Engines for Vessels with Displacement Hulls
The basis of the following guide is that the maximum displacement hull speed is to be achieved by a clean and polished hull in calm sea with no windage.
Racing yachts may wish to opt for more limited horse power as the engine is primarily only used to reach or leaving moorings, motor sailors or yachts used in heavier seas and or tides may opt for up to 30% more horse power to ensure an adequate safety factor is available.
Engine power, rpm, gearbox reduction ratio, propeller diameter and pitch are all variables that will affect a displacement hulls performance and as a consequence is a specialist subject that for new builds, the vessels designer should be consulted.
For vessels which require repowering, there are existing guidelines and or parameters that are already in place to help in making the correct replacement engine choice. The existing engine, gearbox reduction ratio and propeller performance compared to theoretical maximum hull speed for the vessel will be a good indicator on whether it can be improved or not.
Additional there may be some set parameters such as the distance between the centre line of the vessels propeller shaft and the hull that may or may not allow a larger more optimised propeller to be used. If the propeller`s diameter is restricted, this can in part be compensated for by changing the propeller pitch, but there are limitations. For more detailed information on propellers visit Propeller Options.
- A general engine guideline for yachts with a displacement hull is the weight in tonne multiplied by 4 hp.
- Theoretical maximum displacement hull speed in Knots = 1.3 (or 1.4 for a sleek hull) x √ Water Line length (in feet).
|Engine||BHP||Yacht Displacement Weight in Tonnes||LWL in Feet (Water Line Length)|
|Beta 10||10 bhp @ 3,000 rpm||1 - 2||22 - 24|
|Beta 14||13.5 bhp @ 3,600 rpm||2 - 3.5||24 - 26|
|Beta 16||16 bhp @ 3,600 rpm||3 - 4||25 - 28|
|Beta 20||20 bhp @ 3,600 rpm||4 - 5||26 - 32|
|Beta 25||25 bhp @ 3,600 rpm||5 - 6||27 - 34|
|Beta 30||30 bhp @ 3,600 rpm||6 - 7||32 - 36|
|Beta 35||35 bhp @ 2,800 rpm||8 - 9||36 - 38|
|Beta 38||38 bhp @ 3,600 rpm||9 - 10||37 - 39|
|Beta 45||45 bhp @ 3,000 rpm||9 - 11||38 - 40|
|Beta 43||43 bhp @ 2,800 rpm||10 - 12||38 - 42|
|Beta 50||50 bhp @ 2,800 rpm||12 - 15||42 - 45|
|Beta 60||56 bhp @ 2,700 rpm||15 - 17||43 - 46|
|Beta 70||70 bhp @ 2,800 rpm||17 - 22||44 - 47|
|Beta 75||75 bhp @ 2,600 rpm||17 - 22||46 - 50|
|Beta 85||83 bhp @ 2,800 rpm||21 - 23||48 - 51|
|Beta 90||90 bhp @ 2,600 rpm||22 - 24||50 - 52|
|Beta 105||99 bhp @ 2,600 rpm||24 - 26||51 - 53|
|Beta 115||115 bhp @ 2,600 rpm||27 - 33||52 - 55|
Beta Marine Limited will not accept any responsibility or liability for an incorrect engine selection.