Trim Tabs Zinc anodes

[Streff]

Hello,

During the survey haul-out, I did not notice zincs on the Bennett trim tabs. At that time the trim tabs showed no sign of corrosion whatsoever. I did not think much of it as our 45' Californian has been moored in freshwater for the last several years. There were anodes elsewhere as usual.

The boat has now been moored in saltwater for several months, I had a diver come out to change all the original anodes including the zincs for Bennett trim tabs and showed the diver how they should be installed based on diagrams from the Bennett site.

At the end of the dive, the diver indicated that he does not think that zincs were ever installed on the tabs and it would be best if I installed them at the next haul-out. It may have been a challenging process for him.

My question is: what is the forum experience with Zincs for trim tabs?. I could not reach anyone at Bennett's but I will keep trying but thought I would ask the wise knowledge of the members here.

Thank you all

Streff

 

[Jon H]

Trim tab anodes should be the same alloy as the others on your boat.
If hull anodes are zinc then the trim tab anodes should be zinc.

My boat is welded aluminum thus I have aluminum alloy anodes on
both the hull and trim tabs.

A classic case of mismatched anodes is a outboard powered, fiberglass boat - where the aluminum alloy anodes on the outboard waste away while
the zinc anodes on the stainless steel trim tabs never do anything.

Between the stainless steel tabs and the zinc anodes on them, the aluminum alloy anodes on the outboard are fighting a loosing battle.

 

[Keysdisease]

Bennett supplies zincs installed on their trim tabs from the factory to Boat Builders that specify zincs. 1/2 an "R" (Rudder) zinc, an R2 or R3 depending on tab size installed on the top in the middle of the tab with a pan head stainless steel screw. Only the screw head is on the bottom.

There are Bennett trim tab zincs that install between the actuator and the tab. They are convenient, but they are small, not centered, and as they waste the screws holding the actuators to the tabs may become loose.

If you tab has a hole in the middle of the field of the tab it's probably had a factory installed zinc. R zincs are readily available, inexpensive, and have mass in the right place on the tab to be effective.

 

[rslifkin]

My tabs originally had R3 zincs using both the top and bottom halves. I changed this winter to R5s on the top side only, just the screw on the bottom. Mind you, my tabs are huge 48x12s, so a smaller tab may be fine with smaller zincs.

Some companies also sell zincs that attach where the actuators screw to the tab.

 

[Keysdisease]

That's fairly common and understandable. For Trawler type boats the small amount of drag and lost tab efficiency due to flow disturbance from an R zinc on the bottom isn't noteworthy.
But if it is felt that more zinc is needed, your move to a larger zinc or adding an additional zinc on a large tab like your 4 footers, on top, is good practice.

I had 42" x 12" on thee olde Mainship MKI, 1 R4 half on top.

My tabs originally had R3 zincs using both the top and bottom halves.