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Useful Formulas for Boat Owners
These are widely accepted ruleofthumb formulas but the Severnboating page accepts no responsibility for their accuracy.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 overpowered 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 
Index  River Severn  Sharpness Canal  Severn Estuary  Bristol Channel  Tides  Skipper's Reference  Ports of Call  Canal Boats  Severn Links 
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