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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.| 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 |
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