Weather and Sea State Planning

Prerequisites

This lesson is part of the RYA Advanced Powerboat course. You should hold the RYA Intermediate Powerboat certificate or be at that standard: you can obtain and read a forecast, plan a short daylight passage, work out tidal heights and streams from an almanac or tidal atlas, and handle a planing boat in moderate conditions. You should know the Beaufort scale in outline and understand the terms "windward", "lee shore" and "wind against tide".

On the Advanced course you will plan and skipper passages in more demanding conditions, possibly at night. A fast boat can outrun some weather, but it is far more sensitive to sea state than a displacement yacht. A sea that a 12-metre yacht shrugs off can force a 7-metre RIB down to 8 knots, burn twice the fuel and injure the crew. Good planning is mostly about predicting the sea, not just the wind.

Learning Objectives

By the end of this lesson you should be able to:

• Obtain and interpret the forecasts available to a UK small craft skipper.
• Explain the difference between the Beaufort wind force and the sea state, and why a fast boat cares about wave height, period and steepness.
• Explain the effect of wind over tide, shallow water and headlands on the sea.
• Identify likely tidal races and overfalls, and plan around them.
• Make and record a go or no-go decision using clear limits.
• Choose ports of refuge suitable for the wind direction and state of tide.
• Plan fuel, including consumption in a head sea and a reserve using the rule of thirds.

Where to Get the Forecast

A good skipper uses several sources and looks for the general picture first, then the detail.

Forecast terms

The Met Office uses precise words. Learn them so you do not misread the timing:

Visibility terms: very poor (less than 1,000 metres, which is fog), poor (1,000 metres to 2 nm), moderate (2 to 5 nm), good (more than 5 nm).

Wind speed in a forecast is the mean wind at 10 metres height. Gusts are often 30 to 50 per cent stronger, and much more in showers or near cliffs. A forecast of "4 or 5, occasionally 6" means you should plan for 6, with stronger gusts.

Building the picture

1. Look at the pressure charts for the last three days and the next three. Is a front coming? Is the pressure falling quickly?
2. Read the inshore waters forecast and shipping forecast for your area and those upwind.
3. Check the tides: times of high water, springs or neaps, and tidal stream direction and rate along your route.
4. Check live observations on the morning of departure: wave buoys, coastal weather stations, webcams.
5. Write it down: wind direction and force through the day, sea state, visibility, any warnings.

Beaufort and Sea State

The Beaufort scale describes wind, not waves. The waves the wind produces depend also on fetch (the distance of open water the wind has blown over), duration (how long it has blown), the depth of water, and the tide.

The wave heights in the table are typical for the open sea after the wind has blown for a long time over a long fetch. Close to a weather shore (the wind blowing off the land) the waves will be much smaller; on an exposed coast with an onshore wind they can approach these values.

The shipping forecast and inshore waters forecast also give sea state using the WMO terms:

Why a fast boat cares about steepness

A planing boat responds to the shape of the waves, not only their height. Two seas with the same height can feel completely different:

• Long swell with a period of 10 seconds or more: wavelength well over 100 metres. A fast boat rides up and down it comfortably.
• Short, steep wind waves with a period of 3 to 4 seconds: wavelength 15 to 25 metres, close to the length of the boat. The boat launches off one crest and lands on the next. This is where slamming injuries happen.

A useful number: in deep water, wavelength in metres is roughly 1.56 times the period in seconds squared. A 4-second wave is about 25 metres long; a 10-second swell about 156 metres.

The RYA Advanced Powerboat Handbook explains how to handle these seas: trim, throttle and course to keep the boat running smoothly. Planning is about avoiding them in the first place, or choosing a route and time when they are least steep.

Wind Over Tide

When the wind blows against the tidal stream, waves become shorter and steeper, and in strong streams they break. When wind and tide go the same way, the sea becomes longer and flatter.

The reason is that the waves move over the water, and the water itself is moving. A stream running against the waves slows them and compresses them: the same energy in a shorter wave makes it higher and steeper. A stream running with the waves stretches them out.

The diagram below compares the same wind blowing with and against a 2-knot tidal stream.

Click to zoom

Practical effects

• A Force 4 against a 2-knot spring stream can make a "moderate" sea locally, with short steep waves, where a Force 4 with the stream gives a pleasant "slight" sea.
• The effect is strongest where the stream is fastest: in narrows, off headlands, in estuary mouths and over banks.
• The change happens at the turn of the tide. A calm morning passage can become uncomfortable within half an hour of the stream turning against the wind.

Planning around it

1. Look at the wind forecast direction and the tidal stream direction along your route, hour by hour.
2. Find the periods when wind and tide oppose, and the places where the stream is strongest.
3. Plan to pass those places with wind and tide together, or at slack water, or at neaps rather than springs.
4. If you cannot, plan for slower progress and more fuel through those sections.

Headlands, Races and Overfalls

Wind around headlands

Wind is squeezed and accelerated as it flows round a headland, often by a Beaufort force or more, and it bends to follow the coast. Downwind of high cliffs it can be gusty and change direction suddenly. On the far side of the headland you may meet a different wind and sea from the one you left.

Tidal races and overfalls

The tidal stream also accelerates round a headland. Where a fast stream runs over an uneven seabed, a ledge or a shallow bank, the water surface is disturbed into overfalls: steep, breaking, confused waves even in light winds. A tidal race is an area of fast stream, often with overfalls, off a headland. Charts mark these areas with a wavy symbol, and pilot books and almanacs describe them. Well known examples on the south coast of England include Portland Bill, St Alban's Head and the Needles approaches.

The plan view below shows a headland with its race, the wind acceleration and the two common routes past it.

Click to zoom

Options for passing a headland

A fast boat can usually time its arrival precisely, which is one of its great advantages. Plan the passage backwards from the time you want to round the headland.

Shallow Water and Bars

When waves move into shallow water, they slow down, shorten and steepen. Over a shallow bank or a harbour bar, a swell that is harmless offshore can break. The worst combination is an onshore wind or swell meeting an ebb stream running out of the harbour mouth. Many UK harbours have such bars, and the pilot book will say when the entrance is dangerous, for example "do not attempt in strong onshore winds, especially on the ebb". Plan to enter on the flood, and have an alternative.

The Go or No-Go Decision

Set limits before you look at the forecast

The decision is easier and more honest if you decide your limits in advance, when you are not keen to go. Base them on:

• The boat: length, hull type, freeboard, engines (single or twin), range, shelter.
• The crew: experience, fitness, how they cope with motion, whether there are passengers who expect comfort.
• The passage: distance, exposure, number of hazards (headlands, bars), availability of ports of refuge, daylight.

Example limits for a 7-metre RIB with an experienced skipper and a novice crew on a 40 nm coastal passage:

The decision itself

Use the whole passage window, from departure to arrival plus a margin, not just the conditions at departure. Ask:

1. What is the worst condition I expect, and where?
2. Can the boat and crew handle that, with a margin?
3. What if it arrives early, or is one force stronger than forecast?
4. Where can I go if it does?
5. Have I enough fuel for a slower passage?

If the honest answer to any of these is "not sure", the answer is usually "not today", or a shorter passage. Tell the crew why: they will learn from it.

On passage: keep re-deciding

Set decision points, for example at each headland or each port of refuge, where you compare actual conditions with your plan. At each, decide to continue, turn back or divert. Write the decision in the log. The commonest serious mistake is pressing on because you are halfway there.

Ports of Refuge

A port of refuge is somewhere you can get into safely if conditions deteriorate, a crew member is injured or the boat has a problem. A good plan lists one for each section of the passage, with:

• Access at all states of tide, or the times when it is accessible, and any bar.
• Shelter from the expected wind direction. A harbour that is perfect in a westerly may be dangerous to enter in a strong southerly.
• Entrance dangers in onshore wind. A harbour with an exposed entrance becomes unusable just when you need it.
• Night entry. Is it lit, and do you have a night pilotage plan for it?
• Fuel. Is there a fuel berth, and what are its hours?
• Communications. Harbour office VHF channel and phone number.

Write the details on your passage plan. Under pressure there will be no time to look them up.

Fuel Planning

Consumption

Powerboat engines use fuel per hour, and the fuel per nautical mile depends on speed:

• At displacement speed, the boat uses little fuel per hour and per mile.
• Coming onto the plane, the hump, uses a lot of fuel for little speed; avoid running there.
• At efficient planing speed, typically just above the speed where the boat planes cleanly, fuel per mile is lowest of the planing range.
• Flat out, fuel per mile rises steeply.

Know your boat's fuel curve from the manufacturer or from your own measurements with a fuel flow meter. Note speed, rpm and litres per hour, and work out litres per nautical mile.

Allow for conditions

In a head sea the boat cannot hold planing speed, may need more trim and throttle, and slams. Fuel per mile can increase by 30 to 50 per cent or more. A heavy load (passengers, kit, full water tank) and a fouled hull also increase consumption. Strong tidal streams change your speed over ground but not your speed through the water, so allow for them in time and therefore fuel.

The rule of thirds

A simple, conservative rule for fuel:

• One third to get there.
• One third to get back.
• One third reserve.

So the usable fuel must be three times what you need for one leg. For a one-way passage, the reserve is still essential: plan to arrive with at least a third of your usable fuel, more if the route has no fuel stops or the weather may worsen. Usable fuel is less than tank capacity, because the last part may not reach the pickup in a rough sea; assume about 10 per cent is unusable unless you know better.

The diagram below shows a worked example with real figures.

Click to zoom

Worked fuel example

A 7.5-metre RIB with a 200 hp outboard and a 230-litre tank. At 25 knots in calm water the engine uses 45 litres per hour.

1. Fuel per mile in calm water: 45 / 25 = 1.8 litres per nm.
2. Allowance for sea state and load: add 30 per cent. 1.8 x 1.3 = about 2.3 litres per nm.
3. Usable fuel: 230 litres less 10 per cent unusable = 207 litres.
4. Rule of thirds: one leg may use 207 / 3 = 69 litres.
5. Maximum leg distance: 69 / 2.3 = 30 nm.

So a 25 nm trip out and back is within the rule. A 40 nm trip out and back is not, unless you refuel at the destination.

Check your fuel against the plan at each waypoint. If consumption is higher than planned, decide early whether to slow to a more economical speed, divert to refuel or turn back.

Worked Example

You plan a 32 nm daylight passage westwards along the coast in a 7-metre RIB with twin 115 hp outboards (usable fuel 180 litres, about 1.6 litres per nm in calm water). The route passes a headland with a tidal race 14 nm from departure. The forecast at 0600: "Wind east or north-east 3 or 4, increasing 5 later. Sea state slight, becoming moderate later. Good." Tidal stream at the headland runs west from HW minus 1 to HW plus 5, east the rest of the time, up to 3 knots at springs. Today is springs, with HW at 1100.

Wind and tide. The wind is from the east, so a west-going stream runs with the wind: a smoother sea. An east-going stream would be against the wind and would steepen the waves, particularly in the race.

Timing. The west-going stream at the headland starts at 1000. You plan to round it between 1030 and 1130, when the stream is with the wind but before it reaches full strength. At 20 knots, 14 nm takes 42 minutes, so leave at about 0945.

Increasing later. "Later" means more than 12 hours from the time of issue, so it should not affect a morning passage. But the trend is increasing, so plan to arrive before midday and avoid a return trip today.

Fuel. 32 nm x 1.6 litres per nm x 1.3 allowance = about 67 litres. Usable fuel 180 litres. A third is 60 litres, so the passage needs slightly more than a third. That is acceptable for a one-way trip because you will arrive with over 110 litres, well above the reserve, and there is fuel at the destination. You would not plan the return on the same tank without refuelling.

Ports of refuge. You list two: a harbour 6 nm before the headland, accessible at all states of tide, and one 8 nm beyond it with a bar to avoid on the ebb in an onshore wind. With an easterly wind the second is sheltered.

Decision. Go, with a decision point before the headland: if the sea is moderate already, wait in the first harbour.

Common Mistakes

• Reading only the wind force. Sea state, fetch, period and the tide matter more to a fast boat.
• Forgetting wind over tide. A pleasant forecast can still give an unpleasant sea where the stream is fast.
• Taking an inshore passage without exact pilot book guidance. Inshore routes past races are for settled weather, at the right time, with local knowledge.
• Setting limits on the quay. Decide them in advance, when you are not keen to go.
• Ports of refuge that face the weather. Check each one is usable in the wind you expect.
• Fuel planned for calm water. Add an allowance for sea state and load, and keep a reserve of at least a third.
• Pressing on. Use decision points, and turn back or divert before the situation forces you.

Summary

• Use several forecasts, starting with the big picture from pressure charts, then the inshore waters forecast and live observations.
• Know the forecast terms: imminent, soon, later; and plan for the top of the forecast range, plus gusts.
• Beaufort describes wind; sea state depends also on fetch, duration, depth and tide. A fast boat is most affected by short, steep seas.
• Wind against tide makes waves shorter and steeper; plan headlands, narrows and bars for wind with tide, slack water or neaps.
• Headlands accelerate wind and stream and have races and overfalls. Go well offshore, or use an inshore passage only as the pilot book directs.
• Make go or no-go decisions against limits set in advance, for the whole passage window, and keep re-deciding on passage.
• List ports of refuge, checking access, shelter in the expected wind and night entry.
• Plan fuel from your boat's real consumption, add an allowance for conditions, and keep a third in reserve.

Check Your Understanding

1. A forecast issued at 0500 says "gale warning, south-west 8 soon". When is the gale expected?

Answer: "Soon" means within 6 to 12 hours of the time of issue, so between about 1100 and 1700.

1. Why does a fast boat care about wave period as well as height?

Answer: Short-period waves are short and steep, with a wavelength close to the boat's length, so the boat launches and slams. A long-period swell of the same height is much longer and the boat rides over it comfortably.

1. What happens to waves when the wind blows against the tidal stream, and why?

Answer: They become shorter, steeper and may break. The opposing stream slows the waves and compresses them, so the same energy is packed into a shorter, higher wave.

1. Give three ways to plan around a tidal race off a headland.

Answer: Take a wide offshore route as advised by the pilot book; time the passage for slack water or when the stream runs with the wind; go at neaps rather than springs; use an inshore passage only if the pilot book recommends it, in settled weather; or wait in a sheltered place.

1. What makes a harbour a poor port of refuge on a given day?

Answer: An entrance exposed to the wind and swell, especially a bar with an onshore wind on the ebb; restricted access at certain states of tide; no lights or night plan if you might arrive in the dark; no shelter from the expected wind direction.

1. A boat uses 30 litres per hour at 20 knots in calm water. Usable fuel is 150 litres. Using a 30 per cent allowance and the rule of thirds, what is the maximum distance for one leg?

Answer: 30 / 20 = 1.5 litres per nm; with 30 per cent, 1.95 litres per nm. One third of 150 litres is 50 litres. 50 / 1.95 = about 25 nm.

1. Why should you set go or no-go limits before looking at the day's forecast?

Answer: So that the decision is objective. Once you want to go, it is easy to talk yourself into marginal conditions. Limits set in advance take account of the boat, the crew and the passage without that pressure.

1. What is the "hump" and why should you avoid running there?

Answer: It is the speed range where a planing boat is climbing onto the plane, with the bow high and large resistance. Fuel consumption is high for little speed, visibility over the bow is poor and the boat handles badly. Run either at displacement speed or properly on the plane.

Further Reading

• RYA Advanced Powerboat Handbook G108
• RYA Powerboat Handbook G13
• RYA Weather Handbook G1 (Chris Tibbs) and RYA VHF Handbook G31
• RYA Navigation Handbook G6 (Sara Hopkinson)
• R.M. Frampton and P.A. Uttridge, Meteorology for Seafarers
• Dag Pike, Fast Boat Navigation (Adlard Coles), with Reeds Nautical Almanac for pilotage data