A Lust For Thrust

My introduction to bow thrusters was in the Australian Merchant Navy back in 1974, where the first ship I served on was fitted with a direct-reversing DC unit. I remember well a situation in the port of Fremantle, where due to a tug crew dispute our single screw ship was left stranded without assistance out of the harbour. With a daily operating cost of $7000 ($14,000 when at sea) back then, every day lost really ate into the company's profits!

But after having the crew hang tractor tyres from the transom stern to protect it against the wharf, our captain (George Titmarsh) was able to swing the bow 180? to face the sea just by using the bow thruster. And even when we docked in ports such as Melbourne, we needed only one tug to push in the stern.

Bow thrusters have come a long way since those days and now in Australia there are three main brands of bow thrusters on the local market for recreational and commercial vessels from seven metres upwards. AMI Sales imports its Side-Power range, Eastern Engine its Volvo QL, and Bob Littler Agencies' (BLA) Vetus thrusters.

According to Eastern Engine, when the QL range was introduced locally in 1994 the company was lucky to see one or two sales per year due to the fact that clients were not 'ready' to accept thrusters. Now there are several Aussie boatbuilders, such as Riviera Marine, Fairways Marine, Mastercraft Marine and Binks Yachts, that offer QL thrusters as standard equipment or an optional extra. Imported boats equipped with thrusters include Cranchi, Princess and Sunseeker.

According to Eastern Engine, Australia now has the greatest amount of thruster sales per capita in the world - which the company attributes to the 'growing maturity' of the Aussie boating public. But the market is not just limited to bow thrusters and includes compact transom or stern thrusters which enable a boat to move sideways, or by reversing the motor units, spun in its own length without using the main engines.

In crowded European ports, where stern-to berthing is common, bow thrusters are regularly fitted to sportscruisers as small as nine metres. As one experienced fitter of bow thrusters explained to me when researching this article, some owners are unable to manoeuvre their cruisers astern even when twin engines are fitted!

But there's more than meets the eye in this situation. According to AMI, research showed that even experienced boaties who keep their cruisers alongside the jetties of their waterfront houses feel reassured by being able to manoeuvre their boats in strong winds with minimum effort. Also, bow thrusters are particularly useful in sterndrive-powered sportsboats that have an aft weight bias and long bows susceptible to being blown around in crosswinds.

Warren Yachts at Kincumber (NSW) is sold on bow thrusters in its boats and told me that, correctly installed, a unit can create less drag than the antifouling sprayed on a planing hull! In any case, when planing the thruster tunnel opening is out of the water.

SWINGS AND ROUNDABOUTS
To get the maximum advantage from a bow thruster it's drive or impeller (propeller) tunnel should be mounted as far forward as possible. AMI says that even with larger boats (around 20m) as little as a couple of metres difference in mounting position can dramatically reduce the power needed to make a hull rotate around its pivot point.

For example, should the thruster be mounted 16 metres forward of the pivot point, a 220kg (2200 newtons) thrust unit will give 33,792 newton metres (Nm) of torque to rotate the boat. But if the thruster is mounted 17.6 metres from the pivot point, an increase of 10% of torque is available from the same 2200 newton unit.

Selecting the right bow thruster for your boat requires you to know a range of variables. Vetus gives the example of an 11m flybridge cruiser which has a lateral wind draft or area above the waterline of 18sqm. The requirement is that the bow can easily be controlled in a wind strength of Beaufort Scale 5 or 29-39kmh. The wind pressure is therefore 41-74Nm2 with an average of 60Nm2.

The formula for required torque reads:
T = wind pressure x wind draft x reduction factor x distance centre of effort to pivot point (approximately half the hull length)
T =60Nm x 18m2 x 0.75 x 11 divided by 2 = 4455Nm.

The required thrust force is calculated as follows:
F = torque divided by distance between centre of bow thruster and pivot point of boat (the transom being the pivot point) or 4455Nm divided by 10.5m
F =420 newtons.

The most suitable model for this cruiser therefore is the 50 kgf, which produces 500 newtons of thrust and has tunnel diameter of 185mm with a minimum tunnel length of 370mm. The motor is a reversible DC unit that develops 4hp and can operate continuously for five minutes or five out of every hour. The current consumption is 250 amps on 12V, requiring a 100 amp/hour marine starting battery. Excluding the tunnel the 50 kgf weighs 20kg.

Volvo QL also makes a stern thruster for this size of boat, known as the SP600, which develops 589 newtons of thrust from its 4hp motor.

Vetus says that placing the tunnel as deep as possible reduces the possibility of sucking air down from the surface and destroying the thrust generated. It also increases water density for greater power from a given impeller size. And AMI says that depth from the static waterline should be at least equal that of the tunnel diameter, but the distance from the bottom of the forefoot no less than one-third the diameter of the tunnel.

The right tunnel diameter/tunnel length are essential to get the best from a bow thruster unit. AMI says the tunnel should be as short as possible and the opening fairings (where water enters and exits the tunnel) well-rounded to eliminate any eddies forming at the opening which could create bubbles and cause impeller ventilation.

Never be tempted to fit grilles over the tunnel openings to prevent objects being drawn into the blades, as these not only reduce thrust but can also cause cavitation of the impeller blades.

Tunnel drag is not so important with trawler-style powered displacement craft, most of which don't exceed 10kt. However, minimum drag is essential with racing yachts, thus some companies manufacture a range of retractable thrusters which can cost as much as $US50-60K. Whatever, by using a little forethought when installing the tunnel drag can be significantly reduced.

AMI says there are two ways of doing this. Either you can fair in the hull immediately aft of the tunnel, which will stop waterflow over the tunnel opening hitting the flat aft side. Alternatively, by fitting a small lip to the forward edge of each tunnel opening, waterflow will be deflected outwards and not impinge directly on the aft tunnel side. The latter method is recommended for fast powerboats and obviously requires less modification to the hull moulding in this area.

AMI provides comprehensive installation instructions with each of its thruster units, using the tunnels supplied, while Vetus supplies tunnels in steel, aluminium and fibreglass (according to the construction of your boat).

To reduce impeller cavitation or air bubbles on the blades from the gearcase housing, the gearcase itself must be as streamlined as possible. For example, Vetus thrusters use streamlined bronze gearcases with bevel gears (similar to those in an outboard motor gearcase) to not only reduce cavitation but also noise levels for quiet operation underway. Each gearcase, or tailpiece as Vetus calls them, has a large capacity zinc anode fitted to the bottom.

Vetus uses reinforced plastic impellers to reduce the starting load on the motor, which also reduces current draw. However, one problem with plastic impellers is that when they're left constantly immersed fouling will rapidly build-up on the blades, unlike bronze impellers which are far more resistant to fouling.

CHOICE OF POWER
Bow thrusters are available in AC or DC electric and hydraulic, the choice depending on the onboard power supplies. AC electric is mainly used in larger motoryachts and, for example, Warren Yachts has recently completed a 43.9m motoryacht fitted with a 100hp AC electro-hydraulic unit.

DC electric is the most affordable and as the thruster is used for only short periods the 12 or 24V motor can draw directly from the boat's batteries, providing they can handle the hefty current drain.

Thrusters are available for boats as small as, say a Caribbean 26. Vetus' 23 kgf (230 newtons) suits this hull. It requires only a 132mm diameter tunnel, weighs 10kg and to reduce the installation space required the motor may be laid on its side, providing in this case it's supported by a bracket.Volvo QL also makes a model known as the BP300, which again requires a 132mm tunnel and uses a 2.7hp motor that produces 294 newtons.

Hydraulic systems are more torquey for a given size but are only a viable option should your boat already have engine-driven hydraulics aboard, such as in the case of a commercial fishing boat with winches. To install a hydraulic system in a small boat from scratch is expensive, but for boats so equipped AMI has a range from 1000-3000 newtons of thrust requiring tunnel diameters of 185-300mm.

BEAUTIFUL NOISE
Due to straight-cut gears in some models, thruster operating noise can be a problem in luxury motoryachts, where owners want as little intrusion as possible. Warren Yachts has gone to a lot of trouble with NVH testing to reduce this intrusion to a minimum and Volvo QL makes sound-absorption sheets which can be placed around the tunnel area to reduce noise.

But according to AMI, operating noise is a plus with owners of smaller cruisers, who feel reassured the thruster is working when adverse conditions call for it. In any case, as the thruster is running for such a short time out of each trip a bit of noise is no hardship!

According to AMI, thruster maintenance is straightforward. Every time your boat is slipped you should check that the impeller is securely fastened and the bolts holding the motor to its bracket are tight. Make sure there are no leaks around the motor unit and electrical connections are clean. Change the zinc anode once a year and gear oil every second (using SAE 90 oil) and when antifouling the impeller and gearcase make sure the zinc anode, seals and impeller shaft are not painted.

For information on the right bow thruster for your boat, contact AMI Sales, tel (02) 9808 4300; or Eastern Engine, tel (07) 3899 1622.