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5 year report on tiny thruster on a SO39i

Doug Bostrom reviews his installation

I feel as though long-term results on modifications become more valuable in correlation with the amount of hassles, impact of any given modification. As well, living with a modification for a while is helpful for assessing its utility. So here's a retrospective on installation of a bow thruster on our 39ip, how it's looking after some years.


Although the macho man in me sniffed at the idea of a bow thruster, as our boat is in charter service it seemed like a good idea to add one and I was encouraged to do so by our very excellent management firm.


The 39i underbody is tricky for fitting a bow thruster where it'll do the most good, namely as far forward as possible. There's not a lot of interior space to work with.

Working the numbers, it turned out to be the case that shrinking the thruster in trade for more advantageous positioning would likely pay off. More leverage, more rotation per kilowatt. As with real estate, for efficient thruster placement it's "location, location, location."

Adding weight and losing buoyancy to a thruster tube at the bow of course is a negative trade, so these needed to be minimized.


The void immediately behind the anchor locker on a 39i will accommodate a small thruster. Meanwhile, the deadrise of the hull is sufficiently vertical there that the tube apertures don't become absurdly elliptical. We chose a Lewmar 110TT thruster Lewmar Description, which is light and has a small tube. A 140TT might fit but as the tube is quite a bit larger this would push very close to a number of hard limits, such depth of tube below surface etc.

The skilled part of tube installation is above my paygrade. So, I had the yard do the hull cuts, applied my own elbow grease for cutting out liner for filleting, cleaning filler between liner and hull etc. Then the yard returned to do the tube installation, which turned out handsomely.

Power for the thruster is derived from the existing conductors for the windlass. As with all other wiring on Jeanneau boats, the existing conductors powering the windlass were scanty. Voltage stiffening was needed and I didn't want to go to replacement of the existing conductors. Instead, I took the opportunity to help the windlass while also making sure the thruster had adequate voltage by adding a "voltage stiffener" in the form of a small and light Optima AGM battery. This battery is run in parallel with the starter battery aft, with all necessary trimmings in the form of circuit breakers etc. to account for the fact that we now have a power source at both ends of the conductors, split overcurrent trip impacts so that windlass doesn't drop out with thruster or vice versa. The forward battery doesn't need to be large because the actual amp-hour drain during usage is quite small, and this little battery still has plenty of oomph to shore up the juice being delivered by the original conductors.

Total weight of the thruster, battery and accessories is about 65 pounds. About 9 pounds of buoyancy is lost to the tube.


If there's a performance loss due to the tunnel aperture drag it's at a level where it might show up as seconds or minutes lost over many nautical miles, basically not noticeable. This seems to be a frequent "fear factor" over thrusters but perhaps it's the case that ending up with a mostly non-elliptical aperture arrangement helps, or that potential effects are exaggerated. In any case, "de minimus" here.

Since adding this apparatus, the thruster in all probability saved our boat from catastrophic damage or worse on one occasion involving loss of propulsion in a dodgy, rocky location, buying us critical seconds, and almost certainly avoided serious own-anchor engagement with our dock mate's pushpit during a docking back-off-and-retry due to 25 knot crosswind inopportunely puffing up.

So, all testicular considerations aside, operationally speaking it's proven a real boon to have a crew member on board who can walk on water while delivering a solid push. As well, guest skippers do really love the drama of lots of gratuitous thruster operation, so the device is indeed a sales feature.

Over the 5 years since installation of this accessory we've had zero trouble or maintenance requirements in connection with it despite what I know is very enthusiastic usage by a plethora of crew.

The battery at age 5 is in perfect health.

My original intention was to mount the battery aft a bit along side the unfortunately-placed forward water tank, to minimize added weight forward, but I've not "gotten around to it," an indication perhaps that the itsy-bitsy battery isn't really an issue up where it still lives.

With the thickness and solidity of the tube and how nicely it was filleted/tabbed into the hull, I feel as though the bow is perhaps a bit stronger than it was before the addition.







Doug Bostrom


Article from this Jeanneau Owners Forum post 5 year report on tiny thruster add to SO39i