Sterngear corrosion

[tonybarley]

Good morning,

I have a problem about which I hope you may be able to advise me.

In October last year, I had a Bennett Trim Tab system installed on my 10 metre, Sealine power cruiser. At the same time, I had the twin prop sterndrives totally refurbished, anodes replaced and antifouled.

A few weeks ago, I experienced a failure in one of the sterngear legs and, when the boat was lifted out of the water, it appears that both the sterngear legs have corroded due to electrolytic reaction in the marina (saltwater environment).

I am told that, after a total rebuild of the sterndrives, the boat will need to be placed back into the marina in order to establish what is causing the corrosive/electrostatic effect.

I notice over the last few days that the trim tab 'flaps' and Bennett cylinders are unaffected. There is no electricity supply to the mooring; I therefore rule out any 'mains' effect from the marina.

I also notice the trim tabs are able to be operated without switching on the ignition; I am surprised the trim tab system was wired directly into the batteries and not into the ignition!

The question is, would it be possible, due to negligent installation, that there is an electrical potential has been created on the flaps and this has caused the damage to the sterndrives

Any help or advice would be greatly appreciated.

Kind regards

Tony Barley
Tenerife
Canary Isles
Spain

 

[[2068]]

Hmm. What are the trim tabs made out of? What happened to the anodes (why wasn't it spotted that they had disappeared, if that's what has happened)

dv.

 

[cloud7]

On the point of the engine not switched on , i have a 2004 sealine, & can move /operate the trim tabs via the helm trim switch , without the engines on.

 

[tonybarley]

The tabs are aluminium

 

[carlton]

Mine are also wired directly in such that they work independent of the ignition.

Anodes replaced in October - 10 months ago, and no mains power on at all ?

How about your near neighbours in the marina? Do they have 240v. hook-up? I've heard tales of rapid corrosion from the bloke next door leaving 240v. on permanently. Stainless props won't help either, if that's what you have.

Not sure about the electrical potential difference being strong enough if, as you say, your tabs are aluminium. I'm sure one of the resident experts will be along shortly to offer more helpful suggestions.

 

[Peanuts]

I always stand to be corrected, but are you sure they are aluminium, my Bennett trim tabs were stainless steel.
Do you or did you have anodes on the tabs and as asked already do you stainless or alloy props? Do you reverse in and the stern of the boat next to a steel pile? Sorry for all the questions I am only trying to help - honestly.

 

[tonybarley]

Good questions!!

The Bennett system utilises the original tabs fitted on manufacture by Sealine; I guess that they are aluminium.

Your question regarding a steel pile is intriguing. We moor 'nose in'. That way, we have more privacy on the boat from people walking up and down the pontoon. We are right next to a steel pile that is at least 30 feet long!! I notice that the port engine sterndrive is more corroded than the starboard and, of course, the pile is closer to the port side of the boat. Not being too technically minded, what do you suggest I do next? I am au fait with an avometer.

 

[tonybarley]

[ QUOTE ]
Mine are also wired directly in such that they work independent of the ignition.

Anodes replaced in October - 10 months ago, and no mains power on at all ?

How about your near neighbours in the marina? Do they have 240v. hook-up? I've heard tales of rapid corrosion from the bloke next door leaving 240v. on permanently. Stainless props won't help either, if that's what you have.

Not sure about the electrical potential difference being strong enough if, as you say, your tabs are aluminium. I'm sure one of the resident experts will be along shortly to offer more helpful suggestions.

[/ QUOTE ]


Hi Dave

We transferred the boat 70 nautical miles from Santa Cruz to Los Gigantes in Tenerife without a hitch; excellent voyage. However, after 3 days, the local authorities condemned the entire marina's power supply!! There has been no onshore mains since November. One or two boaters, including ourselves, use a portable generator a couple of times a week just to keep the battery systems topped up.

 

[halcyon]

One missing clue, Dave has lost one trim tab indicator. The question I raised was, do they fit a sensor on the trim tab plate to measure plate angle for the gauge ?.
If so, and a saeal fails, you have 12 volt or so leaking. If only the engine is earthed, then only the legs will corrode, more on the faulty tab side.
Sealine tabs when this boat was built would have been stailess.

Brian

 

[tonybarley]

Thanks Halcyon. One of the gauges has failed (under guarantee) and the engineer has agreed to fix this without a fee. Looking at the Bennett Website, I can see fitting instructions for my standard unit but there does not appear to be an electrical diagram to indicate the set up. I agree, if there is some sort of potentiometer below the water line, this could be the root cause. However, there appears to be nothing below the surface apart from the hydraulic cylinder and the nylon (therefore non conductive) hydraulic line.

 

[Its_Only_Money]

[ QUOTE ]
One missing clue, Dave has lost one trim tab indicator. The question I raised was, do they fit a sensor on the trim tab plate to measure plate angle for the gauge ?.
If so, and a saeal fails, you have 12 volt or so leaking. If only the engine is earthed, then only the legs will corrode, more on the faulty tab side.
Sealine tabs when this boat was built would have been stailess.

Brian

[/ QUOTE ]

The Bennet sensor is within the ram itself I think, wiring exits behind the upper hinge so nothing visible outside the transom - don't know whether an electrical leak to seawater could occur though...

 

[Peanuts]

That was my thought as well, the Bennett tabs are hydraulic but like yourself not sure about the sensors. Going back to a previous question, what were the condition of the anodes, both on the tabs and the outdrives, you mention that the outdrives had been overhauled, were all of the anodes electrically bonding to their respective parts, and were the earth bonding wires fitted. I have seen some terrible remains of outdrives/rams because they were left off.
I always double up on the transon anodes as they recommend for stainless props.
When I had a merc outdrive I was going thru a trim tab anode about 1-2 months (iron framed pontoon caused this) I fitted the mercathode system and never had to replace another anode, one day I'll get round to fitting a pair to current boat - 1 set per outdrive and the mercruise set up is better than the VP one

 

[Its_Only_Money]

[ QUOTE ]
I fitted the mercathode system and never had to replace another anode, one day I'll get round to fitting a pair to current boat - 1 set per outdrive and the mercruise set up is better than the VP one

[/ QUOTE ]

In what way better out of interest?

 

[Peanuts]

Simon

From a technical point of view I have no idea, the reason for that statement was that 2 seperate VP technicians came out with it and they worked for different workshops/areas so they weren't biased. All I can tell you is that is certainly worked for me.

Steve