If you ever feel you need attention in the boatyard just cut two huge holes in the bottom your boat. You’ll have people coming from far and wide to see what you are up to.
I recently installed a bow thruster in my boat and I had much more company than I had planned on. I would see them off in the distance checking out my work, they’d come a little closer and ask, “Putting in a bow thruster?” I’d answer affirmatively and they’d give me a puzzled look. “Are you installing that yourself?” Since I was typically up to my elbows in fiberglass and epoxy, I gave them an equally puzzled look and informed them that I was indeed tackling it myself. Most folks would wish me luck and walk away. Others showed more concern and asked if I had gotten some guidance on how to do the job. This exchange happened virtually every day that I was working on the project.

Boats keep getting bigger while crews are not. Today’s typical cruising boat is roughly 45 feet long and is still crewed by a couple. A bow thruster can make handling that boat easier, safer and just more fun. While bow thrusters are common options on new boats, they easily can be added to almost any boat, and for a lot less money than you would think. I sail a 47-foot, 38,000-pound boat, primarily with my family, and I’ll be the first to admit that it is a handful around the docks. The thruster has made things far less stressful and even gives me the ability to singlehand.

Step one
The first step in any major project is planning, and this is especially important in a thruster installation. The placement of the tunnel is critical to the performance of the thruster. You want the tunnel to be as far forward as possible to allow for maximum torque to rotate the boat, and at least one-half the tunnel’s diameter below the waterline to ensure that the thruster will not suck air from above. Of course, you need to make sure that the tunnel
and motor will fit into some interior cabinetry.

Once you have an idea of how much room you have and the possible positions of the tunnel, the next step is to choose the size of the thruster for your boat. Manufacturers can give you an idea of the thruster required based on the boat’s size and design. A flat-bottom, low-windage boat will require less power than a deep-keeled boat with a lot of windage. The manufacturer will typically recommend a few models that will work. My advice is to get the largest recommended model. You can never have too much thruster power, and while a larger thruster will cost you a bit more I feel that the most expensive thruster is the one that will not move your boat correctly. After careful measurements and consultation with the thruster experts at Imtra, I chose a Side-Power model SP125T.

Making holes
The most stressful and potentially difficult part of the job is cutting the hole for the tube. Aside from the obvious psychological challenge of cutting two large holes in a perfectly good boat, you are not cutting a round hole. Of course the tube is perfectly round, but when that round shape is projected onto the compound curve of the hull it becomes an elongated oval shape. The trick to locating and cutting the holes is to first mark and drill a center hole on each side of the hull. These holes must be precisely located because they control the position of the tunnel in the boat. I spent many hours with a tape measure and a level making sure the holes were positioned correctly.

With a good center hole, the tunnel holes become far easier to mark and cut. A quick and dirty method to mark the tunnel holes is to use a length of metal rod that can be bent into rigid J shape with squared corners. The long leg of the J is passed through both holes to keep things centered, the width of the J is set to the radius of the tunnel, and the short leg marks the hole on the hull. Once marked, the hole is simply cut out with a jigsaw or reciprocating saw.

A far easier way to cut the hole is with a purpose-built tool sold by Side-Power. The tool is essentially a large-die grinder with a solid carbide burr attached to a large rod designed to pass the center holes in the hull. I did not have direct access to one of these tools, so I turned to the great team at Manitowoc Marina in Manitowoc, Wisconsin. I drilled the center holes to size, and Jim from Manitowoc Marina was able to cut the holes in about an hour. After just a little grinding, I was able to slip the tube in place.

I had not done a lot of structural fiberglass work before tackling this project, so I started with a consultation with Jack Hall, the fiberglass expert at the boatyard. Jack suggested that I grind the hull back to a 12-to-1 scarf joint on the outside of the hull and grind enough on the inside to get to good, clean fiberglass. He suggested that I should use biaxial fiberglass tape and West System epoxy. Biaxial tape is unwoven fiberglass with fibers oriented 45 degrees opposed, the sandwich of fibers is held together with a few rows of stitching. The design of the tape allows for maximum fiber density per layer, giving maximum strength.
The worst part of the job was the grinding. My hull is about one inch of solid fiberglass in the area of the thruster, so I needed to grind an area of about eight inches around the tunnel opening and grind away a lot of material.

It took me a few days to grind everything, inside and out. I tried a variety of grinding wheels and had the best results with a 36-grit sanding flap disk—essentially a bunch of sandpaper flaps overlapped and glued to a fiber backing disk. Eye protection and a respirator are essential during this very dusty process.

Glassing in
With the tube fitted and the hull ground, it was time to fiberglass. The first step was to glue the tube in the hull with a large fillet of thickened epoxy. The epoxy keeps the tube in place during the lay up and the fillet makes for a nice rounded corner to lay the fiberglass tape over (it is hard to lay fiberglass into a square corner). I had imagined that I would lay up the tape circumferentially around the tube, but Jack informed me that I would be better off to lay up shorter pieces of tape around the tunnel arranged like daisy petals. The fiberglass work was pretty easy, it was a steady rhythm of wetting out the cloth, laying it on, and using a squeegee and roller to remove the excess epoxy and work out the air bubbles. I laid up about 10 layers of fiberglass on the outside of the hull and about 15 on the inside.
After the glass was cured, I cut the tube to length, ground it to shape, sanded everything and faired the area with thickened epoxy. The shape of the tunnel ends is critical to the performance of the thruster. You want the corner where the tunnel meets the hull to be nicely rounded. This rounded edge smooths the flow, keeping things quiet and enhancing the thruster power. Side-Power claims that properly shaped tunnel ends can increase thruster power by up to 40 percent. The front edge of tunnel should be left a little long and faired into a “spoiler.” This shape directs the water flow around the tunnel, keeping things quiet underway and actually reduces drag.

I gave the new epoxy in the thruster installation area the same treatment as the rest of the bottom of my boat, six coats of Interlux InterProtect 2000E primer and three coats of Interlux Micron Extra bottom paint. Much to the chagrin of some of my boatyard colleagues (I am known to go to extremes when doing boat projects) I faired up the fiberglass on the inside of the hull and painted everything with gloss white Interlux BilgeKote.

Mounting the motor
The next step was to mount the motor and gear leg onto the tunnel. The placement of the gear leg is critical, as there is only about an eighth-inch of clearance around the prop tips. Side-Power supplied a template that made the job a lot easier. After drilling the holes, the motor and gear leg were bolted on and a generous application of 3M 5200 was applied to keep everything watertight.

A thruster is likely the largest consumer of electricity on your boat. The motor is very high current, but only used for a few seconds and typically always with the engine running. I had the choice of either a 12- or 24-volt system, with the 24-volt system operating at half the current of the 12-volt system. The lower current allows for smaller cabling and gives longer solenoid life. Of course, a 24-volt system in a boat with a 12-volt charging system is a hassle. Luckily, Side-Power provided a conversion box to allow the batteries to charge from a 12-volt system, but still run the 24-volt thruster. I already had one battery in my bow to run the windlass, so I just added another to supply the 24 volts for the thruster.

Side-Power makes a full complement of controls for the thruster: the classic joystick, a two-button panel, and even a little panel shaped like the boat (you just push the bow in the direction that you want it to go). I chose a simple two-button panel. It was low profile so I’d have little chance of snagging it with the spaghetti in my cockpit. I also opted for a small wireless remote. This is a little indulgent but very handy. I have found it useful when docking singlehanded; I can control the position of the bow while standing anywhere on the boat.

With the thruster install complete, the time had come for the test drive. I carefully backed out of my slip and hit the button to push the bow up into the wind. With a subtle roar (a thruster is a lot of things, but quiet is not one of them) and a gush of water, the bow easily came up into the 15-knot breeze.

Earlier this summer I was cruising in southern Lake Michigan and stopped at a marina in Michigan. I got my slip assignment and proceeded to put the boat in the slip, it was a breezy day and the slip was very tricky to get into—pilings on both sides and very little approach space. With judicious use of the thruster I was able to spin the boat, slip between the pilings and get into the slip. When I went to register, the dockmaster said, “Good thing you have a thruster, most folks don’t make it in that slip without damaging something.” Needless to say, I was unhappy with the dockmaster, but very happy that I had installed
the thruster.

While adding a bow thruster to your boat is an advanced do-it-yourself project, my main intention here was to cover the installation steps and show that it is indeed possible, and even easy, to install a thruster. However, if you are not 100-percent confident in your fiberglass and electrical skills I recommend that you leave the installation to a professional.