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Useful Formulas for Boat Owners

These are widely accepted rule-of-thumb formulas but the Severn-boating page accepts no responsibility for their accuracy.

To calculate the maximum displacement speed of your boat.
(Note that your displacement speed is governed by the waterline length which is less than the overall length of your boat).

 Max Speed in knots =1.4 x (the square root of the waterline length of boat) in feet Max Speed in knots = 2.43 x (the square root of the waterline length of boat) in metres Max Speed in km/h = 4.5 x (the square root of the waterline length of boat) in metres

Many owners of light glass fibre cruisers may be surprised when they apply these formulas to their boat. They may well find that their engines are grossly over-powered for river cruising. If you cannot get on the plane, or are prevented by speed restrictions, then trying to go faster than your hull speed just wastes expensive fuel.

Time, Speed and Distance
(In all these equations, speed is in knots, distance is in nautical miles and time is in minutes)

To find how fast you have travelled over a known distance

 Speed = Distance x 60 divided by Time

(Example: If you have travelled 6 miles in 1 hour and 10 mins then your speed is 6.0 x 60 divided by 70 = 5.1 knots)

To find how far you have travelled at a given speed

 Distance = Speed x Time divided by 60

(Example: If you have been going at 5 knots for 50 mins then the distanced travelled = 5.0 x 50 divided by 60 = 4.2 nautical miles.)

To find how long it will take to travel a given distance

 Time = Distance x 60 divided by Speed

(Example: How long will it take to travel 4 miles at 5.5 knots? Time = 4 x 60 divided by 5.5 = 44 mins.)

Approximate conversions between Mph, Knots (nautical mph) and Kmh

 Mph Knots Kph 1.2 1 1.9 2.3 2 3.7 3.5 3 5.6 4.6 4 7.4 5.8 5 9.3 6.9 6 11.1 8.1 7 13.0 9.2 8 14.8 10.4 9 16.7 11.2 10 18.5 17.0 15 27.8 23.0 20 37.0

Example: At 5 knots (nautical miles per hour) your speed is 5.8 mph or 9.3 kph.

Distance of horizon for various heights of eye

 Metres feet Horizon distance 1 3.3 2.1 2 6.6 2.9 3 9.8 3.6 4 13.1 4.1 5 16.4 4.7 6 19.7 5.1 7 23.0 5.5 8 26.2 5.9 9 29.6 6.2 10 32.8 6.6 15 49.2 8.1 20 65.5 9.3

Example:  If on your fly bridge the height of your eye is 5 metres above water level then the horizon is 4.7 nautical miles away.

Conversion Factors

 To convert Multiply by To convert Multiply by knots to miles per hour 1.1515 miles per hour to knots 0.8680 knots to km per hour 1.852 km per hour to knots 0.54 gallons to litres 4.546 litres to gallons 0.22 feet to metres 0.3048 metres to feet 3.2808