Martyr CM875821M Magnesium Duo Prop Ring Anode – Volvo Penta DuoProp | Freshwater
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Martyr Magnesium Duo Prop Ring Anode — Volvo Penta DuoProp | Part No. CM875821M
Replace the ring anode on your Volvo Penta DuoProp (counter-rotating dual propeller) sterndrive with this magnesium-alloy unit from Martyr Anodes. The ring anode fits around the propeller hub assembly on the DuoProp drive and provides sacrificial protection for the propeller hub, shaft, and surrounding hardware against galvanic corrosion in freshwater environments.
Magnesium is the correct alloy for freshwater use — zinc and aluminum anodes are too electrochemically passive in low-conductivity water to provide adequate protection. If you operate in saltwater or brackish water, see the aluminum alternative linked below.
Part Information
- Part Number: CM875821M
- Brand: Martyr Anodes
- Anode Material: Magnesium
- Anode Type: Duo Prop Ring Anode
- OEM Reference: 875821
- Best Use: Freshwater environments
Compatible Applications
- Volvo Penta DuoProp Sterndrive — propeller hub ring anode position
- Freshwater operating environments only
- Confirm OEM reference 875821 against your drive’s parts diagram before ordering
Choosing the Right Alloy
- Magnesium (this part) — Freshwater use only
- Aluminum — CM875821A — Saltwater and brackish water; preferred alloy for Volvo Penta aluminum drive components
Related Products
- Martyr CMT-290DPKITA Aluminum Anode Kit – Volvo Penta 290 Dual Prop — complete DuoProp anode kit
- Volvo Penta DuoProp Front Propeller Nut 852196 — DuoProp front propeller hardware
- Volvo Penta DuoProp Rear Propeller Nut 21631162 — DuoProp rear propeller hardware
- Volvo Penta DuoProp Propeller Mounting Kit 854667 — DuoProp propeller mounting hardware
Service Interval
Inspect DuoProp ring anodes at every haulout and replace when worn to approximately 50% of original size. Magnesium anodes in freshwater deplete at a moderate rate — inspect annually and replace promptly to maintain continuous protection on the propeller hub assembly.
Parts Desk Insights
The DuoProp ring anode is easy to overlook during routine anode checks because it sits within the propeller hub assembly rather than on an exposed surface — but it’s one of the most important anodes on the drive. The counter-rotating hub assembly involves dissimilar metals in close proximity, making it particularly vulnerable to galvanic corrosion if the anode is depleted. Magnesium is the right alloy for freshwater DuoProp installations — the most common mistake we see is customers installing zinc anodes in freshwater lakes, where zinc doesn’t corrode fast enough to provide meaningful protection. If your boat operates in saltwater or moves between fresh and salt water, the aluminum CM875821A is the correct choice. For a full DuoProp anode service, the CMT-290DPKITA complete anode kit covers all positions in one purchase.
Shipping information
In-stock and OEM parts: ship same or next business day.
Locally warehoused items: ship within 1-3 business days.
Special-order items: timeline confirmed after order.
Returns & warranty
Returns & Satisfaction Guarantee
We stand firmly behind the quality of every product we sell. Your confidence matters to us — and we back it with a straightforward 30-Day Satisfaction Guarantee.
If you are not fully satisfied, you may request a return within 30 days of delivery. All returns must receive pre-approval prior to being sent back. Unauthorized returns will not be accepted.
To qualify for a return:
Item must be in new, unused condition
Product must be in its original packaging
All parts, components, hardware, manuals, and accessories must be included
Packaging must be undamaged and suitable for resale
Important:
Electronics are strictly inspected. No returns on open-box electronics. Exceptions may apply only with prior written approval. Any electronic unit that has been installed, powered, activated, or altered may not be eligible for return.
We are serious about product integrity — and equally serious about taking care of our customers. If there is ever a product issue, contact us immediately so we can make it right.
For return authorization, please contact our support team before shipping anything back.