Over the period I've been messing about in boats I've never owned one with twin screws (two motors). There's not been any particular reason for this, although my bias towards displacement hulls means most of the boats I could afford had only one engine.

When you get used to the idiosyncrasies of your single screw craft, manoeuvring in tight spots isn't too difficult. The odd blunder keeps the ego under control and humility levels in balance.

After one such self-esteem-shattering experience - when I couldn't get the bow around in time and gave the jetty a severe clout - the subject of a bowthruster sprang firmly to mind.

So just what is a bowthruster? Well, imagine a small outboard motor attached to the front of the boat, but mounted so the propeller is at right angles to the centreline of the hull.

In other words, when the propeller turns it pushes the bow either right or left, depending on its rotation. Think now of the added manoeurvability such a device lends, particularly if the boat only has one engine. It allows the skipper to approach a crowded marina with a lot more confidence, and leaving a jetty against a pressing wind is a dream.

Until a few years ago, bowthrusters for small boats, (below 20m) were rare. Now the situation has changed dramatically with the introduction of small yet powerful tunnel thrusters driven by electric motors.

Although a thruster can be mounted in a boat's bow in a number of ways, the tunnel method is by far the most common and popular. In a typical installation, two holes are cut into the bow and a tube (or tunnel) inserted. The actual propeller is mounted in the centre of the tunnel and driven through a gearbox by an electric motor mounted on the outside. The propeller will either thrust the bow to the right or to the left, depending on the rotation of the motor.

SIZE MATTERS
The first question most people will ask when considering a bowthruster ishow big?

Be guided by your supplier is the answer.

As in most things, power is everything, and fitting too small a unit is pointless. Fitting a thruster larger than you really need just means you have a little more grunt than necessary - never a bad thing.

As a rule of thumb, for boats of 10-20m, 0.75kW per 3m of boat is a good guide. In other words, 1hp for every 10ft.

In my case, I have 4hp for 32ft of boat. That may seem a trifle large, but remember I live onboard, and as a consequence, the boat sits deep in the water. In other words, I'm about two tons overweight and need a bit of power to shove the nose around!

The second question generally asked is how much? On the expensive toy list, bowthrusters really rate. A 4hp unit costs between $2000-3000, and nearly as much again to be installed professionally. One of the reasons installation is expensive is the amount of time the boat is out of the water incurring hardstand fees.

The level of skill required is quite high. Accurate measurements, fibreglassing skills and electrical ability will all be needed if you attempt the installation yourself.

Having said this, I installed a thruster on my boat a few years ago and just recently helped a friend to install a unit on his. Both worked well, and both boats are still afloat, so it's not rocket science. The dollar saving was substantial, and the sense of satisfaction in a job well done is beyond value.

Thus, let's have a look at the procedure from start to finish. But remember, this is the way I did it, not necessarily the best method. It suited me, and I was happy with the results, but ultimately you should follow the manufacturer's recommendations at all times.

WILL IT WEAKEN THE BOW OR SLOW DOWN THE BOAT?
First, let's get a few worries out of the way.

Will the installation make the boat weak in the bow area? No, in fact, it tends to strengthen it because a stiff transverse member has been glassed across the bow.

Will it slow down the boat? Yes, there is an element of drag, and I suppose this will be more noticeable at higher speeds than lower speeds. Personally, I couldn't discern one iota of difference. Then again flank speed in my boat is only 8kt anyway - I owned a yacht for years, and still think of 8kt as fast! A 25kt planing hull might experience greater speed loss.

Lastly, in the event of a severe collision, could the tunnel fracture? I suppose it could. The tunnel supplied is only about 6mm thick, more than adequate for the task, but if the boat was to hit a reef in that area at top speed, then a Titanic re-enactment could result.

The way to allay this concern is to build a watertight bulkhead behind the thruster. If any water leaks into this area, it's contained by the bulkhead. The volume of water won't be enough to lower the bow so that water overflows the bulkhead.

I also fitted a pump, float switch and an alarm in the bowthruster area, so if anything ever lets go and water starts seeping in, I'm warned about it. (I know, belts as well as braces, I can't help it.)

The usual way to install a bowthruster is to place the boat on a hardstand and cut the holes for the tunnel through the bow from the outside. This is the easiest method and is recommended by most suppliers.

The disadvantage from a cost point of view is the time spent out of the water. Also, the work must be done all in one go. For most of us that means taking some of our holiday time and putting in three or four days of very hard work.

Hence my 'inside-out method', which has the advantage of allowing most of the installation to be done while the boat is in the water. This meant I could take a more leisurely approach, and do most of the work on a part-time basis over a few weekends.

The first step is to clear the bow area in the intended area of installation. This may necessitate the removal of items such as timber bunks, because the whole area needs to be as clear as practicable - grinding and glassing are difficult enough without having to do them in close and confined areas.

The tunnel should be mounted in the bow as low and as far forward as possible. If the vessel is pushed sideways at the bow, the hull tends to pivot around the rudder, so the further forward the thruster is, the more leverage it has over the pivot point.

Mount the tunnel as low as possible. The minimum recommended distance from the centreline of the tunnel to the waterline is 200mm (8in). Be careful not to select too low a position, as the tunnel will be extremely difficult to glass to the hull if there is insufficient room between it and the hull.

DOWN TO THE WATERLINE
In order to get the correct location, it will be necessary to know exactly where the waterline is from the inside of the boat.

To ascertain this, take a length of clear tube, 10mm or so, and fill it with water. Place one end of the tube in the water and bring the other into the forward area of the hull through a window or hatch. Lower the tube until water starts to come out of the end due to s iphon action, then raise and note where the water sits in the tube. By holding this against various parts of the hull and marking, an accurate internal waterline can be established.

The major difficulty to be faced when installing by the inside-out method is determining the correct length and angle to cut the tunnel.

This is overcome by making a template of the bow using light craftwood approximately 4mm thick. Two pieces of this are cut to a rough shape of the area and placed against either side of the hull in the location where the tunnel is to be located. Light pieces of timber are attached across the hull to these pieces of craftwood - either glue, glass or screw, whatever is easiest. The idea is that these pieces of craftwood can be lifted out as one item, and will form a perfect template of the hull's internal shape where the thruster is going to be mounted.

When the template is finished and in position, mark the centre of the tunnel hole on each side. Take care that the height is the same on each side. The best way to establish this is to mark down from our internal waterline.

Make sure that this centre mark is on, or below, the recommended distance below the waterline.

Check also that the tunnel centreline is at right angles to the centre (keel) line of the boat, generally this is fairly easy to determine by eye.

Finally, check that you will be able to properly glass the underside of the tunnel if it's located in the desired position. Ideally, you should be able to fit your hand all the way around the tunnel.

Carefully lift the template from against the sides of the hull and take it somewhere where you can work comfortably. Using the marked centres as a reference, mark a line that represents the lowest point of the tunnel and make sure it's the same on both sides. Now cut a hole through each piece of craftwood to the diameter of the tunnel. Under no circumstances should the hole descend below the lowest-point-of-the-tunnel line. Keep nibbling away at the holes until the tunnel can pass right through the template. Don't be too concerned at the shape or raggedness of the holes in the template, they could be square and it wouldn't matter as long as the bottoms are the same height - this is the only important thing.

Once the tunnel pipe has been passed through the template, mark around the tunnel tube from the outside of the template and remove. Using a grinder with a cut off disc or a diamond wheel, cut the tunnel tube around this mark.

The tunnel can now be fitted to the bow.

GLASS ACT
Now check the tunnel tube's position, and trim with the grinder as necessary until a reasonable fit is attained. The odd small gap here and there is of no consequence.

When the location is satisfactory, mark around the tunnel on both sides of the hull and remove. On each side, there should be the shape of an ellipse.

Mark roughly the centre of each and, positioning the drill parallel with the water level, drill right through the hull with a very small drill - no bigger than one-eighth of an inch! As the hole you have made is below the waterline, water will come in to the boat. Use a self-tapping screw to seal off the hole and stop the water. Cover the screw with silicon or similar to ensure a seal. Do the same for the other side.

This hole provides a reference point for when the boat goes onto the hardstand... More on that later.

Now is the time to glass the tunnel to the hull. Scuff the inside of the hull as well as the end of the tunnel with a grinder to provide a good key for the glass mat.

Use 300gram mat cut into small pieces and thoroughly saturated with resin.

Glass completely around the pipe for a distance of 120mm along the tunnel and hull. A minimum of at least eight layers of mat will be needed, and this should be built up over a matter of days. The whole area should be finished with flow coat.

Now, the motor base can be attached to the tunnel as per the manufacturer's instructions. Generally, the motor can go in any position from horizontal to vertical.

If it's in the horizontal position, a motor support will be necessary.

All the wiring can now be completed. Remember that a four horsepower thruster will use 3000W of power, which is 300amps at 12V, so use very heavy cables to the unit - 70mm2 or better.

With the high power demands of a thruster, clean, tight electrical joints are essential to eliminate voltage drop, as this will rob the unit of energy.

Any further timberwork can be completed at this point, and if a watertight bulkhead is to be fitted, along with pump and high water alarm, this can also be done.

ON THE HARD
The installation is nearly finished. The final thing is to complete the underwater work, and for that we have to bring the boat out of the water. It's nice if this can be arranged to coincide with the annual lift.

I allowed one day to do the bowthruster, but I'm in Queensland where it's warm and resins, paint, etc, cure quickly. Elsewhere it's much better to stay up over a weekend.

With the boat out of the water, the bow can be inspected for the previously drilled holes. These give a reference to the centre of the tunnel.

Use a 50mm hole saw and bore through the hull in the location of these holes. From there, using a small grinder, break out the hole to the edge of the tunnel and grind this edge as fair as possible.

Use a good quality filler to fill any voids found and sand the filler fair.

Scuff the whole area and glass the interior of the tunnel to the hull, using the same technique as described previously. Lay up at least four layers of mat and, when cured, grind fair and fill if necessary.

Fit the propeller leg to the motor unit as per the manufacturer's instructions. Apply antifouling paint to the whole unit, and then fit any supplied anodes. Paint the whole area, including the inside of the tunnel.

Oh, and don't forget to test the unit before going back into the water.

THE VERDICT
So there it is: installation by the inside-out method. The savings on hardstand time are substantial, and by doing the installation yourself, the labour savings reduce the overall cost to under $3000.

Finally, are bowthrusters worth the time, effort and $$$?

Definitely. But if you weren't much of a boat handler before fitting a bowthruster, then one of these won't help all that much.

What they allow you to do is control the situation to a higher degree, but you still need the confidence to move the boat around with authority. Generally that confidence only comes with experience - good and bad.

Like most things in life, the more you do it, the easier it becomes.