The Bow Thruster

Since these are some early posts and I have no following yet, I’m going to assume that some of my early readers might not be quite as fluent in boatspeak, as say my neighbor, who lives on a 56′ yacht.  So when the title says Bow Thruster, you might not be sure if Bow rhymes with how or low.  Therefore, I’ll explain what this handy device is, when ours stopped working, and greatest of all, how we fixed it.

First, it rhymes with how, and is a reference to an extra little motor and prop that push the front of your boat from side to side.  If you had one on your care, parallel parking would be much easier.  Imagine if it was on the back of your car, where you didn’t have wheels that steer.  Well much like that, it makes parking or maneuvering a boat much easier, especially since wind, current, prop walk and an extra 20 or 30 tons conspire to make it complicated.

Now imagine using your bow thruster every time you park, like a mirror in your car, then discovering it’s not working.  That happened as we left Galveston a couple weeks ago.  Suddenly, no bow thruster.  We didn’t need it much leaving that dock, but we would sure miss it pulling back in our home slip.  We radioed some friends to ask for a couple hands on the dock when we got back, and as boaters are, we had plenty of help upon our return.  We took one black dock rub on the starboard beam as we came in, battling winds which were gusting to 1.7 mph.  Yes it was calm, and yes, we’d want to fix that bow thruster for when it wasn’t.

Imagine a small shoebox is your boat.  Now take a big, fat straw, like the ones you get for the 128 ounce soda at your convenience store, and jam it all the way through the shoebox.  That’s the tube of your bow thruster, and inside it are small propellers that are connected to a gearbox that is connected to a shaft that goes through the side wall of the tube and connects through a coupling to a motor that is connected by really fat wires to big ass batteries via relays that open and close when you move a little joystick in the pilot house.  I think it’s powerful enough to fly a helicopter for about 5 minutes.  About 1/3 of the system is under water, 1/3 is under the bathroom floor, and with the exception of the little joystick at the helm, the rest is under the side of the bed that’s about as easy to reach as the lighter that falls between the center console and driver’s seat in your car. What could possibly go wrong?


At first, I thought it was the batteries, which are under the bed.  After removing all the bedding and mattress, I put a load tester to the batteries, and they seemed fine.  Next my diver, who periodically cleans gunk and barnacles off the bottom of Cirila, checked to make sure there wasn’t something jamming the little props in the tube (remember the straw?).  All was clear and the props turned freely.  Next up, the motor, which sits on the tube under the bathroom floor.  I could see and feel enough to know that I had an issue with the coupling that connects the motor shaft to the gearbox shaft.

The challenge was getting everything apart and the motor removed, without a) breaking something else, b) sinking the boat, or c) breaking several other things just prior to having a stroke.  After much deliberation, I asked for some help.

Brent Hodges, one of the many mechanics that feast on all the boats here, was up for the challenge, and had the motor out in an hour.  The coupling was definitely the problem, as the elastomer spider looked like it was tumbled in a garbage disposal for an hour, and the motor side of the metal coupling had slipped up the shaft, essentially losing its embrace with its mate on the gearbox shaft.  Amazingly, I found a replacement spider for $4.79 at an industrial supply store 3 miles away, and Brent was back the same afternoon to put it all back together, which took him another hour.

The excitement of heading up to the pilothouse to bump the control joystick turned to despair in the same moment that not a damn thing happened.  No whirring sound below, no gargling sound outside, no lateral movement of the bow.  Nothing.  After checking the wiring to the motor, it was time to strip off the bed again to get at the batteries and relays.  We quickly discovered that the voltage was dropping from 25 to 5 when the bow thruster was calling for power, and that bad batteries were the next most likely culprit.  I pulled the old ones, maneuvered them off the boat, into the car, and headed to West Marine.   Thirty minutes, two new AGM 31’s,  $700, and a very sore back later, the bow thruster is working like a champ.


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