Engine Selection for Boats with Displacement Hulls

Engine Selection for Boats with Displacement Hulls

The basis of the following guide is that the maximum theoretical displacement hull speed is to be achieved by a clean and polished hull in calm sea with no windage.

Racing yachts for example may wish to opt for smaller, lighter engines with more limited horse power as the engine is primarily only used to reach or leave moorings and is not required to achieve maximum theoretical hull speed. Propellers will be selected that deliver the least amount of drag when under sail rather than optimum efficiency when under power.

A motor sailor or yacht used in heavier seas and or stronger tides may often opt for an engine with up to 30% more power to ensure an adequate safety factor is available and easily be capable of achieving maximum theoretical hull speed in harsher conditions. Propellers will be selected for high efficiency when under power and drag when under sail is not such a primary consideration.

Engine power, speed (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 pre-set parameters such as the distance between the center line of the vessels propeller shaft and the hull that may or may not allow a larger more efficient 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 Guidance.

Displacement Hulls

Beta Marine specializes in propulsion engines for use in; Seagoing Yachts, Motorsailors, Fishing & Work Boats, Inland Waterway Dutch Barges and Narrowboats.

A displacement hull will displace its own ‘Total Laden Weight’* in the water, when it moves it has to displace and push an equal amount of water out of the way. As the vessel increases speed it has to push through more water in less time which increases the size of the wave building at the bow

*The ‘Total Laden Weight’ will include all fuel, provisions and crew / passengers.

Additionally as you approach the maximum design speed of the hull, the wave will grow so the crest remains at the bow and the stern wave comes up under the counter. At this point the vessel effectively digs a large whole in the water which traps the hull at the bow by a very large wave it can neither push through nor plan over.

 

Heavy seas and or tides can also impact a displacement hulls ability to reach maximum speed.

The following example highlights the speed to power ratio required for a 25 foot yacht.

 

Approximate Boat Speed for a 25 Foot Yacht Approximate HP Required
4 Knots 4.2
5 Knots 4.4
6 Knots 4.8
6.5 Knots (Maximum Hull Speed) 10.0

 

If absolute maximum hull speed is not a priority the above also highlights a significantly reduced power requirement and corresponding fuel saving is available for cruising at 6 knots.

 

Engine Power (HP) Yacht Displacement Weight in Tons LWL in Feet (Water Line Length)
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 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 70T 70 bhp @ 2,800 rpm 17 – 22 44 – 47
Beta 75 75 bhp @ 2,600 rpm 17 – 22 46 – 50
Beta 85T 83 bhp @ 2,800 rpm 21 – 23 48 – 51
Beta 105T 98 bhp @ 2,600 rpm 24 – 26 51 – 53

 

Beta Marine will not accept any responsibility or liability for an incorrect engine selection. Please contact us to discuss your specific boat in detail.