SAILING FROGSLEAP

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Updated Aug2023 DIY

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 in sweaty palmed composed panick asthe crew is unable to hold the bow. Wind loads are heavy on a 50 foot boat and this one is less than some more modern designs.

I took the plunge, after talking to many skippers. Unnecessary for most docking situations, a thruster is like extra help against skipper and crew mix ups. And that does not even include the dock hands who seem to be trained only in self professed expertise in all matters vessel handling.

I came to be known in the yard as the crazy guy who installed his thruster. Trust me, it was not without fear and ample planning time was taken.

I must say it was audacious to cut the hull, a mistake would have been costly in time and effort. Yet, the work was straight forward fiberglassing with epoxy, once you got over the idea of making two 8 inch giant holes at the bow which could sink your boat in 5 minutes. It took a few weekends to get this done, and I will explain how.

The first challenge is that there was a holding tank, and a structural bulkhead which extended slightly above the waterline, in the area where the tunnel should go. So the first step was a new holding tank of fiberglass epoxy, which is compatible with acids, fomaldehyde, sea water, etc. ( I am amazed actually that some manufacturers like Island Packet use aluminum for holding tanks).

The Sidepower SE100 is sufficient, tp 20-25 knot breezes. After that, you hope your skills and timing is perfect and must decide if you shoud anchor and wait.

For power, most thrusters use a deicated battery bank, in this case two would be typically used. This allows a 24 V thruster, and the batteries are replenished with a DC/DC charger feeding off the house bank, or AC, or the engine. The consequence is a high current demand on two batteries, and a deep and fast depletion of charge. I decided to tie the thruster to the house bank, and this allows normal charging of all batteries at the same time, but required a heavy wire bus ( 3/0 or 4/0) from the aft bank to the forwrad thruster batteries. And this also allowed me to dedicate three batteries to the thruster, so all the batteries suffer equal stresses. A cutoff switch is in the area of the thruster. It is switched off whenever we dive the bottom. The wire bus is fused, and each battery in the house bank ( 8 of them) is individually fused.

A few pictures of the Installation

The holding Tank that was removed, The new holding tank, The thruster tunnel glassed in place. The thruster motor mounted on the tunnel. The battery area below the forward cabin.

  • Select a unit that will fit properly, based on measurements:
  • Avoid retractable units. I have seen too many yards fixing them over the years.Simple is best. for example, one retractable used aluminum casings in underwater components, Also, structurally, I preferered a tunnel to a large 13 inch cut in the Vee of the bow.

  • Measure carefully, taking in accound tube diameter and motor dimensions, as well as specifications from the thruster manufacturer. Proper depth below water is important. This is the dilemna; The further the thruster is forward, the better the leverage to turn the bow, but on the negative side, the shallower the bow is.
  • 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.
  • Drill a small ( 1/8 inch) pilot hole and observe the location both inside and outside the hull
  • If the pilot hole is at the correct location proceed, other wise drill another at some offset height and horizontal distance, using the first hole as a reference..
  • If the hole was in the wrong place by more than one tunnel radius, consider hiring a professional.
  • 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.
  • Ideally, the specialized tool as in this article: Power and Motoryacht Article But this is the expensive solution. There are also installation tools advertised which hold grinders in the proper circle distance, these are on the web and can be searched readily.

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  • The low cost solution: Dill a block of wood ( 4 inches thick to prevent wobble) preferably on a drill press to ensure squareness. Use the same size hole as the threaded rod. Glue a pencil to the wood, at the same distance as the outer diameter of the tunnel rube. Rotate the wood to mark the projection of the tunnel edge to the outside of the hull. Now you have a circle projected onto a three dimentional arbitrary surface.
  • Once the hole was marked, use a sawzall to cut out the bulk of the material, staying at a respectable distance from the line and detail it with a carbide bit on a die grinder. finishing with a grinder. In order to get the fit perfect, I used 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. The easier way is a specialized tunnel cutting
  • 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 hull. 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.

    • St Augustine Current

    Bow Thruster may not help here

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