Stern thruster installation: four routes, from the manuals

The short answer

There are four ways a thruster goes on at the stern.

1. On the drive leg. Sideshift makes kits that bolt to the cavitation plate of an outboard or sterndrive. The thruster turns with the engine, and its bolts go through the engine’s plate, not the hull. Sideshift says it can go on with the boat afloat. We recommend fitting it with the boat out of the water, where the leg is easier to reach.
2. On the transom. Most stern units bolt here. DockStar’s track and Sideshift’s ST350, on its standard mount, keep every hole above the water, so you can fit them with the boat afloat. ezDrive, Sleipner and Yacht Thruster are haul-outLifting the boat out of the water on a travel lift or a trailer so work can be done below the waterline. It is billed separately from the job itself. jobs. Sideshift’s wake boat kit goes on boats that live on a trailer, and we recommend fitting it on land.
3. A pod on the hull. Max Power’s stern pods sit below the waterline, which means a haul-out, and its manual strongly advises a qualified installer.
4. A stern tunnel. A tunnelA tube bored straight through the hull below the waterline, with a propeller in the middle of it. It is the conventional thruster. Fitting one is mostly a labor job rather than a hardware one. through the hull at the stern. It is a yard job, and the stern is where the machinery lives, so it is often the hardest of the four.

Two things decide which route is yours. The first is what drives the boat. The second is whether you are willing to have holes drilled below the waterline. The bow or stern page helps if you are still deciding which end needs the thruster.

What each maker asks of your boat

Each row is one maker’s own document. The last column is the document’s own sentence on who does the job, or how long it takes. The “Boat out of the water?” column is each maker’s own answer, as recorded in our product list.

Only ezDrive’s guide prints a torque for the bolts that hold the unit to the transom. Read your own unit’s manual before you drill; this table is only a summary of it.

On the drive leg: Sideshift’s outboard and sterndrive kits

These kits bolt to the cavitation plate, the flat fin just above the propeller on an outboard or a sterndriveAn engine inside the boat driving a steerable leg out through the back of the boat. It steers like an outboard with the motor inboard. It is common on express cruisers, and it leaves no room for a stern tunnel.. The manual has you drill four 3/8 in holes through that plate, using the mounting plate as the template. The bolts go in with the nut on top and the supplied blue thread locker on every thread.

Which way it faces matters. For a single engine, Sideshift says:

“Attach the thruster to the mounting plate with the thruster prop facing to the left (port) side of the engine.”

Sideshift installation manual, p. 34

The cables need slack for the engine to steer and tilt. The manual says to leave room for the motor to move side to side and up and down, and it routes the cables through an opening in the stern with the nylon sleeve and clamps in the kit.

Sideshift says these kits can go on with the boat in the water or on land. We recommend land. Many boats with an outboard or a sterndrive live on a trailer, and the drive leg is far easier to reach with the boat out.

There is one rule for running at speed. Sideshift gives it in the drive-kit section:

“When operating the boat at high speed, trim the outboard motor or I/O to ensure the thruster is just above the water surface when the boat is at planing speed. Failure to trim properly at high speed can cause heavy steering and may damage the thruster.”

Sideshift installation manual, p. 32

On the transom, step by step

The transom manuals broadly share one order, and the six steps below follow it. Where one maker adds a rule, the step names it.

1. Find how deep it has to sit. Every maker here but Yacht Thruster sets a depth, and they differ. Measure from the waterline of an empty boat, which is how Sideshift asks you to mark it.
2. Keep it clear of the drive, the exhaust and the trim tabs. The thruster pushes water sideways, and anything in that path robs it: the drive, the exhaust, the trim tabsTwo flat plates hinged along the bottom edge of the transom, one each side, that tilt down to lift the stern and level the boat at speed.. It does not have to sit on the centerline.
3. Look inside before you drill. Every bolt through the transom has a nut on the inside, so someone has to reach it. Find out now what sits behind the spot. In my case it was a mattress.
4. Drill, seal and bolt. Each maker names its sealant, and Sideshift’s transom kits use a backing plateA plate of metal or plywood on the inside of the hull or deck, under the bolts. It spreads the load over a wide area, so the bolts do not pull through the fiberglass. inside.
5. Run the cables and wire it. Every unit but DockStar’s needs heavy cable to a battery. DockStar’s carries its own battery.
6. Test it in the water. A thruster run in air spins free. Each maker gives a limit on how long it may run dry.

Step one: how deep each maker wants it

Step two: what to keep clear of

Sideshift puts the rule for a transom unit in one sentence:

“The thruster must not interfere with the main drive propeller, rudder, or be placed directly in the flow of the main propeller.”

Sideshift installation manual, p. 23

DockStar adds the engine’s exhaust to that list. Sleipner adds a limit of its own: the unit must not stick out past the outline of the transom on any side. Both DockStar and Sideshift say the thruster can sit off the centerline, and DockStar says a stern unit usually does.

Step three: what is behind the transom

This is the step that made my own install a two-person job. The hull shape put two of the four bolts above the headboard in the aft cabin, and the other two behind the mattress.

Sleipner’s manual puts the check this way: “Ensure enough space inside and outside the transom when planning the thruster’s position.” ezDrive’s guide says it more briefly: “Check for access and clearance inside the boat.”

Sleipner also rules out one kind of spot altogether. Its manual says: “Do not install the thruster system in any position that requires modification of hull stiffeners or stringers.”

Step four: drilling and sealing

The holes for each unit are in the table above. The sealant each maker names is below. If the boat is in the water while you drill, Sideshift has a warning:

“If performing an in-water installation, use a cordless drill to avoid the risk of electrocution.”

Sideshift installation manual, p. 28

Step five: the cables

Sideshift runs its cables into the boat through one hole per cable, above the highest waterline. Its wake boat kit uses a through-hull cable seal that the manual says is not for use below the water. ezDrive’s guide warns: “Thruster cables CANNOT be extended.”

For the cable size, the fuse and the battery, pick your stern unit in the wiring calculator. It prints the cable the maker calls for at your run length. The battery calculator checks the bank you already have against the maker’s figure.

Step six: the first test

Sideshift says not to run a thruster out of the water for more than 5 seconds. ezDrive allows a test of 1 to 2 seconds. Sleipner’s manual says: “Never run the thruster out of water.” The proper test is with the boat afloat, at the dock, in short bursts.

My own install: a DockStar on the transom track

I fitted a DockStar to the stern of my 41 ft trawler, on DockStar’s stern hull track. DockStar’s stern installation guide calls this the hull-mount configuration. I own the unit, so read this as one owner’s account. Here is how it went.

“It took me about two hours. The hardest part was reaching the inside of the four bolts that go through the transom, and that made it a two-person job. My daughter was inside with the socket while I held each bolt tight from outside.”

“Because of the way the hull is shaped, the top two bolts had to go above the headboard in the aft cabin. They show, but they don’t bother me. The bottom two are hidden, but I had to take the mattress out to reach them.”

“The only other tools I needed were a drill and bit, a tape measure and a little sealant for the bolt holes. The instructions were simple and easy to follow. Once the track was bolted on, we slid the thruster onto it, added the small pulley for the rope that raises and lowers it, and we were ready to go.”

Brian, Aim 2 Try, 41 ft CHB trawler, Port Townsend. More on the About page.

The guide describes the same job. Its hull-mount kit bolts on with four 10-32 through-boltsA bolt that passes right through the hull or deck with a nut on the inside, instead of a screw that only bites into the surface., fender washers and nylock nuts (the kind with a nylon ring that stops them working loose), with a marine sealant under the fittings. The guide sums it up this way:

“Four screws installed into the stern and you’re done except for the lanyard.”

DockStar Stern Installation Guide, p. 3

The guide sets two heights. The bottom hinge sits no less than about 4 in above the waterline, and the motors should be at least 8 in under the surface when the unit is lowered. The rope and pulley are the last step: the rope runs from one cleat on the track, through the pulley on the unit and back up to the other cleat.

The pod and the tunnel: yard jobs

A Max Power stern pod bolts to the outside of the transom, but below the waterline. Its manual asks for the top of the turbine to sit one diameter under the water, and it has two rules for the transom itself. The first: “Make sure no part of the STERN POD is lower than the transom of the boat.”

The second is about strength: “If necessary it’s better to strengthen the transom of the boat to support the weight and the thrust.” On who fits it, the manual says: “The use of qualified marine personnel, with experience in bow thruster installation, is strongly advised.”

A stern tunnel is a tunnel install at the other end of the boat. The install page walks through the steps the makers print for a tunnel at the bow. The stern adds a problem the bow does not have: the stern is where the machinery lives. The stern cost page has one owner’s account of what had to move to make room.

How long it takes

Two makers print a time: Sideshift on its installation page and FAQ, and ezDrive in its guide. The other 6 rows are owners’ own reports of their stern jobs, including mine.

The owners’ reports run from my two hours to a couple of months of weekends. The longest one covers a bow and a stern unit fitted with the boat in the water.