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Boat outdrive units - Reliable marine propulsion systems

Improve your boat's propulsion with durable, compatible outdrives

What is a boat lower unit and what is its role?

The lower unit is the sealed submerged housing containing the reduction and gear-shifting system. On inboard engines (Sterndrive), it is referred to as a “Z” drive or Z-drive passing through the transom. On an outboard engine, the architecture is vertical and integrated into the engine leg.

Its mechanical role is threefold: transmitting the rotational movement from the vertical driveshaft to the horizontal propeller shaft through a set of bevel gears; allowing gear shifting (Forward / Neutral / Reverse) through a sliding clutch dog; and finally, drawing in raw water through intake grilles to cool the engine block via the water pump housed inside it.

The orientation of this lower unit, controlled by the steering and adjusted in angle by external hydraulic trim rams, makes it possible to steer the boat and adjust its trim while underway for clean planing.

How to choose the right boat lower unit

Choosing a replacement engine lower unit requires precision: it is not a simple accessory, but the drivetrain that absorbs all the engine torque. At DAM Marine, we help you identify the equipment strictly compliant with the original specifications of your engine, in order to ensure reliable propulsion at sea.

Technical criteria: power, shaft and rotation

Replacing a lower unit leaves no room for approximation. Three fundamental technical parameters must be checked on your engine identification plate:

  • The gear ratio: The internal gear reduction varies depending on engine power, propeller and the torque of your engine block. A V6 engine will not use the same ratio (e.g. 1.47:1) as a 4-cylinder inline engine, in order to optimize engine rpm in relation to thrust in the water.
  • The driveshaft length: It is essential to measure the engine shaft (short, long, extra-long or ultra-long) so that it matches the height of your transom. An incorrect shaft length will prevent any coupling with the engine block or cause severe cavitation problems on your outboard engines.
  • The direction of rotation and the number of splines: The propeller shaft rotates in standard rotation (right-hand - RH) or counter-rotation (left-hand - LH). This criterion is vital, especially on twin-engine installations where the flows must cancel each other out. In addition, the exact count of splines on the vertical driveshaft is essential to ensure proper engagement in the crankshaft.

Compatibility with major engine brands

There is no such thing as a universal lower unit. Each lower unit has its own casting specifications, mounting center distance and cooling circuit. Attempting to fit a non-compatible model is bound to result in mechanical failure. The goal is to replace your equipment with parts that are strictly equivalent to manufacturer dimensions. DAM Marine distributes replacement solutions perfectly calibrated for the leaders of the marine market. Whether you are looking for a Mercruiser lower unit (Alpha One, Bravo) for your Mercruiser Z-drive installations, a lower unit for a Honda outboard, or a pump body and housing for a Yamaha engine block, checking by engine serial number remains the only foolproof method. Browse our dedicated catalogs to identify your reference.

Signs of a worn or damaged lower unit

Wear on an engine lower unit is reflected in clear mechanical symptoms. A sharp metallic clunk when engaging forward or reverse gear often indicates pronounced wear of the clutch dog or an incorrect adjustment of the shift cable. A whistling sound while underway may indicate excessive play in the bearings or incorrect gear alignment.

The most critical symptom remains water intrusion into the lower unit housing. If, during draining, the oil has a milky appearance (commonly called “mayonnaise”), this confirms a sealing failure. The oil seals (lip seals) on the propeller shaft, driveshaft or shift rod are compromised. This defect directly endangers the bearings and gears of your lower unit due to lack of lubrication.

Following severe grounding (impact on a shoal), a bent propeller shaft or a cracked aluminum housing often makes reconditioning economically or technically unsuitable for ensuring onboard safety. Installing a new or reconditioned Mercury lower unit from Mercury becomes the most reliable solution for getting back to sea with confidence.

Maintaining your lower unit: oil change and prevention

The longevity of your transmission relies on rigorous preventive maintenance. The essential operation is changing the lower unit oil (marine gear oil, often SAE 90 or High Performance grade), to be carried out every 100 hours or during annual winterization. Inspecting magnetic drain plugs is crucial: the presence of fine grey metal dust is normal (running-in wear), but the presence of metal chips indicates internal gear failure in your lower unit.

Sealing cannot be checked with the naked eye. In case of doubt, or every three years on average, a mechanic will carry out a pressure and vacuum test on the housing. If the test fails, replacing the lower unit seal kit (oil seals, O-rings) is necessary before launching the boat again.

Finally, maintenance includes systematic inspection of the propeller hub (checking for fishing line wrapped around it, which can cut the seals) and annual replacement of zinc, aluminum or magnesium anodes depending on your navigation area, to protect your lower unit from destructive galvanic corrosion.

Equip and maintain your transmission system

To work efficiently in your workshop or boatyard, DAM Marine supplies all the required technical consumables. Lubrication and sealing quality allow no compromise. Check our categories to find bottom-fill oil pumps, drain plug seals (to be replaced each time they are opened), and water pump kits (turbine/impeller) that must be changed regularly. Explore our catalog to discover our full range of marine parts and validate your service kit at the best price, with express delivery so your boat is not stuck in the yard.

When should lower unit oil be changed?

The standard industry rule requires changing gear oil every 100 operating hours, or at least once a year during winterization preparation. It is essential to carry out this oil change before storing the boat for winter.

If, during draining, you notice “mayonnaise”-colored oil (water/oil emulsion), do not simply refill with new oil. Water has entered the housing. You must remove the lower unit, carry out a leak test, and replace the propeller shaft or water pump oil seals, otherwise you risk completely destroying the gear system during your next trip.

To understand how the sterndrive fits into the different propulsion layouts and compare their advantages, read our guide to choosing between an inboard and an outboard engine.

The sterndrive must suit both the engine and the way the boat will be used. To compare the main propulsion options, read our guide to choosing a boat engine.

To distinguish between the propeller shaft, sterndrive and the different short- or long-shaft configurations, read our guide to propeller shaft and sterndrive terminology and diagnosis.

The sterndrive transfers engine power to the propeller and plays a central role in moving the boat. To understand the complete system, read our guide to how boat propulsion works.

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397 item(s)

JOINT OMC 0909515

JOINT OMC 0909515

RÉF. 33900 | GLM

Intermediate crankcase joint for base OMC 400 - 800.

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GLM GLM
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COLLE A SOUFFLET MERCRUISER...

COLLE A SOUFFLET MERCRUISER 42.5GRS

RÉF. 92/86166Q1 | MERCRUISER
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JOINT HUILE MERCURY OEM

JOINT HUILE MERCURY OEM

RÉF. 26/45577 | MERCRUISER
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VOLVO BASE AXLE BEARING

VOLVO BASE AXLE BEARING

RÉF. 3853423 | ORBITRADE

Volvo Penta cylinder shaft bearing to be used in addition to bushings 3853424.

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JOINT DE POMPE MERCURY

JOINT DE POMPE MERCURY

RÉF. 27/822189 | MERCRUISER
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CACHE AXE VERIN EMBASE...

CACHE AXE VERIN EMBASE MERCRUISER

RÉF. 19/815951 | MERCRUISER
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DERIVE BRP 0338742

DERIVE BRP 0338742

RÉF. AP3104 | EMP
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EMP EMP
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VOLVO WATER PIPE BASE - 854031

VOLVO WATER PIPE BASE - 854031

RÉF. 19766 | SLP

Metal water intake for Volvo Penta outdrive types:

AQ200, AQ250, AQ270, AQ275

AQ280, AQ280DP, AQ285A, AQ290

AQ290A, AQ290DP, SP-A, SP-A/MT

SP-A1, SP-A2, SP-C, SP-C1

DP-A, DP-A1, DP-A2, DP-B

DP-B1, DP-C, DP-D

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SEAL SPI BASE MERCRUISER BRAVO

SEAL SPI BASE MERCRUISER BRAVO

RÉF. 26/12709 | EMP
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EMP EMP
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OIL SEAL

OIL SEAL

RÉF. OMC0330137 | EMP
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KIT RESSOR TRUCK EMBASE VOLVO

KIT RESSOR TRUCK EMBASE VOLVO

RÉF. 22056 | ORBITRADE

Lock kit for Volvo Penta base:

250, 270, 275, 275A, 280, 280DP, 280L, 285A,

290, 290A, 290DP

SP-A, SP-A/MT, SP-A1, SP-A2

DP-A, DP-A1, DP-A2

DP-B, DP-B1

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LOWER HOUSING FOR WATER...

LOWER HOUSING FOR WATER PUMP ALPHA G1-MR BASE

RÉF. 46/44292A3 | EMP

Mercury Lower Seawater Pump Housing - Mercruiser.

Lower housing for Mercruiser Alpha One G1 drive serial number 6854393 to OD469858.

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OIL SEAL

OIL SEAL

RÉF. OMC0981196 | EMP
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CACHE AXE VERIN EMBASE...

CACHE AXE VERIN EMBASE MERCRUISER G1/BRAVO

RÉF. 19/14842 | MERCRUISER

Couvercle de protection pour axe de vérin d'embase Mercruiser Alpha One G1 et Bravo.

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VOLVO BASE CYLINDER SHAFT

VOLVO BASE CYLINDER SHAFT

RÉF. 872126 | SLP
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BRP SHAFT OIL SEAL 0342787

BRP SHAFT OIL SEAL 0342787

RÉF. AP6150 | EMP
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