ZINC TALK
I have a 1982 Marine Trader that we keep on the Bohemia River, which is mostly fresh water, maybe slightly brackish. A few years ago I noticed a lot of pitting on my bronze prop and stainless steel rudder. The zincs looked quite good. I then took my ohmmeter and did a continuity check, which revealed that my bonding system had a break in it. I tracked this down and repaired it. Now, the zincs are wearing at a normal rate, but I think the pitting may have increased slightly. I am about ready to replace the zincs, and I don't know what brand or type to buy.
Ronald L. Klimes
York, Pennsylvania
The anode selection process is fraught with confusion for both boat owners and professionals. Many folks simply use the same anodes they've always used, without really knowing whether they're effective for the boat's hull material, its mix of underwater metals, and the water in which it operates.
First let me clarify that the following advice refers to boats with fiberglass hulls. While your propeller may be made of bronze, it's likely that it's made of manganese bronze, which is a type of brass in that it contains zinc as an alloying agent to copper. This means that if it's galvanically unprotected or improperly protected, it may be the first component to corrode. There are props that don't contain zinc; these are made using an alloy called nibral (nickel-bronze-aluminum). However, nibral props still require anodic protection (note that I didn't say "zinc protection").
The protection that is chosen-both the type and quantity of anodes-is a function of the type and quantity of underwater metal that is being protected and the kind of water in which the boat operates. Vessels that operate exclusively in salt water may use zinc or aluminum anodes. Boats that run in salt and fresh water, or a mixture of the two, should use aluminum anodes. Vessels operating exclusively in fresh water often use magnesium anodes. In your case, it sounds as if aluminum anodes would be the logical choice. Whatever brand is recommended to you and you choose to buy, ensure that the anodes carry a mil-spec rating.
Finally, the only way to be certain that your anodes are doing their job correctly is to have your boat tested using a silver/silver chloride reference electrode. This test should be performed by someone who is experienced in corrosion analysis, preferably someone who has attained ABYC Corrosion Certification. Look for a detailed article on the subject of corrosion and its prevention in an upcoming PMM issue.
-Steve D'Antonio
VIBRATION PROBLEM SOLVED
Just a short note of thanks to Steve D'Antonio for his "Gearhead" article "Installing a Prop the Right Way" (PMM Nov./Dec. '09). We own a Fleming 55 and for years experienced a vibration in the drivetrain that nobody could identify. Steve's article mentioned vibration and shaft installation issues, so we asked our winter storage yard to check it out. They found the problem, and we wound up buying new shafts. It worked.
Ron Michalak
West Bloomfield, Michigan
PMM NEWSLETTER: GOOD READING
I started to read the article by Sharon Kirchner in Volume 1 of the June electronic newsletter, and I am now behind schedule for the rest of the day. "Considering the Cruising Life?" was wonderful reading and contained very helpful advice. Thanks to Sharon and PMM for the great stuff.
Larry Kilbride
Kirkland, Washington
HEADED WEST AND LOOKING FOR COMPANY
We are getting ready to make a trip from Stuart, Florida, to San Diego on our 62 Nordhavn and thought PMM readers might be interested in our blog site: http://www.adventure-earth-harbor.blogspot.com/. We are avid readers and would like to meet others who may be making the same trip.
Mark Holt
Adventure
Mason City, Iowa
BATTERY BANK SIZE AND CHARGE CAPABILITY
I have a question about battery bank size. I have a 58-foot cat and a bank of 20 6-volt, 225Ah batteries set up for 24 volts, which I believe would be 1,125Ah at 24 volts. My charging system consists of a 120-amp/24-volt alternator on each engine. I usually run one engine when motoring. The majority of charging is with the genset, which puts out 100 amps/24 volts.
In one of his "Gearhead" columns (PMM July/Aug. '09), Steve D'Antonio stated that the charge source's output should be no less that 25 percent of the battery bank's amp-hour capacity. That would require me to have an output of some 280 amps! Is there a reason other than extended charging time that would affect the batteries' life span? I am planning on using flooded cell batteries to replace my 5-year-old gel cells, since I see no real advantage to using gel or AGM batteries, other than less maintenance.
Pete Tuls
S/Y Downtime
Indeed, the reason for the 25 percent rule is establishing a reasonable recharge time. For flooded cells, that's the limit of their charge acceptance rate, so you couldn't go any higher. However, AGM and gel batteries (especially AGMs) can accept more charge. In short, yes, you would need a larger charge source-280 amps-to meet the 25 percent guideline.
Using a 120-amp alternator or 100-amp charger to charge your 1,000-plus-amp house bank will be a slow process, tantamount to using a 40-amp alternator to charge a 400Ah house bank, a ratio that would be considered woefully low for a common house bank on smaller vessels. You should limit your discharge to 50 percent, which means you'll be replacing about 500Ah. You can do it; it will simply take a very long time, on the order of seven to eight hours, depending on the temperature (your charger/alternator is temperature compensated, right?) and that will probably not be a 100 percent charge. Remember that the energy replacement ratio isn't necessarily linear. The batteries will likely not accept that 100 amps for the entire charge time.
If you are using a brushless-capacitor-style generator to operate the 100-amp charger, this, too, will likely diminish its output, often by 20 percent. The result is likely to be chronically undercharged batteries, which will lead to sulfation and further diminished capacity and life span. Finally, a flooded bank of this sort, especially one that's prone to sulfation, should be equalized periodically. Your 100-amp charger should have this function.
-Steve D'Antonio
