Don't Rock the Boat | trawlertraveller

Don't Rock the Boat!

Beam sea in Bass Strait in 35kts - without stabilizers!

Let's talk about stabilizers

One of the most frequent questions I get asked is "Does she roll much?". To which my answer is "Every boat rolls when there are waves, and the bigger the waves the more they roll! Ada Hardy is no different." Most often we then end up in a discussion about stabilizers and I am frequently surprised how many people (including experienced boaters) have little idea about the various types of stabilizers and how they work, so I thought a chapter on the stabilizer systems on Ada Hardy would be of interest to some.

It was no surprise to find that Ada Hardy, like any small boat, pitches and rolls in a sea way. Her heavy displacement, deep draft and full length keel help make the motion at sea much more comfortable than boats of comparable size with lighter displacement and shallower draft - but there is much improvement to be had by employing some form of stabilization system both for when underway and when at anchor. There are many available options including bilge keels, towed para-vanes, gyroscopic stabilizers, active fin stabilizers and 'flopper stoppers'.

I will only explain in detail here the two systems installed on Ada Hardy - active fin stabilizers for when underway and flopper stoppers for when at anchor.

Active Fin Stabilizers - Installation

As described elsewhere, Ada Hardy was originally fitted with outrigger arms and towed para vanes which I removed soon after purchasing her. I thought they were cumbersome, ugly, potentially dangerous and reduced our speed by more than 1/2 kt due to their drag through the water. I quickly zeroed in on active fin stabilizers by Naiad Dynamics being the most appropriate solution for this boat and had them ordered and fitted before our first trip to Lord Howe Island.

Installing active fin stabilizers on an existing vessel is no small job.

Essentially each fin is similar to a spade rudder in that it comprises a blade connected to a shaft passing through the hull and rotated by a hydraulic ram attached to the top of the shaft inside the boat. A gyroscope measures the rate and direction of roll and activates hydraulic servo valves to adjust the angle of the blades which as a result provide a hydrodynamic force to counteract the direction of the vessel's roll. A source of power needs to be provided for the hydraulic pump which drives the system. This could be via a PTO off the main engine or genset, an electric motor powered by a genset or a belt drive off the main engine crank shaft.

When planning an active fin stabilizer installation, locating the fins in the optimum position to provide maximum efficacy is the most important consideration. Ideally the fins should be located at the point of maximum beam and as far outboard as possible to provide the largest possible lever arm to counteract roll, but the tips of the fins should not protrude beyond a vertical line projected down from the waterline to ensure the fins are not damaged when mooring alongside a wharf.

In the slings - showing stabilizer fins either side. Note how the fins do not project beyond the vertical plane of the waterline.

The position where the fins are installed also needs to be accessible from inside the boat, not only for the installation but also for future maintenance, as the main moving parts and driving hydraulics are located directly above the shaft of each fin.

The position where the fins are to be located will most likely need to be structurally reinforced to spread the significant loads generated by the fins when active. An experienced shipwright or naval architect should be consulted to make sure this is done correctly. In the case of Ada Hardy, the ideal position for the stabilizers was where the original freezer well was located. This made access for installation very straight forward and simply required that any future fitout / accommodation was designed to provide reasonable access for maintenance.

After analyzing the various options for powering the hydraulic pump I determined that a belt drive off the main engine crank shaft would be the simplest option. The hydraulic header tank and heat exchanger were conveniently located nearby within the engine room.

Stabilizer hydraulic pump driven by a pair of 'B' size belts off the main engine crankshaft

Hydraulic header tank and oil filter mounted on the engine room bulkhead. The oil is cooled by seawater pumped through a heat exchanger coil within the oil tank.

Active Fin Stabilizers - How do they work?

The stabilizers installed on Ada Hardy are designed to work only when the vessel is underway. The hydrodynamic forces which counteract the boat's rolling motion are generated by rotating the fins either side of centre line so that the force of the moving water against the angled blade exerts a force in the opposite direction of the roll direction. The roll direction (and magnitude of the roll) is sensed by a small gyroscope which the electronic control system uses to control the movement of the stabilizer fins. The image and explanation below will (hopefully!) make it clear how these rotating fins counteract a rolling motion.

In this image the stabilizer blade on the port side of the boat is shown angled to starboard (trailing edge down). This is the position the fin would be in during a roll to port. In this position the hydrodynamic force exerted on the fin as it moves through the water is pushing upwards - trying to make the boat roll to starboard and thus counteracting and reducing the roll to port.

The fin on the starboard side of the boat would at the same time also be angled to starboard but because it is on the opposite side of the boat it's trailing edge is up, thus trying to pull the starboard side down.

By rotating the fins in the same direction in unison they are working together - one trying to push one side of the boat up whilst the other, at the same time, is trying to pull the other side down

The really clever feature of this system is that the gyroscope not only senses the direction of the roll (to port or to starboard) but also the acceleration of the roll which gives an important indication of the intensity of the roll. The beauty is that the maximum acceleration occurs at the very beginning of a roll so the system knows not only the moment a roll changes direction but how intense the roll is going to be and can apply the appropriate amount of fin rotation to counteract it.

So how effective are they? The bottom line is that in any sort of seaway the difference between the fins being active or turned off (fixed in a central position) is immediately apparent. The fins will pretty much eliminate any roll in beam seas of up to about 1.0 - 1.5m. In bigger waves, the fins reduce the degree of the roll, but do not eliminate it. The bigger the waves the more amount of roll is beyond the capability of the system, but irrespective of the conditions, the system does an amazing job in reducing the acceleration of the rolling sequence - it dampens the motion making it much more tolerable. The analogy that comes to mind for me is the scenario of lifting one end of a table. If you do it quickly (high acceleration) the smallest movement will cause havoc with anything that is on the table. If on the other hand you lift the end of the table slowly you can go a long way before anything starts to to topple or slide.

All in all, I am extremely pleased with how effective my active fin stabilizers are and immediately appreciate the enormous contribution they make to comfort at sea when, on rare occasions, they break down.

At Anchor Stabilizers - Flopper Stoppers

As discussed above, the active fin stabilizers installed on Ada Hardy rely on the boat being underway to be effective. There are other systems, such as zero speed active fins and gyro stabilizers, which are capable of being effective both whilst underway and stationary, but which for various reasons (briefly discussed later) I do not think are appropriate for Ada Hardy. So when in a rolly anchorage we deploy our 'flopper stoppers'. I think 'flopper stoppers' is an Australian colloquialism and have never been able to find what they are called elsewhere in the world.

Ada Hardy with 'flopper stoppers' deployed

The flopper stoppers on Ada Hardy were supplied by Ocean Torque in Australia (http://www.boatstabilisersaustralia.com/). The key components include two 4.0m long outrigger arms deployed each side of the boat from which is suspended a circular 'valve' disk.

Assembled Flopper Stopper ready to deploy (one each side)

Components of the suspended Flopper Stopper

The key component is the circular 'valve' with the remainder being weights to keep the valve submerged and a strong spring to eliminate impact loads. The assembled flopper stoppers are suspended from the ends of the outrigger arms and lowered into the water so that the valve is approximately 2.0m below the surface. The valve comprises a circular aluminium frame fitted with rubber flaps that open and allow water to flow through freely when the valve moves in a downward direction but close (against the aluminium frame) and prevent the flow of water when moving in an upward direction. As the boat rolls, one valve will open and sink freely with the motion but the valve on the opposite side will close and create a powerful drag as it is pulled upwards through the water, thereby resisting and lessening the roll of the boat. When the boat rolls in the opposite direction the roles of the valves are reversed.

The valves supplied by Ocean Torque come in many different diameters to suit the size of the boat. The larger the diameter the greater the drag exerted when being pulled up through the water.

We have found these flopper stoppers to be amazingly effective and have often resulted in turning an otherwise untenable anchorage into a peaceful stopover. In larger swells they cannot eliminate all roll but, similar to our active fin stabilizers, they dampen the motion significantly. We also use them frequently in anchorages subject to wash from passing vessels where they pretty much eliminate any movement.

Other Stabilizer Systems

There are some other stabilizer systems available which I considered but rejected for various reasons:

Zero Speed Active Fin

These are in principal the same as the active fins I have installed except with much larger fins (blades). The fins remain active when stationary and flap back and forth to counteract any rolling motion. I am not a fan of these because of the ongoing need to supply power to the system, meaning running a generator continuously whilst at anchor (we like to run 'quiet ship' as much as possible) and the potential danger to swimmers.

Gyro-stabilizer

Gyro stabilizers date back to the early 1900's but lost favour to active fin stabilizers due to reduced cost, size and weight. They have recently regained some popularity and a number of manufacturers are producing models especially for yachts. A gyro-stabilizer comprises a heavy flywheel spinning at high speed which is rotated about its axis to counteract roll. The unit is fitted as low as possible inside the boat with nothing protruding through the hull. I don't understand exactly how it works but it is proven technology and is claimed to work just as effectively underway or stationary.

The main down sides for me are:

To be effective the flywheel is required to have a mass of approximately 3-5% of the boat's displacement. At 90t displacement a unit for Ada Hardy would have to be in the order of 3-4 tonnes and requires a space to accommodate a sphere of approximately 1.2m - 1.4m diameter. This is a big unit to try and accommodate.

Like zero speed active fins, a continuous power supply (both electric and hydraulic) is required which means running a generator continuously.

The cost is 2 - 3 times that of active fin stabilizers.

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