Lewmar 185TT Bow Thruster Installation in Mariposa

As far as modifications to your boat go, few are as satisfying as installing a bowthruster. Anybody can feel comfortable cruising around in open water, but docking in adverse conditions can be stressful for even the saltiest captain.
This winter we added a bow thruster to a 2006 SeaRay Sundancer 40 called Mariposa and now we're going to walk you through how we did it.



Step 1: Choose an appropriate thruster and install location


Given our experience with Lewmar Thrusters, we decided to find a Lewmar model that would suit the boat. Our first decision was deciding on a tunnel diameter. The smallest tunnel, at 140mm, seemed a little under-sized, so we moved up to the next option, the 185mm tunnel. We then decided on the beefier of the motor options, moving from a 3.0kw to a 4.0kw motor. This provides the boat with 5.4hp of directed thrust.



The location for the Thruster Installation

Install location is the next priority and often can be the downfall of a thruster install estimate. Bow Thrusters obviously need to be mounted as far forward as possible with the tunnel completely below the waterline for them to work effectively. Some boats will have them installed under a forward bed or even near the anchor locker. Some sailboats have a head in the bow and many things must be moved, rebuilt and re-routed to fit a bow thruster. Luckily, the layout of Mariposa allowed us full access to the subfloor with plenty of space for the thruster and all required accessories without any interior modifications required. An added bonus of the Lewmar 185TT is that the motor does not need to remain vertical and can be mounted nearly 90 degrees from vertical as long as it is supported by a bracket.

Step 2: Measure, measure, measure!

When cutting an 8" hole straight through the boat, it's very important to make it in the right place, especially when you have to cut from both sides and have them be perfectly aligned.
First we made sure the boat was level on its jack stands. Then we measured from the bow and the stern to give ourselves fore-aft marks for locating our center holes. We double-checked this by finding thru-hulls near the install location and measuring the distance from the thru-hulls to our ideal pilot hole location. Following this we triple-checked all of our measurements using plumb bobs hung from the deck as well as measuring with string from the bow eye. Once we were satisfied that we were correct in our holes (remember, we have to drill through the same location on both sides of the boat), it was time mask off and drill a pilot hole.

Step 3: Mask off the interior!

Mariposa has a beautiful interior complete with white carpeting, Corian countertops, one of the shiniest dining tables I've ever seen and spotless upholstery. The last thing we want to do when working on a boat is to ruin the interior so we masked off the entire floor with paper and hung and taped plastic to keep dust contained to the work area. We then cut the masking paper to allow access only the places we needed to work.



Step 4: Cut Holes in the Boat!

This is without a doubt the most stressful part of the process, but it has to be done. We started with a pilot hole in each side. We drilled both to 1/4" and inserted a long, threaded rod to gauge our alignment. Luckily all our measuring paid off and our pilot holes were perfectly aligned and level. If we had been off a little bit, it would not have been the end of the world. We would simply have re-measured and corrected the error. As long as our offset is less than the radius of the tunnel, the holes won't matter since all that material will be removed anyway. Since we're professionals, we were proud not to have that problem.

With the pilot holes drilled and aligned, the fun could really begin: removing an 8" diameter circle of hull. We created a crude compass using our guide rod to give us a guide line for the cut. We then fitted an electric drill with an 8" hole saw, used our threaded rod as the guide bit and went to town. Mariposa is a very well-built boat so we had to cut through nearly 2" of solid fiberglass in spots, so we took turns wrangling the drill, alternating sides from time to time so that our guide rod could do its job while we got our cut started and pretty soon we could see straight through the boat.

Step 5: Fine Tune the Hole

Once the hole was drilled it was time to switch from demolition mode to rebuilding mode. We ground out any uneven bits around the circumference of the hole and opened up a few pinch points that kept the tunnel from sliding through. We got the tunnel to fit nice and snug, but with enough wiggle room to rotate it, which is important for dry-fitting the whole installation later. Just because the tunnel fits doesn't mean that we're done with preparation for install. We still had to remove paint and barrier coat around the area and bevel the fiberglass to accept the tabbing we would be gluing on to secure the tunnel without disturbing the lines of the hull. We also took this opportunity to sand the interior of the install area to accept fiberglass and epoxy  for the tunnel install as well as the support block.

Step 6: Dry Fit

Before we glued everything up, we wanted to get everything lined up properly, so we cut holes in the tunnel for the drive unit to attach to the thruster motor and attached the thruster and drive to the tunnel. We shifted the tunnel side to side through our hole until our prop was at the boat's centerline. On a Lewmar single-prop thruster, the prop faces to Port, so the motor assembly must be shifted to Starboard to allow for proper alignment. Satisfied with our lateral alignment, we then decided how far from vertical we would need to lay the motor in order to comfortably hide and support the unit. We cut our support block, assembled everything and checked for issues.

Marks were placed on the tunnel and the interior of the hull to make sure we could repeat the exact angles when it came time to glue. We took the support block out and coated it in epoxy and over-drilled and re-filled the holes for the screw mounts to keep the block dry. We also installed the prop in the tunnel and ensured that it could rotate freely without any risk of striking the edges of the tunnel. This is an important thing to verify when there is still an opportunity to re-align the drive holes in the tunnel.

Step 7: Bond the Tunnel!

After triple-checking and disassembling the unit, we reinserted the tunnel, aligned our marks and put thickened epoxy around the tunnel, giving it an initial bond to the hull and locking in our alignment for good.

The thickened epoxy secured the tunnel but doesn't offer strength to the hull, so the next few steps are standard fiberglass steps used when attaching an appendage to a hull. We cut the tunnel near the hull and ground it close to flush. Notice in the next picture that we left a flange of about 1.5" on the forward edge of the tunnel. This is required for water flow when the boat is in normal use. Without this flange, water will force itself into the tunnel, creating noise and drag when underway. It becomes more pronounced as we continue to build up the fiberglass that bonds the tunnel to the outside of the hull.

We also need to bond the tunnel on the interior of the hull. We laid additional fiberglass tabbing around the tunnel inside the boat. This is one of those times that you remember just how important that masking step was. Running in and out of the boat with cups of epoxy can be worrisome enough, so it is nice to have confidence that any mishaps can be contained.

On the exterior of the boat, the tunnel bonding is now treated much like a standard fiberglass repair. We sanded it to shape, applied fairing compound, sanded that back down to blend our flange and restore the original strakes, applied barrier coat and antifouling. This concluded the fiberglass portion of the install. We then moved back into the boat for assembly and wiring.

Step 8: Controller Install

Lewmar Bow Thrusters can be controlled by either push-button controls or joysticks. The owners of Mariposa opted for a joystick controller, so we had to choose where on the helm to place it. Thinking of the most common docking scenarios, we decided that the controller should be as close to the steering wheel as possible for fingertip control. We also decided that when docking, the shifters would be used rather than the throttles, so the joystick should be mounted opposite the shifters to allow for the smoothest possible interface. Directly in our way was an ACR Spotlight controller which was a cinch to move and with a quick hole saw cut, a new joystick was installed on Mariposa's helm. 

Step 9: Electrical Install

Bow thrusters draw a lot of juice in short bursts, so a dedicated battery is a good idea. Again, we lucked out with Mariposa having plenty of room for a battery, charger and battery switch just aft of the thruster. The wiring is very straightforward, and the boat was already equipped with a spot on the 110V panel for a bow thruster battery charger, so all we had to do was fish the wires, install the battery, charger and switch, and plug it all in.

Step 10: Final assembly

With everything in place, it was time to mount the thruster motor and drive unit to the tunnel with sealant, lock-tite the bolts and tension them to specifications, install the propeller and anode and properly torque them. We rolled white gelcoat over our interior installation to make it a bit prettier if anyone decides to look at our handy work and cleaned up our cabling.

Step 11: Clean Up
We want our owners to return to a boat that is in better shape than they left it, so after pulling all the masking, it's time to give a cleaning to the interior to get any stray dust and leave the boat confident that everything is back in place.

Step 12: Enjoy!
The owners picked up Mariposa on Sunday and demonstrated how well the bow thruster worked for them as they pulled away from the dock.