Many boatbuilders do an inadequate job of bowthruster installations, and Mainship is one of them. The installation manual details proper installation with regard to the bow thruster location/positioning and tunnel end fairing. This is critical to proper function of the thruster. Following is an excerpt from the Imtra manual:

We recommend to round the tunnel connection to the hull-side as much as possible.
The optimum rounding has a radius of 10% of the tunnels diameter. Important advantages over sharp tunnel to hull connections are:
1. The rounded tunnel end will prevent creation of turbulence / cavitation that will come from a sharp tunnel end when water passes by fast, thereby preventing a double negative impact on the thrust and noise level (Fig. 1&2 ).
- The turbulence / cavitation blocks the outer area of the tunnel and thereby reduces the effective tunnel diameter and thrust.
- The turbulence / cavitation hits the propeller and thereby reduce the propellers performance and creates noise.
2. The angled tunnel end makes the thruster take water also from along the hull-side, creating a vacuum that will suck the boat sideways and thereby give additional thrust (Fig. 3&4). With a sharp tunnel end, the thruster will be unable to take water from along the hull-side, and you will not get the desired vacuum and additional thrust.
This "free" additional thrust can in optimal installations be as much as 30-40% of the total thrust.


The installation on Stellablue was unhappy as in figure 1, not only for the sharp corner of the tunnel ends, but for a poor filling and sealing of the tunnel end/hull joint. This led to water seeping into the joint and and could have led to greater problems down the road.

The picture above was taken in the fall of 2005 when I notice water leaching out of the tunnel end/hull joint. After sanding the bottom paint away, cracking of the resin coating and filler was evident. After grinding a little deeper, large voids in the joint were found. The filler was a soft "Bondo" type filler and was a poor choice for this use here. This was the subject of another article in 2006, so I won't go into much detail here.



Mainship agreed to pay for a repair and a local fiberglass guy did the job. He ground out all the Bondo and used fiberglass and polyester resin to fill the joint. At that time I had not looked into the Imtra manual about the correct fairing of the tunnel ends. All seemed well other that my thruster seemed very noisy and seemed to cavitate at times. At the end of the 2013 boating season, I decided to do a bottom job. Some of the bottom paint was delaminating from the hull and I could see the factory had done a poor job of breaking the gloss and prepping the gelcoat for bottom paint.

I had the bottom paint removed by blasting with walnut shells. It did a wonderful job of removing the ablative paint, but still left the bottom shiny and in need of sanding before painting. The guys doinig the blasting commented that this was the best bottom they had ever seen. The only issue they saw was a little moisture at the bow thruster tunnel ends. Having gone this far, I decided I would barrier coat the bottom as a preventative action, and also properly finish the bow thruster installation. 

By now I had done some research and I wanter to get the most out of my bow thruster. I started by grinding back the tunnel ends to achieve the minimum corner radius which for a 7" tube would be .7 inches. In doing so, I uncovered some more voids and thus the reason for moisture in that area as the blaster had mentioned.

I also wanted to build out the forward portion of the tunnel to make for a smoother flow as the boat moved through the water and the lower portion were the angle between the hull and tunnel are so acute it would have taken excessive grinding to get the proper .7 inch radius.

I started building out using Pettit Spash Zone epoxy putty #A788. I made a simple radius gage out of cardboard and sanded the epoxy build out into shape.


 After getting the rough form into place, I used some Pettit EZ Fair compound to do some final fairing and sanding before covering it all with Pettit High Build Epoxy Primer.

I completely sanded the entire bottom with 60 grit and applied 3 more coats of the barrier coating. Then finished it off with Hydrocoat SR.