info 0. Conceptual assumptions – a new boat

Prior to selecting a boat, every prospective buyer should take the following issues into account:

  1. Aesthetic issues and additional equipment should be put aside: come back to them after establishing the following issues:
  2. The most fundamental question you need to ask concerns the purpose of the boat: leisure or commercial one. If leisure, then is it going to serve the purpose of spending your free time in a port (as is often the case)  and sometimes sailing in still weather conditions, or would you like to be able to travel comfortably also in a moderate sea state, i.e.  1 to 3? Or would you perhaps like to feel the spirit of adventure at sea and sail confidently (fast) also in rough waters? All these questions need answering in order to determine specific technical solutions.
  3. Do you care about maximum speed  (at sea state 0 or maybe 3-4),  or are other parameters important, such as high travelling speed and high manoeuvrability? Or would you like to be able to operate in the shallows without running the risk of damaging the boat, with mooring on a beach (type of drive)?
  4. What waters are you going to sail – the Gdańsk Bay (short waves) the Baltic Sea, lakes, or maybe the Mediterranean (longer waves plus higher level of salinity)?
  5. How many people are going to use it (weight, arrangement of deck), how far are you going to sail (fuel – weight, safety measures); do you intend to install additional equipment? Underestimating the weight is the most frequent cause of disappointment with the performance.
  6. Do you intend to dive with full equipment or only for leisure purposes (different access to the boat, different load on the platform, equipment)?
  7. How often do you wish to use the boat, what distances do you intend to cover and at what loads (selection of engines and drives)?
  8. How much can you invest; do you care about low maintenance cost, reliability, long life of the drive system elements?

boat-ship-speed-ocean-hd-wallpaper-217838All the factors mentioned above have an influence on the vessel`s design, selection of drive systems and subsequent satisfaction of the owner. This is why those issues have to be discussed with the shipyard/ distributor in detail. Each client should, prior to the purchase, devote some time to familiarising themselves with the main technical issues related to motor boats in order to be able to make an informed choice of their boat. We suggest reading the descriptive material we collected for you on this site. When we are involved in the selection/ decision-making process, we ask the client to precise their choice at every instance, presenting variants and possible configurations – from selecting the hull through drive systems to the vessel`s equipment.

In the case of corporate and commercial clients, the process is similar, although in the majority of the cases we deal with experienced clients aware of their expectations, which makes the cooperation much easier.


info I. Surface Drive


Surface Drive is dedicated to fast motor boats as the most effective of all the types of drives; however, as every drive, it has its limitations of use. It may not be used for bay watch rescue boats due to the risk of injury (as other drives, except for water jet). It is precisely due to the risk of injury in the case of falling out of the stern of the boat that the SDS drive (just like stern-drive) requires installing the platform to the transom.


The hull of the boat must operate in planing. There are two types of such hull: fixed angle  (monohedron) where the maximum draft of the boat is on the transom, and variable angle  (warped) where the maximum draft of the vessel is on the amidship.


Each of these hulls has its own work specifics related to the shape and hydrodynamic resistances. The hulls may work in a single system (monohull), double system (cat) and triple  (trimaran).

Monohedron hull is dedicated to fast motor boats, especially to operating in rough waters. The maximum speed for this type of hull is approximately  90 kn, above which the hull starts chain walking and it is endangered by losing stability, especially at shifts of the centre of gravity, which in principle is located at between  25 % and 38 % of load waterline length (LWL).

The second type of hull to be used with Surface Drive is “warped hull”. The maximum speed for this type of hull is 40 kn, above which it is prone to losing stability, especially when  LCG (centre of gravity) is over 40 % LWL . A positive feature of this hull is its higher resistance to bouncing up in rough waters in comparison to the previous one. This hull also has a greater tolerance for changes in weight distribution on deck to the specified speed.

The surface drive may work with any type of stationary engine (via reduction gearbox). It is most frequently used with diesel engines, which in principle have lower rotation speeds (lower reduction on gearbox – lower cost).

As it has already been mentioned and described further in detail, there are two types of SD drives: fixed and trimmed, which we offer. You will find a comparison of producers of both drive types as you read on. There is also a description of all the most common types of drives for motor boats.

Surface drive works effectively at the speed range from 20 to over 80 kn, and it generates the lowest hydrodynamic resistances of all the types of drives (no stern). Replacing a drive with SDS means at least  15 % of increased vessel performance and a decrease in fuel consumption. In contrast to water jet, which has considerable power losses on drive and a high probability of failures resulting from dirty material pulled to the dredge box, SD drive does not have these defects. Its massive propeller with a fin easily handles all dirt or sand in the shallows. Each type of drive has its advantages, but SDS is unbeatable when it comes to performance. In addition, it has the easiness of dealing with various loads, e.g. while operating at rough sea.

Each SDS model has the same components. The drive is a mechanical system which transfers kinetic energy to the propeller. The system consists of 6 main elements: a steering cylinder, a trim cylinder, a lower sphere unit, a thrust box unit, a drive shaft and a frame.






[one_half_last]The pictures of a demo version of the SDS2 show how the steering and trimming functions work:

The hydraulic cylinder, working perpendicularly to the drive shaft, is responsible for the horizontal movement  (inclination of 19 degree angle) and turning.

This cylinder is connected with the cylinder responsible for lifting and lowering of the unit (trimming).






The picture shows a complete drive from the engine room side, with lubrication points.



Examples of installations:











Trimming and turning:  SDS3 model (video)





info II. Types of drives and their short description


  1. Traditional propeller drive on shaft – simple construction and high reliability, for speeds below 30kn, and also for big and heavy vessels. The drive is tolerant of high load differences and it works well in difficult conditions. It is cheap to maintain and the cheapest to purchase.
  2. Stern drive  –  Z – type transmissions for built-in engines (Bravo 1, Brawo 2, Brawo 3, Volvo): a complex construction; high failure frequency and prone to damage due to inappropriate use and maintenance (seals, oil). This drive may not be used to operate in the shallows. It may be used for vessels with little load, below 8m, including single configuration. Z-type drive works well in vessels operating at lower speeds, up to 25 kn. High manoeuvrability in double configuration.  KONRAD type drive (the most reliable on the market) is dedicated to commercial (loaded) use.  As far as purchase price is concerned, the cost of transmission is comparable to SDS, more expensive than the drive integrated with outboard engines and cheaper than water jet.  The drive is expensive in operation and it requires specialised knowledge to operate.
  3. Outboard – the most popular drive integrated with outboard engine suspended on transom; compact construction facilitates deck arrangement; simple installation. It works well for vessels up to 12m and speed up to 40kn. It may operate in the shallows, but it is sensitive to damage. It is used only with petrol engines and is sensitive to damage due to wrong use (flooding, too strong shocks). It is quite cheap to purchase due to its integration with the engine; however, its life is short, which may make it expensive to use (the cost may be higher than the purchase price) if it is frequently operated (over 100h a year).
  4. Surface Drive – this drive works best for vessels over 8m, speed over 30kn and double configuration. It has high manoeuvrability and may operate in the shallows (beaching) and it dirty water. The construction is simple and reliable, to be used in fast motor boats (including leisure ones) and also in patrol, racing, special purpose vessels, RIB. It may be used with diesel and petrol engines; it requires a separate gearbox for transmission reduction and operating the gears: forwards, neutral, reverse. The key issue for the right use of the drive is its appropriate configuration – suited for the specific vessel. The purchase price is comparable to stern drive, or it is lower (Konrad), and the operation cost is low.
  5. Water Jet drive.  It is dedicated to vessels where using a traditional propeller would be dangerous  – Rescue boats, dive boats. It may operate in the shallows, but without the beaching option – drawing sand into the dredge box means an expensive failure. The drive is sensitive to water pollution; its structure is complicated and prone to failures. Repair and servicing require qualified personnel. Manoeuvrability is poor at low speeds; it may be used up to the speed of 45kn.  It is necessary to over dimension the power of drive units due to considerable power drops on drive  (20-30%). The purchase price is comparable to SD for small propellers, and over the double cost for the bigger ones.



info III. Comparison of producers of surface drive systems




Trimability – it is the main feature of surface drive, making it possible to fully use its potential. It is this feature that enables easy operation of the reverse gear, obtaining maximum speeds in different sea conditions and sailing in the shallows. Thanks to trimability we can eliminate or significantly reduce bouncing up the hull by waves. Surface drive on fixed shaft is perfect for obtaining maximum speeds in racing conditions on still waters, but it does not work well in changeable conditions mentioned above, or for military purposes.

Steerability  – possibility of turning is achieved like in Z-type transmissions or overboard engines, via a turn of the shaft with propeller. The difference is, however, that due to shifting the turn axis towards the stern (moving the propellers away from the transom), manoeuvrability is much higher. The fact that there is no traditional stern, which has to be used in the fixed shaft, eliminates resistance and turbulence in water flow caused by the stern. Some producers introduce additional steering wheels for directing the water flow towards the propeller, but our experiments in the tunnel have shown that these make the propeller efficiency worse (they introduce turbulence in laminar water flow, which obviously results in efficiency loss).

Emergency steering  – FH is the only producer that gives the third hydraulic pump as standard, which secures steerability in case of failure of the other pumps

Pre –wired installation – the client receives full cable bundles with connectors in the package, which facilitates assembly.

Propeller manufacturer – the propeller is the key element for the effectiveness of the entire system. FH is the only provider that has its own production and experiment facilities. Manufacturing of propellers is a complex process, starting from the design, through its moulding, treatment and subsequent tests in a special cavitation tunnel.  Even if we use an identically looking propeller in the specific gear unit, this may lead to a dramatic fall in efficiency and frequently result in damage of the propeller and components.

Shock absorbers – the fact that vibrations carried by the drive are muffled significantly increases the comfort of use.

Electronic steering – the system communicates in all accepted standards, which makes it possible to use various electronic and hydraulic solutions. In addition we provide an automatic trim control system on request.

Tunnel tested –  All FH drives and propellers are tested in a special cavitation tunnel, which cost over 1 million Euro to build. This enabled us to obtain full certification, necessary for commercial/ military uses.

Major class approval – as mentioned above, we are the only provider on the market that may be classified thanks to testing. We have ABS, BV and Lloyds classes.

Naval architecture – every design of a naval vessel requires holistic view on performance determined by the hull, the weight, its distribution, engines, gear units and drive with propeller. This requires advanced engineering expertise and considerable experience, which is why each design is assessed by a team of designers.

Warranty of performance  – thanks to the holistic approach to drive systems, we are the only producer that offers the clients not only the standard production warranty, but also the warranty of performance.




info IV. Information that is necessary to select an appropriate drive


In order to select an appropriate drive, it is necessary to adopt a holistic approach, taking into account the operating environment, as well as hull, engine and gear unit data.

tabele tech roboczy

  1. Type of hull – monohull (single), catboat or trimaran; each of these hulls has a different influence on hydrodynamic resistance and stability.
  2. Displacement , weight – a very important coefficient determining the power of drive units. It is necessary to precisely specify the basic and maximum weight.  Underestimating the weight is the most frequent cause of not obtaining the assumed performance of the boat due to a simple relation: the higher the weight, the higher the resistance and hence the lower the speed.
  3. LOA, LWL, BEAM – length overall, waterline length, transom beam. These dimensions have an influence on resistance and speed. The longer the hull, the faster and more stable it is at rough sea.
  4. Draft – another important element influencing total resistance of the boat and the possibility of using SDS (propellers must work in free water flow)
  5. Deadrise angle – the angle specifying the shape of the bottom of the boat. The wider it is (as V-type hull), the higher the resistance and the later the moment of planing.
  6. LCG – centre of gravity has an influence on the behaviour of the hull – its inappropriate location is responsible for chain walking of the boat or riding a high bow.
  7. Transom angle – to check the right arrangement of SDS
  8. Spray rails – they direct the water flow along the hull, reducing resistance
  9. Transom thickness – determines SDS solutions
  10. Steps or no steps – a correctly designed stepped hull reduces the “wet” part of the hull in planing, which reduces resistance and increases speed
  11. Operating environment – performance is also affected by such factors as water salinity and pollution (the higher it is, the higher the resistance). Also important is the sea status at which the boat is to operate: only at 0 or 1 (flat) or 2, 3. Another factor is temperature range of the air or water (how much power is lost to cool the engines).
  12. Selection of engines and gearboxes – we require specifying the number of engines and their parameters.  It seems that the higher the power, the better the performance. It is often forgotten, however, that using engines with a higher power entails a significant increase in the weight of the engines. For example,  Volvo D3 220 kM weighs 300 kg, and D4 260 kM (300 kM) as much as 550 kg: in a double installation, this is already  500 kg more.  There are exceptions to this rule in our offer –  V8 diesel engines – VGT 350 kM to 500 kM, which weigh 500 kg. Apart from engine power, the crucial factor is the torque and its availability at consecutive engine rotation speeds. This is why we ask the client every time about the purpose of the boat.

pdfClick here to see an example of selected drive and performance for a heavy duty RIB type vessel, using a set of two diesel engines VGT 500 hp


info V. Selection of gearbox units


The right choice of engine revolutions transmission is another key element from the point of view of performance and drive effectiveness. The following elements have an influence on transmission:
– engine revolutions (rpm)
– power to weight in tonnes ratio (hp/t)
– speed of advance or  Froude coefficient (V/LWL0,5)
V- speed in kn
LWL – waterline length (in feet)
P – total power of drive units
W – total weight in tonnes
RPM – engine revolution speed
Assuming that  V/LWL0,5 coefficient is higher than 1 and the total power is higher than 100, the following formula may be applied:

R= 1,45 x exp((0,0034xT))

Where T=RPM/P/W
There are several significant gearbox providers  – the most renowned one is the German ZF, and Kanzaki, a slightly cheaper manufacturer cooperating with Janmar.



info VI. Detailed comparison of SDS and Water Jet


propellersThe most frequent question asked by potential clients is why SDS should be used instead of Water Jet. We always give the same answer to this question: it depends on the purpose of using the boat.

In certain areas, SDS has considerable performance advantage, but there are also areas where this advantage belongs to Water Jet. For vessels operating mainly in speed ranges above 30 kn, it is undoubtedly SDS that makes a tangible difference as far as performance is concerned. For vessels operating at the range 15 – 30kn, water jet is a better alternative due to effectiveness.

The table below shows comparative values of operating features for both systems: the red colour indicates undesirable features, and the green colour desirable ones.