Martyr CMDIVERM Hull Plate Anode — Magnesium | ½″ × 6″ × 12″ | Fresh Water

Martyr CMDIVERM — Hull Plate Anode | Magnesium | ½″ × 6″ × 12″ | Fresh Water

Part Number: CMDIVERM (SKU: 194-CMDIVERM)
Brand: Martyr Anodes
Official Product Type: Hull Plate Sacrificial Anode — Magnesium
Common Name: Diver’s Dream Hull Anode
Dimensions: ½″ × 6″ × 12″ (approx. 12.7 mm × 152 mm × 305 mm)
Supersedes: No known superseded number
Replaced By: CMDIVERM is the current active part number

The Martyr CMDIVERM is the fresh water version of the Diver’s Dream hull plate anode — a flat-profile magnesium sacrificial anode designed for direct mounting to a boat’s hull below the waterline in fresh water lakes, rivers, and marinas. Magnesium is the only anode alloy that generates sufficient electrochemical potential to provide effective cathodic protection in the low-conductivity conditions of fresh water. Zinc and aluminum anodes will not work correctly in fresh water — this is the correct choice for fresh water hull protection.

Fresh water use only. Do not use magnesium anodes in salt or brackish water — magnesium is too electrochemically active for salt water and will deplete very rapidly, potentially causing damage to surrounding metals.

Compatible Applications

• Steel and aluminum boat hulls in fresh water lakes, rivers, and marinas
• Vessels kept in fresh water year-round where diver-serviceable anodes are preferred
• Any hull surface in fresh water requiring a large flat-plate anode
• Supplemental hull protection alongside shaft and drive anodes in fresh water

System & Location

Cathodic protection system — hull-mounted plate anode. Mounts flush to the hull below the waterline using through-bolts or studs. The flat profile minimizes drag and allows installation in areas where a streamlined anode profile is important. Designed to be replaced in-water by a diver.

Anode Material Guide

• Magnesium (this anode): Fresh water only ✓
• Aluminum: Salt water & brackish water — see Martyr CMDIVERA Aluminum Hull Plate Anode
• Zinc: Salt water — see Martyr CMDIVER Zinc Hull Plate Anode

Kit Contents

• One (1) magnesium hull plate anode, ½″ × 6″ × 12″
• Mounting hardware not confirmed included — verify at time of order; reuse existing studs or through-bolts if not supplied

Dimensions & Weight

• Dimensions: ½″ × 6″ × 12″ (12.7 mm × 152 mm × 305 mm)
• Weight: Approx. 900 g / 1.98 lb (estimated)

Service Interval

Inspect at every haul-out or have a diver inspect at least annually if the vessel is kept in the water. Replace when the anode has been consumed to approximately 50% of its original mass. Magnesium anodes can deplete faster than zinc or aluminum anodes in high-conductivity fresh water conditions — inspect more frequently in those environments. Do not allow the anode to deplete fully before replacement.

Installation Notes

• Mount flush to the hull using through-bolts or studs — ensure direct metal-to-metal contact between the anode and the hull
• Do not apply paint, bedding compound, or sealant between the anode and the hull surface
• Bare metal contact is essential for the anode to function correctly
• Torque fasteners securely — a loose anode will not make proper electrical contact and may be lost
• Fresh water use only. Do not deploy in salt or brackish water

Technical FAQ

Q: Why do I need magnesium for fresh water instead of zinc or aluminum?
Fresh water has much lower electrical conductivity than salt water. Zinc and aluminum anodes do not generate enough electrochemical potential to provide effective protection in fresh water. Magnesium has a higher driving voltage and is the correct alloy for fresh water cathodic protection.

Q: Can this anode be used in salt water?
No. Magnesium is too electrochemically active for salt or brackish water. It will deplete very rapidly and can over-protect surrounding metals. Use zinc or aluminum anodes in salt and brackish water.

Q: Is there a heavier 1″ version of this anode?
A 1″ heavy version is available in zinc and aluminum, but not currently in magnesium. If you need extended service life in fresh water, contact us to discuss options.

Q: How many anodes does my hull need?
The number and total mass of hull anodes required depends on hull material, surface area of bare metal, and water conditions. A marine surveyor or corrosion specialist can calculate the correct anode system for your vessel.

Related Parts

• Martyr CMDIVERA — Aluminum Hull Plate Anode, ½″ × 6″ × 12″ (Salt & Brackish Water)
• Martyr CMDIVER — Zinc Hull Plate Anode, ½″ × 6″ × 12″ (Salt Water)
• Martyr CMDIVERHA — Aluminum Heavy Hull Plate Anode, 1″ × 6″ × 12″
• Martyr CMDIVERHZ — Zinc Heavy Hull Plate Anode, 1″ × 6″ × 12″

Parts Desk Insights

The magnesium hull plate anode is the correct solution for fresh water hull protection where a bolt-on plate anode is needed — dock boats, inland lake vessels, and river craft are the most common use cases. The most important thing to get right is water type: magnesium in salt water is a costly mistake, as the anode will deplete in a fraction of the expected time. If a vessel moves between fresh and salt water, the anode alloy needs to match the primary mooring environment, and we recommend confirming before ordering. There is no magnesium version of the heavy 1″ plate currently available — for fresh water vessels needing extended service life, contact us to discuss the best approach.