Martyr CMGROUPERM Grouper Hanging Anode with 15' Cable — Magnesium | Fresh Water

Martyr CMGROUPERM — Magnesium Grouper Hanging Anode with 15' Cable | Fresh Water

Part Number: CMGROUPERM (SKU: 194-CMGROUPERM)
Brand: Martyr Anodes
Official Product Type: Sacrificial Hanging Anode — Magnesium
Supersedes: No known superseded number
Replaced By: CMGROUPERM is the current active part number

The Martyr CMGROUPERM is a magnesium sacrificial hanging anode designed to protect submerged metal structures from galvanic corrosion in fresh water environments. The 15-foot cable allows the anode to be suspended freely in the water column, maximizing surface exposure and protection coverage. 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.

Compatible Applications

• Boat hulls (hanging/suspended protection) in fresh water lakes, rivers, and marinas
• Dock pilings and marina structures in fresh water
• Crab pots, fish traps, and wire mesh traps in fresh water
• Underwater frames, cages, and aquaculture equipment in fresh water
• Any submerged metal structure in fresh water requiring cathodic 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 rapidly, potentially causing damage to surrounding metals.

System & Location

Cathodic protection system — standalone hanging/suspended anode. Deployed by suspending the anode from the cable into the water column adjacent to the structure being protected. Not mounted directly to the hull or drive.

Anode Material Guide

• Magnesium (this anode): Fresh water only ✓
• Aluminum: Salt water & brackish water — see Martyr CMGROUPERA Aluminum Grouper Anode
• Zinc: Salt water — see Martyr CMGROUPER Zinc Grouper Anode

Kit Contents

• One (1) magnesium grouper hanging anode
• One (1) 15-foot (approx. 4.6 m) suspension cable

Dimensions & Weight

• Weight: Approx. 1,800 g / 3.97 lb (estimated)
• Grouper / cylindrical hanging form factor
• Cable length: 15 feet (approx. 4.6 m)

Service Interval

Inspect at every haul-out or seasonal maintenance. Replace when the anode has been consumed to approximately 50% of its original mass. Magnesium anodes in active fresh water environments may deplete faster than zinc or aluminum anodes in salt water due to their higher electrochemical activity — check more frequently in high-current or high-conductivity fresh water conditions. Do not allow the anode to deplete fully before replacement.

Installation Notes

• Do not connect to a battery or any electrical source. This is a passive cathodic protection device only.
• Suspend the anode freely in the water column using the cable — do not allow it to rest on the bottom or against the hull.
• Ensure good electrical continuity between the anode cable and the metal structure being protected.
• 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 I use this anode 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, potentially causing hydrogen embrittlement in some alloys. Use zinc or aluminum anodes in salt and brackish water.

Q: Can I use this anode in fresh water?
Yes — this is the correct anode for fresh water applications. It is the only grouper-style hanging anode in this range suitable for fresh water use.

Q: How do I know when to replace it?
Replace when approximately half the anode mass has been consumed. A fully depleted anode offers no protection. Magnesium anodes may deplete faster than other alloys — inspect more frequently in high-activity fresh water environments.

Related Parts

• Martyr CMGROUPERA — Aluminum Grouper Hanging Anode with 15' Cable (Salt & Brackish Water)
• Martyr CMGROUPER — Zinc Grouper Hanging Anode with 15' Cable (Salt Water)

Parts Desk Insights

The magnesium grouper anode is the right tool for fresh water dock and hull protection where a bolt-on anode isn't practical. The most important thing to get right with this product is water type — magnesium in salt water is a common and costly mistake, as the anode will deplete in a fraction of the expected time. If a customer is unsure of their water type or moves between fresh and salt water, we recommend confirming before ordering. For mixed-use or brackish water, the aluminum grouper anode (CMGROUPERA) is the better choice.