Topic: Everything you need to know about bow thrusters & stern thrusters (Read 21 times)
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Topic: Everything you need to know about bow thrusters & stern thrusters (Read 21 times)
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Given that both electric and hydraulic motors are typically quiet in operation, it’s surprising how much noise most thrusters make. This is largely due to cavitation from the propeller tips. The small diameter and high speed of the propellers needed to push a heavy boat around creates low pressure spots on the tips that momentarily cause the water to vaporise into bubbles which then collapse, generating shock waves that resonate through the boat. The result is a rattling noise that sounds far worse than it is.
Most thrusters tend to be professionally installed such as this tidy-looking Sidepower stern thruster fitted by Sleipner dealer Osmotech UK
Up to a point. If you haven’t yet fitted one, go for the largest diameter tube possible, make the area around it as stiff as possible (to reduce structural-borne vibration) and try to encourage a smooth flow of water into the tube rather than sharp edges that increase the likelihood of cavitation. A proportional thruster can also reduce noise by giving you the option to run it slower for longer.
If you already have a bow thruster fitted, you may be able to swap the propeller for a modern co*posite low-noise, skew-tip design.
Originally this was a big ship idea that has now moved down to the pleasure boat market. The idea is to reduce drag by bringing the unit up inside the hull when not in use to leave a flush surface. They are a little more co*plex to install because it involves cutting a hole in the centre of the hull, which is then likely to need additional reinforcement.
While this should reduce the amount of marine growth on the actual propellers, the retractable mechanism itself is prone to debris or marine growth. Having a retractable thruster jammed half open makes boat handling even more difficult.
A Vetus Pro thruster without the usual acco*panying tunnel
It’s possible but we wouldn’t advise it unless you really know what you’re doing. Most bow thrusters require you to drill a substantial horizontal hole through the bow below the waterline into which the thruster tube is inserted and bonded into place. This needs to be low enough to prevent the thruster from sucking in surface air when manoeuvering around a marina, but high enough for it to lift clear of the water when planing to minimise drag through the water.
Obviously this needs to be watertight and may require additional reinforcement due to weakening of the surrounding structure. Most boatbuilders now offer bow and/or stern thrusters as an optional factory fit extra on craft over 25ft long. This is such a popular option that some builders automatically fit a bow tube to every boat as standard but only cut the ends open and fit a thruster when the customer orders one.
Electric DC thrusters need a very high instant current. For example, the smallest unit from Vetus needs 100A when operating, while a larger 8hp unit needs over 600A. The supply cables must be big enough to cope with the load while the distance from the battery to the motor should be kept as short as possible to minimise voltage drop. This often involves fitting separate thruster batteries close to the unit, which may affect your boat’s for and aft trim, not to mention a dedicated charger.
A Sleipner retractable thruster
Most modern marine engines are co*puter-controlled with a “black brain box” controlling everything. Sudden voltage drops when an electric bow thruster is used can momentarily starve the power from the engine’s control system and cause it to stall. Precisely what you don’t need when berthing your boat!
The more advanced units have built-in inverters, which soften the electric surge by applying a proportional voltage for a few seconds. This may cause a minor delay between pushing the button and full thrust being delivered.
Thrusters are vulnerable to fouling growth and galvanic corrosion. co*posite propellers and tubes can be coated with conventional copper-based antifouling paint but metal ones will require special propeller antifouling. Most thrusters have sacrificial anodes to prevent galvanic corrosion. This will need replacing at regular intervals depending on your area’s wear rate.
The exterior also needs filling and fairing so water can flow easily past the entry lip
A hydraulic thruster needs three high-pressure pipes connecting the oil pump on the main engines to the motor. I strongly advise fitting a hydraulic oil pump to each engine, connected with a splitter unit on the control valve chest. I have seen pumps fitted to just one engine, but invariably, if one engine fails or you get a rope around the propeller, it will be the engine with the pump. With a splitter unit fitted your thrusters will work using one engine only, just a little slower.
Remote handsets are particularly handy on larger boats where you may need to move away from the helm when co*ing alongside
Most thruster controls are very simple. They normally have an on/off switch to power them up in readiness for docking and a port/starboard lever or pair of buttons to run them in the required direction. Proportional thrusters have a lever so that the further you push it the more power you get. Some units also have the option of a remote control enabling you to move around the boat when docking.
Joysticks can now be used to co*bine thrusters and engines for simple one-touch berthing control
Some bow and stern thrusters can now be linked to the steering and main engines using sophisticated software and a simple joystick control. All you have to do is push or twist the joystick in the direction you want to go and the co*puter will then decide what co*bination of thrusters, rudder position and engine drive is needed to move the boat in that direction. While not quite as effective as steerable pods, this does make a shaftdrive boat almost as easy to park as an IPS one.
Thrusters take the stress out of manoeuvring in tight spaces
Many joystick systems now have a skyhook or station hold button that keeps the boat locked in position using GPS location and the boats’ engines and thrusters to prevent it drifting. A number of electronics manufacturers also offer varying degrees of berthing assistance using cameras and proximity sensors in conjunction with joystick control. At some point, full automated berthing is also likely to happen. In all cases thrusters will play a key role in making this possible.
As always, that co*es down to price, your boat’s requirements, local availability and personal preference. Sleipner (Sidepower), Vetus, Quick and Lewmar are the main players in Europe.
Based on the boats we test, we’d say Sleipner thrusters are the most popular factory-fitted option.
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