Most thruster manufacturers have phased out their hydraulic motor thrusters and have started producing gearbox thrusters. Shipwrights Inc. has been manufacturing gearbox thrusters since 1980 and has never produced a hydraulic motor thruster. There are several problems with simply putting a hydraulic motor inside a tunnel in order to create a thruster:

LEAKS CAUSED BY SHAFT SEAL WEAR.
In order to prevent water from entering the thruster system and to prevent fluids in the thruster system from being lost into the seawater, all thrusters need to have some kind of shaft seal. And... all shaft seals wear out sooner or later. Friction is a factor that can't be ignored. If the shaft journal where the seal makes contact is made of a material that is not hardened, even low pressure, relatively soft, shaft seals will gradually wear a groove into the shaft until water enters through it. Higher pressure shaft seals will wear a groove into the shaft even more quickly, unless the shaft is hardened. Even if the shaft is hardened to a high level, all shaft seal materials will, themselves, gradually wear to the point at which water will leak through. Finally, the material used for the shaft must not only be hardenable to a high level, it must also be very corrosion resistant, because any pitting or crevices will produce leaks.

WHEN a leak does develop, there are essentially two scenarios:

A slow leak could develop by any of the processes discussed above. Seawater would gradually enter the hydraulic fluid over a period of weeks or months and, although the system may appear to operate normally, every component which used that hydraulic fluid would be permanently destroyed through corrosion.
 The second scenario is a major seal failure in which most of the hydraulic fluid in the system is emptied into the ocean, under pressure, in a matter of seconds. This pollutes the environment and is an offense which can incur very large fines.

In contrast, if a shaft seal in a gearbox thruster wears out, the worst that can happen is that the gearbox is damaged, leaving the entire hydraulic system intact. If this happens to a Shipwrights thruster, the unit can be removed from the tunnel without hauling the boat. Others have tried to copy this feature, but beware of the procedures necessary to accomplish this. Additionally our tunnel is designed to be easily sealed, in order not to disturb your cruise while repairs are made at the factory.

SIZE OF HOUSING IN THE TUNNEL.
Hydraulic motor thrusters tend to be bulkier than gearbox thrusters, taking up valuable space in the tunnel that could otherwise be used for water flow. The volume of water that is able to get past their thruster housing is severely reduced. This restricted flow path results in higher horsepower requirements in relationship to the desired level of thrust, increasing the size of the tunnel necessary to produce a particular amount of thrust. Many of our competitors manufacture gearbox thrusters almost as large as their hydraulic motor thrusters, resulting in flow restriction comparable to their previous generation of hydraulic motor thrusters.

NOISE.
Hydraulic motor thrusters produce a shockingly loud sound. Shipwrights gearbox thrusters produce a smoother sound similar to a well-muffled automobile engine. The proprietary connection between our tunnel and thruster significantly reduces sound levels due to its strength and stiffness. To our knowledge, no other company manufactures a similarly strong connection. Before buying a thruster, we recommend actually listening to it in use. We maintain a 43 foot sailing vessel with a 15 hp thruster system to allow our potential customers to experience the sound and power of our thrusters. Shipwrights thrusters operating at 50 hp very often are not audible in the pilot house.

HYDRAULIC SYSTEM DESIGN FLEXIBILITY.
There is less flexibility in the design of the hydraulic system because hydraulic motor thrusters 'lock in' the size of the hydraulic motor. This can have more serious consequences which might not be obvious. Typically hydraulic motor thrusters work at low pressures and high flow rates resulting in large hoses and large reservoirs. Large hoses lead to difficult routing during installation as well as much more space being used in the bilge. Large reservoirs waste valuable engine room space. Equipment necessary to handle large flows is much heavier and bulkier, making it more difficult to install and service.

SUMMARY.
Taking into account the foregoing, it is easy to see the advantages of a gearbox thruster as well as the particular advantages that ONLY SHIPWRIGHTS GEARBOX THRUSTERS OFFER.