FUNCTIONING AND COLLATERAL EFFECTS
CMC Marine opened the doors of the new HQ situated in Cascina Italy to which the company moved to in the beginning of 2017. The move was dictated by the impellent need to face up to the substantial increase in production and by the need to take on more st
Bow thrusters are proven useful aids while manoeuvring yachts in restricted waters, they become absolutely necessary when talking about super and mega yachts. Often enough though bow thrusters are installed either approximately well which is barely enough or even clearly in the wrong place to the extent that efficiency is reduced and hydrodynamic drag is enhanced, so are vibrations and furthermore structural damage can result from this as well. The text that follows aims to analyze causes and precautions which need to be taken into account and other considerations to ensure correct installation. Use of a side thruster installed in the bow and / or the stern which will at the push of a button or joystick move the bow or stern of the yacht to one side or the other accordingly to enhance manoeuvring in the smallest of slots while berthing for example. And this is the reason for which almost every yacht above a given size will be equipped with a bow thruster and sometimes a stern thruster too. Deploying this technology makes things simple enough when wanting to berth for example in really restricted spaces even, and/ or with cross winds and or currents push the boat in the opposite direction you want to go, this technology becomes more than handy and a lot safer to have than not. But like everything else the installation of the whole, meaning thrusting propeller and housing tunnel must be carried out correctly while knowing that a hull conceived and designed to move through water which is a thousand times denser than air will be modified. Careless, thoughtless installations can be seen often, as you observe housing tunnels, as the yachts are high and dry on their cradles.
The thought being the same one always: well after all its nothing but a “hole” ! But it’s a hole and it’s below the waterline underwater! What’s more it’s inside a moving hull and when done badly it can cost an arm and a leg and more, specifically it will impair propeller efficiency and overall performance due to greater drag effect which means added fuel consumption, noise and induced vibrations. Let’s therefore take a closer look at some of the hydrodynamic aspects involved in connection to the bow propeller which is the most commonly used and its housing tunnel. In actual fact the propeller installed into the housing can be a fixed bladed one, a variable pitch or even a counter rotating system with two props.(anything is more effective than a fixed bladed prop. But the purchasing cost is higher and is more complex to run and maintain: be wary though of extraordinary performances being promised lightly). Whatever the choice of propeller it lodges into a coduit which must possess given features in terms of wear and tear, structure and must be water tight and must comply to a given set of hydrodynamic requisites according to the hull shape. LONGITUDAL POSITIONING So as to deliver maximum manoeuvrability bow thrusters must be installed as forward as possible. In fact the longer the distance to the housing tunnel from the stern, the greater the manoeuvrability. A simple example is supplied in the technical data section attached to this article. Nevertheless there’s a downside to installing bow thrusters as far forward as possible: since, when the yacht is proceeding forwards slowly the bow thrusters loose efficiency by comparison to one which is installed further astern. Tests carried out on several models indicate that the bow thrusters’ effect at just two knots are reduced by as much as 50%. With a speed between 1 and 2 knots 40% loss of efficiency can be recorded but by moving the housing and prop further astern the effect is reduced and the propeller maintains higher efficiency even when the yacht is not moving. Obviously this detail is of relative importance because when the boat is not moving there’s no rudder ( in reality it works passively as deviator of the flux
created by the propeller) as speed increases the rudder blade becomes more effective: and there’s less need for lateral thrust produced by the bow thrusters. Certainly in some cases, in motor yachts for example where rudder blades are very much on the small side ( this is done so as not to have added drag at high speeds) and are therefore practically useless at very low speed. This data can be useful when assessing where the best compromising line or cut off point is between the greatest possible distance from stern to bow thruster or maximum leverage and the minor loss of thruster efficiency at progressing lower speeds.
IMMERSION The rule is simple: to work correctly the bow propeller is be installed as far down as possible below the waterline. This is for two reasons: 1.In such a way to prevent air from the surface from being sucked in. This would greatly reduce thrust produced by the propeller ( the propeller would no longer be pushing water but a mix of water and air) 2.To have water enough to guarantee sufficient pressure to avoid propeller cavitation and consequent loss in propeller efficiency. In practical terms bow thruster producing companies recommend that propellers be installed so that the upper part of the housing tunnel be placed below the waterline for at least ¾ of the housing tunnel’s diameter, or at any rate at least one foot (30.48 cm) below the waterline whichever is the greater. I wish to underscore this is a minimum value and it is better to place the propeller as low as possible under water. Furthermore housing tunnels on planing hulls should be installed so they are out of the water above waterline when the yacht is cruising in planing mode. LENGTH OF HOUSING TUNNELS When positioning the bow propeller, it is not enough to find the right compromise between the longitudinal positioning ( generally as far forward as possible with the reminders already mentioned) and the immersion depth. A third aspect must also be taken into due consideration: the tunnel’s minimum length across into which the propeller will be installed. To do this the propeller must be installed in an area of the hull which is sufficiently wide across or beamy enough, which probably cannot be found neither in the extreme bow area nor at the bottom of the hull in the bow section. In fact the more we move towards the bow and towards the bottom of the hull, the more the volumes shrink meaning that beam decreases. The ideal tunnel length across is comprised between 2 and 4 times the diameter of the tunnel itself. Greater lengths cause loss of efficiency due to water attrition flowing along the sides and walls of the same. The loss of efficiency incurred
is however slight up to tunnel lengths equal to six- seven times the size of the diameter. Lesser lengths below 2 to 4 times the diameter cause loss of efficiency because the propeller, conceived to work in water with a uniform flux and parallel to the axis of thrust, finds itself working partially inside the tunnel: for the lower part of the tunnel, which is considerably smaller, does not permit to have a uniform and parallel flux as in the upper section. Consequently the propeller’s thrust is less efficient and diminishes significantly due to whirling and cavitation. To sum up when positioning a thruster three Cartesian axis need to be taken into account: longitudinal ( length), vertical, ( immersion) transversal ( hull beam). Therefore normally a ‘ possible’ area in which to install the thruster has to be selected prior to commencing installation .
JOINING TUBING TO HULL To prevent efficiency loss of the thrusters and to reduce noise to minimum levels it is very important to perform a rounded cut out between tunnel and hull which is sealed and not less than 10% of the diameter. Should this detail be omitted the resulting edge between tunnel and hull will cause turbulence and cavitation along the tunnel’s walls when water enters at high speed. This phenomena reduces the size of the ‘ efficient’ diameter limiting the volume of water and the consequent thrust. The same turbulence and cavitation can also affect the propellers thereby reducing performance and increasing noise. Furthermore the rounded extremities of the tunnel enhance taking water in and the discharge of the water inside the conduit. The water is ‘ sucked in’ along the sides of the hull and create a large area in depression which tends to draw in, the boat laterally. Such depression which often enough covers a larger area in the case of ‘ soft’ conduits, can be considered as an additional source of thrust: when installations are performed flawlessly, such extra thrust can be significantly high as much as 20% more. HULL SET UP NEAR THE TUNNEL We are talking about a minor detail when compared to the whole yacht and one which is negligible to many ( even to some of those who install side bow thrusters). Nevertheless in terms of drag effects and noise increase it is worth mentioning. Basically we’ll be looking at a minor modification of the hull lines near the thruster. There are two ways of doing this: either by building up a small mould forward of the tunnel which acts as a water deflecting element or water flow deviator, or by creating a small concave dip in the hull just in line with the stern end of the
tunnel to attract the water flow coming from the bow. In both cases the goal is to prevent that flowing water along the hull sides hit the stern walls of the tunnel creating a brake effect or generating more drag, whirling and noise. Both solutions work but the first one is best suited to planing yachts, while the second one befits displacing hulls more . LET’S NOW TAKE A CLOSER LOOK. • Hull setup forward of the tunnel – We’ve claimed that this solution is more appropriate for planing hulls since their bows are significantly out of the water when cruising normally. Consequently the tunnel is generally above the surface. The setup in this case must deliver two functions: the prime one is to deviate water flow at low speeds as long as the bow and tunnel therefore are immersed below the waterline. The second function is to deviate the water impacting the bow while in planing mode in the presence of waves, and to prevent rising waters
from crashing into the tunnels’ walls. While planing along, the water flow is made up of two factors comprising boat speed and the way in which the boat hits the waves in a head sea, or a cross sea and so on. This is where the mould comes into play as it will deviate both the waters coming from the bow and the ones coming from below ( as seen in photo 1) when properly installed. But, setting up the hull with this ‘ mould’ for the entire circumference of the tunnel ( as seen in photo 2) will cancel at least in part the desired effects. In fact some of the water deviated will in any case impact against the opposite internal wall of the tunnel. Photo 3 shows a tunnel built with no protective setup: it is easy to imagine how water rising from below and from the bow will hit up against the inside wall of the tunnel. • Hull setup aft of the tunnel – This solution is generally deployed on ships because it reduces drag more. As said it means creating a concave or shell like dip in the hull which will ‘ collect’ the flow aft of the tunnel preventing it from crashing into the tunnel’s stern walls. The concave dip starts from the aft side of the tunnel and tapers off to- wards the stern. This concave dip must be at least equal to the tunnel diameter and must slant downwards towards the stern to best adapt to the water flow. This solution will reduce drag brought about by the tunnel generally from 1 to 2 % but can be as much as 5% in case of small ships and / or when tunnels sport large diameters. Physical and virtual tests are carried out to determine the best angle of slant at diverse speeds to obtain the best compromise because the drag coefficient is high enough to warrant these. The flow angle is determined by the shape and size of the bow wave which varies accordingly with the speed of the hull. Photo 4 shows an impressive bow wave format generated by a large motor yacht cruising at 16 knots. Photo 5 shows the level of consistency of the flow on the hull, at the same speed using paint or ‘ drop’ methodology which translates into applying plenty of paint on the hull which is then towed at a given set speed. The ensuing water friction along the hull will drag the paint depicting the direction of the flow. The resulting ‘ picture’ made by the flowing water along the hull gives a good indication as to where to install the tunnel housing and the bow thruster ( in corre-
spondence with rib number 18) as paint tracks, show the angles of slant which can be as much as 20°. Pictures 6,7,8 show a fast military vessel proceeding at 29 knots: where the wave is not as steep and the tunnel is situated further astern ( rib number 17). Creating a concave shell like dip into the hull near the tunnel is after all easy enough to do and not expensive when compared to the benefits obtained specially on large yachts. Unfortunately this concept is nevertheless not clearly understood even by those who install bow thrusters. Consequently seeing very costly yachts worth millions, equipped with bow thrusters that have not been adroitly sealed and whose hulls have not been properly set up is no rare thing. In picture 9 there’s a large 30 metre luxury yacht which has no hull setup in proximity of the tunnel whatsoever and what’s worse, is a series of vertical rods have been soldered onto the bulb in the bow which is completely contrary to whatever principal concerning hydrodynamics: whoever thought this up has clearly no idea as to how hulls below the waterline function. Seeing concave shell like dips on planing hulls is another frequent eyesore, and is certainly not so befitting as the mould situated forward of the tunnel. In picture 10, thanks also to the way the picture was taken it is possible to make out why: when hitting a wave vertically water is not induced to slip off, but it is held in by the concave dip in the hull as if it were a spoon. This translates into a high pressure area concentrated locally engendering vibration and an immediate increase of drag. • Hybrid Solutions – Finally planing hulls are often seen sporting hybrid solutions which entail having both solutions installed: the shell like, spoon like concave dip aft of the tunnel as well as the mould installed forward of the tunnel ( photo 11). Even when these solutions can work well they should be installed accordingly with adequate project design work preceding installation. Many times these solutions are mere fantasy work by improvised designers and have no realistic justification. In photo 12 a mould forward of the tunnel is definitely visible but it continues also aft contouring the shell like dip to end up on the skates: a clear stylistic exercise! But will it be enough! It is obviously clear that solutions as the one in photo 13 cannot be justified technically speaking and are merely fruit of ignorance. What can be seen is a planing hull without a mould forward of the tunnel, while a concave shell like dip can clearly be seen but it is too short and therefore of no use at all. The actual concave dip is very deep and extends well above the tunnel. The result of this work translates into a sort of giant water collecting spoon which withholds water coming from below and from forward of it and produces vibration locally and increases drag. Surely a plain hole deployed as a tunnel would have been better and nothing else!
CMC is planning to present three innovative systems to broaden the firm’s current range, by transferring Stabilis Electra technology (electric stabilisers) to yachts under 20 metres with Short Range plants as well as HS , High Speed and LR Long Range ones. These have all been specially designed to suit planing yachts with speeds in excess of 24 knots, semi- displacing ones and considerably slower displacing ones too. This new machinery derives from much of the experience gained in recent years when Stabilis Electra first began to hit the market with a small but considerable revolution in 2009: to overcome the problem of low efficiency mechanical, hydraulic gyros applicable to smaller yachts, with a new electrical actuator which moves stabilising fins generated by DC electric motors which are lighter, highly efficient and more compact than previous systems. CMC’S intuition paid off and they are now the most important company in the sector. The range of Stabilis Electra – SE models has been
growing exponentially since 2013. Today there are about 30 possible configurations thanks to 7 diverse series of actuators coupled to as many as 14 different sizes of fins and to 12 classes of motors. SE systems can currently be installed into a wide range of diverse yachts from fast planing ones to slow displacing ones measuring from just 20 metres up to 80. Latest generation engineering means that SE guarantees easier to install more flexible systems. The latest series sports an auto or self adapting software under Italian patent called Dialogue which guarantees excellent performance and enhanced efficiency. Since Dialogue possesses more versatility it can integrate SE stabiliser systems with other CMC Marine gear such as electric thrusters of the company’s Dualis Electra series run through a single CMC Marine control station situated in the wheelhouse. This presents obvious advantages in terms of ease of use and monitoring. The integration into a single control station translates into accrued efficiency which is energy saving. At entry level of the SE range there’s the 40 model for yachts between 20 to 25 metres through to the largest 200 SE which is dedicated to yachts from 60 metres up with fins which vary accordingly from 0.4 square metres to 5.00 square metres. Flanking this, CMC Marine as mentioned earlier, to satisfy growing requests has produced three diverse systems divided
into three separate categories: the SR Short Range, the HS High Speed and the LR Long Range. The SR Short Range – registered in 2016 currently covered by temporary patent is suitable for yachts less than 20 metres. This product features great innovation, it involves a Brushless Torque electrical engine coupled to a reducer (There are three different power houses available). The design is unique, compact, weight saving, of low consumption and is easy to install. The system can also be powered by 24V DC. The HS system has been designed for planing yachts with cruising speeds above 24 knots.this latest, sports several novelties when compared to the previous range among which: optimized profiling of the fins which perform better and reduce drag. The adoption of new smaller more performing electronic components which can monitor the position of rudder blades and other performance related surface areas as well as the stabilizing fins themselves. The new LR system is ideal for displacing and semi-displacing yachts. Here again more performing and more compact actuators by comparison to previous SE systems delivering the same output offer obvious advantages flanked by new fin designs for displacing yachts with surface areas of up to 5 square metres. Thanks to new electronics offering more capacity it is now possible to integrate, monitor and run up to 8 separate moving parts below the waterline such as two pairs of stabiliser fins, rudder blades
and so on. Sensors to
monitor Roll, Pitch and Yaw can also be installed on request so as to further optimize the degree of comfort perceived on board. Alessandro Cappiello MD at CMC Marine commented as follows: “The new systems derive from constant feedback, owners’ requests and shipyards’ requests have highlighted their need for what was up until very recently an unaddressed request in the field of stabilisers. Well CMC Marine took on the challenge and responded by creating three specially devised new lines. When I mention my electrical plants I always say there are three advantages to be had: one for the shipyard, one for the end user and one for us. The yard because in installing electric stabiliser systems is by comparison easier, time saving in man hours. To which we can add lower consumption, less purchase cost, less bulk. For owners this last point is also relevant when
talking about size which plays a much more important role then otherwise. The owners find themselves with easy to run, cheap to use in terms of consumption, little maintenance and last but not least low noise levels. Talking of which, our plants’ average noise levels measured in decibel are around 40/45 decibel, which is considerably less than the 58/60 decibel hydraulic stabilising plants produce. But what’s important to us what is a great advantage for CMC is to have been able to produce a reliable long lasting product, which is also easily adaptable to diverse types of yachts, and is also simple enough to handle in terms of servicing”. CMC Marine plants have already been chosen among the most renowned Italian shipyards as Benetti, Sanlorenzo and Rossinavi, Azimut, Mangusta and Overmarine and by some European ones like Moonen and Sunseeker. For further information www.cmcmarine.com The Mangusta Grandsport 54 which is currently in construction at the Overmarine Pisa shipyard,is the first flag ship initialled by Alberto Mancini and is due to hit the water in mid 2018. It has been installed with CMC Marine electric stabilisers and bow thrusters. The advantages reported by the shipyard, highlight the low noise impact first and foremost since the noise levels near the guests’ quarters don’t exceed 45 decibels while preceding hydraulic models produced 80 decibels.
Photo 13 – What must not be done: no ‘mould’ forward of the tunnel while aft of it a shell like concave dip which is not long enough and therefore useless. The concave dip extends well above the tunnel and is very deep. The result is a sort of huge...
Photo 11 –Hybrid solutions on planing hulls: where concave shell like dips are performed aft of the tunnel as well as a ‘mould’ forward of the tunnel. It is conceivable this solution may work if carried out professionally following due project design...
Photo 12 – Bow end mould which continues aft contouring the shell like concave dip and tapers off. A clear stylistic exercise! But will it be enough?
Photo 10 – concave shell like dips in planing hulls: these are not recommended in planing hulls because when hitting waves vertically water is held back by the concave dip which acts like a spoon.
Photo 9 - Housing tunnel for the Bow thruster of a 30 metre luxury yacht with no hydrodynamic shield where no hull setup has been made.
On the left - Diagram of the flow without setup. In the middle - Diagram of the flow with setup forward of the tunnel. Better suited to planing hull. On the right - Diagram of the flow with setup aft of the tunnel. Better suited to displacing hulls....
On the left - Diagram of the water flow sucked into the tunnel when it hasn’t been adequately rounded off with the hull (whirling and cavitation). On the right - Diagram of the water flow inside a tunnel with flawless rounding off. Drawings taken from...
On the left – Diagram of the flow inside a tunnel which is too short. (whirling and cavitation) On the right - Diagram of the flow inside a tunnel which is adequate.
Selected positioning area for the thruster in the bow (Drawings taken from SIDE-POWER Thruster System catalogue)
Photo 2 - “Mould”installed on the tunnel’s entire circumference: where the effect of the lower/ bow part of the mould is partially reduced
Photo 3 – Tunnel with no hull setup: it iseasy to imagine how the water rising up from below and from the bow hits on the internal walls of the tunnel.