Key Questions to Ask When Ordering RIM Drive Thruster

My wife is rebelling about handling lines since she hurt her elbow in France (doing a flight of 12 locks - not exactly easy on anyone.) We are considering a larger boat than we've had in the past and I will need to be able to moor it myself much of the time. I don't expect much trouble most of the time, but once in a while ...

So I need to learn about bow and stern thrusters. Are they on/off or variable thrust? Are there any that you can leave on for longer periods than just for short bursts?

My idea is that when the wind is wrong, or whatever else can go wrong, I could put the boat up against the dock and apply just enough thrust to hold it there while I go ashore with the lines. This would require that they have variable thrust so I don't snap off a slip finger and would require that they could remain on for a few minutes while I took over a couple of lines.

Does this make sense?

Alternatively, I understand that there are wireless remotes for some thrusters. Perhaps I could use something like that to keep the boat against the dock, though it would be really bad if something went wrong while mooring to a side-tie with the wind blowing the boat out.

Are hydraulic thrusters more reliable than electric and can they vary their thrust and do they have longer run times than electric? I just remember seeing an engraved placard next to an electric bow thruster control limiting it to 15 seconds every minute. Plus, I like hydraulics. Please feel free to correct me if you think I should hate them.

Thanks.

If your primary interest in thrusters is your wife's understandable reluctance to handle lines I'll offer another solution. Have her learn to be a competent boat handler so that you can handle the lines. I've seen that work out quite well.

Ain't gonna happen. I've tried to get her to learn to drive smaller boats (18' and 28') and she will not. She likes riding and such, but does not have any interest in driving. Her choice, not mine.

Also consider the risk of stepping off to make up the lines while depending on thrusters and possibly remotes to keep the boat to the dock. If she's not able to handle the boat and something goes wrong you both now have a very serious problem.

Yes, I have thought about that, too. I have pretty good situational awareness. If one or both thrusters die and the boat is not in gear, it isn't going to go zooming off immediately and I should be able to get back on board quickly enough. Especially if I carry a line over with me when I step onto the dock or toss one, first. Worst case, as long as she doesn't put the boat into gear, I know how to swim. If she does put the boat into gear with me on the dock, well I guess that would be a sign of a bigger and non-nautical problem.

Back to your questions regarding hydraulics. So much depends on what drives the hydraulic pump. If electric will you require the gen running? If a PTO will it be off the gen or the main engine. If off the main will you need to keep the RPMs up to effectively power the hyd pump?

Setting up electric thrusters on a boat that doesn't already have a hydraulic system is much simpler.

Thanks, that all makes sense. All of my larger boat experience has been on much larger boats such as 100'+ tugs and larger schooners. They all had hydraulics with dedicated powerheads run either off of an electric motor (most) or a small diesel (one of them.) I wondered about running a pump off of the main engine of a 40' or so boat, but you are right that it would have to keep its RPMs up which would interfere with maneuvering to the dock or into the slip. Electric wins for pleasure boats of this size.

You are NOT stepping off the boat before it's secured to the dock are you? If so, that's a HUGE no/no.

Depending on circumstances, yes.

I do understand the risks, but as I said, I have good situational awareness and also a bit of professional experience (a long time ago!) stepping across with lines. My method on the tugs was to do as I was told. I once almost had to spend the night on a barge in the middle of a bay in the middle of nowhere (the middle of the Aleutians is pretty much the middle of nowhere) during a storm with gusts well over 135MPH because the old tug was getting blown away. I had been sent over to catch a line they were going to toss to me. Even at full throttle the boat couldn't make headway against the gusts, which is pretty much the same as an uncrewed boat drifting away. I would not have survived the night. (It was also pretty cold.) I was told to jump for the boat and I jumped. I barely caught the gunnel near the bow and two other crewmembers caught me and pulled me in. Their eyes were really big. Mine probably were, too. I remember the bruises that healed pretty quickly in a 20 year old body and the relief at not dying or having to swim for the boat. I am 45 years older now and much wiser. No way I'm going to let some moron (barely more than a child, himself) tell me to risk my life like that.

My preferred method mooring the 28' boat alone was to put a line across by tossing a bight of line over a cleat (or flipping it onto the cleat when I miss) and making it back up on the boat. That is probably what I will do most of the time. But I want insurance. On the water, like in many things, if you only have or only know one way to do a thing, you will not be prepared to cope with the variety of situations that will be presented to you. So in some circumstances I will probably use the thrusters to hold the boat against the dock while I get a line or two onto a cleat, in other circumstances I will step across with the line or throw it first.

One example of such a situation might be the kind of dock that has horizontal 4x4s along the edge rather than cleats. There are a lot of those in Oregon. The 4x4s are spaced up off the dock level on short 2x4 or 4x4 spacers. You can't toss a line over a cleat or bollard, you have to actually fish the line under and wrap it around the 4x4. The idea is partly that there is a perfect cleat spacing for any boat, but I think mostly because it's cheaper than putting on a bunch of cleats. Someone has to be on the dock to do a tie-up like this and I will be that someone if we run into a dock with this setup. I'm willing to spend a bunch of money to make sure the boat stays against the dock while I am doing it. When you think ahead and prepare things are far more likely to go well. But there are no sure things. To believe so is hubris.

I can throw a line around a 4x4, I'd be you can too, and can always use a hook.

A horizontal 4x4 that is spaced an inch and a half or three and a half inches off of the dock? (I'll put a photo below.) Of course I can throw a line around a vertical 4x4. I will be in total awe of anyone who can throw a line around the type I am talking about. And I have thrown lines from 3/8" heaving lines up to 7" towing hawsers (for very short distances!) and around little cleats, big cleats, bollards, bitts, ship's railings, etc.

Even with no cleats, no pilings there's always my portable dock cleats, but have always been able to secure at least on prior to getting off.

See: now you've introduced something interesting. What is a portable docking cleat? How does it work from the boat? I see the picture. It looks like something you would slip down between two planks. (Better not get any big wakes or you'll pull up planks.) It does look very handy, but I don't see how you would insert it without getting off of the boat.

There's "might be" a situation where you jump off first, but would be with zero wind/current.

You are fixating on something that is unimportant. I was lazy about how I wrote the part about getting off of the boat because I wasn't asking for docking advice. That might come in the future (my ego does actually still fit inside of my head) but for now I was asking about a certain functionality with bow thrusters.

I lived for 10 years right next (8 feet away) from a yacht club dock where large boats came and went all of the time. It is amazing how few big boat operators know anything about mooring. I'd say less than 10% are really good at it. One thing that almost nobody understands is where the control of the lines should be. If you are mooring a boat, you should retain control of the lines on the boat. Don't cleat them off on the dock, cleat them off on the boat. If you are towing, pass the eye to the boat that is under tow and cleat it off to the boat that is doing the towing. (Unless you are on the boat being towed and you think the operator of the towing boat is a moron. In that case you will want to keep control for yourself!)

Most people have mooring lines which are less than half the length that they should have. Or if they have long ones, they don't know how to use them to their best advantage. If you have a mooring line that is three times as long as the length where it will typically be used, you can keep an eye around a cleat on your boat, make a bight from the line, and when you get close enough you can throw it towards a cleat or bollard on a dock or other boat. If it misses going around it will still usually lay close to the cleat or bollard and then you give one side of the bight a flip and it will go over. Now you have a line that goes out, around, and back to your boat where you can cleat it off. You don't have to do this doubling with every line (though it is a good idea in nasty weather) but mostly just when coming alongside. Maybe with a spring line, maybe just with a breast line as circumstances dictate. As I mentioned before, this would be my preferred method. But I want the thrusters to be my second pair of hands because things just don't always go well. Insurance. It's good for the boat and good for the marriage.

And your example with 135 kt winds "might be" that situation..... however, I'd bet that we don't see that very often.

Depends on where you are. But I don't ever want to live anyplace like that again! Or maybe I should say not at my age - though I wish I were still young enough to take on such adventures.

Here is a picture I found that sort of illustrates the horizontal 4x4 tie-ups. I've actually seen this same arrangement on many ship's docks, but with 12x12s instead of 4x4s. On those big docks they are really used just for edging, so vehicles don't roll off into the water and such. There are cleats or bollards on the dock for mooring. But in Oregon they are used for actual tie-ups on small docks for pleasure craft. You have to wrap your mooring line around the horizontal 4x4s. There isn't anywhere else to tie up. And if there is nobody else to help you, you have to get out on the dock and do it yourself. Before you've tied up to anything. I haven't seen them down here in southern CA, but I have seen them on the Sacramento.

Bow thrusters are a type of propeller-shaped system fitted on the bow (forward part) and stern (known as stern thruster) of the ship. They are smaller in size than the ship’s propeller and help the vessel be more manoeuvrable at lower speeds.

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Bow thrusters are generally used to manoeuvre the ship near coastal waters and channels or when entering or leaving a port during bad currents or adverse winds.

Bow thrusters help tugboats berth the ship to avoid unnecessary time and, eventually, money wastage because the vessel stayed less in the ports. The presence of bow thrusters on a vessel eradicates the need for two tugs while leaving and entering the port, thus saving more money.

Nowadays, ships have both bow and stern thrusters, which makes them independent of tugboats for manoeuvring in the port limits (if the port regulation does not make it compulsory to use tugboats).

Related Read: A Detailed Explanation of How a Ship is Manoeuvered to a Port

Installation Of Bow Thruster

Generally, side thrusters are transverse thrusters placed in a duct at the ship’s forward and aft end. The thruster set at the forward end is known as the bow thruster, and the one placed at the aft end is known as the stern thruster.

The requirement for the number of thrusters to be installed depends on the ship’s length and cargo capacity. The vessel’s route also plays an important factor as many countries have local regulations that stipulate the compulsory use of tugboats to enter or leave their port limits.

For the installation of the side thrusters, the following things are essential:

• The thruster compartment, also known as the bow thruster room, should be easily accessible from the open deck by the ship’s crew
• Most seagoing vessels use an electric motor for the thruster, which is heat-generating machinery and must, therefore, be positioned in a dry and well-ventilated area.
• The bow thruster room should have a high-level bilge alarm, and the indication should be provided in the engine control room and bridge.
• The thruster room should be well-lit
• The room should have at least one light from the emergency source.
• In the case of installation of more than one panel, make sure to operate the thruster from only one panel at a time.
• The thruster room should not be used to store flammable products in the area of the electric motor.
• The tunnel or conduit containing the propeller must be installed perpendicular to the ship’s axis in all directions.
• The propeller should not protrude out of the conduit
• Grid bars may or may not be fitted at both ends of the tunnel (taking into account how much debris the ship bottom will experience in its voyage). The number of bars for to be kept at a minimum as they tend to reduce the thrust force and overall performance of the bow thruster (or stern thruster)
• Sharp edges on the grid bars are to be avoided. A trapezoidal shape with no sharpness is a good choice of design for grid bars installed perpendicularly to the direction of the bow wave
• The design and position of the thruster tunnel should not interfere with the water flow under the hull or should not add to hull resistance
• Ensure that the material used for the installed thruster does not foul existing equipment inside the ship, such as steering links.

Related Read: Understanding Design Of Ship Propeller

Construction and Working of Bow Thrusters

The bow and stern thrusters are placed in the through-and-through tunnels on both sides of the ship. There are two such tunnels at the forward and aft ends of the ship.

The thruster takes suction from one side and throws it out at the other side of the vessel, thus moving the ship in the opposite direction. This can be operated in both directions, i.e. port to starboard and starboard to port.

The bow thrusters are placed below the ship’s waterline. For this reason, the bow thruster room should be checked for water accumulation at regular intervals.

The bow and stern thrusters can be electrically, hydraulically, or diesel-driven. However, the most commonly used are electric-driven thrusters, as in hydraulic-driven thrusters, there are many leakage problems.

Also, diesel-driven bow thrusters require more maintenance, and someone needs to go to the thruster room every time before starting to check the thrusters.

The thrusters used are usually of the CPP type, i.e., the blades on the propeller boss can be moved to change the direction of the thrust.

The boss, which carries the blades, is internally provided with a movable shaft (operated by hydraulic oil), also known as a Hydraulic Pod Motor-driven Thruster.

Once the signal to change the pitch is given, hydraulic oil will be supplied to operate the internal shaft (within the boss) to change the blade angle of the thruster (as shown in the video).

Related Read: 10 Precautions to Take Before Operating Controllable Pitch Propeller (CPP) on Ships

The motor shaft drives the thruster shaft via a pinion gear arrangement. The sealing gasket is provided in the motor casing which holds the water which is in the tunnel.

The Thruster assembly consists of the following components:

• The electric motor with safety relays
• The flexible coupling between the motor and thruster
• Mounting and casing for the electric motor
• The connecting flange and shaft
• Motor casing seal
• The tailpiece with shaft seal
• Bearings
• The propeller shaft
• The zinc anodes
• Grid with bars at both ends of the tunnel

Operation Of Bow Thruster

A bow thruster consists of an electric motor mounted directly over the thruster using a worm gear arrangement. The motor runs at a constant speed, and whenever a change is required in the thrust or direction, the controllable pitch blades are adjusted.

These blades are moved, and the pitch is changed with the help of hydraulic oil, which moves the hub on which the blades are mounted. As the thruster is of controllable pitch type, it can be run continuously, and when no thrust is required, the pitch can be made to zero.

The bridge controls the thruster, and the directions are given remotely. In remote failure, a manual method for changing the pitch is provided in the thruster room and can be operated from there.

Related Read: How Bow Thruster is Used for Maneuvering a Ship?

Usually, the hydraulic valve block, which controls the pitch of the blades, is operated in the BT room to change the blade angle in an emergency.

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When the Bow Thruster is operated alone and the signal is given to operate the pitch at the port side, the thrust will turn the ship towards the starboard side from the forward part.

Similarly, when the Bow Thruster is operated alone, and the signal is given to run the pitch at starboard side, the thrust will result in turning the ship towards the port side from the forward part.

When the stern and bow thruster are operated together on the same side, the ship will move laterally towards the opposite side.

As seen in the above diagrams, the bow thruster and the stern thruster provide excellent manoeuvrability to the ship.

Things To Note While Operating Side Thrusters

• Ensure that the motor is started well ahead of the thruster operation and that the hydraulic lines are opened.
• Never operate the thruster beyond its rated load, else it may lead to tripping of the motor.
• Gradually increase the capacity and shift the pitch. Avoid sudden changes in the BT movement.
• The side thrusters are considered as an “on load” starting device, i.e. they should only be operated when they are submerged in water.
• Before operating the thruster, check for small craft, swimmers, boats and tugs adjacent to the thruster tunnel.
• Never touch any moving parts or the electric motor in operation
• In the case of installation of more than one panel, ensure the thruster is operated from only one panel at a time

Maintenance Of Bow Thrusters

1) The insulation must be checked regularly and kept dry. This is done because bow thrusters are not used frequently and thus there are chances of damages by moisture. Moreover, because of the frequent idle state of the bow thrusters, there can be a reduction in insulation resistance, especially in colder regions.

2) The space heater is checked for working condition so that the insulation can be kept dry.

Related Read: Importance of Insulation Resistance in Marine Electrical Systems

3) The bearings of the motor and the links are to be greased every month.

4) The condition of hydraulic oil is to be checked every month for water in oil and samples should be sent for lab analysis for further checking.

5) The thickness of the contactors is to be checked from time to time.

6) Checks are to be made for any water leakages in the bow thruster room which is an indication of seal leaking.

7) The flexible coupling between the motor and thruster should also be checked.

8) Check and inspect all the cable connections for cleanliness and tightness

9) Vacuum or blow clean the motor grid to remove the carbon grid, which may increase the operating temperature

Major Maintenance Of Bow Thrusters 

The major overhauling and maintenance of the bow and stern thrusters are done during the dry dock when the ship’s hull is out of the water, and the thruster blades and tunnel can be easily accessed.

Following maintenance is usually done in the dry docking:

• Replacement of the O’ rings and the sealing rings
• Removal of the pinion shaft
• Inspection and maintenance/ replacement of gear set
• Replacement of the bearings
• Repairs, cleaning and replacement of the blades
• Inspection of hub and repair if needed
• Inspection and overhauling of the oil distribution box (for operating propeller blades)

Advantages Of Using Bow Thrusters

1) Better manoeuvrability at low speeds of the ship.

2) Safety of the ship increases when berthing in bad weather.

3) Saves money due to the reduction of stay in port and less usage of tugboats.

Disadvantages Of Using Bow Thrusters

1) a huge induction motor is required, which takes a lot of current and load, and thus large generator capacity is required.

2) Initial investment is high

3) Maintenance and repairs are costly when there is damage.

The thrust force produced by the motor to move the ship will depend on various parameters, such as the hull design, power source, tunnel design, use of grids, draft and load of the vessel, etc.

The weather’s condition and the water’s state also play a vital role in BT performance.

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