Adding a Bow Thruster.

I thought it was a toyish addition to the boat and resisted adding it. The thrill of docking in a slip with a single screw boat, with a cross wind of 20 knots and current athwartship, watching the crew unable to hold the bow due to the heavy wind loads, all these factors contributed to the decision to bite the bullet and install a thruster. They are not critical at moorings, but so critical around docks. Oddly, people in the yard started knowing me as the "guy who installed his own bow thruster. And, I must say it was not difficult, the only nervous moment of course is to cut the hull, eventhough everything is repairable, a mistake would have been costly in time and effort.

Probably half of the total effort was spent dealing with clearing the volume of the thruster and building a new holding tank out of epoxy/glass as well as the support shelf for the tank (the tank weights about 320 lbs full at 40 gallons, necessitating a good support), and replumbing the discharge and vent systems including diverter valves.

A little advice for those who are interested:

In order to ensure a minimal disturbance to the bow wave of the boat, and to reduce turbulance from the large tunnel, a NACA foilshape was designed to fit the geometry of the hull. Abott and Doenhoff "The Theory of Wing Sections", Dover press. Templates were plotted and drawn against the hull to ensure symmetry of the system from Port to Starboard.

NACA foil shape

The NACA foil above dfefines the shape outline for the bow thruster intake, in the plane of the hull.

Perpendicular to the plane of the hull, the NACA foils are constrained by the height ot the tunnel above the hull (how much it sticks out) and the desire for a tangeantial fflow at the leading edge of the tunnel cavity.

Height profile foil shape

The NACA foil above once, designed, was used to template the shape outline for the bow thruster "bumps" which enable lamniar flow and reduce cavity drag. The templates were made of thin balsa, and underwater epoxy filler material was used for the build-up. The process took two days and three or four quarts of filler.

  • Select a unit that will fit properly, based on measurements: . In my case, I had hoped to install retractable unit.
  • Upon measurements, I found two candidate retractables. One did not fit, and the other used aluminum casings in underwater components, and these did not fare well in salt water. One unit required a 13 inch deep V cut in the bow, and this made it a no go. I had to settle for a tunnel unit.
  • Measure carefully, taking in accound tube diameter and motor dimensions.
  • Measure again, more carefully.
  • Measure inside and outside, using a through hull as a reference point,
    making sure all the measurements are verified and cross check.
  • Did I mention you need to measure with some precision?
  • Drill a pilot hole
  • Observe with great satisfaction that the pilot hole is at the correct depth and location relative to the water line, if you started inside.
  • Observe with great satisfaction that the pilot holw is at the correct location inside, if you started from the outside.
  • If the hole is in the wrong place by more than one tunnel radius, consider hiring a professional.
  • If the hole is in the wrong place by some small offset, drill another hole.
  • Enlarge the hole to 3/4 inch diameter. I used a Forstner bit. Chances are you can throw it away after the two holes, but hey how much is the thruster?
  • Using levels, a laser, scales and squares, drill the opposite side, first with a pilot, then with a 3/4 inch diameter bit.
  • Use a block of wood of the correct diameter, with a 3/4 hole in it, and slide it on the threaded rod. A pencilglued to the wood will mark the outer diameter of the tube..Alternately, buy the special tool which holds a die grinder or procure a very large hole saw.
  • Once the hole was marked, I used a sawzall to cut out the bulk of the material, staying at a respectable distance from the line and finishing with a grinder. In order to get the fit perfect, I used the two disks to hold the tube and was able to mark the exact tube diameter as I slid the tube numerous times for fit and ground small amounts of hull to fit to within 1/16 or 1/32". It took a day for the whole process of cutting, but I went slow and easy and this prevented big mistakes.
  • 0.5" of alternating biax, 45/45 and 0/90 1708 fabric with mat were used with about two gallons (or something like that) of West epoxy 105 resin to reinforce the hull within 15 inches of the cutout. This was done with proper prep to bare glass
  • The tube was glassed in place, and glassed on the outside of the hull
  • I designed NACA foil shapes to reduce turbulent flow on the hulll. These foils shapes seem to have no effect on the bow wave. I think they increase cavitation when the vessel moves forward, and decrease it when it moves backwards. THis is not a bad thing. I am looking for better fairing suggestions from hydrodanamics experts.