Seafury Marine Surface Propulsion Systems
Seafury Marine Surface Propulsion Systems

Design Considerations

Seafury Surface Drives are used on a wide variety of vessel types. As with any propulsion system, the better results will be achieved on the more efficient hull designs.
Seafury respects the experience and knowledge of Naval Architects, Marine Designers, Boat and Ship Builders and is therefore happy to consult with them on design considerations, building and installation. We recommend you consult a qualified naval architect and/or marine engineer before finalising your hull design and propulsion system. The following discussions are offered to assist the designer and builder achieve optimum performance and reliability from their Seafury Surface Drives.
  • · The 45° Transom
  • . New Build and Retrofit
  • · Vacuum Break
  • . Clean Water Flow to the Propeller
  • · Boarding Platforms
  • · Propeller Performance and Trim
  • · Vessel Trim
  • · Engine and Gearbox
  • · Multiple Installations
  • · Driveline Arrangements

  • The 45° Transom

    One of Seafury Surface Drive's special features is the 45° transom. Other surface drives often experience poor reversing performance due to the reversing thrust being deflected back off the relatively square transom, giving the effect of forward thrust.

    Seafury's 45° transom, combined with the unique winged drive unit design, has the effect of directing the water flow down and under the hull thus giving excellent reverse thrust and control.

    The 45° angle can be achieved by building the transom at 45° on a new boat or by adding a 45° mock transom or pods to an existing transom.


    If a Seafury Surface Drive is to be fitted to a vessel that does not already have a 45° transom, then general boat building practices may be employed to attach a 45° mock transom. This transom is often incorporated into a swim platform at the stern of the vessel.
    A recent retrofit on a Malaysian vessel replacing water jets with surface drives and pods achieved an increase of 5 knots in speed, whilst replacement engines saw the horse power reduced by 200HP.

    Clean Water Flow To Propeller

    Vacuum Break

    As shown in the diagram, a vacuum break should be incorporated at the base of the transom and cover the full width of the transom. This is an important feature as it prevents the natural tendency for water to flow up the angled transom. The vacuum break provides clean, hard water, ensuring the propeller is cutting the water surface on the correct plane for optimum drive performance. This is equally important for multi-hulls.

    Through-Hull Fittings and Interference

    The less disturbed the flow of water to the propeller, the better the surface piercing propeller will perform. It is important to ensure that there are no through-hull fittings such as engine cooling intakes, transducers, keels, skegs or planing stakes directly ahead of the propeller. There must be a minimum of 8 feet (2.5 meters) of smooth hull ahead of each propeller. Trim tabs and transom mounted transducers or water pickups must not be installed directly ahead of the propeller. If required, these should be mounted to the sides, out of the water flow to the propeller.

    Boarding Platforms

    300mm clearance between the drive unit and a boarding platform is highly recommended to enable adequate airflow to the propellers. Modern boat design often includes a boarding or swim platform on the transom and can be adapted to a number of uses.

    It is very important when designing for surface drives to ensure the platform does not restrict the supply of air to the surface propellers at take off, when turning, or at normal operating speeds. Surface piercing propellers rely on significant ventilation to operate correctly and require a considerably large volume of air.

    When incorporating Storage Bins and Side Curtains on the drive unit, ensure adequate ventilation is achieved.

    Propeller Performance & Trim

    Trimming the Seafury propeller is not required. Other surface drives have added drive complexity to adjust their vertical trim in order to help match the propeller design to vessel design, Horse Power and weight. Seafury custom-design and build each propeller to suit the vessel weight and engine Horse Power.

    The need to trim the drive system to compensate for poor propeller design and placement is not required with the Seafury system.

    Vessel Trim

    Seafury recommends the installation of trim tabs or interceptors' to alter the vessels operating trim angle if required.

    Engine and Gearbox


    Medium speed, high torque engines (most marine engines) best suit Seafury Surface Drives. Seafury's surface piercing propeller has been designed to take advantage of this high torque to achieve efficient low speed operation, load carrying and maximum operating/cruise speeds.

    When high-speed engines are used, for example higher than 3500 rpm at full throttle, deeper gearbox reduction ratios are required to produce the required torque at the propeller.

    Gasoline engines, piston and rotary, may also be used with Seafury Surface Drives. These engines offer the advantage of higher power to weight ratio and typically take up less space in the vessel. Gasoline engines are higher revving and require deeper speed reduction ratios.

    Engine Exhaust

    All Seafury Surface Drives with the exception of the SF18 model, have an integral exhaust outlet to which the engine exhaust system can be connected. This enables the exhaust gases to be vented directly ahead of the propeller. The exhaust gases assist in ventilating the propeller.

    Marine Gearbox

    Seafury will recommend the most suitable gearbox ratio to optimize vessel performance. Selection of the gearbox remains the responsibility of the owner/designer/builder.

    If a gearbox and ratio has already been acquired, performance may be compromised if the reduction ratio is not optimum for the propeller type, engine HP, and hull configuration. If possible, Seafury will attempt to match your propeller to suit, however, we are happy to discuss your options should this be the case.

    The Seafury shaft angle ranges from six (6) degrees to eight (8) degrees subject to model and needs to be considered when establishing an appropriate drive line arrangement.

    In twin drive applications, specify one gearbox for clockwise propeller rotation and the other for counter-clockwise rotation.

    Multiple Installations

    Multiple installations are possible with the Seafury Surface Drives. Please see the pictures below for more information on what is achievable.

    Driveline Arrangements

    The Seafury Surface Drive units suit a wide variety of driveline configurations.
    Typical driveline and gearbox arrangements include:
    1. Conventional Driveline with horizontally offset gearbox
    2. Conventional Driveline with 7° down angle inline gearbox
    3. Conventional Driveline at 6°or 8° with inline gearbox
    4. Conventional Driveline mid-mount island gearbox
    5. Extended shaft with mid-mount bearing
    6. Vee drive gearbox arrangement
    7. Thrust Bearing with Cardin Shaft

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