full IYT Module 20 standard, then adapts it with a Great Lakes / Lake Ontario water-level addendum, Canadian Hydrographic Service tide tables, and the chart-plotter apps your students actually use.

Spring / neapChart datum / LAT / MLLWDrying height Rule of twelfthsHeight of tideFlood / ebb / slack Tidal stream atlasLake Ontario / IJCCHS tablesNavionics

Delivery-plan hook: Because your local water doesn't rise and fall, tidal height work is delivered as theory + worked exercises + plotter simulation, and reinforced live on the salt-water passages (Caribbean / mileage builder). State this explicitly in the Proposed Delivery Plan — it pre-empts Jon's most obvious question.

Lesson 1 · Interactive

What Causes Tides — Spring & Neap

Tides are the rise and fall of the sea caused mainly by the Moon's gravitational pull, with the Sun contributing about half as much (the Moon is far smaller than the Sun but much closer, so its pull is roughly twice as strong). The Moon raises a "bulge" of water beneath it and a matching bulge on the far side of the Earth — as the Earth turns, each place passes through both. When Sun and Moon line up (new & full moon) their pulls combine into a large range — spring tides. When they're at right angles (quarter moons) the pulls partly cancel into a small range — neap tides. Two springs and two neaps occur each lunar month.

Semi-diurnal
Two highs and two lows each day — the most common pattern (e.g. Atlantic coasts).

Diurnal
A single high and single low each day — fewer places.

Mixed
Two highs/lows of noticeably unequal height (e.g. Pacific coast).

The lunar day: the Moon orbits in 24 h 50 min, so the tides arrive about 50 minutes later each day. Tides originate in the open ocean but are only really noticeable near the shore.

Spring tide — big range
Higher high water, lower low water, and stronger currents (more water moves in the same six hours). Best for clearing a shallow bar; worst for current against you.

Neap tide — small range
Lower highs, higher lows, weaker currents. More forgiving for slack-water timing, but less depth gained over a drying patch.

Try it · Interactive

Tide Curve Explorer

Here's a whole day at a glance. The curve below is a typical semi-diurnal tide — two highs and two lows, of slightly unequal height. Drag the slider (or drag along the curve) to scrub through the day and read off the height at any moment, just like the tide tool in Navionics. The steepest parts of the curve are where the tidal stream runs hardest; the water is near slack around the high and the low.

Height at 12:00

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00:0006:0012:0018:0024:00

Try this: find a high water and the following low — that ~6-hour fall is exactly what the rule of twelfths (Lesson 3) lets you break down hour by hour.

Lesson 2

Datums & Definitions — The Vertical Picture

Every depth question is about a vertical stack of references. Get the ladder straight and the math becomes obvious.

Chart Datum (CD) — the reference all charted depths are measured from. Canadian & UK charts use LAT (Lowest Astronomical Tide); US charts commonly use MLLW (Mean Lower Low Water). The tide almost never falls below it.

Charted depth (sounding) — the depth printed on the chart, measured down from CD. Drying height (underlined figure) is the height a feature dries above CD.

Height of tide — how much water is sitting above CD right now, from the tide tables. Actual depth = charted depth + height of tide.

Range = HW height − LW height. LAT / MHW / MLLW are statistical tide levels; springs give the biggest range, neaps the smallest.

The one equation: Actual depth under the keel = Charted depth + Height of tide − your draft. Over a drying patch, swap charted depth for −(drying height).

Lesson 3 · Interactive

The Rule of Twelfths

A quick mental method to estimate the height of tide at any time between high water (HW) and low water (LW). It assumes a tide that takes about 6 hours to run from low to high and rises and falls in a roughly sinusoidal shape — slow near HW and LW, fastest through mid-tide. That's a good fit for most semi-diurnal ports.

① Look up four numbers

The height of HW and the height of LW (tide table)
The time of that HW and that LW

② Do two tiny sums

Range = HW − LW

One twelfth = Range ÷ 12

③ Start from the nearest turn — LW if the tide is rising, HW if it's falling — and add (or subtract) twelfths as each hour passes. The amount per hour follows the pattern 1 · 2 · 3 · 3 · 2 · 1: tiny near the turn, biggest at mid-tide.

Hour after LW/HW	This hour	Cumulative
1st	1/12	1/12
2nd	2/12	3/12
3rd	3/12	6/12 (half)
4th	3/12	9/12
5th	2/12	11/12
6th	1/12	12/12 (full)

To turn those cumulative twelfths into an actual height of tide, apply them to the level you started from:

Rising (count up from LW)

Height = LW + (n⁄12 × Range)

Falling (count down from HW)

Height = HW − (n⁄12 × Range)

…where n is the cumulative twelfths from the table for the number of hours since the turn (1, 3, 6, 9, 11, 12).

Worked example. LW = 0.8 m at 0900, HW = 4.4 m at 1500.
Range = 4.4 − 0.8 = 3.6 m → one twelfth = 3.6 ÷ 12 = 0.30 m.
Height at 1200 (3 h after LW): by the end of the 3rd hour, n = 6 twelfths → rise = 6 × 0.30 = 1.80 m → Height = 0.8 + 1.8 = 2.6 m (half-range, exactly mid-tide).
Height at 1100 (2 h after LW): n = 1+2 = 3 twelfths → rise = 3 × 0.30 = 0.90 m → Height = 0.8 + 0.9 = 1.7 m.

When it's only rough: the rule assumes a ~6-hour, symmetric tide. Where the rise and fall take very different times, or in diurnal/mixed regimes, treat it as a ballpark — and it doesn't apply at all on the non-tidal Great Lakes (Lesson 7). For an exact answer, read the official tide curve / Navionics.

Set a range and slide the clock — the curve and the running height update live so you can check your mental math:

Low water height (m) High water height (m)

Hours after LW3.0 h

Estimated height of tide

2.60 m

Lesson 4 · Interactive

Height of Tide & Clearance Problems

The exam's classic question: "What is the latest time on the falling tide a yacht can pass over a patch?" It's all the same equation. Required height of tide to float safely:

Required height of tide = Drying height + Draft + Safety clearance − Charted depth.
(Over a charted sounding, drying height is zero. Over a drying patch, charted depth is zero.)

Worked IYT example: a patch dries to 1 ft, draft 4 ft, safety 1 ft → you need 6 ft of tide over it. Try your own numbers:

Drying height (m)

Your draft (m)

Safety clearance (m)

Charted depth (m)

Required height of tide

2.6 m

Then cross-reference the required height against the tide curve (Lesson 3) to read off the time you must be clear by — that's the full exam answer.

Lesson 5

Currents & Tidal Streams

Flood — water flowing in with the rising tide. Ebb — flowing out with the falling tide. Slack water — the near-stationary pause as the stream turns; the moment to cross a strong-current channel.

Spring vs neap rate — currents run faster at springs (more water in the same time). Rates are given for both. Direction (set) is in °True — plot it straight onto the chart with no correction.

Tidal Stream Atlas — a booklet of hourly chartlets, each showing arrows (set) with figures (rate, springs/neaps) for one hour of the cycle. It gives you a picture of where the water is going, hour by hour. Tidal diamonds (lettered ◇ on the chart) tie to a table of set & rate referenced to HW at a standard port.

Source organisations (name these for the exam): Canadian Hydrographic Service (CHS), US National Ocean Service (NOS) / NOAA, UK Admiralty, Australian Hydrographic Service. Reed's Nautical Almanac publishes East-Coast North America tables.

Adaptation · Canada

Reading Canadian Hydrographic Service Tide Tables

The CHS publishes Canadian Tide and Current Tables (and the free online tides.gc.ca) for tidal Canadian waters — Atlantic, Pacific and the St. Lawrence. A skipper trained in PEC who charters in BC, the Maritimes or abroad must read these fluently. The layout mirrors the IYT method: primary (reference) ports list every HW/LW time and height; secondary ports are found by applying time & height differences to the nearest reference port.

Sample reference-port day (heights in metres above chart datum / LLWLT):

Time	Height (m)	Tide
02:14	0.6	Low
08:39	4.5	High
14:55	0.5	Low
21:08	4.7	High

Read it like this: the range of the morning tide = 4.5 − 0.6 = 3.9 m. Use the rule of twelfths between 08:39 HW and 14:55 LW to get the height at any time. Note these are Standard Time — add an hour for Daylight Time.

Secondary port: e.g. "+0:24, −0.3 m" means HW comes 24 min later and 0.3 m lower than the reference port. Apply the time difference to the time column and the height difference to the height column.

Worked example — secondary port HW

Reference-port HW above = 08:39, 4.5 m. Your secondary port lists differences +0:24 (time) and −0.3 m (height). Then:

	Time	Height
Reference port HW	08:39	4.5 m
Difference	+0:24	−0.3 m
Secondary port HW	09:03	4.2 m

Now feed that HW into the rule of twelfths (Lesson 3) to get the height at any time, or into the clearance equation (Lesson 4) to find the safe window over a shallow patch.

Datum note: CHS uses Lower Low Water Large Tide (LLWLT) as chart datum on most charts — very close in spirit to LAT/MLLW: the level the tide rarely drops below, so charted soundings are conservative.

Scope note for the exam: IYT keeps Module 20 descriptive at Bareboat level — its own notes state that calculating current rates and full tidal/secondary-port computation are covered in the Yachtmaster Coastal & Offshore courses. So for Bareboat you must understand and describe these terms and read a table; the worked maths above is a head-start toward Yachtmaster.

★ Addendum · Lake Ontario

Great Lakes Water Levels — the Non-Tidal Story

Lake Ontario has a true astronomical tide of under 5 cm — negligible. But the lake level is not constant, and a competent skipper must understand what does move it. This is the honest answer to "you have no tides, so how do you teach depth and datum?": you teach the same vertical thinking against a different driver.

1 · Seasonal & long-term levels
The lake rises through spring melt and falls into winter — roughly a 0.3–0.5 m seasonal swing, plus multi-year highs and lows. Datum for Great Lakes charts is IGLD 1985 (International Great Lakes Datum), with Lake Ontario's chart datum at 74.2 m above sea level.

2 · Wind setup & seiche
A strong sustained wind piles water against the downwind shore (setup) and lowers it at the upwind end — easily several tens of cm at the east end near Kingston. When the wind drops, the water sloshes back and forth as a seiche, a slow oscillation that can briefly mimic a tide.

Lake Ontario level vs chart datum

metres above IGLD 1985 — chart datum = Low Water Datum 74.2 m

Current level

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Shaded band = water held above chart datum (extra depth over charted soundings). Seasonal curve is the long-term monthly average (approx.); the dot reads live from the NOAA gauge when available. Live official gauges: CHS · NOAA GLERL · IJC.

The US/Canada joint effort — why your students can trust the numbers

Lake Ontario sits on the international border, so its water level is measured and managed jointly by Canada and the United States:

International Joint Commission (IJC) — the binational body created by the 1909 Boundary Waters Treaty that oversees shared waters.
Outflows are regulated at the Moses-Saunders Dam (Cornwall/Massena) under the IJC's Plan 2014, which sets how much water leaves down the St. Lawrence — directly controlling lake level within natural limits.
A coordinated gauge network: Canada's CHS / DFO and the US NOAA (CO-OPS) share a common vertical datum (IGLD 1985) so a level reading at Kingston and at Oswego mean the same thing. This binational coordination is exactly why a single published "lake level" is reliable on both shores.

Teach it as the tidal analogue: chart datum (IGLD 1985) ↔ LAT; actual depth = charted depth + level above datum; wind setup/seiche ↔ height of tide; the published daily level (NOAA / CHS) ↔ the tide table. Students who master this on Lake Ontario can step straight onto a salt-water chart and apply the identical logic — which is precisely how you satisfy IYT Module 20 in fresh water.

★ Adaptation · Tools

Tides on the Plotter — Navionics & Friends

Students will reach for an app before a paper table — so teach the app and the principle behind it. The point is to read, sanity-check and cross-reference, never to trust a single glowing number.

Navionics (Boating app)
Tap a tide-station diamond or current arrow on the chart to open a 24-hour graph: HW/LW times & heights, the live height now, and current set/rate at stream stations. The "sonar/community" depths still reference chart datum — so the height read-off is your "height of tide" layer on top.

Aqua Map / iNavX
Use the same NOAA/CHS station data; good for cross-checking Navionics and for US/Canada cross-border passages. Aqua Map overlays USACE Great Lakes levels.

Windy / lake-level pages
For Lake Ontario: NOAA CO-OPS and the IJC/USACE level dashboards give the daily lake level vs datum; Windy gives the wind forecast that drives setup & seiche.

Instructor drill: have the student predict HW time & height from the CHS table or rule of twelfths first, then open Navionics and compare. When they match, they understand the tool. When they don't, you've found a teaching moment (wrong day, time zone, secondary-port correction missed).

Knowledge Check

Tides & Currents — Knowledge Check

10 questions at IYT exam standard (75% to pass), including the Lake Ontario adaptation.

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