Lewmar 559017A Alloy Shear Pin for 185TT Bow Thruster (2-Pack)

Every bow thruster needs a weak point, and on the Lewmar 185TT that weak point is this pin. The 559017A is an alloy shear pin, and its whole job is to break before anything expensive does. When the thruster prop catches a dock line, a stray stick, a rope, or a thick mat of weed, the load has to go somewhere. Without a shear pin it goes straight into the drive and the motor. With one, the pin snaps first and saves the rest.

Think of it as a fuse for the mechanical side of your thruster. The prop connects to the drive through this pin. Under normal thruster use it transmits plenty of torque to push your bow around in a crosswind or a tight slip. But hit a hard jam and the alloy gives way at a predictable point, letting the prop stop while the motor keeps turning free. You lose thrust until you replace the pin, but you don't lose a gearbox or a motor. That's the trade, and it's a good one.

Lewmar makes this one from alloy, which shears cleanly under a sudden overload. It's the sacrificial part of the system, so it's meant to be replaced. Boaters who run bow thrusters in weedy water or shallow, debris-filled harbors go through them, and that's normal. A sheared pin isn't a failure of the thruster. It's the pin doing exactly what Lewmar designed it to do.

This part is specific to the 185TT tunnel thruster. It works with both the 12V and the 24V 185TT motors. It is not the pin for the larger 250TT, so if you run that bigger thruster this isn't your part. Check the model plate on your thruster before you order. The 185 is common on cruising sailboats in roughly the 28 to 35 foot range, and if you've got one, this is the drive pin it takes.

How do you know you've sheared it? The giveaway is a thruster that sounds like it's running, the motor hums, but the boat's bow doesn't move. No thrust, plenty of noise. That's a sheared pin almost every time. The fix is straightforward mechanically, but on most 185TT installs there's no access port, so you'll be pulling the propeller to reach the pin and swap it. Plan the job for a haul-out or a calm day at the dock rather than mid-docking with wind pushing you onto the pilings.

When you put the new pin in, a lot of installers give the pin ends a light film of grease and nothing more. A dry fit can bind on reassembly, and too much grease just makes a mess and traps grit. Light touch on the ends, seat it, reassemble the prop, and you're back in business. If you boat in salt water, keep an eye on the pin over the season. Alloy corrodes faster than stainless out there, so coastal cruisers tend to carry spares and check the pin as part of routine service.

Which is exactly why Lewmar sells the 559017A as a pack of two. One to fix the problem you've got, and one to stash in the spares kit for next time. If you cruise anywhere with floating weed, current-borne debris, or crab-pot lines, next time is a question of when, not if. Having a spare pin and a few basic tools aboard turns a stuck bow into a fifteen-minute job instead of a ruined afternoon.

Bottom line, this is cheap insurance for a pricey piece of gear. The thruster itself, the motor, the gear leg, all of it costs real money. This pin costs a few dollars and stands between a minor prop strike and a major repair bill. Buy the genuine Lewmar part, keep the spare aboard, and your 185TT keeps doing its job season after season.